Identification of failure modes to apply condition monitoring methods on critical components of a hydraulic stamping press machine
Diogo de Figueiredo Tomás
Maintenance Engineering, master's level (120 credits) 2020
Luleå University of Technology Department of Civil, Environmental and Natural Resources Engineering
ACKNOWLEDGEMENTS
The present thesis would not be possible without the extraordinary team that supported me, during this time. A special thank you to Professor Diego Galar, my external supervisor, that gave me the opportunity to work within an area of passion, in the field of Condition Monitoring, and providing me constant encouragement and guidance To my supervisor in Luleå University of Technology, senior lecturer Mattias Holmgren, a deeply thank you to his special availability, and total support, this thesis was considerably improved, due to his relevant comments, suggestions and revisions. To my examiner and program director Maintenance Engineering in Luleå University of Technology, senior lecturer Johan Odelius, I would like to thank for his guidance and support during the master program and thesis. For all people in Luleå University of Technology - Sweden, in special my colleagues from the master programme as Danial, Johnny, Laila, Aron, Alvaro, Jaya and Ignacio, a big thank you all for this extraordinary journey that we did together, with mutual support. For all people in Portugal, in special to Professor José Torres Farinha, a big thank you for all the opportunities, advises and support during all my master programme. At the end, I would thank to my mother Odília, my father António, and my girlfriend Joana, for their encouragement, and continuous support during all this time. I hope you enjoy reading this thesis. In case of suggestions, comments or questions, please feel free to contact [email protected]
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Abstract
This thesis is the result of a four months internship in a company that aims to transform technology into value for industry. The industries face a wide range of engineering challenges. The requirements and competitiveness of the industry demands high dependability of the assets. During operations of hydraulic stamping presses, unexpected failures may happen, affecting the production and the overall equipment effectiveness, and therefore, the objectives of the company. It might have serious repercussions, both monetary and reputational, this last one normally difficult to recover from. That is the reason why it is important to find solutions which can prevent these situations to happen. One of the challenges with hydraulic press stamping machines is that there are multiple failure modes that affect the machine dependability. There is a big variety of sensor technologies available in the market, that can be used to monitor the health condition of the hydraulic stamping press machines, providing decision makers with reliable information to coordinate maintenance activities. However, it is required to the users to know the specific condition monitoring methods for each failure mode. The present thesis has the purpose to identify and propose strategies in order to leverage the dependability of hydraulic stamping press machines, to achieve the performance objectives. It serves also as base for the creation of future maintenance plans. The first step is to understand the system operation under study, and then identify failure modes that might happen in the asset. The next step is to select and suggest condition monitoring methods to monitor the machine’s failure modes. Based on this information, excellence in the planning of maintenance and production can be achieved. The Reliability Centered Maintenance (RCM) method and the Failure Modes, Effect and Criticality Analysis (FMECA) were used in present thesis to assess failure modes, criticality and prioritize maintenance interventions. The thesis work presents condition monitoring methods for the analysed subsystems of the stamping press machine. The failure analysis of the subsystems and the proposed condition monitoring methods were based on the related literature and technical staff recommendation. How proposed condition monitoring methods can be complemented with further analysis is discussed. A qualitative approach was used to rank the selected subsystems and components based on their criticality. As a conclusion, the slide of the guide rail was considered the most critical component. The reason is that all the press operation is based on force and guiding precision.
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Abstract (Swedish)
Denna uppsats är resultatet av en praktik på fyra månader i ett företag som vill omvandla teknik till värde för industrin. Industrin står inför en lång rad tekniska utmaningar. Behoven och konkurrenskraften i branschen kräver att tillgångarna har hög tillförlitlighet. Vid drift av hydrauliska stanspressar kan oväntade fel inträffa som påverkar produktionen och den totala utrustningens effektivitet och därmed företagets mål. Det kan få allvarliga konsekvenser, både ekonomiska och anseendemässiga, som kan vara svåra att återhämta sig från. Därför är det viktigt att hitta lösningar som kan förhindra att sådana situationer uppstår. En av utmaningarna med hydrauliska pressmaskiner är att det finns flera typer av fel som påverkar maskinens driftsäkerhet. Det finns ett stort antal olika sensortekniker på marknaden som kan användas för att övervaka tillståndet hos de hydrauliska pressmaskinerna och förse beslutsfattarna med tillförlitlig information för att samordna underhållsverksamheten. Användarna måste dock känna till vilken övervakningsmetod som är lämplig för varje typ av fel. Syftet med denna uppsats är att identifiera och föreslå strategier som påverkar driftsäkerheten hos hydrauliska pressmaskiner och som kan forma en grund för framtida underhållsplaner. Det första steget är att förstå driftprocessen och sedan identifiera möjliga feltyper som kan inträffa hos enheten. Nästa steg är att välja ut och föreslå tillståndsövervakningsmetoder för att övervaka maskinens feltyper. Med denna information som grund kan planeringen av underhåll och produktion optimeras. Funktionssäkerhetsinriktat underhåll (reliability cantered maintenance, RCM) och feleffektsanalys (Failure Modes, Effect and Criticality Analysis, FMECA) användes i denna uppsats för att utvärdera feltyper, kritikalitet och prioritera underhållsåtgärder.. Uppsatsens presenterar förslag på tillståndsövervakning metoder för de analyserade delsystemen. Felanalysen av utrustningen och de föreslagna metoderna för tillståndsövervakning baserades på befintlig litteratur och rekommendationer från teknisk personal. Hur den föreslagna tillståndsövervakningen kan kompletteras med ytterligare analyser diskuteras också. Ett kvalitativa tillvägagångssätt användes för att rangordna de utvalda komponenterna utifrån dess kritikalitet. Sammanfattningsvis visade kritikalitetsedömningen att gejdrarna är den mest kritiska komponenten hos pressmaskinen.
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Contents
1. INTRODUCTION ...... 1 1.1. Problem statement ...... 1 1.2. Purpose ...... 1 1.3. Objectives ...... 2 1.4. Approach ...... 2 1.5. Delimitations ...... 2 1.6. Scope ...... 3 2. THEORY FRAMEWORK ...... 5 2.1. Maintenance ...... 5 2.2. Maintenance Objectives ...... 6 2.3. Maintenance Management ...... 6 2.4. Types of Maintenance ...... 7 2.4.1. Corrective Maintenance ...... 8 2.4.2. Preventive maintenance ...... 9 2.5. Maintenance as an investment ...... 10 2.6. Dependability...... 11 2.7. Reliability Centered Maintenance ...... 12 2.8. Failure Modes and Effect Analysis ...... 13 2.8.1. Criticality Analysis (CA) ...... 14 2.9. Defining the System Hierarchy ...... 17 2.10. Hydraulic Stamping Press Machines ...... 18 2.10.1. Mechanical Press ...... 19 2.10.2. Hydraulic Press ...... 19 2.11. Condition monitoring ...... 22 2.11.1. Vibration analysis ...... 23 2.11.2. Motor current signature analysis ...... 24 2.11.3. Thermal Analysis ...... 25 3. METHOD ...... 27 4. RESULTS ...... 31 5. DISCUSSION AND CONCLUSIONS ...... 41 6. RECCOMENDATIONS AND FUTURE WORK ...... 45 7. REFERENCES ...... 47 ANNEX A ...... 49
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Figure 2.1: Relationship of maintenance with other subjects [1]...... 5 Figure 2.2: Stages of a physical asset’s life cycle [1]...... 7 Figure 2.3: Overall view of Maintenance strategies [2]...... 8 Figure 2.4: P-F curve ...... 10 Figure 2.5: PM vs CM costs, and optimum cost [3] ...... 11 Figure 2.6: Roles of reliability and maintainability on system performance [3]...... 11 Figure 2.7: Bathtub curve [1]...... 12 Figure 2.8: Qualitative criticality matrix [10] ...... 16 Figure 2.9: Legend for criticality assessment ...... 17 Figure 2.10: Plant hierarchy of parts [11]...... 17 Figure 2.11: Schematic of system approach in metal forming (using deep drawing as an example) [13]...... 18 Figure 2.12: Relationships between process and machine variables [13]...... 19 Figure 2.13: Hydrostatic principle [13] ...... 20 Figure 2.14: Hydraulic pumps commonly used in presses: (a) external gear pump, (b) bent axis axial piston pump [13]...... 20 Figure 2.15: Arrangement of a press hydraulic sub-system [12]...... 21 Figure 2.16: Condition monitoring methodology ...... 22 Figure 2.17: Charge mass accelerometer a) Construction [17], (b) Product example[18]. . 23 Figure 2.18: Rogowski coil (a) Construction (b) Product example [21]...... 24 Figure 2.19: K-Type Thermocouple (a) Construction, (b) Product Example [23] ...... 25 Figure 4.1: Single-action hydraulic press with active draw cushion for counter drawing. Adapted from [12]...... 31 Figure 4.2: Main sub-systems of a hydraulic stamping press machine ...... 31 Figure 4.3: Main components of each sub-system of a hydraulic press stamping machine...... 32 Figure 4.4: Taxonomy of the hydraulic sub-system, components and items [12]...... 33 Figure 4.5: Taxonomy of the mechanical system, components and items [12]...... 33 Figure 4.6: Taxonomy of the electrical system, components and items [12]...... 34 Figure 4.7: Analysis of the [10]-hydraulic sub-system, and respective selected components [10.1]-hydraulic pumps and [10.2]-cylinders, in terms of its functional failures and corresponding failure modes ...... 34
Figure 4.8: Analysis of the [20]-mechanical sub-system, and respective selected component [20.3]-slide guide, in terms of its functional failures and corresponding failure modes ...... 35 Figure 4.9: Analysis of [10.1]-hydraulic pumps, in terms of its functional failures and failure consequences...... 35 Figure 4.10: Criticality assessment of the [10.1]-hydraulic pumps ...... 36 Figure 4.11: Analysis of [10.2]-cylinder, in terms of its functional failures and failure consequences ...... 36 Figure 4.12: Criticality assessment of the [10.2]-cylinder...... 36 Figure 4.13: Analysis of component [20.3]-slide guide rail, in terms of its functional failures and failure consequences ...... 37 Figure 4.14: Criticality assessment of the [20.3]-slide guide rail ...... 37 Figure 4.15: Criticality assessment of the analysed components ...... 37 Figure 4.16: Suggestion of condition monitoring methods for component [10.1]-hydraulic pump ...... 38 Figure 4.17: Suggestion of condition monitoring methods for component [10.2]-cylinder 39 Figure 4.18: Suggestion of condition monitoring methods for component [20.3]-slide guide rail ...... 40 Figure 6.1:Proposed flowchart for future work [25] ...... 45
Introduction
1. INTRODUCTION
The excellence in the management of assets is paramount relevant on today’s industry, facing a high level of competitiveness. It is vital to ensure high standards of quality and efficiency on the production. The productivity, availability and safety are aspects that need to be ensured and improved, by the use of the best maintenance methodologies. The technological advances that allow it, bring new opportunities that transforms maintenance as a whole, where data is essential to ensure effective planning and decision making. Different approaches have evolved, as the Condition Based Maintenance (CBM), where technologies are essential to monitor the degradation of an asset. There are different technologies, each one with its purpose.
1.1. Problem statement
The company where the thesis work took place is a Research and Technological Development Company. A partnership was done, with a company having its core business in stamping press operations. The present thesis has the purpose to identify and propose strategies that can leverage the dependability of hydraulic stamping press machines, to accomplish the performance objectives, serving also as base for the creation of future maintenance plan. As result, Condition Monitoring methods are proposed for selected most critical subsystems, according to technical staff and expertise.
1.2. Purpose
The purpose of this thesis work is to identify and propose strategies in order to increase the dependability of hydraulic stamping press machines, to achieve the performance objectives. It also serves as base for the creation of future maintenance plans. Dependability is the most important function of maintenance. It is the ability to put an asset in a condition so that it is able to perform as and when required. In other words, dependability is a trustworthiness measure that the item will perform as is supposed and when required. In order to achieve the purpose, the first step is to identify failure modes to contribute for that. After, it is proposed different technologies to monitor the most critical components on the stamping press machine. Condition Monitoring of physical assets involve the use of several methods to monitor the different failure modes. By collecting and analysing data, it
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is able to assess the degradation state of the asset. By following this strategy, it generates significant gains in the reliability and performance [1].
1.3. Objectives
The objectives of this thesis work can be summarised as follows: (i) Understand the hydraulic press machine as a system and operation; (ii) Use of FMECA to identify the failures and failure modes of the selected critical sub- systems and components of the press machine; (iii) Use of the RCM method to rank the criticality of the selected critical sub-systems and components of the press machine; (iv) Suggestion of condition monitoring methods for each failure modes and its failure effects of the selected critical sub-systems and components of the press machine.
1.4. Approach
(i) Describe the hydraulic stamping press machine system; (ii) Select the sub-systems and components to be analysed; (iii) Divide the sub-systems in components; (iv) Describe its failures and failure modes; (v) Describe its failure effects (vi) Assess criticality of the selected sub-systems and components; (vii) Recommend condition monitoring methods for each failure mode and its failure effects of the analysed sub-systems and components;
1.5. Delimitations
The present thesis work will analyse hydraulic stamping press machine from a specific manufacturer. The focus is to find failure modes on a particular machine type. Other manufactures may have different configurations and subsystems, resulting in different failures and failure modes, whose are not part of this thesis work, and therefore, not included. Also, for machines with different driving systems, as per mechanical presses or servo presses, failure modes may diverge. Taking this in consideration, the characterisation and suggested condition monitoring methods are specific for hydraulic stamping press machines.
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Introduction
1.6. Scope
The present thesis work is divided into six chapters. Chapter 1, introduction, covers the problem statement, purpose, objectives, approach, and explains the delimitation of the research study. Chapter 2, theory framework, includes essential topics, concepts and assumptions to achieve the objectives settled for this thesis work. Chapter 3, method, is an explanation on how the objectives will be achieved. Chapter 4, results, present the outcomes of this thesis work, in form of explanation and hierarchical figures that explain the different objectives of the analysis of the selected components. Chapter 5, discussion and conclusions, discusses the suggested condition monitoring methods and technologies and methods for fault isolation. Chapter 6, recommendations and future work, provides an idea and highlights the steps necessary for a complete implementation of the whole system.
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Theory Framework
2. THEORY FRAMEWORK
This chapter explores the essential topics, concepts and assumptions necessary to achieve the purposed objective of this thesis work. For that, the essential of maintenance and related vocabulary, objectives, management and investment are presented. The dependability concept is explained, in which this thesis focuses on. The maintenance concept of reliability centered maintenance (RCM) is discussed, as well as the Failure Modes and Criticality Analysis (FMECA) methodology, linking system to component failures. Condition monitoring methods are then presented, serving as base to the recommendations in the result and discussion chapter.
2.1. Maintenance
According to EN 13306:2017 “Maintenance - Maintenance terminology”, maintenance is the combination of technical, managerial, and administrative decisions and actions during a life cycle of an item intended to retain or restore it into a state in which it can perform the required function [2]. Figure 2.1 demonstrate the different disciplines of maintenance.
Figure 2.1: Relationship of maintenance with other subjects [1].
According other authors, the maintenance should not only restore or retain the asset, but also create a set of activities that aims to preserve and/or improve the safety, performance, reliability, and availability of plant structure, systems and components, to ensure the adequate performance of the assets, when it is required. This additional aspect expands the importance of the maintenance subject, expanding its influence, having a direct impact on
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final product quality, process control and safety of operators, conformity with environmental aspects, structural integrity of the system, and at the end, the global efficiency [1],[3],[4].
2.2. Maintenance Objectives
According to the standard, the maintenance objective is the target assigned to accept maintenance interventions and/or related activities [2]. The maintenance strategies and objectives need to be aligned with the overall business mission objectives for the entire organisation. The classification of maintenance objectives can be divided into short-term objectives and long-term plans. The short-term objectives, include the implementation of a specific maintenance strategy for an area or asset, training program, inventory management program, and overtime reduction to meet specified objective. The long-term plans, the effectiveness of production and cost management, personnel development, and technological leadership. Both terms should be aligned, and need to ensure that the general and maintenance business are in consonance, to achieve its mission [3]. The competitiveness of modern industries increased significantly in the past decades with market globalisation. The maintenance function is now paramount relevant to achieve the general business goal. New requirements of effectiveness, combined with minimum cost, has created additional challenges on setting maintenance objectives and on maintenance management activity.
2.3. Maintenance Management
Maintenance management deals with maintenance related company’s assets and resources. It combines strategic, tactical, and operational levels of knowledge to support and implement the most suitable decisions. A correct maintenance plan positively influences the technical state of an object, and may extend its lifetime significantly [2], [3]. According to the standard, maintenance management is necessarily aligned with the overall management, coordinating efficiently a set of activities to achieve business objectives and goals. It involves the implementation of maintenance planning, control, and improving maintenance activities and economics [2]. Maintenance management can be assessed using Key Performance Indicators (KPIs). KPIs are based upon the goals and objectives of maintenance, and are the most important guide to monitor the performance of the maintenance as a whole [1]. However, the standard definition of maintenance only emphasises its importance during the life cycle of an asset, but do not take in consideration the extreme points of an asset’s life.
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Theory Framework
Some authors consider that the acquisition phase and withdrawal, both beginning and final stages of the life cycle, need to be included in the maintenance management decisions, as shown in Figure 2.2, and is further explained in [1].
Figure 2.2: Stages of a physical asset’s life cycle [1].
Having this problem in mind, it was created the asset management standard ISO 55000:2014 “Overview, principles and terminology”, ISO 55001:2014 “Management Systems- Requirements” and ISO 55002:2018 “Guidelines for the application of ISO 55001”. These standards define requirements for a management system for asset management, common language and framework for decision making. It provides an overview of the management of assets, principles, terminology and expected benefits with the adoption of management best practices, for all types and sizes of organisations.
2.4. Types of Maintenance
Strategy is defined differently depending on the objective. According to the dictionary of Cambridge:2020, strategy is defined as “a detailed plan for achieving success in situations such as war, politics, business, industry, or sport, or the skill of planning for such situations”. In the maintenance area, maintenance strategy is a management method that is used to achieve the defined maintenance objectives of each organisation [2]. The maintenance strategy is influenced by both internal and external factors, and it should be aligned to the general objective of the company. The combination of technical, managerial and administrative decisions is paramount relevant to set a maintenance strategy [5]. Maintenance strategies aims to (i) control or prevent the deterioration process that leads to failure of an asset or (ii) restore the object to its operational state through corrective actions after a failure [3].
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The standard defines maintenance types and strategies, as well as its terminology [2]. (i) Preventive maintenance – maintenance carried out according to prescribed criteria of time, usage or condition, in order to reduce the likelihood of failure or degradation of the operation of an asset. (ii) Corrective Maintenance – maintenance activities carried out after the detection of a fault, and it is intended to restore an asset to a state in which it can perform a required function.
For corrective maintenance, the maintenance actions should be taken immediately. However, in some cases it may be deferred, depending on the criticality value of the asset. If the maintenance action is planned based on asset’s condition, i.e. derived from inspections, testing, or condition monitoring, then it is the so-called condition-based maintenance (CBM). If CBM has a predictive module, that derives from a forecast that evaluates key parameters of the degradation of the asset, then is so-called predictive maintenance (PdM) [2] The overall view of maintenance strategies is shown in Figure 2.3.
Maintenance
Preventive Corrective Maintenance Maintenance
Condition Based Predetermined Deferred Immediate Maintenance Maintenance
Monitoring Functional Clock- Usage- and Test Based Based Inspection
Figure 2.3: Overall view of Maintenance strategies [2].
2.4.1. Corrective Maintenance Corrective Maintenance (CM) is carried out after the recognition of a fault, and is intended to restore a failed asset into a state in which it can perform a required function [2]. For an optimal decision, it may have in consideration cost and impact of the action in future failures. The CM actions are divided in two time based further subcategories, immediate (emergency) CM, that is carried out immediately upon the detection of fault state, in critical assets or failures, to restore an item to a state in which it can perform its required function,
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Theory Framework
and avoid unacceptable environmental and safety consequences. The deferred CM is not performed immediately after the detection of a fault state, but is delayed in accordance with given maintenance rules [3].
2.4.2. Preventive maintenance The PM is carried out according to a prescribed criterion of time, usage or condition. PM is divided in two further subcategories, named predetermined, and condition-based maintenance. Some authors, consider a third subcategory, called opportunistic maintenance, that is carried out at convenient moments which are unpredictable [1],[3]. The PM actions can vary from minor servicing with a short downtime, as lubrication, visual inspection and testing, to major overhauls that require a significant amount of downtime, requiring appropriate planning and resources. PM actions aims timely detect failures, onset and hidden failures, before a breakdown or fault occurrence.
2.4.2.1. Predetermined maintenance Predetermined maintenance is carried out at pre-established time intervals, or a measure of the usage, without previous control of the asset’s “health” condition [2]. The Clock Based maintenance, is divided in calendar clock and age clock. The calendar clock, are maintenance actions that should be performed out at predetermined time instants, based on this clock. The age clock, sets to zero the clock when the beginning of operations of the item, and PM interventions should be based on an item reaching some age. The usage-based maintenance is meter-based, and when the usage of an item or machine passes a specific threshold, it triggers a work order. Most of the PM actions are based on usage of an item because it is easier to manage than the clock-based actions.
2.4.2.2. Condition Based Maintenance Condition Based Maintenance (CBM) is a PM strategy that uses continuously or request condition monitoring of relevant parameters and/or function of the asset, correlating it with the degradation over time. The selected variable can provide both direct or indirect measurement of degradation, be monitored continuously or at discrete points in time, online or offline. The success of condition-based maintenance depends on the capability of a maintenance team to detect a potential failure with enough antecedence, before a catastrophic event happens. Usually the CBM approach uses the P-F curve, shown in Figure 2.4, that is support tool, and it illustrates the degradation level of the asset, over the time.
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Figure 2.4: P-F curve
At time t1, a failure starts to develop. At time t2, a failure has the Potential to be detected (P). If the failure is not detected and mitigation actions are not taken, a functional failure (F) may occur a time t3. The available window to carry maintenance actions is between P and F is commonly called the P-F interval. After t3, the cost of some intervention is usually much more expensive than preventive action. At point tf, a catastrophic failure happens, and the asset no longer can perform its required function [1],[3]. In both Functional Test, and Monitoring and Inspection strategies, the data is used to assess the degradation state of the asset. In functional testing, it focuses on deciding whether to take immediate maintenance actions, while in monitoring and inspection, the data is used to predict the degradation into the future, known as predictive maintenance (PdM), allowing time for a proper planning of both activities and required resources.
2.5. Maintenance as an investment
The maintenance activities are usually associated to multiple costs. The different maintenance strategies, CM and PM, from a financial perspective, have complex shapes that indicates that there is an optimal effort of the PM strategy, as it is shown in Figure 2.5. The most recent advancements in the area, now consider that maintenance should be perceived as an investment. A robust maintenance strategy protects and safeguard key assets, increase its efficiency and allows the generation of profits for the company [6].
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Theory Framework
Figure 2.5: PM vs CM costs, and optimum cost [3]
2.6. Dependability
The degradation of engineering objects consists of a natural process along with age and/or usage, leading to a failure after a certain time. A failure may have serious economic and/or safety consequences. An item is said to have a failure when it cannot carry on its intended function, for which it was built or designed. Maintenance activities play an important role in preventing the unreliability of an object during its life cycle. For an effective maintenance, it needs to consider the reliability of the object [3]. According to the standard, reliability is an inherent property of the items and is defined as the ability of an asset to perform a required function under certain conditions, during a given time interval. The maintainability concept, intrinsically related to reliability, is defined as the ability that an item, under specified conditions of use, to be retained in or restored to, a state in which it can perform the function it was designed to, when maintenance is performed under given conditions and using specified procedures and resources [2]. The role of both concepts on system performance is shown in Figure 2.6.
Figure 2.6: Roles of reliability and maintainability on system performance [3].
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Both reliability and maintainability concepts converge to the availability definition. Availability is defined as an ability of an asset, under specified conditions, to be in a state to perform as and when required, if the necessary external resources are provided. The availability not only depends on the combined aspects of the reliability, maintainability, but also from recoverability of the item, and the maintenance supportability. The terms referred above converge to the dependability concept. Dependability is the ability to perform when required and include availability and its influencing factors, as the reliability, maintainability, and supportability [2] The reliability of equipment is symbolically represented by the so-called “bathtub curve”. It is a simple representation of the failure rate function. Failure rate is the frequency with which a system or component fails, expressed in failures per unit of time. It can have distinct shapes/regions, as it is shown in the Figure 2.7.
Figure 2.7: Bathtub curve [1].
Each region has implications for the appropriate type of maintenance. The proper categorisation of a particular asset is fundamental for the implementation of the correct maintenance strategy and measures [1]. (i) Region A (decreasing failure rate), the failure is due to manufacturing and/or assembly errors. (ii) Region B (constant failure rate), the failure is random (and is not affected by age). (iii) Region C (increasing failure rate), failure is due to the wear out or aging effect.
2.7. Reliability Centered Maintenance
The Reliability Centered Maintenance (RCM) is a well-established method that leads to the increasing of cost efficiency, and dependability of the equipment. It is also a great tool to understand the level of risk of the equipment under study, allowing to prioritize interventions
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Theory Framework
in the system’s equipment. RCM is an engineering framework that allows the definition of a complete maintenance system. It enables to monitor, assess, predict and comprehend the operation of the physical assets. The RCM method is the initial part of identifying the operating context of an asset, and identify a Failure Mode, Effect and Criticality Analysis (FMECA). Additionally, it presents the logic process to be followed, when creating a maintenance program to be implemented [7]. The RCM process aims to answer the following seven basic questions regarding the assets or systems under analysis: (i) What are the functions and associated performance standards of the asset in its present operating context? (ii) In what ways can it fail to fulfil its functions? [Functional Failures] (iii) What causes each functional failure? [Failure Modes] (iv) What happens when each failure occurs? [Failure Effects] (v) In what way does each failure matter? [Failure Consequence] (vi) What can be done to predict or prevent each failure? [Recommendations] (vii) What should be done if a suitable proactive task cannot be found? [Mitigation]
The questions from (i) to (iv) are answered when performing a FMEA or FMECA (C for Criticality). On question (v), the failure consequence is described, allowing the prioritisation of the equipment in terms of the consequences of the failures. Therefore, the higher the ranking of each failure, should be addressed firstly. RCM is considered fundamental in the design phase, especially for components that are difficult to access or not accessible. If the equipment is already in use, design modification for those components is recommended [7].
2.8. Failure Modes and Effect Analysis
In the context of quality assurance and reliability of equipment and processes, it is important to link system failures to component failures. It can be done using the Failure Modes and Effect Analysis (FMEA) methodology [3]. The manner in which the asset in unable to perform the required function is known as Failure Mode [2]. The failure of a system is a result from the failure of one or more items in the system [8]. The FMEA a methodology review as many components, assemblies, and subsystems as possible, in order to identify
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failure modes, their causes, and resulting effects. It establishes corrective action priorities, intended to mitigate probable failures, that may have prejudicial effects on function or hardware on a mission. The application of FMEA is primarily on demonstration and validation phase, but it also can be applied during ongoing production and deployment, to analyse major modifications or final hardware design .
The FMEA tool can be used to achieve the following objectives: (i) Consider all the failure modes and their effects for the operational success of the system. (ii) Catalogue and identify the magnitude of failures and their effect (iii) Ensure higher reliability and security indexes, and promote the cost reduction, quality and customer satisfaction, by improving projects of equipment and processes. (iv) Develop operational and scientific knowledge about failure modes, to design new/ or improve maintenance procedures. Each FMEA is usually dictated to the specific task or objective. The FMEA procedure may have both top-down or bottom-up approaches. It may be performed for hardware, functional, or combination analysis. For both FMEA of hardware or function, the methodology is similar [9]. For FMEA planning, a set of organisational key documents must be considered as worksheet formats, ground rules, analysis assumptions, identification on indenture levels, coding systems description and failure definitions. A group of experts, composed by people from different areas of expertise, should form an FMEA Team, forming a multidisciplinary team. A precise planning will allow the correct implementation of the procedures. The military standards, also known as MIL-STD, give a set of standards that help on the guidance for a correct FMECA application [9]. The FMEA Team should carefully perform a clear description of the equipment or process under analysis, in terms of internal and interface functions, expected performance at each level and system constraints, definition of each failure and their effects. For each failure mode, a severity (S) index is assigned, that established priority list for corrective actions. Severity classification normally follows MIL– STD–882:2012 “Department of Defense standard practice: system safety”.
2.8.1. Criticality Analysis (CA) When FMEA includes the criticality analysis, it becomes Failure Modes, Criticality and Effect Analysis (FMECA). The criticality module is used to evaluate and sequence the probability of failure modes according to the severity of their consequences. On the CA Worksheet the following information, gathered in FMEA, should be included:
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Theory Framework
a) Identification number b) Item/Functional identification c) Function d) Failure modes and causes e) Mission phase/operational mode f) Criticality Assessment
Criticality, also known as a measure of risk, is a numerical index resulting from the combination of each failure or fault severity, and its probability or frequency of occurrence, called likelihood, as shown in Figure 2.8. It produces a measure of importance, where likelihood and severity are combined [2]. The FMECA Tool uses two approaches for risk identification of a possible failure. The risk is a measure of criticality, that is composed by both probability, and consequence of an impelling failure, that are combined to assess the risk. Criticality is used to determine hardware or function design weaknesses. The existence of data related to availability and failure rate of the parts under analysis, will determine the analysis method to be used. It might be (i) Quantitative Approach or (ii) Qualitative Approach [9]. (i) Quantitative approach As part of FMECA analysis, the following parameters should be calculated. a) Failure probability/failure rate data source b) Failure effect probability (훽) c) Failure mode ratio (훼) d) Part failure rate (휆) e) Operating time (t)
f) Failure mode criticality number (Cm), with 퐶 = 휆 푎훽푡 g) Item criticality number (Cr), with 퐶 = ∑ (퐶 )
The risk priority number (RPN) then calculated, doing quantitative assessments of for values as the consequence, likelihood and detectability. These parameters are respectively the severity (S), occurrence (O), and detectability (D). The formula is RPN = S × O × D The numbers for S, O and D are determined using the ratings tables in which the levels for each parameter are associated with a descriptive sentence that assists the analyst in an accurate and consistent choice of rating. The range of each value may range from 1 to 10. A
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lower likelihood of severity and occurrence contribute positively for a low RPN, while low detection likelihood leads to a higher RPN. As a whole, RPN value varies from 1 to 1.000 For a consistent opinion about the criticality assessment and determining treatment actions, the values assessed to S, O, and D should be reviewed.
(ii) Qualitative approach The existing failure modes identified in the FMEA, are classified in terms of probability of occurrence and severity, when parts configuration or data from failure rate are not available. It uses the criticality matrix, shown in Figure 2.8,which identifies the Severity on x axis, and the Likelihood on y axis, that derives from the failure Occurrence.
Figure 2.8: Qualitative criticality matrix [10]
The four levels on the criticality are category X: "Unacceptable”, and it is recommended changes or modifications to either mitigate the consequences, decrease the likelihood, or both. The category 1: “undesirable" may be managed by PM actions, category 2: "acceptable" can be managed by PM or CM actions, and category 3: "minor”, that should be monitored, and interventions may be deferred. For qualitative approach of criticality, the scale definition should be based on consideration about the significance for the chosen parameters. A large number of categories should be avoided, as it can lead to excessive effort to identify the correct category. The selection of the category descriptions and the meanings of each should be taken into account the manner in which they are to be used. To assess the likelihood value, a numerical explanation should be provided for the range of likelihoods expected, depending on the application. In order to simplify FMECA, Figure 2.9 was used as the representation of the criticality assessment of the different selected components of the considered critical sub-systems.
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Theory Framework
Figure 2.9: Legend for criticality assessment
2.9. Defining the System Hierarchy
According to ISO/IEC/IEEE 15288:2015 “Systems and software engineering — System life cycle processes, a system is an ordered relationship between items in a system”, the failure of a system is result from the failure of one or more sub-systems in the system. A system is defined from the top level (hierarchy), as shown in Figure 2.10, that establishes an ordered relationship between the items in a system, represented as being ‘above’, ‘below’ or ‘at the same level’ another system.
Figure 2.10: Plant hierarchy of parts [11].
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2.10. Hydraulic Stamping Press Machines
Forming and forging processes are among the oldest and most important of materials-related technologies. The competitiveness among the industry leads to manufactures to pursue and develop new methodologies and technologies, boosting the research and innovation in this area. The scientific advances allow benefits in the whole process, having as outcome a higher production output, dimensional control and product quality [12]. A metal stamping press machine is a tool that transforms a given material into a useful part, producing a complex geometry with well-defined shape, size, accuracy and tolerances, appearance, and properties [13]. The standards DIN (Deutsches Institut für Normung), German Institute for Standardisation, standardise various aspects of manufacturing processes on presses. The Standard DIN 8580:2003 “Manufacturing processes - terms and definitions, division”. A forming press is a machine tool used to modify the shape of a workpiece, by applying pressure, as shown in Figure 2.11.
Figure 2.11: Schematic of system approach in metal forming (using deep drawing as an example) [13].
Both machine press variables, and process variables, influence the outputs. The main characteristics of the press are divided into four types, as the frame type, load and energy requirements, time-dependent characteristics, and dimensional accuracy [13]. The behaviour and characteristics of the sheet forming press affect all of the following: (i) Stiffness (C) and accuracy of the press, influences the tolerances of final formed. Stiffness also affects die life.
(ii) Strokes per minute under load (np) capability of a press, influences the production rate (parts/time).
(iii) Available machine load (LM) and machine energy (EM), determine whether the press is able to perform the specific stamping operation
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Theory Framework
These variables have a strong influence in the output of the operation. These are summarised in the Figure 2.12.
Figure 2.12: Relationships between process and machine variables [13].
2.10.1. Mechanical Press Mechanical presses are the most common presses used in batch/mass production of sheet metal parts, due to their high stroking rate and low energy consumption. The feed principle is the main operation method of mechanical presses, having its output capacity determined by the drive speed [12]. The mechanical presses are composed by six components. The (i) electric motor drives a (ii) flywheel, that stores the energy in the rotating mass. A (iii) slider-crank mechanism, that converts rotational motion into reciprocating linear motion, and when engaged to the (iv) clutch. it transmits the torque from the flywheel to the (v) drive shaft. After the clutch is released, the (vi) brake is activated in order to stop the press. A press may have gear to increase the torque or reduce the speed, depending on the design configuration [13].
2.10.2. Hydraulic Press The main difference between a hydraulic and mechanical press is the drive mechanism. Hydraulic presses make use of hydraulic sub-system (oil tank, pump, tubes, valves and cylinders), that generate the necessary ram motion, and forming force. The versatility of hydraulic presses is greater than mechanical presses, as the control of both force and stroke
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is more easily. The load-restriction is a characteristic of these machines, as they have a limited available load to carry forming operations [12],[13]. The working principle of these machines is the hydrostatic pressure, as shown in Figure 2.13, is used on hydraulic presses. The generated pressure is distributed evenly through a system of pipes, and that a pressure p [N/m2], acting on a surface A [m2], produces a force F [N].
Figure 2.13: Hydrostatic principle [13]
Hydraulic pumps transform mechanical energy, generated from electric motors, into hydraulic energy. In hydraulic presses, two type of pumps are usually used. and can be in two types: swashplate type Figure 2.14 a, or bent axis type Figure 2.14 b, that may have variable displacement per revolution, depending on the angle of bent axis or swashplate, that can be adjusted.
(a) (b)
Figure 2.14: Hydraulic pumps commonly used in presses: (a) external gear pump, (b) bent axis axial piston pump [13].
A typical hydraulic press machine moves a slide and cushions, that are actuated by the cylinders, generating forces or lifting loads. The hydraulic sub-system is composed by five components. The (i) oil tank, which stores the hydraulic fluid, and supply suction pipes to the pumps or cylinders. It is usually at top position, to ensure that press closes at high speed, while the cylinders are filled with hydraulic fluid. The hydraulic drive system, comprised of (ii) pumps and (iii) motors, forms a structural unit together with the (iv) clutch and (v) connecting flange. The pumps can be
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Theory Framework
both in immersion or suspended in the oil tank, or mounted on supports in the press foundation. The hydraulic fluid is actuated by the valve control system, to the appropriate units. The slide or cushions are moved, or spindles are adjusted by the consumers, cylinders or hydraulic motors, generating forces, lift loads or clamp the dies. The service unit recirculates the fluid by means of filters and coolers in a distinct circuit, purifying and cooling it to ensure that the press remains ready for operation, at any time. Different pump placement might be found of a hydraulic press machine [12]. The DIN 51524-2:2017, “Pressure fluids - Hydraulic oils - Part 2: HLP hydraulic oils, Minimum requirements” standardise several parameters related to the hydraulic oil on operation. The various components of a hydraulic sub-system are represented in Figure 2.15.
Figure 2.15: Arrangement of a press hydraulic sub-system [12].
The electrical sub-system and their peripherals installed on presses, has as three main areas the operating and visualisation system, electrical control, that may be located centrally in the switch cabinet or decentrally distributed in the unit, and sensors and actuators in the press. The mechanical sub-system of the stamping press machine is composed by four components, as the lower and upper bed, uprights, slide guide, and bolster, as it is shown in Figure 4.1. The upper bed is attached to the slide, and the lower bed is clamped or bolted to the bolster. The stamping operations are done between these components. The stamping operations are standardised on DIN 8580:2003, as referred previously.
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The press control sub-systems include control elements and sensors, which produces information and transmitting it from the machine commands to the actuators present on machines. It also executes machine functions and fault diagnostics. The main areas are conventional part, the Input and output systems, the communication system, and the programmable logic controller (PLC).
2.11. Condition monitoring
Condition monitoring is part of PM strategy, and it monitors relevant parameters from the asset, on request or continuously, showing the present state, and indicating undesired or unpermitted states. Deviation of normal process behaviour may result in failure, which may be due to several causes. The condition monitoring aims to supervise and assess the machine’s heath over time, when required, and avoid unexpected failures. The simplified representation of a condition monitoring methodology, from fault detection, going through fault diagnosis, until the action, follows the methodology present on Figure 2.16.
Figure 2.16: Condition monitoring methodology
A fault is state preceded from failure and is characterised by the inability of an item to perform a required function. For fault diagnosis, it involves actions for fault recognition, fault localisation and the identification of the causes that lead to a fault. The fault localisation are all the actions to identify the item in a faulty state, at the appropriate indenture level of the system. It may include testing of the functional specifications of the item under analysis [2]. In condition monitoring, prior-selected parameters are measured in the asset, and a comparison is done with previously established reference input of class of behaviour, in order to detect abnormal conditions. This process is known as diagnosis process, that study the evaluation of symptoms and signs, in order to identify the cause and the nature of a machine problem. A well performed diagnosis is vital for excellence in maintenance planning and decisions. The condition monitoring devices are intended to collected data from all the measurements, that may be fused together in form of mathematical process models [14].
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Theory Framework
Understand the setting up of a measurement or monitoring system is essential for any correct measurement, and should be done as follows [15]: (i) Planning a set of organisational key documents, as specifications prior to measurement (ii) Knowledge about measurement devices (sensors) (iii) Selection of appropriate measurement devices (iv) Information on how to collect data (v) Signal processing, for the correlation between theory and collected data.
The existing methodologies and devices used on the condition monitoring, with focus to the ones that will be used on the monitoring of the stamping press, are the following;
2.11.1. Vibration analysis The vibration on a body is a mechanical phenomenon, is described as a periodic or random oscillation around a reference position. Depending on the industry, the presence of vibrations may cause undesirable events, as a catastrophic failure, that may have associated high direct and indirect costs. The vibration analysis deals with dynamic events, as forces and displacements. There is multiple interest on inferring knowledge from the vibrating object. To do it, it is necessary to quantify, analyse and characterise time-varying physical quantities, that form vibration. A vibration in the equipment can be monitored by using displacement transducers, velocity transducers, or accelerometers, depending on frequencies that the object is vibrating. The three types of vibration responses (acceleration, velocity, and displacement) are linked through integration or differentiation. Recommendations about the frequency range for different transducers are given in [16]. A vibration in an equipment is typically obtained by using an accelerometer, as represented in Figure 2.17.
(a) (b) Figure 2.17: Charge mass accelerometer a) Construction [17], (b) Product example[18].
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The y(t) and x(t) are the displacements of the vibrating object and the seismometer mass (m), respectively, z(t) is the relative mass displacement, m [kg] is the seismic mass, r [kg.m/s] is a dumping coefficient, and k[N/m] a spring constant [17]. Conceptually, the accelerometer is built with a damped mass on a spring. If the body under monitoring accelerates, the mass is displaced to the point that the spring accelerate the mass at the same rate as the casing. Piezoelectric, piezoresistive or capacitive components then convert the mechanical motion into an electrical signal. The small size, that vary in length from one millimetre down to one micron, and weight of accelerometers make its portability a great feature. An accelerometer may be single or multi-axis, and can detect both magnitude and direction of the instantaneous acceleration, called proper acceleration. A recent technology of accelerometers uses microelectromechanical systems (MEMS).[19]
2.11.2. Motor current signature analysis The motor current signature analysis (MCSA), or sometimes abbreviated as current signature analysis (CSA), is part of non-invasive methods for motor fault diagnosis, together with other techniques, as instantaneous power analysis (IPA), and Park vector analysis (PVA). These different techniques are employed for different purposes, having each its strengths and weaknesses. For the purpose of this thesis work, the one that is considered is the MCSA. The MCSA method may find the several faults in a motor, as broken rotor bar, rotor mass unbalance or air gap eccentricity, stator winding faults, single phasing gault, and bearings damage. The MCSA method is used to measure the electromagnetic properties of a motor, by diagnosing the motor and inverter defects, using the collected information from the motor stator current [20]. The device that measures the electromagnetic properties of the motor is the Rogowski sensor, as shown in Figure 2.18, which is a device to measure alternating current (AC), or high-speed current pulses. It measures the rate of current of change, using a transformer with no magnetic core.
(a) (b) Figure 2.18: Rogowski coil (a) Construction (b) Product example [21].
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Theory Framework
Its construction consists on a helical coil of wire, with the lead from one end, that returns over the centre of the coil to the other extremity, so that both terminals are at the same end of the coil. The of Rogowski coils are usually applied for current monitoring and measurement of harmonic current content [21].
2.11.3. Thermal Analysis Thermal measurement technology measures the absolute or relative temperatures of key asset parts or components under condition monitoring. When abnormal temperatures are detected, it indicates the development of a problem. Temperature can be used as an indicator of the degradation of items, and it is frequently measured and used on condition monitoring. Existing technologies are the contact methods, that use thermometers and thermocouples, or the non-contact methods, that makes use of infrared thermography. Thermocouples are widely used in the industry. Its construction consists of two dissimilar metal wires, forming an electrical junction. It produces a temperature-dependent voltage, as result the thermoelectric effect. The generated voltage is then interpreted as a measure of temperature. [22]. From all the types of thermocouples, the k-type (chromel–alumel), as shown in Figure 2.19, is the class of thermocouple most used, due to its low cost and satisfactory temperature range.
The measured voltage v can be used to calculate temperature Tsense if temperature Tref is known.
(a) (b) Figure 2.19: K-Type Thermocouple (a) Construction, (b) Product Example [23]
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Method
3. METHOD
The presented thesis work began with literature study focusing the fundamentals of maintenance. Different textbooks, standards and publications were studied to understand the principles of maintenance, and maintenance objectives. As the purpose of the thesis is to find and propose strategies in order to increase the dependability of hydraulic stamping press machines, to achieve performance objectives, an explanation of the operation and components of the machine was done. Documentation and standards related to the hydraulic stamping press machines, recommended by the company staff, were studied. The press machine is divided into three different sub-systems, the hydraulic, mechanical, and electrical. A taxonomy system of the three sub-systems is also done, by using the standard ISO/IEC/IEEE 15288:2015, as present on chapter 2.9, in order to document the components of each sub-system. Further analysis was performed according to the RCM method. According to the technical team, and available literature, the focus of the analysis are the hydraulic pumps and cylinders of the hydraulic system, and the slide guide rail of the mechanical system. As failure data was no accessible, a qualitative approach is chosen to rank the selected components to be analysed in the system in terms of its criticality. This classification is a valuable information for the performed FMECA, as items with the higher ranking must be the ones firstly selected when setting up a condition monitoring system. The implementation of RCM and FMECA is carried out according to standard as described below. The stages for FMECA implementations are [9]: 1. Planning and Review the function or hardware; 2. Brainstorm failure modes and list their effects; 3. Identify all potential causes and/or mechanisms of a failure of a failure mode, and define their effects on the function, item, system, or mission to be accomplished, as well as characterisation of current prevention fault controls; 4. Identification and listing of the detection controls that may detect a failure mode; 5. In case of a FMEA to a continuous improvement activity of an existing process, product or equipment, actions should be taken aiming to eliminate or reduce failure modes that represent high-risk.
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According to IEC 60300-3-11:2009 “Dependability management - Part 3-11: Application guide - Reliability centred maintenance” the steps when performing RCM are: 1. Initiation and Planning a) Determine the delimitations and objectives of analysis b) Identification of a team of specialist that fulfils the objectives c) Determine the content of the analysis 2. Identify Functional Failures a) Collect and analyse data from the equipment under study b) Perform a taxonomy of the machine c) Identify function, functional failures, failure modes, effects, and criticality 3. Task Clarification and Selection a) Brainstorm failure consequences 4. Program implementation a) Select the most appropriate condition monitoring method b) Prioritise and implement other actions 5. Continuous improvement a) Measure of KPI to assess maintenance effectiveness
The first step of RCM is to set purpose, objectives, contents, and delimitations of the analysis. A group of expertise should be identified. The second step of RCM, identification of failure modes and functional failures, may be further detailed in FMECA analysis, highlight how the components are connected to each other. A careful analysis is crucial, as it identify the connection between components, and focus on the discussion of each failure consequence, a fundamental step for further criticality assessment. In this step, condition monitoring methods are proposed taking in consideration the failure modes and consequences identified during the FMECA. The RCM uses CBM maintenance strategy as a primary failure management. This thesis work focus in the hydraulic sub-system, analysing their components hydraulic pumps, and cylinders, and in the mechanical sub-system, the slide guide rail will be analysed. The other components of the press machine are not considered to have unexpected failures, and therefore not critical, so it was a consensus in the project team that due to time limitations they would not be analysed. The assessing of criticality of each component of the press machine is determined by using the estimated likelihood of each failure, and the gravity of the consequences of failure. Both assessing of likelihood and severity were based on field experience, and also by taking in consideration the opinions of technical staff. In order to simplify FMECA, Figure 2.9 was used as the representation of the criticality assessment of the different
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Method
selected components of the considered critical sub-systems. It assesses the criticality in terms of expected likelihood of each failure, and it rates in a scale from remote to high. The severity assesses the possible failure consequence of each failure mode, and it rates in a scale from minor to catastrophic. There are different considerations when selecting appropriate condition monitoring techniques, as the control variables of the process. The proposed condition monitoring methods are the result of the analysis of different components in terms of their functional failure, failure modes and failure effects, that were identified during the FMECA methodology. The suggested methods are among the multiple condition monitoring methods that exist in the industry. The relative ease of implementation, and team’s good knowledge about the suggested monitoring methods from data to decision, as they were also used in other projects, were central for the choose among methods. The ability of monitoring methods to detect failure in an early stage of the failure development time, as described earlier by the P-F curve in Figure 2.4 , was also considered. The benefits of a condition monitoring program should be noticed, as the reduction of the number of unplanned failures, and higher dependability [1], [3]. The third step of RCM, focuses on brainstorming the obtained failure consequences and rank the equipment in terms of criticality. The criticality assessment is done according the Figure 2.9, determining the component that should be addressed first, and what is the most appropriate maintenance task to avoid the consequences of failure The fourth step, program implementation, determine the periodicity and when it should start the implementation of condition monitoring plan, and maintenance tasks, according a time schedule. Supplier recommendations for time intervals should be considered when scheduling and planning maintenance activities in the assets. The fifth and final step of RCM, is the implementation of continuous improvement system, by measuring the performance of maintenance, and comparing it with safety, operational, and economic objectives or the company.
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Results
4. RESULTS
This chapter will present the results for the objectives in the chapter 1.3. (i) Understand the hydraulic press machine as a system and operation; The hydraulic press machine operation is explained in chapter 2.10. The main components of the machine, are shown in Figure 4.1.
Figure 4.1: Single-action hydraulic press with active draw cushion for counter drawing. Adapted from [12].
The press machine was divided into three main sub-systems, as shown in Figure 4.2.
Figure 4.2: Main sub-systems of a hydraulic stamping press machine
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The main components of each sub-systems in the hydraulic press are shown in Figure 4.3.
Figure 4.3: Main components of each sub-system of a hydraulic press stamping machine.
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Going down in the analysis, the detailed taxonomy of the hydraulic sub-system, is as shown in Figure 4.4.
Figure 4.4: Taxonomy of the hydraulic sub-system, components and items [12].
The detailed taxonomy of the mechanical sub-system, is as shown in Figure 4.5
Figure 4.5: Taxonomy of the mechanical system, components and items [12].
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The detailed taxonomy of the electrical sub-system is shown in Figure 4.6.
Figure 4.6: Taxonomy of the electrical system, components and items [12].
(ii) Use of FMECA to identify the failures and failure modes of the selected critical sub-systems and components of the press machine; The following section presents the analysis of the selected critical sub-systems, and components of the machine. The hierarchy level in the Figure 4.7, and Figure 4.8 corresponds respectively to sub-system, component, function, functional failures, and failure modes. The analysis of the selected component in hydraulic sub-system, in terms of its component function, functional failures and failure modes, is shown in Figure 4.7.
Figure 4.7: Analysis of the [10]-hydraulic sub-system, and respective selected components [10.1]-hydraulic pumps and [10.2]-cylinders, in terms of its functional failures and corresponding failure modes
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Results
The analysis of the selected component in mechanic sub-system, in terms of its component function, functional failures and failure modes, is shown in Figure 4.8.
Figure 4.8: Analysis of the [20]-mechanical sub-system, and respective selected component [20.3]-slide guide, in terms of its functional failures and corresponding failure modes
(iii) Use of the RCM method to rank the criticality of the selected critical sub-systems and components of the press machine; As illustrated in detail in Figure 4.3, the hydraulic press stamping machine was divided into three sub-systems. A breakdown of the components was done, to see each piece of each component separately in reference to its functional failures, failure modes and failure consequences. The following section corresponds to the analysis of the selected critical sub-systems, and components of the machine. The criticality of the sub-system was assessed in terms of the likelihood of failure and severity of the failure consequences. The analysis of the selected component in hydraulic sub-system, in terms of its component failures modes and failure consequences, is shown in Figure 4.9.
Figure 4.9: Analysis of [10.1]-hydraulic pumps, in terms of its functional failures and failure consequences.
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The criticality of the component [10.1]-hydraulic pumps, was then assessed, as shown in Figure 4.10.
Figure 4.10: Criticality assessment of the [10.1]-hydraulic pumps
The analysis of the component [10.2]-cylinder, in hydraulic sub-system, in terms of its functional modes and failure consequences, as shown in Figure 4.11.
Figure 4.11: Analysis of [10.2]-cylinder, in terms of its functional failures and failure consequences
The criticality of the component [10.2]-cylinder, was then assessed, as shown in Figure 4.12.
Figure 4.12: Criticality assessment of the [10.2]-cylinder.
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The analysis of the component [20.3]-slide guide rail, in mechanical sub-system, in terms of its functional failures and failure consequences, is shown in Figure 4.13.
Figure 4.13: Analysis of component [20.3]-slide guide rail, in terms of its functional failures and failure consequences
The criticality of the component [20.3]-cylinder, was then assessed, as shown in Figure 4.14.
Figure 4.14: Criticality assessment of the [20.3]-slide guide rail
After the analysis in terms of likelihood and severity of failure consequences of the selected components, the Figure 4.15 was obtained.
Figure 4.15: Criticality assessment of the analysed components
The discussion and conclusion of this values are presented in the next section.
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(iv) Suggestion of condition monitoring methods for each failure modes and its failure effects of the selected critical sub-systems and components of the press machine. The following section corresponds to the analysis of the selected critical sub-systems, and components of the machine. The condition monitoring methods to asses each failure are proposed. The Figure 4.16, Figure 4.17, and Figure 4.18 are analysed in terms of component, functional failure, failure modes and failure effects, and then is suggested condition monitoring methods. The proposed condition monitoring methods are discussed in the discussion and conclusions chapter, and alternatives for fault identification and fault isolation are also proposed and discussed. For the component [10.1]-hydraulic pump, as presented in Figure 4.16, the suggested monitoring techniques are the vibration analysis, and current signal analysis, as they can detect an abnormal change on hydraulic pump itself, or in the motor’s pump, indicating some impelling failure happening.
Figure 4.16: Suggestion of condition monitoring methods for component [10.1]-hydraulic pump
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For the component [10.2]-cylinder, as presented in Figure 4.17, the suggested condition monitoring methods are the monitoring of the PLC variables, as they can detect an abnormal change of pressure in the cylinder, indicating some impelling failure happening.
Figure 4.17: Suggestion of condition monitoring methods for component [10.2]-cylinder
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For the component [20.3]-slide guide rail, as presented in Figure 4.18, the suggested condition monitoring methods are thermal analysis, vibration analysis, and the monitoring of the PLC variables, that make part of the protection system of the analysed hydraulic press machines.
Figure 4.18: Suggestion of condition monitoring methods for component [20.3]-slide guide rail
Further condition monitoring may be applied, which are discussed in discussion and conclusions chapter.
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Discussion and Conclusions
5. DISCUSSION AND CONCLUSIONS
The purpose of this thesis work was to identify and propose strategies in order to increase the dependability of hydraulic stamping press machines, to the company to achieve the performance’s objectives, serving also as base for the creation of future maintenance plan. The analysis of hydraulic stamping press machine focused on a specific manufacturer. Other manufacturers may have different configurations and subsystems, resulting in different failure modes, whose are not part of this thesis work, and therefore, not included. Also, for machines with different driving systems, as per mechanical presses or servo presses, failure modes may diverge. The focus was to find failure modes on a particular machine type in order to suggest condition monitoring methods for hydraulic stamping press machines, that meets the purpose of this thesis work. The working principle of hydraulic stamping press machines is the hydrostatic pressure, which makes use of a hydraulic sub-system that generate the necessary ram motion and forming force for operation. The different sub-systems of the hydraulic machine are the hydraulic, electrical, mechanical, and the press control sub-systems. This thesis work focused in the [10]-hydraulic sub-system and the analysed components were [10.1]- hydraulic pumps, and [10.2]-cylinders. For the [20]-mechanical sub-system, the [20.3]-slide guide rail was also analysed. The other components were not considered to have unexpected failures, and due to time limitations were not considered, as explained in method chapter. A FMECA was performed in order to identify failure and failure modes of the selected critical sub-system and components of the press machine. The author own experience in the field, discussions with project team, technical staff, and literature reading helped to describe the various steps from component until failure modes. The FMEA/FMECA is quality tool and the sooner the failures are discovered, the less the impacts will cost. FMECA is performed in different steps preferable by a multidisciplinary team from different expertise. In order to allocate resources and experts efficiently, the different steps must be separated to assure that only the appropriate experts are required to be present on each step. This allow the experts to continue their normal tasks and guaranty the normal running of the company. Too much people at the same time can also lead to useless discussions. As this is a resources and time consuming, the effort putted on the FMECA must be equivalent to the results and value-added expected. FMECA cannot be a way to substitute engineering or technical expert teams. Rather, it is a tool for the engineers to apply the knowledge on a systematic and effective way.
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The failure modes identified in the FMECA were classified in terms of likelihood and severity, in order to assess the criticality of the component. When parts configuration or data from failure rate are available, the quantitative approach may be considered, as it is less dependent from the opinion of who is assessing the different phases of qualitative critical assessment. The qualitative approach assessment should be consistent and based on experience in one type of asset. For the qualitative risk assessment, there are several methods, being each one more appropriate than others, depending on a number of factors, explained in each method. From the qualitative assessment of criticality of the analysed components, the [10.1]-hydraulic pumps and [10.2]-cylinder were in the category 2: Acceptable, meaning that both can be managed by preventive maintenance or corrective maintenance actions. The [20.3]-slide guide rail is in category 1: Undesirable, meaning that the failure modes may be managed by preventive maintenance actions. From the FMECA and criticality assessment result, it is concluded that [20.3]-slide guide rail is the most critical component. The reason is that all the press operation is based on force and guiding precision and all the operations needs necessarily to use this component. Other components, as the tools and slide, are attached to this component, and for this reason, they can have indirectly its condition monitored, and/or indication that degradation might be excessive, as these components affect directly the normal operation of the surrounding components, as the [20.3]-slide guide rail. The best maintenance policy to be applied was based on the advantages and disadvantages of the considered maintenance policies. The main advantage of CM, is that it is considered an easy option, and it does not require high investments, as training and testing equipment. As disadvantages, it may affect the production plans, having high risk on safety and environmental damage, high repair and replacement costs. The main advantages of CBM is that due to a better control of variables that best reflects the condition of the asset, maintenance activities can be planned and carried out when it is more convenient; the risk on health, safety, and environmental is reduced. As disadvantages of CBM, the chosen key performance indicators may not reflect the real health state of the asset, resulting on decisions that might affect the general goals of the company. Condition monitoring equipment and software are needed, demanding high initial investment, and it is difficult to perform, as it requires high qualified people. As conclusion, the CBM shows to be the maintenance type that better meets the objectives of this thesis work to be applied on the analysed critical components, according the advantages and disadvantages of each. The CM may affect the production and has high associated risk, while the CBM, despite the high initial investment, allows improved planning between maintenance activities and production, and has a reduced associated risk, meeting the safety and health policies of modern industries. Taking this in consideration, the characterisation of possible condition monitoring methods for the failure monitoring, it was considered the thermal analysis, vibration analysis, current
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Discussion and Conclusions
signal analysis, and monitoring of PLC variables. Sometimes a combination of methods are used to cover all the failure modes and/or failure consequences that might happen in the analysed sub-system or component. The recommended condition monitoring methods presented, are the result of discussions with technical staff and study of available literature. For the component [10.1]-hydraulic pump, the recommended condition monitoring methods are vibrations and current signal analysis, and they should be used together to mitigate or prevent faults. The vibration analysis deals with dynamic events, as forces and displacements, that are produced by the failure modes and consequences identified in the component. As the pump’s motor is electric, the current signal analysis method was chosen to indirectly monitor the pumps functioning, as the variation of electric consumption may indicating some failure in the pump. Other condition monitoring methods could be taken in consideration, such as acoustic emission analysis, that can be a complement and/or replace the vibration analysis method. Acoustic emission technology is getting attention in the research field, due to its high capability to infer more knowledge from the measured equipment, rather than vibration analysis [24]. For the component [10.2]-cylinder, the PLC variables are suggested to indirectly monitor the condition of the cylinder, by measuring the pressure in the hydraulic circuit, continuously or at discrete time, depending on the configuration. When there is a change on pressure, the most common is pressure drops, this abnormal behaviour is an alert that the normal operation of the cylinder is on a failure state. Further condition monitoring methods may be applied, in order to assess the integrity condition of cylinders. Crack detection can be done using some non-destructive testing techniques like infrared thermography, ultrasonic testing, and radiographic testing. For the Component [20.3]-slide guide rail, the suggested system is a combination of condition monitoring analysis. In order to assess when there is improper lubrification on the component, the suggested condition monitoring methods are thermal analysis and monitoring of PLC variables. When there is improper lubrication, it can be due to a variety of reasons, that leads ultimately to machine stop due to actuation of the detection and protection elements of the machine. The deficient/missing lubrication does not create the lubricant layer between moving surfaces, leading to worn-out of slide guide rail, and consequently to significant increasing of temperature, where thermal analysis is a method that is able to detect this change. The temperature measurement, in this case, is an indirect process control, that allows to infer if there is some failure initiation. The improper quality of lubricant may be inferred by using thermal analysis, as the change of its physical and chemical properties will lead to improper lubrication. Further analysis may be done to isolate the failure, and in this case, laboratory analysis for chemical control of oil inherent
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characteristics may be performed, as infrared spectroscopy to detect chemical changes according specifications, or physical properties of the oil, as the viscosity, by using a viscosimeter. Ultimately, the improper lubrication may cause machine stop, as the protection system, controlled by the monitoring of PLC variables is actuated, in order to prevent major damage, leading to interruption of production. Different errors may be displayed in the human system interface, as the low oil pressure, high oil pressure, pump stopped, low oil level, and potentially other error messages. In order to detect other types of failures, vibration analysis can be used, as direct or indirect process control. As example of direct process control, vibration analysis may detect excessive vibrations when lock nut is loose, as the system is not tight. Vibration analysis may also detect problems in material quality, as it leads to worn-out, increasing the clearances in the system, creating vibrations. For fault isolation, further analysis should be done, as control of tolerances, material quality, finishing, hardening, and potentially others. The suggested condition monitoring methods may differ, depending on the criticality of the asset in the company, and the objectives to be achieved. If other failure modes and consequences are considered, it may also result in different condition monitoring methods. Further, the amount of resources that the company is willing to invest may differ, limiting the condition monitoring methods to the ones that do not requires high investment or many human resources. This thesis work suggested condition monitoring methods that are in accordance with the team’s good knowledge with the suggested methods, and relative ease of their implementation. The suggested methods are suitable for normal operation, and they use limit or threshold checking, raising an alarm if tolerance zone is exceeded. The benefits on the operation, as reduction of number of unplanned failures and higher dependability should be noticed. Otherwise, it is recommended an analysis of the system, as part of the continuous improvement process, in order to choose more adequate methods. A predictive module could be added to predict the behaviour of the monitored variables, which is suggested as future work.
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Recommendations and Future Work
6. RECCOMENDATIONS AND FUTURE WORK
As future work, this thesis work recommends the analysis of all the components of the hydraulic press stamping machine, as a complete system, and the criticality assessment of the different components. For that, it is suggested the creation of a multidisciplinary team, with different expertise. If possible, the criticality analysis should use the quantitative approach, as it is less dependent from the evaluator opinion, as it is based on real data collected from the asset. It is also recommended the completion of the of the RCM method, by identifying the maintenance tasks and intervals of the interventions, and the implementation and continuous improvement steps, in order to build future maintenance plans for the analysed hydraulic press stamping machine. In the CBM module, as in Figure 6.1, it is recommended the complete implementation, from sensor module to the data presentation. Data fusion for the health assessment level may be an interesting challenge. In this work, it is suggested different condition monitoring methods for the same component, but the way on how to do the data fusion is still to be answered. The concept is well explained in the literature, but real-life application may face difficulties on the reliability on the displayed information. If the data could be fused, it would possibly be able to have a better overview of the health state of the asset. When in a continuous running plant, program evaluation and cost analysis should be included, as well as the continuous improvement, resulting in expected improvements on maintenance plan for the totality of components of the hydraulic stamping press machine.
Figure 6.1:Proposed flowchart for future work [25]
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References
7. REFERENCES
[1] J. T. Farinha, Asset Maintenance Engineering Methodologies, CRC Press. CRC Press, 2018. [2] European Standard, EN 13306:2017 - Maintenance - Maintenance terminology. 2010. [3] M. Ben-Daya, U. Kumar, and P. Murthy, Introduction to Maintenance Engineering : modeling, optimization, and management. John Wiley & Sons, Ltd, 2016. [4] P. M. L. Bastos, “Sistema de predição de avarias em máquinas de unidades fabris globalmente dispersas,” 2014. [5] D. J. Edwards, G. D. Holt, and F. C. Harris, “Predictive maintenance techniques and their relevance to construction plant,” J. Qual. Maint. Eng., vol. 4, no. 1, pp. 25–37, 1998, doi: http://dx.doi.org/10.1108/13552519810369057. [6] S. J. Lacey, “The Role of Vibration Monitoring in Predictive Maintenance,” Maint. Asset Manag., vol. 25, no. 2, p. 19, 2010. [7] J. Moubray, “Reliability-centered maintenance,” Nuclear Plant Journal, vol. 9, no. 3. pp. 59–61, 91, 1997. [8] “ISO/IEC/IEEE 15288:2015 Systems and software engineering — System life cycle processes,” ISO/IEC/IEEE, p. 108, 2015. [9] Military Standard - Procedures for Performing a Failure Mode, Effects and Criticality Analysis. DEFENSE, DEPARTMENT OF Washington, DC 20301, 1980. [10] IEC 60812, IEC 60812:2018 - International Standard - Failure modes and effects analysis (FMEA and FMECA). 2018. [11] A. Kelly, Strategic Maintenance Planning, vol. 53, no. 9. Elsevier Ltd, 2006. [12] Metal Forming Handbook. Schuler (c) Springer-Verlag Berlin Heidelberg 1998, 1998. [13] T. Altan and A. E. E. Tekkaya, Sheet Metal Forming - Fundamentals. ASM International, 2012. [14] R. Isermann, Fault-Diagnosis Systems - An Introduction from Fault Detection to Fault Tolerance. Springer Berlin Heidelberg New York, 2006. [15] J. Kumar Sinha, Vibration Analysis, Instruments, and Signal Processing. 2014. [16] R. B. Randall, Vibration-based condition monitoring - industrial, aerospace and automotive applications, vol. 11. John Wiley & Sons, Ltd, 2011. [17] K. Tomczyk, “Problems in modelling charge output accelerometers,” Metrol. Meas. Syst., vol. 23, no. 4, pp. 645–659, 2016, doi: 10.1515/mms-2016-0045. [18] D. Askew, “MEMS: A Brief Overview,” Mouser Electronics. [Online]. Available: https://br.mouser.com/applications/mems-overview/. [Accessed: 17-May-2020]. [19] J. W. Gardner, V. K. Varadan, and O. O. Awadelkarim, Microsensorts, MEMS, and
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Smart Devices, vol. 3, no. 2. John Wiley & Sons Ltd, 2001. [20] S. Karmakar, S. Chattopadhyay, M. Mitra, and S. Sengupta, Induction Motor Fault Diagnosis Approach through Current Signature Analysis. 2016. [21] G. Sudha, K.R.Valluvan, and T. Basavaraju, “Fault Diagnosis of Transmission Lines with Rogowski Coils as Current Sensors,” 2013. [22] D. Galar, Artificial Intelligence Tools - Decision Support systems in condition monitoring and diagnosis. 2015. [23] D. D. Pollock, Thermocouples - Theory and Properties. CRC Press, Inc., 2000. [24] M. Lucas, “Acoustic Emission: The Next Generation of Vibration Techniques.” [Online]. Available: https://www.reliableplant.com/Read/28771/acoustic-emission- techniques. [Accessed: 22-Jun-2020]. [25] G. Niu, B. S. Yang, and M. Pecht, “Development of an optimized condition-based maintenance system by data fusion and reliability-centered maintenance,” Reliab. Eng. Syst. Saf., vol. 95, no. 7, pp. 786–796, 2010, doi: 10.1016/j.ress.2010.02.016.
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ANNEX A
ANNEX A
FMECA of the selected critical sub-systems and components of the press machine;
Criticality Recommended Condition Component Function Functional Failures Failure Modes Failure Effects Failure Consequences L Sev R Monitoring Methods Hydraulic System It creates implosion effect, having as potential It happens when air is consequence the removal Aeration - presence of sucked into the pump of metal from the pressure dispersed air bubbles due to faulty shaft plates, wear plates, etc, on on the system’s sales, and leaky inlet the surrounding area near hydraulic fluid joints. the implosion point, and resulting in high local temperatures It may cause an abrasive Presence of foreign action in the close mating solid particles, liquids Contamination tolerance among or gases on the components, and results in hydraulic fluid accelerated wear and tear. Aeration, cavitation, contamination and It affects the fluid viscosity, over-pressurisation Excessive heat deteriorating it, creating a are factors that chain reaction. contribute to high temperatures The air bubles are Implosion subjected to hydraulic It may cause mallfunction pressure and failure of the pump, Failure on pressure resulting in early failures Over-pressurisation control device
Viscosity can change Mechanical source 1. Internal Pump due to some factors, of power that System variation of the Damage, Wear and as temperature converts Viscosity pressure, resulting in a chain corrosion 1. Vibrations variations or [10.1] mechanical power reaction 2. Working Instability 2. Current Signal Analysis contamination by v B 2 Hydraulic Pump into hydraulic [Indirect Measurement, other fluids. 3. Decreasing of energy, creating Performance done in Pump's Motor} high pressure on a It can be caused by 4. Interruption of hydraulic circuit over speeding of the It causes cyclic production pump, high fluid stress through repeated Cavitation viscosity or restricted implosion may cause or excessive long vibration and wear intake line
1. Body discolored in form of Piston wear dark bands 2. Piston wear
Adhesive wear on slipper Slipper pad wear Lack of lubrication, face improper fluid or If the operating limits are particle contamination exceeded, valve plate and Valve plate wear may result in higher cylinder barrel will get friction, causing wear. separated Surface scratching on Swash plate face wear swashplate Piston stick (piston size) and Cylinder block wear cause wear and separation Input shaft spindle Excessive vibration Fretting corrosion at the end wear/misalignment and bearing wear of the shaft Excessive loads, fatigue failure, Degradation of the Bearing failures corrosion, component itself and (ball/roller) contamination, surrounding elements lubricant failure, and misalignment
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Criticality Recommended Condition Component Function Functional Failures Failure Modes Failure Effects Failure Consequences Monitoring Methods L Sev R Hydraulic System Internal Leakage. Oil flows from the high pressure chamber to the low pressure 1. Safety concerns - chamber. May occur People slipping and Generates the from piston damage or The necessary force for falling; [10.2] required force and Pressure loss seals required function is not 2. Environemental v B 2 PLC Variables Cylinder motion on the External leakage. generated Contamination forming process. The inlet pressure 3. Interruption of decreases when production leakage occurs. May occur from cracks or damage seals
Criticality Recommended Condition Component Function Functional Failures Failure Modes Failure Effects Failure Consequences Monitoring Methods L Sev R Mechanical System
Quality of lubricant Worn-out Increases clearance on the guiding system Contamination
Machine Stop with Errors: Thermocouple and Improper Lubrication Lubricant Temperature 1- High oil temperature; PLC Variables 2- Low oil temperature Lack of Lubricant Machine Stop with Errors: Moving and Guiding Interruption of Production the Slide up and 1- Low Oil pressure; 2- High Oil Pressure; [20.3] down Insufficient Lubricant (vertical movement) 3- Pump Stopped; iv B 1 Slide Guide Rail 4- Low Oil level; Material Finishing Improper clearance Hardening 1. Malfunction Improper Tolerances Worn-Out 2. Tool Damage 3. Early Degradation Excessive vibrations Accelerometers Lock nut is loose Incorrect tightening
Teeter-totter effect Inconsistient parts quality Tool and Die Off Center Load High punch and die wear (Imbalance) / burrs / Chipping
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