Hydropneumatic suspension in a

Installation of a hydropneumatic suspension for a Scania truck

Hydropneumatisk hjulupphängning i lastbil

Installation av hydropneumatisk hjulupphängning till en Scania lastbil

Rasmus Karlqvist

Fakulteten för hälsa, natur- och teknikvetenskap Högskoleingenjörsprogrammet i maskinteknik Examensarbete 22.5 hp Handledare: Anders Wickberg Examinator: Anders Biel Datum: 2020-05-18 Version: 3.0 Foreword

This thesis would not be possible without the help of several people. First and foremost I would like to direct a sincere thank you to my mentors at Scania CV AB Olle Alsing and Johan Parsons for the time and engagement they have directed towards helping me out with this project. I would also like to dedicate a thank you to the all the colleagues at RTC and RTCB for a very pleasant experience and warm welcoming to the group it’s been a blast to be around you throughout this time with the activities, fika and lunches. Last but not least I would like to thank my mentor from Karlstad Universitet Anders Wickberg for guiding me through this project.

Abstract

Investigation and testing of hydropneumatic suspension systems has previously been done at Scania between the year 1992 and 2000. Interest has aroused at Scania CV AB to further test a hydropneumatic suspension. The reason being the new ventures of decarbonised, clean, electrified, automatized and digitalised vehicles. If electrified are to be adopted in the market as an alternative to trucks with combustion engines, solutions for this type of vehicle’s capacity need to be presented. The vehicle’s weight needs to be reduced; the effectiveness of the components needs to be increased and alternatives to increase battery storage needs to arise if it’s going match the traveling distance of a combustion engine.

The mission of the project is to present an installation solution of a hydropneumatic suspension that retains the performance of the current air suspension. The presented material will contain CAD-models of all the brackets that will be designed to fit the suspension, as well as the placement in the vehicle assembly for said brackets.

The results show that as for the front suspension the best solution is a placement of the hydraulic cylinders in front of the vehicles front . Furthermore the rear suspension is best suited for a placement of the hydraulic cylinders behind the vehicles rear axle. However it was concluded that the rear suspension will not be able to retain the current stroke of the vehicle without sacrificing its ground clearance. Parts of the suspension could however be terminated when the air suspension system was replaced by the hydropneumatic system namely: The front suspension anti-roll bar, shock absorbers, air springs and their coexisting brackets.

Sammanfattning

Undersökning och testning av hydropneumatiska upphängningssystem har tidigare gjorts i Scania mellan 1992 och 2000. Intresset har väckts hos Scania CV AB för att ytterligare testa en hydropneumatiska hjulupphängningen. Anledningen är de nya satsningarna på avkolade, rena, elektrifierade, automatiserade och digitaliserade fordon. Om elektrifierade lastbilar ska anta marknaden som ett alternativ till lastbilar med förbränningsmotorer, måste lösningar för denna typ av fordon presenteras. Fordonets vikt måste minskas; komponenternas effektivitet måste ökas och alternativ för att öka batterilagringsutrymme måste uppstå om det skall kunna matcha förbränningsmotorernas räckvidd.

Målet med projektet är att presentera en installationslösning av den hydropneumatiska hjulupphängningen som erhåller den aktuella luftfjädringens prestanda. Det presenterade materialet kommer att innehålla placeringarna av hydraulcylindrarna till det hydropneumatiska systemet i fordonets sammanställning samt CAD-modeller av samtliga fästen som kommer att utformas för att passa till hjulupphängningen.

Resultaten visar att den bästa lösningen för främre hjulupphängningen är en placering av de hydraulcylindrarna framför fordonets framaxel. Den bakre hjulupphängningen är däremot bäst lämpad för placering av de hydraulcylindrar bakom fordonets bakaxel. Det drogs dock slutsatsen att den bakre hjulupphängningen inte kommer att kunna bibehålla fordonets nuvarande slaglängd utan att ge avkall på markfrigången. Delar av hjulupphängningen kunde emellertid avlägsnas när luftfjädringssystemet ersattes av det hydropneumatiska systemet nämligen: Den främre fjädringens krängningshämmare, stötdämpare, luftfjädrar och deras samexisterande infästningar.

Nomenclature

4x2 truck - An axle configuration of two were one is the drive axle.

6x2 truck - An axle configuration of three axles were one is the drive axle.

FMEA - Failure modes and effects analysis.

WBS - Work breakdown structure.

Accumulator - A pressure storage reservoir, accumulators enables the coping of extreme demands using less power and smooths out pulsations in a hydraulic system.

ECU - A box of electronics that provides rules and logic to a specific area of function.

Load handling – The action of controlling the vehicle’s height through the suspension system.

Roll – The load transfer of a vehicle towards the outside of a turn.

Lateral force – The force that acts in the direction parallel to ground and perpendicular to the direction of gravitational pull of the earth and the length of the a body.

Longitudinal force – The force that acts in the direction of the length of a body.

Vertical force – The force that acts in the opposite direction of the gravitational pull.

Level sensor – A component in a vehicle which determines angle of change relative to the point of calibration.

CAD - computer aided design.

Hz - Hertz. kN - kilo newton. Table of contents

1 Introduction ...... 8 1.1 Background ...... 8 1.2 Problem ...... 9 1.3 Project object ...... 9 1.4 Project mission ...... 9 1.5 Project delimitations ...... 9 2 Theory ...... 10 2.1 Hydropneumatic suspension ...... 10 2.2 Air suspension ...... 13 2.3 Current layout ...... 17 2.4 Comparison ...... 23 3 Method ...... 24 3.1 Planning ...... 24 3.2 Concept & Design ...... 26 4 Results ...... 30 4.1 Replacement and removal of the front suspension ...... 30 4.2 Concept generation of the front suspension ...... 31 4.3 Concept evaluation of the front suspension ...... 40 4.4 Final concept and further development of the front suspension ...... 42 4.5 Replacement and removal of the rear suspension ...... 45 4.6 Concept generation of the rear suspension ...... 45 4.7 Concept evaluation of the rear suspension ...... 47 4.8 Final concept and further development of the rear suspension ...... 48 5 Discussion ...... 51 6 Conclusion ...... 52 7 Future work ...... 53 References ...... 54

Appendixes Appendix A: WBS Appendix B: Gantt-Schedule Appendix C: Equilibrium calculation Appendix D: FMEA

1 Introduction

1.1 Background

Investigation and testing of hydropneumatic suspension systems has previously been done at Scania between the year 1992 and 2000. Conclusions have been drawn that the air suspension is superior according to driver’s subjective experience and the project has since then been resigned. Interest has aroused at Scania CV AB to further test the hydropneumatic suspension. The reason being the new ventures of decarbonised, clean, electrified, automatized and digitalised vehicles.

If electrified trucks are to be adopted in the market as an alternative to trucks with combustion engines, solutions for these vehicles’ capacity need to be presented. The vehicles weight needs to be reduced, the effectiveness of the components needs to be increased and alternatives to increase battery storage needs to arise if it’s going match the traveling distance of a combustion engine.

According to Wöhrmann et al. (2013) was the first company to introduce hydropneumatic suspension in the 1950’s. The suspension was used on passenger and was available in the market a while before air suspension was introduced. Now the hydropneumatic suspension is only manufactured by Citroën in larger quantities with respect to passenger cars. The interest of the suspension type diminished when many deficiencies were noticed. Leaks in the system caused parked cars to sink to the ground, which forced the driver of the vehicle to restore the pressure level before departure. It was also known that the hydropneumatic system requires installation of more components than the conventional compression system, namely oil pump, tank and accumulator.

Wöhrmann et al. (2013) goes on to explain that there has recently been an increased interest in hydropneumatic suspension in commercial off-road heavyweight vehicles and that the aforementioned problems with leaks in the system now are negligible. Compared to the conventional compression spring system and air suspension, hydropneumatic suspension offers high energy density at low surface area, therefore providing the possibility of placement near the centre, which is normally a very tight space. Placement near the wheel centre is advantageous as it can eliminate torque that otherwise presents because of the lever effect of the distance between the wheel centre and the spring.

8 1.2 Problem

Today's trucks use compressed air to carry loads in the suspension. Such a solution is space consuming and has relatively high energy consumption. For future chassis, a more efficient solution in terms of space, energy consumption and weight are required.

1.3 Project goal

Present concept solutions for the installation of hydropneumatic suspension system that fits into an existing test truck with minimal alteration of surrounding parts. Where in future work Scania can evaluate this type of suspension solution, meet market demand and expand the company's growth opportunities. The presented material will contain the position in the truck assembly for the hydraulic cylinders as well as CAD-models of all the brackets that will be designed to fit the suspension.

1.4 Project delimitations

The project focuses on concept generation for the positions of the hydraulic cylinders as well as design of brackets for the hydraulic cylinders installation possibilities for a truck with air suspension as existing solution. The project will not contain:

 Testing of the hydropneumatic suspension and the installation of it.  Optimization of the design for production.  Market research.  Mechanical drawing of the solution.  Installation of remaining articles in the hydropneumatic system. (see section 2.1)

9 2 Theory

2.1 Hydropneumatic suspension systems

The hydropneumatic suspension system that is supplied contains a hydraulic cylinder, accumulator, damping block, level sensor, ECU, Oil tank and an Oil pump. Every axle in the truck will require two hydraulic cylinder, two accumulators and two level sensors. The components around the axle share a single damping valve block. The whole system is then controlled by an ECU (electronic control unit), which provides logic and functions to the system. Since the truck for testing is a 4x2 truck the total of the components used will be the following:

 4x hydraulic cylinder  4x accumulator  4x level sensor  2x damping valve block  1x ECU.  Oil tank  Oil pump

2.1.1 Function

Each side of an axle is mounted to a hydraulic cylinder which suspends the axels of the vehicle. The vehicle can adjust the ground clearance by increasing or decreasing the volume of oil in the hydraulic cylinder chambers. In a hydropneumatic suspension the springing medium is the gas, which in most cases as well as this system is nitrogen. The gas is stored in a chamber in an accumulator, as the hydraulic cylinder is compressed by irregularities in the road the gas in the accumulator also compresses from the oil that mediates the motion. The increased pressure from the compressed gas naturally causes a reactive force which springs the vehicle. The return motion of the accumulator causes the oil to pass through the damping valve block which acts as a conventional by restricting the flow of the oil which in turn suppresses the hydraulic cylinders motion. (Supplier internal material 2020).

10

Figure 2.1.1. Schematic of the hydropneumatic suspension.

2.1.2 Secondary spring

Contrary to conventional hydropneumatic suspensions, theses accumulators are equipped with secondary springing. The secondary springing is derived from the extra smaller cylinder compartment on top of the accumulator. The compartment is filled with gas and is therefore pre-loaded. The preloaded accumulator should decrease the difference in stiffness of the system for when the vehicle is laden versus unladen. As the vehicle is unladen the secondary springing compartment is forcing the piston in the accumulator to maintian it’s position, putting pressure on the lower gas compartment making for a greater stiffness of the system in an unladen condition.

Figure 2.1.2. Secondary spring in accumulator.

11 2.1.3 Damping valve block

The black accumulator membrane above the damping valve block in figure 2.1.3 saves information of the load magnitude and varies the damping intensity depending on the static and dynamic pressure in the system. (Supplier internal material 2020).

Figure 2.1.3. Damping valve block.

2.1.4 Previous Testing

Results acquired from previous testing in 1992 of a hydropneumatic suspension at Scania yield that the test drivers experienced the truck to be too stiff while driving on “good” roads were high frequency vibrations occur. The drivers described it as they could feel the road structure up in to the cab.

During cornering the drivers explained that they experienced the truck to be swaying more comparing to the air suspended axle. Although while increasing the cornering intensity the problem seemed to fade.

Scania’s testing also noted the load handling to be significantly faster for the hydro pneumatic suspension. As well that the use of a hydropneumatic suspension on a 6x2 truck could eliminate the need of six air tanks in the truck. They also measured the natural frequency of the front and axle to 1,2 respective 1,4 Hz versus 1,6 respective 1,8 Hz for the air suspension. (Hellström 1992).

12 2.2 Air suspension

An air suspension system often consists of

• air spring • compressed air tank • compressor • level valve or level sensor + ECU

There are many adaptations on air suspensions, where the two most common are of the type roll bellow or convoluted bellow.

The air spring is the component that links the truck’s axle and frame and springs the vertical forces that the truck is exposed to. The roll bellow type air spring displayed in figure 2.2.1 will be focused on in this section as it is the type Scania uses in their trucks. The roll bellow type consists of a rubber bellow which is clamped between a piston in the base and a mounting plate at the top. (Nygren 1991)

Figure 2.2.1. Reversible sleeve air spring.

2.2.1 Function

During operation, the bellows are filled with air that is compressed and expands as the vehicle travels over uneven roads. The compression is caused by the piston pushing up underneath the bellow. The increased pressure inside the bellow pushes the piston back. Thus, forcing the piston to return which causes the air spring to expand to its regular state. (Nygren 1991, p14)

13 2.2.2 Carrying capacity

The carrying capacity of an air spring is dependent on the effective area and the internal overpressure. (Nygren 1991). The carrying capacity F is defined by the formula:

F=pA (1)

Where:

• Po is the internal overpressure

• Ae is the effective area • F is the carrying capacity

(Nygren 1991)

2.2.3 Effective area

The effective area changes during the spring movement, since it is dependent on the dynamic load acting vertically on the top of the bellow and the overpressure from the gas within the bellow. The effective area is defined by the formula A = D (2) where Ae is the effective diameter. In order to measure the effective diameter, the suspension is subjected to a relatively low load. With the load applied the bellow rolls over the edges around the piston and form a fold similar to a semicircle. The effective diameter can be approximated during this process to the distance between the midpoints of the opposing folds. (Nygren 1991)

Figure 2.2.2. Effective diameter of a roll bellow.

14 2.2.4 Spring characteristics

Air springs differ from conventional leaf springs in terms of suspension characteristics. The conventional suspension characteristic has a stiffness which is linear to the deformation in the spring and is described by the relationship between the l oad F and the deformation of the spring s, 푘= (3). The characteristics of air suspension are more complex as the stiffness varies depending on the dynamic overpressure within the air bellow and is therefore progressive. The spring characteristic is described in theory by the formula:

k = +p (4)

Where:

• ktheory is the theoretical stiffness. • n is the polytropic exponent. • p is the absolute pressure.

• po is the overpressure.

• Ae is the effective area. • V is the static volume. • x is the spring displacement.

Plotting the formula shows that the right part of the formula, contribution (b) in Figure 8, affects the spring constant as the effective area is dynamic. (Nygren 1991, p41)

Figure 2.2.3. Graphical representation of kteory.

15 2.2.5 Damping

Damping in a hydraulic shock absorber is created when a fluid flows concentrated through a smaller volume. Pneumatic damping is both non-linear and of turbulent flow, which makes it unpredictable. With high-frequency suspension, less air will flow between the volumes, which will have the consequence of higher spring stiffness as the effective volume decreases while the damping force also decreases. Because of this problem with pneumatic damping, it is rarely used. The most common practise is to use an external hydraulic damper with the pneumatic suspension (Scania CV AB 1999)

2.2.6 Roll

Air springs roll stiffness is usually very low, which means that an anti-roll bar often is required to firm the structure. The roll stiffness is described by the formula:

푅푆 = 푘 (5)

Where:

• RS is roll stiffness • s is spring distance • t is track width • k is spring stiffness.

(Hathaway n.d.)

Figure 2.2.4. Roll stiffness.

16 2.2.7 Natural frequency

According to Inman 2014 the natural frequency is the spring mass system as well as the frequency in which a motion repeats itself. For air suspended trucks the target natural frequency to hit for comfort is about 1.1-1.4 Hz.

According to Hellström 1992 low natural frequencies are considered to be comfortable during vertical suspension travel.

2.3 Current layout

This section of the report serves to grant more understanding about the setup of and around the current suspension.

2.3.1 Front suspension

The Scania front air suspensions main components consist of the air spring, shock absorber, torque rod, air spring link on both sides of the vehicle and an anti-roll bar connected between them. As illustrated in figure 2.3.1. The air spring link is mounted to the leftmost torque rod bracket and front spring bracket which are in turn mounted to the frame. The torque rod is mounted between both torque rod brackets, in which the leftmost bracket is mounted to the frame and the rightmost is fastened with screws to the front axle (not shown in figure 2.3.1). The shock absorber while poorly displayed in figure 2.3.2 is mounted to the lower shock absorber bracket (displayed in red in the figure) and the upper shock absorber bracket. The lower bracket is mounted to the front axle and the upper bracket to the frame. The air spring is mounted directly to the frame at the top and to the rightmost torque rod bracket at the bottom. Finally, the anti-roll bar is mounted on the two anti-roll bar brackets on either side of the vehicle and hung on to the two lower shock absorber brackets, best displayed in figure 2.3.2.

17

Figure 2.3.1. Front suspension sideview.

Figure 2.3.2 and 2.3.3 are overviews that illustrate the amount of parts that must be considered during the installation of the hydropneumatic suspension.

The chamber is a possible obstruction for the installation of the hydraulic cylinder for the hydropneumatic suspension. As the figure 2.3.4 illustrates, the space is quite confined around the front axle. The drag link is fitted very close to the shock absorber and frame leaving next to no room to fit anything. The track rod displayed in figure 2.3.3 is carrying the from the drag link to the passenger side wheel therefore this is the only drag link the truck is equipped with. This means that the space is not obstructed on the passenger side of the vehicle.

Figure 2.3.2. Overview of front.

18

Figure 2.3.3. Overview of back.

Figure 2.3.4. Overview of top.

19

Figure 2.3.5. Front suspension top view.

As explained in 3.2 the air springs function is to absorb the vertical loads between the truck and the road. Although the air spring link looks similar to a it does not assist the air spring much in the matter of absorbing vertical loads. The air spring link serves a different function; its function is to work together with the torque rod to absorb the longitudinal forces asserted on the truck during acceleration and deceleration as well as handling the lateral forces asserted on the truck. The air spring link and torque rod form a parallelogram which reduces the that normally increases during compression of the suspension.

The crossmember serves a multipurpose; it is used to stiffen the frame and to protect the air spring bellows from heat asserted from the combustion engine. According to theory of influence of shear from (Gross et.al. 2011, p 157) the applied forces needs to act in the shear centre in order to prevent torsion of the beam. The air spring asserts vertical load underneath the frame (see figure 2.3.3) which is of a u-shape thus having its shear centre outside of its cross-section (see figure…) it tends to rotate outward, Therefore the crossmember is needed to stiffen the frame in the lateral direction.

20

Figure 2.3.6. The frame’s shear centre.

2.3.2 Rear suspension

The truck that will be used for the installation is equipped with a four bellow air suspension configuration. An illustration of the space around the wheel that can be used to fit the hydropneumatic suspension is displayed in figure 2.3.7 and 2.3.8.

Figure 2.3.7. Back overview rear suspension.

21

Figure 2.3.8. Rear suspension front view.

As illustrated in the figure 2.3.9 the air springs are mounted on each side of the air spring beam which while not displayed in this figure is mounted on the rear axle by the U-bolts. The top of the air springs is mounted to the frame by the air spring brackets. The shock absorber is at its bottom mounted at the side of the separator plate. At the top the shock absorber is mounted at the shock absorber bracket which is mounted to the frame. The V-rod is mounted at its rear directly to the rear axle (as displayed in figure 16) and at its front to a rear cross member. The anti-roll bar is hung under the air spring beam and mounted between the anti- roll bar brackets on either side of the vehicle which are mounted to the frame and the cross member.

Figure 2.3.9. Rear suspension side view.

22 2.4 Comparison

As a summary of theoretical knowledge as well as knowledge from previous testing of the two suspension systems, a comparison of the positive and negative aspects of the hydropneumatic suspension versus the air suspension will be presented in this section.

Hydropneumatic positives

 Good comfort on “bad” roads with frost damage and longwave bumps.  Good shock absorption and well damped oscillations  Integrated anti-roll  Integrated shock absorber  Small design space at  No oil heating in hydraulic cylinder from shock absorption.

Hydropneumatic negatives

 Feeling too stiff on “good”/ high frequency shock roads  Swaying truck while cornering  Expensive.

(Hellström 1992)

23 3 Method

 The project will be divided into two phases, each phase containing a number of sub- stages  Planning  Concept and design. 3.1 Planning

Project plan

According to Astrakan, the project plan is designed to define activities, develop a schedule and set boundaries for the scope of the project. A project plan is also developed for the work to be carried out more efficiently. The project plan was issued in agreement with the client. The project plan included a WBS (work breakdown structure) that was arranged to break down the work into smaller segments. A Gantt- schedule in which the WBS- segments where used as planning blocks. Lastly an FMEA was also created in an effort to anticipate potential situations that may harm the project, the probability that these situations may arise and what can be done to address the situations. This project plan also included the introduction segment of this report.

Light Study

A light review of new material provided during the introduction day at Scania CV AB for the thesis.

WBS (Work Breakdown Structure)

In a hierarchical tree, the various project phases are ranked side by side and their associated parts are subordinate. This is to create an overview of the project's constituent parts and to more easily create the more specified Gantt-schedule.

24

Figure 3.1.1. Work breakdown structure.

Gantt-schedule

A simple method where the diagram is drawn in a coordinate system. The X-axis represents the time and the Y-axis represents the activities. Then a line is passed along the X-axis and orthogonally with the activity. This schedule will represent an overarching planning of the project (Johannesson, et al., 2013, p. 659). See (Appendix B) for the project's gantt schedule.

Figure 3.1.2. Gantt-schedule.

FMEA

FMEA is an acronym for "Failure mode and effect analysis". Evaluates subjective assessments of error events and consequences that may arise in a project. Overall assesses the risk of error, the possibility of detecting the error and the consequences the error produces. (Johannesson, et al., 2013, p. 296)

25

Figure 3.1.3. Failure mode and effect analysis.

Study

This study was conducted to acquire further understanding of the air suspension and the hydropneumatic suspension, in order to aid in the development of this project. The study included a thorough review of background material, reports and other studies. The study can be found in the theory segment of the report.

Requirement Specification

Used to determine the functional requirements by arranging what the product should perform. The requirements specification is developed during the course of the project when it is further decided how the requirements are to be met (Johannesson, et al., 2013).

3.2 Concept and design

Packing Analysis

The work began by analysing how the current front wheel suspension was packed, the space that was available to work with and which articles shared function with the hydropneumatic system and therefore could be replaced.

Kinematic analysis

A number of concepts for the positioning of the hydraulic cylinders were developed by kinematically investigating whether the positions of the cylinder caused collisions with surrounding articles. The positions that were considered to have the least impact on the surrounding articles were then chosen as concepts.

Analysis & observations of concepts

Various different analysis and observations were carried out on the different placements of the concepts namely:

26 • If the cylinder location affects the ground clearance. • The effect of the placement on the reaction forces in the suspension. • If the placement required a reduction in stroke. • The impact of the placement on surrounding articles. • Theoretical weight of the brackets. • Distance to roll centre. • Change of bushing angle during roll. • Change of bushing angle during load handling.

The analysis of the change of bushing angle during rolling and load handling was performed by placing the axles in different positions from a kinematics model of a similar truck. These positions included rolling of the axles +/- 6 degrees (see figure 3.2.2) followed by axles with + 190 / -60 mm offset from driving position, i.e. max and min stroke of the cylinders (see figure 3.2.3). Where the cylinders were then drawn with respect to the coordinates from points determined in the previous phase. The requirement was that the change of bushing angles should not exceed 6 degrees, since that would compromise the life-span of the bushing and damage the hydraulic cylinder.

Figure 3.2.2. Bushing angle analysis during roll.

27

Figure 3.2.3. Bushing angle analysis during load handling.

The analysis of reaction forces was performed with an equilibrium equation about the front and rear wheel-axle of the vehicle in excel. Where the various placements could easily be entered to investigate the impact the placements had over the reaction forces in the suspension. The applied load was determined equal to the load experienced by the air suspension at 6 bars of pressure in the bellows. The applied load for the front suspension was determined to be 35 kN. As for the rear air suspension the bellow in front of the rear axle experienced 20 kN and the bellow behind the rear axle experienced 33 kN. Replacing the combined bellows to a single hydraulic cylinder the applied loads were combined to a total of 53 kN to represent what the hydraulic cylinder would experience. For the equilibrium equations see appendix C.

28 Concept Evaluation

The concept evaluation was carried out with a modification of a Pugh’s matrix where the aforementioned analysis and observations were entered as parameters. The concepts were then graded depending on the impact the concept had over the parameters from where:

++ is very positive

+ is positive

- is negative

-- is very negative

The concept which inherits the highest score is then chosen as the final concept to enter the design phase and be the base in which the brackets are designed around.

Figure 3.2.1. Phug’s matrix.

Design

With a final concept evaluated from previous phase the brackets in which the hydraulic cylinder is to be held is can be designed. The design of the brackets was made in the CAD program Catia V5 where the design is solely based on creativity and experience of solid mechanics.

29 4 Results

4.1 Replacement and removal of the front suspension

According to roll calculations made by the supplier the anti-roll bar seen in figure 4.1.1 should be able to be terminated from the suspension once it’s fitted with the hydropneumatic suspension system. Since the hydropneumatic suspension system should be able to withstand roll without the use of an anti-roll bar. The shock absorber will also be replaced as the hydropneumatic suspension will have integrated dampening. The air springs will of course be removed as their function will be replaced with the hydraulic cylinders together with the accumulators. (See section 2.1).

Figure 4.1.1. Front suspension bottom view.

As the air springs are replaced by the hydropneumatic cylinders, which will cover a greater vertical distance than the air springs, the hydropneumatic cylinders will therefore require a placement on the side of the frame contrary to the air springs which are placed underneath the frame. This will render the cross member unnecessary since the frames cross-section is of a u-shape and therefore has its shear centre outside of the frame. (See section 2.3.1). Furthermore mounts will have to be redesigned to fit the new parts as well as removal of mounts that were rendered unnecessary along with the removal of their supported parts.

30 4.2 Concept generation of the front suspension

4.2.1 Concept 1: Cylinder between frame and draglink

Concept 1 is shown in figure 4.2.1 were the hydropneumatic cylinder would be placed in the space between the drag link and the frame as displayed in figure 4.2.2. An uncertainty with this placement of the hydropneumatic cylinder was if the spacing between draglink and the frame was enough for when the vehicle was set in motion. During a rolling motion of the truck the cylinder will sway either towards the drag link or towards the frame, depending on the direction of the rolling motion. Thus an analysis of the kinematics of the cylinders movement during roll had to be done. The results of this kinematic analysis showed that the hydropneumatic cylinder would in fact collide with the draglink during a 6° roll of the front axle. Figure 4.2.3 shows how the hydropneumatic cylinder moves during different stages of roll.

Figure 4.2.1. Concept 1.

31

Figure 4.2.2. Concept 1 drag link space.

Analysing the change of the angles in the bushings placed at the end of the hydraulic cylinders of concept 1 during +/- 6° rolling of the front axle returned the following changes in bushing angles relative to a neutral front axle:

 +6° roll: 3.103° change  -6° roll: 3.387° change

Figure 4.2.3. Concept 1 roll.

32 Analysing the change of the angles in the bushings placed at the end of the hydraulic cylinders of concept 1 during +190 mm -60 mm load handling of the front axle returned the following changes in bushing angles relative to a neutral front axle:

 -60 mm load handling: 0.711° change  +190 mm load handling: 3.033° change

Figure 4.2.4. Concept 1 load handling.

A heat shield is located above the frame to absorb emitting heat and noise from the engine. Since this heat shield is obstructing the space above the frame, the height in which the cylinder can be placed is confined below this point. Considering these limitations the cylinder would have to extend below the front axle’s lowest point limiting the vehicles ground clearance. (See figure 4.2.5)

33

Figure 4.2.5. Concept 1 overview.

4.2.2 Concept 2: Angled cylinder

In concept 2 illustrated in figure 4.2.6 the hydropneumatic cylinder is placed with a quite extreme angle outward from the frame in an attempt to save vertical space as well as to increase the roll stiffness the hydropneumatic cylinder can obtain on the vehicle with inspiration taken from the equation described in section 2.2.6.

Figure 4.2.6. Concept 2.

34 Placing the hydropneumatic cylinder as showcased in figure 4.2.6 causes a collision with the draglink as seen in figure 4.2.7, thus changing the draglinks position is required to achieve the space needed for the cylinder. In the case of concept 2 the hydropneumatic cylinders vertical placement is instead limited by the brake chamber if the hydropneumatic cylinder is to retain its angled position illustrated in figure 4.2.7. If it were to be raised vertically it would collide with the brake chamber during turning of the vehicle, therefore forcing the hydropneumatic cylinder to traverse the front axle’s lowest point causing it to limit the vehicle’s ground clearance.

Figure 4.2.7. Concept 2 collision.

Analysing the change of the angles in the bushings placed at the end of the hydraulic cylinders of concept 2 during +/- 6° rolling of the front axle returned the following changes in bushing angles relative to a neutral front axle:

 +6° roll: 0.896° change  -6° roll: 1.258° change

35

Figure 4.2.8. Concept 2 roll.

Analysing the change of the angles in the bushings placed at the end of the hydraulic cylinders of concept 2 during +190 mm -60 mm load handling of the front axle returned the following changes in bushing angles relative to a neutral front axle:

 -60 mm load handling: 0.03° change  +190 mm load handling: 1.541° change.

Figure 4.2.9. Concept 2 load handling.

36 4.2.3 Concept 3: Cylinder on top of the front axle

In this concept the total length of the hydraulic cylinder would be reduced by 270 mm. For purpose of illustration the hydraulic cylinder’s lower components have been hidden in figure 4.2.10.

Figure 4.2.10. Concept 3.

Analysing the change of the angles in the bushings placed at the end of the hydraulic cylinders of concept 3 during +/- 6° rolling of the front axle returned the following changes in bushing angles relative to a neutral front axle:

 +6° roll: 5.738° change  -6° roll: 5.299° change

Figure 4.2.11. Concept 3 roll.

37 Analysing the change of the angles in the bushings placed at the end of the hydraulic cylinders of concept 3 during +190 mm -60 mm load handling of the front axle returned the following changes in bushing angles relative to a neutral front axle:

 -60 mm load handling: 0.03° change  +190 mm load handling: 2.366° change.

Figure 4.2.12 Concept 3 load handling.

4.2.4 Concept 4: Cylinder with offset from heat shield

Concept 4 is shown in figure 4.2.13 were the hydraulic cylinder would be placed with an offset from the heat shield in order to gain extra vertical space. The hydropneumatic cylinder would also reserve a relatively small angle to ensure no collisions were made with the brake chamber during turning of the vehicle as illustrated in figure 4.2.17. This placement allows the cylinder to retain the current stroke of +190/-60 while not traversing the lowest point of the axle and is therefore able to maintain the vehicles ground clearance. This placement would however results in an increase of the reaction forces in the torque rod and the air spring link from 2.7 kN to 11.3 kN determined from an applied load of 35 kN.

38

Figure 4.2.13. Concept 4.

The drag link would be obstructing the hydraulic cylinders path as displayed in figure 4.2.14. Therefore the drag link and the drag link arm would have to be redesigned to consider this concept. A suggestion is to design a drag link that goes between the cylinder and the frame. Accompanied with a U-shaped drag link arm that is fitted around the backside of the cylinder, the suggestion is illustrated in figure 4.2.14.

Figure 4.2.14. Concept 4 collision and draglink arm.

39 Analysing the change of the angles in the bushings placed at the end of the hydraulic cylinders of concept 4 during +/- 6° rolling of the front axle returned the following changes in bushing angles relative to a neutral front axle:

 +6° roll: 4.169° change  -6° roll: 3.592° change

Figure 4.2.15. Concept 4 roll.

Analysing the change of the angles in the bushings placed at the end of the hydraulic cylinders of concept 4 during +190 mm -60 mm load handling of the front axle returned the following changes in bushing angles relative to a neutral front axle:

 -60 mm load handling: 1.764° change  +190 mm load handling: 1.41° change.

Figure 4.2.16. Concept 4 load handling.

4.3 Concept evaluation of the front suspension

It was agreed in a meeting with the mentors at Scania CV AB which parameters were to be added to the Pugh’s matrix and that it were to be modified to not include any weights as each parameter was found equally important. The results gathered from the Pugh’s matrix were that concept 4: “Cylinder with offset from heat shield” was the most suitable solution for the problem.

40 Table 1: Pugh’s matrix front suspension

Parameters Concept Concept Concept Concept 1 2 3 4 Impact on - - + + ground clearance Impact on - - + - reaction forces Impact on + + + + steering angle Eligible for + + -- + +190 -60 stroke (working stroke +/- 110) Impact on surrounding articles - - + - Impact on weight - - + - Distance to roll centre - ++ - + Impact on + + - + bushing angle change during roll Impact on bushing angle change during load - + + + handling Quantity + 3 6 6 6 Quantity - 6 4 4 3 Sum -3 2 2 3 Where: ++ is very positive + is positive - is negative -- is very negative

41 4.4 Final concept and further development of the front suspension

Since the Pugh’s matrix deduced that concept 4 (see section 4.2.4) is the best placement for the front suspension. Further development of brackets for the hydraulic cylinders could take place. Since the heat shield above the frame was limiting the available space to fit the top bracket for the hydraulic cylinder. The disc style top mount of the hydraulic cylinder was changed to a radial style top mount. This change of mounting point would make the design of the top bracket an easier task.

Figure 4.4.1. Bushing styles.

As illustrated by the side view of figure 4.6.2 and figure 4.6.3 the bracket has been extended outward in such matter that the bracket does not collide with the heat shield. Furthermore the holes displayed in figure 4.6.2 front view are designed with such spacing to be compatible with the frames already existing holes. This enables the possibility to extend the bracket to the height determined by concept 4. As previously mentioned this height is important for safety of the vehicle to make sure that the bottom of the hydraulic cylinder does not surpass the front axle’s lowest point.

42

Figure 4.4.2. Top front hydraulic cylinder bracket.

Figure 4.63 illustrates how the top bracket wraps around the heat shield. The cylinder had to be rotated by 43° so that the protruding pipe of the hydraulic cylinder was not facing the side of the bracket to avoid making the top bracket excessively wide. This protruding pipe would also obstruct the movement of the brake chamber if facing toward the wheel during turning. The protruding pipe needs to be angled by 43° from the centre of the mounting as well as in the opposite direction from the brake chamber. This implies that the front hydraulic cylinders would need to have mirrored top mountings for each side of the vehicle. Facing the hydraulic cylinders this way would grant an easy access for replenishment of oil during service of the hydraulic cylinders.

Figure 4.4.3. hydropneumatic front suspension sideview.

43 The bottom bracket is designed for the disc style bushings as displayed in figure 4.6.1. This bracket would be placed underneath the front axle in the holes previously assigned for the shock absorbers and the anti-roll bar, which no longer are required for this type of suspension system. The bracket has got an indentation which is displayed by the top view of figure 4.6.4. The function of this indentation is for the bracket to fit the specific shape of the front axle’s mounting point. Furthermore the bracket is asymmetrical thus needing to be fitted on the correct side of the vehicle. Figure 4.6.4 displays a bracket intended for the left side of the vehicles driving direction.

Figure 4.4.4. Bottom front hydraulic cylinder bracket.

Figure 4.6.5 illustrates the final product of the front suspension when the air spring suspension system is replaced with a hydropneumatic suspension system. Comparing to figure 2.3.4 it can be deduced that the air spring, anti-roll bar and the shock absorber are no longer a part of the suspension system.

Figure 4.4.5. Final hydropneumatic front suspension.

44 4.5 Replacement and removal of the rear suspension

The rear suspension displayed in figure 4.4.1 has similarities shared with the front suspension (see section 4.1) in that the following will be either removed or replaced with parts from the hydropneumatic suspension:

 shock absorber  air springs  brackets for previously mentioned articles.

The difference being that the roll stiffness for the hydraulic cylinders intended for the rear suspension could not match the current roll stiffness of the rear anti-roll bar, thus forcing the anti-roll bar to remain in the rear suspension system.

Figure 4.5.1. Rear air suspension bottom view.

4.6 Concept generation of the rear suspension

4.6.1 Concept 1: Cylinder in front of the axle

In concept 1 the hydraulic cylinder would be placed in front of the rear axle illustrated by figure 4.5.1. However the brake chamber is limiting how close to the wheel centre the hydraulic cylinder can be mounted. Because of this the reaction forces in the v-rod and the anti-roll bar are increased from 3.1 kN to 38.3 kN determined from an applied load of 53 kN.

45

Figure 4.6.1. Concept 1 rear suspension.

4.6.2 Concept 2: Cylinder behind the axle

In concept 2 the hydraulic cylinder would be placed behind the rear axle illustrated by figure 4.5.2. The space behind the rear axle is not limited by any parts which present the opportunity to mount the hydraulic cylinder close to the wheel centre. This placement would however results in an increase of the reaction forces in the V-rod and the anti-roll bar from 3.1 kN to 16.7 kN determined from an applied load of 53 kN.

Figure 4.6.2. Concept 2 rear suspension.

46 4.7 Concept evaluation of the rear suspension

In the same meeting with the mentors at Scania CV AB as of section 4.3 the parameters for the rear suspension were established. The same principle applied that the parameters were to be considered equally important.

Table 2: Pugh’s matrix rear suspension

Parameters Concept 1 Concept 2 Impact on -- - ground clearance Impact on -- - reaction forces Eligible for + + +190 -60 stroke (working stroke +/- 110)

Impact on surrounding - + articles Impact on weight - + Distance to roll centre - -

Impact on + + bushing angle change during roll Impact on bushing angle + + change during load handling Quantity + 4 6 Quantity - 6 2 Sum -3 3 Where: ++ is very positive + is positive - is negative -- is very negative

47 4.8 Final concept and further development of the rear suspension

Since the Pugh’s matrix deduced that concept 2 (see section 4.5.2) is the best placement for the rear suspension. Further development of brackets for the hydraulic cylinders could take place.

In order to maximise ground clearance for the current stroke of the hydraulic cylinder, the top disc style bushing were exchanged to a radial style bushing (see figure 4.6.1). However the hydraulic cylinder would still traverse the lowest point of the rear axle by 57 mm making it affect the ground clearance and safety of the vehicle.

The top bracket for the rear hydraulic cylinder is displayed in figure 4.7.1. The bracket is designed to be mounted directly to the frame. The notable thing about the design of this bracket is that the frame mounting holes are not centred in the bracket. This is by design to acquire the minimum distance between the hydraulic cylinder and the rear axle, thus minimizing the reaction forces in the anti-roll bar and v-rod.

Figure 4.8.1. Top rear hydraulic cylinder bracket.

Figure 4.7.2 displays the bottom bracket for the hydraulic cylinder. This bracket is designed to be fitted underneath the separator plate and fastened by the U-bolts that were previously used to fasten the separator plate and air spring beam. The hydraulic cylinder is to be mounted with its disc style brackets in the cylinder mounting hole and with a disc bushing on the loading surface followed by a disc bushing in the bushing pocket fastened with a plate and a nut. Lastly the anti-roll bar cap is to be placed on top of the anti-roll bar illustrated in figure 4.7.4.

48

Figure 4.8.2. Bottom rear hydraulic cylinder bracket.

Figure 4.7.3 illustrates the final product of the rear suspension when the air spring suspension system is replaced with a hydropneumatic suspension system. What should be noted in figure 4.7.3 is that the two air springs and air spring beam have been replaced with a singular hydraulic cylinder and its brackets.

Figure 4.8.3. Final hydropneumatic rear suspension.

49

Figure 4.8.4. Final hydropneumatic rear suspension top view.

50 5 Discussion

As can be observed in section 4.2.2 figure 4.2.8 the change in bushing angle seem to be reduced by angling the cylinder out from the frame. The opposite can be observed from section 4.2.3 figure 4.2.11. Here the hydraulic cylinder is angled inwards to the frame, in which the changes in bushing angle seem to increase during rolling of the vehicle.

Since the lever effect applies the reaction forces affecting the torque rod and the air spring link among the front axle will be of lower amplitude if the hydraulic cylinder is to be placed closer to the axle. Same principle applies around the rear axle where the reaction forces in the v-rod and the anti-roll bar assembly would instead be affected. Because of this the hydraulic cylinder is recommended to be placed as close to the axles as possible to avoid any increased stress in the affected components.

The space around the rear axle seems to be too restricted vertically as the hydraulic cylinder should not traverse above the frame of the truck, thus forcing the hydraulic cylinder to be mounted in such way that it would traverse the trucks lowest point and therefore affecting the vehicles ground clearance and making it unsafe in off-road environments. The solution presented in concept 2 could be valid for the purpose of testing the vehicle on the test track, since it would not involve any off-road instances. However for purposes of series production a minimum reduction of 57 mm to the total length of the hydraulic cylinder would be advised, as to not compromise the safety of the vehicle. This reduction should affect the load handling portion of the hydraulic cylinder as a working stroke of +/- 110 mm is still needed for the compression and extension function of the suspension.

Gathered from the supplier’s calculations and diagrams of characteristics the roll stiffness and the characteristics of the hydraulic cylinders are only able to act as a replacement for the front anti-roll bar, the rear cylinders will not be able to match the roll stiffness and characteristics needed to replace the rear anti-roll bar if current performance is to be retained.

Load handling does not seem to cause an increase to any limiting matter in bushing angles that has to be taken in to account during positioning of the hydraulic cylinders.

51 6 Conclusion

 Front hydraulic cylinder placement is best suited for placement in front of the wheel axle according to concept 4.  Rear hydraulic cylinder placement is best suited for placement behind the wheel axle according to concept 2.  The stroke for load handling of rear hydraulic cylinders needs to be reduced by a minimum of 57 mm.  Angled mounting of the hydraulic cylinder towards the wheel reduces the change in bushing angling of disc style bushings during rolling.  Placement of hydraulic cylinders close to either axle’s wheels centre is preferred to reduce the reaction forces in other parts of the suspension.  The rear anti-roll bar will not be eligible for removal.  Front anti-roll bar is eligible for removal.  Front and rear shock absorbers, air springs and coexistent brackets are eligible for removal.

52 7 Future work

 Refining the design of the brackets to be suited for casting.  Re-design affected surrounding parts: drag link, drag link arms, brake pipes etc.  Finding positions for the accumulators, damping valve block, ECU, oil tanks and routing of oil lines.  FEM analysis of the brackets.  Fatigue calculation/ testing of the torque rod, air spring link, v-rod, anti-roll bar assembly with the hydraulic cylinder mounted.  Testing the vehicle with the hydraulic cylinders mounted.

53 References

Astrakan (2019). Skapa en projektplan. https://www.astrakan.se/projektplan/ [2020-01-29]

Gross, D., Hauger, W., Schröder, J., Wall, W. & Bonet, J. (2011). Engineering Mechanics 2. New York: Springer.

Hathaway, R. (n.d). Spring spacing, Roll Stiffnessand Transverse . http://ismasupers.com/downloads/tech-talk/Tech-03%20Springs-Roll%20Stiffness- 4.pdf/ [2020-03-10]

Hellström, S., 1992. Gashydrauliskt fjädringssystem (Delrapport 1). [Unpublished manuscript] Södertälje: Scania CV AB.

Inman, D., 2014. Engineering Vibration. 4. Harlow: Pearson.

Johannesson, H., Persson, J.-G. & Pettersson, D., 2013. Produktutveckling. 2. Stockholm: Liber AB.

Nygren, N-G., 1991. Luftfjädrar. Stockholm: Kungliga tekniska högskolan.

Scania (2019). Towards a sustainable transport system. https://www.scania.com/group/en/towards-a-sustainable-transport-system/ [2020- 01-30]

Scania CV AB., 1999. Thesis 2. [Unpublished manuscript] Södertälje; Scania CV AB.

Supplier,. 2019. Presentation for Scania 1 [Unpublished manuscript] Haan; Hemscheidt Fahrwerktechnik Gmbh & Co

Supplier,. 2020. Presentation for Scania 2 [Unpublished manuscript] Haan; Hemscheidt Fahrwerktechnik Gmbh & Co

Transportstyrelsen (2018). Lasta lagligt. https://www.transportstyrelsen.se/globalassets/global/publikationer/vag/yrkestrafik /lasta-lagligt/tran045-lasta-lagligt-sve.pdf [2020-02-26]

54 Wöhrmann, M. Holzinger, M & Hauschild, J. (2013). Hydropneumatische federungssysteme für schwere nutzfahrzeuge. ATZ off highway, 3.

55 Appendixes

Appendix A: WBS

Appendix B: Gantt-schedule

Appendix C: Equilibrium equation

Appendix D: FMEA

56 Appendix A: WBS

Appendix B: Gantt-schedule

Appendix C: Equilibrium equations

Appendix D: FMEA