Failure Analysis Gears-Shafts-Bearings-Seals
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Failure Analysis • Installation & Maintenance Gear – Shafts – Bearings – Seals (Page 1 of 20) FAILURE ANALYSIS GEARS-SHAFTS-BEARINGS-SEALS CONTENTS 1 - INTRODUCTION Page The function of the industrial power transmission gear drive is Part 1 Introduction.······························1 to reliably transmit torque and rotary motion between a prime mover and a driven piece of equipment at acceptable levels of Part 2 Gear Distress and Failure Modes ··············2 noise, vibration and temperature. When one or more of the Surface Fatigue – Pitting ····················3 preceding operating characteristics exceeds allowable limits, –Spalling···················5 the drive and its application should be examined to determine Wear ··································6 the cause of the problem. Gear drive components most – Degrees of Wear ·············7 commonly subject to distress are the gears, shafts, bearings and seals. The purpose of this paper is to describe a number – Types of Wear ···············8 of distress and failure modes of each of these components – Miscellaneous Wear Modes ·····9 and to indicate probable causes and possible remedies for Plastic Flow ·····························10 these failure modes. Breakage ······························11 For drives that are properly designed and manufactured, Failure Associated with Processing ·············13 abnormal distress or failure can result from misapplication or Part 3 Shaft Distress and Failure Modes ·············14 poor installation or poor maintenance. Part 4 Anti-Friction Bearing Failures ················16 Continuous steady state overloads can result from erroneous initial power requirement calculations, over motoring, Part 5 Failure of Contact Oil Seals ·················18 increased output demands, etc. Such loading should be detectable by motor overheating or from electrical meter readings on the driving motor. Momentary or transient peak loads are of such short duration that electrical meters do not respond accurately to them. In such cases torquemeter readings of instantaneous loads, as shown in Figure 1.1 may be necessary to determine actual torque loads. NON-RESILIENT COUPLING RESILIENT COUPLING STARTING RUNNING TORQUE TORQUE PEAK TORQUE PEAK TORQUE =7.0 RUNNING TORQUE TIME TIME TORQUE ZERO TORQUE TYPICAL TORQUE IN STEEL MILL DRIVE RUNNING TORQUE VARIATION TIME LESS THAN 5% HIGH TORQUE STARTING - UNIFORM LOAD STARTING RUNNING ZERO TORQUE PEAK TORQUE RUNNING TORQUE TORQUE PEAK TORQUE =1.2 VARIATION LESS TORQUE RUNNING TORQUE THAN 5% ZERO TORQUE TIME TIME EASY STARTING UNIFORM LOAD FLUCTUATING LOAD-TWO CYLINDER GAS COMPRESSOR FIG. 1.1 (253-2) TORQUE METER RECORD FIG. 1.2 (253-1) TORQUE METER RECORD Rexnord Industries, LLC, Gear Group 108-010 3001 W. Canal St., Milwaukee, WI 53208-4200 USA Telephone: 414-342-3131 August 1978 Fax: 414-937-4359 e-mail: [email protected] web: www.rexnord.com Replaces 690801 Installation & Maintenance • Failure Analysis (Page 2 of 20) Gear – Shafts – Bearings – Seals Vibratory loads, or system dynamic loads depend on the tolerances result in loads being non-uniformly distributed interrelation of the components in the entire system with one across the gear tooth surface, thus stressing some areas of the another and torquemeter readings or a study of the system is tooth more highly than others. In the AGMA rating formulas, required to assess instantaneous loads. From properly which are in wide use today, this effect is recognized by the obtained torquemeter readings, see Figure 1.2, the load incorporation of a load distribution factor, designated Cm or characteristics of the system can be determined including Km. The numerical value of this factor is the ratio of the steady state loads, peak loads, accelerating loads, reversing maximum load intensity on the tooth to the load intensity for a and vibratory loads and other dynamic effects that may exist. uniformly distributed load. Figure 1.3 shows several contact Once the nature of the load within a system has been patterns that might be observed on loaded teeth. Load established, the manner in which the load affects each of the intensity diagrams along with values of load distribution drive components must then be assessed. factors are shown for each case. Other more complex intensity Unexpected environmental conditions affecting gear drive distributions often prevail, but for most well designed gear performance can be grouped into three categories: sets, the load distribution factors vary from 1.2 to 1.7. The AGMA rating formulas also recognize a tooth loading 1 – ambient temperature, either high or low effect which is velocity dependent, but is largely independent of the system transmitted load. It is the dynamic impact 2 – airborne abrasive material loading a tooth encounters as it picks up its share of the load 3 – corrosive materials of gaseous, liquid or solid form as it enters the zone of tooth action at the mesh. The AGMA Ambient temperatures that are too high may result in lube formulas account for the usual accuracies of today’s temperatures so high that ineffective oil films are formed, manufacturer, but if exaggerated errors are present, from while too low a temperature may thicken oil so much that flow whatever cause, dynamic loads can be a source of tooth is restricted and certain parts may be starved of lubricant. In surface distress or tooth fracture. either case, abnormal wear may result. Ingress of airborne abrasive material can cause premature bearing failure, 2 — GEAR DISTRESS AND FAILURE MODES abnormal gear wear and can damage shaft journals for oil seals. Infiltration of corrosive gases, liquids (including water) Distress or failure of gears may be classified into four or solids can result in corrosive chemical reactions on bearing, categories: gear and seal contact surfaces that can cause premature 1 – surface fatigue (pitting), failure or abnormally high wear rates. 2 – wear, Last but not least, installation, care and maintenance 3 – plastic flow procedures can influence gear drive performance. Units improperly installed or maintained can result in such things as 4 – breakage. poor tooth contact, preloaded bearings, damaged bearings The appearance of the various distress and failure modes can and improper or poor lubrication and lubrication leakage. differ between gears that have through hardened teeth and A gear drive is one part of a power system which has certain those that have surface hardened teeth. These differences result load characteristics peculiar to the specific application. The from the different physical characteristics and properties and gear drive package itself is a subsystem within the overall from the residual stress characteristics associated with the system and in addition to transmitting the load demands of the surface hardened gearing. Where appropriate, examples of overall system, it is subject to additional load variations distress of both through and surface hardened gears are shown generated or contained within itself. and discussed. The elastic deflection of the gears, shafts, bearings and their supporting structures determine to a large degree the manner in which mating gears will be aligned to one another under operating loads. These deflections along with manufacturing b z y ss óy ó ACTUAL LOAD ACTUAL LOAD ACTUAL LOAD z DISTRIBUTION DISTRIBUTION DISTRIBUTION 4P ó 4 4 4 Scm = óx x b ó 3 UNIFORM LOAD 3 3 S z UNIFORM LOAD UNIFORM LOAD cm ó DISTRIBUTION DISTRIBUTION DISTRIBUTION y 2 2 2 –.25b –.393b x LOAD INTENSITY 1 1 LOAD INTENSITY 1 LOAD INTENSITY FFF F/2 F TENSION TIP RADIUS OF CURVATURE PITCH COMPRESSION OF TOOTH LINE ROOT SHEAR STRESS .434b .434b AT A=±0.25 Scm Km = 1.2 Km = 2.0 Km = 4.0 AT B= 0.30 Scm SHADED AREA IS PROFILE CONTACT AB C FIG. 1.3 (253-3) CONTACT PATTERNS AND LOAD DISTRIBUTION FIG. 2.1 (312-10) HERTZIAN STRESSES 108-010 Rexnord Industries, LLC, Gear Group August 1978 3001 W. Canal St., Milwaukee, WI 53208-4200 USA Telephone: 414-342-3131 Replaces 690801 Fax: 414-937-4359 e-mail: [email protected] web: www.rexnord.com Failure Analysis • Installation & Maintenance Gear – Shafts – Bearings – Seals (Page 3 of 20) CONTACT STRESS - Scm y 1.0 0.5 0 0 0.5 b STRESS ALONG x AXIS 1.0 b ACE STRESS ALONG y AXIS STRESS ALONG z AXIS 1.5 b SHEAR STRESS PLANE xy 2.0 b DEPTH BELOW SURF y 2.5 b FIG. 2.3 (683-2) INITIAL (CORRECTIVE) PITTING 3.0 b Scm y x x y b FIG. 2.2 (312-6) VARIATION OF STRESS BELOW SURFACE SURFACE FATIGUE Surface fatigue is the failure of a material as a result of repeated surface or sub-surface stresses beyond the endurance limit of the material. Figure 2.1 indicates the theoretical mutual Hertzian stresses occurring when a gear and pinion mesh. There are compressive stresses at the surface and unidirectional and bi-directional sub-surface shear stresses. Figure 2.2 indicates FIG. 2.4 (683-9) INITIAL (CORRECTIVE) PITTING ENLARGED the magnitude of these stresses. PHOTO For most through-hardened industrial type gears, initial pitting PITTING is considered normal and no remedial action is required. Where necessary, initial pitting can be reduced by special Pitting is a form of surface fatigue which may occur soon after tooth finishing means and sometime by a careful break-in at operation begins and may be of three types: reduced loads and speeds. In some special critical 1 - initial (corrective) applications, teeth are copper or silver plated to prevent or reduce initial pitting. 2 - destructive 3 - normal INITIAL PITTING is caused by local areas of high stress due to uneven surfaces on the gear tooth. This type pitting can develop within a relatively short time, reach a maximum and with continued service polish to a lesser severity. Initial pitting shown in Figure 2.3 usually occurs in a narrow band at the pitchline or just slightly below the pitchline. It is most prominent with through-hardened gears and is sometimes seen with surface-hardened gears. The shape of a classical pit is shown in Figure 2.4.