齿轮失效分析.docx

1、齿轮失效分析How to Analyze Gear FailuresRobert L. Errichello and Jane Muller, GeartechWhen an important gear failure occurs, someone becomes responsible for analyzing the failure, determining its cause and recommending a solution. A company can select its own engineer, an outside consultant or both. If a

2、consultant is called in, this should be done as early in the process as possible.Though similar procedures apply to any failure analysis, the specific approach can vary depending on when and where the inspection is made, the nature of the failure and time constraints. When and where. Ideally, the en

3、gineer conducting the analysis should inspect the failed components as soon after failure as possible. If an early inspection is not possible, someone at the site must preserve the evidence based on instructions from the analyst.If a suitable facility for disassembling and inspecting the gearbox is

4、not available onsite, it may be necessary to find an alternate location or bring the necessary equipment to the site.Nature of failure. The failure conditions can determine when and how to conduct an analysis. For example, if the gears are damaged but still able to function, the company may decide t

5、o continue their operation and monitor the rate at which damage progresses. In this case, samples of the lubricant should be collected for analysis, the reservoir drained and flushed and the lubricant replaced.If gearbox reliability is crucial to the application, the gears should be examined by magn

6、etic particle inspection to ensure that they have no cracks. The monitoring phase will consist of periodically checking the gears for damage by visual inspection and by measuring sound and vibration.Time constraints. In some situations, the high cost of shutting down equipment limits the time availa

7、ble for inspection. Such cases call for careful planning. For example, dividing tasks between two or more analysts reduces the time required.Preparing for InspectionBefore visiting the failure site, interview a contact person located at the site and explain what you need to inspect the gearbox inclu

8、ding personnel, equipment and working conditions.Request a skilled technician to disassemble the equipment under your direction. But, make sure that no work is done on the gearbox until you arrive. This means no disassembly or cleaning. Otherwise, a well-meaning technician could inadvertently destro

9、y evidence.Verify that the gearbox drawings, disassembly tools and adequate inspection facilities are available.Ask for as much background information as possible, including manufacturer抯 part numbers, gear and bearing runtime (hr), service history and lubricant type.Now, it抯 time to assemble your i

10、nspection equipment, including items such as a magnifying glass, measuring tools, felt tip markers, lubricant sampling equipment and photographic equipment. A well-designed set of inspection forms for the gearbox, gears and bearings should be at the top of your priority list.Failure InspectionBefore

11、 starting the inspection, review the background information and service history with the contact person. Then interview those involved in the design, installation, operation, maintenance and failure of the gearbox. Encourage them to tell everything they know about the gearbox even if they feel it is

12、 not important.After completing the interviews, explain your objectives to the technician who will be working with you. Review the gearbox assembly drawings with the technician, checking for potential disassembly problems.Visual examination. Before disassembling the gearbox, thoroughly inspect its e

13、xterior. Use an inspection form as a guide to ensure that you record important data that would otherwise be lost once disassembly begins. For example, the condition of seals and keyways must be recorded before disassembly. Otherwise, it will be impossible to determine when any damage may have occurr

14、ed to these parts. Gear tooth contact patterns should be taken before completely disassembling the gearbox.After the external examination, disassemble the gearbox and inspect all internal components, both failed and undamaged. Examine closely the functional surfaces of gear teeth and bearings and re

15、cord their condition. Before cleaning the parts, look for signs of corrosion, contamination and overheating.After the initial inspection, wash the components with solvents and re-examine them. This examination should be as thorough as possible because it is often the most important phase of the inve

16、stigation and may yield valuable clues. A low-power magnifying glass and pocket microscope are helpful tools for this examination.It is important to inspect the bearings because they often provide clues as to the cause of gear failure. For example: Bearing wear can cause excessive radial clearance o

17、r end play that misaligns the gears. Bearing damage may indicate corrosion, contamination, electrical discharge or lack of lubrication. Plastic deformation between rollers and raceways may indicate overloads. Gear failure often follows bearing failure. Gear tooth contact patterns. (Complete this ste

18、p before disassembling gearbox components for inspection). The way in which mating gear teeth contact indicates how well they are aligned (Figure 1). If practical, record tooth contact patterns under either loaded or unloaded conditions. For no-load tests, paint the teeth of one gear with marking co

19、mpound. Then, roll the teeth through mesh so the compound transfers the contact pattern to the unpainted gear. Lift the pattern from the gear with scotch tape and mount it on paper to form a permanent record.For loaded tests, paint several teeth on one or both gears with machinist抯 layout lacquer. R

20、un the gears under load for a sufficient time to wear off the lacquer and establish the contact patterns. Photograph the patterns to obtain a permanent record.Document observations. Describe all important observations in writing, using sketches and photographs where needed. Identify and mark each co

21、mponent (including gear teeth and bearing rollers), so it is clearly identified in the written description, sketches and photographs. It is especially important to mark all bearings, including inboard and outboard sides, so their location and position in the gearbox can be determined later.Describe

22、components in a consistent way. For example, always start with the same part of a bearing and progress through the parts in the same sequence. This helps to avoid overlooking any evidence.Concentrate on collecting evidence, not on determining the cause of failure. Regardless of how obvious the cause

23、 may appear, do not form conclusions until all the evidence is considered.Gear geometry. The load capacity of the gearset will need to be calculated later. For this purpose, obtain the following geometry data, either from the gears and gear housing or their drawings: Number of teeth Outside diameter

24、 Face width Gear housing center distance for each gearset Whole depth of teeth Tooth thickness (both span and top land measurement) Specimens for laboratory tests. During the inspection, you will begin to formulate hypotheses regarding the cause of failure. With these hypotheses in mind, select spec

25、imens for laboratory testing. Take broken parts for laboratory evaluation or, if this is not possible, ensure that they will be preserved for later analysis.Oil samples can be very helpful. But, an effective lubricant analysis depends on how well the sample represents the operating lubricant. To tak

26、e samples from a gearbox drain valve, first discard stagnant oil from the valve. Then take a sample at the start, middle and end of a drain to avoid stratification. To sample from the reservoir, draw samples from the top, middle and near the bottom. Examine the oil filter and magnetic plug for wear

27、debris and contaminants.Samples from the oil storage drum or reservoir can uncover problems such as excessive water in the oil due to improper storage.Have you got it all? Before leaving the site, make sure that you have everything needed (completed inspection forms, written descriptions and sketche

28、s, photos and test specimens) for completing the failure analysis.Determine Type of FailureNow it抯 time to examine all of the information and determine how the gear (or gears) failed.Several failure modes may be present and you need to identify which is the primary mode, and which are secondary mode

29、s that may have contributed to failure. Table 1 lists six general classes of gear failure modes, of which the first four are the most common. An understanding of these four common modes will enable you to identify the cause of failure.1. Bending fatigue. This common type of failure is a slow, progre

30、ssive failure caused by repeated loading. It occurs in three stages: Crack initiation. Plastic deformation occurs in areas of stress concentration or discontinuities, such as notches or inclusions, leading to microscopic cracks. Crack propagation. A smooth crack grows perpendicular to the maximum te

31、nsile stress. Fracture. When the crack grows large enough, it causes sudden fracture. As a fatigue crack propagates, it leaves a series of 揵each marks?(visible to the naked eye) that correspond to positions where the crack stopped (Figure 2). The origin of the crack is usually surrounded by several

32、concentric curved beach marks.Most gear tooth fatigue failures occur in the tooth root fillet (Figure 3) where cyclic stress is less than the yield strength of the material and the number of cycles is more than 10,000. This condition is called high-cycle fatigue. A large part of the fatigue life is spent initiating cracks, whereas a shorter time is required for the cracks to propagate.Stress concentrations in the fillet often cause multiple crack origins, each producing separate cracks. In such cases, cracks propagate on different planes and may join to for