文献翻译汽车曲轴的失效分析

1、analysis of a vehicle crankshaft failuref.s. silvaabstractthis paper reports an investigation that was carried out on two damaged crankshafts. they were diesel van crankshafts that were sent to be ground, after a life of about 300,000 km each. some journals were damaged on each crankshaft. after gri

2、nding, and assembling on the diesel van, the crankshafts lasted about 1000 km each, and the journals were damaged again. the crankshafts were then sent to be investigated. different laboratory tests were carried out in order to discover what could have been the cause of the damage. different typical

3、 crankshaft failures were assessed, and will be discussed in this paper. the cause of the damaged journals was found to be a wrong grinding process that originated small thermal fatigue cracks at the center of the journals, on both crankshafts. these almost invisible cracks, with sharp edges, acted

4、as knives originating a very quick damaging of the journal bearings, and as a consequence damaged the journals themselves.1. introductionshaft failures in general, and crankshaft failures in particular, may be originated by several causes. mechanical fatigue failures are probably the most common cau

5、se of crankshaft failures 3,4. mechanical fatigue failures may have some sources such as misalignment of the shaft, originating rotating-bending, or vibration due to some problem with the main bearings, or even due to high stress concentrations caused by an incorrect fillet size (lower radii at geom

6、etrical discontinuities). nevertheless, when a crankshaft fails, due to damage to one or more of its journals, the cause of the damage is generally attributed to any feature related to the engine operation. it may be absence of oil, or defective lubrication of one or more journals, or high operating

7、 oil temperature, etc. in a particular case, when a crankshaft is sent to be ground and, after assembly, works for only a few kilometers and one or more journals fail again, the causes are generally attributed to the assembly department or to an incorrect use of the engine in the first few kilometer

8、s. in this case, the fault is either attributed to an incorrect installation of the journal bearings, or to improper lubrication in use. finally the cause may also be attributed to the crankshaft repair shop. traditionally, causes attributed to the repairing work are: misalignment of the crankshaft

9、or of the journals due to incorrect grinding, no proper straightening of the crankshaft, when necessary, no proper surface finishing, or incorrect welding when it is necessary to reconstruct the journals. in the case described in this work, it was not necessary to reconstruct or weld the journals. o

10、nly a grinding operation was demanded. among all the possible explanations for the damaging of the crankshafts the question that always arises is: what was the cause of the damaging, and to whom may be attributed the fault? based on the analysis of the damaged components this paper will try to answe

11、r the above questions.2. methodto determine the cause of the crankshaft failures, the following method was used:presentation of the problem;visual analysis;analysis of possible explanations for the problem based on typical crankshaft failure causes;definition of laboratory tests to be done.after the

12、 analysis of each explanation for the failures was made, and based also on laboratory tests considered necessary, a conclusion has been drawn.3. presentation of the probleman automotive repair shop of a worldwide-accredited automotive brand received a diesel van with a problem with its crankshaft. t

13、wo journals were damaged. the crankshaft was sent to be repaired on a crankshaft repair shop. after being repaired the crankshaft was assembled at the automotive repair shop. the van was working properly but after about 1000 km the journals were damaged again. a few months later another diesel van,

14、with a damaged crankshaft with jagged journals, was sent to be repaired at the same crankshaft repair shop. the same thing happened to this second crankshaft, i.e. after being repaired, it worked for only about 1000 km. two crankshafts repaired at the same crankshaft repair shop that had problems ju

15、st after a few kilometers of use led the automotive repair shop to think that there was some problem with the repairing process. on the other hand the crankshaft repairing shop said they had decades of practice on repairing crankshafts and that the problem was probably on the assembly of the compone

16、nts or on improper use (for example absence of oil). as a consequence, the automotive repairing shop asked minho university mechanical engineering department to prepare a technical report to assess what were the causes of the crankshaft damage. the crankshafts are shown in figs. 1 and 2. on cranksha

17、ft 1, journal 4 is badly damaged, as shown in detail in fig. 3. cracks are painted with white paint to make them obvious. an analysis with a lens showed some cracks (fig. 4), not easily visible with the naked eye, on journal 1 of crankshaft 1 (not jagged). these cracks are on the surface of the jour

18、nal and dispersed along the whole circumference of the journal, but aligned with the axis of the crankshaft. journal 2 of crankshaft 2 is badly damaged (fig. 5). some small cracks can be observed at the center of the journal, all aligned with the axis of the crankshaft. on journal 4 of the same cran

19、kshaft (fig. 6) some small cracks are also visible with a magnifying glass. these cracks are on the center band of the journal and are dispersed along the circumference of the journal, but are parallel to the crank axis. a rougher surface is visible near these cracks, and the journal is beginning to

20、 become jagged. a finer detail of the cracks is shown in figs. 7 and 8, respectively of journal 1 of crankshaft 1 and of journal 4 of crankshaft 2. it is clear that there are cracks, almost invisible with naked eye, but easily detectable with some non-destructive testing methods such as magnetic par

21、ticle testing. after a visual and a microscopical analysis, some possible explanations for journal damage were admissible. they will be analyzed consecutively4. possible explanations/typical failures in vehicle crankshaftsthe analysis was separated into three kinds of possible sources for the journa

22、l damage:operating sources;mechanical sources;repairing sources.in the first item (operating sources) were included:1. oil absence;2. defective lubrication on journals;3. high operating oil temperature;4. improper use of the engine (over-revving)on the second item (mechanical sources) were included:

23、5. misalignments of the crankshaft on assembly;6. improper journal bearings (wrong size);7. no control on the clearance between journals and bearings;8. crankshaft vibrations.finally, in the third item (repairing sources) were included:9. misalignments of the journals (due to improper grinding);10.

24、misalignments of the crankshaft (due to improper alignment of the crankshaft);11. high stress concentrations (due to improper grinding at the radius on both sides of the journals);12. high surface roughness (due to improper grinding, originating wearing);13. improper welding or nitriding;14. straigh

25、tening operations;15. defective grinding.a brief discussion of each of the possible explanations for the damage, based on typical failures, will be done successively.4.1. operating sourcesoperating sources such as oil absence or defective lubrication of journals would be enough to damage the journal

26、s. nevertheless, the contact between the bearings and the journals would essentially promote a rougher surface of the journals. the heat and stresses during the contact would also eventually promote the propagation of existing cracks, such as those shown in fig. 3, but would not nucleate cracks. the

27、re was no contact between the bearing and journal 1 of crank 1 (fig. 4) but there exist some small cracks. thus, cracks developed on the journals due to reasons other than contact between the bearing and the journal.another operating source is high oil temperature. if this were the case, the oil wou

28、ld be working below its specifications, i.e. there could exist contact between journals and bearings. however, as previously concluded, the small cracks were not a consequence of contact between surfaces.the improper use of the engine (over-revving) could be included in the previous points because i

29、ts consequence would be contact between journals and bearings4.2. mechanical sourcesmisalignments of the crankshaft on assembly, improperly straightened cranks due to improper repairing, or worn bearings would create a bending moment and higher stresses at the crankshaft. this would also be the effe

30、ct of vibrations. however these higher stresses would originate mechanical fatigue cracks at the weakest points of the crank, namely at the stress concentrations. cracks would develop at those weaker points, and the crank would fail by fracture. fig. 9 shows an example of a train crankshaft that fai

31、led due to mechanical fatigue. as can be seen the crack developed at the radius at the geometrical discontinuity. cracks observed on the crankshafts of figs. 1 and 2 developed at the surface of the journals, e.g. at surfaces that are not critical in terms of stresses. thus, a second conclusion that

32、can be drawn is that the cracks observed on crankshafts 1 and 2 are not this kind of mechanical fatigue crack. another consequence of a misaligned crankshaft could be the contact of bearing and journals. however, in the previous operating sources discussion it was concluded that the cracks were not

33、originated by contact between surfaces. this conclusion also eliminates the possibility of the incorrect or even non-existent control of the clearance between journals and bearings to be the cause of the cracks. notwithstanding the previous conclusions, a measurement of the straightness of crank 1 w

34、as made. the crank was placed on v-blocks, and with a dial of resolution 1 mm placed at the center journal, measurements were made during one revolution (fig. 10). the result was 135 mm. this result, according to the engine assembly manual 6 is acceptable. the limit is 150 mm. thus, it can be conclu

35、ded that crankshaft 1 is straight.4.3. repairing sourcesthe repairing sources 9, 10, and 11 are considered by the previous explanations. item 13 is not considered since it was not necessary to weld the cranks or to nitride them. high surface roughness would also originate a contact between surfaces

36、(journal-bearing), and neither can be the reason of the cracks, as explained before. some crankshaft repair shops, in order to straighten the cranks, use a hammer or a press to bend the cranks 5. both of these methods could fracture the crank by bending it. if cracks appear due to the bending, they

37、would also appear at the stress concentrations. that is not the case for the cracks at the surface of the journals (as explained in the mechanical sources item). another possible explanation for the damaging of the journals could be the absence of a nitrided layer. measurement of the diameter of the

38、 journals showed that 0.25 mm in diameter was the removed material in the grinding operation. the limit is, according to the engine assembly manual 6, 1 mm in diameter. rockwell hardness measurements showed that there exists a nitrided layer. measurements gave 60 hrc on the journals, and the minimum

39、 is, according to the manual 6, 52 hrc. finally, item 15 points to the grinding process. would this be able to originate such cracks? as the author of this work believes that the cracks were created due to thermal fatigue, some further considerations on this item will be done consecutively.4.4. the

40、grinding processthe grinding process is basically explained in fig. 11. the journal rotates with a low rotation speed and the grinding wheel rotates with a high rotation speed. some features can occur during the grinding process that may cause some small cracks, as observed in this work. those featu

41、res are as follows.excessive depth of cutif during the rst few revolutions there is an excessive depth of cut the material may burn. there are cases where the surface of the material even changes color, becoming blue 7. it is common that, when the grinding process is nished, the journals seem normal

42、 because on the last revolutions, the operator works with a normal depth of cut, and the blue color (burnt surface) disappears. this happens very often in crankshaft repair shops. this burning may cause small cracks 7 which do not disappear with the last revolutions. an important feature that also o

43、ccurs when heat is developed at the contact surface between the grinder wheel and the journal is that heat expands diamond dressing tools. thus, the excessive depth of cut becomes even bigger and consequently more detrimental.defective lubricationthis is another source of heat at the journal surface

44、. the type and quantity of the coolant 7,8 are very important in the grinding process, in order to prevent the surface of the journal burning. defective lubrication may cause small cracks.dressing of the grinding wheel the grinding wheels must be dressed from time to time. if this operation is not o

45、bserved the wheels may become pasted, and this effect will produce poorer cutting characteristics and a higher generation of heat at the contact surface 8,9. again the heat may produce small cracks. there were exposed three aspects responsible for heat generation at the contact surface between the j

46、ournal and the grinding wheel. the question that arises is how does this heat create small cracks, and what would the cracks look like? a thermal gradient in a body creates thermal stresses due to expansion in warmer areas. if this temperature gradient is repeated for several times, e.g. an area is

47、heated and then cooled, and heated and then cooled, and so on, these stressed areas will suffer thermal fatigue. after tens or hundreds of cycles small thermal fatigue cracks appear at these stressed areas. this is exactly what may be occurring during the grinding process. each time an area of the j

48、ournal comes into contact with the grinding wheel that area is heated. during the rest of the rotation the area undergoes cooling. this means that each portion of the journal surface (as fig. 12 shows) will be under a thermal gradient, or under a stress gradient, which will be repeated. when the sur

49、face warms, compressive thermal stresses occur at that surface position a on fig. 12(b), and when that same surface cools, tension stresses occur at the same surface position b on fig. 12(a). as a consequence, small fatigue cracks, as shown in fig. 13, will develop at the journal surface. this also

50、explains the orientation of the cracks. thermal fatigue cracks develop parallel to the thermal gradient. as is observed in figs. 48 cracks are parallel to the thermal gradient. another important feature is that thermal fatigue cracks may be long but with a small depth. mechanical fatigue cracks that

51、 develop at stress concentrations in crankshafts are approximately semicircular, as can be seen in fig. 9(b). thus, if those cracks in journals of figs. 48 were mechanical fatigue cracks, their depth would have caused the crankshafts to fracture. as they are thermal fatigue cracks (superficial crack

52、s) they can be long and the crankshaft is not fractured yet. an example of this phenomenon of thermal fatigue cracks is that which occurs on disk brakes. fig. 14 shows a damaged disk brake. fatigue cracks are long, with small depth, and the position is parallel to the heat front (caused by the brake

53、 pads). escobar 10, in his msc thesis, found some cracks at the lobes of two camshafts. after an extensive examination he concluded that those cracks were thermal fatigue cracks due to an improper grinding process. observation of those cracks showed that they are small cracks, with small depth, and

54、all in the direction parallel to the grinding wheel. from these considerations about the grinding process it seems clear that those cracks developed at the journals; surfaces (figs. 48) are thermal fatigue cracks. only cracks shown in fig. 3 seem to have been developed during the contact of the surf

55、aces (bearing with journal). it seems that these cracks developed during the surface contact, but from small existing thermal fatigue cracks. these cracks nucleated due to thermal fatigue from the grinding process, and propagated due to heat and stresses. in a similar way, escobar 10 found some othe

56、r cracks at the lobes of the camshafts 10 that were not attributed to the grinding process. he concluded that those cracks were initiated by the grinding process, but propagated due to contact stresses generated by the roller follower, on the camshaft lobes. there is no doubt that the damaging of th

57、e bearings and of the journals of the crankshafts was originated by those small thermal fatigue cracks and should not be attributed to operating sources or mechanical sources. a last aspect that must be explained is that these crankshafts are forged. one could ask if those cracks could be developed

58、during the casting process. on one hand these cranks were not obtained by the foundry, and on the other hand cracks due to residual stresses after casting do not have the shape and location of cracks on these crankshafts. as a final comment it should be said that if crankshaft repair shops would have non destructive testing methods such as magnetic particle testing, improper use of the grinding process would have been already overcome.5. conclusionsthe main conclusions of this work can be drawn as follows: the damage of the journals weas originated by the small cracks at the surface