Fretting phenomenon

fretting in bearings and its stages

1. FRETTING PHENOMENON TRIBOLOGY ASSIGNMENT DONE BY: GIRISH RAGHUNATHAN 1RV18MMD07

2. OUTLINE FRETTING DEFINITION STAGES OF FRETTING IN POROUS BEARINGS CASE STUDY ON FRETTING PHENOMENON

3. FRETTING DEFINITION Fretting is surface damage that occurs between two contacting surfaces experiencing cyclic motion (oscillatory tangential displacement) of small amplitude. At the contact areas, lubricant is squeezed out, resulting in metal-to-metal contact . Fretting wear is understood as the removal of material on contact surfaces due to fretting action , while the fretting fatigue represents reduced of life due to cracks caused by fretting.

4. Fig.1 a) The fretting corrosion has caused the cracking of spherical roller bearing inner ring; b) fretting corrosion has caused longitudinal crack in a deep groove ball bearing outer ring . Fig. 2 a) Wear damage in crowned teeth (on the left) and straight teeth (on the right) spline coupling. Fig. 3. Fretting of aluminium lining

5. STAGES IN FRETTING PHENOMENON The first stage is the metallic contact between two surfaces. The surfaces must be in close contact with each other. The contact occurs at few sites, called asperities (surface protrusions). Fretting can be produced by very small movements, as little as 10– 8 cm. Fig 4. fretting wear initial stage

6. CHARACTERISTICS OF FIRST STAGE Adhesion In order for the metals to be in physical contact with each other, there must be no protective oxide layer. The breakdown of the protective layer is essential for the onset of fretting. The asperities are bonded together at adhesion sites created by the relative slip of the surfaces. Fretting may occur at amplitude as small as 10 −8 cm . The coefficient of friction can increase from 0.2 to 0.55 within 20 cycles. If two metals in intimate contact are similar, protective films on both shall be disrupted, however, if one metal is soft and the other is hard, the layer on the soft metal will be destroyed, and on the other metal it would not be disrupted.

7. The second stage is oxidation and debris generation. It is possible that oxidation may occur before or after debris removal, each process being controlled by conditions which lead to fretting. In either case, the debris is produced as a result of oxidation. Fig 5. oxidation and debris generation

8. CHARACTERISTICS OF SECOND STAGE Generation of Debris The material removed from the metal surface due to fretting is called debris. The debris produced by low carbon steel consists of mainly ferric oxide, Fe 2 O 3 . The debris can also contain unoxidized particles in the case of non-ferrous metals. The composition of the debris differs from one metal to another metal. If the particles of oxide becomes embedded in the softer material, the rate of wear is reduced and hence fretting is minimized. Loose particles increase the rate of wear and hence fretting proceeds at a high rate .

9. Crack Initiation Cracks grow in a direction perpendicular to the applied stress at the fretting area. Some of the cracks may not propagate at all at low stresses because the impact of stress on a fretted surface extends to a shallow depth only. The propagation of cracks is either restrained or prevented by the presence of favorable compressive stresses . The stage of crack initiation is called fretting fatigue . Crack propagation at higher stresses is of practical importance as it can lead to failure of components, such as shafts and axles . The crack originates at the boundary of a fretted zone and propagates. During propagation, if a corrosion medium contacts the crack, corrosion fatigue also contributes to the crack propagation. Outside the sphere of the surface contact stress, the crack propagates as a fatigue crack, and upon fracture, a characteristic lip is observed.

10. Fig. 6 Process of fretting wear

11. Factors affecting Fretting Contact load Wear is a linear function of load and fretting would, therefore, increase with increased load . Amplitude No measurable threshold amplitude exists below which fretting does not occur. An upper threshold limit, however, exists above which a rapid increase in the rate of wear exists. Amplitude oscillations as low as 3 or 4 nm are sufficient.

12. Number of cycles The degree of fretting increases with the number of cycles. The appearance of surface changes with the number of cycles. An incubation period is reported to exist during which the damage is negligible. This period is accompanied by a steady-state period, during which the fretting rate is generally constant. In the final stage, the rate of fretting wear is increased .

13. Temperature The effect of temperature depends on the type of oxide that is produced. If a protective, adherent, compact oxide is formed which prevents the metal-to-metal contact, fretting wear is decreased. For example, a thick layer of oxide is formed at 650° C on titanium surface. The damage by fretting is, therefore, reduced at this temperature. The crucial factor is not the temperature by itself, but the effect of temperature on the formation of oxide on a metal surface. The nature and type of the oxide is the deciding factor

14. Relative humidity The effect of humidity on fretting is opposite to the effect of general corrosion where an increase in humidity causes an increase in the rate of corrosion, and an increase in dryness causes a decrease in corrosion. Fretting corrosion is increased in dry air rather than decreased for metals which form rust in air. In case of fretting, in dry air, the debris which is formed as a consequence of wear on the metal surface is not removed from the surface and, therefore, prevents direct contact between two metallic surfaces. If the air is humid, debris becomes more mobile and it may escape from the metal surface, providing sites for metal-to-metal contact.

15. CASE STUDY: FRETTING PHENOMENON IN A BALL BEARING Fig 7. Fretting Corrosion device

16. Fretting failure of raceways on 52100 steel rings of an automotive front wheel bearing. The inner and outer rings were made of cold drawn 52100 steel tubing . On physical examination, it was discovered that serious fretting of the raceway in the ball contact area has occurred. Fretting and pitting occurred at spacings equivalent to the spacings of the balls in the retainer. Examination of the inner raceway showed lesser attack . During transportation of vehicle by sea, the body of the vehicle was continuously vibrating without any rotation of the bearing.

17. Sufficient preventive measures were not taken during the transportation of the above vehicle. The rolling elements should be taken off during packing. Vibrations should be eliminated, as far as possible, during transportation of vehicles by sea.

18. Prevention of fretting Increase the magnitude of load at the mating surfaces to minimize the occurrence of slip. 2. Keep the amplitude below the level at which fretting occurs, if known for a particular system. There is lower threshold limit below which fretting does not occur. Use materials which develop a highly resistant oxide film, at high temperature to minimize the adverse effect of temperature on fretting. 4. Use gaskets to absorb vibration.

19. Increase the hardness of the two contacting metals, if possible, by shot-peening. Compressing stresses are developed during shot-peening , which resist and increase fretting resistance . Use low viscosity lubricating oils . 7. Use materials to resist fretting corrosion.

20. REFERENCES Adriana Urs (Zara ), “STUDIES REGARDING SEVERAL ASPECTS IN FRETTING WEAR”, International Journal of Modern Manufacturing Technologies, VOL 2, 89-93, 2010 R.K. Upadhyay ,“Rolling element bearing failure analysis: A case study”, Case Studies in Engineering Failure Analysis, VOL 1, 15-17, 2013 Wikipedia ( https:// en.wikipedia.org/wiki/Fretting)

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