fretting in bearings and its stages

1. FRETTING
PHENOMENONTRIBOLOGY 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, Fe2O3.
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
1. 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.
3. 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. 5. 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.
6. 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)

21. THANK YOU