Fretting is the small-amplitude oscillatory movement that may occur between contacting surfaces, which are usually nominally at rest.
One of the immediate consequences of the process in normal atmospheric conditions is the production of oxide debris, hence the term fretting wear or fretting corrosion is applied to the phenomenon.
The movement is usually the result of external vibration, but in many cases it is the consequence of one of the members of the contact being subjected to a cyclic stress (that is, fatigue), which gives rise to another and usually more damaging aspect of fretting, namely the early initiation of fatigue cracks. This is termed fretting fatigue or contact fatigue. Fatigue cracks may also be initiated where the contacting surfaces are under a very heavy normal load or where there is a static tensile stress in one of the surfaces.
There are cases where the movement is not simply tangential, but is complicated by the normal force also oscillating to the extent that the surfaces lose contact in each cycle. This leads to a hammering effect, which is termed impact fatigue.
Fretting Wear in Mechanical Components:
The contacts between hubs, shrink- and press-fits, and bearing housings on loaded rotating shafts or axles are particularly prone to fretting damage, but because the movement arises from alternating stresses in the shaft surface, the problem is more one of fatigue than wear.
The contact between bearing and housing leads to wear rather than fatigue. Thin-shell bearings are universally used in diesel engines, and such bearings involve an interference fit between the bearing and the housing. If the contact pressure is not high enough, movement can occur, giving rise to the fretting damage.
Flexible couplings and splines, particularly where they form a connection between two shafts and are designed to accommodate some slight misalignment, can suffer severe fretting wear.
Turbines, both steam and gas, operate under conditions that subject these components to fretting damage. There are three main sites:
1. Where the turbine disk is either shrink-fitted onto the driving shaft or attached to the shaft by means of a bolted flange.
2. Where the blades are fixed into the disk by either a dovetail or fir-tree type fixture.
3. Where snubbers at the outer ends of the blades contact those on adjacent blades.
The first two sites are more likely to be areas for fatigue crack initiation, but loss of material by wear of the snubbers will give rise to increased vibration of the blades and the onset of impact fretting.
The rotors of electrical generators have slots along their lengths in which the windings are located. These are held in place by wedges. Fretting can occur between the wedges and the undercut portion of the winding channel with the development of fatigue cracks in the rotor.
Steel ropes are used widely as mooring ropes, haulage ropes, mining ropes, and on cable cars. Overhead power lines are in effect ropes made up to aluminum wires wound on a steel support rope; more recently, they have been made up entirely of aluminum (the all-aluminum conductor).
Any bending movement or alternating tension can result in local movement in the numerous interwire contacts. Most ropes include some form of internal lubrication to lessen the effects of such movement. The accumulation of debris also leads to a reduction in flexibility at this point and, in the case of a mining rope, can cause jolting when this part of the cable passes on to the winding drum.
Parameters Affecting Fretting:
Amplitude of Slip: The variations in the slip amplitude determines the fretting wear rate on the material surface. If the slip is lower then this would cause the nucleation and the crack initiation at the surface undetected. If the slip amplitudes are increased then this causes the effects similar to the direct abrasion and the fretting wear rates are increased.
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