Wear mechanisms of machine elements
1. Which statement is untrue?
Scuffing between intermeshing gear teeth is caused by a critical contact temperature at which the lubricant film fails and micro welding between asperities takes place.
A stainless steel screw joint may become blocked during tightening as a result of seizure. Disassembling the screw joint than becomes problematic.
The material transfer of a tire rubbing against the road is a good example of scuffing.
When the aluminum piston of a pneumatic cylinder initially comes into contact with the aluminum bore of the cylinder the surfaces may become ruined by galling in a single pass.

2. Which statement is untrue?
Three body abrasion between surfaces of high hardness may reduce the coefficient of friction and accelerate the wear rate.
Hard particles only abrade relative softer surfaces.
Hard wear particles that abrade the sliding surfaces may originate from the sliding surfaces itself.
In polymer – metal contacts, the wear rate of the polymer surface will always exceed that of the metal surface.

3. Which statement is untrue?
The incubation period for surface fatigue is also called the life duration or service life.
The L10 life duration is the duration that can be achieved with a probability of 10%
During the incubation period surface cracks are formed by surface fatigue.
Cracks formed by surface fatigue are generally the result of repetitive elastic deformation in the contact region.

4. Archard’s equation can be applied to calculate the wear rate of machine parts subjected to
Abrasive wear, adhesive wear, corrosive wear and fatigue wear.
Abrasive wear, adhesive wear and corrosive wear, not for fatigue wear.
Abrasive wear and adhesive wear, not for corrosive wear and fatigue wear.
Abrasive wear, not for adhesive wear, corrosive wear and fatigue wear.

5. Calculate the service life t of a polymer bearing. Consider a bearing with a shaft diameter D=20mm, a relative bearing length L/D=1, sliding velocity v=0.5 m/s, load F=100N, rotating load vector, and specific wear rate k=1 10-15 m2/N. The service life has ended when the radial bearing clearance is increased with h/R=0.01, with R=D/2.
t = 350 h
t = 700 h
t = 1400 h
t = 2800 h