Lezione 11: Friction and wear

Question 1

What happens to materials when there is friction?

Answer 1

When surfaces slide, they wear: material is lost from both surfaces. The coefficient of friction µ does NOT depend on the surface area of contact.

Question 2

How do we measure the ability to resist static contact pressure?

Answer 2

It’s measured by looking at the surface’s hardness. The maximum bearing pressure (Pmax) is just the hardness H of the softer surface.
ka (archard constant) and H are material properties. Their product is K.

Question 3

How do we distingue the best materials for bearings?

Answer 3

The best materials for bearings are the ones in the lowest ka. Safe bearing indeed are loaded to fraction of their maximum bearing pressure (H). Best materials have the lowest K=kaH

Question 4

Talk about friction

Answer 4

No surface is perfectly flat, so when 2 surfaces are placed in contact they touch only at points where asperities meet. Real contact area (Ar) is tiny compared to the nominal area. On real contact area, Fn will tend to yield the material.

Question 5

What is wear?

Answer 5

There are 2 kinds of wear:

• adhesive wear, where adhesion at the work hardened junctions causes shear-off of materials when surfaces slide
• abrasive wear, where asperities of the harder materials, or embedded hard particles, plough through the softer one, grinding off wear particles.

Question 6

What are bearings?

Answer 6

Bearing allow two components to move relative to each other in one or two dimensions while constraining them in others. They must have low friction and the ability to support the bearing load without yielding.
We have mainly 2 kinds of bearing:
-Plane bering: good for strength, corrosion resistance and conformability
-Roller bearing: the deformation must remain elastic, they have high hardness and are clean and inclusion free

Question 7

What is lubricant used for?

Answer 7

Lubricant allows for easy shear between surfaces. The coefficient of friction is reduced to 0.001 - 0.01
Hydrodynamic lubrication: this is better suited for higher performance applications. The rise in pressure as the oil is compressed forces the surfaces apart.
For bearings, the rate of wear increases with the bearing pressure (P) and the sliding velocity (v).