Topic 4_1 Roughness, Hardness, and Friction

Question 1

Is a smooth engineering surface still rough?

Answer 1

Yes on a microscopic scale all surfaces are rough

Question 2

What happens when two bodies come into contact

Answer 2

The peaks of the surface roughness come into contact, the rough the surface the lower the real area of contact and the higher the contact stress.
Peaks may yield plastically, elastically eventaully get asperity fatigue and peaks will wear away

Question 3

Draw a diagram of lay, flaws, waviness and roughness

Answer 3

see powerpoint

Question 4

What is lay?

Answer 4

Pattern created by machining marks - pattern of peaks - by moving across one way of surface will get more wear than the other

Question 5

What are flaws?

Answer 5

Deviation from the lay of a surface

Question 6

What is waviness and roughness

Answer 6

Waviness is the macrosurface finish, rougness is the microsurface finish

Question 7

What parameter can describe surface geometry

Answer 7

No single parameter can

Question 8

What are some commonly used measures of roughness

Answer 8

Ra - value of centre line average (CLS)
Rq - value of root mean square (RMS)
Ra is most commonly quoted

Question 9

How is Ra calculated

Answer 9

1/L * intergral from 0 to L of the absoulte value of surface height above the mean level

Question 10

What is the disadvantage of Ra

Answer 10

values can’t distinguish between a sharp surface and a wavy undulating surface, same Ra different tribological properties

Question 11

Give some typical Ra values

Answer 11

Ball bearing Ra = 0.01micrometers
Sand cast Ra = 10 - 20 micrometers
between 0.5 and 1 micrometeres for most applications, less than 0.4 for low wear and long life

Question 12

How does Rq solve the problems of Ra

Answer 12

Uses a squared term to give greater significance to surface variations some way from the mean

Question 13

How is Rq defiend

Answer 13

sqrt(1/L * intergral from 0 to L of the absoulte value of surface height above the mean level squared)

Question 14

How is surface roughness measured

Answer 14

Using a none contact - optical profilometer (shinning a light on surface to measure when it bounces back)
contact - profilometer (running a needle along surface)

Question 15

Whats the adv/disadv of a contact vs non contact profilmeters

Answer 15

Non contact are more expensive and require more training
Contact profilometer if the diamond tip is bigger than surface roughness you are unable to measure the variation
Neither more accurate depends on operator

Question 16

Effects on roughness on wear

Answer 16

• initial roughness is critical
• influences initial and therefore subsequent wear mechanisms
• IT determines initial surface conditions and contact stresses (less uniform pressure distribution and high max pressures)
• Influences lubricant wettability

Question 17

Why is surface roughness important to lubricants

Answer 17

too rough wont spread and get even coating
too smooth can retain lubricant
Need to know lubricant film you want to begin with
May have surface texturing in hard to lubricate areas to help retain lubricant

Question 18

Ho does roughness impact wear

Answer 18

usually rougher something is the more wear

Question 19

in a worst case approximation approach what is wear depth proportional to

Answer 19

Ra