Topic 4_3 Roughness, Hardness, and Friction Flashcards
Why do we care about friction
100 million terajoule is used to over come friction - 1/5 of all energy produced
What is friction
A force that must be overcome to initiate or sustain motion
The energy that is dissipated during relative motion
What are the three laws of dry friction
- Frctional force is propertional to the applied load
- the frictional force is independent of the apparent area of contact
- the frictional force is independent of the sliding velocity
What are the issues for the three laws of dry friction
Friction is dependent on real area which is dependent on apparant area
Frictional force is dependent on motion as have static vs dynamic friction
What are the three generic mechanisms usually proposed for friction
- adhesion - interlocking and shearing of junctions
- Deformation - displacement of material (elastic or plastic)
- Hysteresis - lag in response (viscoelastic materials)
What is the coefficient of frction
used to characterise friction
= F/N
Is system parameter not material, depends also on load, speed, temp etc
Which is great static friction or kinectic friction
static friction is always higher (up to 2 times) all junctions need to reach slip point
static required to initiate motion
kinectic required to maintain motion
How does static friction change with time
increases logarithmically, adhesion is a chemical process that increases with time - more junctions adhering more force to move
What happens to coefficient of friction when lubricated
considerable drop - order of magnitude
Whats the issue with looking at table of friction coefficients
values are very generic, dont know conditions created under
only as rough guide or comparative between two if trying to design system
cant take as absolute
Why do dissimilar materials tend to have lower friction values
lower friction as less adhesion
How does kinematic friction change with velocity
decreases with increasing velocitity logarithmically
(when slower more time for microcontacts to form and more time before break)
direct contradiction of classic theory of friction
Why is classic theory of friction wrong about being independant of area and load
Higher area more adhesion = more friction
Higher load more deformation = more friction
How does friction relate to wear
Not a direct relationship, with adhesion wear does not reult if shearing occurs are the original interface, with defromation wear results directly from plastic deformation or cutting (very efficient high wear little friction)
What % of energy dissipated by friction goes into wear
Only 10%
