Principles of Friction
You’ve been asked to move a one ton, smoothly polished block
of granite to another location. During your initial attempt to move
the block, the two surfaces in contact (the base of the granite
block and the ground) resist movement. This is called static
friction. Trying harder, you exert greater force, enough
so that the surfaces begin to slide against one another. Once in
motion, the resisting force is from kinetic,
or sliding friction, rather than static
friction.
If that same block is now placed on five equally spaced rollers
the force required to move the block is significantly decreased.
Why? The rollers, in contact with both the surfaces of the roadway
and block, still encounter friction; however, the rotating action
of the rollers carries the block forward with less effort. The rollers
eliminate the need to slide the block and have eliminated the resisting
force of kinetic friction: the friction encountered is now classified
as rolling friction. Rolling
element bearings are designed to take advantage of this principle.
They eliminate sliding friction and utilize the efficiencies of
rolling friction to carry a load.
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