ABSTRACT
Rotation and Friction There is nothing different in the behavior of friction due to the differences between linear and rotational motion-friction is friction-but how it is taken into account does differ, since friction will appear as a torsional rather than linear resistance to movement. Some component of the total torque applied to rotate an object will be the torque needed just to overcome friction, and this component, , is found in the rotational version of the formula used to estimate the maximum power needed by a given effect:
There is also nothing about rotational motion that negates the complexity involved in accurately predicting friction. The interaction of different materials, in sliding or rolling situations, with or without a lubricant, at different speeds, temperatures, and loads, will all affect the frictional losses of a particular setup. Add in the everyday possibilities of dirt, wear, overloading, and misalignment and accurate predictions become even harder. Fortunately the same rough approximations made in Chapter 4 relative to linear friction will serve us well here too. So then:
These assumptions will lead to conservative, worst case values for the situation being analyzed, but as always with friction other not easily quantified factors might completely throw off your best made estimates. For example, fixed mounted casters are used for most turntables, but on a turtle (a scenic unit that both spins and tracks) swivel casters might be used. As the unit goes from tracking to rotating, or as it changes its direction of rotation, the wheels will scrub against the floor as they swivel to align to the new direction of movement. This can take a surprising amount of effort for both the sliding friction of the wheel pivoting in place, and for the swivelling action as some casters spin clockwise, and others counterclockwise.
