Pure rolling involves two surfaces moving over each other without sliding (sliding page), there are two scenarios in a rolling contact, one where both are moving with the same surface velocity; and other where one is stationary and the other rolling along the surface. The latter is typified but a wheel rolling along a road or rail. Both are seen in rolling element bearings – one race is always stationary, whether it be the inner or outer race, the element will roll along the surface. While for the moving race, the surface has the same velocity of the rolling element; if the velocities are different this will introduce sliding and in bearings this is referred to a ‘slip’. In the first rolling scenario, sliding can occur during initial start-up if the traction, which is rolling equivalent of friction, is too low that the wheel just spins without generating motion and this is commonly referred to ‘wheel spin’. Conversely sliding can happen during braking if the wheel is locked in a stationary position.
nCATS has a number of tribometers that can produce rolling motions, two have two motor to drive the two surfaces (Twin Roller (Plint TE74S) and Mini Traction Machine (MTM). nCATS has also developed a reciprocating rolling head design for the Reciprocating (Plint TE77).