The Importance of Raceway Groove Shape

The Mechanism behind Differential Slip The kinematic performance of an LM Guide is greatly affected by the way the balls rotate as they move. As they rotate, the balls roll, but the surface speed at which they do so changes based on where they are. This is because their distance from the axis of rotation changes as they move. The farther a ball is from the axis of rotation, the longer its travel distance (circumference of travel) becomes, and the faster it moves. Conversely, the closer a ball is to the axis of rotation, the shorter its travel distance becomes, and the slower it moves. Because balls move at a set speed, forcing them to do so inside a contact structure with a defined contact width causes some of them to slip. You can think of it like trying to roll a paper cup on its side. The lip of the cup and the base of the cup have different circumferences, so you would need to slide part of the cup sometimes to keep it rolling forward in a straight line. The idea here is basically the same.

With the four-point contact structure of Gothic-arch grooves, any installation error or misalignment has considerable consequences. If the balls don’t make contact with the grooves at all four points simultaneously, it becomes impossible to keep them moving the way they should. What ultimately happens is that they end up sliding instead of rolling, in a phenomenon called differential slip.

Too much differential slip creates friction that generates heat and accelerates wear, diminishing the kinematic performance of an LM Guide and reducing its service life. With the two-point contact structure provided by circular arc grooves, though, there’s less of a difference between the circumferences of travel for each ball, which makes it possible to keep them all rolling naturally. This reduces differential slip and provides three crucial characteristics sought from an LM Guide: 

 Smooth motion 

Reduced heat generation 

Reduced wear