Machine Design

Increasing Rolling Contact In Linear Guides

This shows a double-sided way with two single-sided bearing blocks.

Friction-type linear bearings with flat sliding surfaces provide high load capacity and stiffness between bearing surfaces. Unfortunately, they also exhibit the stick/slip phenomena caused by the difference between static and dynamic friction. Stick/slip occurs particularly at low velocities, which is troublesome in numerically controlled servosystems when equipment approaches an intended location. Stick/slip also causes vibration and chatter that shows up on the workpiece.

Rolling elements, on the other hand, exhibit much less of a difference between static and dynamic friction, which gives them an advantage in linear-bearing systems. However, they have a small contact area so surface finish, hardness, and the conformity of rolling elements to the race are critical.

Localized stresses must be considered when designing linear rolling-element bearings. When rolling elements are so loaded that their deformation exceeds the elastic limit of the bearing material, the ball race can permanently deform. One way to avoid this is to ensure close conformity of the ball race to the ball radius. This increases the area of ball contact and substantially increases the system's load capacity and stiffness. Considerably larger balls are necessary to get the same load capacity and stiffness with point contact on a rod or flat surface as with a close-conformity race.

Elliptical ball contact is one way to create close conformity. There is a trade-off, however, between scuffing and pure rolling. During elliptical contact, the ball purely rolls only at the center of the ellipse while scuffing occurs along the major axis of the ellipse.

The following equation is used to calculate the degree of conformity:
C = R/B
where C = % conformity, R = ball-race radius, and B = ball diameter.

Based on the formula, 50% conformity means that there is full contact between the ball and race. Although this produces high load capacities, it also creates excessive scuffing. The ball contact angle also influences wear because it leads to bearing-ball stress.

A gothic shape on the race is another way to create close conformity. Linear guides with a gothic shape in guide ways and on bearing blocks have four-point contact that produces low-friction motion and a low profile, compact linear-ball system. In addition, there is no travel limit with this type of system. Bearing blocks and guide ways are available in single and double-sided configurations.

This information supplied by Precision Motions Co., West Hartford, Conn.

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