In the special-shaped guide rail linear guide using balls or rollers, the geometry and arrangement of the bearing raceways play an important role in the bearing capacity, friction, rigidity, and ability to withstand installation errors. But another aspect of bearing design also affects its load-carrying capacity and stiffness-contact angle.

The ISO 14278 standard for linear motion rolling bearings defines the nominal contact angle as:

The angle between the load direction on the linear bearing and the nominal line of action of the resultant force transmitted by the bearing raceway member to the rolling element

Linear guides using Gothic arched geometry (including miniature guides and most roller bearing guides) have four contact points between the balls and the raceways, resulting in a 45-degree contact angle.

The benefit of the 45-degree angle is that it provides the bearing with equal load capacity in all four main directions-radial (downward and lift-off) and lateral (side) loads. This means that the guide rail can be used in any direction without reducing the load capacity.

On the other hand, linear guides using circular arcs or offset Gothic arch geometry can be designed with different contact angles to produce higher rated load in one direction, but at the expense of load capacity in the other direction .

For example, a circular arc design uses a 90-degree contact angle on the top volleyball and a smaller 30-degree angle on the bottom volleyball. This provides the bearing with a very high radial (downward) load carrying capacity-which is the main load direction in many applications-because the load is transferred directly from the top row of balls down to the raceway. When a radial load is applied, it also provides a very high stiffness (low deflection) for the bearing. The trade-off between higher radial load capacity and stiffness is the opposite load capacity and stiffness in the radial and lateral directions.

Another design is based on offset Gothic arch geometry, using a 50-degree contact angle for all four rows of balls. This provides a higher load capacity in the radial and reverse radial directions, but a lower load capacity in the lateral direction.

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