How to Select a Rotary Stage

Rotary Stages consist of the following parts

• drive motors
• bearing supports
• drive mechanism

Drive Motors
The selection of they type of drive motor depends on compatibility with the rest of the system and personal taste. Drive motors can be anything from open loop stepper motors to brushless servo motors with absolute encoders. It is important to understand that the accuracy, repeatability and capacity of the rotary stage does not depend on the drive motor and is determined soley by the bearings and drive mechanism.
Bearing Supports
The rotary stage table itself is supported by one of the following

• simple bushing
• rotating ball bearing
• rotating cross roller bearing
• specialized bearing like fluid or magnetic bearing

The simple bushing usually consists of a flat surface made of material with good wear or lubrication properties such as TEFLON or DELRIN. The advantages of this type of bearing are that it usually can support a large weight or moment load and they tend to be the lowest cost type of bearing. The drawback is that these bearings tend to wear, often unevenly, especially with moment loads. They also exhibit the greatest friction and so expend more oof the available drive motor torque in overcoming the inherent friction. These bearings tend to be simple to set up but often need adjustment because of the wear.

Rotating ball bearings consist of precision diameter ball bearings running in a raceway and held apart by a retainer that prevents the balls from rubbing against each other. The improvement over a bushing comes from the fact that the balls rotate around the raceway rather than have a surface dragged over another surface. This leads to far greater efficient usage of the drive motor torque as well as significantly reduced wear and need for adjustment. Rotating ball bearings are relatively easy to seal and tend to be fairly immune to dirt and dust.

Rotating cross roller bearings are similar to ball bearing type bearings except the balls are replaced with precision cylinders that alternate in orientation. This provides a very stiff support with virtually no play. The cylinder provides line contact on the support surface versus point contact on the ball bearings and so there is significantly more load and torque capacity.