Motion System Design
MSD 101: Cage creep

MSD 101: Cage creep

Rail guides are ready-to-mount linear bearings used for limited, high precision travel. As a rule they consist of two tracks separated by intermediate rolling element assemblies. In the case of crossed roller linear rail blocks, the rolling assemblies consist of a series of orthogonally aligned rollers that run back and forth in a guideway. This guideway is made up of two halves that are machined with vee-shaped grooves, attached to block and rail, and aligned; the rollers then lay crisscross in the resulting vee-vee space. A cage spaces adjacent rollers to prevent rubbing friction and drag.

If it weren’t for the tendency of the cage to migrate toward one end of the guideway, the story would end here. But the spacer rarely stays put, and as it moves further from its centered position it jams up the rollers, increasing friction at stroke limits. Ideally, the cage fully travels its stroke, contacting end stops only at completion. If the cage housing hits a stop prematurely, the cage either drags, or is dragged by the guideway half still trying to move. Its rollers spin in place, skidding rather than rolling for the rest of the stroke ... reducing the effective work and life of the rail guide assembly. This leads to high torque spikes at the end of travel, or even limited motion. Possibly caused by uneven pre-loading and poor guideway alignment, the movement, called cage creep, is exaggerated by gravity when the guide is mounted vertically.

Questions & answers

Q: How misaligned can a cage become?
A: For 100 km of travel, systems can experience up to 30 mm of creep.

Q: Are there additional performance drawbacks to cage creep?
A: Decreased roller-vee contact makes for lower system stiffness.

Q: Can cage creep be corrected?
A: Machines must either be shut down to reset cages, or the system motor has to be powerful enough to force resetting. Some mechanisms have been developed to hamper cage creep.

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