Preload in linear guides

2024-07-08

Why do linear guides have preload?
Several reasons. A preloaded carriage gives you:

• Zero backlash
• Less deflection under load
• Higher natural frequency (makes machines more stable)
• Forces the balls to roll even during fast accelerations, preventing sliding (which causes major wear)

During manufacturing, there are always some tolerances. So they measure the internal clearance between the guide and the carriage’s raceways and then insert balls that are slightly too big – pressing them in and creating preload.

Some brands use balls of different sizes in different tracks. Others use the same size throughout. And if you ever accidentally drop a ball from a carriage, you can usually clean it and pop it back in without issues.

Preload is calculated as a percentage of the dynamic load rating (C). Preload classes are typically:
C0: No preload (max 10 micron play)
C1: ~2% of dynamic load rating
C2: ~8%
C3: ~13%

The formula is: Fpr = Xpr × C
So if C = 25,000 N and you want 2% preload, then Fpr = 0.02 × 25,000 = 500 N.

Check out the video with Carry Lindholm from Bosch Rexroth for more.