Bearing friction is typically the largest single force term in an actuator sizing calculation, and getting the friction coefficient right matters more than any other input.
Catalog values by bearing type:
- Profiled linear rail (recirculating ball) — μ = 0.003 to 0.005
- Cam rollers on steel plate — μ = 0.002 to 0.005
- Plain bronze bushings — μ = 0.10 to 0.20
- PTFE-lined bushings — μ = 0.04 to 0.10
- Roller bearings (cylindrical) — μ = 0.005 to 0.010
The critical reality: installed friction is 2 to 5 times catalog values.
Catalog friction coefficients represent clean, properly lubricated, perfectly aligned bearings under controlled conditions. In a production environment, actual friction is higher due to:
- Contamination (dust, web debris, adhesive residue)
- Inadequate or degraded lubrication
- Rail or bearing misalignment from installation tolerances
- Preload variation from mounting surface flatness
- Side-loading from web tension or mechanical misalignment
The Detailed calculator includes a kinstall multiplier (default 2.0 to 3.0) that scales catalog friction to installed conditions. This is the single most important correction factor in the model.
Best practice: spring-scale pull test. Attach a calibrated spring scale to the carriage and pull horizontally at a slow, steady rate. The peak reading is your actual installed friction force. This 5-minute test eliminates the largest source of sizing uncertainty and is far more reliable than estimating from catalog values. If you can measure it, measure it.
Use the actuator sizing calculator with measured pull-test values in Detailed mode for the most accurate sizing result.