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How can I choose the right linear bearing type for my linear shaft?

How can I choose the right linear bearing type for my linear shaft?

2025-11-23

Choosing a linear bearing is not only about shaft diameter. You must check:

Shaft hardness and tolerance (basic conditions)
Bearing format – raw LM vs housed SC/SBR units
Open vs closed type – floating shaft vs supported rail
Standard vs long type – stability and moment rigidity

1. Check shaft hardness and tolerance (basic conditions)

Hardness
- LM linear ball bearings are designed to run on hardened shafts (typically HRC 60+).
- If you use soft mild steel or soft 304 stainless with steel balls, the balls will quickly dig grooves into the shaft and destroy accuracy.
Tolerance
- The ID of LM bearings is made to fit precision ground shafts with g6 or h6 tolerance.
- If the shaft is too small (e.g. generic cold-drawn rod with big minus tolerance or poor roundness), the bearing will feel loose and wobble.
- If the shaft is too large (e.g. k6 or positive tolerance chrome bar), the bearing may jam, run rough or even break the ball cage.