When an automated cylinder or stepper-driven carriage binds mid-stroke, technicians usually blame grease or air pressure. The real breaker is localized shaft scoring from edge-loading.
If you mount an LM or LMK bushing onto a shaft with a surface hardness below 58 HRC, the bearing steel balls bite right into the rod. That action creates microscopic grooves along the travel line. Once those metal flakes break off, they drift inside the plastic ball retainer. The balls stop rolling, slide against the shaft, and lock up the axis completely.
Another failure point comes from press-fitting the bushing into an undersized housing bore. Crushing the thin outer sleeve squeezes the ball tracks, leaving zero radial clearance for the balls to recirculate.
2. Mounting, Shaft Alignment, and Lube RulesShaft Hardness Check: Match induction-hardened SUJ2 or CF53 steel shafts ($58 \text{ to } 64\text{ HRC}$) with a surface finish under $0.4\text{ Ra}$. Soft stainless or unhardened shafts peel apart under load.
Parallelism Dial Test: Bolt down the primary shaft. Mount a dial indicator on the carriage and sweep the secondary shaft along its full travel. Keep non-parallelism under $0.01\text{ mm}$ per $500\text{ mm}$ stroke to avoid ball binding.
Greasing Action: Never pack a linear bushing solid with thick grease. Fill only 30% to 50% of the internal volume using lithium soap grease (NLGI No. 2). Over-packing pops the rubber end-seals out of their grooves during fast strokes.
A: The shaft is running out of parallel with the housing bore, or the carriage is hitting hard mechanical stops at speed. The side loads force the balls against the cage tabs until the plastic snaps.
A: No. The hardened steel balls will groove aluminum within hours. Use linear plain bushings (PTFE/ceramic lined) if you must run soft aluminum rods.
Proper shaft hardness, correct housing bore tolerances, and parallel rod alignment keep linear bushings rolling smooth and stop shaft scoring for good.
