Linear Bearing Life and Shaft Hardness
The service life of a rolling element linear bearing can be affected by many variables, but one of the biggest factors to consider is the shaft or running surface hardness. Here is an explanation and calculation for how bearing life can vary with shaft hardness.
The optimal hardness for a round shaft, V-Guide rail, or other linear raceway for steel rolling element bearings is Rc60. If hardness is below that number on the Rockwell scale, performance can be greatly affected. This is accounted for in the standard formula for calculating bearing life. The fH, or hardness factor, can be determined from the graph and plugged into the life formula to account for the softer shaft surface.
Below is the standard life formula for a rolling element linear bearing.
L: Rating life (km)
fH: Hardness factor
fT: Temperature coefficient
fC: Contact coefficient
fW: Load coefficient
C: Basic dynamic load rating (N)
P: Load (N)
Below are 3 examples of similar type rails with varying hardness measurements. When working through the life calculation formula, considering all other factors to equal at a value of 1, with only the hardness factor changing, we can see what impact it has on bearing performance.
- 420 sst, hardened to Rc40 -> fH = 0.27
- L = (0.27)3 x 50 = 0.98 km
- 440 sst, hardened to Rc50 -> fH = 0.53
- L = (0.53)3 x 50 = 7.44 km
- 420 sst, hardened to Rc60 -> fH = 1.0
In this comparison, a 50% increase in Rockwell hardness = 50x the life