Shaft Hardness and Linear Bearing Life
Bearing life is usually expressed as the number of hours an individual bearing will operate before the first evidence of metal fatigue develops. The usual life rating for industrial applications is called “L-10” life. The L-10 life is the number of hours in service that 90% of the bearings will survive.
The service life of a rolling element linear bearing can be affected by many variables. One important factor 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 that accompanies a steel rolling element bearing 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. Using the graph, the fH or hardness factor can first be determined and then plugged into the life formula to account for the softer shaft surface.
Below is the standard life formula for a rolling element linear bearing, where:
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, we consider only the hardness factor to change, leaving all other factors equal at a value of 1. We can observe the impact that the hardness factor 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
L = (1.00)3 x 50 = 50 km
Customers can avoid previous industry misconceptions and discover how to properly calculate the statistical probability of life for a range of linear roller bearing solutions.Our in-depth Roller Bearing Life Calculations white paper provides a detailed analytical approach to calculating life, and is designed to give engineers confidence when applying our broad range of linear motion solutions to their applications. This method applies to all products within the Cam Roller Technology (CRT) family including Redi-Rail, V-Guide wheel, Commercial Rail, and Hevi-Rail, along with Integral-V Technology (IVT).
All factors and formulas are based on data that has been proven over time by sources such as the International Organization for Standardization (ISO), as well as its membership bodies that include, but are not limited to, the American National Standards Institute (ANSI), Deutsches Institut für Normung (DIN), and the Japanese Industrial Standards Committee (JISC).
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