Static load capacity is an important factor in linear bearing selection. While dynamic load capacity is used to calculate bearing life, static load capacity indicates how much load the bearing can withstand before permanent damage occurs to the balls and/or the raceways. Even when they’re not moving, linear guides often experience a combination of both forces and moments, all of which contribute to the combined static load that the bearing sees.
The combined static load (sometimes referred to as the equivalent static load) takes into account the various types of loading that a bearing is subjected to at standstill.
A linear guide rail with one bearing can experience forces in both the Y and Z directions, and moments can occur about any of the three axes: X, Y, and Z. This means that there can be as many as five different components to the static loading.
However, most applications use more than one linear bearing on a rail, and demanding applications typically use two rails with one, two, or sometimes three or more bearings per rail. The benefit of multiple bearings per rail, and multiple rails used in parallel, is that moment loads are resolved into forces. Because linear bearings generally have higher load capacities than moment capacities, using multiple bearings to counteract moment loads will typically yield increased bearing life.
For one rail with one bearing:
For one rail with two bearings:
For two rails with two bearings:
For two rails with four bearings:
Fcomb = combined static load
Fy, Fz = external forces in Y (lateral) and Z (vertical) directions
Mx, My, Mz = external moments about the X (roll), Y (pitch), and Z (yaw) axes
C0 = static load rating
Mt0 = static roll moment rating (moment about the X axis)
ML0 = static pitch and yaw moment rating (moment about Y and Z axes)
For many linear bearings, the moment ratings about the Y (pitch) and Z (yaw) axes are equal and are denoted as ML0.
Once the combined static load has been calculated, it can be used to determine the static load safety factor. The static load safety factor is calculated by dividing the basic static load rating (C0) by the static equivalent load of the most heavily loaded bearing. The recommended safety factor is given by the manufacturer, based on the type of loading and the usage conditions—low, moderate, or high impact loads and vibrations. It often ranges from 1 to 8, and is used to ensure that the bearing never experiences a load that will cause permanent plastic deformation of the balls or raceways.
Application Note: Preload creates an internal force on the bearing. This internal force is counteracted as an external load is applied. For most linear bearings, when the applied load is equal to or greater than 2.8 times the preload force, preload can be disregarded in bearing life and static loading calculations.
For example, if a bearing has a dynamic load capacity of 25,000 N and a preload of 2%, an external load of 1,400 N (25,000 * 0.02 * 2.8) or greater will be sufficient to overcome the internal preload force.
Feature image credit: PILETEST