How do you measure the preload of a bearing?

During the use of bearings, we often find that their service life is shorter than expected. We suspect that this minor wear and deformation is partly related to the bearing’s preload. Next, let’s take a closer look at how to better control the bearing’s preload and what are the appropriate methods for measuring it.

There are three main methods for bearing preload:

First, let’s discuss the starting torque method. We can begin by using a dynamometer to measure the tangential force required to initiate rotation, thereby calculating the starting torque. It’s important to note that this method is particularly well-suited for measuring large pre-tension forces. The two figures below illustrate the schematic diagram of the starting torque method as well as the relationship between the starting torque and the pre-tension force applied during installation.

The second method is the thrust static stiffness method. By applying an axial force, we can measure the axial displacement and ultimately determine the static stiffness. This method is particularly well-suited for measuring the preload of ball bearings, as the stiffness of ball bearings varies significantly with changes in preload. The two figures below show a schematic diagram of the thrust static stiffness method and the relationship between axial displacement and axial force.

Finally, there’s the natural frequency method. This method involves measuring the frequency by applying an axial vibration excitation and is particularly well-suited for angular contact ball bearings. Although it offers relatively high measurement sensitivity, it is susceptible to influences from assembly conditions and the clamping fixtures used. The three figures below illustrate the schematic diagram of the apparatus suitable for the natural frequency method, the relationship between the axial elastic coefficient and the axial natural frequency of the spindle, and the relationship between the preload during assembly and the axial elastic coefficient. By analyzing these two relationships, we can determine the correlation between the axial natural frequency and the preload.