In order to design a plain bearing correctly, you need two main basic parameters: the sliding speed (v in m/s) and the load to be carried (p in N/mm²). These two parameters are added together to give the pv value, which is an important quantity in the selection of a plain bearing. Calculating the load or surface pressure is relatively simple, but calculating the surface velocity requires more formulas and parameters. This blog post will describe the most commonly encountered motion situations and the calculations they require.
Calculating surface velocities at bearing points for rotational motion
For applications where the shaft undergoes full rotation in the bearing, the peripheral velocity at the inner diameter of the bearing must be calculated. This requires the relevant bearing diameter and the number of rotations per unit time (preferably rpm). We then simply plug these values into the formula.
Formula: v(u) = π * d * n
Since the standard units used in the field of plain bearings are mm and rpm, the formula requires conversion factors.
Formula with conversion: v(u) = ( π * d * n ) / ( 1000 x 60 )
For a bearing with a diameter (d) of 20 mm and a speed (n) of 100 rpm, the surface speed is calculated as.
v(u) = π * d * n => v(u) = 3.142 * 0.002m * 1.667 1/s = 0.01m/s (numbers are rounded to two decimal places)
One source of error is unit conversion. In practice, bearing diameters are usually given in millimeters, while rotational speeds are given in rpm. The former must be converted to meters and the latter must be converted to revolutions per second. This formula returns the peripheral speed in meters per second.
Calculating the surface velocity of a bearing point with rotational or oscillating motion
Another common mode of operation for plain bearings is rotational or oscillating motion. This occurs in applications with hinges or other types of rotational motion. The shaft rotates in an alternating direction at the bearing point. The angle representing the difference in position between the two ends is the pivot angle. In these applications, the frequency of motion and the pivot angle are often the only known quantities. However, calculations for these applications also require the peripheral velocity, or the velocity of the axes relative to each other in motion. This formula takes into account the angles and is therefore a bit more complicated.
Equation: v(u) = ( n * ꞵ * d * π ) / 360
Here, ꞵ is the pivot angle. adapted to the bearing diameter (mm) and the pivot period per minute.
v(u) = (n * ꞵ * d * π) / (60 * 1000 * 360)
This equation can be used to calculate the surface velocity for pivoting applications.
Polymer plain bearings and their potential service life.
Do you need help with the design of your plain bearing? We will be happy to advise you, help you determine the various parameters, and recommend a viable bearing solution.