The P, or effective load from Equation 1, on the bearing is calculated several ways. One is the weight of the rotor on the bearings — basically, half the rotor weight across two bearings — and the rest are loads based upon other components attached to the shaft, unbalance, misalignment, belt tension, air gap eccentricity, and other conditions.
In effect, the calculation of bearing life can be as simple, but inaccurate, or complex, and accurate, as the analyst or engineer wishes. The primary purpose is to determine reliability. This can also be used as a way to determine the impact of different conditions. For instance, if we assume all other conditions are the same between bearings, we can change one condition and determine the overall impact of the L10.
We often hear of vibration in terms of mils (displacement), in./sec (or mm/sec — both are velocity) or g (acceleration). In the case of determining the amount of force due to unbalance alone, you need to determine the pound-inch (lb-in.) unbalance of the motor and attached components. This can be calculated as shown in Equation 3, where Flb is force in pounds.
Equation 3
Flb = 1.770*(rpm/1000)^2*lb-in.
Therefore, an 1,800 rpm rotor with 1/16 lb-in. unbalance and a diameter of 24 in. (12 in. radius) would have a radial force of 68.76 lb.