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Bearing Mobility

 

Consider an electric motor driving a pump, and we are measuring the free end motor bearing. The force acting at the 1X frequency is generated by the centripetal force due to the imbalance in the rotor. The magnitude of this force is dependent on how stiff the bearing housing is at the 1X frequency. The actual force in the bearing load zone is the centripetal force plus the weight of the rotor. In other words, if the motor is spring mounted and is quite free to move, a relatively small force will result in a large vibration level. The residual imbalance in this case could cause fairly high 1X component in the spectrum, but because the motor is not very rigid, the actual force acting on the bearing elements may be quite small, and the likelihood of bearing damage will be greatly overestimated.

Also, if the excitation frequency (1X) is near a natural frequency of the structure, the vibration amplitude will be very high, but the force required to cause the vibration will be low.

 

This machine has flexible shock mounts between itself and the foundation.


On the other hand, if the same motor, with the same residual imbalance, is rigidly mounted to a solid foundation so it is very stiff and not free to move, the vibration level will go down. The forces acting in the bearing, however, will have increased, for they are now pushing against the entire foundation and earth, rather than just the mass of the motor itself. The lower vibration level would lead one to believe the motor is well balanced, but in fact, the bearing is in danger of being damaged by the high forces involved. From this, we see that the measured vibration level is not a good indicator of what the bearing itself is experiencing.

Here, the machine is solidly connected to the foundation.

This is one reason that many machines with high vibration levels run for years without bearing failures, while other machines with low vibration levels go through bearing after bearing.

If the force is measured in pounds and the response is measured in inches per second, the mobility is a spectrum of inches per second per pound as a function of frequency. The resulting vibration at any frequency is equal to the force times the mobility at that frequency. Suppose the mobility at 1X is .01 inch per second per pound, and suppose the measured vibration level at 1X when running is .1 ips. Then the centripetal force acting on the housing is (.1) ¸ (.01) = 10 pounds. On the other hand, if the mobility were .0001 ips per pound, the bearing force would be 1000 pounds for the same measured vibration level.

In a properly designed machine installation, the mechanical mobility at the bearings should be reasonably high to avoid excessively high bearing forces due to imbalance conditions. This is one area where two-channel measurements provide valuable information that is almost impossible to obtain otherwise.





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