Occasionally I receive requests to perform a "bump test". It is also called an "impact test", which is just as it sounds-you hit something with a hammer! Preferably use a rubber, or plastic, face hammer so you don't damage anything. The trick is to have your accelerometer mounted on the machine and connected to your FFT vibration analyzer so you can record/measure the movement/reaction of the equipment.

Why? To determine the natural frequency of a piece of equipment. If the motor/pump unit(or blower, or gearbox, or turbine) is operating at a given speed(1785 rpm), you don't want the operation to excite the 'natural frequency' of the assembly. The result is resonance, and high vibrations that can damage, and/or shorten the life, of the machine.

Everything has a natural frequency. By impacting it(think tuning fork, or the "gong" on the Gong Show), it will 'ring' at its' natural frequency. Each motor, baseplate, and pump would have its' own natural frequency, and when all components are assembled together, installed, and possibly bolted down, the complete assembly will have a natural frequency. Engineers try to calculate this when designing equipment by putting this safely above, or below(20%-30%), the running speed.

If the natural frequency is within 10% of operating speed, the machine 'excites' the natural frequency resulting in excessive vibrations(resonant condition). When starting new equipment, high vibrations can easily be caused by resonance. With the right equipment, and a qualified technician, it is usually easier to do a bump test before sending a new pump, blower, motor back to the factory for inspection.

After determining the natural frequency, correction is easy. To lower the frequency, you dampen, or add more mass to the machine. Example: A motor operating at 3580 rpm(60 Hz) was found to have a natural frequency at 123 Hz.-almost exactly at 2x line frequency. Adding a 1/8" plate beneath the motor increased the mass and thereby reduced the vibration levels from .41 in/sec to .13 in/sec. To increase the frequency, stiffen the machine. Example: a pump operating at 3588 rpm was found to have a natural frequency of 3751 cpm. Adding braces to stiffen the baseplate increased the resonant frequency to 4082 cpm. Vibrations decreased from .53 in/sec to .12 in/sec.

Something must excite the natural frequency. Imbalance can excite it. Machinery vibrations transferred through the hard piping to a building can excite the natural frequency of the building. Reducing vibrations caused by resonance all starts with the Bump Test.  

As equipment ages, it loosens up, loses mass to rust & corrosion, and can easily become resonant. Use the bump test to confirm the cause of your increased vibration. Invest in PM, it will save you money!