Any machine running with anti-friction bearings and gears will, to a certain degree, exhibit high frequency vibration.
Predictive Maintenance: Vibration Analysis of Screw Compressor Bearings
Tips on Detecting Bearing Faults in Rotary Screw Air Compressors
By Scott D Mercer
President, PdM Specialists
Any machine running with anti-friction bearings and gears will, to a certain degree, exhibit high frequency vibration. Rotary screw air compressors are an example of this type of machine. Single stage rotary screw compressors are designed with two rotors that have anywhere from four to six lobes on each rotor. The rotors turn in opposite directions with the lobes nearly touching to create a “screw” which compresses the air. The compressor is usually driven by a motor directly coupled to the male rotor which has a gear on the coupling and that in turn drives the gear on the female rotor. The gears are timed in such a fashion that the rotors never touch.
Given this mechanical configuration, vibration at several different high frequencies can or will occur:
• Rotor Mesh Frequency — equals the number of lobes of the male rotor multiplied by the running speed of the motor. Therefore, this frequency will be synchronous with the motor running speed.
• Gear Mesh Frequency — equals the number of gear teeth multiplied by the running speed of the motor. In the case of multiple stage rotary screw compressor, there will be more than one set of gear mesh frequencies. This vibration will also be synchronous with motor running speed.
• Bearing Frequencies — which depend on the particular bearing used in the machine. In general, the bearing frequencies will be non-synchronous with motor running speed. The inner and outer race frequencies will be the ones most commonly seen.
• Other Frequencies — which could be induced by resonances of machines or compressed air flow.
Standard alarming methods often do not work well on machines that naturally exhibit high frequency vibration, such as rotary screw air compressors, even when they are running properly. Overall vibration levels are sometimes insensitive to the small changes in vibration levels caused by bearing deterioration. This is due to the large amount of vibration that is naturally occurring in a rotary screw compressor. The usual high frequency bearing detection tools or techniques will be triggered with each measurement, once again due to the vibration from the gears and rotors. This results in “nuisance” alarms.
Given these problems in using the standard alarming techniques, it is common to compare spectral data on rotary screw air compressors. Spectral data are usually compared to the previous measurements and to a reference measurement. This can be a time-consuming and intimidating task, given many measurement points and the number of different frequencies that could occur.
One technique for speeding up this process is to compare the percentages of synchronous and non-synchronous vibration between measurements. Many vibration software programs calculate this data for each spectrum. Bearing deterioration will normally be non-synchronous vibration. In other words, it is not a whole number multiple or harmonic of machine running speed.
Scott Mercer may be contacted at firstname.lastname@example.org
Originally printed in the March/April 1994 issue of Reliability Magazine.