Practical Solutions to Machinery and Maintenance Vibration Problems
Chapter 3, Detuning and Proving Resonance
Section 18, Positioning the Vibration Shaker on the Test Part of Structure
When conducting tests, positioning of the shaker motor is sometimes critical to the success of the resonance shaker test. If the weight of the motor is a significant proportion of the weight of the part being analyzed, then the resonant frequency of the test part may be altered due to the weight of the shaker motor. On the other hand, if the part of structure being tested is relatively large, then it is all right for the shaker motor to be mounted directly on the test part or structure.
If the shaker motor as illustrated in Fig. 1 is heavy compared to the test pipe rigidity, then it should be mounted on a section of pipe that is not being tested but is physically connected to the test pipe.
If as shown in Fig. 2, the shaker is light compared to the test pipe's large rigidity, then it can be mounted directly on the section of pipe being tested, since it would have a negligible effect on pipe resonance. If mounted on the suspect part, place at a location where its weight will have a minimum affect on the resonant frequency.
The above recommendations apply not only to pipes, but also to shut down rotors, shafts, plates, bases, building structural members, floors (including concrete floors), and so on.
The vibration pickup could be placed on the suspect resonant part but most often should also be placed on the bearing that is vibrating the most. Phase changes can be observed digitally or with a strobelight directed at the shaker rotor.
This textbook contains only part of the information in our Practical Vibration Analysis seminar.