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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 3, Detuning and Proving Resonance

Section 19, Using Shaker to Determine whether Fault is Due to Weak Structure or Vibration Source

The shaker test is the best method to prove whether or not the support structure is the reason for excessive vibration (foundation, building beams, and so on). It can be used to determine if the suspect part of the structure is too flexible, weak or otherwise at fault, or if instead, the vibration from the source is too great (such as from unbalance, misalignment, and so on).

When a specific machine is vibrating badly, the following comment is often heard: "The reason this vibrates so badly is that the support structure is too weak." Is this usually true? Almost always the answer is no! Although it is occasionally correct to blame the structure (beams, floors, columns, etc.), it is also true that the source vibration (due to misalignment, unbalance, bent shaft, etc.) may be excessive. The structure is usually at fault only if the support structure is relatively flexible as well as resonant. If flexible but not resonant, the support structure should not vibrate excessively unless the vibration source is excessive. How do you determine which is the cause of the excessive vibration? The solution to this question is to perform the simple shaker test.

Procedure:

1. Unbalance the shaker pulley to the upper limit of the unbalance tolerance as specified by the manufacturer.

2. Mount the shaker motor to the machine to be shaken at a relatively rigid section (such as near the bearing housing, etc.).

3. With all in position, run the shaker motor up to the service speed of the machine.

4. While the shaker is operating at the desired frequency, take readings on both the structure and the machine.

5. Determine if the structure is responding excessively or if the vibration is greater at the machine itself.

Rarely will the shaker produce excessive vibration in the support structure unless it is a relatively flexible resonant structure. If it is not a resonant structure, vibration should be very small. In other words, the shaker that has been unbalanced to the standards of the machine manufacturer very rarely will cause excessive vibration in the structure. This means that the machine itself has more vibratory forces in it that it is supposed to have. Sometimes the writer adds enough unbalance to double the unbalance tolerance. Even then, the structures almost never vibrate excessively. This shows that the machines have more source vibration than originally specified as acceptable by the manufacturer. Therefore, all speculation ceases and concentration is focused on reducing vibration at the source. (For further guidance on the subject, see section on "Detuning Resonant Part vs. Reducing Vibration at the Source.")

 

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