Practical Solutions to Machinery and Maintenance Vibration Problems
Chapter 7, Misalignment
Section 6, Axial Vibration Symptoms Due to Shaft/Coupling Misalignment
Most practical vibration "wisdom" indicates that high axial vibration indicates coupling misalignment. Some use the rule that when the axial vibration reaches at least 50 percent of the radial vibration (horizontal or vertical), the source for the vibration is coupling misalignment. This rule is a reasonably good one but, unfortunately, is not always reliable. Coupling misalignment doesn't always create high axial vibration. There are many instances whereby coupling misalignment is the source of the problem, and axial vibration as low as only 20 to 30 percent of the radial vibration. It is supposed that the amount of axial vibration depends on the coupling type and the type of misalignment (parallel or angular). Resonance can magnify lower levels of axial vibration from other sources so that it looks like large vibration due to misalignment.
Precaution: When axial vibration is high, first check to see if there are symptoms for other sources for higher than usual axial vibration, such as:
1. Resonance of some part in the axial direction, such as a pedestal or pipe. When diaphragm (flexible disc-type) couplings are used, the disc or discs may be resonant in the axial direction.
2. The couple component of dynamic unbalance.
3. Bent shaft or resonant whirl.
4. Misaligned bearings (rare). Sometimes bearings can be misaligned by a frame twisted by improperly shimmed foot pads.
5. Motor with sleeve bearings, hunting for its magnetic center.
If the items in the list are not probable, then it can be fairly and safely assumed that the axial vibration is due to misalignment.
This textbook contains only part of the information in our Practical Solutions seminar.