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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 10, Gear Vibration

Section 1, Gear Vibration Frequency of 1 x rpm

For those designing gears and geared machinery, completely covering all aspects of gear vibrations would include many subjects beyond the scope of this course. However, for those working with already built and running machinery, the following is offered.

Vibration at 1 x rpm of the "offending" gear is by far the most common gear vibration. In some vibration instruction book this 1 x rpm is blamed on the gear's machined eccentricity, but Update believes an eccentric gear is rarely the source of the problem. Of course, it cannot be ruled out, but the other sources for gear 1 x rpm vibration should first be checked.

For example, a gear can be well balanced within itself. But it can be mounted on a rotor that is out-of-balance. The out-of-balance rotor creates its usual 1 x rpm vibration, orbiting at that frequency. Then the pitch circle of the gear must also be "orbiting" or running eccentrically. In other words, the gear may be running true and concentrically on a slow roll. But when running at operating speed, the unbalance causes an orbit, and the orbit causes the gear to run as if it is eccentric. The same situation can result from the 1 x rpm due to coupling misalignment, or any other source of 1 x rpm vibration that affects the gear's centerline and axis of rotation.

When a gear is eccentric, or running eccentrically (as describe above), it creates its own additional 1 x rpm vibration, with its amplitude high in the direction of the imaginary line connecting the centers of the two mating gears. The additional vibration in the direction 90° to this line should be very low. Therefore, a characteristic of gear vibration at 1 x rpm will be high 1 x rpm in one direction and considerably lower 1 x rpm vibration in the other. The resulting orbit will be a relatively flat ellipse.

If the gear is not eccentric within itself but "running eccentrically," then reducing the original shaft orbit will have a drastic effect on the gear 1 x rpm vibration. This is done by getting at the source for the original orbit such as unbalance, coupling misalignment, bent shaft, etc.

 

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