Many drive system problems can be traced to mechanical vibrations. But, a basic understanding of vibration principles and the benefits of elastomeric couplings can help you avoid many of the unwanted effects of vibration.

Noise and vibration sources

Mechanical vibrations occur in both rotating and reciprocating equipment. In rotating equipment, torsional pulsations initiated by either the driving or driven equipment are the predominant cause of vibration.

Among types of driving equipment, four-stroke engines generate pulsations equal to the number of cylinders times half the engine speed, and two-stroke engines generate pulsations equal to the number of cylinders times the engine speed. Even seemingly smooth-running electric motors generate torsional pulsations that are a function of the number of stator poles and the motor operating speed.

Driven equipment can also generate torsional vibrations, as in the case of a horizontal mixer where the paddle loading varies as it rotates through the mixing medium. Here, the vibration frequency equals the number of blades times the rotational speed.

Abrupt starts and stops can create vibration in the form of damaging shock waves. Shock is a high-intensity noncyclic vibration.

Where vibrations generate audible signals, the result is called structure-born noise. This noise usually originates from the relative motion of structural components or from sound waves generated by high-frequency vibration in structural members. And, it occurs most often in flat plates and equipment enclosures.

If mechanical vibrations areunchecked, they can cause human discomfort and premature wear. In the mixer example, if the cyclic loading is transmitted back into the driving gearbox, resonances may occur within the gearbox, which can reduce the service life of drive components. Vibrations can also be amplified by the surrounding structure, causing fatigue failure of support members.

Control methods

There are two basic ways to control noise and vibration:

Minimize them. The primary method of reducing noise and vibration is to minimize their generation. This can be accomplished by:

• Balancing rotating components.
• Reducing the mass of reciprocating members.
• Loading the driven device as uniformly as possible.
• Incorporating a soft-start device to minimize start-up shock.

Accommodate them. Once vibrations are minimized at the source, the next step is to reduce their harmful effects. Here, the key issues are system design and drive coupling selection. Vibrations can be either amplified or isolated depending on the mass and stiffness characteristics of the system. The main parameter to control is the relationship between natural and disturbing frequencies of the system.

Natural frequency is the rate at which a spring-mass system vibrates in an unrestrained state. All systems exhibit a natural frequency depending on their stiffness and inertia characteristics. Vibration frequency is usually expressed in cycles per second or Hertz (1 cps = 1 Hz). For a rotational system the natural frequency is:

Fn = (1/2π)(K/J)^1/2

where:
J = Mass moment of inertia of driven equipment, in.-lb-sec²/rad
K = Torsional stiffness of the system, in.- lb/rad

Disturbing frequency is the rate at which torsional pulsations are applied to the system. They can be either steady state, such as firing pulses created by a diesel engine, or transient, as created by an abrupt change in shaft speed.

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