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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 3, Detuning and Proving Resonance

Section 17, Use of Variable Speed Vibration Shaker

Machinery test laboratories often use electronic or electronic/ hydraulic variable frequency shakers to vibration test new machine models to determine actual resonant frequencies of the various parts. Such devices offer many advantages over home-constructed shakers but are not usually available at the typical plant. Therefore, a simple shaker made from a variable speed motor is shown.

Try to obtain a variable speed motor of approximately ½ hp with top speed near 10,000 rpm or higher. The channel base should have motor mounting holes positioned so that the motor can be placed in line with the channel's axis or perpendicular to it, depending on the desired direction for the vibrating force.

Separate the motor from the channel with washers, so that stainless steel hose clamps (used to tie the shaker to piping or large rotor shafts) will put stress on the channel and not the motor frame.

The vibration may cause the clamps to loosen; therefore, make sure a safety chain or belt is used. Surprisingly, a ½ hp motor is sufficiently large to act as a shaker for about 80 percent of the typical process plant's machinery, piping or structures. For shaking very large machinery or sections of building structures, including concrete floors and bases, a 2 to 3 hp motor (with a much larger pulley and unbalance) is required. Ideally this larger shaker will be a dc motor, as it is much easier to adjust its speed to desired rpm. If dc is not available, it usually can be supplied by the dc arc welder engine driven generator, commonly available at most plants.

When first using the shaker, the vibration amplitude will most often seem disappointingly low. However, when the shaker rpm approaches the resonant frequency of the part, the vibration will increase to amplitudes that are quite noticeable and easy to use.

Observe both amplitude and phase as the shaker rpm resonates different parts at different shaker speeds. In some situations, if the shaker is inadvertently mounted on a resonant part, it may be difficult to force the motor to reach the exact desired speed. This is true because much more energy is required, since the shaker motor vibrates much more when it is mounted on the resonant part. Then when the resonance "peak" is passed, the motor tends to drastically increase in rpm. If this is the situation, the shaker will have to be moved to a non-resonant part (or the amount of unbalance that was initially used will have to be reduced).

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