Page 107 - Reciprocating Compressors Operation Maintenance

94 Reciprocating Compressors: Operation and Maintenance

PRIMARY FORCES

Primary forces are composed of rotating and reciprocating masses and
occur at crankshaft speed.
In effect, the primary force can be likened to a piece of string weighted
at one end, with the opposite end tied to a ring that rotates about a fixed
steel rod (see Figure 2-53).

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FIGURE 2-53. Unbalanced and balanced forces acting on a shaft system.

As long as the weight (mass) revolves about the rod, the string will be
in tension, and the effect of centrifugal force exerted by the rotating
weight will be to pull the rod in the direction of the weight. Since the
weight is revolving, the direction of the force will be constantly changing
through a full 360° circle. If the weight (or mass) rotates at a constant
speed, the force exerted on the rod will be constant also.
If the steel rod were, instead, a crankshaft without a counterweight, the
unbalanced force of the sum of the reciprocating and rotating masses
would cause the crankshaft to slam violently about within its bearings.
The vibrations would be transmitted to the frame and into the foundation.
An excessively large foundation would be required to restrain these
unbalanced forces.
But, suppose a second string were tied to the ring on the rod and an
equal weight put on its end, this weight being the same distance from the
center line of the shaft as the first weight. If both weights are caused to
rotate in the same direction and at the same speed, with an interval of