Page 103 - Reciprocating Compressors Operation Maintenance

90 Reciprocating Compressors: Operation and Maintenance

If the tail rod were to be omitted, the frame load would be 4M times
greater. Worse yet, the load does not reverse in direction and is, in fact,
not much reduced, making it mandatory to provide a greatly increased
size of crosshead pin bushing.

DISTURBING OR SHAKING FORCES

When a reciprocating compressor runs, moving parts such as pistons,
piston rods, crossheads, and connecting rods are repeatedly accelerated
and retarded. These velocity changes set up pulsating inertia forces. The
forces and couples are of the first and second order.
The first order forces have the same frequency as the compressor shaft
speed and the second order forces have a frequency equal to twice the
shaft speed. It is possible, but in most cases not economical, to design the
compressor in such a way that these inertia forces cancel each other. By
careful design, with equal piston masses and by fitting counterweights, it
is possible to reduce these inertia forces to low values.
THE EFFECT OF UNBALANCE

Compressor inertia forces may have two effects. One is a force in the
direction of the piston movement, and the other is a couple or movement
that is developed when there is an offset between the axis of two or more
pistons on a common crankshaft. The interrelation and magnitude of
these forces will depend upon such factors as number of cranks, their
longitudinal and angular arrangements, cylinder arrangement, and extent
of counterbalancing possibility.
Two significant vibration (movement) periods are set up:

«Primary—at rotative speed
• Secondary—at twice rotative speed

There are others that normally can be neglected.
Aside from the direct inertia forces, there are also forces set up by the
torque variations on the compressor shaft.
Although the forces developed are sinusoidal, only their maximum
values are considered in analysis. Figure 2-50 shows relative values of
the inertia (shaking) forces for various compressor arrangements. The