Page 106 - Reciprocating Compressors Operation Maintenance

Design and Materials for Reciprocating Compressor Components 93

FIGURE 2-52. Rotating mass visualization on crankshaft web.

The centrifugal force created by these masses is an exciting force, hav-
ing the same intensity in all directions. It is the product of mass, crank
radius, and square of angular speed.
By selecting proper counterweights and placing them on the opposite
side of the crank, the rotating forces can be balanced.
Therefore, the inertial forces of rotating masses produce a centrifugal
force of constant magnitude that can be completely balanced by using
properly sized counterweights.

Inertia Forces Due to the Reciprocating Masses

The inertia forces of the reciprocating masses are by far the most
important. These forces are the result of the masses or one-third the
weight of the connecting rod, plus the weight of the crosshead, piston
rod, and piston. The weights of fasteners forming a part of the various
assemblies must also be considered.
These forces, which are the result of acceleration and deceleration of
the reciprocating weights, exert a variable force on the crankpin, acting
along the axis of the cylinder.
As may be seen, reciprocating forces are variable (depending on crank
position) and act at fundamental and even multiples of rotating speed.
The forces resulting from the rotating and reciprocating masses can be
resolved into force systems consisting of two parts: primary forces and
secondary forces. These are expressed in both horizontal and vertical
directions, and, in the instance of multi-crank compressors, as moments
or force couples.