Page 108 - Reciprocating Compressors Operation Maintenance

Design and Materials for Reciprocating Compressor Components 95

180° between the weights, it will be seen that the ring does not pull the
rod as the weights revolve about the rod. The centrifugal force of weight
No. 1 is counteracted by the centrifugal force of weight No. 2, creating a
situation of forces in equilibrium.
In actual practice, weight No. 2 would be a counterweight. The mass
of this counterweight acts through its center of gravity at a distance suffi-
cient to offer a force equivalent to the effective weight of the reciprocat-
ing and rotating masses acting through a point in the crankpin.
In a single cylinder compressor, the addition of counter-weighting
allows a part of the primary unbalanced forces acting in the direction of
the cylinder center line to be transferred into the perpendicular direction.
It is important to remember that the counterweights in a single compres-
sor do not "balance" the inertia force. They only "transfer" the force in a
perpendicular direction.
In a two-cylinder "Y" compressor, if the reciprocating weights of both
cylinders are equal, then the sum of the primary forces of both cylinders is
a constant magnitude force rotating with compressor speed. Consequently,
this force can be balanced by the counterweight. Practically, however, the
same counterweights are used for several cylinder combinations and,
therefore, a small unbalanced primary force is sometimes present.
We know from experience that this resultant force does not produce
any difficulties in actual installations.

SECONDARY FORCES

The secondary forces are the result of reciprocating masses only, and
because of acceleration and deceleration of these masses, occur at twice
crankshaft speed. They act only in the plane of the cylinder axis, and can
only be balanced by counterweights rotating at twice crankshaft speed,
and in opposite directions. Such an arrangement requires a separate
counterweighting system, which is gear or chain-driven. Because of
costs, space requirements, and design difficulties, balancing of single
crank compressors has not yet been successfully employed. It may there-
fore be concluded that there is no practical way for reduction or elimina-
tion of secondary forces, except by suitable arrangement of cranks of
multi-crank compressors.
The balanced-opposed type compressor with its multi-crank design is
well suited for secondary force considerations. Assuming equal recipro-
cating weights in an opposed, two-cylinder compressor arrangement, all