DETROIT DIESEL 53
1.7
ENGINE BALANCE AND BALANCE WEIGHTS
In the balance of two-cycle engines, it is important to
consider disturbances due to the reciprocating action of
the piston masses. These disturbances are of two kinds:
unbalanced forces and unbalanced couples. These
forces and couples are considered as primary or
secondary according to whether their frequency is equal
to engine speed or twice engine speed. Although it is
possible to have unbalanced forces or couples at
frequencies higher than the second order, they are of
small consequence in comparison to the primary forces
and couples. Even the secondary forces and couples
are usually of little practical significance.
The reciprocating masses (the piston and upper end of
the rod) produce an unbalanced couple due to their
arrangement on the crankshaft. On a V-type engine,
this unbalanced couple tends to move the ends of the
engine in an elliptical path.
This couple is cancelled by incorporating an
integral crankshaft balance component and by placing
Fig. 1. - Removing Front Balance Weight (Pulley
balance weights at the outer ends of the camshafts (V-
Type)
type engine)
This
balance
arrangement
2. Force the balance weight off the end of each shaft,
produces a couple that is equal and opposite in
using two screw drivers or pry bars between the balance
magnitude and direction to the primary couple.
weight and the upper front cover as shown in Fig. 1.
On the camshafts (V-type engine)
Install Front Balance Weights
each set of weights (weights on
the outer ends of each shaft comprise a set) rotates in
1. Reinstall the Woodruff keys in the shafts, if they
an opposite direction with respect to the other.
were removed.
When the weights on either end of the engine are in a
vertical plane, their centrifugal forces are in the same
2. Align the keyway in the balance weight with the key
direction and oppose the primary couple. When they
in the shaft; then, slide the weight on the shaft. If the
are in a horizontal plane, the centrifugal forces of these
weight does not slide easily onto the shaft, loosen the
balance weights oppose each other and are, therefore.
thrust washer retaining bolts at the opposite end of the
cancelled. The front balance weights act in a direction
shaft; then, to prevent possible damage to the thrust
opposite to the rear balance weights; therefore, rotation
washer, support the rear end of the shaft while tapping
will result in a couple effective only in a vertical plane.
the weight into place with a hammer and a sleeve.
This couple, along with that built into the crankshaft,
Retighten the thrust washer retaining bolts to 30-35 lb-ft
forms an elliptical couple which completely balances the
torque. Install the other weight in the same manner.
primary couple.
3. Wedge a clean rag between the gears. Refer to
The balance weights are integral with the gears and the
Fig. 1, Section 1.7.2, and, tighten the gear retaining
circular balance weights (pulleys) on the shafts.
nuts to 300-325 Ib-ft torque. Then tighten the front
balance weight retaining nuts to 300-325 Ib-ft torque.
Both the rotating and primary reciprocating forces and
Remove the rag from the gears.
couples are completely balanced in the engines.
Consequently, the engines will operate smoothly and in
balance throughout their entire speed range.
Remove Front Balance Weights
1. Remove the nut at each end of both shafts as
outlined in Section 1.7.2.
1970 General Motors Corp.
December, 1970
SEC. 1.7 Page 1
