The pistons, rings, pins, locking rings and the upper half of the rods are considered as reciprocating parts. These parts make up a mass weight that is difficult to off set because this weight moves inward and outward (to and fro) and at varying speeds from zero to maximum and back to zero again, twice for each revolution of the flywheels. The pistons actually stop at both top and bottom dead centers, then change their direction of travel. Their rate of travel is, therefore, constantly variable. A motor that is running 3000 r.p.m. would cause each piston and rod to make 6000 strokes per minute. Where one revolution of the flywheel would require but 1/50 second, each piston stroke would be only 1/100 second of time in duration. We can appreciate that this terrific reciprocating action must be offset by balance by some opposite force, else the motor would jump out of the frame.
Since only the upper half of the connecting rod is considered as reciprocating mass, then the lower half tends to revolve with the crankpin and add weight to the rotating mass creating more centrifugal force. The action of the connecting rod is peculiar in that it starts from zero (dead center in the cylinder and crankshaft plane) and reaches a maximum speed of travel when it is at right angles (90°) to the crank throw. If we were to plot a curve of the mass movement of the rod we would develop a sort of pear-shaped design with the weight or body increasing toward the crankpin rotation. The center of the rod describes an elliptical orbit in its motion.
Most formulae call for balancing one half of the reciprocating mass. In practice, however, this may vary from .45 to .82 of the reciprocating mass weight. As already mentioned, the actual balancing weight selected depends upon the purpose for which the motor was designed.
By studying the figures in Illustration No. 2, you will get a fair idea of how the mass weight of pistons and rods is acted upon by the revolving mass weight of the flywheels. Note, especially, the position of the pistons when either rod is at right angles with the crank throw.
Besides the to-and-fro motion of pistons and rods and the revolving flywheel counterweight, we have the side thrust of the pistons against the cylinder walls on explosion and compression strokes to consider. When pressure is exerted on the piston head due to the combustion in the power stroke, the piston is forced violently against the side of the cylinder. At the same time, there is the outward force on the rod, driving the flywheels. The tendency of these torque reactions is to cause the cylinders to sway back and forth, vibrate. To check torque reaction we must have strong crankcases and heavy cylinders securely mounted in a rigid frame. Thus the motor mounting has much to do with controlling vibration
Torsional Flexibility of Flywheels
This means simply the tendency of flywheels to weave or flex under the extreme pressure of combustion and compression. These rapid changes, however, set up a sort of vibrating couple, lateral in its action. Only very strong flywheels will minimize this torsional force. Excessive vibration at this point will tend to break the flywheels or crankpin.
A Study in Counterweights