Crankshafts are one-piece forgings or castings, although provisions are usually made for removable counterweights.
Reciprocating machines have two kinds of motion – rotational and reciprocating (translational). In rotational motion, a rotating force is caused when there is an unbalanced weight at some distance from the center of rotation. The imbalance involved in the rotational motion of one crankthrow of the compressor consists of the weights of the crankpin, crankshaft webs, and a portion (usually about 2/3) of the connecting rod. Counterweights are sometimes used to compensate for the offcentered weights of these components. (Figure 300-21 illustrates these terms.)
The components involved in translational motion are the piston, piston rod, crosshead, and the remaining portion (usually about 1/3) of the connecting rod. A fluctuating force results when these parts are accelerated and decelerated as the piston travels back and forth.
For a single-cylinder compressor, the forces caused by both kinds of motion can be resolved into two sets of forces, primary and secondary, acting both horizontally and vertically.
Primary forces result from the rotational motion, and their frequency is that of running speed. Secondary forces result from translational motion, and their frequency is two times running speed due to the acceleration and deceleration during each stroke of the piston. Secondary forces act only along the axis of the cylinder.
Now, if a horizontally opposed compressor has two cylinders, a force couple can be generated by the unbalanced force of each cylinder acting in opposite directions and separated by the distance between the crankthrows. Figure 300-22 shows a primary couple for a two-throw machine having equal reciprocating weights on each throw. It also shows how counterweights can be added to the crank webs to reduce the primary couple.
Pistons on opposite adjacent throws are often not of the same diameter, so their weights are unequal. Figure 300-22 shows the location where a balance weight could be added to equalize the reciprocating weights. Dissimilar piston materials can also be used to equalize the weights.
It is seldom practical to fully compensate for forces and couples with counterweights and balance weights. The design becomes more complex where the machine has more than two throws. The resultant magnitudes of the unbalanced forces and couples, then, depend on:
• the number of throws,
• the angular orientation of the crankpins with respect to each other,
• the distance between the throws,
• the difference in reciprocating weights, and
• the amount of counterweighting that can be applied.
It is probably possible to balance the reciprocating weights on a pair of adjacent throws, but to have identical weights for all throws of a machine with several stages is seldom practical.
Figure 300-22 shows the simple case of a two-throw machine with a crankpin orientation of 180 degrees. As the number of throws increases, the effect of crankpin orientation on forces and couples gets quite complicated. Figure 300-23 qualitatively shows these effects for some of the more common crank arrangements with equal and unequal reciprocating weights.
Selecting the best arrangement from Figure 300-23 is not always the complete answer to the matter of shaking forces and couples. For example, for a four-throw machine, the 180 degree “flat” crankshaft is obviously the best choice from the standpoint of shaking forces and couples, and it eliminates the need for counterweights. However, the “flat” crankshaft causes all cylinders to be compressing at the same time. Hence, the torque-effort diagram (see Section 100, Figure 100-35) of the compressor may have undesirable oscillations from the standpoint of the driver. The “flat” crankshaft might require the addition of a large flywheel effect to the driver system, larger crankshaft diameter, and a special coupling to attenuate the oscillations. Therefore, the 90-degree arrangement might be more economical for an application that is not sensitive to shaking forces and couples. On the other hand, if the application is offshore, or onshore with undesirable soil conditions, the “flat” crankshaft will likely be the best choice.
A question that often arises concerns the number of main bearings. There are two configurations in Figure 300-21. One has two main bearings and a common web between the throws; the other has three main bearings, both for a two-throw crankshaft.
The advantage of the two-bearing design is that the throws are closer together, so the magnitude of the couple is less than that of the three-bearing design. With the three-bearing design, however, the crankshaft is more uniformly supported so that shaft deflection and bending stresses are less than that of the two-bearing design. The three-bearing arrangement has more places to attach counterweights to reduce the primary couple, but can the added complexity be justified? Opinions vary, but the fact remains that both designs are common, and both work.
Note that with a main bearing between each throw, the frame can have odd and even numbers of throws. When two throws have a common web, only even numbers of throws are possible.