The allowable rod load depends on rod diameter and material, and will be quoted by the manufacturer. The actual load can be calculated from the following equations if the geometry is known:
The calculations shown above provide the gas load imposed on the rod (and crosshead bushing) by the compressor cylinder piston. To provide a reasonable crosshead pin bushing life, the rod loading at the crosshead bushing must change from compression to tension during each revolution. This is commonly referred to as “rod reversal” and allows oil to lubricate and cool one side of the bushing while load is being applied to the other side of the bushing.
A single-acting, head end cylinder will not have load reversal if suction pressure is applied to the crank end. Similarly, if discharge pressure is applied to the head end of a single-acting, crank end cylinder, load
reversal will not occur.
In addition to the gas load, the rod and crosshead pin bushing is subject to the inertia forces created by the acceleration and deceleration of the compressor reciprocating mass. The inertia load is a direct function of crank radius, the reciprocating weight, and speed squared. The total load imposed on the crosshead pin and bushing is the sum of the gas load and the inertia load and is referred to as the “combined rod load.”
The combined rod load should be checked anytime the gas loads are approaching the maximum rating of the compressor frame or anytime rod reversal is marginal or questionable.