Reciprocating Compressor Rod Load Calculator (API 618)
Determine combined gas and inertia rod loads in tension and compression, rod reversal angle, and crosshead pin lubrication compliance per API 618.
Cylinder Dimensions & Pressures
Peak Forces & Rod Reversal Angle
Recommended Tools & Equipment
Tested hardware and components for high reliability
API 618 Reciprocating Compressor Rod Loading
In a double-acting reciprocating compressor cylinder, the piston rod transmits forces created by differential gas pressure between Head End (HE) and Crank End (CE) chambers, vectorially summed with reciprocating inertia forces ($F_{inert} = - m_{rec} r \omega^2 [\cos \theta + \lambda \cos 2\theta]$).
The API 618 Rod Reversal Requirement
The crosshead pin bushing oscillates within a narrow angular rocking motion without full rotation. For lubricating oil to penetrate between the pin and bushing, the net combined force must cross zero (reversing from tension to compression) for a minimum of 3° of crank angle and sustain at least $3\%$ of peak load. Without rod reversal, the oil film squeezes out, causing catastrophic pin seizure and rod ejection.