Reciprocating Compressor API 618 Rod Load & Pin Reversal Calculator
Calculate combined gas and inertia piston rod loads, tension and compression safety margins, and crosshead pin lubrication reversal degrees per API 618 standards.
Cylinder Dimensions & Operating Pressures
Kinematics & Reciprocating Mass
Combined Rod Load & Reversal Compliance
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Frequently Asked Questions
What is crosshead pin reversal in a reciprocating compressor?
Crosshead pin reversal occurs when the combined rod load alternates between compression and tension during a single revolution of the crankshaft. This periodic shift allows the clearance between the crosshead pin and its bushing to open on alternate sides, drawing in fresh lubricating oil.
Why does API 618 mandate at least 15 degrees of pin reversal?
Unlike rotating journal bearings which pump oil hydrodynamically, crosshead bushings oscillate through a limited angular arc. If the rod remains in compression without reversing for at least 15° of crank rotation, lubricating oil cannot squeeze between the pin and bushing, causing rapid metal-to-metal galling and seizure.
How does reciprocating inertia affect the combined rod load?
At high RPM, the acceleration of the heavy piston, rod, and crosshead generates massive inertia forces that oppose the gas pressure load at dead centers. High inertia can either assist pin reversal or, if unbalanced, dramatically elevate tensile stress on the piston rod threads.