Rider ring load check
Six numbers tell you whether the rider rings on a reciprocating compressor piston carry their load within the limit set by API 618, and how wide the rings would have to be if they don't. Also called rider bands or wear bands.
Enter the six values to see the result.
- Load carried by the rings
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- Projected bearing area
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- Largest load these rings may carry
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- Narrowest ring width that still passes
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The calculation
How the number is worked out
The rings carry the piston assembly plus half the rod, because the rod is held at the crosshead at one end and the piston at the other. That load is spread over the projected area of a 120° arc of every ring:
p = (mpiston + ½ mrod) · g ÷ (0.866 · Dpiston · b · n)
API 618 caps that pressure on non-metallic wear rings at 0.035 N/mm² for non-lubricated cylinders and 0.070 N/mm² with lubrication. The 0.866 is the chord of the 120° arc. Ring makers note these are a guide for filled PTFE; some polymer alloys are run well above them.
Limits
What it doesn't tell you
Bearing load is one of five checks a rider ring has to pass. This page leaves out the other four:
- Side clearance in the groove at operating temperature
- Radial clearance, so the ring never seals gas
- End gap against thermal growth of the ring
- Radial thickness against the groove depth
A ring can pass here and still fail on any of those. Vertical cylinders are a separate case: the rings guide rather than carry, so this calculation doesn't apply to them at all.
Replacing a ring you have no drawing for
We reverse-engineer piston rings, rider rings and packing from a worn sample and the machine data sheet: verified geometry, tolerances to ISO 286, material equivalence and a manufacturing drawing.
Results are indicative and follow the bearing-pressure guidance in API 618. They do not replace a full design review.