Application guide · Hydraulics
There is no single hydraulic O-ring. Choose it in three steps: where the seal sits, which fluid and temperature it sees, and how the gland handles pressure. The size comes from the hardware.
Keep EPDM away from mineral oil and petroleum grease, including the grease used at assembly.
A hydraulic cylinder has several sealing positions, and each one fails for different reasons. Identify the position before you choose size, material or hardness.
Typical sealing positions in a double-acting cylinder. The numbers match the cards below.
PositionWhat movesCheck firstNext step
Engineer Pro's Dynamic mode looks up a reference gland depth and width for metric cross-sections and marks the result for engineering review. If the cylinder uses a dedicated rod or piston seal profile, such as a U-cup, cap seal, T-seal or energized seal, size it from that seal's own data rather than as an O-ring.
Hydraulic fluid is not one chemical. The fluid family decides the starting material, and the exact product decides the final compound.
| Fluid | NBR | FKM | EPDM | Start with | Check |
|---|---|---|---|---|---|
| Mineral (petroleum) hydraulic oilAbove NBR's typical upper limit, use FKM. EPDM swells in mineral oil, so keep it out. | AGood | AGood | UAvoidvaries | NBR | Check mineral oil |
| Mineral oil above about +120 °CNBR is past its typical upper limit. | UAvoid | BFair | UAvoid | FKM | Check hot oil |
| Water-glycol (HFC)EPDM also rates well, as long as no mineral oil is present. FKM needs a check. | BFairvaries | RReviewvaries | AGood | NBR | Check water-glycol |
| Oil-in-water and water-in-oil emulsions (HFA, HFB)The compatibility data marks emulsions for review, so confirm the exact product and compound. | RReview | RReview | CLimited | NBR | Check an emulsion |
| Phosphate-ester fire-resistant fluid (HFD-R)NBR is not suitable. FKM rates well with some phosphate esters and poorly with others. | UAvoid | UAvoidvaries | AGood | EPDM | Check phosphate ester |
| Synthetic and biodegradable fluidsSynthetic base oils and polyglycols rate differently. Screen the exact product. | CLimitedvaries | CLimitedvaries | UAvoidvaries | Depends on the base fluid | Check a synthetic fluid |
Grade keyAGoodGoodBFairFairRReviewReviewCLimitedLimitedUAvoidAvoidEach chip is the lowest grade across the fluids in that row; "varies" means they differ.
These are family-level screening ratings from the compatibility check. Confirm the exact fluid and compound for critical service. Silicone is a poor fit for hydraulic seals: it is weak in moving seals and under pressure.
Pressure is mostly a question of hardness and clearance. As pressure or the extrusion gap grows, the usual steps are a harder compound, 80 or 90 Shore A instead of 70, and a backup ring.
As pressure or the extrusion gap grows
NBR
FKM
Shore A. Availability varies by size.
Hardness on its own is not a pressure rating. The extrusion gap, eccentricity, pressure spikes and temperature decide whether the ring extrudes, so check the gap, squeeze and gland fill in Engineer Pro before you change the compound.
These details decide most of the checks above. Send them with any engineering review.
There is no single hydraulic O-ring size standard. Identify the ring by inside diameter and cross-section, or by a reference you can verify: an AS568 dash number, an ISO 3601 metric size or the equipment drawing. For ports and fittings, use the size the fitting documentation specifies, because a ring that physically fits is not automatically the right fitting seal.
A used ring is often flattened, so its cross-section reads smaller than it was new. If it falls between two standard sizes, the larger one is usually the original. Where you can, measure the groove instead, then pick material and hardness from the configurations the Finder lists.
The removed ring and the hardware around it are your strongest evidence. Trace the damage before you change the material.
What you seeCheck firstUsual fix path
For mineral hydraulic oil at moderate temperature, NBR is the usual starting point, and FKM is the usual step up when the oil runs above NBR's typical upper limit of about +120 °C. Fire-resistant fluids change the answer: for phosphate esters, EPDM is the usual starting point and NBR is not suitable. Check your exact fluid in the compatibility check.
Often, but not on its own. As pressure or the extrusion gap grows, the usual steps are 80 or 90 Shore A instead of 70 and a backup ring. Whether a ring extrudes depends on the gap, eccentricity, pressure spikes and temperature, so check the gland first.
There is no single hydraulic O-ring size standard. Use the inside diameter and cross-section, a verified AS568 dash number or ISO 3601 metric size, or the size the fitting documentation specifies, and look it up in the size charts or the Finder.
Do not assume so. The rod seal and the piston seal slide on different surfaces and sit in different glands, so check each one separately.
An immediate leak usually points to the wrong size, damaged hardware, a sharp lead-in, or a ring twisted or cut during assembly, not only to the material.
No. A wiper or scraper keeps dirt away from an exposed rod. It does not replace the pressure seal.
Know the size and the fluid? Find the ring in the catalog. Not sure? Send the conditions above for an engineering review.
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