Hydraulic Cylinder O-Ring Design Guide
Rod and piston seal material selection, groove dimensions, compression rates, and anti-extrusion solutions for mobile and industrial hydraulics.
Hydraulic cylinders are one of the most demanding dynamic sealing environments. O-rings must resist petroleum-based hydraulic oils, tolerate pressure spikes, and maintain elasticity across a wide temperature range. This guide covers rod seal and piston seal design for mobile equipment, industrial presses, and agricultural machinery. We explain material selection (NBR vs HNBR vs FKM), recommended groove dimensions for reciprocating motion, compression rates that balance leakage control against friction, and when to add PTFE backup rings to prevent extrusion at pressures above 150 bar.
Application Requirements
Recommended Materials
NBR 70-90 Shore A
General-purpose hydraulic oil sealing, cost-effective
Temp: -40°C to +120°C
Standard choice for most hydraulic cylinders. Use 90 Shore A for high pressure.
HNBR 80-90 Shore A
High-temp hydraulics, bio-oils, sour gas exposure
Temp: -40°C to +150°C
Superior abrasion and ozone resistance versus NBR.
FKM 75-90 Shore A
High-temp synthetic fluids, phosphate esters
Temp: -20°C to +200°C
Use when continuous operating temperature exceeds +120°C.
Design Tips
- 1.Dynamic groove width should be 1.15–1.25 × CS to allow the O-ring to roll rather than twist during stroke.
- 2.Keep surface finish at 0.2–0.4 μm Ra for dynamic sealing surfaces to minimize abrasion.
- 3.Install a lead-in chamfer of 15–20° on rod and bore entries to prevent damaging the seal during assembly.
- 4.Use PTFE backup rings when pressure exceeds 150 bar or when radial clearance gaps are greater than 0.1 mm.
Common Sizes
| Size | Typical Use |
|---|---|
| AS568-210 to -222 | Small bore hydraulic cylinders (up to 38 mm bore) |
| AS568-325 to -341 | Medium bore cylinders (40–100 mm) |
| AS568-425 to -445 | Large bore cylinders and rod seals (100–250 mm) |
Frequently Asked Questions
What is the best O-ring material for hydraulic cylinders?
NBR 70–90 Shore A is the standard for mineral oil-based hydraulics. HNBR is preferred for bio-oils, higher temperatures, or when ozone resistance is required.
At what pressure do I need backup rings?
PTFE backup rings are recommended at pressures above 150 bar or whenever the radial clearance gap exceeds 0.10 mm to prevent O-ring extrusion.
What compression rate should I use for a dynamic hydraulic seal?
Target 10–15% compression for rod and piston seals. Too much compression increases friction and heat; too little causes leakage.
Can I use the same O-ring for rod and piston sealing?
Yes, provided the groove design respects the different motion directions. Rod seals experience outward pressure; piston seals experience inward pressure. Backup rings should be placed on the low-pressure side in both cases.
Need hydraulic cylinder seals?
We supply NBR, HNBR and FKM O-rings in standard and custom sizes for hydraulic applications. MOQ 1 piece.