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Hydraulic Seals: Types and How to Choose
A seal is not a rubber ring. It is a system of parts with separate jobs. A rod seal fitted to a piston may match the diameter and still fail within hours. This guide covers the parts, materials and dimensions that make a replacement work.
RFQs often give only the rod diameter or a photo. That is not enough: housing diameter, groove height, pressure direction and compound all matter. One wrong value means ordering twice.
The seals inside a hydraulic cylinder
A double-acting cylinder uses several seal groups at once. They differ in profile, lip direction and load capacity.
| Component | Where it works | Job | Typical material |
|---|---|---|---|
| O-ring | Static joints, covers, flanges | Seals a joint by controlled compression in a groove | NBR, FKM, EPDM |
| Rod seal (U-ring, chevron pack) | Dynamic rod-to-housing joint | Keeps oil in and works with the wiper | PUR, NBR, PTFE |
| Piston seal | Dynamic piston-to-barrel joint | Separates the chambers and holds pressure both ways | PTFE with NBR energiser, PUR |
| Wiper (scraper) | Rod entry at the gland | Removes dirt before the rod seal | PUR, NBR |
| Back-up ring | Beside an O-ring or seal | Stops the seal extruding into the clearance at high pressure | PTFE, polyamide |
| Guide ring | Piston and rod | Carries side load, prevents metal contact | Bronze-filled PTFE, fabric composite |
O-rings: two dimensions, not one
O-ring size is written as inside diameter × cord thickness, for example 50 × 3.53 mm. Groove diameter, depth and width matter just as much. Above roughly 100–160 bar, or with a wider clearance, the ring may extrude into the gap; add a back-up ring. Cleanliness and compression set drive service life.
Rod and piston seals
A rod seal is usually single-acting, with the lip facing the pressure. Large rods and high pressures may use a chevron pack with a tension sleeve. It is adjustable but takes more axial space and creates more friction. A piston seal works both ways, so its profile is symmetrical or has two lips.
Compact PTFE seals with an elastomer energiser save space and tolerate high sliding speeds, but need a cleaner surface and careful fitting. Do not substitute one without checking the groove.
Wipers, back-up rings and guide rings
A wiper does not hold pressure; it protects the rod seal. A scored rod will damage a new wiper lip and let contamination through. Guide-ring wear is often mistaken for a leaking rod seal.
Our catalogue includes seal kits and seals, rod seals, piston seals and hydraulic cylinders for complete assemblies.
Material selection: NBR, PUR, PTFE, FKM
Choose the compound by fluid, temperature, pressure and sliding speed. There is no universal material: a compound that works well in mineral oil may fail in water or a fire-resistant fluid.
| Material | Typical temperature | Strengths | Limitations |
|---|---|---|---|
| NBR (nitrile rubber) | −30 to +100 °C, short excursions to +120 °C | Mineral oils, many water-based fluids, good strength, low cost | Not for phosphate esters, ketones or strong solvents; ozone ageing |
| PUR (polyurethane) | −30 to +80 °C, some grades to +90 °C | Excellent abrasion resistance; standard for rod seals | Hydrolyses in hot water and steam; limited temperature range |
| PTFE | −50 to +200 °C | Chemical resistance, low friction, high sliding speeds | Cold flow, low elasticity; needs an energiser and careful fitting |
| FKM (fluoroelastomer) | −20 to +200 °C, special grades to +230 °C | Heat resistance, mineral oils, fuels and many acids | Not for ketones, amines, hot water or steam; relatively expensive |
Figures are typical for standard compounds, not a guarantee. EPDM suits water and steam but not mineral oil. For biodegradable or fire-resistant fluids, check the fluid specification.
Hardness comes next. PUR rod seals are commonly 90–95 Shore A; static NBR O-rings 70–80 Shore A. Colour is not a reliable identifier.
How to read seal dimensions D × d × L
Seal catalogues generally use D × d × L, where:
- D is the outside diameter, which fits the housing or cylinder barrel;
- d is the inside diameter at the sealing surface: the rod, shaft or groove base;
- L (sometimes H) is the axial height, or groove width.
| Type | Example | What the figures mean |
|---|---|---|
| O-ring | 50 × 3.53 | 50 mm inside diameter, 3.53 mm cord |
| Rod seal | 60 × 45 × 10 | 60 mm housing, 45 mm rod, 10 mm height |
| Piston seal | 80 × 65 × 7.5 | 80 mm bore, 65 mm groove base, 7.5 mm height |
| Wiper | 60 × 45 × 7 | 60 mm housing, 45 mm rod, 7 mm height |
Some suppliers list d × D × L. State the application as well as the figures: “rod seal, 45 mm rod, 60 mm housing, 10 mm height”.
Practical rule: measure the groove and the rod, not the swollen old seal. A distorted seal no longer represents the housing. Check groove radii and sharp edges, which can cut a new lip during installation.
Six installation mistakes that cause leaks
- Replacing the seal without checking the rod and groove. Scores, corrosion or ovality will defeat even the correct part.
- Fitting dry. Lubricate the lip with clean hydraulic oil; other greases may be incompatible.
- Twisting an O-ring. Roll it into place rather than pulling. Use a tapered sleeve for deep grooves.
- Reversing the lip. A single-acting rod seal has its lip towards the pressure. Fitted backwards, it will leak.
- Reusing old back-up rings. An extruded ring no longer controls clearance. Replace it and check the gap.
- Ignoring cleanliness. One grain of sand under the lip creates a leak path. Wash the parts and cap the cylinder.
Run two or three slow cycles without pressure, then test under load. If it leaks, do not just tighten the gland: a twisted seal or damaged lip is more likely.
Frequently asked questions
Can you match a seal from a photo?
A photo can identify the profile, not the size. We need D × d × L, the rod or bore diameter and the application. A readable nameplate photo helps.
Are rod seals and piston seals interchangeable?
No. They run on different surfaces and see pressure from different directions. A rod seal is usually single-acting and prevents external leakage; a piston seal separates two chambers and holds both ways. Equal diameters do not make them interchangeable.
Which material should I use at 120 °C?
It depends on the fluid and duration. Standard NBR is at its limit; FKM or PTFE are more likely. FKM is unsuitable for hot water or steam, where EPDM may be better.
Can you supply non-standard sizes?
Yes. We can source custom seals made to a drawing or sample. Send groove dimensions, compound, fluid, temperature and quantity; lead time is longer than for stock.
Not sure about the size or material?
Send D × d × L, a profile photo and the fluid specification. We will match a seal or kit and help identify the part if the number is unknown.
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