Hydraulic seals are sealing elements that prevent fluid leaking out of hydraulic cylinders, pumps, valves and rotating shafts, and prevent dust, water and dirt getting in. Cone crusher tramp release and setting cylinders, impact crusher apron adjustment cylinders, the folding and track cylinders of mobile crushers and the closing cylinders of filter presses all depend on correctly selected and correctly fitted seals. The failure of a single seal can lead to pressure loss, oil leaks, environmental contamination and the machine stopping.
In this guide we cover comprehensively the main types of seal (U-cup seals, T-seals, piston and rod seals, wiper seals, O-rings, back-up rings and guide strips), seal materials, selection criteria, fitting practice and failure analysis. Note: In general, seals on moving surfaces are called dynamic seals and those on stationary surfaces static seals or gaskets.
Which Seals Are in a Hydraulic Cylinder?
| Element | Position | Function |
|---|---|---|
| Wiper seal (scraper) | Outer end of the cylinder head | Wipes dust and dirt off the rod as it retracts |
| Rod seal | Inside the cylinder head | Prevents oil leaking along the rod |
| Buffer seal | In front of the rod seal | Takes pressure peaks, protects the rod seal |
| Piston seal | On the piston | Prevents leakage between the two sides of the piston |
| Guide strip (wear ring) | On the piston and in the head | Centres the piston and rod, prevents metal contact |
| Static seals (O-ring + back-up ring) | Between head and barrel | Sealing in fixed joints |
Types of Seals
U-Cup Seal
A U-cup seal is a lip seal whose cross-section resembles the letter U. System pressure acts on the inside of the U and presses the lips against the surfaces; as pressure increases, so does the sealing. It works in one direction and is usually used as a rod seal, and in some applications as a single-acting piston seal. Polyurethane U-cups offer high wear and extrusion resistance; NBR U-cups are an economical solution at lower pressures.
T-Seal
A T-seal consists of an elastomer ring with a T cross-section and back-up (anti-extrusion) rings on each side. It works in both directions and is usually used as a piston seal in double-acting cylinders, and sometimes as a rod seal. The T shape stops the seal rolling in its groove (spiral failure); the back-up rings prevent the elastomer being squeezed into the gap at high pressure.
O-Ring and Back-Up Ring
The O-ring is the simplest and most common sealing element, with a circular cross-section. It is widely used in static joints (covers, flanges, fittings). At high pressure or with large gaps, back-up rings are added to stop the O-ring being extruded into the gap.
Compact and Combined Piston Seals
Piston seals made from a combination of elastomer and PTFE or polyamide parts, combining guiding and sealing functions. They provide low friction and long life.
Wiper Seal (Scraper)
The wiper seal is the first line of defence preventing dust and mud from the crushing and screening environment getting into the cylinder. In a cylinder with a damaged wiper, the rod seal soon wears and the oil becomes contaminated. In dusty environments, double-lip or metal-cased heavy-duty wipers are preferred.
Rotary Shaft Seals (Oil Seals)
Spring-loaded lip seals used on rotating shafts such as gearboxes, pumps and motors. A wear groove on the shaft surface causes a new seal to leak too; in this case the shaft surface should be repaired or a repair sleeve used (shaft sleeve guide).
Seal Materials
| Material | Characteristics | Typical use |
|---|---|---|
| NBR (nitrile rubber) | Resistant to mineral oils, economical | General hydraulics and O-rings |
| PU (polyurethane) | High wear and extrusion resistance | U-cups, wipers, heavy duty |
| FKM (fluoroelastomer) | High temperature and chemical resistance | Hot oil, aggressive fluids |
| HNBR | Higher temperature and wear resistance than NBR | Harsh conditions |
| PTFE and bronze-filled PTFE | Very low friction, chemical resistance | Combined piston and rod seals |
| Polyamide, POM | Hard; guide and back-up rings | Guide strips, back-up rings |
When choosing material, the type of hydraulic fluid (mineral oil, biodegradable oil, water-glycol), temperature range, pressure, speed and environmental conditions must be considered.
Seal Selection Criteria
- Dimensions: Rod diameter, cylinder bore (tube diameter), groove width and depth.
- Pressure: Working and peak pressures; pressure surges.
- Speed: The cylinder’s speed of movement.
- Temperature: Oil and ambient temperature.
- Fluid: Type of hydraulic oil and compatibility.
- Surface quality: The surface roughness and hardness of the rod and barrel directly affect seal life.
- Single or double acting: Determines the seal type.
Correct Fitting
- Dismantle the cylinder in a clean environment; clean all parts.
- Check the chrome surface of the rod and the inner surface of the barrel for scratches, corrosion and wear; damaged surfaces soon ruin new seals.
- Clean the grooves; remove sharp edges and burrs.
- Fit the seals in the correct direction (U-cup lips must face the pressure side).
- Fit the seals with installation tools; do not force them with sharp tools such as screwdrivers.
- Lubricate the seals with clean hydraulic oil during fitting.
- After assembly, run the cylinder at low pressure to bleed the air and carry out a leak test.
Seals should always be replaced as a kit; replacing only the leaking seal leads to the other seals failing soon afterwards.
Seal Failures and Their Causes
| Appearance | Possible cause |
|---|---|
| Wear and scratches on the lip | Dirty oil, damaged rod surface, damaged wiper |
| Extrusion (seal squeezed into the gap and torn) | Excessive pressure, large clearance, missing back-up ring |
| Hardening and cracking | Excessive temperature, ageing |
| Swelling or softening | Material incompatible with the fluid |
| Spiral failure | Seal rolling in the groove, side load |
| Installation cuts | Sharp edges, wrong installation tool |
Most seal failures are caused by contaminated hydraulic oil. For oil cleanliness and filter maintenance, see our hydraulic system maintenance guide.
Seal and Gasket Applications in Crushers
- Cone crusher tramp release and setting cylinders: High pressure and shock; heavy-duty seal kits (hydraulic cylinders).
- Cone crusher dust seal: Prevents dust from the crushing chamber entering the lubrication system (dust seal).
- VSI lid seal: Dust sealing (lid seal).
- Impact crusher apron adjustment cylinders: Shock during tramp events (apron adjustment system).
- Mobile crushers: Folding, track and setting cylinders (mobile crusher maintenance).
Frequently Asked Questions
What is the difference between a U-cup seal and a T-seal?
A U-cup is a single-acting lip seal usually used for rod sealing. A T-seal works in both directions with back-up rings and is usually used as a piston seal in double-acting cylinders.
What types of hydraulic seal are there?
Wiper seals, rod seals, buffer seals, piston seals, guide strips, O-rings and back-up rings are the main hydraulic sealing elements.
Why should the seal kit be replaced completely?
Seals working under the same conditions age in a similar way; replacing only one soon leads to new leaks and a second strip-down.
Why does a cylinder leak despite new seals?
The main causes are scratches or corrosion on the rod surface, barrel wear, seals fitted the wrong way round or damage during fitting.
Information Needed to Order a Seal Kit
- The equipment the cylinder is used on, make and model (crusher, mobile machine),
- Cylinder bore (piston diameter) and rod diameter,
- Groove dimensions (width, depth),
- Working pressure and temperature,
- Type of hydraulic fluid,
- Photos or samples of the existing seals.
If the original seal kit number is known, a suitable equivalent kit can be identified quickly.
The Importance of the Rod and Barrel Surface
One of the factors that most affects seal life is the surface the seal runs on. If there are scratches, impact marks or corrosion on the chrome-plated rod surface, even the best seal will wear quickly. If the rod is damaged, re-chroming and grinding or rod replacement is required. Scratches on the inner surface of the barrel should also be removed by honing. Cylinder bellows and guards can be used to protect the rod surface from external damage.
Glossary
- Dynamic sealing: Sealing on moving surfaces.
- Static sealing: Sealing on stationary surfaces.
- Extrusion: The seal being squeezed into a gap under pressure.
- Back-up ring: A hard ring that prevents extrusion.
- Guide strip: A wear ring that centres the piston and rod.
- Buffer seal: A seal that protects the rod seal from pressure surges.
More Questions
How is a hydraulic piston seal chosen?
According to whether the cylinder is single or double acting, pressure, speed and temperature; in double-acting cylinders, T-seals or compact piston seals are usually preferred.
How long can seals be stored?
Elastomer seals should be stored in a cool, dark place away from ozone, in their original packaging; seals stored for a long time should be checked for flexibility and cracks before use.
Support from CSP Mühendislik
CSP Mühendislik supplies seal kits, gaskets and sealing elements for cone and impact crusher hydraulic cylinders, mobile equipment cylinders and hydraulic power units, and carries out cylinder overhauls. See our cone crusher spare parts page for cone crusher seal kits and our impact crusher spare parts page for impact crushers, and find out about our service, maintenance and repair.
