A hydraulic cylinder starts leaking oil along the rod, pressure drops, and the equipment loses force in the middle of the working cycle. The cause is almost always the same: a worn or hardened seal. A rubber seal wears against the rod, takes a permanent set and stops sealing, so it has to be replaced again and again. TIMOL polyurethane seals have wear resistance 2-5 times higher than standard rubber, hold their shape under pressure and resist oils and petroleum products. Below we look at how a seal is built, how polyurethane outperforms rubber and PTFE, how to choose the hardness and how to order custom manufacturing to size.
What a seal is and where it works
A seal is a sealing element of hydraulic and pneumatic machines that prevents leakage of the working fluid and protects the assembly from dust and dirt. It works on the rod or piston of a cylinder, pressing its working lip against the surface, and provides leak-tightness under pressure during reciprocating motion. The force and precision of the cylinder depend directly on its condition.
By design, seals are divided into symmetrical, where both ends are identical, and asymmetrical, where one end is longer for a tighter fit after installation. In a moving seal, the lip is constantly rubbing against the rod, so the wear resistance of the material becomes decisive. This is exactly where polyurethane beats rubber the most: abrasion per DIN 53516 is 38-39 mm³ for polyurethane versus 80-150 mm³ for NBR nitrile rubber.
A polyurethane seal also holds its shape under pressure better thanks to mechanical memory: after the load is removed, the material returns to its original geometry and continues to seal, whereas rubber develops a permanent set over time. Ready-made sizes and manufacturing to drawing are presented in the polyurethane products catalogue.
Polyurethane versus rubber, silicone and PTFE
For hydraulic seals the material is chosen by three criteria: wear resistance under friction, oil resistance and lip strength under pressure. Polyurethane surpasses the classic alternatives in exactly this combination. Rubber is leak-tight but wears quickly; silicone is heat-resistant but weak; PTFE is inert but creeps under load.
Against NBR nitrile rubber, the main material for oil seals, polyurethane has lower abrasion (38-39 mm³ versus 80-150 mm³) and double the tensile strength (39-87 MPa versus 10-25 MPa), so the seal lip keeps its geometry longer. Silicone rubber has a wide temperature range, but its abrasion is 150-350 mm³ and strength only 5-12 MPa, so it is unsuitable as a loaded moving seal. PTFE is chemically inert and has minimal friction, but its abrasion of 200-500 mm³ and creep under pressure limit its use to anti-friction rings rather than load-bearing seals. In terms of oil resistance, polyurethane and PTFE are on the same excellent level, whereas SBR and natural rubber do not withstand oils.
Seal material comparison table
| Material | Abrasion DIN 53516, mm³ | Working t, °C | Hardness | Tensile strength, MPa | Oil resistance |
|---|---|---|---|---|---|
| Polyurethane TIMOL | 38-39 | -60…+100 | 85A-95A | 39-87 | excellent |
| NBR rubber | 80-150 | -30…+100 | 40A - 90A | 10-25 | good |
| SBR rubber | 100-200 | -40…+80 | 40A - 80A | 8-20 | poor |
| Silicone rubber | 150-350 | -60…+230 | 20A - 80A | 5-12 | moderate |
| PTFE | 200-500 | -200…+260 | 50D - 65D | 15-35 | excellent |
The table explains why polyurethane is chosen for loaded hydraulic seals: it combines the lowest abrasion among elastomers (38-39 mm³) with high tensile strength (39-87 MPa) and excellent oil resistance. Silicone and PTFE win only on temperature, so they are used for narrow heat-resistant conditions where wear under pressure is not critical.
Engineer’s tip: for high-pressure seals use a harder polyurethane (65-70 Shore D) so the working lip does not extrude into the gap between the rod and the guide. For moderate pressure with an emphasis on tight contact, a softer grade in the 85-95 Shore A range is suitable. Before installation, check the rod for burrs and sharp edges: a damaged rod surface will cut even the most durable seal within the first cycles of travel.
How to choose a seal for a cylinder
Selecting a seal starts with geometry. You need the rod or piston diameter, the dimensions of the seal groove and the seal profile, symmetrical or asymmetrical. These dimensions determine the interference of the working lip and the leak-tightness of the assembly. The most reliable approach is to provide a drawing or a sample of the old seal for accurate reproduction.
Next, the operating conditions are assessed. The working pressure and rod travel speed determine the hardness: the higher the pressure, the stiffer the seal needed to resist extrusion into the gap. The ambient temperature must fall within the range of -60 to +100 °C, and the fluid type (mineral oil, hydraulic fluid, water) affects the choice of polymer formulation.
Installation is done without twisting or damaging the lip, with a proper chamfer at the cylinder entry. A clean rod surface without burrs and the recommended interference multiply the seal’s service life. Technical recommendations on selection and installation are collected in the reference information section.
Custom-size seal manufacturing
Seals of non-standard sizes are made by free casting to the customer’s drawing or sample. First the profile, diameters and hardness are agreed for the pressure and medium, then the tooling is made and the polyurethane is poured. Polymerization takes place at a controlled temperature, after which the product is checked for dimensional compliance.
This approach makes it possible to reproduce a discontinued or imported seal for which a replacement is hard to find, from a sample alone. The order volume is flexible: from a single part for an urgent cylinder repair to serial batches for scheduled maintenance.
To order seals for your cylinder or get advice on choosing hardness and formulation, get in touch through the TIMOL plant contacts. The engineers will help select the profile, calculate the lip interference and arrange manufacturing to your requirements.