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What Is a Hydraulic Seal?

Lori Kilchermann
Lori Kilchermann

A hydraulic seal is a gasket-like device used to seal the hydraulic fluid and pressure inside of a hydraulic cylinder. There are two types of hydraulic seals commonly used on a hydraulic cylinder: a static seal and a dynamic seal. There also are two types of material used to manufacture a hydraulic seal: rubber and polyurethane. The type of material used to manufacture the seal is typically mandated by the amount of pressure or heat that the seal is subjected to.

As hydraulic fluid flows into a hydraulic cylinder, the pressure is contained by a hydraulic seal that is fitted inside of the cylinder. The hydraulic seal is commonly fitted onto the inner cylinder shaft and sealed against the outer cylinder case. The static-type of seal is a non-moving seal that is typically used in cap seal applications or as a flange-type seal or gasket. The static seal is usually fitted within a groove and is able to withstand a great amount of pressure due to its stationary mounting design.


The dynamic style of hydraulic seal is able to withstand movement against its inner opening and is typically used on the inner shaft of the hydraulic cylinder. The dynamic seal allows the inner hydraulic cylinder to slide through its opening while still containing the fluid within the cylinder itself. Another type of dynamic seal is known as a piston seal. The piston seal allows movement against its outer dimension, such as inside of a hydraulic cylinder case. In either version, the seals are machined to such a close tolerance that the cylinder is able to slide through the seal while the hydraulic fluid remains sealed inside of the cylinder.

Rust and erosion on the hydraulic cylinder are the enemy of the hydraulic seal. Rough surfaces due to pitting and chaffing of the typically smooth cylinder can cause the flexible surface of the seal to rip or tear. This can result in a leak or complete blow out of the seal, resulting in loss of hydraulic fluid as well as a weak hydraulic cylinder. A poor seal can allow the pressure to slowly bleed off of a hydraulic cylinder. This will often result in a sagging cylinder or a cylinder that has a reduced lifting or holding capacity.

Occasionally, a replacement hydraulic seal can be purchased that is designed to fit in a recessed position on the hydraulic cylinder. This creates a new sealing surface for the face of the seal to rest on that may be void of pitting and wear. This prevents leaking and eliminates the need to completely rebuild or replace a worn cylinder.

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