Seals For Hydraulics (CSI for PSI)


Seals For Hydraulics (CSI for PSI)

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Jesse Thomas



Hydraulic Seals are a widely used component in hydraulic systems, as they help to seal the opening between various components. Let’s spend some time going over the basic categories of hydraulic seals, the materials, and the specific seals for the most common conditions.


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TWO Categories of Hydraulic Seals:

A hydraulic cylinder is a linear actuator that transfers pressurized fluid power to a piston which drives a rod. The most important part of the cylinder may be the seals which make sure that all or most of the force is transferred only to the piston. The seals in a hydraulic cylinder can be split into two categories:

  1. STATIC: which does not see movement
  2. DYNAMIC: which will see movement

Under static are classified o-rings and packings, whereas dynamics seals would be the seals, bearings, and wipers.


These seals, bearings, wipers, and o-rings can be made of many different materials. Some of which could be elastomers or rubbers, synthetic or natural, urethanes which are more resilient or engineered plastics such as PEEK and PTFE, also known as Teflon.

Piston & Rod Sealing:

The piston and the rod seals are similar in most cases and are classified under low, medium, or high-pressure. Pressure is not the only application detail required to design a seal. Among others would be the surface finish on the bore and on the rod, where a rougher bore or rougher rod might cause premature wear on the seal.

LOW-Pressure Conditions

In low-pressure conditions, rubber can be commonly used as a sealing material. That’s because rubber doesn’t have the tensile strength and resilience to damage that many of the other engineered materials do and it’s quite a bit cheaper usually. If a bearing is required such as in this diagram here to stop the rod from doing damage to the cylinder or vice versa, it can be made of PTFE which has internal lubricity which makes it easier to slide without causing too much friction.

MEDIUM-Pressure Conditions

Medium pressure seals are often made of urethane which has more resiliency to wear. And in that case, also pressure. Back up rings are often used in medium pressure seals because they decrease the clearance between the piston and the rod and the bore which makes it more difficult to extrude material through the gap.

HIGH-Pressure Conditions

In a high-pressure seal, backups are mostly required. And in some seals, they’re actually included in the assembly, such as this T-Seal here which includes two backups stopping the rubber from extruding out through the gap. Furthermore, advanced materials such as PEEK might be used. PEEK has extremely high hardness and has great resiliency to being extruded out.


Amongst these materials, each has its own drawbacks. For instance, elastomers which are required to resist compression – forming a seal – can undergo compression set after a certain amount of time, pressure, and heat. This will render the seal failed and it will no longer be able to form that seal against its resisting surface.

Urethanes suffer the same but are also used in higher pressure applications. For that reason, it’s common to have a failed urethane seal be extruded through the small gap between the meeting components.

Plastics are also subject to extrusion, but in addition, they can cold flow. That’s when the geometry of the seal might change even though it’s only time that’s been acting on the material.

THREE Common Seals:

Three common seals are shown here. Starting from the beginning would be the most common U-Cup.

  1. The U-Cup is an extremely versatile seal that forms when under pressure from the left side – a seal on the top and bottom as the lips are pressurized out due to the gap.
  2. To the right, you have the Capped T. Although the PTFE or other material perhaps nylon seal on the top does not have its own internal resistance and ability of form a seal, the elastomer that energizes it from below forces it against the bore and creates a great seal. The internal lubricity of the PTFE helps it float with low friction.
  3. As before, the Capped T-Seal performs a perfect seal – a static seal – on the base of its gland. Whereas on the top it will form a seal using an o-ring like profile and it’s protected by two PEEK backups. The potential pressure rating of this seal might 10,000 psi.

And that’s seals for hydraulics.

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