Sealing Shielding

Optimum Sealing Performance, Even in Low-Pressure Conditions With the HL Rod Seal

HL Stangendichtung

Operators of fluid power systems increasingly demand friction-optimised piston and rod seals for hydraulic cylinders. High friction not only means high use of energy, but can also lead to premature seal failure due to wear.  Both of these aspects are costly as well as harmful to the environment.

The HL rod seal by Parker Prädifa is a friction-optimised sealing solution featuring an effective functional principle: the single-acting seal has pressure-activated, cascading dynamic sealing edges. This design clearly reduces static and dynamic friction in hydraulic cylinders and increases the efficiency of hydraulic systems. The P6030 polyurethane material superbly supports the profile geometry
and its action principle. P6030’s main advantages are excellent temperature,
extrusion and wear resistance. 


Before: the lower the pressure, the more critical the influence of friction

Various parameters influence friction of hydraulic seals. The size of the contact area between the seal and the respective sliding partner is one of the key factors: the larger the contact area of the seal, the higher the amount of static and dynamic friction. The system pressure inside a hydraulic system determines the friction properties required of the seal. For example, friction at lower system pressures or in differential cylinders with low pressure differences is significantly more critical than in cylinders subjected to higher pressure loads. 


With the HL rod seal: low friction even in low- or no-pressure conditions

When conventional U-rings are used, a larger portion of the dynamic sealing area will typically contact the piston rod surface even at lower system pressures. By contrast, in the case of the HL rod seal, the sealing edges of the dynamic sealing lips will successively contact the mating surface as system pressure rises. This significantly reduces friction in the pressure-less state and in low-pressure conditions. The small contact area reduces the generation of heat and friction as well, thus enabling higher travel speeds.

Depending on the system pressure applied, the individual sealing lips are activated by the deformation of the seal’s cross-section. This in turn reduces the amount of drag oil on the rod surface that occurs during the cylinder stroke, and thus increases the sealing effect. Additionally, in case of slow travel speeds, the risk of stick-slip is nearly excluded.

HL Stangendichtung HL






Result: optimum friction, sliding and sealing performance in all pressure conditions

Hence the sealing performance increases with the number of sealing edges and their effects. At the same time, dynamic friction slightly increases as the contact area enlarges. Overall, however, it remains at a very low level.
Since only the primary sealing edge is engaged in pressure-less conditions the new functional principle minimises the high break-away friction typically occurring after prolonged downtime. This phenomenon is also known as the “Monday morning effect”.


Einbauzeichnung HL

Installation examples

The new HL rod seal can be used as both a single seal with a wiper and as a secondary seal in a sealing system behind a primary or buffer seal.


Advantages of the HL rod seal

•          Extremely low friction
•          No stick-slip, even in case of low travel speeds
•          Optimum sealing function
•          High wear resistance
•          High extrusion resistance
•          High temperature resistance
•          Easy fitting
•          Short fitting space
•          Also ideally suited for use as a secondary seal in a sealing system




More information:

Brochure HL Rod Seal Low friction - pressure controlled

HL Rod Seal Product Detail Page

More details on our Prädifa Technology Division


marketing communications managerArticle contributed by

Christine Stehmans
Marketing Communications Manager,
Engineered Materials Group Europe






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