Piston Cups

Types and Definitions

Piston Cups

Piston Cups are primarily used on piston heads, plungers or rams in hydraulic or pneumatic service. Its mechanical sealing surface is at its periphery.

Piston Cups are commonly used because of economical cost, excellent service life, efficient assemble, universal acceptance, and simple accessories.


Homogeneous Piston Cups are recommended to handle low pressures (under 150 psi) in air, water, hydraulic fluids, and lubricating oils. Homogeneous Piston Cups stocked by American Seal & Packing are 80 Durometer Nitrile. They are also available in other homogeneous materials such as EPR, fluorelastomer, and silicone.


Fabric reinforced Piston Cups prevent extrusion in medium pressure installations (up to 1000 psi) or where diametrical clearances are greater.

Hydraulic seals

Piston Cup Installation Data

Figure 1 shows a Piston Cups packing (P) attached directly to a ram without the use of a separate backing plate.


This is a proper method, as long as the rules of good design are observed. The cylinder should be correctly finished to close tolerances and the ram of a size and material that can be finished to leave only a minimum working clearance between it and the cylinder wall. The cylinder and ram must be of suitable material so that no scoring of the cylinder will take place.


Due to the nature of the work being done by a ram of this type, wear is inevitable and usually occurs to a great extent on one side. When this wear has progressed to a certain degree, the Piston Cup packing will not be properly supported.


To insure lip contact, Piston Cups generally are designed with the heel .010"-.015" under nominal O.D., and the lip diameter is .020"-.030" over nominal O.D. This insures heel clearance and full lip contact.


Heel clearance is needed so full pressure is not carried by this narrow area, or it wears excessively. Clearance between the piston and cylinder wall must be minimized to prevent extrusion at the cups heel. The seal between the bottom of the cup and the piston head is a compression seal, created by bolting or otherwise fastening the cup between the piston head (or backing plate) and follower plate. Do not over-tighten the follower nut especially when using homogeneous Piston Cups. Over-tightening will cause heel to extrude cause premature failure.

Piston Cup packings are installed with the sealing lips toward the pressure medium, thus the mechanical seals between the piston and the cylinder wall for which the cup is designed is created by the pressure applied against the cup. In the dynamic seal area indicated in Figure 2, make sure there is enough compression, with no gaps, to insure a seal. It is also important to be sure that the heel of the Piston Cup is thick enough to create compression between the areas of B and C.

Figure 2 illustrates two Piston Cup packings (A and A) which are installed back to back against a common backing plate C in a double-acting cylinder. B serves as the follower plate to compress the static seal. A good solid plate is provided which is a piston head. Packings should not be installed without a backing plate between them.

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