Page 54 - HydraulicSealsCatalog

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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 ser-
vice 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 Packing are 80 Durometer Nitrile. They are also avail-
able 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.
Figure 1 shows a Piston Cups packing (P) attached directly to a ram
(R) 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 toler-
ances 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 its wears excessively. Clearance between
the piston and cylinder wall must be minimized to prevent extrusion at the cup’s heel.
The seal between the bottom of the cup and the piston head is a compression seal, created by bolting or otherwise fas-
tening the cup between the piston head (or backing plate) and follower plate. Do not over-tighten the follower nut espe-
cially when using homogenous Piston Cups. Over-tightening will cause heel to extrude cause premature failure.
Piston Cup Installation Data