LD Design Piston Cup Design & Installation John Tomme 2017-02-08T11:50:25+00:00
EPM Piston Cup Packings are designed specifically for pistons in reciprocating pumps and pneumatic and hydraulic cylinders. They offer the best service life for this type of application, require a minimum recess space, minimum recess machining and are simply and quickly installed.
Homogeneous piston cups are available in a variety of compounds for specific applications. Fabric piston cups have cotton, asbestos or other suitable fabric frictioned with synthetic elastomers to meet various service conditions.
Homogeneous piston cups are generally recommended for low pressure service applications since their greater flexibility at the sealing lip makes them more responsive to light pressures. Fabric reinforced cups with their more rigid construction are recommended for higher pressure services up to 3000 psi.
Design & Installation
EPM Piston Cup Packings incorporate an exclusive square cut heel design that eliminates binding in service and at the same time strengthens the hinge area at the heel. Special lip design provides proper interference (when installed) to hold low pressures.
By designing the back-up plate with a shoulder of proper diameter and height, not only is the centering of the packing automatic, but a controlled squeeze is applied on the packing to prevent it from working loose under pressure.
Where shouldered piston rods are used, it is recommended that an O-Ring static seal be installed to prevent leakage along the rod. This eliminates the need for employing the more expensive tapered piston rod ends and piston assembly. When large diameter cups are used (4″ O.D. or over), several equally spaced bolts or screws are recommended in order to secure the cup installation firmly in place.
To ensure proper operation and long service life of piston cup packings, avoid any design in which the packing must pass over corners, holes, ports or over a step diameter arrangement. For best results cylinder bores should be finished to 16 RMS.
It is recommended that the back-up plate be made with close clearances and with square corners to prevent cup extrusion. Excessive clearance between it and the cylinder walls results in early cup failure, because of the extrusion of the heel of the cup into the clearance gap.
A controlled squeeze or compression to the bottom of a piston cup is essential. Over-compression is the most common cause of premature failure of piston cups. When the bottom or base of the cup is compressed the material moves outward radially. If more squeeze than required to seal is used, the cup material is forced into the bore of the cylinder causing high friction, excessive wear and possible toe-in of the lips.
An installation squeeze of .005″ to .025″ is all that is necessary to seal the bottom of the cup to the piston. This squeeze can be controlled with a metal shoulder on the piston. A metal spacer washer should be used if the piston or follower does not incorporate the shoulder.
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