Banded piston and method of making

A heat-formable fluoropolymer band hot-formed from a first solidified state to a second flowable state from a relatively thin, flat circular washer to jacket a cylindrical piston having at least one inwardly-directed annular groove, said washer, after hot-forming, comprising a tubular sleeve having an outer all for engaging a cylindrical bore of a fluid-actuated shock-absorbing device and an inner wall in surface contact with a pair of lands comprisin8 an outer peripheral surface of said piston and said sleeve further including at least one integral inwardly directed rib extending circumferentially of and essentially filling said annular groove between the sides thereof, at least one side of said rib forming an acute angle with the adjacent inner wall of said sleeve so as to form an interlocking connection between said one side and a correspondingly acute-angled side of said groove, whereby axially-directed fluid forces tending to expand said band relative to said piston are resisted,

• said bend being produced by the process of initially preheating said washer to a temperature substantially greater than ambient temperature but not exceeding the temperature at which said material reaches its second flowable state, promptly thereafter hot-forming said band onto said piston at a temperature sufficiently high to achieve said second state while simultaneously pressing said material radially inward to cause the material to after essentially fill said groove and finally cooling said to a lower temperature at which said material solidifies, whereby it remains interlocked with said acute angle side wall of said groove.

In a method of forming a relatively flat circular fluoropolymeric washer with a central circular opening therethrough into a band over a cylindrical multiple-grooved piston so as to provide a relatively friction-free piston adapted to be reciprocable in a cylindrical bore of a fluid cylinder, said central opening being slightly smaller in diameter than said piston diameter but essentially the same diameter as said piston at the bottom of an anchoring groove provided adjacent one end of said piston, comprising the steps of rapidly expanding said washer at its central opening, pressing said expanded washer over the end of said piston and relaxing the washer to enable the central opening to contract and grip said piston in said anchoring groove;

• drawing said piston and washer into a heated cylindrical chamber essentially of the same diameter asthe bore of said fluid cylinder to hot-form the washer to the cylindrical shape of said piston;
• maintaining the piston and washer in said heated chamber for a time period and at a flow-inducing temperature sufficient to cause the fluoropolymer to flow into other grooves of said piston and thereafter cooling the band while it is maintained at said diameter to cause the fluoropolymer to set and firmly grip the outer periphery of said piston, the improvement comprising:
• initially heating said washer to a preheating temperature substantially greater than ambient temperature, but not exceeding said flow-inducing temperature prior to drawing the piston into said heated chamber, whereby to enhance flowability of the fluoropolymer into said grooves and thereby reduce the elastic memory of said band to return toward its flat washer state.