This invention relates to the construction of a self lubricating shaft or pivot pin suitable for operating at high temperatures such as in tenter frame apparatus.
U.S. Pat. No. 5,067,214 issued Nov. 26, 1991 to Christopher E. Hosmer and John F. Whaley for a Tenter Framer Apparatus and Method shows a self-lubricating bushing made of a polyamide resin.
U.S. Pat. No. 5,797,172 issued Aug. 25, 1998 to Christopher Eugene Hosmer for a Tenter Frame and Method discloses tenter frame apparatus of the type in which this invention has particular application.
Conventional self lubricating friction or wear devices such as bushings and/or plain bearings are typically composed of a metal housing in which a plain plastic bushing is inserted and secured in a manner preventing rotation of the bushing. Typically a metal shaft is carried in the interior diameter of the bushing. In the case of a tenter frame clip jaw application the currently issued bushings result in the bushing locking down on the metal shaft due to thermal expansion and the bushing wears out prematurely or it locks up preventing the desired function of the jaw.
Attachment of the self lubricating plastic bushing is achieved by insert injection molding, at high pressure and temperature, a plastic sleeve around the metal pin. To enhance adhesion and provide sufficient material for wear, the plastic is molded in a groove of a specific depth which has a knurled surface. The self lubricating bearing member is thus on the shaft which rotates in the metal housing. With this design, thermal expansion at elevated temperature results in contraction of the internal diameter of the plastic sleeve. Because the internal diameter cannot decrease, the forces holding the plastic sleeve on the pin are increased thereby preventing the sleeve bearing from loosening on the metal pin. To minimize the outside diameter expansion of the plastic bearing sleeve, a low coefficient of thermal expansion material with good self lubricating properties is used.
The process by which the hybrid steel and plastic pin is produced is also unique and very cost effective. Although CNC turning of the hybrid pin to the desired outside diameter and dimensional tolerance is possible, a more cost effective and dimensionally and surface finish superior part is produced by centerless grinding the outside diameter. The process of centerless grinding metal and plastic simultaneously is unique and requires a plastic that is dimensionally stable and rigid enough to grind properly and not melt and smear or have metal particles inbedded.
A polytetraflouraethylene is preferred for use in tenter frame jaw pins for its physical, thermal and tribological properties which are required to achieve a long wearing, stable product with reliable performance.
One embodiment of the invention is illustrated in the drawings in which:
Referring to
The tenter clip 11 is a clamping device which includes a clip body 14 which is open on one side with a surface 16 for supporting web material being fed into it. The clip body 14 includes a pair of support arms 17, 18 pivotally supporting a pivot pin 21 for a jaw 22. The pivotable jaw 22 is operated by an external mechanism, not shown, to allow entry and exit of the web material. The jaw 22, and the clip 11, are designed to automatically clamp and hold the web material. The jaws movement is a limited pivotal movement about the axis 23 of the jaw pivot pin 21 which is nonrotatably secured to the jaw 22 by a transverse pin 24 extending through aligned bores in the pin 21 and the jaw 22.
Referring to
Number | Date | Country | |
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Parent | 10352194 | Jan 2003 | US |
Child | 11129229 | May 2005 | US |