Abstract:
A tenter clip has a jaw pivot shaft having a pair of molded bearing surfaces of superior strength, wear resistance and stability under high temperature operating conditions. The method for manufacture of the jaw pivot shaft includes knurling recesses formed in the shaft prior to applying plastic bearing material to the recesses by injection molding and finishing machining the pin by centerless grinding.

Description:
TECHNICAL FIELD 
   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. 
   BACKGROUND OF THE INVENTION 
   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. 
   BRIEF SUMMARY OF THE INVENTION 
   Attachment of the self lubricating plastic bushing is achieved by insert injection molding, at high pressure and temperature, a thin, self lubricating plastic sleeve around the metal pin. To enhance adhesion and provide sufficient material for wear, the plastic is molded in a groove or recess of a specific depth which has a roughed up or knurled surface. The  self lubricating bearing member is thus on the shaft which rotates in the metal housing. Thermal expansion at elevated temperature results in expansion of the internal diameter of the plastic sleeve; however, the roughed up surface prevents rotation of the plastic sleeve. Expansion of the plastic bearing sleeve is minimized by using a thin layer of plastic which has a low coefficient of thermal expansion. 
   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 in-bedded. 
   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. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One embodiment of the invention is illustrated in the drawings in which: 
       FIG. 1  is a perspective view of a tenter frame with parts separated and broken away for illustration purposes; 
       FIG. 2  is an end view of the tenter frame in an assembled condition; 
       FIG. 3  is a side view of a metal pivot shaft initially machined to provide two reduced diameter recesses;  
       FIG. 4  is a side view of the shaft after the recessed areas of the shaft have been knurled; 
       FIG. 5  is a side view of the shaft after a plastic material has been applied to the recessed areas by injection molding; 
       FIG. 6  is a side view of the shaft after it has been machined by centerless grinding; 
       FIG. 7  is a section view taken along the line VII—VII in  FIG. 6 ; 
       FIG. 8  is a side view of a centerless grinding apparatus, and 
       FIG. 9  is a top view of the apparatus shown in  FIG. 8 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIGS. 1 and 2 , the invention is illustrated in a tenter clip  11  which with other clips, are connected in series by a link chain  12  thus providing a tenter chain  13 . Tenter chains are provided in opposing pairs to grip the edge of a continuous web of flat material such as woven cloth and to simultaneously stretch the material in a transverse direction while conveying it through a process oven as a finishing operation to its manufacture. 
   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  FIGS. 3–7  the process for manufacture of the jaw pivot pin  22  is illustrated by showing the steps of manufacture.  
     FIG. 3  shows the pin  22  after the pin is rough finished to a predetermined oversize diameter and a pair of reduced diameter recesses  31 ,  32  are formed near its axially opposite ends. The recesses  31 ,  32  having an axial dimension matching the bearing surfaces of the arms  17 ,  18  of the tender clip  11 . 
     FIG. 4  shows the pin  22  after the reduced diameter surface of the recesses  31 ,  32  have been knurled. 
     FIG. 5  shows the pin  22  after sleeves  34 ,  36  of high temperature resistant plastic, such as polytetraflouraethylene has been applied by injection molding to the reduced diameter areas  31 ,  32  of the pin  22 . The oversized pin  22  is next machined by centerless grinding to a desired finished diameter as illustrated in  FIGS. 6 and 7 . 
     FIGS. 8 and 9  illustrate the centerless grinding step. The shaft  22  is supported by a work blade  41  and is sandwiched between a grinding wheel  42  and a regulating wheel  43 . As shown in  FIG. 9  the pin  22  is guided by work rest guides  46 ,  47 ,  48 ,  49  during a machining operation.  
   PRACTICAL APPLICATION OF THE INVENTION 
   This application has particular application in stenter clips which are connected in series by a chain to form a tenter chain. Tenter chains are used in opposing pairs to grip opposite edges of a continuous web of flat material, such as a woven cloth, and simultaneously stretch the material in opposite directions while conveying it through a relatively high temperature process oven as a finishing operation in the manufacture of the processed material. 
   Wear on the bearing surfaces has long been a problem in tenter clip design because lubrication of the bearing surfaces is impractical, mainly because the bearing surfaces are above the web material and it is virtually impossible to lubricate the bearing surfaces without lubrication spilling onto the web material. Since these bearing surfaces are typically not lubricated, they are made of metal which resists high temperature and are expected to wear significantly. Frequent repair of the clip is required to keep the clips within operating tolerances. The repair of some clip designs require reboring the pivot pin hole and a new larger diameter pivot pin. The repair of clips using replaceable steel bushing are repaired by replacing the bushing and it is often to replace the pin as well. 
   Various plastic materials with improved strength and wear characteristics and the ability to withstand high temperature have been developed. They have been tried and tested with limited success. Very close tolerances are necessary for proper operation of a tenter clip. Steel bushings for tenter clips are commonly ground on their outside diameter and honed on their inside diameter to achieve the required tolerances. It is difficult and expensive to obtain these desired tolerances with plastic bushings. Most plastic materials have a much higher coefficient of thermal expansion than metals used in tenter clip components. This tends to cause the  bushings to lose clearance and lock on the pin preventing rotation. Increasing the cold clearance solves this problem but creates manufacturing and functional problems. 
   The normal method of installation of bushings is by interference press fit. Plastic bushings tend to deform when press fit, thereby losing most of the interference press fit holding ability. This allows rotation and or axial movement of the bushing, which is unacceptable. 
   The tenter pin of this invention solves all the beforementioned problems in a cost effective manner. Tolerances are not a problem because the plastic bearing material is molded into a groove or recess in the pin and the pin and the plastic inserts are centerless ground as a unit. By molding the plastic into a recess with a knurled bottom surface the plastic insert is restrained from rotation and axial movement relative to the pin. Thermal expansion of the plastic insert is minimized because the plastic insert is radially thinner than that of a stand alone plastic bushing. By forming the thin plastic bearing in recesses in the pin, the pin and inserts can be centerless ground as a unit to a final diameter. This facilitates manufacture and pin installation.