Abstract:
An improved shiftable clamp ( 12 ) adapted for connection to a fixture ( 10 ) is provided, wherein the clamp ( 12 ) is selectively operable to engage and hold a workpiece ( 14 ) to the fixture ( 10 ). The clamp ( 12 ) includes a tubular body ( 22 ) and a piston ( 24 ) telescopically received within the body ( 22 ). Movement of the piston ( 24 ) is guided and controlled by way of a cam assembly ( 26 ) including a cam track ( 64 ) formed in the piston ( 24 ) and an associated cam track follower ball ( 66 ) mounted on the body ( 22 ). A spring unit ( 74 ) is also provided on the body ( 22 ) and includes a spring ( 82, 96 ) serving to self-center and bias the ball ( 66 ) into the track ( 64 ). This insures that the ball ( 66 ) smoothly rotates during movement of piston ( 22 ) and reduces wear and premature clamp failure.

Description:
RELATED APPLICATIONS 
   This application is a continuation of and claims priority benefit to U.S. patent application Ser. No. 10/802,229, filed Mar. 17, 2004 entitled SWING CLAMP APPARATUS WITH SPRING BIASED CAM ASSEMBLY, which is hereby incorporated into the present application by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is broadly concerned with improved clamps used for clamping workpieces to fixtures, and especially so-called swing clamps which simultaneously move in axial and rotational directions to allow easy placement and removal of workpieces. More particularly, the invention is concerned with such clamps including a shiftable piston equipped with a workpiece-engaging outer head, where piston movement is guided and controlled via an internal cam assembly made up of a specially configured cam track and cam follower ball arrangement. Such cam assemblies are provided with spring units serving to bias and self-center the follower balls into the associated tracks, providing many operational advantages including increased clamp speeds and reduction in clamp wear and damage. 
   2. Description of the Prior Art 
   Hydraulic clamps are commonly used in manufacturing operations to hold and clamp workpieces to stationary fixtures, so that the workpieces may be machined or otherwise worked upon. Typical hydraulic clamps include a cylinder body adapted for attachment to a fixture and a piston telescopically received within the cylinder body for movement between an retracted, clamping position and an extended, release position. A clamping head is attached to the distal end of the piston for holding and clamping workpieces to the fixture when the piston is in its retracted, clamping position. Commonly, several such clamps are mounted to a single fixture so that a workpiece may be securely held at several locations while it is being worked upon. 
   Swing clamps are hydraulic clamps that include swinging mechanisms serving to swing the clamping heads away from the workpiece when the pistons are extended to their release positions. Swing clamps make it easier to load and unload workpieces from fixtures, especially in confined spaces. 
   One type of swinging mechanism used in swing clamps is a cam assembly having a curved cam track or groove formed in either the piston or the cylinder body and a corresponding cam follower ball attached to the other of the piston and cylinder body. The follower ball moves along the curved cam track when the piston is shifted which serves to rotate the piston and clamping head as described. 
   Conventional cam assemblies in swing clamps are subject to premature wear over time that interferes with the swinging operation of the clamps. Specifically, when the cam follower ball moves in the track, it is subject to circumferential forces tending to push the ball to the sides of the groove. Over time, the cam ball wears down the edges of the track and creates dimples along the length thereof. The dimples and worn regions of the cam track often catch the ball during piston movement, creating a “choppy” clamp operation. When a clamp is used in severe conditions, its cam ball may completely wear down the edges of the track, causing the ball to completely roll out of the groove. 
   Excessive wear on the cam grooves of a clamp can be a serious problem. In many clamping operations, it is important for the clamping head to swing to a precise location away from the workpiece, and then return to the same exact starting position when the clamp is shifted to its clamping position. When the cam groove on a clamp become worn, the swing clamp can no longer achieve this precise and repeatable swinging movement. Thus, the entire swing clamp must be replaced, even though the remaining parts of the clamp are in good condition. 
   U.S. Pat. No. 5,820,118 describes a decided improvement in the swing clamp art. In this patent, uses may of a special cam track design which inhibits the cam follower ball from prematurely wearing the cam track edges. Specifically, the cam track described in the &#39;118 patent includes a central arcuate region and a pair of substantially planar side faces extending tangentially from the central arcuate region. This construction forces the cam follower ball to be more centrally seated within the cam track without pushing up against the edges of the cam track. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to further improvements in shiftable clamps, and particularly the swing clamps described above. Broadly speaking, the clamps of the invention include a hollow body for attachment to a fixture, with the body presenting an interior wall. A piston is telescopically received within the body for movement between clamping and released positions. A cam assembly is used for guiding and controlling relative movement between the piston and body, with the cam assembly having a cam track formed in one of the interior wall of the body and the outer wall of the piston, and a cam follower received within the cam track and attached to the other of the interior wall of the body and the outer wall of the piston. The specific improvement of the invention involves the use of a spring for biasing the ball toward the cam track. It has been discovered that use of such a spring affords a number of operational advantages, including improved clamping speeds and reduced wear. 
   In preferred forms, the biasing spring forms part of a spring unit having a follower-engaging component with a spring remote from the follower, thereby biasing the follower through the component. The spring may be of any desired construction, for example a bellville spring or a small coil spring. In the usual case, the cam follower is a ball and the spring unit is mounted within a recess on the clamp body; the component has an arcuate face in direct engagement with the ball, whereas the spring is within the recess. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an isometric view of a fixture equipped with a plurality of clamps in accordance with the invention, shown with the clamps engaging and clamping a workpiece to the fixture; 
       FIG. 2  is a vertical sectional view of a preferred clamp of the invention; 
       FIG. 3  is a fragmentary sectional view taken along line  3 — 3  of  FIG. 2  and illustrating the construction of a bellville spring assembly used for biasing the cam follower ball into the cam groove of the piston; 
       FIG. 4  is a sectional view similar to that of  FIG. 3 , but depicting the use of a coil spring assembly; and 
       FIG. 5  is a sectional view similar to that of  FIG. 3 , but illustrating the use of a removable sleeve forming a part of the overall clamp body. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Turning now to the drawings,  FIG. 1  illustrates a fixture  10  equipped with a plurality of clamps  12  adapted to releasably hold a workpiece  14  in position on the fixture  10 . As illustrated, the exemplary fixture  10  includes a base  16  supporting an upright mounting box  18 , the latter having a workpiece-supporting wall  20 . The clamps  12  are threadably secured within threaded bores provided in wall  20  as will be described. Briefly, in operation the clamps  20  are selectively movable between the clamping position depicted in  FIG. 1  to thus hold workpiece  14  in place, and a retracted, swung-away position allowing removal of the workpiece  14  after it is worked upon, and positioning of another workpiece  14  in its place. 
   In more detail, the body  22  has an elongated segment  30  presenting an inner wall  32  as well as a threaded exterior wall  34 . Each clamp  12  includes an elongated, tubular body  22  together with a piston  24  telescopically received within the body  22 , and a cam assembly broadly referred to by the numeral  26  for guiding and controlling relative movement between piston  24  and body  22 . As shown, each piston  24  supports an outer clamping head  28  adapted to engage workpiece  14  which mates with the clamp bores in wall  20 . A recess  33  is formed in segment  30  and extends outwardly from wall  32  as shown. The base of segment  30  is internally threaded at  36  and receives a correspondingly threaded cup-shaped plug  38 . The body  22  also has a somewhat enlarged outer portion  40  remote from plug  38  which has an inner wall  42  concentric with wall  32 , thus defining an annular stop shoulder  44 . The portion  40  has an inner sealing ring  46  and retainer  48 . Finally, the portion  40  includes a hydraulic fluid port  50  which-communicates with passageway  52 . 
   Piston  24  includes a base  54  equipped with a sealing ring  56  engaging surface  32 , a guide section  58  presenting an outer surface  59  and extending upwardly from base  54 , and a rod  60  extending beyond portion  40 . A relatively large translation spring  62  is seated within plug  38  and engages the underside of base  54  as shown. As illustrated in  FIG. 2 , the section  58  has a slightly reduced diameter as compared with base  54  but has a greater diameter than rod  60 . 
   The assembly  26  includes a plurality (here three, two of which are shown) of circumferentially spaced apart cam tracks  64   a,    64   b . . .  formed in the outer surface  59  of piston section  58 . The preferred tracks  64  are configured for guiding the piston along different paths during piston movement. For example, the track  64   a  is configured so as to cause piston  24  (and thereby head  28 ) to swing during retraction and extension of the piston, whereas track  64   b  is essentially rectilinear so that the piston  22  merely reciprocates without any swinging movement. In addition, the assembly  26  includes a cam follower ball  66  which is secured to body segment  30  adjacent inner surface  32 ; the ball  66  is seated within one of the tracks  64  as will be readily apparent from a consideration of  FIGS. 2 and 3 . 
   One possible geometry of the cam tracks  64  and the follower ball  66  is described in detail in the referenced U.S. Pat. No. 5,820,118, incorporated herein by reference. Briefly however, the cam follower has an outer peripheral surface presenting a radius of curvature R, whereas the cam track includes a central arcuate region  68  having a radius of curvature R′ substantially equal to the radius R. Moreover, the track  64  has a pair of opposed, substantially planar side face  70 ,  72  extending from arcuate region  68 , with the side faces  70 ,  72  each having a proximal end converging into the region  68  and an opposed distal end that diverges from the region  68 , with the distal ends also diverging from one another. In other embodiments, the cam track has a geometry which matches that of the cam follower. Specifically, the cam track has essentially the same radius of curvature as the corresponding cam follower. 
   The preferred assembly  26  also has a spring unit  74  seated within the recess  33  which biases the ball  68  toward and into the adjacent track  64 . Referring to  FIG. 3 , the unit  74  includes a force-transmitting annular component  76  having an arcuate face  78  engaging ball  66 , and an opposite, substantially planar face  80 . In the depicted embodiment, a bellville spring  82  is disposed between the inner surface of recess  33  and face  80 , and thereby biases ball  68 . 
     FIG. 4  illustrates a somewhat modified embodiment wherein a resilient elastomeric plug  84  is used to house a spring unit  86 . In this case a through-bore  88  is provided in the segment  30  and is configured to receive plug  84 . The latter includes an annular wall  90  defining a recess  92 . The unit  86  is similar to unit  74  in that it includes a component  94  identical with component  76 . However, in this case a coil spring  96  is seated within recess  92  and engages the planar face of component  94 . 
     FIG. 5  illustrates a still further embodiment of the invention wherein the body  22   a  is formed using an outer tubular wall  98  together with an inner, replaceable sleeve  100  the latter being equipped with a recess  33   a.  The recess  33   a  houses the identical spring unit  74  described with reference to  FIG. 3 . Use of a replaceable sleeve  100  permits ready repair of a clamp  12  in the field. 
   Each clamp  28  is in the form of an elongated element  101  presenting a workpiece-engaging underside  102 . A screw  104  is employed to attach each element  101  to the outer end of each rod  60 . 
   After the clamps  12  are installed on wall  20  of fixture  10  by threading the segments  30  thereof into the pre-drilled holes in wall  20 , the clamps may be used for holding workpieces  14  in place. Turning to  FIG. 2 , it will be seen that the spring  62  of each clamp  12  serves to bias the corresponding piston  22  to its extended position where, in the illustrated embodiment, the head  28  is swung laterally to a clearing position allowing removal and replacement of a workpiece  14  onto the fixture. When this is done, the individual clamps are actuated by application of hydraulic fluid through the ports  50 , whereupon the pressurized fluid passes downwardly between the walls  32 ,  59  and engages base  54 , thereby moving the piston downwardly against the bias of spring  62 . During such movement of the pistons, the heads  28  are swung laterally owing to the configuration of cam tracks  64   a  and follower balls  66  until the heads come into proper holding relationship with the workpiece  14 . After operations on workpiece  14  are completed, the pressurized hydraulic fluid is relieved, thereby permitting the springs  62  to return the individual pistons  22  and clamps  28  to their extended and swung-away positions. 
   The provision of spring units in accordance with the invention provides a number of significant operational advantages. First, the spring units insure that the biased cam follower balls  66  self-center in the associated tracks  64   a.  Thus, the balls  66  are constrained in both vertical and horizontal planes, providing a stationary point for the cam tracks  64   a  for proper guidance through both axial and rotary motion. In essence, the components  76  act as bearing races allowing the balls  66  to rotate as the pistons move through their strokes, while at the same time biasing the balls  66  so that they remain fully engaged in the tracks  64   a.    
   This construction reduces the static and dynamic frictional forces generated between the balls  66  and the track  64   a,  especially during starting movement of the pistons, allowing smoother tracking and essentially eliminating the tendency of the balls to drag within the tracks, rather than to rotate. The spring units give an even load distribution and, owing to the self-centering action of the spring units, the balls  66  are inhibited from riding up on the edge of the tracks. At the same time, the design allows a degree of ball float within the tracks to compensate for manufacturing and operational variations. It has been found that cam damage during inadvertent arm contact, a frequent problem in the art, is reduced with the present invention. Consequently, higher clamp speeds are possible as compared with current designs, while at the same time eliminating the wear and operational problems commonly encountered with conventional clamps. 
   Although not shown in detail, it will be appreciated that the clamps of the invention may assume a variety of different configurations. For example, while in the illustrated embodiment the hydraulic clamp is single acting, making use of the translation spring  62 , the invention is not so limited. Thus, it is well within the skill of the art to employ a double acting hydraulic design wherein pressurized hydraulic fluid is used to move the piston  22  in both directions. Additionally, while a rotatable cam follower ball is preferred, other follower designs could be employed.