Abstract:
A fixture and technique for testing a coil spring selected from a supply of springs. Each spring has an unstressed generally helical configuration and the test determines an excess departure from the helical configuration of a spring under a compressive load. The first end of a selected spring is radially fixed in a first guide while a second end of the spring end is axially retained in a movable second guide that is supported on rollers to allow omni-directional transverse radial motion of the second end. An axial compressive force is applied to the selected spring while allowing the second end relatively free radial motion. When compression induced radial motion of the second end exceeds a predetermined threshold, the second end the selected spring closes a normally opened electrical circuit to inform an operator that the selected spring is excessively out of square and should be rejected.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to quality control systems and more particularly to a concentricity test fixture for helical springs and similar resilient members, and to a technique for assuring that a lot of such resilient members exhibits acceptable compressive attributes such as squareness of the ends under a load. 
   2. Description of the Related Art 
   It is desirable to monitor coil spring quality by checking for departure from a helical configuration and fixtures for accomplishing this for springs in their unstressed state is well documented. For example, U.S. Pat. No. 4,472,882 discloses a test fixture for measuring the squareness of unstressed coil springs by positioning a candidate spring adjacent a calibrated contoured surface of unique geometry and positioning the spring to touch the fixture at various locations. The patented arrangement does not address variations in spring squareness which might occur when the spring is compressed or stretched. 
   Departure from a truly helical configuration under load can have adverse effects. For example, in a mechanism having coil spring loaded poppet valves, excess departure can side-load the valve train increasing friction, affecting performance and decreasing wear life. The ability to test a coil spring or similar resilient member for squareness under load is a salient feature of the present invention. 
   SUMMARY OF THE INVENTION 
   The present invention provides solutions to the above identified problems by facilitating a measurement of a side load imparted by a coiled spring under stress when a first end and a second thereof are held parallel to one another. 
   The invention comprises, in one form thereof, a coil spring testing fixture having a pair of spaced apart spring end engaging jaws that are relatively movable toward and away from one another along a primary spring axis. One jaw is adapted to engage a first end of a spring and the other jaw is adapted to engage a second end of the spring to axially fix the ends relative to the respective jaws while allowing relatively free motion of one end relative to the other end in directions orthogonal to the primary spring axis. An electrical circuit detects excess motion of one end in directions orthogonal to the primary spring axis induced by motion of the jaws toward one another and the resulting compression of the spring. The first end of a spring is laterally restrained by the one jaw so that the second end and other jaw move together in directions orthogonal to the primary spring axis. 
   An advantage of the present invention is that it is simple to operate and provides an answer to the question of spring suitability. 
   Another advantage of the invention is that the fixture does not require optical comparators or other sophisticated equipment but uses a simple hand-held voltmeter with continuity checking capability to obtain information necessary to determine the acceptable characteristics of a spring. 
   A further advantage is that the same fixture may be adapted to various spring sizes by providing pairs of spring guides in pre-designed sizes. 

   
     BRIEF DESCRITPION OF THE DRAWINGS 
       FIG. 1  shows a squareness under load test fixture, partially in cross-section, according to one form of the invention; 
       FIG. 2  is a simplified partially cross-sectional view of  FIG. 1  with the fixture in an opened position and ready to receive a spring for testing; 
       FIG. 3  is a simplified partially cross-sectional view of  FIG. 1  wherein a spring to be tested is located within the fixture; 
       FIG. 4  is a simplified view of  FIG. 1  that includes electrical components for evaluating a test for an acceptable spring under a compressive load; 
       FIG. 5  is a simplified view of  FIG. 1  that includes electrical components for evaluating a test for an un-acceptable spring under a compressive load; and 
       FIG. 6  is a flow chart illustrating a process for sampling a lot or supply of springs for squareness. 
   

   Corresponding reference characters indicate corresponding parts throughout the several drawing views. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings and particularly to  FIG. 1 , there is shown an illustrative test fixture  11  for performing a simple “accept” or “reject” test for coil spring squareness under compressive load for a spring  27 . The test fixture  11  includes a housing made of a top portion or cover  13  and a bottom portion or base  15 . The top portion or cover  13  and a lower or base portion  15  are electrically conductive. However, the cover  13  contains a contoured insulative upper spring guide  17  while the base  15  for the housing supports a lower spring guide  19  that is located on a set of three equiangularly spaced low friction rollers,  21 ,  22  and  23  located on a same radii with respect to an axis  51  and are fixed to the base  15  to engage and support the lower spring guide  19 . The configuration of the upper spring guide  17  is generally circular and is centered on the axis  51  by a tapering frustro conical region or surface  75  located in an upper section of interior sidewall  73  of cover  13 . The upper spring guide  17  is rigidly fixed in place with respect to axial axis  51  through an adhesive interface  53 , as illustrated in  FIG. 2 . The rollers  21 ,  22  and  23  provide a low friction electrically conductive support for the jaw or spring guide  19  and each may include a primary roller ball  77  supported in a ball bearing race illustrated at  79 . The rollers  21 , 22 , and  23  allow the lower spring guide  19  relatively free lateral movement within the cavity  16  formed within the housing fixture and may be made of stainless steel or similar electrically conductive material commercially available as Ball Transfer Units that are known and used in industry, for example, as photocopier slide supports. 
   A non-electrical conductive annular guide  25  functions to align the cover  13  and base  15  and provide electrical isolation from each other when they are closed on one another. A vent  45  is located in cover  13  to allow air to escape from within cavity  16  of the housing when the cover  13  is located on the base  15 . The annulus guide  25  and the upper spring guide  17  may be made from a nylon, acrylic, or similar insulating material while the cover  13 , base  15  and lower spring guide  19  are preferably made of a stainless steel or similar good conductive material. A screw  29  in cover  13  and a screw  31  in base  15  form electrical terminals to connect the electrically conductive portions of cover  13  and base  15  to a continuity testing arrangement such as shown in  FIGS. 4 and 5 . 
   Except for the screw  31  and vent  45 , the cover  13 , base  15 , guide  17  and guide  19  of the fixture of  FIG. 1  exhibit cylindrical symmetry about axis  51 . Of course, the exterior of the fixture may depart from such symmetry, for example, for manufacturing convenience, or to suit a specific installation. The interior sidewall portion  73  of cover  13  and the peripheral portion of lower spring guide  19  are also preferably cylindrically symmetric with respect axis  51  to insure that a resulting test for angular spring orientation of a spring to be tested independent. Note, that the lower spring guide  19  is spaced a peripherally uniform distance from the inner cylindrical sidewall  73  of the cover  13  and is not restrained in that position, but rather is free to move in any direction orthogonal to the axis  51  on the rollers  21 ,  22  and  23  until spring guide  19  comes in contact with the cylindrical sidewall  73 . 
   In  FIG. 2 , cover  13  and base  15  are shown as having been moved axially away from one another to accept a spring  27  that has been selected from a spring lot. A selected spring  27  is precisely located to have its axis collinear with the fixture axis  51  by annular groove  41  on spring guide  17  and annular groove  43  on spring guide  19 . The annular groove  41  acts as a jaw to fix an end of spring  27  to the upper spring guide  17  while annular groove  43  acts as a jaw to fix an opposite end of spring  27  to the lower spring guide  43  such that initially spring  27  is located along the fixture axis  51 , as illustrated in  FIG. 3 , when the cover  13  and base  15  begin to close on one another. Continued closure of the cover  13  and base  15  is achieved by a compressive force being applied to cover  13  and base  15 , as illustrated by arrows  47  and  49  in  FIG. 4  to compress the spring  27  with a preferred stress. When the structure in  FIG. 2  is compared with that of  FIG. 4 , it should be apparent that the cover  13  and base  15  and respective supported spring guides  17  and  19  movable along the fixture axis  51  between the open position for accepting a spring  27  to a closed position wherein spring  27  is compressed. In an acceptable coil spring  27 , the lower spring guide  19  is retained in the generally cylindrical cavity  16  defined by side wall  73  of the cover  13  as shown in  FIG. 4 . 
   In  FIGS. 4 and 5 , an electrical circuit including a galvanometer  33 , battery  35  and leads  37  and  39  is shown as being connected with screw  29  in cover  13  and screw  31  in base  15 . A simple volt/ohm meter or continuity tester as well as any other suitable indicator and/or power source could function in a similar manner to achieve a similar result. If the spring  27  is square (does not exhibit any induced stress in directions perpendicular to the primary fixture axis  51 ), the lower spring guide  19  remains centered on the fixture axis  51  as shown in  FIG. 4 . So long as the lower spring guide  19  is located as in  FIG. 4 , there is no current flow through an indicator such as galvanometer  33  from battery  35  by way of the leads  37  and  39  attached to cover  13  and base  15  because of an open circuit is located between cover  13  and base  15  as represented by the gaps  81  and  83 . However, should the compressive stress  47 ,  49  induce a lateral stress in spring  27 , the lower spring guide  43  is free to move in directions orthogonal to the fixture axis  51  and the spring  27  will experience a resulting lateral strain that moves the lower spring guide  19  toward the cylindrical sidewall  73  of cover  13 . A sufficient lateral travel component in spring  27  will cause the lower spring guide  19  to contact and make an electrical connection with the inner surface of cylindrical sidewall  73 , see  FIG. 5 , to close the circuit and enabling an indicator such as galvanometer  33  to inform an operator that the spring  27  is not square and should be rejected as not possessing a desired squareness characteristic. 
   The process of determining spring squareness of a spring  27  of the present invention is set forth in the flow chart of  FIG. 6  when viewed in conjunction with the structure illustrated in  FIGS. 2–5 . In this process, a sample spring  27  is randomly selected from a lot or supply of springs  55 , as indicated at by step  57 . The number of sample springs  27  selected for testing or sample size may be determined by statistical considerations such as lot size and the desired degree of confidence as to the acceptability of the lot  55  based on acceptability of individual tested springs. An individual spring of the number selected to be sampled is indicated by step  59 . Of course, the lot may be sufficiently small such that an individual spring  27  may comprises the entire selected sample. Step  61  indicates the selected spring  27  has been gripped by the jaws or spring guides  17  and  19  in a manner as shown in  FIG. 3 . A compressive force  47 ,  49  is then applied to cover  13  and base  15  to axially compress the selected spring  27  as illustrated in  FIG. 4  and shown by step  63  in  FIG. 6 . Excess radial motion is sensed for by the electrical circuit through step  65  and, if such radial motion is detected as illustrated in  FIG. 5 , a spring is rejected as indicated by step  67 . Otherwise, the lower end of a spring  27  and lower jaw or spring guide  43  remain spaced from the surface sidewall  73  as illustrated in  FIG. 4 , the indicator  33  remains un-energized and the spring  27  is accepted as shown at  69 . Once a spring  27  has been rejected, another spring  27  from the selected number  59  in the sample or lot  55  for testing, a new sample (which may comprise the entire lot) may be selected from the lot  55 , or the entire lot may be rejected as excessively “un-square.” If a spring  27  that is first selected is accepted as indicated at  69 , further springs  27  may be selected from the number to be tested until the sample select number  59  has been exhausted. Acceptance of an entire sample (or a statistically determined percentage of the sample) dictates acceptance of the entire number of springs in the lot  55  as shown at  71 . If the test is particularly critical (as dictated by a particular application, spring compression force  47 ,  49 , and the dimensions of gap  83 ), rejecting a single spring  27  my dictate an entire lot  55  must be tested and only those individual springs that pass the test for squareness of the above process be retained as acceptable.