Abstract:
A method of monitoring lateral force of a coil spring having a body and opposing end coils is disclosed. A fixture includes a base defining a planar surface with a shaft that extends from said planar surface at a normal angle to said planar surface. An axis defined by the coil spring is aligned with the shaft. Angular displacement from the shaft of an end coil is measured, and the angular displacement is correlated with a lateral force value.

Description:
PRIOR APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 61/973,454 filed on Apr. 1, 2014. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally toward coiled compression springs. More specifically, the present invention relates toward a method of optimizing performance of a coiled compression spring. 
       BACKGROUND 
       [0003]    Compression springs have been used for many years in various capacities. For example, coiled compression springs are used in suspension systems, brake actuators, and various other mechanical devices where axial force generated through the compression of the spring is used. Generally, the desired force, or K-value of a spring is used to affect location and movement of the mechanical device. Hooke&#39;s Law is a principle of physics that defines the actual force of a compressed spring based upon the distance a spring is compressed. The axial force is a desired result of compressing a coiled spring. However, a phenomenon known as lateral force, has been an historic problem that heretofore has not been solved. 
         [0004]    Lateral force is the force generated by a compressed spring that is lateral to the axial force along a spring axis, which is defined by a body of a coiled compression spring. Lateral force is known to cause premature failures in brake actuators and other mechanical devices that make use of the axial force generated by the compressed spring. It has been an unknown phenomenon as to what mechanical feature of the spring has caused unwanted lateral force. Therefore, no solution to the lateral force phenomenon has, to date, been developed. Therefore, it would be desirable to identify a cause of lateral force of a coiled compression spring and solve the problem of lateral force on mechanical devices. 
       SUMMARY 
       [0005]    The method of monitoring lateral force of a coil spring having a body and opposing end coils is disclosed. A fixture having a base that defines a planar surface with a shaft extends from the planar surface at a normal angle to the planar surface. An axis defined by the coil spring is aligned with the shaft. Angular displacement from the shaft of one of the end coils is measured and correlated with a lateral force value of the spring. 
         [0006]    It has been determined that the phenomenon known as lateral force generated during compression of a coil spring is the result of angular displacement of an end coil of the coil spring from the axis defined by the coil spring. The cause of lateral force generated by a coil spring was previously unknown. Therefore, it was determined that maintaining the angular displacement of at least one of the end coils of a coil spring would control the amount of lateral force generated by compression of the coil spring. For example, maintaining an angular displacement of at least one of the opposing end coils from the spring axis of between about −1.5° and 2.5° from a plane that is normal to the spring axis controls the lateral load generated by the coil spring below a predetermined threshold believed to substantially eliminate damage caused to mechanical devices from lateral force. As such, a desirable manufacturing process has been developed that solves the historical lateral force problem of compression coil springs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Other advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
           [0008]      FIG. 1  shows one embodiment of a coil compression spring that is subject of the present invention; 
           [0009]      FIGS. 2A and 2B  show cross-sectional views of a coil compression spring in use in brake actuators; 
           [0010]      FIG. 3A  shows a front view of an apparatus of the present invention; 
           [0011]      FIG. 3B  shows a side view of the apparatus of the present invention; 
           [0012]      FIG. 4  shows a plan view of a measurement assembly; and 
           [0013]      FIG. 5  shows chart indicating statistical test data. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Referring to  FIG. 1 , an exemplary spring that is subject to the present invention is generally sown at  10 . The spring  10  is of the coiled, compression-type where stored energy as the result of compression is generated along spring axis a. The spring  10  includes a first end coil  12  and an opposing second end coil  14 . A body  16  is disposed between the first end coil  12  and the second end coil  14 . While the spring  10  represented in  FIG. 1  is of the barrel-type having a body  16  with a greater diameter than the end coils  12 ,  14 , it should be understood by those of ordinary skill in the art other coil springs are subject to the present invention including, for example, tubular springs where the diameter of the entire expanse of the spring is generally constant. 
         [0015]    The first end coil  12  is identified at a first distal end  18  and extends around the spring axis about 180°. Likewise, the second end coil  14  extends from a second distal end  20  to about 180° around the spring axis a from the second distal end  20 . 
         [0016]    As set forth above, the primary function of the coil spring  10  is to store energy upon compression that is translated along spring axis a. However, lateral force L has historically been an uncontrolled phenomenon of the coil spring  10 . Lateral force L generated by a coil spring  10  is known to cause structural defects in a related component such as, for example, brake actuators, suspension systems, and other mechanical devices making use of spring force K in the axial direction. 
         [0017]    One such example will now be described as shown in  FIGS. 2A and 2B . A relevant portion of a brake actuator is generally shown at  22 . The coil spring  10  is disposed within power spring housing  24 . The housing  24  is secured to a flange case  26  by way of a crimp  28 . A diaphragm  30  is secured within the crimp  28  between the power spring housing  24  and the flange case  26 . A power spring chamber  32  is defined between the diaphragm  30  and the power spring housing  24 . The coil spring  10  is shown in an expanded state in  FIG. 2A  and a compressed state having stored energy in  FIG. 2B . 
         [0018]    A spring piston  34  is disposed between the diaphragm  30  and the coil spring  10  to guide the coil spring  10  as it is compressed and expanded and provides structural support to the spring  10 . A pneumatic chamber  36  is depressurized as shown in  FIG. 2A  allowing the coil spring  10  to expand forcing plate  38  and piston rod  40  to actuate in a known manner. The pneumatic chamber  36  is pressurized at pneumatic port  42  with sufficient pneumatic pressure to compress the coil spring  10  in a known manner as shown in  FIG. 2B . A return spring  42  causes the plate  38  and piston rod  40  to extend toward the spring chamber  32  to retract the plate  38  and piston rod  40  when the pneumatic chamber  36  is pressurized. 
         [0019]    As set forth above, the coil spring  10  not only exerts force along the spring axis a, a coil spring is known to exert a lateral force L known to cause damage by forcing the spring piston  34  to actuate in an inconsistent angle damaging the diaphragm  30 , in addition to other defects. Therefore, the present invention endeavours to reduce or eliminate lateral force associated with the coil spring  10  to reduce or eliminate defects caused by unwanted lateral force. 
         [0020]    Referring now to  FIGS. 3A and 3B , applicant has developed a unique apparatus to identify that which has been determined to cause the lateral force in a coil spring. The inventors of the present application have determined that angular displacement of one of the end coils  12 ,  14  from a plane perpendicular to the spring axis a is the cause of lateral force generated by the coil spring  10 . In addition to this discovery, an apparatus for measuring this angular displacement has been invented and is best represented in  FIGS. 3A and 3B  generally shown at  44 . 
         [0021]    A shaft  46  extends from a base  48  that defines a planer surface  50 . The shaft  46  extends at a normal or perpendicular angle from a planer surface  50  of the base  48 . A conical member  52  is received by the shaft  46  and is disposed upon the planer surface  50  of the base  48  so that the conical member  52  is co-axial with the shaft  46  having its narrower portion directed upwardly. A second conical member  54  is inverted relative to the first conical member  52  and supports a measurement assembly  56 . 
         [0022]    The measurement assembly  56  includes a pivot arm  58  that is pivotally secured to a tubular member  60  at pivot point P as best seen in  FIG. 4 . The tubular member  60  is aligned concentrically with second conical member  54  so that, when received by the shaft  46  the second conical member  54  is co-axial with the shaft. A stop  62  is positioned at the pivot point P the purpose of which will be described further herein below. An angle gauge  64  is disposed upon the arm  58  in a manner that a zero angle is measured relative to a plane that is perpendicular to the shaft  46 . The angle gauge  64  is contemplated to be a Wixey Visual Angle Gauge, part number WR300 or equivalent. 
         [0023]    When measuring angular displacement of the end coils  12 ,  14 , the coil spring  10  is placed onto the apparatus  44  so that the second end coil  14  engages the first conical member  52 . Subsequently, the second conical member  54  is positioned into the first end coil  12 , thereby aligning the spring axis a with the shaft  46  at a same axis defined by the first and second conical members  52 ,  54 . The measurement assembly  56  is placed on shaft  46 , thereby aligning the measurement assembly  56  with spring axis a. A reference arm  66  which is secured to tubular member  60  along axis which intersects pivot point P will make contact with first end coil  12 . The reference arm  66  is used to rotate the measurement assembly  56  until the first distal end  18  of the first end coil  12  abuts the stop  62 . In this manner, the arm  58 , and therefore the angle gauge  64  are disposed at a location on the end coil  12  that is 90° from the first distal end  18 . At this point, the arm  58  is allowed to pivot around to the point P so that the angle gauge  64  measures the angular relationship of the end coil  12  relative to a plane that is normal to the axis a defined by the coil spring  10 . In a similar manner, the coil spring  10  is inverted and the angle of the second end coil  14  is measured by the angle gauge  64 . 
         [0024]    Previously, the lateral force of compression springs were measured resulting in upwards of a 40% of coil springs  10  having a lateral force threshold higher than a desirable value. Some applications, it&#39;s proved to be a lateral force of about 400 Newtons. Therefore, applicant set upon optimizing an end coil angle relative to a desired target lateral load. Referring to  FIG. 5 , it was determined that an optimized angular displacement of between about −1.5° and +2.5° from a plane that is normal to the spring axis A. It was further determined that a more optimal range is between −1° and 0° from the plane that is horizontal to the spring axis a. Still further, a target value of about −1° from a plane horizontal to the spring axis a is a target value for minimal lateral force. 
         [0025]    Various steps were made to tighten tolerances during spring manufacturing, the use of pre-tempered wire prior to folding the coil proved most beneficial in establishing a consistent angular displacement between about −1.5° and 2.5° from the plane that is normal to the spring axis a. For an SAE J2318 spring or the like, a target ratio of axial force to lateral force measured at an extended disposition is between about 67 and 13 to 1. A target ratio is about 53 to 1. A spring that exceeds SAE J2318 standards by about 20% includes an axial force to lateral force measured at an extended position between about 80 and 60 to 1, with a target ratio of about 64 to 1. 
         [0026]    The invention has been described in an illustrative manner, and is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. It is now apparent to those skilled in the art that many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that the invention may be practiced otherwise and is specifically described, and still be within the scope of the present application.