Abstract:
A long travel, varying spring rate combination spring includes two differing rate springs connected end-to-end in such a way that the lower rate spring is compressed until positive engagement of a plunger with an intermediate flange member occurs with the higher rate spring thereafter compressed. Each spring may be assembled separately and connected together with a coupling to create a combination spring, allowing various springs to be combined to meet the requirements of a particular application.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention concerns springs and more particularly mechanical compression springs as are used to provide a return force such as in cam operated dies.  
         [0002]     In cam operated tooling installed on dies in forming presses, a slide is operated by the press motion to drive a tool used to form a feature on a part.  
         [0003]     One or more return springs are compressed as the tool is advanced by a camming action produced by the slide. When the press platens are moved apart, the compressed spring or springs act to retract the slide.  
         [0004]     In metal forming, it sometimes takes considerable force to withdraw a piercing tool from the part, such that a high return spring force is required at the start of the return motion. At the same time, a substantial slide travel is required. Very stiff coil springs have limited available travel for a given space due to the thick cross section required to develop a high spring rate.  
         [0005]     This required combination of long travel and high peak forces has led to the development and use of “nitrogen” springs in which compressed nitrogen is sealed in a chamber, and further compressed by a piston having a projecting rod subjected to a load, creating a fluid spring.  
         [0006]     Nitrogen springs are capable of generating considerable forces while allowing substantial travel, and are widely used as return springs in die cam units.  
         [0007]     However, nitrogen springs have several disadvantages, including their initial high cost and their need to be serviced regularly, and the fact that high cycle rates are not possible due to excessive heat build up.  
         [0008]     These nitrogen springs must be precharged with nitrogen under high pressure so as to have an initial high spring rate.  
         [0009]     This initial high spring rate creates a high shock force when the cam unit is impacted by the cam driver since there is a relatively high initial resistance to cam movement caused by the precharging of the nitrogen spring.  
         [0010]     These disadvantages of nitrogen springs led to the development of a variable force spring assembly described in U.S. Pat. No. 5,390,903, assigned to the same assignee as the present application.  
         [0011]     The design disclosed in U.S. Pat. No. 5,390,903 houses springs of different stiffness in telescoped casings which are arranged to successively compress the springs, which are nested together, allowing a large range of movement with low forces developed initially and very high forces developed at the end of the stroke. A stack of Belleville springs is used as the high rate spring.  
         [0012]     The inclusion of machined casings and other components makes that device relatively high in cost to manufacture, and is not easily adapted to varying applications since the casings and other components must be reengineered for each application. Also, the Belleville springs used are not well suited for high cycling rates, as fatigue failure commonly occurs when Belleville springs are subjected to such service.  
         [0013]     For this reason, there has heretofore not been any mechanical spring arrangement available which has become commercially successful as a replacement for nitrogen springs.  
         [0014]     It is the object of the present invention to provide a mechanical spring combination having a large travel, with an initial low spring rate during an initial range of compression and developing a high spring force through second shorter range of compression of the spring combination.  
       SUMMARY OF THE INVENTION  
       [0015]     The above recited object and other objects which will become apparent upon a reading of the following specification and claims are achieved by combining two springs in an end-to-end in series relationship, each spring having a substantially different spring rate.  
         [0016]     A plunger is slidable within the lower rate spring as the lower rate spring is compressed by a flange on the upper end of the plunger. The plunger in turn is slidable on a pin projecting from the other end of the combination spring through the higher rate spring and into the plunger. The pin has a first flange affixed thereto abutting the lower end of the high rate spring and an intermediate flange above the higher rate spring is held against the other end of the higher rate spring by being seated on a shoulder on the pin.  
         [0017]     The plunger moves into abutment with the second flange held against the upper end of the higher rate spring after a predetermined length of travel, and thereafter begins to compress only the higher rate spring for the remainder of its stroke.  
         [0018]     The higher spring rate spring is normally preloaded by the second flange to a degree such that it is not deflected by the pressure exerted by the lower rate spring and is not further compressed, until the plunger engages the intermediate flange. Alternatively, the higher rate spring may be preloaded to a lesser extent such as to be allowed to be deflected at the same time as the lower rate spring after the precompression level of the higher spring rate is reached when a composite spring rate is affected until abutment of the plunger against the intermediate flange occurs, and the only higher rate spring compressed thereafter.  
         [0019]     In a second embodiment, separate spring assemblies each having a plunger, connecting pin, and precompression flanges are connected together end-to-end as by a press fit coupling pin mated with bores in adjacent flanges. This allows different combinations of various springs to be readily provided to suit different applications. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a side view of a combination spring according to the present invention.  
         [0021]      FIG. 2  is a partially sectional view of the combination spring shown in  FIG. 1 .  
         [0022]      FIG. 3  is a partially sectional view of the combination spring shown in  FIGS. 1 and 2  with the plunger within the lower rate spring moved into abutment with the upper flange to thereafter create compression of the higher rate spring by continued travel of the plunger.  
         [0023]      FIG. 4  is a partially sectional view of the combination spring shown in  FIGS. 1-3  with both springs fully compressed.  
         [0024]      FIG. 5  is a partially sectional view of a variation of the combination spring shown in  FIGS. 1-4 .  
         [0025]      FIG. 6  is a partially sectional view of another variation of the combination spring shown in  FIGS. 1-4 .  
         [0026]      FIG. 7  is a partially sectional view of a die assembly having a combination spring according to the present invention installed therein.  
         [0027]      FIG. 8  is an exploded view in partial section of a second embodiment of a combination spring according to the present invention.  
         [0028]      FIG. 9  is a diagram depicting the force-deflection characteristics of a typical combination spring according to the present invention and for comparison a characteristic of a conventional nitrogen spring.  
         [0029]      FIG. 10  is a diagram of another force-deflection curve for a spring combination creating three stages of effective spring rates. 
     
    
     DETAILED DESCRIPTION  
       [0030]     In the following detailed description, certain specific terminology will be employed for the sake of clarity and a particular embodiment described in accordance with the requirements of 35 USC 112, but it is to be understood that the same is not intended to be limiting and should not be so construed inasmuch as the invention is capable of taking many forms and variations within the scope of the appended claims.  
         [0031]     Referring to the drawings, and particularly  FIGS. 1-4 , a combination spring  10  according to the present invention is shown, including two elongated, differing rate helically wound coil compression springs  12  and  14  connected together end-to-end.  
         [0032]     Each spring  12 ,  14  is held compressed between respective end flanges  16 ,  18  and a common intermediate flange  20 .  
         [0033]     End flange  16  is held to the common intermediate flange  20  by a stepped diameter pin  22  having a head  24  slidable within a plunger  26  connected to the associated end flange  16 .  
         [0034]     The plunger  26  has an opening in its lower end slidably fit to a larger diameter section  28  of the pin  22 , allowing the plunger  26  to slide down when the flange  16  is advanced to compress the lower rate spring  12  until moving into abutment with the upper surface of the intermediate flange  20 .  
         [0035]     The smallest diameter section  30  of the pin  22  extends through a hole in the intermediate flange  20  with a slidable fit, allowing the flange  20  to move down when compressing the higher rate spring  14 .  
         [0036]     The smallest diameter pin section  30  extends within the higher spring rate  14  to the end flange  18 , to which it is affixed as by a plug weld. This prevents the intermediate flange  20  from moving away, which allows a precompression of the higher rate spring  14 . The end flange  18  has a pilot plug  32  extending up into the lower end of the higher rate spring  14 .  
         [0037]     The combination spring  10  is mounted in an installation so that the two end flanges  16 ,  18  are able to be compressed together.  
         [0038]     Different rate springs can be readily substituted into the combination to enable different spring requirements to be engineered at very low cost, and different end fittings easily provided as required for a given application.  
         [0039]     In the first stage of compression, the lower rate spring  14  is compressed against the intermediate flange  20 , held in position by the precompression of the higher spring rate spring  14 . When the lower end of the plunger  26  contacts the intermediate flange  20 , as seen in  FIG. 3 , after a traveling a distance X, this positive abutment prevents further compression of the lower rate spring  12  and compression only of the higher rate spring  14  thereafter occurs with continued compression of the combination spring  10 .  
         [0040]     The plunger  26  pushes the intermediate flange  20  off the shoulder on the stepped diameter pin  22  as seen in  FIG. 4  to initiate compression of the higher rate spring  14 .  
         [0041]      FIG. 9  shows the force-deflection plot for the combination spring  10  of the present invention. If the precompression of the higher rate spring  14  is high enough to be in excess of the peak force existing at the point when compression of the lower rate spring  12  ceases, then two different segments  32 A,  32 B of the curve result.  
         [0042]     A plot  34  of a conventional nitrogen spring is also shown for comparison.  
         [0043]     In  FIG. 10 , the initial compression of the lower rate spring  12  is plotted in segment  36 A.  
         [0044]     If the precompression of the high rate spring  14  is set to be less than in the above example such that the force in the lower rate spring  12  exceeds the higher rate spring force at some point during the compression of the lower rate spring  12 , deflection of the higher rate spring will begin at point X as seen in  FIG. 10  with deflection of the lower rate spring  12  continuing at the same time.  
         [0045]     Thus, lesser composite spring rate exists from point X to point Y depicted as segment  36 B. Abutment of the plunger  26  occurs at point Y.  
         [0046]     Thereafter, the higher spring rate of the spring  14  alone controls, depicted by segment  36 C.  
         [0047]     The combination spring  10  can be easily redesigned for other applications, by merely changing the dimensions of the relatively simple hardware items, i.e., the plunger  26 , the pin  22 A, and substituting different springs  12 A,  14 A as in the combination spring  10 A shown in  FIG. 5 .  
         [0048]     A different plunger travel distance X, can also easily be set. A slight clearance to eliminate any precompression of the lower rate spring  12  may be provided, if desired.  
         [0049]     Different end features can be included, as the pilot end feature  32  shown in  FIG. 5 .  
         [0050]     Such a pilot feature  32  can be provided at one end and a tubular extension  34  at the other end in the combination spring  10 B shown in  FIG. 6 .  
         [0051]      FIG. 7  shows another variation of the combination spring  10 C according to the invention installed in a cam unit  40  adapted to be installed in a press (not shown). Such cam units  40  include a driver  42  on an upper press platen which cams a tooling body  44  slidable on a plate  46  mounted on a lower press platen in the manner well known in the art.  
         [0052]     The body  44  moves relative the driver  42  as the press is operated closing the space  45 .  
         [0053]     A return stripping force is created by a combination spring  10 C installed in a pocket  50  formed in the tooling body  44  extending in the direction of relative movement between the driver  42  and body  44  so that this relative motion will compress the combination spring  10 C.  
         [0054]     The combination spring  10 C includes a boss  52  integral with the intermediate flange  20 C extending within the high rate spring  14 C, and a boss  54  integral with the end flange  18  also extending within the high rate spring  14 C.  
         [0055]     The combination spring  10 C is largely confined and supported within the bore  50 . The protruding portion of the low rate spring  12 C is internally supported by the plunger  26 C, to eliminate any tendency to buckle.  
         [0056]     A removable plug  56  is threaded into the lower end of the bore  50  and together with a snap ring  58 , resists the force exerted when the combination spring  10 C is fully compressed.  
         [0057]     Other arrangements for providing a removable closure for the bore  50  are described in copending application U.S. Ser. No. ______, filed on ______, Attorney Docket No. DNY-117.  
         [0058]     Referring to  FIG. 8 , two separate single spring assemblies  60 A,  60 B are shown which are readily connectable together to form a combination spring according to the invention. This allows a wide variety of spring combinations to be conveniently made.  
         [0059]     In this arrangement, each assembly  60 A,  60 B holds a respective compression coil spring  62 A,  62 B between a pair of end flanges  64 A,  66 A and  64 B,  66 B.  
         [0060]     Each flange  64 A,  64 B,  66 A,  66 B has an integral boss  68 A,  70 A,  68 B,  70 B projecting into the associated spring  62 A or  62 B.  
         [0061]     A headed pin  72 A,  72 B is received in a bore in each boss  68 A,  68 B,  70 A,  70 B, the head portion  74 A,  74 B slidable in the outboard boss  70 A,  70 B. The stem portion  76 A,  76 B is plug welded in the opposite boss  70 A,  68 B. This captures the springs  62 A,  62 B between the associated pairs of flanges  64 A,  66 A and  64 B,  66 B.  
         [0062]     Thus, the outside, away flanges  64 A,  64 B are free to collapse to compress the springs  62 A,  62 B respectively to the distance allowed by the spacing A, B.  
         [0063]     Each spring assembly  60 A,  60 B is readily connectable by a coupling pin  76  press fit into counterbores  78 A,  78 B in adjacent flanges  66 A,  64 B.  
         [0064]     Thus, various spring assemblies can be inventoried and combined in any combination suited to a particular application.  
         [0065]     Optional pilots  80 A,  80 B can be press fitted in the bases of the opposite flanges  64 A and  66 B.  
         [0066]     Urethane or other compression springs can be substantial for the coil springs shown, although the coil springs are preferred.