Abstract:
A gripper for gripping a workpiece includes a jaw assembly having an actuator with a housing and an elongate member axially slidably movable within the housing. A force-multiplying mechanism is interconnected between the elongate member and the housing. The force-multiplying mechanism is configured to add a mechanical force to the jaw assembly and thereby increase a gripping force on the workpiece during operation of the gripper.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a non-provisional application based upon U.S. provisional patent application Ser. No. 61/546,656, entitled “GRIPPER WITH FORCE-MULTIPLYING MECHANISM”, filed Sep. 13, 2011, which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to grippers for gripping a workpiece. 
         [0004]    2. Description of the Related Art 
         [0005]    Grippers are mechanical devices characterized by one or more jaws that move together or apart by motive force typically from an electric motor or pneumatic piston. Once moved into a position of contact with the gripped workpiece, the jaws produce a gripping force against the workpiece. It is often desirable for the gripper to provide as large a gripping force as possible while also possessing a minimum weight and physical size. Typically, increasing gripping force increases the size of the motor or piston which increases the weight and physical size of the gripper. 
         [0006]    By it&#39;s nature, operation of a gripper involves two distinct actions. The jaws must first be moved into a position of contact with the workpiece, after which, the jaws apply a force against the workpiece sufficient to affect subsequent movement (e.g., lifting) of the workpiece. Moving the jaws to the workpiece requires each jaw to exert enough force to overcome the mass inertia of any tooling attached to the jaw and any friction between the jaw and the surfaces of the gripper body that support and guide the jaw. The second action requires each jaw to exert the full intended grip force against the workpiece. While gripping, the jaws must only move sufficiently to compensate for any compliancy from the workpiece or tooling to maintain gripping contact. In other words, the actions of the gripper are separable into two regimes; the first being the jaws traveling some distance with low force until contact is made; and the second is the jaws applying high force against the object. 
       SUMMARY OF THE INVENTION 
       [0007]    The present disclosure describes a gripper that incorporates a force-multiplying mechanism to exploit the operational differences between these two regimes. Illustratively an embodiment of the gripper employs an increased gripping force over reduced jaw travel to hold the workpiece, in contrast to gripping the workpiece. 
         [0008]    The invention in one form thereof is directed to a fluid actuated gripper for gripping a workpiece. The gripper includes a jaw assembly having a cylinder, a piston slidably positioned within the cylinder, and a piston rod having a first end coupled with the piston. The piston has a head end positioned adjacent a fluid chamber for receiving a pressurized fluid. A force-multiplying mechanism is interconnected between a second end of the piston rod and the cylinder. The force-multiplying mechanism is configured to add a mechanical force to the jaw assembly and thereby increase a gripping force on the workpiece during operation of the gripper. 
         [0009]    The invention in another form thereof is directed to a method of operating a fluid actuated gripper for gripping a workpiece, including the steps of: providing a jaw assembly including a cylinder, a piston slidably positioned within the cylinder, and a piston rod having a first end coupled with the piston, the piston having a head end positioned adjacent a fluid chamber; positioning the jaw assembly relative to the workpiece; pressurizing the fluid chamber with a fluid and thereby causing extension of the piston and the piston rod from the cylinder, and generating a gripping force on the workpiece using the jaw assembly; generating a mechanical force using a force-multiplying mechanism interconnected between a second end of the piston rod and the cylinder, using the extension of the piston rod from the cylinder; and applying the mechanical force to the jaw assembly and thereby cumulatively increasing a gripping force on the workpiece during operation of the gripper. 
         [0010]    The invention in yet another form thereof is directed to a gripper for gripping a workpiece, including a jaw assembly having an actuator with a housing and an elongate member axially slidably movable within the housing. A force-multiplying mechanism is interconnected between the elongate member and the housing. The force-multiplying mechanism is configured to add a mechanical force to the jaw assembly and thereby increase a gripping force on the workpiece during operation of the gripper. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
           [0012]      FIG. 1  is a perspective view of an embodiment of a gripper of the present invention; 
           [0013]      FIG. 2  is another perspective view of the gripper shown in  FIG. 1 , positioned relative to an exemplary workpiece; 
           [0014]      FIG. 3  is another perspective view of the gripper shown in  FIGS. 1 and 2 , gripping the workpiece; 
           [0015]      FIG. 4  is a schematic view of a prior art gripper; 
           [0016]      FIG. 5  is another schematic view of the prior art gripper shown in  FIG. 4 ; 
           [0017]      FIGS. 6 and 7  are schematic views of the gripper of the present invention shown in FIGS.  1 - 3 , including an embodiment of the force-multiplying mechanism of the present invention; 
           [0018]      FIG. 8  is a partially schematic view of the gripper of the present invention shown in  FIGS. 1-3 , including another embodiment of the force-multiplying mechanism of the present invention; 
           [0019]      FIGS. 9   a  and  9   b  illustrate another embodiment of the force-multiplying mechanism of the present invention; 
           [0020]      FIG. 10  illustrates a potential interference condition that can occur between the rack and pinion of the force-multiplying mechanism; 
           [0021]      FIGS. 11   a  and  11   b  illustrate a keyway and detent arrangement that can be used to obviate the interference shown in  FIG. 10 ; 
           [0022]      FIG. 12  shows a perspective view of another embodiment of a force-multiplying mechanism of the present invention; 
           [0023]      FIG. 13  shows a partially schematic view of another embodiment the gripper of the present invention; 
           [0024]      FIG. 14  is an exploded perspective view of the gripper shown in  FIGS. 1-3 ; 
           [0025]      FIG. 15  is an exploded perspective view of the force-multiplying mechanism shown in  FIGS. 1-3 ; 
           [0026]      FIG. 16  is an exploded perspective view of the brake assembly shown in  FIGS. 1-3 ,  14  and  15 ; 
           [0027]      FIGS. 17   a,    17   b  and  17   c  are cross-sectional views illustrating operation of the force-multiplying mechanism of  FIGS. 1-3  during operation. 
       
    
    
       [0028]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    Referring now to the drawings, and more particularly to  FIG. 1 , there is shown a gripper  2  that includes a perspective view of a force-multiplying system of the present invention. Gripper  2  includes jaw arms  4  and  6  each attached to a jaw bridge  8  and  10 , respectively. Bridges  8  and  10  are slideable in directions  12  and  14  along cover  16 . Jaw bridges  8  and  10  are also supported by plate  18  capped by end plates  20  and  22 . Fasteners  24  attach end plates  20  and  22  to each respective end of plate  18 . Similarly, fasteners  26  illustratively attach jaw arms  4  and  6  to their respective bridges  8  and  10 . It is appreciated that jaw arms  4  and  6  may be of any variety of configurations to hold a desired workpiece. The illustrative design of arms  4  and  6  in this embodiment demonstrates their ability to hold a tubular workpiece such as workpiece  28  shown in  FIG. 2 . A brake assembly  30  is configured to decelerate a moving jaw to rest and hold a stopped jaw in a stationary position. This may become useful during operation of gripper  2  if emergency stopping or retention of the gripped workpiece occurs. 
         [0030]    Another perspective view of gripper  2  is shown in  FIG. 2 . This view depicts how jaw arms  4  and  6  move in directions  14  and  12 , respectively, in anticipation of gripping onto workpiece  28 . As previously discussed, this movement is the first stage where only the inertia of the components of the gripper itself such as arms  4  and  6 , bridges  8  and  10 , and piston assemblies  53 A and  53 B, jaw assemblies  56 A and  56 B, and driven racks  15 A and  15 B shown in  FIGS. 13 and 14 , must be overcome in addition to friction in order to at least move jaw arms  4  and  6  against workpiece  28 . 
         [0031]    A perspective view of gripper  22  shown in  FIG. 3  depicts the second stage which is multiplying the force in directions  14  and  12  by jaws  4  and  6  to create a firmer grip on workpiece  28 . As previously discussed, although conventional grippers are designed to grip and hold a workpiece, this gripper is configured to apply a multiplying force to that holding function to create a more substantial hold force on the workpiece. 
         [0032]    Operational schematic views of a prior art gripper  200  is shown in  FIGS. 4 and 5  and current gripper  2  is shown in  FIGS. 6-8 . The view of  FIG. 4  represents the prior art two jaw pneumatic gripper  200  wherein each jaw consists of a movable cylinder  201   a  and  201   b  fitted around pistons  202   a  and  202   b  held stationary by rods  203   a  and  203   b  and connected to the body of the gripper  200  (denoted by the “ground” symbol). When compressed air fills volumes  205   a  and  205   b  between the closed end of the cylinder and the piston, the cylinders move in directions  14  and  12 , respectively until the jaws contact workpiece  206 . Cylinders  201   a  and  201   b  continue apply a force against the object. The force applied to the workpiece is balanced by an equal and opposite force  208   a  and b applied to the body of gripper  200  via their respective piston rods  203   a  and  203   b.    
         [0033]    A view of the prior art in  FIG. 5  depicts one side of gripper  200  with cylinder force  207   b  acting against gripped object  206  and an equal, but opposite piston force  208   b  transmitted through piston rod  203   b  to ground. 
         [0034]    In contrast to gripper  200 , the schematic view of gripper  2  in  FIG. 6  shows lever  209  free to rotate about a mid-point  210  to move piston rod  203   b  via pivot  211 . The opposite end of the lever  209  is attached to cylinder  201   b  via pivot  212 . The piston force  208   b  no longer travels to gripper portion  2 , but is redirected by lever  209  to cylinder  201   b.  In the illustrated embodiment, the pivot point of lever  209  is positioned at the mid-point  210  between pivots  211  and  212  at the opposite ends of lever  209 . However, it will be appreciated that the relative position of the pivot point between pivots  211  and  212  can be selected to provide a desired degree of force magnification using the force-multiplying mechanism of the present invention. 
         [0035]      FIG. 7  shows, in schematic form, the relative movements of piston  202   b  and cylinder  201   b  under the influence of lever  209 . For any distance “d” along which cylinder  201   b  moves, piston  202   b  moves an equal, but opposite distance “d”. 
         [0036]    The schematic view in  FIG. 8  illustrates how the lever  209  can be effectively replaced by rack and pinion arrangement including a driving rack  213 , pinion gear  214 , and driven rack  215  system with the force  208   b  applied by the piston to the rack adding to the force  207   b  applied by cylinder  201   b  to the gripped workpiece  206 . The lengths of the two rack segments  213  and  215  may be chosen to provide for any practical length of force-multiplied jaw travel. 
         [0037]      FIG. 9   a  illustrates how driving rack  213  is held stationary by a “shot-pin” cylinder  217  which includes a piston assembly  216  free to move vertically, but constrained from horizontal motion by cylinder  217 . A portion of the piston assembly  216  engages a mating notch  221  in driving rack  213 . (See also  FIG. 9   b ). A stripped area  220  of driving rack  213  has no teeth. This allows driven rack  215  to rotate pinion gear  214  unencumbered as driven rack  215  translates under the influence of cylinder  201   b  as it moves to contact workpiece  206 . (See  FIG. 8 ). 
         [0038]    As depicted in  FIG. 9   b,  after cylinder  201   b  contacts workpiece  206 , compressed air is allowed to fill volume  222  between the closed end of shot-pin cylinder  217  and piston assembly  216 . This forces piston assembly  216  to travel in direction  218 . Piston assembly  216  is, thus, retracted from notch  221  allowing rack  213  to move in direction  219  until one tooth of rack  213  contacts a mating tooth on pinion gear  214 . Once in contact, driving rack  213  is free to transfer force applied to the rack in direction  219 , through pinion gear  214  to driven rack  215 . 
         [0039]      FIG. 10  shows a potentially problematic condition that may occur while driving rack  213  moves to engage a tooth on pinion gear  214 . The orientation of pinion gear  214  relative to driving rack  213  is controlled by the stopping position of driven rack  215  as cylinder  201   b  contacts workpiece  206 . (See, also,  FIG. 5 .) It is possible that pinion gear  214  may be stopped by driven rack  215  such that the position of the engaging tooth of the pinion gear  214  will contact the mating tooth of driving rack  213  somewhere within interference zone  223 . Within zone  223 , the flank of the driving rack tooth does not mesh with the involute flank of the pinion gear, but contacts the top of the gear tooth instead. In this condition, torque cannot be transmitted from driving rack  213  to pinion gear  214  causing the gear to jam. 
         [0040]      FIGS. 11   a  and  11   b  illustrate how this jamming condition can be obviated by including a ball-detent with ball  224  and spring  225  located within an appropriate sized bore in driving rack  213 . The ball-detent may be used in conjunction with a second pinion gear (not shown) attached to shaft  226  to drive driven rack  215  (see  FIG. 8 ). A machine key  227  couples pinion gear  214  to shaft  226  via a mating sector shaped key-slot in the pinion gear so that rotation of the machine key simultaneously rotates shaft  226  and the attached second pinion gear. If pinion gear  214  should stop anywhere within interference zone  223 , as shown in  FIG. 11   a,  the sector shaped key-slot in the pinion gear allows pinion gear  214  to rotate, illustratively counter-clock-wise, relative to shaft  226  by contact with ball  224  acting under the influence of spring  225  to a position beyond interference zone  223 , as shown in  FIG. 11   b.    
         [0041]      FIG. 12  is a perspective view of a force-multiplying mechanism having a rack and pinion arrangement with two pinions  214   a  and  214   b  interconnected by a shaft  226   a.  Each pinion  214   a  and  214   b  has a different gear pitch, which in turn results in a different force amplification factor which is transmitted back to the cylinder (not shown in this view). It will be appreciated that a desired force amplification factor can be selected by appropriately selecting the pitch diameters of pinions  214   a  and  214   b.    
         [0042]      FIG. 13  demonstrates how the driving rack  213 , pinion gear  214 , and driven rack  215  system is operable on cylinder  201   a  and piston rod  203   a  without changing the substance of the embodiment. 
         [0043]      FIG. 14  shows a partially exploded view of a preferred embodiment for gripper  2  with a force multiplying mechanism. Center plate assembly  50  mounts to base plate  18  with threaded fasteners  52 . Cylinder assemblies  53 A and  53 B span the volume between center plate assembly  50  and end plate assemblies  54 A and  54 B, respectively. Fasteners  24  attach end plate  20 ,  22  of assemblies  54 A and  54  B to base plate  18 , respectively. Jaw assemblies  56 A and  56 B are respectively retained in channels  57 A and  57 B of base plate  18  by wedges  58 A and  58 B, which allow identically constructed jaw guides  60 A-D to translate longitudinally, while preventing vertical and lateral movement, with respect to the base plate  18 . The positions of the wedges, relative to base plate  18 , are adjustable via threaded fasteners  59 A and  59 B, respectively to remove any clearance between the jaw guides and wedges and jaw guides and base plate. The included angle of the wedge is chosen to be less than the self-locking wedge angle determined by the coefficients of friction between the wedge and abutting surfaces to prevent the wedges from locking in place during adjustment of fasteners  59 . Cover  16  is disposed between jaw assemblies  56 A and  56 B and base plate  18 . 
         [0044]    Way covers  62 A and  62 B are constructed from a magnetic ferrous alloy and are held by magnetic attraction to magnetic strips  63 A and  63 B, respectively. Strips  63 A are illustratively adhesively bonded to base plate  18 , while strips  63 B are illustratively adhesively bonded to cover  16 . Way cover  62 A passes over a curved portion of jaw guides  60 A and  60 C and under rollers  64 , which force the way cover to conform to the curved portion of the jaw guides. Rollers  64  are retained upon their respective jaw guides by dowel pins (not shown) that are press fit into the jaw guides. In an analogous manner, way cover  62 B passes over a curved portion of jaw guides  60 B and  60 D and under rollers  64 , which force the way cover to conform to the curved portion of the jaw guides. Protrusions  65 , located on each of the jaw guides, fit into mating notches  66  in oval profiled cylinders comprised within cylinder assemblies  53 A and  53 B to couple the longitudinal motion of the cylinders to the respective jaw assembly. Scraper bands  67 , surrounding each jaw guide  60 A-D, help to prevent contaminant ingress from underneath the jaw guide. Elastomeric cords  68 , apply pressure to the top of each scraper band to force the band tightly against the surface of way guides  62 A and  62 B. 
         [0045]    Jaw bridge  8  is attached to jaw guides  60 A and  60 B with threaded fasteners  70 , to complete jaw assembly  56 A. In an analogous manner, threaded fasteners  70  attach jaw bridge  10  to jaw guides  60 C and  60 D to complete jaw assembly  56 B. Threaded fasteners  71  retain the ends of way covers  62 A and  62 B in end plate  20 . A similar pair of fasteners (not shown) may retain the opposite ends of way covers  62 A and  62 B in end plate  22 . Piston assembly  73 A is disposed within cylinder assembly  53 A with a similar piston assembly (shown exploded in  FIG. 15 ) disposed within cylinder assembly  53 B. Cylinder  74 A surrounds piston assembly  73 A. Seals (not shown) may seal the periphery of piston  75 A against a complimentary oval bore in cylinder  74 A to prevent the flow of motive compressed air around the piston. The ends of rods  81 A,  82 A, and  83 A pass though seals (not shown) contained within seal retainers  76 A and  76 B to prevent the flow of motive compressed air around the rods. Another seal (not shown) seals the periphery of each seal retainer against the mating oval bore in cylinder  74 A. Threaded fasteners  77  retain cylinder covers  78 A and  78 B onto cylinder  74 A. Threaded fastener  79  passes through spacer  80 A to fasten driven rack  15 A onto cylinder cover  78 A. The end of rod  82 A passes through seal  84 A and into bearing bushing  85 A, which are both retained within complimentary bores within end plate  20 . The end of rod  81 A passes into bearing bushing  86 A, which is similarly retained in a bore within end plate  20 . It is understood that cylinder assembly  53 B is constructed and constrained in an analogous manner to that described for cylinder assembly  53 A. Brake assemblies  30  and  31  thread into thread bores in center plate  118 . 
         [0046]    A partially exploded view of the force-multiplying mechanism components of gripper  2  is shown in  FIG. 15 . Threaded fasteners  100  join hollow rods  82 A and  83 A to piston  75 A and join hollow rods  82 B and  83 B to piston  75 B, respectively and prevent the flow of motive compressed air between the joined hollow rods. Retaining rings  101  retain solid rod  81 A within piston  75 A and retain solid rod  81 B within piston  75 B, respectively. Seals (not shown) within the pistons prevent the flow of motive compressed air around the rods and through the holes in the pistons through which rods  81 A and  81 B pass. Seals  102 A and  102 B, disposed in glands within base plate  118 , seal the periphery of hollow rods  83 A and  83 B, which pass into mating bores in base plate  118 . Dowel pin  87  passes through a hole through the side of driving rack  13 A and into a mating hole in the end of solid rod  81 A to couple the rod to the rack. In a similar fashion, another dowel pin  88  passes through a hole through the side of driving rack  13 B and into a mating hole in the end of solid rod  81 B to couple the rod to the rack. Driving racks  13 A and  13 B are disposed into mating slots in center plate  118  and prevented from vertical movement by covers  104  that are retained on the center plate by fasteners  105 . Ball-detent assemblies  103 A and  103 B, comprising ball  24  and spring  25  (see, also,  FIGS. 11   a  and  11   b ), are press-fit into mating holes in driving racks  14 A and  14 B, respectively. 
         [0047]    Illustrative woodruff machine keys  106 , inserted into mating keyseats in shafts  107 A and  107 B, key pinion gears  17 A,  108 A, and  109 A to shaft  107 A and key pinion gears  17 B,  108 B, and  109 B to shaft  107 B. Radial bearings  110 , retained by retaining rings  111 , support shafts  107 A and  107 B within complimentary bores within center plate  118 . Synchronizing pinion gears  108 A and  108 B are in mesh so as to couple the rotation of shaft  107 A to that of shaft  107 B. Driven pinion gears  109 A and  109 B engage driven racks  15 A and  15 B, respectively, so that the translation of one driving rack is synchronized to the other by the action of pinion gears  108 A and  108 B being in mesh. The beveled ends of rack-locking pins  112 A and  112 B engage mating angled notches in driving racks  13 A and  13 B, respectively. The cylindrical body of each rack-locking pin passes through a mating hole in center plate  118  to so as to prevent longitudinal motion of the driving racks until the beveled portions of pins  112 A and  112 B are retracted from the mating notches in the driving racks. Once rack-locking pin  112 A is retracted, driving rack  13 A engages pinion gear  17 A transmitting the force from motive air pressure acting on the face of piston  75 A through rod  81 A, to shaft  107 A and pinion gear  109 A, to driven rack  15 A and cylinder cover  78 B. 
         [0048]    In an analogous manner, retraction of rack-locking pin  112 B allows the transmission of the force applied to piston  75 B through rod  81 B to driving rack  13 B to pinion  17 B, shaft  107 B, pinion gear  109 B, and driven rack  15 B, to cylinder cover  78 C. The rotation of shafts  107 A and  107 B may be controlled by brake assemblies  30  and  31 , respectively. Engagement of the brake prevents the associated shaft from rotating, subsequently locking the driven rack, cylinder assembly, and jaw assembly associated with that shaft. 
         [0049]      FIG. 16  shows an exploded view of brake assembly  30  shown in  FIG. 15 . A plurality of disks  151  are inter-disposed between a second plurality of disks  150 . A tab on bottom of disk  150  engages a slot  113  within the shaft bore of center plate  118 , to prevent rotation of the disk. Opposing slots in disk  151  engage feather machine keys  119  that are disposed into mating keyways in shaft  107 A so as to couple rotation of the shaft to the disk. Housing  152  threads into center plate  118  to retain the brake assembly onto the gripper. Piston  154  is coaxially located within housing  152 . A seal (not shown) seals the periphery of piston  154  against a complimentary bore in cylinder housing  152  to prevent the flow of motive compressed air around the piston. Rod seal  153  seals a cylindrical rod portion that protrudes from piston  154  through a hole in housing  152  to contact the closest disk  150 . A plurality of coned spring washers  156  are disposed between split washer  155  and washer  157  to apply a force against piston  154  and subsequently, against the stack of disks  150  and disks  151 . Spiral retaining ring  158  engages an annular groove in housing  152  to retain washer  155 , coned spring washers  156 , and washer  157  within housing  152 . 
         [0050]    In operation, the brake assembly is disengaged by applying compressed air into the cavity formed between the underside of piston  154  and housing  152  creating a force that acts on the face of piston  154  sufficient to overcome the force applied to the opposing face of the piston by coned spring washers  156 . The brake assembly is engaged by removing the applied compressed air, allowing coned spring washers  156  to apply a force against piston  154  and in-turn, against the stack of disks  150  and  151 . Retaining ring  111  prevents movement of the stack of disks along the axis of shaft  107 A. (See, also,  FIG. 15 .) As the force applied by coned spring washers  156  passes through each consecutive disk-to-disk interface, frictional forces are created at the interface that oppose the rotation of one disk relative to the adjoining disk. These frictional forces effectively couple the rotation of disks  151 , keyed to shaft  107 A, to that of disks  150 , prevented from rotation by the engagement of the tab of each disk into slot  113 , to impede rotation of the shaft. It is understood that brake assembly  31  operates in an analogous manner to the operation described for brake assembly  30  allowing brake  31  to control the motion of shaft  107 B. 
         [0051]      FIGS. 17   a - 17   c  show a series of cross-sectional views taken through the centerline of rack-locking pins  112 A and  112 B and illustrate the sequence of events that occur during engagement of the force multiplying mechanism. (See, also,  FIG. 15 ).  FIG. 17   a  shows the relationship of components with the force multiplying mechanism disengaged. Arrows  175  indicate the direction of forces acting on driving racks  13 A and  13 B. The angled contact surfaces  176 A and  176 B between rack-locking pins  112 A and  112 B and driving racks  13 A and  13 B, respectively, impart vectoral components of the forces  175  acting on the racks to produce forces that act to push the rack-locking pins towards the center of control cam  114 . The bases  177 A and  177 B of rack-locking pins  112 A and  112 B, respectively, rest against horizontal surfaces of control cam  114 , which prevent axial movement of the pins and the associated translation of the rack engaged by each pin. Compressed air, filling the volume  178  between cushion piston  115 A and bore plug  116 A, forces the piston against annular shoulder  179  in center plate  118 . Seals (not shown) seal the periphery of control piston  115 A and the periphery of bore plug  116 A against the walls of center plate  118 . Bore plug  116 A is retained in center plate  118  by retaining ring  117 A which engages a complimentary annular groove in the center plate. Helical spring  180  (shown schematically in  FIGS. 17   a - 17   c  and not shown in  FIG. 15 ), is disposed between a bore in cushion piston  115 A and a coaxially aligned bore in control cam  114  so as to force control cam  114  against control piston  115 B and bore plug  116 B, which is retained by retaining ring  117 B. 
         [0052]      FIG. 17   b  shows the relationship of components with the force multiplying mechanism activated, but prior to the driving racks engaging the driving pinion gears. Compressed air has been directed into the volume  181  between bore plug  116 B and control piston  115 B with the resulting force of the air pressure acting of the face of the piston sufficient to overcome the force of spring  180  and the frictional forces applied by rack-locking pins  112 A and  112 B. Seals (not shown) seal the periphery of control piston  115 B and the periphery of bore plug  116 B against the walls of center plate  118 . The motion of control piston  115 B away from bore plug  116 B and into contact with annular shoulder  182  of center plate  118  moves control cam  114  into a position where the ends  177 A and  177 B of rack lock pins  112 A and  112 B, engage angled surfaces  183 A and  183 B of the control cam, respectively. The angle of surfaces  183 A and  183 B is chosen so that vectoral components of the forces that act to push the rack-locking pins towards the center of control cam  114  combine to exert a force that pushes control cam  114  into contact with cushion piston  115 A. A double-acting valve, used to control activation of the force-multiplying mechanism, is configured to simultaneously exhaust air pressure from cavity  178  as air pressure is applied to cavity  181  to activate the mechanism. Forces  175  acting upon driving racks  13 A and  13 B can have large magnitudes, which can cause the racks to accelerate to large velocities prior to a rack engaging the corresponding driving pinion gear. It is desirable to reduce the engagement velocity of the rack so as to minimize the force generated as the tooth of the rack impacts against the meshing tooth of the pinion. This desirable reduction in engagement velocity is accomplished by controlling the rate at which air is exhausted from cavity  178 . Reducing the exhaust rate creates a back-pressure against the face of cushion piston  115 , subsequently slowing the travel of control cam  114  and the associated retraction rate of rack-locking pins  112 A and  112 B, should driving racks  13 A and  13 B be propelled too rapidly by the action of forces  175 . 
         [0053]      FIG. 17   c  shows the relationship of components with the force multiplying mechanism fully engaged. The compressed air in cavity  178  has been completely exhausted allowing control cam  114  to move into a position allowing rack-locking pins  112 A and  112 B to fully retract from racks  13 A and  13 B, respectively. With the pins no longer engaging surfaces of  176 A and  176 B of driving racks  13 A and  13 B, respectively, the racks are free to move unencumbered in the direction of forces  175 . 
         [0054]    The present disclosure illustratively shows an actuator in the form of a pneumatic piston and cylinder arrangement which generates motive force. This disclosure, however, also contemplates employing an electric or fluid actuated motor to generate the motive force as well. An example of a motor driven actuator is disclosed in U.S. Pat. No. 8,152,214 (Williams et al.), which is assigned to the assignee of the present invention and incorporated herein by reference. 
         [0055]    While this invention has been described with respect to at least one embodiment, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.