Abstract:
A powered pushing unit transfers ads no greater than a predetermined magnitude in one direction along the length of a load transfer system. The pushing unit includes a frame and a source of power for moving the pushing unit back and forth along the length of the load transfer system. A dog for engaging a load is pivotally mounted with respect to the frame. A restricting mechanism applies a load engaging force to hold the dog at a load transfer range of positions until the load is greater than the predetermined magnitude. A toggle joint is mounted between the dog and the frame. The toggle joint forms an angle in a first angular direction to lock the dog in a load engaging position during engagement of a load under the predetermined magnitude. The toggle joint further shifts to form an angle in a second angular direction upon engagement of a load in excess of the predetermined magnitude, thereby allowing the dog to pivot away from the load which is in excess of the predetermined magnitude. When pivoting away from and passing beneath the toad, the dog exerts a force against the load that is substantially less than the predetermined magnitude. A toggle force mechanism returns the toggle joint to a first angular direction after the dog passes the load.

Description:
This application claims the benefit of U. S. Provisional Application Nos. 60/195,642 filed on Apr. 7, 2000 and 60/196,199 filed on Apr. 11, 2000, which are incorporated herein by reference. 
    
    
     BACKGROUND 
     This invention relates generally to powered pushing units for moving loads from one point to another along a selected path, and more particularly, to powered pushing units for moving loads in one direction while transferring back and forth along a selected path. 
     Powered pushing units are used in many applications to transfer loads between two or more different points along a predetermined path. For example, in railroad applications pushing units called dog carriages or table sleds move railway cars from one point to another for various purposes. Pusher conveyors use powered pushing units, sometimes called shuttles, to transfer loads from one point to another along the lengths of the conveyors. Some pusher conveyors incorporate a centrally located, stationary motor for providing power throughout the conveyor system. Loads are engaged and transferred with one or more pushing elements, known as a dog or dogs, that are located apart from the stationary motor. The motor then supplies power to the dogs remotely with the use of a separate power track or with the use of a roller and pulley or a drive chain arrangement. The dogs may either contact the load directly or contact one or more load bearing trolleys or carriages supported from a second driving track. Some designs incorporate toggle or pivot mechanisms that allow for variable or one-directional engagement of loads with a dog. 
     For example, U.S. Patents to Hoehn (U.S. Pat. No. 4,072,111) and Curry, et al. (U.S. Pat. No. 3,451,352) each disclose multiple-track conveyor systems where two or more independent, closed loop driving tracks include a plurality of pushing dogs that independently engage one or more load-bearing trolleys. The trolleys themselves have power engaging dogs along a separate load-bearing track. Mechanisms variably disengage the pushing dogs by retracting power engaging dogs when contact is made with adjacent trolleys. In their construction, these systems require room and added expense for at least three separate tracks and many individual dogs, along with additional maintenance time and costs for a large number of components. Since each driving track is a closed loop, breakage of any component of the closed loop necessarily making the entire driving track inoperative. Furthermore, since each driving track is also solely responsible for transferring loads along a particular leg of the conveyor, inoperativeness of any one driving track normally leads to inoperativeness of the entire conveyor system. 
     Some of the inherent shortcomings of these closed loop systems can be overcome with the use of unitary, individually powered pushing units. Such units typically incorporate a toggle or pivot mechanism for actively engaging or for bypassing a load along the length of the conveyor track. U.S. Patents to Saxonmeyer (U.S. Pat. No. 3,556,011) and Hunt (U.S. Pat. No. 3,522,772) each disclose railroad car movers having pushing units capable oftwo-directional travel and incorporating toggle mechanisms for allowing dogs to rigidly engage loads in a first direction and for bypassing loads in a second direction. U.S. Patent to Morikiyo (5,695,044) discloses a pushing unit allowing for the same one-directional engagement but incorporating a more simple dog and pivot mechanism. Power for the pushing unit is provided externally, such as by the use of a chain or pulley. In all of the foregoing pushing unit designs, forward load engagement in the first direction is rigid, and no mechanism for releasing excessive loads is disclosed. The lack of such a mechanism may present a problem relating to the dependability and service life of a car moving or other conveyor system. For example, in the case where a conveyor is used to transfer variably loaded pallets, accidental overloading of one or more pallets can result in a pushing unit encountering a force greater than the pushing unit&#39;s intended transfer capacity. Such an excessive load force can result from one or more loads becoming jammed along the path of the pushing unit. In the absence of a suitable release mechanism, excessive wear or damage to the pushing unit or power source may also result. Although a suitable trip mechanism may be incorporated into a system to terminate power to the pushing unit upon the occurrence of such an overload, such a solution normally requires a temporary shutdown of the entire system and for some systems may lead to the need for further repairs or maintenance of the conveyor. Where multiple pushing units in a system depend upon a single, centralized power source, other pushing units, as a consequence, remain inoperative during the period of temporary shutdown. 
     One previous attempt to incorporate an overload release mechanism into a conveyor system does not deal with these shortcomings. U.S. Patent to Janzen, et al. (U.S. Pat. No. 5,437,231) discloses a two-track, continuous conveyor system, the upper track having powered claws pivotally mounted for engaging load-bearing carriages along the second track. The claws are positioned with a simple spring loaded pivot to engage the load bearing carriages and can bypass a carriage when a carriage is jammed or overloaded on the second track. After bypassing a jammed carriage, the claws are incapable of returning to their load engaging position without manual resetting. Since Janzen discloses a continuous conveyor, such resetting necessarily requires a shutdown of the entire system before the claws are again able to engage loads. In addition, Janzen requires additional space and the added expenses that are associated with multiple track systems. 
     SUMMARY 
     The present invention is a powered pushing unit for transferring loads no greater than a predetermined magnitude in one direction along the length of a load transfer system. The pushing unit includes a frame and a source of power for moving the pushing unit back and forth along the length of the load transfer system. 
     A dog for engaging a load is pivotally mounted with respect to the frame. A restricting mechanism applies a load engaging force to hold the dog at a load transfer range of positions until the load is greater than the predetermined magnitude. A dog spring which has a dog spring force pivots the dog toward a load engaging position on the dog support arm to engage and transfer loads when the frame is traveling in a first direction. The dog pivots against the dog spring force to bypass loads when the frame is traveling in a direction opposite that in which it pushes loads. 
     The pushing unit further includes a toggle joint connected between the dog and the frame. The toggle joint forms an angle in a first angular direction to lock the dog in a load engaging position during engagement of a load under the predetermined magnitude. The toggle joint further shifts to form an angle in a second angular direction upon engagement of a load in excess of the predetermined magnitude, thereby allowing the dog to pivot away from the load which is in excess of the predetermined magnitude. 
     A toggle force mechanism is interconnected between the frame and the pivot of the toggle joint to apply a toggle force, which is substantially less than the predetermined magnitude, to bias the toggle joint in the first angular direction with respect to the pivot. When the dog is pivoted away from the load due to a load in excess of the predetermined magnitude, the toggle force mechanism exerts a force substantially less than the predetermined magnitude on the dog and from the dog to the load. The toggle force mechanism then returns the toggle joint to the first angular direction after the dog passes the load. 
     Various features, advantages, and characteristics of the present invention will become apparent to one of ordinary skill in the art while reading the following specification. This invention does not reside in any one of the features of the powered pusher unit disclosed above and in the following Detailed Description of Preferred Embodiments and claimed below. Rather, this invention is distinguished from the prior art by its particular combination of features which are disclosed. Important features of this invention have been described below and shown in the drawings to illustrate the best mode contemplated to date for carrying out this invention. 
     Those skilled in the art will realize that this invention is capable of embodiments which are different from those shown and described below and that the details of the structure of this powered pusher unit can be changed in various manners without departing from the scope of this invention. Accordingly, the drawings and description below are to be regarded as illustrative in nature and are not to restrict the scope of the invention. The claims are to be regarded as including such equivalent powered pusher units as do not depart from the spirit and scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding and appreciation of this invention and many of its advantages, reference should be made to the following, detailed description taken in conjunction with the accompanyin drawings wherein: 
     FIG. 1A-E are sectional views of the intercomponent operation of pushing dog support arm and toggle release mechanisms according to one embodiment of the invention; 
     FIGS. 2A-C respective top, side, and end views of an independently powered I-beam track pushing unit having a pair of pushing dog, support arm and toggle release mechanisms according to one embodiment of the invention; 
     FIG. 3 illustrates an alternative cog-wheel and chain arrangement for externally powering a power pushing unit according to one embodiment of the invention; 
     FIG. 4 is a sectional view of a pushing dog and toggle release mechanism incorporating an alternative, non-stationary load block according to one embodiment of the invention; 
     FIG. 5 is a sectional view of a pushing dog and toggle release mechanism having operational characteristics similar to the embodiment illustrated in FIG. 4, but positioned at the front of a pushing unit according to one alternative embodiment of the invention; 
     FIG. 6 illustrates and adjustable, two-cog drive system for powering a pushing unit made in accordance with this invention; and 
     FIG. 7 illustrates a single-cog drive system for powering a pushing unit made in accordance with this invention, 
     FIG. 8 is a sectional view of a pushing dog support arm and toggle release mechanism placed at the front end of a pushing unit frame according to one embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to the drawings, identical reference numerals and letters designate the same or corresponding parts throughout the several figures shown in the drawings. 
     FIG. 1A shows a powered pushing unit  1  constructed according to this invention for transferring a load no greater than a predetermined magnitude along the length of a load transfer system such as a conveyor. For the purpose of describing this invention, the size of a load is the amount of force required to push that load along the conveyor. Since conveyors normally use rollers, ball bearings or similar devices which minimize friction, the amount of force required to push a particular load will normally be significantly less than the weight of the load itself Additionally, while FIGS. 1A-F show single dog, support arm and toggle release mechanisms for pushing a load, FIGS. 2A-2C show the use of a pair of each such mechanism used to push heavier loads. If desired, more than two of the dog, support arm and toggle release mechanisms could be used to push even heavier loads. 
     The powered pushing unit  1 , often referred to as a shuttle, has a dog support arm  4  which is pivotally mounted on a pushing unit frame  8 . The frame  8 , including its various components, can be welded or bolted together or made as a casting. The dog support arm  4  is biased to an upright load transferring position by a restricting mechanism which is shown in FIG. 1A as a support spring  12 . As will be explained more fully below, the support spring  12  holds the dog support arm  4  within a range of load transfer positions as long as the load is no greater than a predetermined magnitude determined by an operator of the powered pushing unit  1 . 
     The support spring  12  is mounted over threads  15  of a support bolt  16  and is compressed between an adjusting nut  13  and the support arm  4 . The adjusting nut  13  is mounted on the threads  15  of the support bolt  16  and is locked into a preselected position with a lock nut  14 . 
     The support bolt  16  can be connected to the pushing unit frame  8  by extending within a threaded hole through a metal connector  20  which is welded directly to the pushing unit frame  8  as shown in FIG.  1 A. The threads  15  of the support bolt  16  are threaded into mating threads within the connector  20 . Alternatively, the support bolt  16  can be threaded directly into a hole within the pushing unit frame  8 . 
     A spacer element  24  or other padding device is interposed between each support arm  4  and metal connector  20 . The spacer element  24  may be constructed of metal or preferably a flexible material such as rubber, resilient plastic and the like, and serves to orient the support arm  4  to an upright position while under continued compression by support springs  12 . As shown, the spacer element  24  may be cylindrical in its construction to accommodate a support bolt  16 . 
     The force of the support spring  12  can be varied by adjusting the amount of its compression between support arm  4  and adjusting nut  13 . This is accomplished by varying the position of the adjusting nut  13  on the threads  15  of support bolt  16 . As will be more clearly explained below, the spring force required will vary according to the maximum load which the pushing unit  1  is to push along the conveyor. The size of the support spring  12  can also be changed to accommodate different sized loads. The purpose of the support spring  12  is to limit the pivotal movement of the support arm  4  to within a range of load transfer positions until the load is greater than the predetermined maximum load to be pushed. Those skilled in the art will recognize that any movement restricting mechanism which can exert a force against the support arm may be substituted for the support spring  12  to restrict the movement of the support arm  4  in accordance with the principles of this invention. Support spring  12  could be replaced by a component made of rubber or other elastic material. 
     A dog  28  is pivotally mounted on a pin  36  positioned at the top of the support arm  4 . A dog spring  32  is also mounted on the pin  36  and has one spring end  32   a  contacting the dog  28  and the other spring end  32   b  contacting the support arm  4 . Thus, the spring  32  exerts a load engaging force between the support arm  4  and the dog  28  to cause the dog  28  to rotate in a counterclockwise direction. 
     The dog  28  includes a curved slot  34  that is in sliding engagement with a sliding pin  44  of an upper toggle link  40 . The sliding pin  44  can travel along the curved path of the curved slot  34  to allow the dog  28  to pivot in a clockwise direction about the pin  36  and against the force of the spring  32 . In view of FIGS. 1A-1E, the force of the spring  32  rotates the dog  28  in a counterclockwise direction until the top of the curve slot  34  contacts the sliding pin  44  to prevent the dog  28  from rotating beyond its upright, load engaging position. 
     A toggle joint  56  includes the upper toggle link  40  and a lower toggle link  48  which are interconnected at a toggle pivot  52 . As explained above, the upper toggle link  40  is connected through its sliding pin  44  to the curved slot  34  of the dog  28 . The lower toggle link  48  is pivotally connected between both the toggle pivot  52  and a pin  64  on frame  8 . The support arm  4  is also pivotally connected to the pin  64 . 
     The toggle joint  56  forms an angular orientation with respect to the pivot  52  which is pointed away from the powered pushing unit frame  8  during the normal operation of the powered pushing unit  1  as shown in FIG.  1 A. As long as the upper and lower toggle links  40  and  48  of the toggle joint  56  maintain this angular orientation, the dog  28  remains locked in its upright position due to contact between the upper end of the curved slot  34  and the sliding pin  44  as shown in FIG.  1 A. Thus, in FIG. 1A, the dog  28  can only rotate in a clockwise direction about the pin  36 , against the force of the dog spring  32 . 
     A central adjustable link  66  is interconnected between the toggle pivot  52  and a pin  67  mounted on an extension  69  of the powered pushing unit frame  8 . The central adjustable link  66  includes a slot  70 . The toggle pivot  52  is mounted within a rectangular slide  71  which is shaped to enable the pivot to move along the slot  70 . 
     A toggle biasing mechanism  68 , shown as a coil spring, is mounted within the central adjustable link  66  so as to be effectively mounted between the extension  69  of the powered pushing unit frame  8  and the toggle pivot  52 . The spring  68  has a spring force which is substantially less than the spring force of the coil spring  12  and is thus substantially less than the predetermined maximum load which the powered pushing unit will push. As can be seen in FIGS. 1A-1E, one end of the toggle spring  68  is mounted within an extension of the slot  70  and is stationary throughout the operation of the toggle joint  56 . The other end of the toggle spring  68  contacts one end of the slide  71  in which the toggle pivot  52  is mounted. An adjustable toggle bolt  72  is inserted through a threaded hole at the free end of central adjustable link  66 . The end of toggle bolt  72  which extends into slot  70  contacts an end of the slide  71  opposite to the end of the slot  70  which is contacted by the toggle spring. As a result, rotating the bolt  72  so that a greater portion of its length extends within the slot  70  compresses the toggle spring  68  to increase its force on toggle pivot  52  that pushes or biases the toggle joint  56  in the first angular direction with respect to the toggle pivot  52 . Decreasing the length of the toggle bolt  72  in the slot  70  decreases the force of the toggle spring  68  on the toggle pivot  52 . 
     Consider engagement of a load  60  in a forward direction  2  by the powered pushing unit  1  when the load  60  is less than a maximum predetermined magnitude. The dog  28  contacts the load  60  and the load  60  exerts a force on the dog  28  and dog pivot  36 . The dog  28  will be naturally inclined to rotate on its pivot  36  in a counterclockwise direction, but will be restricted from doing so by sliding pin  44 . A force will also be exerted on the dog&#39;s pivot  36  and support arm  4 , which will in turn exert a force on support spring  12 . The spring force of support spring  12  on the support arm  4  has been adjusted by the operator to be commensurate with the predetermined maximum size of the load which is to be pushed. Thus, there will be no significant compression of the support spring  12 , by lesser loads, and support arm  4  will remain substantially in its upright position as shown in FIG.  1 A. So long as support arm  4  remains upright, upper and lower toggle links  40  and  48  of the toggle joint  56  continues to retain an angular orientation with respect to toggle pivot  52  away from powered pushing unit frame  8 . As a result, the dog  28  as locked and prevented from rotating away from the load  60 , allowing the load  60  to be conveyed in the forward direction  2 . 
     Now consider engagement of a load  60  in the forward direction  2  where the load  60  is greater than a maximum predetermined magnitude. The dog  28  again contacts and receives the force of the load  60 . The dog&#39;s pivot  36  and support arm  4  bear the load from dog  28 , and as a result, support arm  4  exerts a force on support spring  12 . Since the magnitude of the load  60  is in this case greater than the predetermined magnitude, support spring  12  is no longer capable of bearing the load as exerted by the dog  28  and support arm  4 . As a result, support spring  12  begins to compress and the dog  28  and support arm  4  begin to rotate about support arm pivot  64 . 
     Referring now to FIG. 1B, as the support arm  4  continues to rotate about the support arm pivot  64 , the sliding pin  44  rotates the upper toggle link  40  about toggle joint  52 , while each lower link  48  remains stationary. Support spring  12  continues to compress and toggle joint  56  becomes increasingly elongated, varying the joint&#39;s angular orientation with respect to the powered pushing unit frame  8  until the toggle joint  56  is fully elongated as shown in FIG.  1 B. This full elongation of the toggle joint  56  releases the joint&#39;s locking effect on the dog  28 . FIG  1 C illustrates the beginning rotational movement of the dog  28  resulting from the unlocking effect of the toggle joint  56 . The toggle joint  56  assumes a new angular orientation with respect to the toggle pivot  52 , pointed toward powered pushing unit frame  8 . With the termination of the locking action by the toggle joint  56 , the sliding pin  44  no longer restricts the rotation of the dog  28  about its pivot  36  in a counterclockwise direction. Thus, the toggle  56  permits the dog  28  to rotate and thereby yield to the force of the load  60 , as illustrated in FIG.  1 D. The termination of the toggle joint&#39;s locking action also permits support arm  4  to return to its upright position due to the released compression of support spring  12 , as further illustrated in FIG.  1 D. 
     The force of the toggle spring  68  should be a fraction of the magnitude of the strength of support spring  12 . Thus, while the toggle joint  56  is in the position shown in FIG. 1D, the force of the dog  28  against the bottom of the load  60  is substantially less than the force of the support spring  12 . This substantially lower force minimizes any risk that the dog  12  will upset or damage the load  60  as it bypasses this load. When the dog  28  bypasses the load  60  so that it no longer contacts the bottom of the load  60 , the force of the toggle spring pushes the toggle joint  56  to its previous locked position shown in FIG.  1 A. Thus, after bypassing a load greater than the predetermined maximum level, the powered pushing unit automatically resets itself to engage subsequent loads along its path. 
     Those skilled in the art will understand that although the maximum predetermined magnitude of the load will be commensurate with the spring force of the support spring  12  used, characteristics inherent to the design of this invention permit the adjustment of the maximum load capacity of the pushing unit. For example, further compression of support spring  12  with adjusting nut  13  increases the load force required to compress the spring with support arm  4 , thereby increasing the load required to effect the initial rotation of dog  28 . The maximum predetermined magnitude can also be adjusted by altering the relative angle between the toggle links  40  and  48  of toggle joint  56  to increase the amount the support arm  4  must rotate to cause the toggle joint  56  to assume the angular orientation shown in FIG.  1 E. 
     Adjustment of return spring bolt  72  repositions the toggle pivot  52  against the return spring  68 , variably increasing or decreasing the angle shown in FIG. 1A of the toggle joint  56  with respect to the powered pushing unit frame  8 . Increasing the angle of the toggle joint  56  results in a load of a greater predetermined magnitude being required to compress support spring  12  to the point in which the toggle joint  56  will take on a linear orientation shown in FIGS. 1B-1D so as to unlock the dog  28 . Reducing the angle of the unloaded toggle joint  56  results in a lesser force being required to unlock the toggle joint  56 , thereby lowering the predetermined magnitude. 
     Referring to FIG.  1 A and then to FIG. 1E, consider engagement of a load  60  while the pusher unit  1  is traveling in the reverse direction  3 . The toggle joint  56  continues to lock the dog  28 , preventing rotation in a counterclockwise direction. However, contact by the dog with the load  28  when the pusher unit  1  is traveling in direction  3  permits springing engagement between the dog  28  and the load  60  and rotation in a clockwise direction against the force of the dog spring  32 . Thus, the shuttle will effectively bypass all loads it encounters while traveling in the reverse direction  3 . The dog spring  32  will cause the dog to rotate in a counterclockwise direction to its upright position shown in FIG. 1A after the dog passes the load  60 . 
     In traveling in either the forward direction  2  or reverse direction  3 , the powered pushing unit follows a predetermined path along a stationary rack. The stationary rack can be any one of a number of appropriate embodiments, including for example, an I-beam, gutter, rails and the like. 
     FIGS. 2A-2C show one preferred embodiment of this invention with two parallel I-beam tracks  76  which the powered pushing unit  1  engages with a combination of drive wheels  80  and reaction wheels  84 . The drive wheels  80  effect movement of the shuttle in both the forward and reverse directions, powered by an electric motor  88  connected through a gearbox  92 , each of which is incorporated into frame  8  of the powered pushing unit  1 . FIGS. 2A-2C also show the use of two sets of dogs  28 , toggle joints  56  and other components of the pushing unit  1  for pushing heavier loads. One or more additional sets of components could be used for even heavier loads if desired. 
     In alternative embodiments, one or more cogs can be used in lieu of a drive wheel for effecting movement of the pushing unit along a substantially stationary chain, tooth bar, rack, or other grooved track device. FIG. 7 illustrates such an embodiment that includes a powered cog-drive wheel  96  for effecting movements of the pushing unit  1  along a rigid chain  100 . An additional adjustable cog  104  may be added to the arrangement so that chain tension can be adjusted with a cog-adjusting bolt  108 , as illustrated in FIG.  6 . 
     Additional embodiments of the invention may effect movement of the pushing unit through an external power source connected to the pushing unit through a chain, cable, flexi-band, or other apparatus. In such an arrangement, chain-cogs  112  can be situated at either end of the powered pushing unit&#39;s path as illustrated in FIG.  3 . Although the invention is shown in FIG. 3 as incorporating chain-cogs  112  and a drive chain  116  attached to either end of the pushing unit, it will be appreciated that other embodiments using non-toothed apparatuses such as cables and flexi-bands may incorporate devices such as pulleys, drums, and the like in lieu of cogged elements. 
     Although the invention is shown and described hereinbefore with a toggle release mechanism incorporating a described arrangement of levers and adjustable spring elements, it will appreciated that variations in the construction and orientation of the toggle mechanism may be incorporated without departing from the spirit and scope of the invention. For example, FIG. 4 illustrates an alternate embodiment using an appropriate toggle mechanism wherein each dog  28   a  and toggle mechanism  56   a  is entirely disposed on a non-stationary load block  120 . The entire load block  120  is in sliding engagement with two or more support bolts  16   a  and  16   b  extending from the powered pushing unit frame  8   a . The restricting mechanism comprises separate support springs  12   a  and 12 b  that are mounted on support bolts  16   a  and  16   b , respectively, for providing an appropriate load engaging force and for pushing a load  60   a . Support bolts  16   a  and  16   b  also have separate spacer elements  24   a  and  24   b  mounted on them. This embodiment of this invention also includes a return toggle spring  68   a  interposed between toggle joint  52   a  and the powered pushing unit frame  8   a.    
     Upon engagement of a load  60   a  in excess of the predetermined magnitude, as determined by the force set for the support springs  12   a  and  12   b , while the pushing unit  1  is moving in the forward direction  2   a , load block  120  slides along support bolts  16   a  and  16   b , compressing support springs  12   a  and  12   b . Upper toggle link  40   a  then rotates about toggle joint  52   a  until toggle joint  56   a  becomes filly elongated and unlocks, thereby allowing rotation of the dog  28   a  so the pushing unit  1  can bypass the excessive load  60   a . Here, again, the lower force of the toggle spring  68   a  in comparison to the force of the support springs  12   a  and  12   b , determines the force the dog  28   a  places on the bottom of load  60   a . After bypassing the load  60   a , toggle spring  68   a  automatically repositions the dog  28   a  to its upright position shown in FIG.  4 . 
     The invention is shown and described above in embodiments which have the dog, toggle and associated components positioned behind the powered pushing unit frame. Those skilled in the art will understand that these components may be mounted in other positions on the frame without departing from the spirit and scope of this invention. For example, FIG. 5 illustrates an alternative embodiment using a non-stationary load block toggle mechanism similar to that of FIG.  4 . In FIG. 5, the toggle mechanism is positioned in front of the powered pushing unit frame  8   b  for conveyance of load  60   b  in a forward direction  2   b . Thus, the respective dispositions of the dog  28   b , toggle joint  56   b , support springs  12   c  and  12   d , and spacer elements  24   a  and  24   b  are a mirror image opposite of those shown or described in FIG. 4, although this embodiment operates in substantially the same manner. 
     FIG. 8 also illustrates a powered pushing unit  1  having a pushing dog support arm and toggle joint mechanism which operates to push a load in the opposite direction of the powered pushing unit show in FIGS. 1A-1E. Most of the components of the powered pushing unit  1  shown in FIG. 8 are the same as those of the powered pushing unit shown in FIG.  1 . However, a spring  12  in FIG. 8 is located and compressed between support arm  4  and block  69 . The total length of the support spring  12  is not shown in FIG. 8 due to the illustration of the toggle links  40  and  48 . 
     The toggle spring  168  which is shown in FIG. 8 is a watch spring that is mounted on toggle pivot  52 . One end of the watch spring  168  is attached to the upper toggle link  40 , while the other end  168   b  is attached to the lower toggle link  48 . As a result, the force of the toggle spring  168  biases the toggle pivot  52  toward the frame  8 . The toggle spring  168  operates in the same manner as the toggle spring  68  shown in FIG.  1 . 
     This invention has been explained with respect to the details, arrangements of components and certain specific embodiments shown in the accompanying drawings. Many modifications can be made to these embodiments to those skilled in the art without departing from the spirit and scope of this invention. Thus, the appended claims are intended to be interpreted to cover such equivalent powered pushing units which do not depart from the spirit and scope of this invention.