Abstract:
A dock leveler having a frame and a deck pivotably mounted at one end thereof to the frame. A lip is pivotably mounted to the deck at another end thereof. A lip latch and lip extension mechanism are mounted to the leveler and comprises a lip latch pivotably connected to the deck by a crank mechanism and a latch bar pivotably connected to the crank mechanism. The latch bar has one end selectively engaging a latch bracket mounted to the deck. A bar is connected at one end to the crank mechanism and another end is operably connected to the lip. A first spring is operably connected to the crank mechanism and the frame. A second spring is operably connected to the bar and the frame. A third spring operably couples another end of the latch bar to the second spring. Upon upward movement of the deck the first spring urges the crank mechanism in a first direction to move the bar so that said lip is raised from a pendant position to an extended position and the latch bar moves based on movement of the crank mechanism and engages the latch bracket at a first point to hold said lip in the extended position and is maintained in engagement by the third spring.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to loading dock equipment and in particular to dock levelers that are used to span the distance between a loading dock and the bed of a vehicle. Specifically, it deals with an improved mechanical latch for the lip of a dock leveler.  
           [0003]    2. Prior Art  
           [0004]    A conventional dock leveler has a deck assembly which typically stores in a position level with the dock floor, and has a pivoting lip assembly which extends outward to rest on the vehicle which is being loaded. The lip must hinge downward approximately 90 degrees for the lip to be removed from the vehicle and to store the dock leveler with the lip hanging in a pendant position. To move the dock leveler from the stored position to the operative position, the leveler is raised, the lip is extended from the pendant position and the leveler is then lowered until it is supported by the lip resting on the transport vehicle. The use of various mechanisms as a mechanical latch to hold the lip in the extended position until it rests on the transport vehicle is well known in the industry. U.S. Pat. No. 2,974,33 discloses a pawl mounted to the dock leveler engaging a lug on the lip. U.S. Pat. No. 3,249,956 discloses a releasable lip latch that is supported by a spring which will yield to allow the lip to fold if it is inadvertently struck by a backing truck. Both U.S. Pat. No. 3,662,416 and U.S. Pat. No. 4,398,315 show over-center toggle mechanisms as a latch which is yieldable to allow the lip to fold if it is inadvertently subjected to an excessive downward load.  
           [0005]    U.S. Pat. No. 4,937,906 discloses a lip counterbalance spring attached to the frame instead of the deck as is conventional in this technology. The purpose is to provide extra force to extend the lip. The advantage of this system is that the lip is at least partially counterbalanced throughout the operating range of the leveler. Another more complicated system is disclosed in U.S. Pat. No. 6,112,353 disclosing a yieldable lip latch.  
           [0006]    A major limitation of prior art mechanical lip latches has been that the lip latch does not automatically disengage and allow the lip to fall to the pendant position if the lip is extended when a transport vehicle is not present and when safety legs or cross traffic legs are engaged. Safety legs or cross traffic legs are well known in the dock leveler industry and are used to limit the distance that the deck will fall if the transport vehicle inadvertently pulls away when the leveler is supported by the lip resting on the vehicle. However the presence of safety legs can cause problems for prior art mechanical lip latches. Several designs including U.S. Pat. No. 3,662,416 and U.S. Pat. No. 5,475,888 disclose a means to release the lip latch when the dock leveler descends to its lowest position. However when safety legs or cross traffic legs are engaged, the dock leveler is prevented from descending to its lowest position and the latch will remain engaged until the lip is manually lifted to allow the latch to release. A second problem with mechanical lip latches is referred to in the industry as “stump out” and occurs when the bed of the transport vehicle is lower than the lip when the safety legs engage the frame of the leveler. Unless the dock operator notices the problem and retracts the safety legs, the lip will be supported by the lip latch and not by the bed of the vehicle. A fork truck driven over the lip will force it down and cause severe damage to the lip latch.  
           [0007]    One attempt to address this problem has been the use of a viscous damper commonly referred to as a “hydrashock” to replace the lip latch. Such a device is shown in U.S. Pat. No. 5,323,503. The lip is able to freely extend but the rate of fall of the lip is retarded by the viscous resistance of the damper. Thus if the lip is left extended without the support of a transport vehicle, the lip slowly falls by gravity. While eliminating some of the problems associated with mechanical lip latches, the viscous damper has its own significant limitations. The viscosity of the oil in the damper changes with temperature. As the viscosity decreases in warm weather the rate of fall of the lip increases and the lip may not remain extended long enough to properly engage the bed of the transport vehicle. Conversely as the viscosity increases in cold weather, the rate of fall of the lip may be so slow that it impedes the ability to move the leveler from the transport vehicle to the stored position with the lip pendent. Most dock levelers with such devices provide multiple mounting positions of the damper so that the force resisting lip falling may be modified for large changes in ambient temperature.  
           [0008]    Another attempt to provide a yieldable latch is set forth in U.S. Pat. No. 4,398,315. The configuration disclosed is a latch that releases by buckling within the link to the lip rather than by a latch mounted to the dock leveler. Another proposed solution is found in U.S. Pat. No. 6,112,353 which employs a yieldable lip latch with a compensating link supporting the lip bellcrank.  
           [0009]    Dock levelers use various means to raise the deck and extend the lip. Dock levelers which are upwardly biased with springs are typically “walked down” from the elevated position by dock worker placing his weight on the deck and the rate of decent is relatively rapid. Dock levelers which use powered means such as an electric actuator, hydraulic cylinder or inflatable bag to raise the leveler have a slower rate of decent. While the viscous damper may provide satisfactory performance for a “walk-down” type of mechanical leveler, it is much less suitable for use with power actuated levelers having a slower rate of descent. If the viscous damper were stiff enough to hold the lip extended until the leveler lowered the lip to the transport vehicle then an unacceptably long time would be required to allow the lip to fall while restoring the leveler.  
         SUMMARY OF THE INVENTION  
         [0010]    This invention is a mechanical lip latch that automatically disengages at multiple positions of deck height depending on whether the safety legs are engaged. The latch is disengaged at the lower limit of downward travel of the dock leveler. The lower limit is determined by whether the safety legs are engaged or retracted. The latch also has multiple positions of engagement to ensure that the lip is supported even if it is not fully extended. The latch is also designed to yield and disengage to protect it from damage if excess force is applied to the lip.  
           [0011]    The first preferred embodiment has a lip extension structure suited for the faster activation speed of an upwardly biased “walk down” dock leveler. The second preferred embodiment has a lip extension method better suited for the slower activation speed of a powered up, a downwardly biased dock leveler. In each of these embodiments the ability to vary the lip tension is a significant benefit. For example the ability to increase the tension may be limited so that the lip can fall when the leveler is raised from a high truck.  
           [0012]    In the third preferred embodiment a single lip spring is attached to the deck to maintain support for the lip and additionally is releasably attached to the frame. This spring is engaged to the frame only when the lip nearly fully pendant and therefore the spring tension may be increased as the deck is raised to extend the lip without the necessity of using a lip cam as in the second preferred embodiment. This embodiment also uses a lip latch which is biased toward the release position only when the deck is lowered to the working position. Thus a second spring to overcome the release spring when the deck is raised is unnecessary.  
           [0013]    In the first and second preferred embodiments the lip spring tension is varied but the increase in tension has a limit or else the lip may not fall when the leveler is raised from the bed of a truck that is high. In the third embodiment a single lip spring is employed, attached to the deck to maintain support for the lip. It is releasably attached to the frame. The spring is engaged with the frame only during the period of time when the lip is nearly fully pendant and therefore the spring tension may be sufficiently increased as the deck is raised to extend the lip without requiring the lip cam of the second embodiment. The third embodiment also has a lip latch which is biased toward to release position only when the deck is lowered to the operative position and thus does not require a second spring to overcome the force of the release spring.  
           [0014]    In accordance with this invention there is a provision for a multi-position latch trip. This allows the release of a mechanical lip latch at multiple positions of deck height as a function of the deployment state of the safety legs.  
           [0015]    This invention will be described more completely by reference to the drawing and the description of the preferred embodiments that follow. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0016]    [0016]FIG. 1 is a sectional side view of the first preferred embodiment of this invention with the leveler raised and the lip in the pendant position;  
         [0017]    [0017]FIG. 2 is a sectional side view of the first preferred embodiment of this invention with the leveler raised and the lip held by the lip latch in a partially extended position;  
         [0018]    [0018]FIG. 3 is a sectional side view of the first preferred embodiment of this invention with the leveler raised and the lip held by the lip latch in a fully extended position;  
         [0019]    [0019]FIG. 4 is a sectional side view of the first preferred embodiment of this invention with the leveler lowered to an operative position and the lip latch deflected by an external force on the lip;  
         [0020]    [0020]FIG. 5 is an exploded view showing the safety legs, lip latch trip rod and trip bar;  
         [0021]    [0021]FIG. 6 is a sectional side view of the first preferred embodiment of this invention with the leveler lowered to rest on the safety legs and the lip latch disengaged;  
         [0022]    [0022]FIG. 7 is a sectional side view of the first preferred embodiment of this invention with the safety legs retracted and the leveler almost fully lowered;  
         [0023]    [0023]FIG. 8 is a sectional side view of the first preferred embodiment of this invention with the leveler fully lowered and the lip latch disengaged;  
         [0024]    [0024]FIG. 9 is a partial sectional side view of the second preferred embodiment of this invention with the leveler raised and the lip in the pendant position;  
         [0025]    [0025]FIG. 10 is an enlarged partial sectional view of the latch assembly of the second preferred embodiment of this invention;  
         [0026]    [0026]FIG. 11 is a partial sectional side view of the second preferred embodiment of this invention with the leveler raised and the lip held by the lip latch in a partially extended position;  
         [0027]    [0027]FIG. 12 is a partial sectional side view of the second preferred embodiment of this invention with the leveler fully lowered and the lip latch disengaged;  
         [0028]    [0028]FIG. 13 is a sectional side view of the third preferred embodiment of this invention with the lip extended and resting on a transport vehicle;  
         [0029]    [0029]FIG. 14 is an enlarged view of the latch bar;  
         [0030]    [0030]FIG. 15 is an enlarged view of the hook assembly for the lip spring;  
         [0031]    [0031]FIG. 16 is a sectional side view of the third preferred embodiment of this invention with the deck raised to remove the lip from the transport vehicle;  
         [0032]    [0032]FIG. 17 is a partial sectional side view of the third preferred embodiment of this invention with the lip latch forcing the hook into engagement with the frame;  
         [0033]    [0033]FIG. 18 is an enlarged view of the latch release rod;  
         [0034]    [0034]FIG. 19 is a partial sectional side view of the deck raised and the hook providing increased tension for the lip spring;  
         [0035]    [0035]FIG. 20 is a sectional side view of the third preferred embodiment of this invention with the leveler lowered to an operative position and the lip latch deflected by an external force on the lip;  
         [0036]    [0036]FIG. 21 is a partial sectional side view of the third preferred embodiment of this invention with the leveler fully lowered and the lip latch disengaged.  
         [0037]    [0037]FIG. 22 is a perspective view of the lip latch of the fourth preferred embodiment of this invention;  
         [0038]    [0038]FIG. 23 is a sectional side view of the fourth preferred embodiment of this invention with the leveler raised and the lip held by the lip latch in an extended position; and  
         [0039]    [0039]FIG. 24 is a sectional side view of the fourth preferred embodiment of this invention with the with the leveler lowered to the working range, the lip held by the lip latch in an extended position, and the lip latch spring biased toward the disengaged position. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0040]    Referring now to FIGS. 1 through 8 the essential components of the first preferred embodiment of this invention will be described, it being understood that a typical dock leveler has other constructional features, not illustrated. A loading dock is shown with a driveway approach  1 , a dock face  2 , and a dock floor  3  with a recessed pit  4 . A transport vehicle  5  is shown in front of the dock. The dock leveler  10  is typically mounted in the pit  4 . A frame has horizontal members  14  and a vertical brace  15  both of which rest in the pit. The leveler frame also has stop blocks  16  and lip keepers  17  at the forward end of the horizontal members  14 . A trip plate  18 , shown in broken lines in FIG. 1, is attached to the far side of one horizontal member  14  at a predetermined distance below the top of the stop block  16 . The leveler  10  has a deck  20  which has a top plate  21 , a plate  22  that forms a front header and a plate  23  that forms a rear header. Deck beams  24  attached to the top plate and header bars provide structural strength to the assembly. The deck  20  is pivoted to the frame at pivot  25 .  
         [0041]    A lip  30  is pivoted to the deck on a pin  26  inserted in hinge tubes  27  attached to the front header bar  21  and hinge tubes  32  attached to the lip plate  31 . Control arms  33  are attached to the lip plate  31 . Although not illustrated, the leveler is held horizontal in the stored position with the lip  30  in the pendant position and retained in the lip keepers  17 . The lifting of the dock leveler to the position shown in FIG. 1 may be accomplished by any means including mechanical linkage and springs, electric actuator, hydraulic cylinder or inflatable bag. Such is not material to the functioning of this invention.  
         [0042]    Two pairs of brackets  29  are attached to the front header plate  22  to carry the safety legs  70  on pivot pins  76 . As shown in FIG. 5 the safety legs comprise two vertical bars  71 . Each bar  71  is attached to a pivot boss  73  by an arm  72 . A cross bar  74  joins both vertical bars  71  to ensure that they move in and out of engagement together. One bar  71  carries a pin  75  to control the lip trip mechanism that will be described later. When in the forward position the vertical bars  71  are placed between the front header plate  22  and the stop blocks  16  to limit the downward travel of the deck  20  as shown in FIG. 6. The safety legs  70  are urged forward to the operative position by a spring, not illustrated. To allow the deck to fall lower the safety legs must be manually retracted, typically by the operator pulling on a chain that is also not illustrated.  
         [0043]    The deck  20  also carries a support bar  27  with a pivot pin  28 . A crank assembly  35  pivots on the pin  28  and carries pins  36  and  37 . A bar  40  is attached at one end to the pin  36  and at the other end to the lip control arms  33  by a pin  38 . A spring  41  is attached to the pin  37  on the crank assembly  35  by an adjusting bolt  44  and a nut  45 . The other end of the spring  41  is attached to two chains  42  and  43  which are attached to the deck  20  and a vertical frame brace  15  respectively.  
         [0044]    As shown in FIG. 4, when the leveler is lowered to an operative position, near horizontal, the chain  43  is slack and the spring  41  is held by chain  42  attached to the deck  20 . The adjusting bolt  44  is positioned by the nut  45  to tension the spring  41  so that most of the weight of the lip  30  is counterbalanced. The tension of the spring must allow the lip to fall by gravity to the pendent position for storing.  
         [0045]    Referring now to FIGS. 1, 2 and  3 , rotation of the deck  20  to the raised position causes the chain  43  to increase the tension of the spring  41  and thereby provide greater assistance in rotating the lip  30  to the extended position.  
         [0046]    Referring now to FIGS. 1 through 8 the operation of the first preferred embodiment of the lip latch and extension mechanism will be described. As shown in FIG. 1, a latch bar  50  pivots on the pin  36  of the crank assembly  35 . One end of the latch bar  50  has a notch that provides two engagement surfaces  51  and  52 . The other portion of the latch bar  50  carries a release arm  53  and a pin  54 . A latch bracket  55  is attached to the deck and has a slotted opening which guides the end of the latch bar  50  yet allows some limited vertical travel. A latch plate  60  is attached to the latch bracket  55  by a bolt  56 , nut  57  and spring  61 . A chain  65  has one end attached to the bar  40  and the other end to a spring  66  which is then attached to the frame member  14 . A spring  67  has one end attached to the chain  65  and the other end attached to the pin  54  on the latch bar  50 . As the dock leveler is lifted toward the position shown in FIG. 1 the chain  65  is stretched taut and pulls the front of the bar  40  against the pin  38 , causing the lip  30  to rotate rapidly towards the extended position. The spring  66  stores energy and limits the force exerted on the chain  65 . The chain  65  and spring  66  also limit the upward travel of the deck  20 . Because the latch bar  50  heavier than the control arm  53  gravity urges the latch bar to fall out of engagement. The spring  67  is pulled taut by the chain  65  and causes the latch bar  50  to rotate clockwise into engagement with the latch plate  60 .  
         [0047]    As the lip  30  approaches the extended position shown in FIG. 2 the line of the force exerted on the pin  38  moves much closer to the lip pivot pin  26  and the rotational moment exerted by the chain on the lip is greatly reduced. The lip  30  is urged toward the fully extended position by rotational inertia and by the force exerted on the lip bar  40  by the lip spring  41  acting on the crank assembly  35 . Because resistance to extension of the lip is dependent on the factors such as wear, debris and lack of lubrication, the energy available may not always be sufficient to fully extend the lip. FIG. 2 shows the lip  30  almost fully extended with the surface  51  of the latch bar  50  engaging the latch plate  60 . Although not fully extended, the lip  30  is still held in a position where it can properly engage the bed of a transport vehicle. Without the alternate latch position provided by the surface  51 , the lip would fall back to the pendant position as the deck is lowered. FIG. 3 shows the lip  30  fully extended with the surface  52  of the latch bar  50  engaging the latch plate  60 . Because the tension of the spring  41  is increased when the deck  20  is fully raised, the weight of the lip  30  may not be sufficient to overcome the spring  41  and hold the latch bar  50  in contact with the latch plate  60 .  
         [0048]    The spring  67  maintains the latch bar  50  in the engaged position. As the deck  20  is lowered and the tension of both springs  41  and  67  is reduced and gravity urges the latch bar  50  to fall out of engagement with the latch plate  60 . The weight of the lip  30  acting on the bar  40  holds the end of the latch bar  50  against the latch plate  60  and the lip  30  is prevented from falling. When the deck  20  lowers and the lip  30  is supported by a transport vehicle, the load is removed from the latch bar  50  and it falls out of engagement with the latch plate  60  allowing the lip to fall when the dock leveler is stored.  
         [0049]    [0049]FIG. 4 illustrates the dock leveler with the lip  30  extended and an external force “F” exerted essentially horizontally on the end of the lip. The spring  61  has sufficient compression to withstand the force exerted on the latch bar  50  by the weight of the lip  30 . When the force on the latch bar  50  exceeds the compression load of the spring  61  the spring will deflect and allow the latch plate  60  to rotate. The end of the latch bar  50  will then slip out of engagement with the latch plate  60  and the lip  30  will fall pendent.  
         [0050]    The components that automatically disengage the lip latch  50  will now be described. As shown on FIG. 5 a trip bar  80  has a formed member  81  with its rearward end attached to a pivot boss  82 . An angle bracket  83  is attached near the forward end of the member  81 . A control surface  84  is formed into the middle part of the member  81 . FIG. 5 also shows a trip bar  85  that has a pivot hole  86  and an elongated hole  87 . As shown in this exploded view, the end of the rod  81  engages the hole  86  to carry the trip bar  85 . The pin  75  on the safety leg assembly  70  engages the elongated hole  87  in the trip bar  85 .  
         [0051]    As illustrated in FIGS. 1, 2 and  3  the trip rod pivots on pin  28 . The forward end of the trip rod  80  is supported by the trip bar  85  which is supported at the top of the elongated hole  87  by the pin  75  on the safety legs  70 .  
         [0052]    [0052]FIG. 6 illustrates how the lip latch is automatically disengaged as the deck  20  is lowered and the safety legs  70  rest on the stop blocks  16 . In FIGS. 1 through 4 the vertical position of the stop bar  85  is determined by the pin  75  on the safety legs  7  supporting the top of the elongated hole  87 . FIG. 6 has the stop block  16  cut away to show the stop plate  18  attached to the frame member  14 .  
         [0053]    As the deck  20  falls the lower end of the trip bar  85  rests on the trip plate  18 . Thus the forward end of the trip rod  80  is held at a predetermined height above the trip plate  18  as the deck  20  is lowered to rest on the safety legs  70 . As the deck  20  moves down the trip rod  80  rotates upward relative to the deck  20 . The control surface  84  on the trip rod  80  engages the pin  54  on the latch bar  50  forcing the control arm  53  upward and the end of the latch bar  50  downward and out of engagement with the latch plate  60 . The lip  30  is now free to fall to the pendent position.  
         [0054]    [0054]FIG. 7 illustrates the condition where the dock leveler with the safety legs moved to a retracted position. The pin  75  on the safety legs  70  causes the trip bar  85  to rotate rearward and expose the angle bracket  83 . Thus, as the deck  20  falls to the fully lowered position shown in FIG. 8, the trip bar  85  does not engage the trip plate  18  and the lip latch does not disengage prematurely. However when the deck  20  is fully lowered the angle bracket  83  engages the trip plate  18  and this causes the lip latch  50  to disengage from the latch plate  60  as described herein.  
         [0055]    A second preferred embodiment of the invention is illustrated in FIGS. 9, 10,  11  and  12 . This embodiment is better suited for a powered dock leveler where the rate of lifting the deck is much slower and there is insufficient rotational inertia of the lip to ensure that it is fully extended. With the exception of the lip latch and release components, the dock leveler has the same components as the first preferred embodiment.  
         [0056]    [0056]FIG. 9 shows a roller  88  on the pin  28 . A latch bar  90  pivots on the pin  38 . The latch bar  90  has a cam surface  91 , a stop surface  92  and a latch surface  93 . The chain  65  is attached to the latch bar  90  rather than to the lip bar  40  as in the first preferred embodiment. The trip rod  180  is similar to the trip rod  80  of the first preferred embodiment except for having a different formed shape.  
         [0057]    In FIG. 10 the latch housing  95  and spring housing  96  are mounted to the deck  20 . A latch assembly  100  has an adjustable bolt  101 , flange  102  and latch block  103  which is free to move axially in the latch housing  95 . A latch spring  97  is supported in the spring housing  96  and acts against a nut  98  to urge the flange  102  of the latch assembly  100  against the end of the latch housing  95 . The latch housing  95  also carries a latch release spring  99 .  
         [0058]    When the lip is in the pendent position as shown in FIG. 9, tension in the chain  65  acts on the latch bar  90  to pull down on the pin  38  and cause the lip  30  to rotate. As the lip approaches the extended position shown in FIG. 11 the line of force exerted on the pin  38  moves much closer to the lip pivot pin  26  and the rotational moment exerted by the chain  65  on the lip  30  is greatly reduced. The lip  30  is urged toward the fully extended position shown in FIG. 11 by force exerted on the lip bar  40  by the lip spring  41  acting on the crank assembly  35  and by the cam surface  91  bearing on the roller  88 . Any increase in resistance to extension of the lip caused by factors such as wear, debris and lack of lubrication may be overcome by increasing the tension on the chain  65 . The lip  30  is fully extended when the stop surface  92  of the latch bar  90  contacts the roller  99 . The stop surface  92  also deflects the latch release spring  99 . The latch surface  93  is positioned against the end of the latch block  103 .  
         [0059]    As in the first preferred embodiment, in this embodiment, the weight of the lip  30  may not be sufficient to overcome the spring  41  and hold the latch bar  90  in contact with the latch block  103 . The spring  67  maintains the latch bar  90  in the engaged position until the deck  20  has lowered. The tension of the spring  41  decreases and the weight of the lip  30  is sufficient to hold the latch surface  93  against the end of the latch block  103 . When the deck  20  lowers and the lip  30  is supported by a transport vehicle, the load is removed from the latch bar  90 . Because forward travel of the latch block  103  is limited by the flange  102  bearing against the end of the housing  95 , the latch bar  90  moves away from the latch block  103 . The release spring  99  lifts the end of the latch bar out of engagement and the lip is free to fall. The latch spring  97  has sufficient compression to withstand the force exerted by the weight of the lip  30  acting on the latch bar  90 . However an external force exerted on the end of the lip will cause the latch spring  97  to deflect. The cam surface  91  acting on the roller  88  will cause the latch arm  90  to be lifted out of engagement with the latch block  103  and the lip will be free to fall.  
         [0060]    [0060]FIG. 12 illustrates how the lip latch is automatically disengaged as the deck  20  is lowered and the safety legs  70  rest on the stop blocks  16 . As in the first preferred embodiment, in this embodiment, the lower end of the trip bar  85  contacts the trip plate causing the trip rod  80  to rotate upward as the deck  20  is lowered. The trip rod  80  engages the bottom surface of the latch bar  90  forcing the end of the latch bar out of contact with the latch block  103  and allowing the lip  30  to fall by gravity. As in the first embodiment, when the safety legs  70  are retracted the pin  75  on the safety legs  70  causes the trip bar  85  to rotate rearward and expose the angle bracket  83 . Thus as the deck  20  falls to the fully lowered position the angle bracket  83  engages the trip plate  18  and causes the lip latch  50  to disengage from the latch plate  60  as described previously.  
         [0061]    A third preferred embodiment of the invention is illustrated in FIGS. 13 through 21. This embodiment is also suited for a powered dock leveler where the rate of lifting the deck is much slower and there is insufficient rotational inertia imparted to the lip  30  by the lip chain  65  to ensure that the lip is fully extended. FIG. 13 shows a dock leveler with the lip  30  resting on a transport vehicle  5 . The frame member  15  carries an anchor pin  19 . The deck  20  has a bracket  105  having a slotted hole  106 . A pair of link bars  107  are attached to the bracket  105  by a pin  108 . FIG. 15 shows a hook assembly  110  having a hook  111  and attachment holes  112  and  113 . A lever arm  114  projects upward and carries a cantilevered spring  115 . The hook assembly  110  is attached to the end of the link bars  118  by a pin  109  passing through the hole  112 . The spring  41  has one end attached to the hook assembly  110  through the hole  113  and the other end to the pin  37  on the crank  35  with the adjusting rod  44  and nut  45 . The rod  44  is adjusted so that most of the weight of the lip  30  is counterbalanced by the tension of the spring  41  while still allowing the lip to fall by gravity when the deck  20  is raised from the transport vehicle  5 . As shown in FIG. 13 the spring  41  pulls the hook assembly  110  into alignment between the end of the bracket  105  on the deck  20  and the pin  37 . The hook assembly  110  is thus held so that the hook  111  is positioned above the pin  19  on the frame member  15 .  
         [0062]    When the deck  20  is raised to allow the lip to fall as shown in FIG. 16, the hook  111  does not engage the pin  19 . Consequently, the tension of the spring  41  is not increased as the deck  20  is raised.  
         [0063]    [0063]FIG. 17 illustrates an enlarged partial view of the dock leveler in the stored position. A latch housing  120  has a latch plate  121  and is pivoted on the deck with a pin  122 . A bracket  123  is anchored to the deck  20  and the latch housing  120  is held in a forward position by the spring  61 , bolt  56  and nut  57 . The latch bar  125 , shown in FIG. 14, has two latch surfaces  126  and  127 . The latch bar  125  is attached to the pin  36  on the crank  35  and passes through the latch housing  120 . As shown in FIG. 13, when the lip is extended the latch bar  125  is moved forward away from the arm  114  on the hook assembly  110 . Because the lip is supported on the transport vehicle there is no load on the latch bar  125  and the spring  115  urges the latch bar  125  upward to lift the latch surfaces  126  and  127  out of engagement with the latch plate  121 . FIG. 16 shows the end of the latch bar  125  moving closer to the lever arm  114  of the hook assembly  110  as the lip  30  rotates toward the pendent position.  
         [0064]    [0064]FIG. 17 shows the dock leveler in the stored position. When the lip  30  is fully lowered the end of the latch bar  125  contacts the lever arm  114  to rotate the hook assembly  110  and force the hook  111  to a position where it will engage the pin  19  when the deck  20  is raised.  
         [0065]    [0065]FIG. 19 shows the deck fully raised with the hook  111  engaging the pin  19 . The deck  20  has rotated forward relative to the hook assembly  110  and the end of the spring  41  has been pulled rearward relative to the bracket  105 . The pin  108  has moved in the slotted hole  106  to allow the link bars  108  to move rearward with the spring  41  and hook assembly  110 . Thus the tensional force of the spring  41  may be increased to exceed the weight of the lip  30  so that the lip can be fully extended by the force of the spring  41 . As the lip  30  is extended the latch bar  125  moves forward and out of contact with the spring  115  allowing the latch bar to fall with the latch surface  127  placed to engage the latch plate  121  as shown in FIG. 19.  
         [0066]    As described in the second preferred embodiment, in this embodiment the alternate latch position  126  will allow the latch to engage even if the lip does not full extend. The cantilever spring  115  will not engage the end of the latch bar  125  until the deck  20  has lowered to a position where the hook assembly  110  no longer exerts extra tension on the spring  41 . Thus the latch bar  125  will remain in the engaged position until the weight of the lip  30  forces the latch surface  127  into contact with the latch plate  121  and the lip will remain extended. There is no requirement for a spring  67  attached to the chain  65  to hold the latch in the engaged position as in the first and second embodiments. Because the lever arm  114  is not in contact with the end of the latch bar  125 , the hook  111  will disengage the pin  19  when the deck is lowered to a working position as shown in FIG. 13.  
         [0067]    [0067]FIG. 20 illustrates how an external force exerted on the end of the lip  30  will cause the latch plate to disengage the latch bar  125 . The bolt  56  and nut  57  can be adjusted so that the compression of the spring  61  will support the lip  30  in the extended position. An excessive force on the lip will cause the spring  61  to deflect and allow the latch housing  120  to rotate about the pin  123 . The latch bar  125  will then be supported by the rear edge of the latch plate  121 . The front edge of the latch plate will rotate downward to disengage the latch surfaces  126  and  127  and the lip  30  will be allowed to fall.  
         [0068]    [0068]FIG. 20 also illustrates a third embodiment of this invention that will release the lip latch  125  in multiple positions of the deck  20  depending on the position of the safety legs  70 . A latch release rod  130  is shown in FIG. 18 with a pivot boss  131 , a guide loop  132  and a contact bar  133 . FIG. 20 shows a latch trip angle  135  with a vertical leg  136  and horizontal leg  137  mounted on the frame member  14 . The release rod  130  is carried by the boss  131  mounted on the pin  75  of the safety legs  70  and the guide loop  132  carried by the latch bar  125 . When the safety legs  70  are forward in the engaged position the latch release rod  130  is held in a forward position with the contact bar  133  above the vertical leg  136  of the angle  135 . As the deck  20  lowers to bring the safety legs  70  into contact with the stop blocks  16 , the contact bar  133  will engage the vertical leg  136  and cause the release rod  130  to lift the latch bar  125  out of engagement with the latch plate  121 .  
         [0069]    [0069]FIG. 21 shows the safety legs  70  retracted so that the deck  20  can be fully lowered. The latch release rod  130  is moved to a rearward position where the contact bar  133  will not engage the vertical leg  136  of the angle  135 . As the deck  20  reaches the fully lowered position the contact bar  133  will engage the horizontal leg  137  and cause the release rod  130  to lift the latch bar  125  out of engagement with the latch plate  121  and allow the lip  30  to fall pendent.  
         [0070]    A fourth preferred embodiment of the invention is illustrated in FIGS. 22 through 24. This embodiment is also suited for a powered dock leveler where the rate of lifting the deck is much slower and there is insufficient rotational inertia imparted to the lip  30  by the lip chain  65  to ensure that the lip is fully extended. FIG. 22 shows the latch bar  190  having a cam surface  91  and a stop surface  92 . A latch surface  193  is recessed slightly from the cam surface  91 . A trip bar  195  projects horizontally from the side of the latch bar  190  and has a trip rod  196  attached at a downward angle.  
         [0071]    [0071]FIG. 23 shows the deck  20  in the fully raised position and the lip  30  fully extended. The latch bar  190  has engaged the roller  88  on the pin  28 . FIG. 23 also shows a latch release spring  160  attached at the front end to a pivot bushing  161  mounted on the front header bar  22 . The rear of the latch release spring  160  is supported by a chain  162  attached to the upper lip spring chain  42 . Because the chain  42  is slack when the deck  20  is raised the latch release spring  160  does not engage the trip bar  195  on the lip latch bar  190 .  
         [0072]    As in the first and second preferred embodiments, in this embodiment the tension of the spring  41  increases as the deck  20  is raised and the weight of the lip  30  may not be resting on the latch bar  190 . However in this embodiment the latch bar falls by gravity to the engaged position and there is no need of a spring  67  to hold the latch bar engaged as shown in FIG. 11.  
         [0073]    In operation, as the deck lowers the latch bar remains engaged by gravity and there is no danger that the latch bar will release accidentally even though the weight of the lip  30  may not be urging the latch surface  193  into contact with the roller  88 . As the deck  20  continues to lower the chain  43  attached to frame  15  causes the tension of the spring  41  to decrease until the spring is supported by the chain  42  attached to the deck  20 . FIG. 23 shows the deck  20  lowered to the working range. As the chain  43  is slackened the chain  42  is tightened and the rear of the latch release spring  160  is raised until it engages the trip bar  195  on the lip latch bar  190 . Because the weight of the lip  30  is resting on the latch bar  190 , the force of the latch release spring  160  cannot lift the latch bar  190  from the engaged position. However as the deck continues to lower and the end of the lip  30  is supported on the bed of the transport vehicle  5  then the spring  160  will lift the lip latch bar  190  from the engaged position and the lip  30  will fall when the deck  20  is raised. If no transport vehicle is in position as the dock  20  is lowered with the lip  30  held extended then the end of the trip bar  196  will engage the floor of the pit  4  and cause the lip latch bar  190  to disengage and allow the lip  30  to fall.  
         [0074]    While this invention has been described with respect to the preferred embodiments, it will be apparent to those skilled in this art that modifications of this invention may be practiced without departing from the scope of the invention.