Patent Publication Number: US-2007107269-A1

Title: Dipper door latch with locking mechanism

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to power shovels and, more particularly, to power shovels having a dipper adapted for excavating earthen material. Specifically, the present invention relates to latches for dipper doors.  
      Large electric mining rope shovels utilize a digging attachment consisting of a stationary boom with a combination handle/dipper structure which mounts on the boom and actively crowds and hoists into a bank in order to dig with and fill the dipper. The dipper is rigidly connected to the handle. After digging through a bank face and filling with material, the dipper is lifted and the upper portion of the shovel is rotated relative to the lower portion of the shovel so that the dipper is positioned above a waiting dump truck. The operator then activates a tripping mechanism that opens a dipper door latch on the bottom of the dipper and allows a door to swing down and open. The contents of the dipper are then dumped into the truck bed.  
      The heavy dipper door is pivotally mounted on a lower end of the dipper. The conventional mechanical latch mechanism secures the door in its closed position and, when released, allows the door to open under the force of gravity. The conventional latch mechanisms, as shown in  FIGS. 2 and 3 , typically include a trip wire  1  or cable assembly which has one end adapted for control by a power shovel operator and another end connected to a moveable latch lever  2  which is generally located on the dipper door  3 . The latch lever is typically coupled to a slidable rod or latch bar  4  that is selectively engaged in a latch keeper opening  5  (see  FIG. 3 ) in a front wall  6  of the dipper body of the dipper. The dipper door  3  is held closed when the latch bar  4  is within the latch keeper opening  5 . The dipper door is caused to open by tripping the trip cable  1  which moves the latch lever  2  which causes the latch bar  4  to slide away from the latch keeper opening  5  and disengage the latch keeper opening  5 , whereby the dipper door  3  will open under its own weight plus the weight of any material contained within the dipper body. The latch lever  2  provides significant mechanical leverage to slide and pull the latch bar  4 , which is under very high load.  
      Normally, the door is thereafter closed by swinging the dipper in such a direction so as to cause the dipper door to move by inertia towards its closed position until the latch bar reengages the latch keeper. More particularly, the latch bar is forced away from the dipper front wall by contacting the latch keeper wall with a sloping surface that causes the latch bar to push up, and then the latch bar drops into the slot by gravity, locking the door.  
      This is a simple device including mainly two bars pulling on each other with dry, sliding friction contact that has worked well for many years; however, with the increase in dipper size over the past few years, its reliability has become compromised because dry sliding friction levels have increased under higher contact loads making behavior less predictable.  
      There are maintenance problems with this system, especially when it is used with 100-ton or larger payload dippers. Latch bars and related operating equipment are a significant part of dipper maintenance cost.  
      The maintenance problems include broken pull chains and clevis. The pull chain is the chain that connects a tugger rope to the latch bar lever. The trip ropes have to be replaced constantly as they become frayed and strands break. The snatch block (sheave) and bracket of the trip mechanism break or have to be replaced on a regular basis. The tugger motor, gearing and drum take a continuous beating causing repeated failures. And lastly, the latch bar has to have shimming added, and this is a constant ongoing maintenance issue that in the long run becomes labor intensive and costly.  
      The amount of tension required to trip or move a latch bar on a fully loaded 120-ton payload dipper nearly exceeds the mechanical ability of this system. Larger motors are usually the solution, but the net result is a very high cost maintenance area.  
      Another problem with conventional mechanical latch closure mechanisms is the tendency for such mechanisms to quickly wear out and require replacement in only a short period of time. Each time the slidable latch bar engages the latch keeper or the like, the tip of the slidable latch bar naturally wears down. In many conventional latch mechanisms, the slidable latch bar is only moved about a half inch to about an inch in order to allow the dipper door to open. Thus, only a very small portion, i.e., the tip, of the slidable latch bar comes into contact with the latch keeper. Since the latch bar is under very high load and the contact area is very small, the tip experiences very high contact forces that cause an accelerated rate of wear. As the tip of the slidable latch bar wears down over time, it becomes possible for the dipper door to prematurely open before the power shovel operator is ready for the dipper door to open. This, as can be appreciated by those skilled in the art, can create a hazardous and unsafe condition if the power shovel is not properly maintained.  
      To account for this wear, the latch bar length of engagement with the latch keeper must be frequently adjusted by adding or removing shims to the latch lever pivot mechanism  6  (see  FIG. 2 ). This requires the lifting of the heavy latch lever and latch bar to insert and remove the shims, usually with the assistance of a crane or forklift. Thus, conventional latch closure mechanisms exhibit operational shortcomings that must be addressed with more frequent, hazardous, and costly maintenance activities.  
      Examples of other past dipper latch approaches include U.S. Hilgeman U.S. Pat. No. 2,544,682 that illustrates a pivoting latch with a primary locking mechanism and a secondary latch, and U.S. Brown Jr. U.S. Pat. No. 6,467,202 that illustrates a dipper door pivoted and held by a linkage mechanism.  
     BRIEF SUMMARY OF THE INVENTION  
      One object of the invention is to provide a new system that is less maintenance intensive.  
      Another object of the invention is to provide a new dipper door latch system that does not require replacement after a short amount of time.  
      Another object of the invention is to provide a dipper door latch mechanism that will eliminate almost all dry sliding contact surfaces by replacing translational sliding motion with rotational motions.  
      Another object of the invention is to provide a dipper latch mechanism that replaces the dry sliding latch bar approach with a new rotational door latch where there are no members that see high loads and dry sliding friction at the same time. Dry sliding friction is replaced with greatly decreased lubricated rotational friction. Members rotate relative to each other and the dipper but the rotation does not take place while undergoing high loads.  
      Another object of the invention is to provide a dipper latch mechanism that will latch with high reliability even when there is a “soft” impact between the dipper door and the dipper body.  
      Another object of the invention is to provide a dipper latch mechanism that will usually unlatch only when the operator wants to unlatch, avoiding any false and unwanted trips.  
      This invention provides a dipper including a dipper door and an impact-actuated jaw having a “C” shape defining a lip and a chin. The jaw is rotatably mounted on the dipper door for rotation between a door-opened position and a door-closed position and positioned so that when the jaw is in the door-opened position, the jaw chin can be impacted by the dipper body when the door pivots to the door-closed position. The dipper also includes a hold open mechanism for releasably holding the door latch in the latch open position when the latch is in the open position, and a locking mechanism for releasably locking the latch when the latch is in the door-closed position.  
      In one embodiment, the locking mechanism includes one bar pivotally attached to the door, and another bar pivotally connected to and extending between each of the one bar and the latch.  
      Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings in which like numerals are used to designate like features. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a side elevational view of a power shovel embodying the present invention.  
       FIG. 2  is a bottom view of a prior art dipper with a latch assembly to open and close a dipper door of a dipper.  
       FIG. 3  is a side cross sectional view of the prior art latch assembly shown in  FIG. 2 , illustrating how the prior art latch slides into the latch keeper.  
       FIG. 4  is a perspective view of part of the bottom of the dipper door and the latch assembly of this invention.  
       FIG. 5  is a partial cross sectional view of the latch assembly and latch keeper of this invention with the dipper door opening.  
       FIG. 6  is a partial cross sectional view of the latch assembly and latch keeper of this invention with the dipper door closing.  
       FIG. 7  is a schematic illustration of the hydraulic circuit of this invention.  
       FIG. 8  is a partial exploded perspective view of the jaw of the latch assembly of this invention.  
       FIG. 9  is a perspective view of the dipper door and latch keeper of this invention.  
       FIG. 10  is a perspective view of the latch keeper of this invention.  
       FIG. 11  is a partial broken away side perspective view of another embodiment of the latch mechanism of this invention, with the latch in a door-closed position.  
       FIG. 12  is a partial broken away side perspective view of the embodiment of the latch mechanism shown in  FIG. 11 , with the latch in a door-opened position.  
       FIG. 13  is a side view of the secondary latch mechanism of the latch mechanism shown in  FIGS. 11 and 12 , with the secondary latch in a bar holding position.  
       FIG. 14  is a side view of the secondary latch mechanism of the latch mechanism shown in  FIGS. 11 and 12 , with the secondary latch in a bar released position.  
       FIG. 15  is a bottom view of the locked secondary latch mechanism shown in  FIG. 13 , with the bumper stop  220  removed.  
       FIG. 16  is a bottom view of the dipper door of this invention.  
       FIG. 17  is a perspective view of the bar holder  194  in  FIG. 13 . 
    
    
      Before the embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting. The use of “including” and “comprising” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items and equivalents thereof. The use of “consisting of” and variations thereof herein is meant to encompass only the items listed thereafter and the equivalents thereof.  
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Illustrated in  FIG. 1  is a power shovel  10  embodying the present invention. It should be understood that the present invention is capable of use in other power shovels known in the art and power shovel  10  is only provided as an example of one such power shovel. The power shovel  10  comprises a frame  14  supported for movement over the ground. Specifically, frame  14  is a revolvable upper frame mounted on a mobile base such as crawler tracks  18 . A fixed boom  22  extends upwardly and outwardly from the frame  14 . A dipper handle  26  is mounted on the boom  22  for movement about a rack and pinion or crowd drive mechanism (not shown) for pivotal movement relative to the boom  22  about a generally horizontal dipper handle axis  32 , and for translational (non-pivotable) movement relative to the boom  22 . The dipper handle  26  has a forward end  34 . A dipper  38  is mounted on the forward end  34  of the dipper handle  26  in a conventional manner. An outer end  42  of the boom  22  has thereon a sheave  46 , and a hoist cable or rope  50  extends over the sheave  46  from a winch drum (not shown) mounted on the frame  14  and is connected to the dipper  38  for pivotal movement relative thereto about a horizontal pivot axis  58 .  
      The dipper  38  is generally of a box shape having a body  62  which includes a back wall  66 , opposite side walls  68  extending forwardly from and substantially perpendicular to the back wall  66 , and a front wall  70  which is generally parallel to the back wall  66 . In other embodiments (not shown), other dipper body shapes can be used. Digging teeth  74  extend outwardly from an upper end of the front wall  70 . The main body or dipper body  62  defines a material receiving opening  78  and a material discharging opening  82 . The dipper  38  further includes a dipper door  86  pivotally connected to the back wall  66  adjacent the lower end thereof about a dipper door axis  90 . The dipper door  86  is movable between opened and closed positions, as will be further described below. The back wall  66  of the dipper  38  is connected to the forward end  34  of the dipper handle  26 . The back wall  66  (and thus the dipper  38 ) is rigidly connected to the dipper handle  26 .  
      As further explained below,  FIGS. 1, 5  and  6  illustrate how the dipper body  62  also has a latch receiving opening  100  adjacent one end of the discharging opening  82 .  
      In order to keep the door  86  in its closed position until it is desired to open the door  86  to drop the dipper&#39;s contents, the dipper  38  includes an impact actuated latch  104  in the form of a jaw having a “C” shape, as shown in  FIGS. 4, 5 ,  6  and  8 , thus defining a lip  108  and a chin  112 . Each of the lip  108  and chin  112  can incorporate a removable wear plate  113 , as shown in  FIG. 8 . The jaw  104  is pivotally, and more particularly, rotatably mounted on the dipper door  86  for rotation between a door-opened position ( FIG. 5 ) and a door-closed position ( FIG. 6 ). Further, the jaw  104  is positioned so that when the jaw  104  is in the door-opened position, the jaw chin  112  can be impacted by the dipper body  62  when the door  86  pivots to the door-closed position, and so that impact by the dipper body  62  on the chin  112  rotates the jaw  104  into the door-closed position where the jaw lip  108  is in the latch opening  100  and prevents door opening. In other words, the jaw lip  108  is out of the way of the dipper body  62  when the jaw  104  is in a door-opened position, as shown in  FIG. 5 .  
      More particularly, as shown in  FIG. 4 , the dipper door  86  includes spaced apart structural support ribs  116 , and the jaw  104  is rotatably mounted between two of the support ribs  116 . In other less preferred embodiment, a simple bar latch (not shown) can be used that rotates into and out of the latch receiving opening  100 .  
      The dipper  38  further includes a hold open mechanism  120  for releasably holding the jaw  104  in the door-opened position when the jaw  104  is in the open position, and a locking mechanism  124  for releasably locking the jaw  104  when the jaw  104  is in the door-closed position. When the locking mechanism  124  is released, the latch  104  rotates open as a result of the weight of the door  86  and the material pushing against the dipper door  86 .  
     First Embodiment  
      In one embodiment, as illustrated in  FIGS. 5, 6  and  7 , the hold open mechanism  120  and locking mechanism  124  is a hydraulic cylinder assembly  128  pivotally attached to and extended between the dipper door  86  and the latch  104 . More particularly, as illustrated in  FIGS. 5 and 6 , the hydraulic cylinder assembly  128  is pivotally attached at one end to between two of the support ribs  116 .  
      The hydraulic assembly  128  is provided with a hydraulic cylinder  132  and a piston  134  that is movable within the hydraulic cylinder  132 . The piston  134  divides the hydraulic cylinder  132  into a first chamber and a second chamber wherein the volumes of the chambers change as the piston  134  moves back and forth within the hydraulic cylinder  132 . Either the hydraulic cylinder or the piston can be connected to the latch and the other connected to the dipper door  86 .  
      More particularly, in this embodiment (as shown in  FIG. 7 ), the hold open mechanism  120  and locking mechanism includes a closed loop hydraulic circuit  140  including at least one check valve  142 , a heat sink  146 , a locking valve  150  and at least one accumulator  154 . In this embodiment, there are two check valves  142 , and they are incorporated into the piston  134 . The accumulators  154  are used to maintain pressure and accommodate the volume differences between the top (shaft) and bottom (open) portions of the cylinder  132 .  
      The locking valve  150  controls the hydraulic fluid flow through the conduit from one of the chambers to the other chamber. In this way, when the valve  150  is closed, the hydraulic fluid is prevented from flowing between the chambers so that the latch  104  remains locked and the dipper door  86  is prevented from moving. When the valve  150  is opened, the hydraulic fluid is allowed to flow between the chambers and the dipper door  86  is allowed to move.  
      Preferably, the valve  150  is a solenoid valve that is controlled by way of a remote switch (not shown) operated by the power shovel operator. Such valves are commonly known to those skilled in the art and are also readily available from numerous commercial sources. Power can be supplied by a 24V rechargeable battery pack mounted in the top of the dipper door  86  and activated by an RF signal. Another method would be to mount a cable take-up reel on the boom and attach the cable to the top of the door. This would allow the control valve  150  to be hard wired.  
      When the dipper door  86  is latched the jaw  104  is biased to rotate to release from the latch opening  100  by the pressure difference in the cylinder assembly  128  caused by the greater area in the non-rod end of the cylinder  132 , but is held in place by the control valve  150 . And the hold open mechanism  120  comprises the at least one accumulator  154  that provides the residual pressure that extends the cylinder assembly  128  to its maximum extension. Any excessive heat is accounted for through the heat sink  146 .  
      In operation, when the door  86  slams shut, the hydraulic cylinder assembly  128  will be displaced transferring fluid through its piston  134  via the series of internal check valves  142 . Once engaged, the piston  134  is fully retracted. The cylinder assembly  128  cannot extend, due to the hydraulic lock. And the cylinder assembly  128  is under tension when the jaw  104  is engaged.  
     Second Embodiment  
      In another and more preferred embodiment, which is shown in  FIGS. 9 through 16 , like numerals identify items described previously. As illustrated in  FIGS. 11 and 12 , the locking mechanism  124  in this embodiment includes a primary locking mechanism  160  including one bar  164  pivotally attached at  165  to the door  86 , and another connecting bar  168  pivotally connected to and extending between each of the one bar  164  at  166  and the latch  104  at  167 . The jaw  104  is pivotally connected to the door  86  at  169 . Further, the hold open mechanism  120  in this embodiment is means biasing the latch  104  into its open position in the form of a spring  172  attached between the one bar  164  and the connecting bar  168 . More particularly, in this embodiment, there is one spring  172  on one side of the one bar  164  and the connecting bar  168  and a similar spring (see  FIG. 12 ) on the opposite side of the one bar  164  and the connecting bar  168 . When locking the locking mechanism  124 , the pivot connection between the one bar  164  and the connecting bar  168  travels through the spring  172 , which helps to drive the latch  104  into the locked position and hold the latch  104  closed, as further explained below. The latch  104 , the springs  172  and the locking mechanism  124  are located between two of the support ribs  116  (only one of which is shown in  FIGS. 11 and 12 ), and are protected by a protective cover  173 .  
      In this embodiment, as illustrated in  FIGS. 9 and 10 , the latch opening or keeper  100  in the dipper body  62  is in the form of a cutout  174  formed in a striker plate  176  that is a part of, such as by being welded, the dipper body front wall  70 . A striker bar  180  extends across the cutout  174  parallel to the plane of the dipper discharging opening  82 , the latch receiving opening being defined by the striker bar  180  and the striker plate  176 . The jaw chin  112  impacts the striker bar  180 .  
      The dipper striker bar  180  serves the important purpose of anchoring the dipper door  86  to the dipper body  62  through the latch link engagement. The construction of the striker bar  180  is a round pin, and the pin is allowed to rotate under load because it has bushed supports  184 . This is significant because sliding friction between the latch  104  and the striker bar  180  is eliminated. In addition, the large contact load between the latch  104  and the striker bar  180  is relieved instantly when the door is unlatched. That is, there is no constant high and dry contact load acting over a significant length like there is in a sliding friction door latch. Shims (not shown) can be placed between the supports  184  and the cutout  174  to adjust the position of the striker bar  180 , when desired.  
      The rotatable latch jaw  104  that acts as the forward link of this four bar primary locking mechanism  124 , rotates and curls around a striker bar  180  when the chin  112  of the latch  104  strikes the dipper striker bar  180  upon door re-latch. The tension springs  172  are positioned such that the rotatable latch  104  is driven with extra rotational impetus to provide a controlled and positive re-latch. Re-latch is achieved in this manner as the latch jaw  104  curls around the striker bar  180  and is positively held in that position by the locking and holding power of the primary locking mechanism  160 .  
      The various pivot points in the primary locking mechanism  160  have the added advantage in that they can be lubricated with the lubricant held locally in place with pin retention systems. Also, these pin joints can be lined with replaceable bronze bushings to protect the parent linkage material.  
      The mechanism remains in the unlatched, collapsed condition thanks to the springs  172  that effectively control and hold the mechanism bars or links in the door-opened position as long as the door  86  is unlatched. The springs  172  also prevent the latch jaw  104  from extending back into the latched position, for if the latch jaw  104  were in the latched position, the latch jaw  104  would not be in the proper position to permit the door to shut again. As the empty shovel dipper  38  is lowered and swung backward to start a new dig cycle, the dipper  38  with the dipper body strike bar  180  swings into the hanging dipper door  86 , so that the bottom portion or chin  112  of the jaw strikes the dipper body striker bar  180  and is driven back into the closed position. The springs  172  again serve their dual purpose because the springs now act to drive the latch  104  into engagement as the jaw  104  curls around the striker bar  180 .  
      In order to lock the locking mechanism  124 , the locking mechanism also includes a secondary latch  190 , as illustrated in  FIGS. 11 through 15  and  17 . The secondary latch  190  is under much less load but controls the engagement of the primary latch  104  holding the dipper door  86  to the dipper body  62 . The secondary latch  190  comprises a bar holder  194  pivotally mounted at  195  on the dipper door  86 , the bar holder having a bar holding indentation  198 , the indentation  198  receiving the one bar end or tail  200  and preventing movement of the one bar end  200  when in a bar holding position ( FIG. 13 ) and permitting movement of the one bar end  200  when in a bar releasing position ( FIG. 14 ). The locking mechanism  124  is locked by impact of the one bar end or tail  200  into the secondary latch bar holder  194 , and movement of the bar holder  194  into the bar holding position, as further explained below.  
      When the dipper door is latched and under heavy load due to the material within the dipper, the primary latch mechanism is under heavy load and the bars want to collapse so the latch wants to disengage the dipper striker bar allowing the door to open. This tendency of disengagement exists because the four bar linkage approaches the toggle but is held by mechanical stops  201  and by the secondary latch  190  to be about 7 degrees short of toggle. The “tripping” or unlocking of the secondary latch  190  allows the bars of the primary locking mechanism  160  to collapse and the latch jaw  104  to rotate out of engagement with the body  62 .  
      As best illustrated in  FIG. 15 , the bar holder  194  is biased by a weak spring  204  toward the bar releasing position (see  FIG. 14 ) but is held in locked position by a plunger  205  that engages an end  208  (see  FIG. 13 ) of the bar holder  194 . The plunger  205  slips into the bar holding position when the secondary latch moves into the bar holding position, as shown in  FIGS. 13 and 15 . The plunger  204  is biased toward the bar holder  194  by a strong compression spring  212  (see  FIG. 15 .  
      When the bar end  200  strikes the bar holder  194  on its way to the latch position, the weak spring  204  allows the bar holder  194  to rotate into a position to “catch” the bar end  200  in the latched position in the indentation  198  of the holder  194 . If the bar holder  194  travels past its bar holding position, the weak spring  204  brings the bar holder end  208  back into engagement with the plunger  205 . As the bar holder  194  rotates to its latched position, the bar holder has an inclined surface  232  (see  FIG. 17 ) that engages the end of the plunger  205 . The bar end  200  striking the bar holder  194  drives the bar holder end  208  to forcibly push the trip plunger  205  out of the way. The bar holder end  208  then over travels slightly to allow the trip plunger  205  to extend under force of the torsion spring  212  to thereby prevent the bar holder end  208  from any rotational movement that would unlatch the locking mechanism  124 . The trip plunger  204  therefore locks the secondary latch  190 , which in turn locks the primary latch  160 .  
      This weak spring  204  effect guarantees a successful latch even when the latching force is very small, thus giving the locking mechanism  124  a “soft latch” feature. The soft latch feature is desirable because the shovel operator does not have to deliberately try to slam the door shut for a successful door latch. The strong spring  212  on the trip plunger  204  is stiff enough to resist inadvertent inertial loads on the trip cable. This helps to eliminate unintended false trips due to cable slack take up when the dipper handle is suddenly extended or retracted. The stiff spring, however, can be overcome by the very deliberate operator act of tripping the cable via cable pull from a motor driven drum.  
      In operation, the latch jaw  104  is held in the latched position by the secondary latch  190  that holds onto the tail end  200  of the primary locking mechanism  160  until the operator trips the secondary latch  190 . The primary locking mechanism extends to a position just short of a full toggle position. As a result, the mechanism wants to collapse away from the toggle position under gravity. The secondary latch prevents it from doing so and the door is locked.  
      The instant the latch jaw  104  curls around the striker bar  180 , the secondary latch  190  grabs the tail  200  of the one bar  164  and keeps the locking mechanism  124  from collapsing again until the operator trips the secondary latch  190 . The impact is not considered great during routine shovel digging but the momentum of the heavy door and dipper drive the relatively light linkage to the latched position just in time for the dipper to start a new dig cycle through a bank of material.  
      The secondary latch mechanism can take on many different mechanical configurations (not shown) and can be triggered by many different methods including pneumatic, hydraulic, electromagnetic solenoid, and mechanical cable pull. In the preferred embodiment, the secondary latch plunger  204  is remotely triggered with an RF signal, as is done with a garage door opener. This eliminates all maintenance intensive trip motors, trip cables, and cable pulleys.  
      The fixed pivot location of the secondary latch is located directly above and in line of the bar indentation  198  that holds the bar end  200  in the latched position. As long as this position holds, or that the indentation is somewhat to the right of this line of action, then the latch will not rotate and is locked in place. The end  216  of the bar holder  194  opposite the end with the bar indentation  198  engages a mechanical stop in the form of a bumper stop  220  and insures the bar indentation  198  is in the proper position to receive the bar end  200 . The bar holder  194  will press against this bumper because of torsional tension supplied by the spring  204 .  
      Rotation of the secondary latch is limited in the latching position by the bumper stop  202 . The stop  220  is located precisely in a position to allow the bar holder  194  to over travel slightly to allow the trip plunger  204  to fully extend to lock the bar holder from rotating back into the unlatched position.  
      In an alternate embodiment (not shown) the location of the secondary latch can be placed at the toggle pivot  166  between the two bars, if needed. It is shown at the end of the link extension to keep it out of harms way. As you locate nearer the tip of the door  86 , the greater the possibility of seeing material plowing and contamination during use.  
      The secondary latch  190  is tripped by pulling the trip plunger  204  out of engagement with the bar holder  194 . This allows the bar holder  194  to release the tail end  200  of the primary locking mechanism. The weight of the door, and the weight of the material in the dipper which exerts additional weight on the door, produces a rotational moment on the latch jaw forcing it to rotate out of engagement. The linkage system will accommodate the rotation of the latch because the linkage is just short of the toggle point, not at the toggle point, and not beyond the toggle point where it would fail to move. The mechanical stop  202  insures that the mechanism will not reach toggle nor go beyond toggle. When the latch rolls out of engagement, the mechanism collapses and the door opens. Latch rotation out of engagement does not generate any significant sliding friction because the striker bar is allowed to rotate within the bushings that support it in the cradle of the dipper body. The sudden latch rotation results in the sudden release of latch loads that do not have to be resisted by any objects in contact. Sliding friction under large sliding loads is eliminated.  
      The door  86  falls open releasing the load within the dipper  38 . The mechanism is held in the collapsed state by the pair of tension springs  172 . This is necessary to hold the latch jaw with the proper orientation for the jaw chin  112  to strike the dipper striker bar  180  upon re-latch. The tension springs mounted uniquely with respect to the mechanism toggle point, therefore, hold the latch jaw in the proper position after unlatching and drive the latch jaw closed upon re-latching. In a less preferred embodiment (not shown), torsional springs could be incorporated at the toggle pin joint to achieve the same behavior.  
      The addition of the springs  172  to the linkage is an important controlling feature of the mechanism. Without the springs the linkage would not be held in the collapsed position with the latch jaw wide open and ready for re-latch. The springs, one spring located outboard on either side of the bars, serve a dual purpose in that upon re-latch the spring force line of action transfers from one side of the pivot pin to the other side. As the lower latch jaw strikes the dipper striker bar upon re-latch, the spring line of action passes across the pivot and now serves to drive the latch jaw rotation to curl around the striker bar and remain in the latched position.  
      As the jaw  104  curls around the striker bar  180 , the tension spring line of action now acts on the other side of the pivot and “kicks in” and provides impetus to drive the latch  104  into engagement with the striker during re-latch.  
      The unique rotational dipper door latch does not require the high forces generated by hydraulics to open and close. The rotational dipper door latch uniquely closes under gravity, latches with a positive lock on the dipper striker bar, and requires a relatively small tripping force to unlatch.  
      The secondary latch in essence, controls the toggle point of this four bar linkage because the bar involved in the toggle is extended away from all the link motion toward the back of the dipper, but sees the same reduced load. The secondary latch therefore takes advantage of the inherent mechanical advantage of a pivot-approaching toggle. Any dry sliding friction loads in the secondary latch are miniscule compared to the original design friction loads in the primary latch arm, and therefore the invention eliminates the maintenance and reliability problems posed by the dry sliding friction.  
      Various features of the invention are set forth in the following claims.