Apparatus and method for a rotatable tailgate assembly having service and out-of-service positions

A subassembly is provided for the pivotal body of a heavy-duty truck comprising a tailgate assembly which rotates between service and out-of-service positions in order to provide a truck body having a dual volume capacity. The tailgate assembly includes opposing side beams attached at their first ends to a tailgate which forms a back wall of the truck body when the assembly is in its service position. Pivot means couple the opposing side beams to the opposing side walls of the truck body such that the tailgate assembly may be rotated about an axis of rotation between its service and out-of-service positions. Means are provided for rotating the tailgate assembly about its axis of rotation wherein the rotating means advantageously utilizes the changing position of the center of gravity of the tailgate assembly as it rotates between positions.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates generally to heavy-duty, off-road trucks and, 
more particularly, to the construction of the bodies for these trucks. 
BACKGROUND 
Often, off-road trucks are subjected, during their routine use, to weight 
loads which differ greatly because of different material density and/or 
the ability of some material to more tightly pack when loaded into the 
truck body. In the working environment of a coal mining site, a truck may 
be called upon to haul either coal or overburden. Overburden is the earth 
material which must be removed in strip mining to expose the coal for 
removal. Typically, the overburden material has a greater density than the 
coal it covers. As a result of this difference in density, truck bodies 
which are always filled to their full volume capacity may carry weight 
loads which exceed the weight capacity of the truck. 
By reducing the volume capacity of the truck body, the weight overloading 
problem can be eliminated. But, by reducing the volume capacity of the 
truck body, the truck hauls less than its full weight capacity of the 
lighter density coal. Therefore, the mine operator faces the unpleasant 
choice of either operating his trucks inefficiently (i.e., smaller volume 
bodies which hold less coal by weight than the truck is capable of 
holding) or, alternatively, risk overloading the truck (i.e., larger 
volume bodies which hold the full weight capacity of coal but which can be 
weight overloaded if fully loaded with heavier overburden). 
One solution to the foregoing problem has been to employ a fleet of trucks 
comprising two types of truck bodies. One group of trucks is equipped with 
with smaller volume bodies for use in connection with hauling overburden. 
A second group of trucks is equipped with larger volume bodies for use in 
connection with hauling coal. Unfortunately, the dedication of a truck to 
a particular type of hauling task (i.e., hauling overburden or coal) 
reduces the versatility of the operator's fleet. During particular times 
of a mining cycle there may be a need for more trucks to haul overburden 
and at other times a need for more trucks to haul coal. When these times 
occur, the mine operator is in a situation where he must use a truck 
intended to haul one type of load (e.g., overburden) to haul another type 
of load (e.g., coal). Obviously, dedicating the truck fleet to groups 
intended for different types of hauling reduces the flexibility of the 
fleet and introduces the possibility of inefficient operation of the truck 
fleet. 
One prior art solution to the problem of reduced flexibility of a truck 
fleet using truck bodies of different volume capacities has been to add a 
tailgate to the smaller capacity bodies in order to provide the bodies 
with means to increase their capacity if desired. Specifically, off-road, 
heavy-duty trucks are commonly equipped with bodies having beds which are 
angled with respect to a horizontal plane. The truck beds slope from a 
high point at the rear of the truck bodies to a low point at the front of 
the bodies. By providing bodies having the foregoing beds, loads can be 
carried by the bodies without necessitating separate tailgates for the 
bodies. By adding tailgates to the bodies, the load carrying capabilities 
of the bodies can be substantially increased. Therefore, many mine 
operators have equipped their truck fleet with smaller volume truck bodies 
which include tailgates for increasing the volume capacities of the 
bodies. When the tailgates are not used to retain loads, the trucks are 
operating with a smaller volume body suitable for safely hauling 
overburden. By using the tailgates to retain loads, the trucks are 
operating with a larger volume body suitable for the efficient hauling of 
coal. 
Unfortunately, the addition of a tailgate to a truck body makes it 
difficult to control the exact loading of the truck when overburden is 
being hauled. Without the tailgate, the volumetric loading limit could be 
easily determined by visually monitoring the level of overburden by way of 
the rear opening of the truck body. With the addition of a tailgate, the 
loader operator can no longer make a visual estimation. Instead, he must 
now guess at the appropriate load. 
In addition to the foregoing disadvantages of a tailgate, the tailgate also 
hinders the loading of a truck since a loading shovel must lift its bucket 
above the lowered tailgate and down into the bed of the body. For loading 
coal, this up and then down loading movement of the shovel is justified 
because of the additional load capacity provided by the tailgate. But, 
when the body is carrying overburden, the tailgate is not providing any 
additional capacity. In fact, since the loading shovel must raise the 
heavy buckets of overburden an additional height equal to that of the 
tailgate (e.g., approximate seven feet), the tailgate is actually a 
hindrance to the hauling of overburden. Moreover, because the tailgate 
requires overburden be raised relatively high above the body bed in order 
to clear the tailgate and load the truck, a risk is created that the 
overburden will be dropped into the truck body from a height which may, 
with continual usage, damage the truck frame and/or body. 
SUMMARY OF THE INVENTION 
It is a general object of this invention to provide a truck body for 
heavy-duty, off-road trucks which efficiently and safely hauls loads of 
different volumes and material densities. In this connection, it is a more 
specific object of this invention to provide a truck body for heavy-duty, 
off-road trucks which utilizes a tailgate without introducing 
inefficiencies into the hauling routine and without creating a risk of 
overloading the truck with high density material. 
It is another object of the invention to provide an improved tailgate for 
the truck body of a heavy-duty, off-road truck which does not interfere 
with the loading of overburden and which does not create a risk of 
damaging the truck when loaded with overburden. 
It is yet another object of the invention to provide the foregoing 
objectives while providing a tailgate which increases the volumetric 
capacity of a truck body for a heavy-duty, off-road truck. 
Other objects and advantages of the invention will be apparent from the 
following detailed description and the accompanying drawings. 
In accordance with the invention, a subassembly for the pivotal body of a 
heavy-duty truck is provided that includes a tailgate assembly which 
rotates between service and out-of-service positions. The tailgate 
assembly includes opposing side beams attached at their first ends to a 
tailgate. Pivot means couple the opposing side beams to opposing side 
walls of the truck body such that the tailgate assembly is capable of 
rotating about an axis of rotation intersecting both side walls. The 
tailgate assembly has a center of gravity intermediate the tailgate and 
the axis of rotation of the assembly. Means are provided for rotating the 
tailgate assembly about the axis of rotation from its service position to 
the out-of-service position in which the tailgate assembly rests on the 
canopy of the body.

While the invention will be described in connection with certain preferred 
embodiments, it will be understood that it is not intended to limit the 
invention to those particular embodiments. On the contrary, it is intended 
to cover all alternatives, modifications and equivalents as may be 
included within the spirit and scope of the invention as defined by the 
appended claims. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning to the drawings, and referring first to FIG. 1, a truck is 
illustrated with a tailgate assembly 11 mounted on the side walls of the 
truck body 13. The truck body 13 is mounted on an off-road truck frame 15 
for pivoting about an axis 17. A hydraulic piston 19 operates the truck 
body 13 about the axis 17 between raised and lowered positions. 
Structurally, the truck body 13 consists of steel panels which form the 
shape of the body and beams 21a and 21b which provide the structural 
framework for the body. The truck body 13 consists of side walls 21 (only 
one is shown in FIGS. 1-12), a front wall 23, a bed 25 and a canopy 27 
integrally attached to the top of the front wall and extending over the 
cab 31 of the truck. 
Supporting the truck frame 15 are tires 33 connected for rotation to the 
frame by wheel axles (not shown). The tailgate assembly 11 includes side 
beams 35 (only one is shown) connected to a tailgate 37 which cooperate to 
rotate the tailgate assembly about an axis of rotation 38 defined by the 
pivot means 39. The tailgate assembly 11 has a center of gravity 41 
located intermediate the tailgate 31 and the axis of rotation 38 for the 
tailgate. 
Since the truck of the illustrative embodiments is symmetrical about its 
longitudinal axis, many of the elements identified in the side views of 
FIGS. 1-12 have complementary elements on the opposite side of the truck. 
For example, the truck body has two opposing side walls 21 and the 
tailgate assembly 11 has two opposing side beams 35 and pivot means 39. It 
will be understood that reference to plural elements where only one is 
show (e.g., side beams 35 or side walls 21) indicates a complementary 
element is located on the side of the truck not shown. 
In accordance with the invention, the tailgate assembly 11 is moved from 
its service position (shown in FIG. 1) to an out-of-service position 
(shown in FIG. 3) by rotating the tailgate assembly about its axis of 
rotation 38 to a resting position on the canopy 27 of the truck body 13. 
By providing an out-of-service position for the tailgate assembly 11 atop 
the canopy 27, the truck body 13 can be quickly and easily converted to a 
smaller-capacity body for use in hauling overburden or other high density 
material. In order to convert the smaller-capacity body back to a 
larger-capacity body having a tailgate, the tailgate assembly 11 is 
rotated in a clockwise direction about the axis of rotation 38 from its 
out-of-service position illustrated in FIG. 3. Continued clockwise 
rotation as illustrated by the raised tailgate assembly 11 in FIG. 4 
results in the assembly returning to the service position illustrated in 
FIG. 1. 
The tailgate assembly 11 has a center of gravity 41 at a location 
intermediate the axis of rotation 38 and the tailgate 37. Accordingly, in 
a raised position, the tailgate assembly 11 has a natural tendency to 
rotate in a clockwise direction when the center of gravity 41 is in a 
position rearward of an imaginary vertical plane 45 passing through the 
axis of rotation 38. In contrast, the tailgate assembly 11 has a natural 
tendency to rotate in a counterclockwise direction when the center of 
gravity 41 is in a position forward of the imaginary vertical plane 45. 
Means for rotating the tailgate assembly 11 are provided which rotate the 
assembly by controlling the net moment on the assembly about the axis of 
rotation 38. In order to control the rotation of the tailgate assembly 11, 
the means for rotating must overcome the gravitational force in order to 
raise the assembly and, after the center of gravity 41 of the assembly has 
passed through the vertical plane 45, the means for rotating must reduce 
to a force less than the gravitational force in order to continue rotation 
of the assembly in the same direction. But the reduced force must be such 
as to allow the gravitational force to lower the tailgate assembly without 
the assembly slamming into the canopy 27 (out-of-service position) or 
truck bed 25. 
A line joining the axis of rotation 38 of the tailgate assembly 11 with the 
center of gravity 41 of the tailgate assembly can be considered a 
reference line 46 for the purpose of examining the relative forces 
controlling the movement of the tailgate assembly. As the means for 
rotating the tailgate assembly 11 applies a downward force on the side 
beam 35 of the tailgate assembly at a location opposite the center of 
gravity 41 with respect to the axis of rotation 38, the net moment about 
the axis of rotation is in a counterclockwise direction, i.e., the 
gravitational moment is less than the moment associated with the means for 
rotating. As the tailgate assembly 11 rotates in a counterclockwise 
direction in FIGS. 1-3, the moment at the center of gravity 41 decreases 
until, at the zenith of the tailgate's rotation, the moment switches from 
the rearward side to the forward side of the reference line 46. When this 
switch occurs, the natural tendency of the tailgate assembly 11 to fall 
from its raised position switches from a rearward direction to a forward 
direction in accordance with the change in the moment about the center of 
gravity 41. 
In order to raise the tailgate assembly 11, the force associated with the 
center of gravity 41 is overcome by the means for rotating. After the 
tailgate assembly 11 has reached its zenith of rotation and the 
gravitational force has switched to the forward side of the reference line 
46, the means for rotating compensates for the switch in direction of the 
gravitational force. Without compensating for the switch in direction of 
the gravitational force, the new direction of the force of gravity would 
complement the force from the means for rotating and, thereby, cause the 
tailgate assembly 11 to slam into its lowered position (either the service 
position or the out-of-service position). Therefore, the means for 
rotating reverses the direction of its force at approximately the same 
time during the rotation of the tailgate assembly 11 as the force of 
gravity changes from one side of the vertical plane 45 to the other side. 
In addition, the force from the means for rotating must now have a moment 
which is less than the moment associated with the force of gravity since a 
net moment is required in the same direction as the direction of the net 
moment during the lifting of the tailgate assembly 11. This continuity of 
the net moment assures that the tailgate assembly 11 will continue 
rotating to its resting position either atop the canopy 27 or on the truck 
bed 25 (depending on the direction of rotation). 
Specifically, from the service position of the tailgate assembly 11, the 
means for rotating applies a force whose moment overcomes the moment at 
the center of gravity 41 until the tailgate assembly reaches its zenith of 
rotation. As the reference line 46 passes through the vertical plane 45, 
the means for rotating responds to the change in direction of the moment 
at the center of gravity from rearward to forward by reversing its moment 
and adjusting the magnitude of the moment in order to create a continuous 
net moment about the tailgate assembly such that the counterclockwise 
rotation of the tailgate assembly continues. In addition, the means for 
rotating must return the tailgate assembly 11 from its out-of-service 
position to its service position (clockwise rotation of the tailgate 
assembly) in the same manner it rotates the assembly from its service 
position to its out-of-service position. 
In order to provide for a means for rotating the tailgate assembly 11 in 
the foregoing manner, three illustrative embodiments are shown in the 
drawings. Referring first to the illustrative embodiment shown in FIGS. 
1-4, a cable or chain 47 is connected between the truck body and the 
tailgate assembly 11 and operates as the means for rotating the tailgate 
assembly in response to movement of the truck body from its lowered 
position to its raised or dump position. Specifically, the cable or chain 
47 is fixedly attached at its ends to fixed points 49 and 51 on the side 
beam 35 and the truck body, respectively. In between these two endpoints, 
the cable or chain 47 is biased by a roller 53 carried on an outrigger off 
of the truck frame 15 and also by a roller 55 carried by a support off of 
the truck body 13. 
As the truck body 13 pivots to its raised position, the cable or chain 47 
is pulled about the rollers 53 and 55, and the tailgate assembly 11 
responds by rotating in a counterclockwise direction as illustrated in 
FIG. 2. As the truck body is raised, the tailgate assembly rotates into a 
position such that the reference line 46 has passed through the vertical 
plane 45. Since the cable or chain 47 remains relatively taut during this 
rotation of the tailgate assembly 11 and pivoting of the truck body 13, 
the tailgate assembly does not fall toward the canopy 27 as it otherwise 
would if it were not supported in its raised position by the cable or 
chain 27. Accordingly, as the truck body 13 is moved from a raised 
position to its lowered position the cable or chain allows the tailgate 
assembly 11 to lower toward a rest position on the canopy 27 of the truck 
body. 
In order to place the tailgate assembly 11 back into its service position, 
the cable or chain 47 is biased on the opposite side of the roller 55 on 
the truck body 13. With the cable or chain 47 repositioned, the tailgate 
assembly 11 is in a position, when the truck body 13 is raised, such that 
the center of gravity 41 of the assembly passes from a forward position of 
the imaginary vertical plane 45 to a rearward position of the plane. With 
the center of gravity 41 in a rearward position of the vertical plane 45, 
the lowering of the truck body 13 allows the tailgate assembly 11 to 
lower, under control of the cable or chain 47, to its service position. 
Referring now to the second embodiment of the invention illustrated in 
FIGS. 5-8, the means for rotating the tailgate assembly 11 is provided by 
hydraulic piston assemblies 61 attached at their first ends to the side 
walls 21 of the truck body 13 and at their second ends to levers (arms) 63 
of the side beams 35 of the tailgate assembly. As the arms 65 of the 
hydraulic piston assemblies 61 are retracted, the hydraulic piston 
assemblies and the arms 63 provide a net moment about the axis of rotation 
38 of the tailgate assembly 11 which rotates the tailgate assembly in a 
counterclockwise direction. 
At the end of the stroke of the arms 65 of the hydraulic piston assemblies 
61, the center of gravity 41 of the tailgate assembly 11 has passed from a 
rearward position of the vertical plane 45 to a forward position of that 
plane. Accordingly, when the arms 65 of the hydraulic piston assemblies 61 
are extended from their fully retracted position, the tailgate assembly 11 
continues to be moved in a counterclockwise direction until the assembly 
rests upon the canopy 27 of the truck body 13. Because the center of 
gravity 41 of the tailgate assembly 11 passes to a position forward of the 
vertical plane when the arms 65 of the hydraulic piston assemblies 61 are 
in a fully retracted position, the subsequent extension of the arms 65 of 
the hydraulic piston assemblies will not return the tailgate assembly to 
its original position, but instead, continues to rotate the tailgate 
assembly in its counterclockwise direction. 
In order to move the tailgate assembly 11 from its out-of-service position 
on the canopy 27 of the truck body 13 to its service position, the truck 
body is raised slightly from its lowered position. By slightly raising the 
truck body 13 from its lowered position (as shown in FIG. 7), the 
retraction of the arms 65 of the hydraulic piston assemblies 61 to their 
fully retracted positions will result in the center of gravity 41 of the 
tailgate assembly 11 passing through the vertical plane 45 thereby 
allowing a subsequent extension of the arms 65 to continue rotation of the 
tailgate assembly in a clockwise direction. 
In the third alternative embodiment of the invention, the means for 
rotating the tailgate assembly 11 is provided by a cable 71 connected at 
one end to the side beam 35 of the tailgate assembly 11 and at another end 
to a winch 73. At an intermediate point of the cable 71, the cable is 
biased by the roller 55 (used in connection with the first embodiment of 
the invention). As the winch 73 coils the cable 71, the tailgate assembly 
11 is rotated in a counterclockwise direction until the center of gravity 
41 passes through the vertical plane 45. The tailgate assembly 11 then 
continues the counterclockwise rotation under the effect of gravity until 
the cable 71 is taut. At that point, the winch 73 then begins to uncoil 
the cable 71 in order to allow the tailgate assembly 11 to continue its 
counterclockwise rotation toward its final position atop the canopy 27 of 
the truck body 13 shown in FIG. 11. 
Referring to FIG. 12, in order to return the tailgate assembly 11 to its 
service position, the cable 71 is biased against a roller 75 held on a 
support mounted to the truck body 13 wherein the roller is positioned 
differently from the position of the roller 55 used to rotate the tailgate 
assembly 11 in a counterclockwise direction. By changing the location of 
the biasing roller, the coiling of the cable 71 by the winch 73 locates 
the center of gravity 41 of the tailgate assembly 11 at a position 
rearward of the vertical plane 45 as the cable is coiled. Therefore, when 
the winch 73 uncoils the cable 71, the force of gravity causes the 
tailgate assembly 11 to continue rotating in a clockwise direction toward 
its service position. As an alternative to the biasing roller 75 for 
returning the tailgate assembly 11 to its service position, the truck body 
13 is raised from its lowered position (as shown in FIG. 7) such that the 
coiling of the cable 71 by the winch 73 will result in the center of 
gravity 41 passing clockwise through the vertical plane 45 thereby 
allowing a subsequent uncoiling of cable 71 by winch 73 to continue 
rotation of the tailgate assembly 11 in a clockwise direction. 
Still referring to FIG. 12, a canopy locking mechanism 80 is illustrated 
which holds the tailgate assembly 11 in its out-of-service position on the 
canopy 27. The locking mechanism 80 is fixedly attached to the front wall 
23 of the truck body 13. As illustrated in FIG. 13, the locking mechanism 
80 is mounted on a support 81 off of the truck body 13 in order that it 
may pivot between locked and unlocked positions. In order to lock the 
tailgate assembly 11 on the canopy 27, an L-shaped arm 83 of the locking 
mechanism 80 (as viewed in FIG. 13) is moved into an engagement position 
(solid lines) with a dog 85 extending from the side beam 35 of the 
tailgate assembly. The L-shaped arm 83 locks the tailgate assembly 11 over 
the canopy 27 by swinging into a position (as shown by the arrow in FIG. 
13) wherein the arm 83 is over the dog 85 such that the tailgate assembly 
is biased against clockwise rotation as viewed in FIG. 12; therefore, the 
tailgate assembly 11 cannot lift away from its resting place on the canopy 
27. By providing the locking mechanism 80, the tailgate assembly 11 can be 
rotated to its out-of-service position without fearing that the assembly 
may unintentionally and uncontrollably swing back to its service position. 
Without the locking mechanism 80, a combination of circumstances, 
individually or combined such as rough road conditions, steep grades, and 
at or near fill dump of truck body 13 could create a significant risk of 
unintentional clockwise movement of the tailgate assembly 11 from the 
canopy 27. Under extreme circumstances the tailgate assembly 11 may even 
uncontrollably rotate to its service position. 
From the foregoing illustrative embodiments, it will be appreciated that 
the tailgate assembly 11 can be alternatively placed in a service position 
or an out-of-service position on the canopy 27 of the truck body 13 by way 
of rotating the tailgate assembly about the axis of rotation 38. The means 
for rotating the tailgate assembly 11 advantageously utilize the change in 
direction of the moment produced by the gravitational force as the 
tailgate assembly is rotated. Furthermore, the tailgate assembly 11 can be 
safely carried on the canopy 27 of the truck body 13 by providing the 
locking mechanism 80 described in connection with FIGS. 12 and 13. By 
utilizing the invention to provide a tailgate assembly with an 
out-of-service position, a more versatile heavy-duty truck is provided 
which can be safely used for both hauling overburden and coal at a mining 
site.