Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~::OV7438
This invention relates to hydraulic platform
lifts for use on truck trailers and the like for
loading or unloading cargo when no loading dock is
available.
S Previous lift platforms are generally of two
types. A first type is often referred to as a TUK-A-
WAY ( a trade mark) where the platform is tucked away
underneath the rear end of the trailer when not in use
and through hydraulic means it can emerge from beneath
the trailer for use in raising or lowering cargo. The
TUK-A-WAY lift has one hydraulic cylinder which can be
used to provide a positive force to the platform in an
upward direction but no force is applied to the
platform in the downward direction. The platform
simply lowers by gravity as the hydraulic pressure is
released. The TUK-A-WAY lift cannot be used where the
rear axle of the trailer is located close to the rear
of the trailer (as is often the case) as there is then
no space for the TUK-A-WAY to be stored when it is not
in use. A second type of lift has a horizontal
hydraulic cylinder that is used in conjunction with
several complex connectors and cables to move a
platform located at the rear of a trailer upward.
This type of platform lift also moves the platform
downward by gravity as hydraulic pressure is released.
There is no positive force moving the platform
downward. This type of lift stores the platform in an
upper vertical position when the truck is travelling
down a highway. The platform can be moved into an
upper horizontal position, a lower horizontal position
or a lower vertical position for loading or unloading
cargo, as desired. In the travel position, the
cylinder is extended and is thus exposed to the
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elements, such as road salt. With previous lifts,since there is no positive downward force that can be
exerted on the platform hydraulically, if the lift is
not working perfectly, the platform will often not
lower unless it is loaded with cargo. Further, if one
of the hydraulic lines breaks when the platform is in
an upper position, it will crash to the ground, with
or without cargo as the hydraulic pressure is
released. Obviously, personal injury or damage to the
cargo can result. Previous platform lifts are too
complex, subject to frequent mechanical failure or too
expensive to manufacture. Further, with previous
platform lifts, when the lift is located at the rear
of the lead unit of a B-train, many lifts are
completely unsuitable as they prevent the connection
of the fifth wheel to the B-train trailer so that the
rear trailer can be connected. A B-train is a
trucking system where a tractor pulls two trailers
simultaneously. The pivot pin of the second trailer
rests on a fifth wheel secured to the rear of the
first trailer. Previous platform lifts are designed
for a trailer of a specific width and cannot be easily
adapted to a trailer with a different width.
A platform lift for use with a cargo door of
a truck or trailer has a platform with an upper
surface and two ends. The cargo door has a lower edge
and two sides and the truck or trailer rests on a
supporting surface. There are two vertically mounted
hydraulic cylinders, a first cylinder being mounted at
one side of said door and a second cylinder being
mounted at the other side of said door, each cylinder
containing a rod and piston. The first cylinder has a
larger cross-sectional size than the second cylinder
such that a cross-sectional area of the first cylinder
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at a rod end minus a cross-sectional area of the rod
of the first cylinder is substantially equal to a
cross-sectional area of the second cylinder at a cap
end. The cylinders are connected by a connecting line
so that hydraulic fluid can flow through the rod end
of the first cylinder and the cap end of the second
cylinder as the cylinders are activated. Control
means and pumping means for hydraulic fluid are
connected to form a hydraulic system to activate the
cylinders. Each rod of the cylinders has a free end
opposite to an end that is connected to the piston,
the free ends of the rods and the platform being
connected so that the platform can be moved upward or
downward between a lower edge of said door and said
supporting surface with a positive force by
appropriately activating the cylinder so that the
pistons move upward or downward. The platform is
pivotally connected so that its upper surface can be
moved from a vertical position to a horizontal
position and vice-versa, as desired. The platform is
capable of being located at a vertical position
entirely below the door of the truck or trailer by
forcing the cylinders downward.
In the drawings:
Figure l is a partial perspective view of a
platform lift located at the rear of a trailer where
the platform is in a lower horizontal position;
Figure 2 is a partial perspective view of a
platform lift located at the rear of a trailer where
the platform is in an upper vertical position;
Figure 3 is a schematic view of an hydraulic
circuit for the lift of the present invention;
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Figure 4 is a partial schematic view of a
self-levelling system when a second cylinder reaches
an uppermost position before a first cylinder;
Figure 5 is a partial schematic view of a
self-levelling system when the first cylinder reaches
the uppermost position before the second cylinder; and
Figure 6 is a front view of a double port.
Referring to the drawings in greater detail,
in Figures 1 and 2, there is shown a platform lift 2
mounted at a rear 4 of a trailer 6 tonly part of which
is shown). The lift 2 has a first cylinder 8 and a
second cylinder 10, each of the cylinders being
mounted vertically, one on each side of a door 12 of
the trailer 6. Each cylinder 8, 10 has a rod 14 that
is connected to a piston (not shown in Figure 1). A
free end of each rod 14 opposite to the piston is
pivotally connected to an inside corner 16 of a
platform i8. The platform is spring-mounted (not
shown) in a conventional manner so that it can be
easily moved from a horizontal position to a vertical
position by pivoting the platform upward from the
position shown in Figure 1. For example, as is
conventional, the platform is counterbalanced with a
portion bar 19. Flexible cables or chains 20 prevent
the platform 18 from pivoting beyond the horizontal
position. Supports 22 are vertically mounted on
either side of the door 12 at the rear of the trailer
to hold the platform in a vertical position through
interlocking means 24 on the platform 18 and
interlocking means 26, 27 on the supports 22. The
interlocking means 24 is a projection extending
outward from either end 28 of the platform 18. The
interlocking means 26 are two lower channels, one in
each of the supports 22. Channels 26 are designed for
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holding the platform in a vertical position when the
platform has been lowered. The interlocking means 21
are two upper channels 27, one in each of the supports
22. The upper channels 27 are designed to hold the
platform in an upper vertical position as shown in
Figure 2. Each of the upper channels 27 has
disengagement means 28 for disengaging the projections
24 from the channel 27. Once disengaged, the platform
18 can be pivoted from the vertical position shown in
Figure 2 to a horizontal position with the chains 20
fully extended.
It will also be noticed that there are
control means 30 for activating the hydraulic
cylinders 8, 10 to raise and lower the platform 18.
The position of the platform shown in Figure 2 is the
travel position, (i.e. for use when the truck is
travelling). The position of the platform shown in
Figure 1 is the loading or unloading position. There
are two other distinct positions, one occurring when
the platform is in an upper horizontal position and
the other occurring when the platform is in a lower
vertical position. The upper horizontal position and
all intermediate horizontal positions are positions
for unloading or loading cargo. The lower vertical
position is used when a loading dock is available and
it is desired to get the platform out of the way so
that the trailer can be loaded or unloaded directly
from the loading platform without using the platform
lift 2.
In Figure 3, a schematic of a hydraulic
circuit for the lift platform 2 is shown. Each of the
cylinders 8, 10 has a rod end 32 and a cap end 34
together with a piston 36. The rods 14 are much
longer in actual use than their lengths as shown in
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Figure 3. Each rod 14 has a free end 38. The first
cylinder 8 has a larger cross-sectional size than the
second cylinder 10. The area of the first cylinder 8
at the rod end 32 minus the area of the rod 14 is
equal to or at least substantially equal to an area of
the second cylinder 10 at the cap end 34. The closer
these relative cross-sectional areas are to being
equal, the better the lift system will operate.
The cylinders 8, 10 are interconnected by a
connecting line 40 so that hydraulic fluid can flow
between the rod end 32 of the first cylinder 8 and the
piston 34 of the second cylinder 10 as the cylinders
are activated. Control means 42 and pumping means 44
are connected to activate the cylinders 8, 10. The
connection of the free ends 38 of the rods 14 to the
platform 2 is not shown in Figure 3. The control
means 42 can activate the cylinders 8, 10 either
upward or downward.
An electric solenoid activated four-way
directional control valve or double acting power pack
46 controls the flow of fluid through two lines in two
different directions. A velocity fuse 48 is located
in the connecting line 40 to control the flow rate of
the hydraulic fluid through the connecting line.
Also, if the connecting line is severed or develops a
leak, the flow of hydraulic fluid through the velocity
fuse will be automatically shut off. A counterbalance
valve 50 is connected in the line extending between
the rod end of the small cylinder 10 and the control
means 32 and pumping means 44. The purpose of the
counterbalance valve is to prevent any cavitation from
occurring in the hydraulic system and also to prevent
the platform from coming down too fast (i.e. faster
than the flow of the pump) during normal operation or
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from crashing down if a line breaks. A first pressurerelief valve 52, connected to the same line as the
counterbalance valve 50, is set at a pressure above
the system pressure so that it will protect against
overload of the platform. A second pressure relief
valve 54 is connected to the line between the four-way
directional control 46 and the cap end 34 of the first
cylinder 8. The relief valve 54 is set below system
pressure in order to limit downforce of the platform
and cylinder to a predetermined maximum.
An electric solenoid activated first check
valve 56 is located between the pressure relief valve
52 and the four-way directional control 46. The first
check valve 56 can be activated between two positions,
one position permitting flow upward to the second
cylinder but preventing flow downward from the second
cylinder to the control 46 and the other position
permitting flow downward but preventing flow upward.
Numerous reservoirs 58 for hydraulic fluid are shown
for ease of illustration.
In operation, the platform moves upward or
downward as the pistons move upward or downward
respectively. If it is desired to move the pistons
upward, the electric solenoid four-way directional
control 46 is activated to permit an upward flow to
the second cylinder 10 and a downward flow from the
first cylinder 8. Similarly, the first check valve 56
is activated to permit upward flow to the second
cylinder 10 through the counterbalance valve 50 as
shown in Figure 2 but to prevent any downward flow.
The counterbalance valve 50 has two components, a
second check valve 60 and a three-way valve 61 with
one port blocked. In an upward mode for the cylinder
the hydra~llic fluid flows through the check valve 60
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to rod end 32 of the second cylinder 10. This in turn
causes the piston 36 and rod 14 to move upward toward
the cap end 34. The upward movement of the piston 36
in the second cylinder 10 causes the hydraulic fluid
above the piston 36 to be forced through the
connecting line 40 and the velocity fuse 48 and into
the rod end 32 of the first cylinder 8. The more
accurate the relative sizing of the two cylinders, the
more precisely one cylinder will track with the other.
The hydraulic fluid entering the rod end 32 of the
first cylinder 8 causes the piston 36 to move upward,
thereby displacing the hydraulic fluid above the
piston and forcing it out of the cylinder 8 and
through a line 62. The fluid flows through the line
62 back through the four-way control 46 to the
reservoir 58. In this manner, the pistons are moved
upward until the platform attains the desired level.
A third pressure relief valve 64 is
connected to the line between the pump and tank or
reservoir S8. The valve 64 determines the maximum
pump pressure which corresponds to the maximum system
pressure when the pump is operating.
Alternatively, in order to move the pistons
and the platform downward, the four-way directional
control 46 is moved to a second position which
prevents hydraulic fluid from flowing upward into the
rod end of the second cylinder 10 but allows hydraulic
fluid to flow upward through the line 62 into the cap
end 34 of the first cylinder 8. As hydraulic fluid
enters the cap end 34 of the first hydraulic cylinder
8, the piston 36 is forced downward and the fluid
beneath the piston is forced through the connecting
line 40 and the velocity fuse 48 into the cap end 34
of the second cylinder 10. This in turn forces the
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piston 36 of the second cylinder 10 downward, thereby
forcing hydraulic fluid located beneath the piston 36
out of the second cylinder 10 at the rod end 32. The
fluid flows through the three-way valve 61 of the
counterbalance valve 50 and back to the first check
valve 56 which has been activated to a second position
to permit downward flow to the four-way directional
control valve 46 and back to the reservoir 58. In
this way, the pistons are forced downward.
As can be readily seen, the movement of the
pistons within the cylinders is caused by forcing
hydraulic fluid from the pumping means 44 directly
into one of the cylinders only. The hydraulic fluid
flowing through the connecting line activates the
other cylinder. For upward movement of the pistons,
the fluid is forced directly into the rod end of the
second cylinder. For downward movement of the
pistons, the fluid is forced directly into the cap end
of the first cylinder. Thus, there is a positive
force exerted by the cylinders in both an upward and
downward direction.
The counterbalance valve S0 prevents ~he
platform from coming down too quickly. The dotted
lines shown in Figure 3 extending from the three-way
valve 61 are pilot lines that carry pressure signals
to censors within the various valves but little, if
any, hydraulic fluid flows through these pilot lines.
It can be seen that the three-way valve 61 has three
different pilot lines connected thereto, one being
connected into the line 62 of the first cylinder 8,
the other being connected to the line between the
three-way valve 61 and the rod end of the second
cylinder 10 and the third pilot line being connected
between the up pressure line and the three-way valve
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,
61 to ensure that fluid coming upward passes through
the check valve rather than through the three-way
` valve 61. The three-way valve 61 is set so that it
permits lowering of the platform until pressure is not
less than one-tenth on the down side of the pressure
on the upside. The components of the hydraulic
circuit are well known to those skilled in the art.
In Figures 4, 5 and 6, there is shown a
self-levelling feature for the platform. The cap end
34 of each cylinder 8, 10 contains a double port 66
for the ingress and egress of hydraulic fluid. Each
double port has an upper orifice 68 and a lower
orifice 70 which are spaced vertically apart from one
another. The lower orifice 70 is tiny in size when
compared to the upper orifice 68. As can be seen from
the Figures, the orifices, 68, 70 are located so that
when the piston 36 is in the uppermost position within
one of the cylinders, the lower orifice 70 is not
sealed off from the hydraulic fluid in the rod end 32
of that cylinder by a seal 72 on each piston 36.
Further, it can be seen that the double port 66 of the
second cylinder 10 is connected into the connecting
line 40 between the two cylinders. The double port 66
of the first cylinder 8 is connected to the line 62 of
said first cylinder. For ease of illustration the
cylinders 8, 10 are shown in Figures 4 and 5 to be
much shorter than they actually are.
In operation of the self-levelling system,
if the circumstances shown in Figure 4 are considered
first, when the piston 36 of the second cylinder 10
reaches the uppermost position before the piston 36 of
the first cylinder 8, the piston in the first cylinder
8 can be forced continually upward until it too
attains the uppermost position within the cylinder.
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The continued upward force on the piston of the first
cylinder 8 can occur because the control means can
continue to be activated in an upward mode and
hydraulic fluid can continue to pass from the cap end
34 of the cylinder 10 through the lower orifice 70 and
hence through the connecting line 40 to the rod end 32
of the first cylinder 8. This will cause the piston
in the first cylinder to continue to move upward and
hydraulic fluid will continue to be forced through the
double port 66 of the first cylinder 8 to the line 62
until the piston reaches the uppermost position within
the first cylinder 8. Then, when it is desired to
lower the platform, the two pistons will start from
the same level. It is important that the lower
orifice 70 be very tiny relative to the upper orifice
68. For example, fifteen one-thousandths of an inch
has been found to be satisfactory in one embodiment
for the diameter of the lower orifice 70. The lower
orifice 70 shown in the drawings is much larger than
its actual size when compared to the size of the upper
orifice 68. If the lower orifice is too large, the
cylinders will not start in a downward motion.
The self-levelling system also works when
the piston in the first cylinder 8 attains the
uppermost level before the piston in the second
cylinder 10. This situation is shown in Figure 5.
When the piston in the first cylinder 8 attains the
uppermost position, the piston in the second cylinder
10 can continue to move upward because hydraulic fluid
is being forced into the rod end of the second
cylinder 10. This will cause the piston in the second
cylinder 10 to continue to move upward. Hydraulic
fluid above the piston of the second cylinder 10 will
continue to move through the double port 66 of that
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cylinder through the connecting line 40 to the rod end
of the first cylinder 8. Hydraulic fluid can continue
to flow out of the cap end of the first cylinder 8
through the lower orifice 70 and to the line 62.
Thus, the pistons can be levelled at their uppermost
position whenever they become unlevel simply by
continuing to activate the cylinders through the
control means 42 in an upward mode. During normal
operation of the lift 2 the pistons will almost always
be level with one another. In the event that the
pistons are not level at any time, the problem can be
corrected simply by moving the pistons and the
platform to their uppermost position. The correction
is automatic and can be done each time the pistons are
moved through the uppermost position, thereby
correcting the problem before it becomes serious.
The two cylinders, which can be referred to
as phasing cylinders, have a size relationship that is
based on the formula for the area of a circle:
A = ~r2.
The cross-sectional area of the fluid in the
rod end of the first cylinder is as follows:
A1 = (D12 - dr2) 1.27
where D1 is the inside diameter of the
first cylinder; and
where dr is the outside diameter of the
rod of the first cylinder.
Similarly, the cross-sectional area of the
fluid in the second cylinder is:
A2 = D22 . 1.27
where D2 is equal to the inside
diameter of the second cylinder.
As long as these cylinders are designed so
that A1 is equal to A2 or that A1 is at least
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substantially equal to A2, the system will work. For
example, where the inside diameter of the first
cylinder is 2.015 inches and the outside diameter of
the rod in that cylinder is 1.0 inch, the inside
diameter of the second cylinder is 1.749 inches. The
lift takes up a minimum of space at the rear of a
truck or trailer as the inside diameter of the first
cylinder would not exceed three inches for most
applications.
It can readily be determined that some of
the features of the hydraulic circuit can be deleted
while still achieving the main advantages of the
present invention. For example, some of the pressure
relief valves could be omitted or a different control
means could be substituted. Also, it is not necessary
to design the system with the self-levelling feature
even though it would be advisable to do so. Further,
it is not absolutely essential to have both the four-
way control 46 and the first check valve S6. Though
it is not recommended, the lift will work as long as
there is a valve to direct the hydraulic fluid in
either of two directions through the hydraulic
circuit. It should be noted that all filters have
been omitted from the hydraulic circuit shown in
Figure 3. Filters are conventional in any hydraulic
system and their use will be readily apparent to those
skilled in the art. If the filters had been included,
the circuit diagram would have been unnecessarily
complex.
The hydraulic system for the platform lift 2
is also designed for thermal relief. If the trailer
is left in the hot sun and the hydraulic fluid expands
from the heat, it can be expunged from the system
through the third pressure relief valve 64 or one of
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the other relief valves. The system is located
entirely on either side of the door at the rear of a
trailer. Therefore, for trailers of different widths,
it is a relatively simple matter to construct a wider
or narrower platform and to space the cylinders either
further or closer to one another depending on the
width of the trailer. In the travel position, the
cylinders are in a closed position and thereby
protected from the elements such as road salt as much
as possible. While the platform shown in the drawings
is of fixed construction, it is possible to use the
system with a bifold platform.
The system can be used easily on the first
trailer of an A-train, B-train or C-train as the
platform lift of the present invention, except for the
platform itself, is located entirely along each side
of the trailer and not underneath the trailer or in
the central area of the rear of the trailer. Of
course, the platform lift can easily be used with any
door of any truck trailer or truck of appropriate
size. Since the cylinders are direct acting, the
trailer or truck must be of sufficient height to allow
the cylinders to be long enough to move the platform
between the ground and the floor of the trailer.
