A hydraulically operated dockboard including a ramp which is hinged at its rear edge to the frame of the dockboard, and a lip is hinged to the forward edge of the ramp and can be pivoted between a downwardly hanging pendant position and an extended position. A main hydraulic cylinder unit interconnects the frame and the ramp and by operating the cylinder unit the ramp can be pivoted upwardly from a horizontal cross traffic position to an upwardly inclined position. A lip cylinder unit interconnects the ramp and the lip, and operation of the lip cylinder unit will pivot the lip from the pendant to an extended position. The hydraulic system is constructed so that fluid is initially supplied to the main cylinder unit by a pump to raise the ramp, and when the main cylinder unit is fully extended, the increased pressure will shift a valve to supply fluid to the lip cylinder unit. The valve is constructed so that the valve will be maintained in the shifted position by substantially less pressure than that required to initially shift the valve. When operation of the pump is discontinued, the ramp will descend and the back pressure generated by the weight of the lip acting against the valve will retain the valve in the shifted position to maintain the lip in the extended condition until the lip engages the bed of a truck.

BACKGROUND OF THE INVENTION 
The conventional hydraulic dockboard is mounted in a pit or depression in 
the loading dock and includes a ramp which is pivoted at its rear edge to 
the frame or supporting structure. In addition, a lip is hinged to the 
forward edge of the ramp and is movable between a downwardly hanging 
pendant position and an extended position where it forms an extension to 
the ramp. When in use, the extended lip will engage the bed of a truck 
positioned in front of the loading dock to bridge the gap between the dock 
and the truck. After the loading operation is completed and the truck 
pulls away from the dock, the lip will pivot downwardly to the pendant 
position, and the ramp will pivot downwardly to a stored position. 
In a conventional hydraulically operated dockboard, a main hydraulic 
cylinder unit interconnects the frame and the ramp, and acts to pivot the 
ramp from a horizontal dock level position to the upwardly inclined 
position. In addition, a lip cylinder unit interconnects the ramp and the 
lip and pivots the lip from the downwardly hanging pendant position to the 
extended position. 
U.S. Pat. Nos. 4,365,374 and 4,641,388 describe hydraulically operated 
dockboards in which the flow of hydraulic fluid to the lip lifting 
cylinder is controlled by, and supplied through, the main hydraulic 
cylinder unit. In accordance with U.S. Pat. No. 4,641,388, a two-way 
control valve is mounted in the pressure line extending between the main 
hydraulic cylinder and the lip lifting cylinder and the valve is also 
connected to a return line leading to the reservoir for the hydraulic 
system. Thus, the valve connects the lip lifting cylinder to either the 
pressure supply line or to the return line. When the main hydraulic 
cylinder unit is operated to raise the ramp, the piston rod of the 
cylinder is extended, and as the piston approaches the end of its stroke 
of travel, a passage is opened which supplies hydraulic fluid from the end 
of the main cylinder unit through the pressure line to the lip lifting 
cylinder to thereby operate the lip lifting cylinder and pivot the lip to 
the extended position. 
As described in the aforementioned patent, when the flow of pressurized 
fluid to the main cylinder unit is terminated, the combined weight of the 
ramp and lip will cause the main cylinder unit to retract and the ramp 
will pivot downwardly until the extended lip engages the bed of a truck 
parked in front of the loading dock. A restricted orifice in the return 
line extending between the main cylinder and the reservoir will control 
the rate of descent of the ramp, resulting in the pressure being 
maintained in the main cylinder and the lip cylinder until the descent of 
the ramp is arrested. 
When the extended lip engages the bed of the truck, the pressure in both 
the main cylinder unit and the lip lifting cylinder will drop to near 
ambient pressure and the drop in pressure will enable a biasing force to 
move the control valve to a position where the lip cylinder is connected 
to the reservoir via the return line. After the loading operation has been 
completed, the lip will pivot downwardly by gravity, forcing the fluid 
from the lip cylinder through the valve and back to the reservoir. 
With the construction of U.S. Pat. No. 4,641,388 the lip will return to its 
downwardly hanging pendant position without the necessity of the ramp 
being moved to a below dock position. 
SUMMARY OF THE INVENTION 
The invention is directed to an improved hydraulic system for a dockboard. 
In accordance with the invention, a supply line or conduit interconnects 
the reservoir of the hydraulic system with the main cylinder unit. On 
operation of the pump, the increased pressure will pivot a shuttle valve 
connected in the supply line to thereby supply pressurized fluid to the 
main cylinder unit to raise the ramp to its upper inclined position. When 
the main cylinder unit reaches its fully extended position, the pressure 
will increase and the increased pressure acts through a bypass conduit to 
shift a second valve against a biasing force and permit flow through the 
second valve to the lip cylinder to thereby extend the lip from the 
pendant to the extended position. 
The second valve is constructed such that once shifted by a predetermined 
pressure, a reduced pressure will maintain it in a shifted condition, 
allowing the system pressure to be low while extending the lip. 
On discontinuation of operation of the pump, the ramp will descend and the 
pressure in the system will decrease, but the reduced pressure will 
maintain the second valve in the shifted position. As the back pressure 
created in the main cylinder unit by the weight of the ramp is greater 
than the back pressure generated by the weight of the lip, the lip 
cylinder is maintained in an extended position. 
When the extended lip engages the truck bed, the pressure drops to near 
ambient which acts to shift the second valve back to its original position 
to enable fluid to be returned from the lip cylinder to the reservoir. 
When the truck pulls away from the loading dock, the lip will then be able 
to fall by gravity to its pendant downwardly hanging position. 
The invention also includes a novel piston construction for the main 
cylinder unit in which the piston is provided with a longitudinal passage 
which communicates with opposite ends of the cylinder. A valve is mounted 
within the passage and is constructed so that it is biased to an open 
position and fluid can freely flow through the passage as the piston rod 
is extended and retracted in raising and lowering the ramp. However, the 
valve will close and act as a velocity fuse to prevent rapid descent of 
the ramp in the event a predetermined external downward force is applied 
to the ramp as for example, if a truck accidentally pulls away from the 
loading dock when an added load, such as a fork lift truck, is on the 
ramp. The piston construction enables the cylinder to have only a single 
port which is located in a position which allows any air in the cylinder 
to escape as the ramp is lowered, thus providing a self-bleeding 
operation. 
The invention provides a simplified and less costly hydraulic system for a 
hydraulically operated dockboard. Only two fluid lines or conduits are 
required, one extending to the main cylinder and the other to the lip 
cylinder. The construction as used in the past required six separate 
hydraulic lines and a corresponding number of fittings or connections. 
Thus, the invention provides a more aesthetically attractive unit, as well 
as substantially reducing the potential for fluid leakage. 
By building the velocity fuse directly into the piston of the main cylinder 
unit, a less costly construction is achieved. Moreover, as the velocity 
fuse is located internally of the cylinder, tampering is eliminated as can 
occur when the velocity fuse is locate externally. 
In a modified form of the invention, the pressure required to unseat the 
lip control valve to supply fluid to the lip cylinder is adjustable 
independently of the pressure required to return the lip control valve to 
its seated position. Thus, the unseating pressure can be adjusted 
according to the size and weight of the ramp of the dockboard without 
altering the return pressure for the valve. This provides a more precise 
control of the lip action. 
Other objects and advantages will appear in the course of the following 
description.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
FIG. 1 illustrates a dockboard 1 that is mounted within a pit or depression 
2 in a loading dock 3. The dockboard is intended to bridge the gap between 
the loading dock and the bed of a truck which is located in front of the 
dock. 
Dockboard 1 includes a supporting frame 4 and the rear end of a ramp or 
deckplate 5 is pivoted to the frame so that the ramp is movable from a 
generally horizontal dock level position, where the ramp is generally 
flush with the upper surface of the dock 3, to an upwardly inclined 
position, as shown in FIG. 1. 
Hinged to the forward edge of ramp 5 is a lip 6 which can be pivoted from 
between a downwardly hanging pendant position and an outwardly extended 
position as shown in FIG. 1, where the lip forms an extension to the ramp. 
The lip is hinged to ramp 5 by a series of lugs 7 which are connected to 
the underside of the lip and are mounted for rotation on a hinge pin 8 
that is connected to the forward end of the ramp. The construction of the 
frame, ramp and lip is conventional and is of a type such as shown in U.S. 
Pat. No. 4,068,338. 
A main cylinder unit 9 interconnects the frame 4 and the ramp 5 and serves 
to pivot the ramp from the horizontal dock level position to the upwardly 
inclined position, while a second lip lifting cylinder unit 10 
interconnects the ramp and the lip 6 and serves to pivot the lip to the 
extended position. 
Main cylinder unit 9 includes a cylinder 11 and a bracket mounted on the 
lower end of the cylinder is pivotally connected to lugs 13 on frame 4. 
Cylinder 11 is composed of a generally cylindrical outer shell 14, the ends 
of which are enclosed by an upper head 15 and a lower base 16. A piston 17 
is mounted for sliding movement in shell 14 and carries a piston rod 18 
that extends through upper head 15. The outer end of piston rod 18 is 
pivotally connected to hinge pin 8. 
Hydraulic fluid is supplied to the upper end of cylinder 11 through a 
fitting 19. 
As best shown in FIG. 2, piston 17 is provided with a longitudinal passage 
20 which is offset laterally from the center axis of the piston. Passage 
20 includes a small diameter section 21 and a large diameter section 22 
which are separated by a valve seat 23. A valve ball 24 is mounted within 
the section 22 and cooperates with the valve seat 23. Valve ball 24 is 
normally maintained in an open condition with respect to valve seat 23 by 
a pair of springs 25 and 26. Spring 25 is interposed between the end of 
passage section 21 and the valve ball 24, while spring 26 engages the 
opposite side of the valve ball and is retained within passage section 22 
by annular retainer 27 which is threaded within the end of the passage 
section 22. 
With the construction of the valve 24, hydraulic fluid can flow freely 
through the passage 20 as the piston moves within the cylinder 11. 
However, if a predetermined downward force is applied to the ramp, as for 
example, in the event a truck accidentally pulls away from the loading 
dock when an added load, such as a fork lift truck, is on the ramp, the 
flow across the valve ball 24 causes a pressure differential, resulting in 
the pressure in the lower end of cylinder 11 being greater than the 
pressure in the upper rod end, thereby closing valve 24 and preventing 
further descent of the ramp. 
As shown in the hydraulic circuit of FIG. 3, a line 28 connects fitting 19 
of cylinder 9 with a shuttle valve 29 that is located in the pump valve 
body. In addition, a supply line 30 connects shuttle valve 29 with a pump 
33 driven by motor 34 is connected in supply line 30. Also located in 
supply line 30 between pump 33 and shuttle valve 29 is a check valve 35. 
A return line 32,45 connects shuttle valve 29 with reservoir 31 and an 
adjustable orifice 36 is connected in return line 32. Orifice 36 acts to 
control the flow of fluid return through line 32 and thus the rate of 
descent of the ramp as will be hereinafter described. 
In addition, a line 37 interconnects the supply line 30, at a location 
downstream of pump 33, with reservoir 31 and a conventional relief valve 
38 is connected in line 37. 
Pilot line 39 interconnects supply line 30 with shuttle valve 29 and a 
second pilot line 40 interconnects line 28 with the shuttle valve. 
Pressure in line 30 generated by pump 33 will operate through pilot line 
39 to move the shuttle valve to the right as shown in FIG. 3, and connects 
the supply line 30 with the cylinder line 28 to thereby supply fluid to 
the main cylinder 11. On lowering of the ramp, the back pressure in line 
28 will act through pilot line 40 to move valve 29 to the left as shown in 
FIG. 3, and thereby connect line 28 to return line 32 to return fluid from 
the main cylinder to the reservoir 31. 
The hydraulic system also includes a by-pass line 41 which connects 
cylinder line 28 with a control valve 42, and a fixed orifice 43 is 
located within line 41. In addition, a lip cylinder line 44 connects valve 
42 to the lip cylinder 10 and a return line 45 connects the valve 42 to 
the reservoir 31. 
Pilot line 46 interconnects by-pass line 41 and valve 42 so that a 
predetermined pressure in line 41 will shift valve 42 to the right, as 
shown in FIG. 3, to connect line 41 with line 44 to supply fluid to the 
lip cylinder 10. In addition, a second pilot line 47 acts to maintain 
valve 42 in the shifted position after the pressure in line 41 has been 
decreased. 
A construction of valve 42 is best illustrated in FIG. 4. Valve 42 includes 
an outer body 48 having a central cavity 49 and a valve 50 is mounted for 
sliding movement within cavity 49. One end of the valve body 48 is 
provided with a first opening 51 which defines a valve seat and is 
connected to line 41, while the body 48 is also provided with a second 
opening 52 which is connected to line 44 and a third opening 53 which is 
connected to return line 45. As shown in FIG. 4, a pair of collars 54 and 
55 are mounted in spaced relation on the valve 50. 
The tip 56 of valve 50 is biased to a closed position with respect to the 
valve seat 51 by a spring 57 which is interposed between collar 55 and a 
spring retainer 58 which is threaded within the end of the valve body 48. 
Valve 42 is constructed so that a pressure in by-pass line 41, in excess of 
that required to raise ramp 5 acts on the exposed area of tip 56, 
resulting in a force of sufficient magnitude to overcome the force of 
spring 57 and unseat valve tip 56 from seat 51. Once the tip 56 of the 
valve has been unseated, a larger area of the end of the valve, including 
the face of collar 54, will be exposed to the fluid pressure so that a 
substantially lesser pressure will be required to maintain the valve in 
the opened or shifted position, as compared to the pressure required to 
initially unseat the tip 56 from the opening 51. 
When the valve 50 has been shifted to the right, as shown by the dashed 
lines in FIG. 4, fluid can then flow from the line 41, through cavity 49 
to line 44, to the lip cylinder to thereby extend ram 59 of the lip 
cylinder to pivot the lip upwardly. With the valve 50 in the shifted 
position, the surface area of the valve exposed to the fluid pressure is 
increased so that a lesser pressure is required to exert a force 
sufficient to overcome the force of spring 57 to retain the valve in the 
shifted position. In practice, a lesser force, about 1/12 the force 
required to initially shift the valve, will hold the valve in the shifted 
position. 
Ramp 5 is normally stored in the horizontal dock level position. After a 
truck pulls into position in front of the dock 3 for loading, the operator 
will push the "raise" button on the control panel which operates the motor 
pump unit to supply pressurized fluid from the reservoir through supply 
line 30 to the valve 29. The pressurized fluid acting through pilot line 
39 will shift valve 29 to thereby connect line 30 with cylinder line 28 to 
supply pressurized fluid to the main cylinder 9. The pressure on both 
sides of piston 17 will be balanced due to he free flow through passage 20 
in the piston. However, because the pressure acts against a larger surface 
area on the lower face of the piston than on the upper face, due to the 
presence of piston rod 18, the differential force will move the piston 
upwardly to extend the piston rod 18 and pivot the ramp 5 to the upwardly 
inclined position, as shown in FIG. 1. As the piston moves upwardly, fluid 
will pass through the passage 20 from the upper end of the cylinder 11 to 
the lower end. 
When the piston 17 reaches the end of its upward stroke and bottoms out, 
the pressure in the line 28 will increase and the increased pressure 
acting through by-pass line 41 will be sufficient to unseat the tip 56 of 
valve 50 from the seat 51, to thereby shift the valve 50 to a position 
where the by-pass line 41 is connected to line 44, thereby supplying fluid 
to the lip cylinder. Supplying fluid to the lip cylinder will extend the 
ram 59 to thereby pivot the lip 6 from the pendant to the extended 
position. 
As valve 42 shifts to connect line 41 to the lip line 44, the pressure is 
reduced until the ram 59 of the lip cylinder bottoms out. At this time the 
relief valve 38 will open to release the pressure in the system. 
With the ramp 5 fully elevated, operation of the motor 34 and pump 33 is 
discontinued and the ramp will then descend by gravity. The back pressure 
in line 28 will pilot the valve 29, so that the fluid will then be 
returned through valve 29 and return line 32 to the reservoir 31, through 
the adjustable orifice 36 which will act to control the downward movement 
of the ramp. However, the back pressure in lines 28 and 41 will be 
sufficient to maintain the valve 42 in the shifted position so that the 
lip will remain in the extended position. 
When the extended lip engages the truck bed, the descent of the ramp and 
lip is terminated, with the result that the pressure in lines 28 and 41 
will be reduced to ambient. This reduction in pressure will enable the 
biasing force to shift the valve 42 back to its original position, 
connecting lip line 44 with the return line 45, to thereby permit the 
fluid within the lip cylinder to return to the reservoir. 
When the loading operation is complete, the operator may push the "raise" 
button on the control panel. As before, this will result in the extension 
of main cylinder 9, and pivot the ramp to an upwardly inclined position. 
The lip which was supported by the truck bed will be allowed to fall by 
gravity because lip cylinder 10 is ported to the reservoir through line 
44, valve 42 and line 45. 
Also when the truck pulls away from the dock, the lip is then free to fall 
by gravity to its original pendant position. 
The invention provides a substantial simplification of the hydraulic system 
over conventional types and reduces the overall cost of the hydraulic 
system. With the system of the invention, only a pair of external 
hydraulic lines are required, one being connected to the main cylinder 9 
and the other to lip cylinder 10. Not only does this system reduce the 
overall cost, but also provides a cleaner appearance and substantially 
reduces the number of potential leakage sites. 
As a further advantage, the velocity fuse, which prevents the rapid descent 
of the ramp, is built into the piston of the main cylinder unit. This 
reduces the cost of the velocity fuse over conventional types. As the 
velocity fuse is located internally of the cylinder, tampering or 
re-adjustment of the velocity fuse is prevented, as can occur if the 
velocity fuse is located externally. 
In a modified form of the invention, as shown in FIGS. 5-7, the pressure 
required to unseat the second or lip control valve to supply fluid to the 
lip cylinder is adjustable independently of the pressure required to 
return the lip control valve to its seated position. With this 
construction, the unseating pressure can be adjusted according to the size 
and weight of the dockboard without altering the return pressure for the 
valve, thus providing more precise control of the action of the lip. 
In this embodiment, valve 60 is substituted in the hydraulic system, as 
shown in FIG. 5, for valve 42 of the first embodiment. Valve 60 includes a 
hollow valve body 61 having an externally threaded section 62 that is 
threaded to pump block 63. O-ring assemblies 64 provide seals between 
valve body 61 and block 63. The open inner end of body 61 communicates 
with pressure passage 65 which is connected to pressure line 41. 
A tubular valve spool 66 is mounted for sliding movement within the central 
bore 67 of body 61. The outer end 68 of spool valve 66 is generally flat 
and is disposed normal to the axis of the spool valve and contains a small 
diameter central orifice 69 which provides communication between pressure 
passage 65 and the internal passage 70 of the spool valve. Valve spool 66 
is adapted to slide in bore 67, and the central portion of valve spool 66, 
spaced from end 68, has a reduced diameter to provide an annular clearance 
between spool 66 and bore 67, as indicated by 71. 
The inner end of body 61 is provided with a plurality of radial ports 72 
which communicate with annular chamber 73 in block 63 and chamber 73 is 
connected through passage 74 and line 44 to lip cylinder 10, as shown in 
FIG. 5. 
Valve spool 66 is biased to an inner position where the inner end of the 
valve closes off the ports 72 by a compression spring 75. One end of 
spring 75 bears against an annular shoulder 76 formed in valve spool 66, 
while the opposite end of spring 75 is engaged with a central recess 77 in 
valve seat 78, which is mounted in body 61. 
Valve seat 78 is provided with a central opening 79, which is enclosed by a 
poppet valve 80. Valve 80 includes a small diameter end 81, which is 
received within opening 79, and an the enlarged head 82. 
To bias valve 80 to a closed position, a coil spring 83 is interposed 
between head 82 and an adjusting screw 84 which is threaded within a 
central opening in cap 85. Cap 85, in turn, is threaded within the outer 
end of valve body 61. By threaded adjustment of screw 84, the force of 
spring 83 can be adjusted, thereby selectively varying the force required 
to open valve 80. This adjustment is independent of the biasing force 
which biases valve spool 66 to the closed position. A lock nut 86 can be 
engaged with the outer end of screw 84 to lock the screw in the desired 
position. 
Valve body 61 is provided with radial ports 87, as well as a radial port 88 
which is located axially of ports 87. Ports 87 and 88 communicate with an 
annular chamber 89 in block 63, and chamber 89 is connected through 
passage 90 and line 45 to reservoir 31. 
In addition, valve spool 66 is provided with a radial port 91 which, as 
shown in FIG. 6, is closed off by valve body 61 when the spool valve is in 
the inner closed position, as seen in FIG. 6. However, axial shifting of 
the valve spool 66 will bring port 91 into communication with port 88 in 
the valve body. 
A further passage 92 is formed in body 61 and valve seat 78 and provides 
communication between the interior of cap 85 and chamber 89 in block 63. 
When poppet valve 80 is opened, fluid can flow from the interior of the 
valve spool 66 through opening 79 and then through passage 92 and chamber 
89 to reservoir 31. 
With the valve construction shown in FIGS. 5-7, the ramp is elevated in the 
manner described in connection with the first embodiment. When the piston 
17 of the main cylinder 9 reaches the end of its upward stroke and bottoms 
out, the pressure in line 28 will increase and the increased pressure 
acting through line 41 will be applied to the valve spool 66 through 
passage 65. Due to orifice 69 and passage 70, the pressure will be applied 
to both ends of the spool valve. As the force of the fluid pressure acting 
on the exposed area of the lower end of spool valve 66 is equal to the 
force acting on the upper end of the spool valve (the sum of the force of 
spring 75 plus the force of the fluid pressure acting on the exposed area 
of the upper end), the valve will be maintained in the closed position, as 
shown in FIG. 6. 
The fluid pressure acting through spool valve 66 will also be applied to 
the inner small diameter end 81 of poppet valve 80, which is biased to a 
closed position by spring 83. When the pressure applied to poppet valve 
end 81 is great enough to overcome the force of spring 83, the poppet 
valve will open, allowing fluid to flow through passage 92, chamber 89, 
passage 90, and line 45 to the reservoir 31 As fluid flows through the 
spool valve, it passes through the small diameter orifice 69, causing a 
pressure drop across the orifice, which is sufficient to produce a higher 
force on end 68 of the valve, as opposed to the opposite end. This 
resulting differential in force will move the valve spool 66 axially in 
body 61 against the force of spring 75 to open the ports 72 and supply 
fluid from passage 65, through ports 72 to chamber 73 and then through 
passage 74 and line 44 to the lip cylinder to extend the lip. When the 
ports 72 to the lip cylinder are open, the pressure will drop in the 
pressure line, which will cause the poppet valve 80 to move to the closed 
position, under the force of spring 83. Fluid will continue to flow from 
passage 65 through orifice 69 into passage 70, and out port 91 through 
port 88 into chamber 89 and through passage 90 and line 45 to reservoir 
31. This flow maintains the pressure differential through orifice 69, 
causing higher pressure on face 68, keeping spool 66 in the shifted 
position. 
When the ram 59 of the lip cylinder 10 bottoms out, the relief valve 38 
will open to release the pressure in the system. With the ramp 5 fully 
elevated, operation of the motor and pump 33 is discontinued and the ramp 
will then descend by gravity. The back pressure in line 28 will pilot the 
valve 29, so that the fluid will then be returned through valve 29 and 
line 32 to the reservoir 31 through the adjustable orifice 36, which will 
act to control the downward movement of the ramp. However, the back 
pressure in lines 28 and 41, and the resulting flow through passage 65, 
orifice 69, passage 91, port 88, chamber 89, passage 90, and line 45 to 
reservoir 31, will result in a pressure differential across orifice 69 
sufficient to maintain the valve spool 66 in the shifted position, so that 
the lip will remain in the extended position. 
When the extended lip engages the truck bed, the descent of the ramp and 
lip is terminated with the result that the pressure in lines 28 and 41 
will be reduced to ambient. Valve spool 66 will then shift to its original 
position, as seen in FIG. 6. When the truck pulls away from the dock, the 
lip is then free to fall by gravity to its original pendant position, and 
as the lip falls, fluid will be returned from the lip cylinder 10 through 
line 44 and will flow through passage 74, chamber 73, ports 72 and 
clearance 71 to ports 87, chamber 89, passage 90 to reservoir 31. 
The construction shown in FIGS. 5 and 6 has distinct advantages. While the 
lips of different dockboards may be somewhat uniform in size and weight, 
the ramps of dockboards may vary considerably in size and weight. The 
sequence pressure, which is the pressure in excess of that required to 
raise the ramp and supply pressure to the lip cylinder, will necessarily 
vary depending upon the weight of the ramp. As the sequence pressure and 
the return pressure, which is the pressure at which the lip control valve 
will return to its original position, are usually in a ratio of about 
10:1, an increase in sequence pressure, in a conventional system, will 
necessarily result in an increase in return pressure. Increasing the 
return pressure will mean that the lip will fall by gravity at a higher 
pressure, which is undesirable, for it is preferred that the return 
pressure be as low as possible. However, the construction shown in FIGS. 
5-7, overcomes this problem by enabling the sequence pressure to be 
independently adjusted through operation of the adjusting screw 84 and 
this adjustment is independent and will not effect the return pressure, 
which is determined by the force of spring 75. Thus, with the construction 
of the invention, a low return pressure can be obtained while providing an 
independent adjustment of the sequence pressure to accommodate the weight 
of the ramp of the dockboard. 
Various modes of carrying out the invention are contemplated as being 
within the scope of the following claims particularly pointing out and 
distinctly claiming the subject matter which is regarded as the invention.