Composite pneumatic-hydraulic system in a dockleveller installation

A dockleveller installation is provided having a dockboard hinged for pivotal movement and a composite pneumatic-hydraulic system for pivoting the dockboard. The composite system includes a fluid-filled cylinder attached to the dockboard, a pressurized air supply, and a multiplier connected therebetween. The multiplier is a closed cylindrical member comprising an air cylinder segment connected to the air supply and a fluid cylinder segment connected to the hydraulic cylinder. The interior cross-sectional area of the air segment is greater than that of the fluid segment. The multiplier also includes a double headed piston, one end of which reciprocates and seals in the air segment, the other end of which reciprocates and seals in the fluid segment. This composite system enables pressure to be transferred through the multiplier from the air supply to the hydraulic cylinder, causing the cylinder to open and the dockboard to pivot. The pressure transferred from the air to the fluid is increased by an amount proportional to the ratio of the piston head diameters.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a dockleveller installation. More 
particularly, it relates to a dockleveller installation having a hydraulic 
cylinder, for pivoting the dockboard of the installation between 
cross-traffic and bridging positions, and to the system for actuating the 
cylinder. 
2. Description of the Prior Art 
A dockleveller installation is a device which provides a bridge to span the 
gap between the surface of a loading dock and the bed of a truck backed up 
to the dock. The installation includes a dockboard, which is a flat, 
heavily reinforced platform. This dockboard is hinged at its inner or rear 
edge to the frame of the installation. Thus it can be pivoted between a 
horizontal cross-traffic position (in which it forms part of the loading 
dock surface) and an inclined bridging position. 
Means must be provided to pivot or raise the heavy dockboard from the 
cross-traffic to the bridging position. In most cases such means comprise 
a hydraulic cylinder, a pneumatic cylinder, or a spring. The present 
invention is concerned with the cylinder-biased type of installation. 
Each of the prior art pneumatic and hydraulic pivoting systems are 
characterized by both advantages and disadvantages. 
Let us consider some of the disadvantages of the hydraulic pivoting system. 
This system normally comprises an electric motor, a hydraulic pump driven 
by the motor, a fluid reservoir, and suitable lines and connections 
completing the system. If the facility (such as a warehouse) requires a 
number of docklevellers, then a separate motor and pump must be provided 
for each unit. This is expensive. Also, many working parts are involved, 
so the likelihood of mechanical failure is high. Another disadvantage of 
this system is that the reservoir is usually vented to the atmosphere in 
the facility. This is undesirable in a food-processing plant, as the 
reservoir emits polluting compounds. Still another disadvantage is that 
moisture may enter the system through the open reservoir; this can be 
troublesome if the lines are exposed to freezing temperatures. If the 
system is to be installed in a harardous area, it is usually necessary to 
locate the motor and pump outside this area, unless an expensive 
explosion-proof unit is used. This results in having to provide a long 
hydraulic line extending from the pump to the dockleveller; there is thus 
a high pressure drop across this line and the output pressure at the 
cylinder is correspondingly diminished. Still another disadvantage has to 
do with the capability of an electric-hydraulic system to build up a high 
output pressure at the cylinder. If a forklift truck has been left parked 
partly on the dockboard, and the cylinder is activated, the pressure 
generated can be sufficient to overturn the truck. 
A hydraulic pivoting system is, however, characterized by some positive 
aspects as well. For example, it may be accurately monitored, controlled 
and adjusted with respect to the output delivered by the cylinder. This is 
so because the amount of fluid moving through the line can be closely 
controlled. Also, it is a reliable system when exposed to freezing 
temperatures (although, as previously pointed out, problems can arise from 
the entry of moisture into the lines through the open reservoir). 
Use of a pneumatic pivoting system does away with many of the shortcomings 
of the hydraulic system. For example, a single air compressor can be used 
to power all the cylinders of a multiple dockleveller facility. Thus lower 
capital and maintenance costs can be obtained by use of the pneumatic 
system. In many buildings, an air compressor is already in place and part 
of its output can be used to power the new dockleveller cylinders. Also, 
there is no reservoir associated with a pneumatic system, so atmospheric 
pollution is not a problem. Furthermore, pressure drops due to friction 
are much reduced in a pneumatic system, as compared to a hydraulic system. 
Thus long supply lines can be used without disadvantage. In addition, an 
air system has a built-in accumulator capability which assists in 
producing a constant and safe pressure output at the cylinder. 
Now, while the pneumatic system does have a number of advantages relative 
to the hydraulic system, it has some disadvantages which have prevented it 
from achieving commercial success. More particularly, in cold climates 
customers fear condensation and freeze up in exposed lines. Also, the 
system does not lend itself to close control and adjustment of the output 
pressure at the cylinder. 
With this background in mind, it is the object of this invention to provide 
a dockleveller installation, having a cylinder actuating system, which 
incorporates many of the best features of both the hydraulic and pneumatic 
systems and which does not have their disadvantages. 
SUMMARY OF THE INVENTION 
In accordance with the invention, a composite pneumatic-hydraulic system is 
provided to actuate the cylinder of a dockleveller installation. 
More particularly, there is provided an air compressor and a conduit or 
line connecting it with an interfacing element, which I term a multiplier. 
The multiplier is a closed cylindrical member having first and second end 
portions. The first end portion has a larger internal cross-section than 
the second end portion. The air line is connected with the first end 
portion for the entry or discharge of pressurized air thereinto or 
therefrom; a hydraulic conduit or line is connected with the second end 
portion for the entry or discharge of hydraulic fluid thereinto or 
therefrom. The hydraulic line is connected with a hydraulic cylinder, 
which biases the dockboard between cross-traffic and bridging positions. A 
double-headed piston reciprocates within the multiplier cylindrical 
member. This piston has a first head member at its first end, which slides 
within the first end portion and sealably engages it; at its opposite end, 
the piston has a second head member which slides within the second end 
portion and sealably engages it; means, such as a rod, rigidly 
interconnects the head members. The volume between the second head member 
and the piston of the hydraulic cylinder is filled with hydraulic fluid. 
In operation, the air compressor delivers air at a particular 
pressure/square unit into the first end portion of the multiplier. The 
double-headed piston is shifted in response to this pressure and forces 
hydraulic fluid at a higher pressure into the hydraulic cylinder, thereby 
raising the dockboard. 
There are two features incorporated in the invention. Firstly, a novel 
combination has been made by splicing together pneumatic and hydraulic 
systems. The composite system is characterized by desirable features of 
the components but omits undesirable ones. Secondly, this splice has been 
effected by using a multiplier which converts the relatively low pressure 
obtained from the compressor into a relatively high pressure--this allows 
for the higher pressure drop encountered in the hydraulic line. 
Broadly stated, the invention involves, in a dockleveller installation, the 
combination comprising: a dockboard hinged at its inner edge for pivoting 
movement between cross-traffic and bridging positions; a hydraulic 
cylinder connected with the dockboard for biasing it between said 
positions; pressurized air supply means and a first conduit extending 
therefrom for supplying and exhausting pressurized air; a multiplier 
comprising connected air and fluid cylinder segments, said segments 
combining to form a closed cylindrical member, the interior 
cross-sectional area of the air segment being greater than that of the 
fluid segment; a double-headed piston disposed in the cylindrical member, 
said piston having a first piston head which reciprocates within the air 
segment and sealably engages it, a second piston head which reciprocates 
within the fluid segment and sealably engages it, and means rigidly 
interconnecting said piston heads; said first conduit interconnecting the 
air supply means with the air segment, for supplying pressurized air into 
said segment and exhausting it therefrom; and a second conduit, 
interconnecting the fluid segment with the hydraulic cylinder, for 
supplying pressurized fluid into the cylinder and exhausting it therefrom.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In general, a conventional dockleveller 1 is shown having a dockboard 2 
hinged at its inner end for pivotal movement. A hydraulic cylinder 3 is 
attached to the dockboard 2 and the base frame 4, for raising and lowering 
the dockboard 2. Pressurized air supply means 5 are connected through a 
conduit 6 to one end of a multiplier 7. The other end of the multiplier 7 
is connected through a conduit 8 to the hydraulic cylinder 3. 
As stated, the dockleveller 1 is conventional; therefore it will only be 
described generally. It includes the base frame 4 which is positioned in a 
recessed pit formed in the loading platform P. The dockboard 2 is hinged 
at 9 for movement between cross-traffic and raised positions. A lip 
extension member 10 is hinged to the outer end of the dockboard 2 for 
bridging the gap between the dockboard 2 and the truck bed (not shown). An 
articulated lifting arm 11 pivotally interconnects the dockboard 2 and 
base frame 4. The piston 12 of the hydraulic cylinder 3 is connected to 
the lifting arm 11. Expansion and contraction of the cylinder 3, acting 
through the articulated lifting arm 11, raises and lowers the dockboard 2. 
A tension spring 13, connecting the rear end of the dockboard 2 with the 
lifting arm 11, balances the dockboard 2 in the cross-traffic or 
horizontal position. 
Turning now to the multiplier 7, it comprises an air cylinder segment 14 at 
one end and a fluid cylinder segment 15 at the other end. The interior 
cross-sectional area of the air segment 14 is greater than that of the 
fluid segment 15. The cylinder segments 14, 15 are sealed and connected at 
their inner ends 16, 17 by a junction block 18. The junction block further 
holds the cylinders 14, 15 in concentric alignment. Together, the segments 
14, 15 form a closed cylindrical member. The air segment 14 is connected 
through the conduit 6 with the air compressor or supply means 5. The fluid 
segment 15 is connected with the base end chamber 19 of the hydraulic 
cylinder 3 by the conduit 8. 
Operative within the multiplier 7 is a double-headed piston 20. This piston 
20 has first and second piston heads 21, 22 interconnected by a rigid rod 
23. The first piston head 21 moves within and sealably engages the side 
wall 24 of the air cylinder segment 14. The second piston head 22 moves 
within and sealably engages the side wall 25 of the fluid cylinder segment 
15. 
The pressurized air supply means 5 normally includes a conventional air 
compressor (not shown), equipped with an air strainer (not shown) and 
lubricator (not shown) and a two position N/C (normally closed) air 
control valve 26. The valve 26, in its closed position (FIGS. 1 and 3) 
permits the air cylinder segment 14 to be vented through an exhaust port 
27. In its open position (FIG. 2) the exhaust port 27 is closed, and the 
air compressor can supply air, through the valve 26 and conduit 6, to the 
air cylinder segment 14. 
The air supply means 5, the conduit 6 and the multiplier 7 may be located 
inside a warehouse, while the conduit 8 leads outside to the dockleveller 
1. Alternatively, the multiplier 7 could be mounted below the dockleveller 
1 in the loading platform. 
OPERATION 
The dockboard 2 is shown in FIG. 1 in its cross-traffic position. The air 
control valve 26 is closed, and thus air from the air compressor does not 
reach the multiplier 7. To raise the dockboard 2, the valve 26 is pushed 
open, as shown in FIG. 2, to allow air from the air compressor to flow 
through the valve 26 and conduit 6 to the air cylinder segment 14. The 
pressure of the air acting against the first piston head 21 causes the 
double-headed piston 20 to move, displacing hydraulic fluid from the fluid 
cylinder segment 15, through conduit 8, to the hydraulic cylinder 3 in the 
dockleveller 1. The pressure transferred through the multiplier 7 from the 
air to the fluid is increased by an amount proportional to the ratio of 
the two piston head diameters, multiplier by the air pressure applied, 
neglecting any frictional losses. Generally, a multiplier factor of 1.66, 
obtained from a ratio of piston areas of 5:3, has been found to be 
sufficient. The fluid displaced to the hydraulic cylinder 3 causes the 
latter to expand, pivoting the dockboard 2 upwardly. 
The dockboard 2 is initially raised to a height above the level of vehicle 
bed to be loaded, to enable the lip member 10 to be extended. Thereafter, 
the dockboard 2 needs to be lowered to a loading position (FIG. 3), 
wherein the lip member 10 rests on the vehicle bed. To accomplish this, 
the air control valve 26 is pulled closed, the hydraulic fluid returns to 
the fluid cylinder segment 15, and air from the air cylinder segment 14 is 
vented through the open exhaust port 27. When the vehicle drives away, the 
dockboard 2 falls back to its cross-traffic position. 
Generally, an air compressor, to operate a dockleveller, should be capable 
of producing about 100 psi air pressure. The necessary cubic foot capacity 
of the compressor depends on the demands on the compressor. Each 
dockleveller normally requires about 120 cubic inches of air at 90 psi to 
take it through one lift cycle. 
FEATURES AND ADVANTAGES OF THE PREFERRED EMBODIMENT OF THE INVENTION 
1. The use of one air compressor, especially in those cases in which 
existing air compressor is in place, to operate a plurality of 
docklevellers, can significantly reduce the costs of each dockleveller 
installation over, for instance, the conventional electric-hydraulic 
systems. 
2. The combined air-hydraulic fluid system of the present invention permits 
one to isolate the air filled components inside a warehoue and run only 
hydraulic fluid to the outdoor dockleveller. This overcomes the 
condensation problems which buyers worry about in the straight air 
systems. 
3. The hydraulic fluid is contained in a closed system, moving back and 
forth between the multiplier and the hydraulic cylinder. Therefore the 
fluid does not need to be vented to the atmosphere, as it does in a 
conventional electric-hydraulic system. Venting a hydraulic fluid 
reservoir can cause contamination problems in, for example, a food 
environment. 
4. The conventional electric-hydraulic dockleveller installations, as 
previously mentioned, need an electric pump for each dockleveller. These 
pumps are designed with a high peak power capacity to overcome the initial 
inertia of the pump itself. This surge power is transmitted to the 
dockboard, and is sufficient to lift both the dockboard and any equipment 
which happens to be on or partially on the dockboard. This often results 
in damage to the equipment. With the combined air-hydraulic system, air 
pressure builds up slowly to gradually overcome the initial inertia of the 
dockboard. Further, the air pressure can be set at a level sufficient to 
lift only the dockboard. 
While the present invention has been disclosed in connection with the 
preferred embodiment thereof, it should be understood that there may be 
other embodiments which fall within the spirit and scope of the present 
invention as defined in the following claims.