WO1995003951A1

WO1995003951A1 - Dual power pack for double acting tailgate lifts

Info

Publication number: WO1995003951A1
Application number: PCT/CA1994/000432
Authority: WO
Inventors: Ralph Edward Gray
Original assignee: Ralph Edward Gray
Priority date: 1993-08-02
Filing date: 1994-08-02
Publication date: 1995-02-09
Also published as: GB9315980D0; AU7380694A

Abstract

A double acting hydraulic lift system has two power packs that form part of a hydraulic circuit. The circuit is formed so that one power pack raises a platform and the other power pack lowers the platform. If one power pack stops working, the remaining power pack can be used to raise or lower the platform by moving a directional control valve between a first and second position. Previous double acting lift systems have only one power pack or a second power pack that sits idle until a first power pack fails.

Description

DUAL POWER PACK FOR DOUBLE ACTING TAILGATE LIFTS BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

This invention relates to a hydraulic lift system for truck trailers and the like and to a method of use thereof. More specifically, this invention relates to a double acting hydraulic lift system having a double power pack and to a method of use thereof. DESCRIPTION OF THE PRIOR ART Previous hydraulic lift systems for use with truck trailers and the like have one or two double acting hydraulic cylinders. A two cylinder system is described in the Gray U.S. Patent Number 5,110,251 issued on May 5th, 1992. That system has only a single power pack. Previous lift systems having a dual power pack are known but these systems are designed so that the system operates by utilizing only one power pack. Previous lift systems having a double power pack contain relatively elaborate, expensive and complex manual switch-over procedures when switching from one power pack to the other. With these known dual power pack systems, it is sometimes recommended that use of the two motors be alternated on a monthly basis. In actual practice, one motor is often used until it fails and then the system is switched over to use the other motor. Since the second motor has not been used while the first motor was operable, the second motor can be in a state of disrepair when it is desired to be used for the first time. Further, the second motor is often of inferior quality when compared to the first motor as its usage is intended to be on an emergency basis only.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a lift system and a method of operating said system where the system has a dual power pack where, in normal operation, one power pack lifts the platform and the other power pack lowers the platform.

A hydraulic lift system for use with a truck trailer and the like has a cylinder portion containing at least one cylinder. Each cylinder has a piston therein and one rod extending from said piston. A platform is connected to each rod. The cylinder portion is hydraulically connected to lines of a hydraulic circuit so that each cylinder in said cylinder portion is double acting to raise and lower said platform of said lift. The system is characterized by a first pump and a second pump for hydraulic fluid, each pump being connected to a separate power source. The circuit is formed so that said first pump can be activated to cause hydraulic fluid to flow through said cylinder portion in one direction to raise said platform and said second pump can be activated to cause hydraulic fluid to flow through said cylinder portion in an opposite direction to lower said platform. The cylinder portion has an entrance and an exit for hydraulic fluid, a location of said entrance and exit being dependent on whether said platform is being raised or lowered. The exit when said platform is lowered has a flow limiter to limit flow from said cylinder portion through said exit. Preferably, the cylinder portion has two cylinders.

A method of operating a hydraulic lift system for use with a truck trailer and the like where said system has two pumps and each pump has a separate power source, said pumps being connected into a hydraulic circuit and said system having a platform said method comprising operating said first pump and said first power source to raise said platform and operating said second pump and said second power source to lower said platform without switching said system. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, which describe a preferred embodiment of the invention:

Figure 1 is a circuit diagram of one embodiment of a dual power pack in accordance with the present invention;

Figure 2 is a prior art circuit diagram showing two hydraulic double acting cylinders that are connected in series; and Figure 3 is a circuit diagram showing a variation in the embodiment of Figure 1.

DESCRIPTION OF A PREFERRED EMBODIMENT As shown in Figure 1, two identical motors 2, 4 are preferably utilized and connected identically to a two-way electrical switch 6. The motors 2, 4 preferably have identical pumps 8, 10 with identical pressure relief valves 12, 14 and identical starter solenoids 15. The pumps 8, 10 are connected into a double acting hydraulic system (see Figure 2) through a manual directional control valve 16 so that when the valve 16 is in a first position, the pump 8 will always move the lift downward and the pump 10 will always move the lift upward. Located between each pump 8, 10 and the valve 16 are check valves 18, 20 respectively. These check valves prevent hydraulic fluid from flowing into the respective pumps after the fluid has passed through the check valve and are often built into the pumps.

Each pump 8, 10 has an output line 22, 24 which extends through the check valves 18, 20 respectively and through the control valve 16 to a prior art double acting hydraulic lift system as shown in Figure 2. On a side of a valve 16 opposite to the pumps, there is located in each of the lines 22, 24 a pilot operated check valve 26, 28 respectively. The check valve 26 in the line 22 has a pressure line 30 extending to the line 24. Similarly, the check valve 28 has a pressure line 32 extending to the line 22. Since the line 22 is the line that causes the lift to move downward, a downward force limiting valve 34 is located between the check valve 26 and the hydraulic lift system of Figure 2 in the line 22. The valve 34 has a pressure line 36 connected to the line 22 and is set so that hydraulic fluid flowing through the line 22 into the hydraulic system will not exceed a pre¬ determined maximum pressure. The valve 34 protects the cylinder rods against buckling by limiting the hydraulic pressure during the push cycle (i.e. when the platform is being lowered). If the pressure into the hydraulic lift system gets too high, the cylinder rod or rods might buckle. The hydraulic system has one reservoir 38 which is shown in the figure as being located in several locations for ease of illustration. A filter 41 is located between each of the pumps 8, 10 and the reservoir 38.

It can be seen that the prior art hydraulic system of Figure 2 fits onto the hydraulic circuit of Figure 1 to complete the circuit. While the circuit shown in Figure 2 defines a cylinder portion with two cylinders that are connected in series to be double acting, other cylinder portions could be utilized as well. For example, the cylinder portion could have a single hydraulic cylinder where hydraulic fluid is caused to flow in one direction into the cylinder to lift the platform and in an opposite direction into the cylinder to lower the platform. Further, double acting cylinders could be connected in parallel rather than in series. Other variations will become readily apparent to those skilled in the art. Each cylinder 40, 42 shown in Figure 2 has a rod end 44 and a cap end 46 together with a piston 48. The cylinders 40, 42 are oriented so that their rod ends 44 extend downward.

Rods 50 extend from each of the pistons 48 and each rod 50 has a free end 52. A platform 54, only partially shown in Figure 2, extends between the two free ends 52. The first cylinder 40 has a larger cross-sectional area than the second cylinder 42. The cross-sectional area of the first cylinder 40 at the rod end 44 minus the cross-sectional area of the rod 50 is equal to or at least substantially equal to a cross-sectional area of the second cylinder 42 at the cap end 46.

The cylinders 40, 42 are interconnected by a connecting line 56 so that hydraulic fluid can flow between the rod end 44 of the cylinder 40 and the cap end 46 of the cylinder 42. A velocity fuse 58 is located in the connecting line 56 to control the flow rate of the hydraulic fluid through the connecting line. Also, if the connecting line 56 is severed or develops a leak, the flow of the hydraulic fluid through the velocity fuse will automatically be shut off. The velocity fuse 58 is not a necessary component of the platform lift and could be eliminated from the hydraulic circuit entirely. A flow limiter 60, which in this case is a counterbalance valve is connected into the line 24. During downward movement of the platform 54, the flow limiter 60 is located at an exit from the cylinder portion and limits the flow of hydraulic fluid from the rod end 44 of the cylinder 42. The purpose of the limiter 60 is to prevent any cavitation from occurring in the hydraulic system and to prevent the platform from coming down too fast (i.e. faster than the pump can pump fluid into the entrance of the cylinder portion) during normal operation or from crashing down if a line breaks. The limiter 60 or counterbalance valve has two components, a check valve 62 and a two-way valve 64. When fluid is flowing into the rod end 44 of the cylinder 42, fluid flows through the check valve 62. When fluid is flowing out of the rod end 44 of the cylinder 42, the fluid flows through the two-way valve 64. The valve 64 modulates, if necessary, between an open and closed position or a partially open and partially closed position to control the flow of fluid. In Figure 3, the check valve 28 is replaced with a counterbalance valve 66 having a check valve 68 and a two-way valve 70. Another check valve 72 is located in the line 24 immediately following the valve 16 to prevent fluid from flowing into the valve 16 during downward movement of the platform 54. The counterbalance valve 66 is simply a relatively inexpensive replacement for the check valve 28 of Figure 1. The check valve 28 has a 4 to 1 ratio whereas the counterbalance valve 66 has a 10 to 1 ratio.

The present system has several advantages over the dual power packs of previous systems. Firstly, one motor is used for moving the lift upward and the other motor is used. for moving the lift downward. Therefore, the motors are each used regularly and generally are used by about the same amount. Secondly, because of this regular usage, when one of the motors fails, that failure will be noticed almost immediately. It is highly unlikely that both motors will fail on the same day. Thirdly, the switch to use one motor or the other is a simple two-way switch and there are no other switches that must be tripped. Fourthly, if one motor fails, the manual directional control valve can be moved to the second position to move the lift during that part of the movement when the failed motor would ordinarily be used. For example, if the motor 2 were to fail, the control valve 16 would be left in the first position for upward movement of the lift by motor 4 and moved to the second position (not shown) for downward movement of the lift by the motor 4. Preferably, the motors 2, 4 are identical, the pumps 8, 10 are similar or identical and have a common or linked reservoir and the system relief valves 12, 14 are set to the same pressure. The downward force limiting valve sets a maximum pressure for fluid flowing from the exit of the cylinder portion during downward movement of the platform regardless of which motor is causing hydraulic fluid to flow. Self- levelling means on the hydraulic circuit shown in Figure 2 are preferred but have been omitted as self- levelling means are known. One example of self- levelling means is described in U.S. Patent #5,110,251.

Claims

I CLAIM:

1. A hydraulic lift system for use with a truck trailer and the like, said system having a cylinder portion containing at least one cylinder, each cylinder having a piston therein and one rod extending from said piston, a platform being connected to each rod, said cylinder portion being hydraulically connected to lines of .a hydraulic circuit so that each cylinder in said cylinder portion is double acting to raise and lower said platform of said lift, said system being characterized by a first pump and a second pump for hydraulic fluid, each pump connected to a separate power source, said circuit being formed so that said first pump can be activated to cause hydraulic fluid to flow said cylinder portion in one direction to raise said platform and said second pump can be activated to cause hydraulic fluid to flow through said cylinder portion in an opposite direction to lower said platform, said cylinder portion having an entrance and an exit for hydraulic fluid, a location of said entrance and exit being dependent on whether said platform is being raised or lowered, said exit when said platform is being lowered having a flow limiter to limit flow from said cylinder portion through said exit.

2. A system as claimed in Claim 1 wherein there are two cylinders in said cylinder portion.

3. A system as claimed in Claim 2 wherein said cylinders are connected in series with an interconnecting line extending between a rod end of a first cylinder and a cap end of a second cylinder, said exit being located at a rod end of said second cylinder when said platform is being lowered.

4. A system as claimed in Claim 3 wherein there is a control valve located between said pumps and said cylinders, said valve having a first position wherein said first pump moves the platform upward and said second pump moves the platform downward and a second position wherein said first pump moves the platform downward and said second pump moves the platform upward.

5. A system as claimed in Claim 1 wherein a first output line from said first pump contains a check valve to prevent hydraulic fluid from flowing into said first pump and a second output line, said second pump contains a check valve to prevent hydraulic fluid from flowing into said second pump.

6. A system as claimed in Claim 1 wherein said flow limiter is a counterbalance valve.

7. A hydraulic lift as claimed in Claim 1 wherein said cylinders have self-levellers located therein.

8. A method of operating a hydraulic lift system for use with a truck trailer and the like where said system has two pumps and each pump has a separate power source, said pumps being connected into a hydraulic circuit and said system having a platform, said method comprising operating said first pump and said first power source to raise said platform and operating said second pump and said second power source to lower said platform.

9. A method as claimed in Claim 8 wherein there are control means having two positions, said method including the steps of, in a first position, raising said platform using said first pump, switching said control means to a second position, lowering said platform using said first pump.

10. A method as claimed in Claim 8 wherein there are control means having two positions, said method including the steps of, in a first position of said control means, using said second pump to lower said platform, switching said control means to said second position and using said second pump to lower said platform.