Hydraulic arrangement for a lifting arm pivotably mounted on a vehicle
A hydraulic arrangement is described comprising a lifting cylinder 10 for a lifting arm 12 pivotably mounted on a vehicle. A hose 22 selectively connects a working chamber 10 a of the lifting cylinder 10 to a supply 16 and a return 18 line to raise and lower the lifting arm, respectively. A safety check valve 46 is connected between the hose 22 and the working chamber 10 a to maintain the working chamber under pressure and avoid collapse of the lifting arm 12 in the event of rupturing of the hose 22 . An accumulator 70 is connectable to the working chamber 10 of the lifting cylinder 10 by way of the safety check valve 46 to provide ride control. A solenoid 50 overrides the operation of the safety check valve 46 while the hose 22 is isolated from both the supply 16 and the return 18 line to allow fluid to flow in both directions between the accumulator 70 and the working chamber 10 a to cause the lifting cylinder to act as a spring supporting the weight of the lifting arm. A pressure switch 82 prevents overriding of the operation of the safety check valve 46 if the pressure in the hose 22 should drop below a threshold.
1. A hydraulic arrangement comprising: double acting lifting cylinder, having first and second working chambers and piston faces of different sizes, for a lifting arm pivotably mounted on a vehicle, a first hose for selectively connecting the first working chamber of the lifting cylinder to a supply line and a return line to raise and lower the lifting arm, a first safety check valve connected between the first hose and the first working chamber, a second hose for selectively connecting the second working chamber to the supply line and the return line, a second safety check valve connected between the second hose and the second working chamber, an accumulator connected to the first working chamber of the lifting cylinder by way of the first safety check valve, and a respective electrically actuated solenoid for overriding the operation of each of the safety check valves while either hose is isolated from both the supply and the return line and allowing fluid to flow in both directions between the accumulator and the working chamber to cause the lifting cylinder to act as a spring supporting the weight of the lifting arm, wherein a pressure sensitive switch, arranged to inhibit the supply of current to the respective of each overridable safety check valve monitors the pressure in the connection between the accumulator and the lifting cylinder and prevents the overriding of the operation of the safety check valve when the monitored pressured drops below a threshold.
2. A hydraulic arrangement as claimed in claim 1 , wherein the two safety check valves are interconnected by pilot lines such that when one of the hoses is connected to the pressurized supply line, the check valve connected to the other hose is opened to permit fluid to flow from the other working chamber to the return line.
3. A hydraulic arrangement as claimed in claim 1 , wherein an electrically operated accumulator valve is provided to connect the accumulator to a working chamber of the lifting cylinder, which accumulator valve is connected in the same electrical circuit as the solenoid of each safety check valve whereby the operation of each safety check valve can only be overridden when the accumulator is connected to a working chamber.
4. A hydraulic arrangement as claimed in claim 3 , wherein an UP/HOLD/DOWN valve is provided to control the connection of the lifting cylinder to the supply and return lines, and wherein a safety interlock is provided to inhibit operation of the accumulator valve when the cylinder is connected to either one of the supply and return lines.
5. A hydraulic arrangement as claimed in claim 4 , wherein the UP/HOLD/DOWN valve is a hydraulically operated spool valve having control chambers connected by control lines to a joystick and wherein the interlock comprises a pressure sensitive switch connected to the control lines.