Closed center hydraulic system for lift trucks

• US 3,956,891 A
• Filed: 12/30/1974
• Issued: 05/18/1976
• Est. Priority Date: 12/30/1974
• Status: Expired due to Term

• Current Assignee: AC Material Handling Corporation
• Original Assignee: Allis Chalmers Corp (Archer Ltd.)
• Inventors: Karazija, Arvin
• Primary Examiner(s): Geoghegan, Edgar W.
• Application Number:
  US05/537,610
• Time in Patent Office: 505 Days
• Field of Search: 60/413, 60/414, 60/418, 60/484, 60/DIG. 2, 60/468, 91/420
• US Class Current: 60/413
• CPC Class Codes: B66F 9/22 Hydraulic devices or systems
• CPC Class Codes: F15B 21/14 Energy-recuperation means
• CPC Class Codes: Y10S 60/911 Fluid motor system incorporating electrical system

Abstract

A closed center hydraulic system is provided for a lift truck which includes an accumulator, a motor driven pump for supplying fluid under pressure to the accumulator, a plurality of closed center proportional electrohydraulic control valves for a plurality of hydraulically operated devices on the truck such as the lift jack, the mast tilt jack and the hydraulic powered steering mechanism. The pressure to the steering control valve is reduced by a pressure reducing valve and the pressure fluid supply circuit from the accumulator to the lift and tilt circuits is controlled by a blocking valve which is opened when the manual control member for either the lift or tilt control valve is moved from neutral to an operating position. Initial movement of either manual control member causes the blocking valve to open and further movement meters flow, thus permitting the operator to regulate lift and tilt speeds. A return line interconnects the lift jack and reservoir in bypassing relation to the lift control valve and a pilot operated bypass valve in the return line has its pilot line connected to one of the lift control valve delivery ports. An energy recovery system is disclosed in the return line from the lift jack.

Claims

1. A hydraulic control system for a lift truck having a reservoir, a source of pressure fluid, and a hydraulic lift jack, said system comprising:

a closed center control valve (51) having

a flow control element (53) shiftable between neutral, raise and lower positions,

a fluid receiving port (57) and

a pair of fluid delivery ports (58,

     59),

a fluid supply line (44) interconnecting said source of pressure fluid and said fluid receiving port,

a fluid delivery line (71) interconnecting one (59) of said delivery ports and said lift jack,

a fluid return line (74) interconnecting said lift jack (11) and said reservoir (18) in bypassing relation to said control valve (51),

a pilot operated bypass valve (76) interconnecting one (59) of said delivery ports and said lift jack,

a pilot line (77) interconnecting the other (58) of said delivery ports and said bypass valve (76),

said lift jack (11) being expanded when said flow control element (53) is shifted to said raise position and

said pilot operated bypass valve (76) being adjusted from its closed condition to its open condition by pressure fluid in said pilot line (77) when said flow control element (53) is shifted to said lower position whereby fluid from said jack (11) flows to said reservoir (18) by way of said return line (74) without passing through said control valve (51).

2. The system of claim 1 and further comprising an energy recovery system including an electric generator driven by a hydraulic motor disposed in said return line.

3. In a closed center hydraulic system for an electrically powered lift truck of the type having a pressure fluid source including an accumulator, a reservoir, a pump, an electric motor driving said pump and pressure sensitive switch means automatically maintaining the fluid pressure between predetermined minimum and maximum values, the combination comprising:

a first hydraulic motor,

a first closed center control valve connected in controlling relation to said first motor,

a first supply line connecting said accumulator with said first control valve,

a second hydraulic motor,

a second closed center hydraulic valve connected in controlling relation to said second motor,

a second supply line connecting said accumulator with said second control valve and

a pressure reducing valve in said second supply line operative to reduce the pressure of fluid delivered to said second valve to a value below said minimum value.

4. The combination set forth in claim 3 wherein said first motor is a lift jack and said second motor is a steering motor.

5. A hydraulic control system for an electric lift truck comprising:

a source of electric energy,

a fluid reservoir,

an accumulator,

a pump connected in fluid receiving relation to said reservoir and fluid delivery relation to said accumulator,

an electric motor drivingly connected to said pump,

electric control means operatively associated with said electric motor and operative to maintain the pressure in said accumulator between predetermined minimum and maximum values,

a lift jack,

a proportional electrohydraulic control valve having a flow control element shiftable from its neutral position to its raise position,

a fluid delivery line interconnecting said accumulator and said control valve,

a supply line interconnecting said control valve and said lift jack,

a return-to-reservoir line interconnecting said control valve and said reservoir,

an electrically actuated blocking valve operatively interposed in said delivery line and having open and closed positions,

electric valve control means including valve control circuits between said source of electric energy and said control and blocking valves, respectively, and,

switch means operatively associated with said circuits and having open and closed operating positions, said circuits being electrically energized in said closed position of said switch means thereby effecting movement of said flow control element of said control valve to its raise position and shifting of said blocking valve from its closed position to its open position.

6. The control system of claim 5 and further comprising a hydraulic steering motor, a fluid transmitting line interconnecting said accumulator and said steering motor and a pressure reducing valve operatively interposed in said fluid transmitting line operative to maintain the pressure of fluid delivered to said steering motor at a predetermined value below said minimum value.

7. The control system of claim 5 wherein said flow control element of said control valve has neutral, raise and lower positions and a pair of fluid delivery ports, one of which is connected to said supply line, and further comprising a return line interconnecting said lift jack and said reservoir, a pilot operated bypass valve in said return line having open and closed positions of adjustment and a pilot line interconnecting said bypass valve and the other of said fluid delivery ports.

8. The system of claim 7 wherein said source of electric energy is a battery and further comprising energy recovery means including an electric generator and a hydraulic motor in said return line drivingly connected to said generator.

9. The system of claim 7 and further comprising a hydraulic steering motor, a fluid transmitting line interconnecting said accumulator and said steering motor and a pressure reducing valve operatively interposed in said fluid transmitting line operative to maintain the pressure of fluid delivered to said steering motor at a value below said minimum value.