Closed loop hydraulic system with variable charge pressure

Abstract

A closed loop hydraulic system having a variable charge pressure capability without parasitic power losses is disclosed. The present system includes a variable displacement hydraulic pump driven by a power source, a tilt control means for controlling a tilt angle of the variable displacement hydraulic pump, and a hydraulic motor having a pair of inlet/outlet ports connected in a closed circuit to a pair of inlet/outlet ports of the variable displacement hydraulic pump by way of a pair of main conduits. The system has a charging system including a charge pump driven by a power source, the charge pump including an inlet connected to a supply of oil, and an outlet port connected to the closed circuit for supplying pressurized oil to the variable displacement hydraulic pump. The charge system further includes an electrohydraulic proportional relief valve including means for varying an operating pressure setting thereof, which electrohydraulic proportional relief valve is connected to the charging system for controlling the pressure level of the pressurized oil supplied to the variable displacement hydraulic pump. The means for varying the operating pressure setting of the proportional relief valve are controlled by control means responsive to at least a pressure condition in the closed circuit. According to another aspect of the invention, the charging system is also connected to a hydraulic circuit of the tilt control means for supplying pressurized oil thereto.

What is claimed is:  
     
       1. A hydraulic system, comprising: 
       a variable displacement hydraulic pump driven by a power source, a tilt control means for controlling a tilt angle of the variable displacement hydraulic pump;  
       a hydraulic motor having a pair of inlet/outlet ports connected in a closed circuit to a pair of inlet/outlet ports of the variable displacement hydraulic pump by way of a pair of main conduits;  
       a charging system including a charge pump driven by a power source, the charge pump including an inlet port connected to a supply of oil, and an outlet port connected to the closed circuit for supplying pressurized oil to the variable displacement hydraulic pump;  
       an electrohydraulic proportional relief valve including means for varying an operating pressure setting thereof and being connected to the charging system for controlling the pressure level of the pressurized oil supplied to the variable displacement hydraulic pump; and  
       control means for controlling the means for varying the operating pressure setting of the electrohydraulic proportional relief valve responsive to at least a pressure condition in the closed circuit.  
     
     
       2. The hydraulic system, as set forth in claim  1 , wherein the operating pressure setting of the electrohydraulic proportional relief valve is variable between a high pressure setting and a low pressure setting. 
     
     
       3. The hydraulic system, as set forth in claim  2 , wherein the control means are operable for controlling the means for varying the operating pressure setting of the electrohydraulic proportional relief valve to maintain the high pressure setting responsive to a high pressure condition in the closed circuit and a low pressure setting responsive to a pressure condition representative of a null condition of the variable displacement hydraulic pump. 
     
     
       4. The hydraulic system, as set forth in claim  1 , wherein the tilt control means comprise a hydraulic circuit, and wherein the outlet port of the charge pump is connected to said hydraulic circuit for supplying pressurized oil to the hydraulic circuit. 
     
     
       5. The hydraulic system, as set forth in claim  1 , wherein the control means are responsive to the pressure condition in the closed circuit and a speed characteristic of the variable displacement hydraulic pump. 
     
     
       6. The hydraulic system, as set forth in claim  1 , wherein the control means comprise a microprocessor operatively connected to a pressure sensor for sensing the pressure condition in the closed circuit. 
     
     
       7. The hydraulic system, as set forth in claim  6 , wherein the pressure condition is a pump discharge pressure. 
     
     
       8. The hydraulic system, as set forth in claim  7 , wherein the control means for controlling the means for varying the operating pressure setting of the proportional relief valve are operable responsive to the pump discharge pressure, a tilt characteristic of said variable displacement hydraulic pump, a speed of the pump, and an operator input.