Hybrid hydraulic systems for industrial processes

1. A hydraulic drive system for driving a load, the hydraulic drive system comprising:

a drive shaft;

first and second hydraulic pumps driven by the drive shaft, at least the second hydraulic pump being a variable displacement bidirectional pump, the first hydraulic pump having an outlet in fluid communication with a supply line that connects to the load and an inlet in fluid communication with a reservoir, the second hydraulic pump having a first pump port in fluid communication with the supply line and a second pump port selectively in fluid communication with the reservoir and with a hydraulic fluid accumulator;

a control system that operates the hydraulic drive system in a plurality of modes including: a) a first mode where the second hydraulic pump pumps hydraulic fluid from the supply line to the accumulator; b) a second mode where the second hydraulic pump pumps hydraulic fluid from the accumulator to the supply line; c) a third mode where the second hydraulic pump pumps hydraulic fluid from the supply line to the reservoir; and d) a fourth mode where the second hydraulic pump pumps hydraulic fluid from the reservoir to the supply line.

2. The hydraulic drive system of claim 1 , wherein the control system operates the hydraulic drive system in the first mode when the load is below a predetermined lower threshold level and the accumulator is not fully charged.

3. The hydraulic drive system of claim 2 , wherein the control system operates the hydraulic drive system in the second mode when the load is above a predetermined upper threshold level.

4. The hydraulic drive system of claim 1 , wherein the control system operates the hydraulic drive system in the third mode when the load is below a predetermined lower threshold level and the accumulator is fully charged.

5. The hydraulic drive system of claim 3 , wherein the control system operates the hydraulic drive system in the fourth mode when the load is between the predetermined upper and lower threshold levels.

6. The hydraulic drive system of claim 1 , wherein the drive shaft is rotated by an electric motor.

7. The hydraulic drive system of claim 1 , wherein the first hydraulic pump is a fixed displacement hydraulic pump.

8. The hydraulic drive system of claim 1 , wherein the control system includes a directional valve having a first valve port in fluid communication with the second pump port, a second valve port in fluid communication with the accumulator and a third valve port in fluid communication with the reservoir.

9. The hydraulic drive system of claim 8 , wherein the valve is a three position valve having a first position where the first valve port is connected to the third valve port and the accumulator is isolated form the first valve port, a second position where the second and third valve ports are isolated from the first valve port, and a third position where the first valve port is connected to the second valve port and the third valve port is isolated from the first valve port.

10. A method for operating a hydraulic drive system to drive a load, the hydraulic drive system including first and second hydraulic pumps driven by a common shaft, at least the second pump being a bidirectional pump, the method comprising:

pumping hydraulic fluid from a reservoir to a supply line with both hydraulic pumps when the load is between an upper threshold level and a lower threshold level;

pumping hydraulic fluid from the supply line to an accumulator with the second hydraulic pump when the load is below the lower threshold level; and

pumping hydraulic fluid from the accumulator to the supply line with the second hydraulic pump when the load is above the upper threshold level.

11. The method of claim 10 , further comprising pumping hydraulic fluid from the supply line to the reservoir with the second hydraulic pump when the load is below the lower threshold level and the accumulator is fully charged.

12. The method of claim 10 , wherein the first hydraulic pump is a fixed displacement pump and the second hydraulic pump is a variable displacement pump.

13. A hydraulic drive system for powering a load, the hydraulic drive system comprising:

a pump arrangement for pumping hydraulic fluid to a supply line, the pump arrangement being a variable displacement pump driven by an electric motor;

first and second proportional flow valves positioned in series along the supply line downstream from the variable displacement pump;

a hydraulic fluid accumulator that connects to the supply line at a location between the first and second proportional flow valves; and

a valve for selectively connecting the hydraulic fluid accumulator in fluid communication with the pump arrangement and for isolating the hydraulic fluid accumulator from the pump arrangement.

14. Then hydraulic drive system of claim 13 , further comprising a relief valve that connects to the supply line at a location between the accumulator and the first proportional flow valve.

15. The hydraulic drive system of claim 13 , wherein the pump arrangement includes the fixed displacement pump driven by the variable frequency drive, wherein a proportional flow valve is positioned on the supply line downstream from the fixed displacement pump, and wherein the accumulator connects to the supply line between the proportional flow valve and the fixed displacement pump.

16. The hydraulic drive system of claim 15 , further comprising a relief valve that connects to the supply line at a location between the accumulator and the fixed displacement pump.

17. The hydraulic drive system of claim 13 , wherein the pump arrangement includes a first pump and a second pump, wherein the second pump is the variable displacement pump driven by the electric motor, wherein the variable displacement pump is a bi-directional pump, wherein the first and second pumps are both driven by a common drive shaft, wherein the first and second pumps both have first ports in fluid communication with the supply line, and wherein the second pump has a second port selectively in fluid communication with the hydraulic fluid accumulator.

18. The hydraulic drive system of claim 17 , wherein the first pump has a second port that is always in fluid communication with a reservoir, and wherein the second port of the second pump is selectively in fluid communication with the reservoir.

19. A hydraulic drive system for powering a load, the hydraulic drive system comprising:

a pump arrangement for pumping hydraulic fluid to a supply line, the pump arrangement including a first pump and a second pump that are both driven by a common drive shaft, the first and second pumps both having first ports in fluid communication with the supply line, the second pump being a variable displacement, bi-directional pump driven by an electric motor, the second pump having a second port;

a hydraulic fluid accumulator with which the second port of the second pump is selectively in fluid communication; and

a valve for selectively connecting the hydraulic fluid accumulator in fluid communication with the pump arrangement and for isolating the hydraulic fluid accumulator from the pump arrangement.

20. The hydraulic drive system of claim 19 , wherein the first pump has a second port that is always in fluid communication with a reservoir, and wherein the second port of the second pump is selectively in fluid communication with the reservoir.