Synchronized hybrid clamp force controller for lift truck attachment

1. A hydraulic control circuit comprising:

an input port configured to receive pressurized fluid from a pump and return unpressurized fluid to a reservoir

a first output port connectable to a first hydraulic actuator and a second output port connectable to second hydraulic actuator, each of the first output port and the second output port configured to simultaneously deliver pressurized fluid to its respectively connected hydraulic actuator and receiving fluid discharged therefrom; and

a selector capable of selectively using fluid discharged from the first hydraulic actuator to pressurize fluid delivered into the second hydraulic actuator; where

the control circuit is configured so that, during simultaneous operation of the first hydraulic actuator and operation of the second hydraulic actuator, when fluid is not discharged from the first hydraulic actuator to pressurize fluid delivered into the second hydraulic actuator, the first hydraulic actuator and the second hydraulic actuator expand and retract synchronously even when fluid supplied into the first hydraulic actuator is at a different pressure than fluid supplied into the second actuator.

2. The hydraulic control circuit of claim 1 where the selector selectively uses fluid discharged from the first hydraulic actuator to pressurize fluid delivered into the second hydraulic actuator automatically based upon the magnitude of fluid pressure provided to the hydraulic control circuit.

3. The hydraulic control circuit of claim 1 where the selector selectively uses fluid discharged from the first hydraulic actuator to pressurize fluid delivered into the second hydraulic actuator automatically based upon which connection of an input port receives pressurized fluid.

4. The hydraulic control circuit of claim 1 connected to a lift truck attachment having clamps, where the selector selectively uses fluid discharged from the first hydraulic actuator to pressurize fluid delivered into the second hydraulic actuator automatically when the clamps engage a load.

5. The hydraulic control circuit of claim 1 where the selector selectively uses fluid discharged from the first hydraulic actuator to pressurize fluid delivered into the second hydraulic actuator automatically during an opening movement of the hydraulic actuators.

6. The hydraulic control circuit of claim 1 where the selector selectively uses fluid discharged from the first hydraulic actuator to pressurize fluid delivered into the second hydraulic actuator automatically during a portion of a closing movement of the hydraulic actuators and during an opening movement of the hydraulic actuators.

7. The hydraulic control circuit of claim 1 where the selector alternates the control circuit between a first mode where the hydraulic actuators are linked in series and a second mode where the hydraulic actuators are not linked in series.

8. The hydraulic control circuit of claim 7 where in the second mode, the hydraulic actuators are driven in parallel.

9. The hydraulic control circuit of claim 8 including a second selector that controls the flow from a flow divider.

10. The hydraulic control circuit of claim 7 where in the second mode, one hydraulic actuator is moved by the control circuit while the other hydraulic actuator is prevented from moving by the control circuit.

11. A hydraulic control circuit configured to receive pressurized fluid from a pump and return unpressurized fluid to a reservoir, the hydraulic control circuit comprising a selector alternately operable between a first state and a second state when the control circuit is connected to a first hydraulic actuator and a second hydraulic actuator, wherein in the first state the first hydraulic actuator and the second hydraulic actuator are hydraulically linked and in the second state the first hydraulic actuator and the second hydraulic actuator are not hydraulically linked, wherein the control circuit is configured so that, during simultaneous operation of the first hydraulic actuator and operation of the second hydraulic actuator, when fluid is not discharged from the first hydraulic actuator to pressurize fluid delivered into the second hydraulic actuator, the first hydraulic actuator and the second hydraulic actuator expand and retract synchronously even when fluid supplied into the first hydraulic actuator is at a different pressure than fluid supplied into the second actuator.

12. The hydraulic control circuit of claim 11 where the selector automatically alternates between the first state and the second state based upon the magnitude of fluid pressure provided to the hydraulic control circuit.

13. The hydraulic control circuit of claim 11 where the selector automatically alternates between the first state and the second state based upon which connection of an input port receives pressurized fluid.

14. The hydraulic control circuit of claim 11 including a resynchronizing valve operable to resynchronize the first hydraulic cylinder with the second hydraulic cylinder.

15. The hydraulic control circuit of claim 11 included in a lift truck attachment having the first hydraulic actuator and the second hydraulic actuator.

16. The hydraulic control circuit of claim 15 where at least one of the first hydraulic actuator and the second hydraulic actuator operates as a valve that selectively permits fluid to pass between a rod side and a head side of the respective hydraulic actuator.

17. The hydraulic control circuit of claim 16 where at least one said valve is a resynchronizing valve configured to resynchronize the first hydraulic actuator with the second hydraulic actuator.

18. The hydraulic control circuit of claim 17 where the at least one said valve permits fluid to pass between a rod side and a head side of the respective hydraulic actuator at an end-of-stroke of a closing movement.

19. The hydraulic control circuit of claim 18 where the at least one said valve permits fluid to pass between a rod side and a head side of the respective hydraulic actuator at an end-of-stroke of an opening movement.

20. The hydraulic control circuit of claim 16 where at least one said valve permits fluid to pass between a rod side and a head side of the respective hydraulic actuator at an end-of-stroke of a closing movement.