Multi-capacity cylinder

1. A multi-capacity hydraulic system, comprising

a hollow cylinder closed by a cylinder head;

an annular piston moveable within the hollow cylinder, the piston dividing the interior of hollow cylinder into a first chamber and a second chamber;

a hollow piston rod coupled to the piston, the piston rod aligned with an opening in the piston; and

a secondary rod moveable within the hollow piston rod, the secondary rod defining a third chamber in the hollow piston rod;

wherein the multi-capacity hydraulic cylinder is coupled to a fluid reservoir with a hydraulic circuit comprising a pressure sensitive switch, the switch movable between a first position directing a hydraulic fluid to the third chamber and second position directing fluid to the first chamber; and

wherein the third chamber has a cross-sectional area that is less than the cross-sectional area of the first chamber.

2. The multi-capacity hydraulic system of claim 1 , further comprising a first mounting lug coupled to the piston rod and a second mounting lug coupled to the cylinder head.

3. The multi-capacity hydraulic system of claim 2 , wherein the multi-capacity cylinder may be extended by pressurizing one of the first chamber, the third chamber, or a combination of the first chamber and the third chamber.

4. The multi-capacity hydraulic system of claim 3 , wherein the multi-capacity cylinder extends at a first rate when the first chamber is pressurized and a second rate when the third chamber is pressurized, the first rate being less than the second rate.

5. The multi-capacity hydraulic system of claim 3 , wherein the multi-capacity cylinder extends with a first force when the first chamber is pressurized and with a second force when the third chamber is pressurized, the first force being greater than the second force.

6. The multi-capacity hydraulic system of claim 5 , wherein the multi-capacity cylinder extends with a third force when the first chamber and the third chamber are pressurized, the third force being greater than the first force and the second force.

7. The multi-capacity hydraulic system of claim 2 , further comprising a coaxial tertiary rod coupled to the piston rod, wherein the secondary rod is a hollow member and tertiary rod is moveable within the secondary rod.

8. The multi-capacity hydraulic system of claim 7 , wherein the tertiary rod defines a fourth chamber in the secondary rod, the fourth chamber having a cross-sectional area less than the cross-sectional area of the second chamber.

9. The multi-capacity hydraulic system of claim 8 , wherein the multi-capacity cylinder may be extended by pressurizing one of the first chamber; the third chamber; the fourth chamber; the first chamber and the third chamber; the first chamber and the fourth chamber; the third chamber and the fourth chamber; or the first chamber, the third chamber, and the fourth chamber.

10. A hydraulic system, comprising:

a multi-capacity cylinder comprising:

a hollow cylinder closed by a cylinder head;

an annular piston moveable within the hollow cylinder, the piston dividing the interior of hollow cylinder into a first chamber and a second chamber;

a hollow piston rod coupled to the piston, the piston rod aligned with an opening in the piston; and

a secondary rod moveable within the hollow piston rod, the secondary rod defining a third chamber in the hollow piston rod;

a fluid reservoir;

a hydraulic pump; and

a hydraulic circuit coupling the multi-capacity cylinder to the fluid reservoir and the hydraulic pump; and wherein the hydraulic circuit comprises a pressure sensitive switch, the switch movable between a first position directing the fluid to the third chamber and second position directing fluid to the first chamber.

11. The hydraulic system of claim 10 , wherein the hydraulic circuit directs fluid to the third chamber below a first threshold pressure in the hydraulic circuit; the hydraulic circuit directs fluid to the first chamber above the first threshold pressure and below a second threshold pressure in the hydraulic circuit; and the hydraulic circuit directs fluid to both the first chamber and the third chamber above the second threshold pressure in the hydraulic circuit.

12. The hydraulic system of claim 10 , wherein the pressure sensitive switch is actuated by fluid passing through a first pressure release valve above the first threshold pressure.

13. The hydraulic system of claim 10 , wherein fluid passes through a second pressure release valve above the second threshold pressure to direct fluid to both the first chamber and the third chamber.

14. The hydraulic system of claim 10 , wherein the hydraulic system comprises an actuator for the boom of one of a mining shovel or an excavator.