Lifting equipment having telescopic boom with automatic extension limiting

Claim 1

In a mobile lifting equipment, a wheeled vehicle having a frame, a boom structure mounted on the frame for rotation about a vertical axis, raising and lowering about a horizontal axis and extension and retraction with respect to the vehicle, said boom structure including an outer boom and inner boom mounted for telescoping movement in the outer boom, first and second extension cylinder assemblies carried by the outer boom for extending and retracting the inner boom with respect to the outer boom, an elevation cylinder assembly connected to the boom structure, each of the cylinder assemblies including a cylinder, a piston slidably mounted in the cylinder and a piston rod connected to the piston, each of the cylinders having openings at opposite ends thereof on opposite sides of the piston in the cylinder, means connecting one of the openings of the elevation cylinder to one of the openings of one of the first extension cylinder so that hydraulic fluid can be transferred between the elevation cylinder and the first extension cylinder, a drain tank, sump means connected to the drain tank for supplying hydraulic fluid to the other of the openings of the elevation cylinder for causing movement of the piston in the elevation cylinder, means connecting the other of the openings of said first extension cylinder to the drain tank so that as the boom structure is elevated, the inner boom section is moved relative to the outer boom section so that the extension of said first extension cylinder assembly is controlled directly by the amount of hydraulic fluid supplied to the elevation cylinder assembly, means connecting the pump means to one of the openings of the second extension cylinder and means connecting the other opening of the second extension assembly to the drain tank so that the inner boom structure can be extended and retracted independent of the action of the elevation cylinder assembly.