Patent ID: 8777545

Claim:
A multi-lift mast assembly, comprising: a vertical mast having a plurality of mast members of which one is stationary and at least one other is extensible, a pair of double acting hydraulic mast lift actuators having a rod end and an opposite bore end, each actuator extending between and connecting the stationary mast member and a slideably extensible mast member for slideably moving that extensible mast member relative to the stationary mast member, a carriage slideably mounted on an extensible mast member, a double acting positively connected carriage hydraulic actuator having a rod end an opposite bore end, such actuator being positively connected to the extensible mast member mounting the carriage and positively connected to the carriage, for positively slidingly lifting and lowering the carriage on the extensible mast member mounting the carriage, and a hydraulic circuit for the actuators comprising first and second hydraulic pressure supply lines for the mast lift and carriage actuators and a pair of pilot operated normally closed valves for each actuator, one valve of the pair fluidly communicating with an orifice at one end of an actuator and the first supply line, and the other valve of the pair fluidly communicating with an orifice at the other end of the same actuator and the second supply line, each valve having an operator piloted by a supply line pressure and operative at a predetermined pressure to release fluid from the orifice with which it communicates, the valve fluidly communicating with the orifice at the one end of the actuator having an operator piloted by pressure in the second supply line to the other end of the same actuator, and the valve fluidly communicating with an orifice at the other end of same actuator having an operator piloted by pressure in the first supply line to the one end of the same actuator, the operator of the valve for the one end of the positively connected carriage actuator operating at greater pressure than the operator of the valve for the other end of the carriage actuator, and the operator of the valve for the one end of each of the mast lift actuators operating at lesser pressure than the operator of the valve for the other end of the same mast lift actuator.