Abstract: A tank for oil or other hydraulic fluid is kept under a few pounds of air pressure during operation to keep air from being drawn into the fluid and to aid its flow to the pump. The pressure is provided dependably and quickly by a small air pump driven whenever the hydraulic pump is driven. A bleed-off discharge causes prompt dissipation of the pressure on shutdown so as not to cause oil leakage while the associated hydraulic pump is idle. The hydraulic level is below the pump shaft level so there will be no leakage by gravity.

Abstract: Greater overall speed of crane operation is made possible by using a plurality of hydraulic piston-cylinder sets together with valving which can direct the pumped hydraulic fluid selectively to either of the cylinder sets or to both of them. Preferably the two cylinder sets are of different speed-load characteristics so that the same supply of pressured oil will raise the boom faster when supplied to one than when supplied to the other. When the boom must be raised with maximum load, the same oil supply will be directed to both cylinder sets, thereby giving the maximum lifting power with a given available oil pressure and the slowest raising speed. The overall time required to accomplish a given task is reduced because most of the time the boom can be raised or lowered at a faster speed than that slow speed which must be provided for handling heaviest loads. Speeds are selected by operating one or both of two handles to the up or down position, each handle controlling its own valve spool.

Abstract: The invention may be used with a boom or the like which is raised and lowered by double-acting hydraulic cylinder means provided with a pilot-actuated safety valve at its bottom such that the boom can only go down when pilot pressure (normally from the downthrust line) is applied to this safety valve. The main manual valve has a four-position spool which provides a float position reached by moving through the lowering position. As the main valve is moved to the float position, its manual actuation mechanism actuates an auxiliary valve to supply pilot pressure to the safety valve to open the safety valve to allow the cylinder to achieve the desired floating action. This pilot pressure supply is through a shuttle valve which alternatively lets pilot pressure be applied to this safety valve as here stated or, from the downthrust, without ever connecting the two sources.

Abstract: The invention is especially suitable when front and rear wheels are hydraulically steered, each set by its own hydraulic cylinder, the two cylinders being connected in hydraulic tandem so that the same angularity of steering will be imparted to both sets of wheels. The self-synchronizing of the present invention overcomes a serious prior fault that occasionally a nonsynchronous relationship would develop. The present invention opens a synchronizing valve on the occasion of each extreme swing of the wheels. A check valve then ensures a synchronized return swing.

Abstract: A tank for oil or other hydraulic fluid is improved both for cooling the fluid during normal operation and for heating it up preliminarily at start-up time by an internal curtain spaced a narrow distance from the side walls and providing a cooling channel through which, during operation, the oil may be passed in immediate contact with the outer side walls of the tank to be cooled before flowing into the main storage chamber, a flap valve being turned at start-up time to a position to direct returning oil directly into the main storage chamber where it is relatively insulated from the outer side walls by the stagnant oil in the cooling channel, so that the heat developed by power-circulation of the oil is conserved and in cold weather the oil is more quickly brought up to satisfactory operating temperature. The peripheral cooling channel normally used also settles foreign particles from the hydraulic fluid, and dissipates any entrained air at its surface.