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Hydraulics has lowered the work intensity of man more than any other single factor in the last three decades. This fact is especially true in agriculture.

Typically, the 1950s was the decade of hydraulics with open-center systems. These systems contained a fixed-displacement pump; a four-way, three-position valve; and an actuator.

Oil simply is pumped through the valve by the pump and when the valve is moved, the oil flow is interrupted and diverted to the actuator.

The optimum hydraulic system would operate at 100% efficiency. In the past decade, a system has come forth that offers more system design versatility, plus an improvement in efficiency, as compared with earlier systems. This system will be termed a pressure and flow-compensated load-sensing system, for the purposes of this paper.

The system is closed center with many distinct features over a typical high pressure closed-center system, or a closed-center valve operating with a fixed-displacement pump with an unloading device.

The pressure flow load-sensing system does just what the name implies. It senses the pressure required to operate the system and sends a signal from the control valve back to the pump. This signal causes the pump to maintain only the required system pressure to do the job plus whatever pressure is required to put flow through the system at a desired pressure drop. The pressure flow load-sensing system allows the valve to limit the amount of flow based ont he desired pressure drop. This gives the system capability of variable flow with no increase or decrease in pressure drops.

Another advantage of the pressure and flow load-sensing system is that, when there is no demand for flow and there is no pressure feedback from the system, the pump goes to low pressure standby operation. In this standby situation, the pump produc es only enough flow to make up for internal losses and maintain a low pressure int he pressure line. The pump remains in this condition as long as there is no demand from the system. The pump absorbs little power in standby, thus producing little heat.

When the valving is activated and the pump is required to provide more flow tot he system, the pump compensator senses the slight pressure imbalance. This causes the pump to increase its volume until pressure balance is achieved. The volume of oil delivered per unit of time is controlled by the size of an orifice in the pressure line. In a typical system, the orifice can be the land opening in a spool valve, a flow control valve, or simply passage size. Whatever method is used, it is sized to provide the desired flow at the desired pressure drop. The pump compensator spring is set for the desired pressure drop and the system pressure is fed back to the compensator through a sensor line. The pump produces the volume of oil per unit of time dictated by the size of the orifice at the desired pressure above the actual working pressure. This creates a very efficient hydraulic system because the pump provides only the flow required at a desired setting above actual system working pressure. Thus, the pump automatically adjusts to the varying pressure and flow demands of a system. Should a pressure and flow compensator load-sensing system stall out under a high load from the actuator, the pump destrokes and goes into high pressure standby.

The pump remains in this condition until the load is overcome or until the orifice in the pressure line is shut off. The heat generated in this condition is low because there is no oil flow at the high pressure.

An additional feature of a pressure flow compensated load-sensing system is the ease with which multiple functions can be operated with proper valving and the constant flow feature of these systems for hydraulic motor drives.

Hydraulic Valves and Systems

Open-Center System