Page 186 - The Mechatronics Handbook

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fluids to the outlet due to the reduction in internal volume near the output port. The pump capacity is
determined by pump displacement (D) and operating speed (n). The displacement of a pump is defined
as the theoretical volume of fluid that can be delivered in one complete revolution of the pump shaft.
Q = Dn (10.15)

The pump output pressure is determined by the system load, which is the combined resistance to
fluid flow in the pipeline and the resistance to move an external load. Unless the pump flow has
egress either by moving a load or by passing through a relief valve back to the reservoir, excessive
pressure build-up can cause serious damage to the pump and/or the connecting pipeline (Reed and
Larman, 1985).
Based on their ability to change displacement, hydraulic pumps can be categorized as fixed-flow or
variable-flow pumps. Based on their design, hydraulic pumps can be categorized as gear pumps, vane
pumps, and piston pumps. Normally, gear pumps are fixed-flow pumps, and vane pumps and piston
pumps can be either fixed-flow pumps or variable-flow pumps.
The choice of pump design varies from industry to industry. For example, the machine tool manu-
facturers often select vane pumps because of their low noise, and their capability to deliver a variable
flow at a constant pressure. Mobile equipment manufacturers like to use piston pumps due to their high
power-to-weight ratio. Some agricultural equipment manufacturers prefer gear pumps for their low cost
and robustness (Reed and Larman, 1985), but piston pumps are also popular.

Pump Controls and Systems

Pumps are energy conversion devices that convert mechanical energy into fluid potential energy to drive
various hydraulic actuators to do work. To meet the requirements of different applications, there are
many types of fluid power system controls from which to choose. The design of the directional control
valve must be compatible with the pump design. Normally, an open-center directional control valve is
used with a fixed displacement pump and a closed-center directional control valve is used in a circuit
equipped with a variable displacement pump.
A fluid power system including a fixed displacement pump and an open-center directional control
valve (Fig. 10.1) is an open-loop open-center system. Such a system is also called a load-sensitive system
because the pump delivers only the pressure required to move the load, plus the pressure drop to overcome
line losses. The open-loop open-center system is suitable for simple “on-off” controls. In such operations,
the hydraulic actuator either moves the load at the maximum velocity or remains stationary with the
pump unloaded. If a proportional valve is used, the open-loop open-center system can also achieve
velocity control of the actuator. However, such control will increase the pressure of the extra flow for
releasing it back to the tank. Such control causes significant power loss and results in low system efficiency
and heat generation.
To solve this problem, an open-loop closed-center circuit is constructed using a variable displacement
pump and a closed-center directional control valve. Because a variable displacement pump is commonly
equipped with a pressure-limiting control or “pressure compensator,” the pump displacement will be
automatically increased or decreased as the system pressure decreases or increases. If the metering
position of the directional control valve is used to control the actuator velocity, constant velocity can
be achieved if the load is constant. However, if the load is changing, the “pressure-compensating” system
will not be able to keep a constant velocity without adjusting the metering position of the control valve.
To solve this problem, a “load-sensing” pump should be selected for keeping a constant velocity under
changing load. The reason for a “load-sensing” pump being able to maintain a constant velocity for
any valve-metering position is that it maintains a constant pressure drop across the metering orifice of
the directional control valve, and automatically adjusts the pump outlet pressure to compensate for the
changes in pressure caused by external load. The constant pressure drop across the valve maintains
constant flow, and therefore, constant load velocity.

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