17-05-2014, 11:55 AM
Hydraulic pumps and pressure regulation
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A hydraulic pump (Figure 2.1) takes oil from a tank and delivers it to the rest of the hydraulic circuit. In doing so it
raises oil pressure to the required level. The operation of such a pump is illustrated in Figure 2.1a. On hydraulic
circuit diagrams a pump is represented by the symbol of Figure 2.1b, with the arrowhead showing the direction of
flow.
Hydraulic pumps are generally driven at constant speed by a three phase AC induction motor rotating at 1500
rpm in the UK (with a 50 Hz supply) and at 1200 or 1800 rpm in the USA (with a 60 Hz supply). Often pump and
motor are supplied as one combined unit. As an AC motor requires some form of starter, the complete
arrangement illustrated in Figure 2. 1 c is needed.
There are two types of pump (for fluids) or compressor (for gases) illustrated in Figure 2.2. Typical of the first type
is the centrifugal pump of Figure 2.2a. Fluid is drawn into the axis of the pump, and flung out to the periphery by
centrifugal force. Flow of fluid into the load maintains pressure at the pump exit. Should the pump stop, however,
there is a direct route from outlet back to inlet and the pressure rapidly decays away. Fluid leakage will also occur
past the vanes, so pump delivery will vary according to outlet pressure. Devices such as that shown in Figure
2.2a are known as hydrodynamic pumps, and are primarily used to shift fluid from one location to another at
relatively low pressures. Water pumps are a typical application.
Should the pump stop, one of the two valves will always be closed, so there is no route for fluid to leak back. Exit
pressure is therefore maintained (assuming there are no downstream return routes).
More important, though, is the fact that the pump delivers a fixed volume of fluid from inlet to outlet each cycle
regardless of pressure at the outlet port. Unlike the hydrodynamic pump described earlier, a piston pump has no
inherent maximum pressure determined by pump leakage: if it drives into a dead end load with no return route
(as can easily occur in an inactive hydraulic system with all valves closed) the pressure rises continuously with
each pump stroke until either piping or the pump itself fails.
Hydraulic pumps are invariably hydrostatic and, consequently, require some method of controlling system
pressure to avoid catastrophic pipe or pump failure. This topic is discussed further in a later section.
A hydraulic pump is specified by the flow rate it delivers (usually given in litres min
or gallons min ) and the
maximum pressure the pump can withstand. These are normally called the pump capacity (or delivery rate) and
the pressure rating.
Pump data sheets specify required drive speed (usually 1200, 1500 or 1800 rpm corresponding to the speed of a
three phase induction motor). Pump capacity is directly related to drive speed; at a lower than specified speed,
pump capacity is reduced and pump efficiency falls as fluid leakage (called slippage) increases. Pump capacity
cannot, on the other hand, be expected to increase by increasing drive speed, as effects such as centrifugal
forces, frictional forces and fluid cavitation will drastically reduce service life.
Like any mechanical device, pumps are not 100% efficient. The efficiency of a pump may be specified in two
ways. First, volumetric efficiency relates actual volume delivered to the theoretical maximum volume. The simple
piston pump of Figure 2.2b, for example, has a theoretical volume of A x s delivered per stroke, but in practice
the small overlap when both inlet and outlet valves are closed will reduce the volume slightly.
Second, efficiency may be specified in terms of output hydraulic power and input mechanical (at the drive shaft)
or electrical (at the motor terminals) power.