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Back To The Basics Part IV: Variable Displacement And Pressure-Compensated Pumps.

Nearly all types of hydraulic pumps used in mobile equipment applications are available in variable displacement configurations. Vane, piston and even gear pumps can offer variable displacement, although variable displacement gear pumps have never really reached commercial fruition.

In our earlier discussion of vane pumps, we indicated that pumping action was due to eccentricity between rotor and cam ring (Fig. 1a). Displacement, the volume of oil delivered per revolution of pump shaft, is dependent upon this eccentricity If the pump is designed so the cam ring can be moved relative to the rotor, displacement can be changed; i.e., the eccentricity is made smaller.

The rotor center stays fixed because it is also the drive shaft center and fixed by bearings. When the ring center coincides with the rotor center, no oil is pumped. If the ring is moved so it crosses over to the other side, the direction of flow is reversed, even though pump shaft rotates in the same direction (Fig. 1b).

We have mentioned the methods for varying the displacement of piston pumps. These include: changing the cam plate angle, varying the angle between the cylinder barrel and drive shaft, short stroking -- or unloading -- individual pistons. In radial piston pumps, eccentricity between the shaft and cam ring centers is varied.

Most controls used to adjust variable displacement pumps are applicable in all cases, only their implementation changes. The simplest application would be as an adjustable unit, but operating fixed once adjusted. Such a control is illustrated in Fig. 2. It incorporates an adjusting screw to shift eccentricity of a cam ring or change angle of a swashplate. Another type is a hand wheel adjustment, which allows greater ease of control while unit is operating.

To change displacement, an air or hydraulic pilot cylinder can be applied. The cylinder can be remotely operated by a manual pilot valve or controlled automatically. Another feature is a mechanical stop to limit amount of shift of pumping elements. It is then possible to get two-position, positive control. With the control cylinder energized in one direction, a given quantity of oil will be delivered. This can be adjusted by changing the stop on that end. When a signal is applied to shift the control cylinder to the opposite end against its stop, a second fixed quantity will be delivered, arriving at a hi-lo type of circuit control for rapid traverse and feed.

Another method of varying displacement includes electric motor controls, in which a reversible motor controls eccentricity or angle. It can be operated open loop (remotely by pushbuttons), or made part of a closed loop system and operated in response to a feedback signal from a transducer.

Pneumatic controllers can be used to shift the pump elements in response to control signals impressed on the operator. The function relative to the pump is still the same -- to vary eccentricity of swash plate angle and change displacement. A full servo-operator can also be used both open and closed loop, (with or without feedback).

Pressure-Compensated Pumps

A pressure-compensated pump is recognized as one with individual characteristics. Essentially, it is one that senses outlet pressure and adjusts displacement accordingly (see Fig. 3). It is a variable pump and the element controlling Output is forced into position of maximum eccentricity by a compensator spring, corresponding to maximum pump delivery or displacement. A compensator piston is placed on the opposite side of the cam ring; any force developed will oppose the force of the compensator spring.

Fig. 4 illustrates a comparable setup for a piston pump. Note that as the compensator spring holding the cam plate at maximum angle, the piston opposes the spring. The pump then delivers oil to the external circuit against a load resistance, a pressure is developed in proportion to this resistance. The compensator piston is exposed to the circuit pressure and develops a force-P x [A.sub.c]. When it equals spring force, the ring is in balance. Any increase in pressure, P, will cause increase in piston force. Total compensator force (P + AP) x [A.sub.c]. The ring is unbalanced and shifts toward center until new spring force balances compensator force and equilibrium is re-established.

A typical flow rate versus pressure curve is shown in Fig. 5. Note that flow rate remains constant over a large part of the pressure range. During this time piston force is less than spring force. The cam plate is thus held at maximum stroke position and the pump acts like a fixed displacement pump. However, where the curve breaks downward, cut-off pressure is reached and piston force overcomes spring force. As pressure increases, displacement falls off due to shifting of the plate as spring is compressed. As pumping element comes to center, output to external circuit is zero. This level is called deadhead pressure.

When this occurs a maximum circuit pressure is maintained; the pump displaced enough oil to make up for internal leakage, but delivers none to the external circuit. Hp consumption is low because no oil is pumped, and only losses must be supplied.

A further modification is the dual-pressure pump (Fig. 6). Functioning as a pressure-compensated pump with two ranges, the low is attained by exhausting the governor spring chamber. Only spring force is brought to bear on the adjusting element -- the ring plate -- and system pressure need only to overcome this force. Increased system pressure is required to effect compensator action.

The pressure-compensated pump can be used as a relief valve. When system pressure reaches compensator setting or an actuator bottoms out, displacement goes to zero; constant maximum pressure is maintained. Power consumption is very small because only losses must be supplied. Therefore, it is unnecessary to use a relief valve in a circuit with the pump except as an overpressure-safety device. This design is such that it represents something of a feedback system within itself. Above the cutoff point, if pressure goes up -- displacement goes down. Thus, any system transients that could be reflected to the pump as pressure surges could cause displacement to change. It is possible if transients occur at the right frequency, the pump may go into oscillation. A relief valve in the system has been known to cause hunting if set too low.

A takeoff on pressure compensation, flow rate compensation, is illustrated in Fig. 7. The control orifice is in the discharge line from the pump. Fluid flowing thing experiences a pressure drop proportional to flow rate. This drop can be imposed across a piston with a bias spring behind. As flow rate goes up, pressure clifferential imposed across piston increases until unbalanced force overcomes bias spring force. Piston and pumping elements then shift to a new equilibrium position. Displacement is varied to match required conditions dictated by spring bias and orifice characteristics.

Pressure compensating can be added to flow control by a second orifice in line between the discharge pressure level and spring chamber in conjunction with a relief valve (Fig. 8). The relief valve's function is to hold the pressure level in the spring chamber to the constant maximum level. As system pressure continues to rise, unbalanced hydraulic force across the piston causes it to shift resulting in decreased output.

It is interesting to note, the pressure-compensated variable volume vane pump does not have a physically separated compensator piston which is the case with its piston counterpart. Examining Fig. 9, you will notice an area of the cam ring on the discharge side exposed to full system pressure. The oil appears trapped between the rotor o.d. and the ring i.d., pushing out on the ring. In fixed displacement, this ring reaction is taken by the housing holding the ring in place. In a variable pump, where the ring is free, tins reaction is used to push the ring toward on-center, or zero output position. The pump no longer develops any pressure since it does not discharge oil into the system.

This is why it was indicated earlier, the pressure compensator pump still moved a small amount of oil to make up losses under deadhead conditions. The ring never quite reaches dead center or pressure and discharge flow would go to zero. The compensator spring is positioned on the side toward winch pressure reaction is directed, just as it was placed opposite the compensator piston in earlier diagrams. Thus, the two forces, spring and hydraulic, oppose each other

In our last few discussions, we've looked at pump design and construction. The next link to understanding the function and proper use of hydraulic pumps concerns pump curves and their interpretation, winch we'll cover next time.
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Comment:Back To The Basics Part IV: Variable Displacement And Pressure-Compensated Pumps.
Author:Henke, Russ
Publication:Diesel Progress North American Edition
Geographic Code:1USA
Date:Sep 1, 2000
Words:1434
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