CONSTANT PRESSURE/VARIABLE DISPLACEMENT PUMP

A Constant pressure/Variable displacement, (Self-idling) pump, only moves an amount of fluid, which the system requires, hence the term: Variable

displacement. As the pressure in the system builds up due to no actuation (no fluid movement), the pump delivery displacement is automatically reduced to no- flow. By varying the pump output, the system pressure can be maintained at a constant, within the desired range without the use of Regulators (Cut-

out/Unloading valves). It allows the pumps to turn without delivering fluid to the system. However, this can cause overheating of the pump. To prevent this, fluid is by-passed back to the reservoir, by the LP spur-gear back-up pump, ensuring a continuous flow of fluid through the HP piston pump at all times, even when there is no fluid delivery to the system. Thus providing cooling of the pump.

Fig. 43 Constant Pressure/Variable displacement, (Self-idling) hydraulic Pump

This type of pump is similar in construction to the fixed volume, axial-piston type, (Figure. 28.) It is normally a 2 stage pump. The first stage usually consists of a low pressure (LP), high volume, spur gear pump, (similar to the Radial pump shown in Figure. 31). This ensures a positive supply of fluid to the second stage, high pressure (HP), axial, Multi -piston pump, the cylinder block of which is driven by a common drive shaft.

JAR 66 CATEGORY B1 MODULE 11.11 HYDRAULIC POWER

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The piston stroke is varied by a Yoke mechanism, sometimes called a Swash- plate, or Cam. (See Figures. 36. & 37.) The pistons are attached to shoes that rotate against the stationary Yoke. The angle between the Yoke and cylinder block is varied, to increase, or decrease the piston stroke. This action is carried out by a Servo Control Piston, which senses “system pressure”. This

pressure pushes the Servo Control Piston against the return spring pressure, and reduces the Yoke angle, thereby, reducing the HP piston strokes. When the Yoke is at 90º to the drive shaft, (Perpendicular to the pistons) the piston stroke is zero and there is no flow of fluid, therefore, no load on the drive-shaft.

JAR 66 CATEGORY B1 MODULE 11.11 HYDRAULIC POWER

engineering

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As previously discussed, some kind of “unloading valve” is required when using a constant displacement pump. But the same force, (system operating pressure) which controls this valve can be used to control the output of the variable

displacement pump. Figures. 35, 36 and 37. show variable displacement pumps, which are controlled by a spring-loaded piston, which moves a pivoted yoke, or swash-plate to adjust the stroke of the delivery pistons, thereby regulating the fluid flow.

Another commonly used variable displacement pump for high pressure aircraft hydraulic systems is the Stratopower demand-type pump illustrated in Figure. 38.

Fig. 38 Constant Pressure/Variable displacement, (Self- idling) hydraulic Pump.

(Stratopower Pump, demand-type)

This pump uses nine axially-orientated pistons and cylinders. The pistons are driven up and down in the cylinders by a fixed-stroke cam. The stroke of the pistons is the same regardless of system demand. In this type, the effective length of the piston stroke controls the amount of fluid delivered to the system. This type of pump usually has a delivery capacity of between 22-37gpm. (gallons per minute) and maintains a nominal supply pressure of 3,000psi.

JAR 66 CATEGORY B1 MODULE 11.11 HYDRAULIC POWER

engineering

uk

The forces which control the pump output and system pressure is between the compensator spring and the compensator stem piston. Pump out-put pressure is ported around the compensator stem which acts as a piston and opposes the compensator spring. As the pressure increases, the stem piston compresses the compensator spring.

JAR 66 CATEGORY B1 MODULE 11.11 HYDRAULIC POWER

engineering

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The spider, which is connected to the compensator stem, moves the sleeves up and down the delivery pistons. When the pressure is high, the stem piston moves the spider, compressing the compensator spring and uncovers the relief holes near the bottom of the delivery pistons during the full stroke. This allows the fluid to be dumped during the compression stroke to the inlet side of the pump,

preventing fluid flow through the check-valves and into the system.

The pump is allowed to deliver a small amount of fluid even at it’s minimum stroke to ensure adequate lubrication and cooling of the pump at all times during operation.

When system pressure drops, the compensator spring forces the stem and spider assembly down the piston, covering the relief holes at the bottom of the delivery piston stroke. This prevents bleed-off of fluid during the compression stroke. The compressed fluid is then forced out through the check valves and into the system to meet the fluid demand. During any intermediate pressure condition the spider sleeves cover the relief holes at some point along the discharge piston’s stroke, thereby maintaining system pressure and fluid flow to the required value. The value of the compensator valve is set by the pressure adjusting screw, which varies the tension of the compensator spring.

JAR 66 CATEGORY B1 MODULE 11.11 HYDRAULIC POWER

engineering

uk