CONSTANT VOLUME FIXED DISPLACEMENT PUMPS

(Non Self- Idling)

The most common type of Constant volume (CV) pump for medium-pressure systems, is the Gear pump type. (See Fig. 34.)

JAR 66 CATEGORY B1 MODULE 11.11 HYDRAULIC POWER

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The left-hand gear is driven by the engine through a splined shaft. This gear rotates in a close fitting housing and drives the right-hand gear housed in the same manner. As the gears rotate in the direction shown, fluid is transported between the teeth around the outside of the gears, from the inlet side of the pump. When the teeth mesh with each other, in the outlet chamber, fluid is displaced into the outlet side of the pump.

A very small amount of fluid is allowed to leak past the gears and around the shaft for lubrication, cooling, and sealing. This fluid drains into the hollow shafts of the gears where it is picked up by the low pressure on the inlet side of the pump.

A relief valve holds the oil in the shafts until it builds up to about 15 psi. This is called; case pressure. This is maintained so that, in the event of the shaft, or seal, becoming scored, fluid will be forced out of the pump rather than air being drawn in.

Spur gear pumps provide a good, non-pulsating, high flow rate, but are limited to pressures up to about 800psi. Because of this, they are more

commonly used on smaller aircraft, but also as pressure back-up pumps for the more powerful, piston-type pumps on larger aircraft, who’s hydraulic systems operating pressures are between: 1,200 to 3,000psi.

Gear Type Hydraulic Pump Figure.34

JAR 66 CATEGORY B1 MODULE 11.11 HYDRAULIC POWER

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Aircraft hydraulic systems that require a relatively small volume of fluid under a pressure of 2,500 psi or more, often use fixed-angle, Multi-piston pumps as shown in fig. 28.

a Axial Piston Pump, (Figure.28.)

This type of pump consists of a bronze cylinder block, rotated by a splined drive shaft, driven by the engine, through a universal link. The cylinder housing is mounted at a fixed angle to the drive shaft and bearing housing. The cylinder block usually has seven, or nine axially-drilled holes, which accommodate, “High precision, close fitting pistons”. These in turn are attached by a ball-jointed rod to a pump drive plate which is rotated by the engine. As the piston and cylinder block assembly are rotated by the drive-shaft, the pistons on one side (upper pistons) are at the bottom of their stroke, and open to the Inlet port. due to the angle of the housing. The pistons on the opposite side (Bottom pistons) are then at the top on their stroke, open to the Outlet port. (See fig. 35)

Fixed Angle, Axial, Piston Type Hydraulic Pump Figure 35

The stroke (Displacement) of the piston is dependent on the angle of the cylinder housing to that of the bearing housing. As the whole assembly is rotated, fluid is

JAR 66 CATEGORY B1 MODULE 11.11 HYDRAULIC POWER

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A valve plate with two crescent-shaped openings cover the end of the cylinders. One above the pistons moving up, thereby pushing fluid through the Outlet port. The other, above the pistons moving down, drawing fluid into the cylinder, through the Inlet port.

b Radial Piston Pumps

In this type of fixed volume pump, the cylinders are arranged radially around an eccentric crankshaft. (See Fig.29A & B). When the crankshaft is rotated, the pistons move outwards in each cylinder, forcing pressurised fluid into the annular outlet port through each cylinder delivery valve. When each piston is at the bottom of it’s stroke, the pistons uncover the inlet port, allowing a fresh charge of fluid to enter each cylinder.

The fresh charge of fluid is then compressed as the piston moves outwards again forcing fluid once more through the delivery valve. This process is repeated with each revolution of the eccentric

crankshaft

Typical Radial, Piston-type, Hydraulic Pump - Side View

Figure 36

Radial Piston Hydraulic Pump – End View Figure 37

JAR 66 CATEGORY B1 MODULE 11.11 HYDRAULIC POWER

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Typical Radial, Piston-type, hydraulic pump Constant Volume/Fixed Displacement

Figure 38

JAR 66 CATEGORY B1 MODULE 11.11 HYDRAULIC POWER

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These pumps are used in systems, which required moving a large volume of fluid, but at relatively low pressures.

The vanes are allowed to float freely in slots machined in the rotor, and are held in place by a spacer. This rotating assembly is attached to a drive shaft and is driven by the engine, or, an electric motor. The rotating assembly is mounted “concentrically” in a ported, steel sleeve which is pressed into a cast, aluminium housing.

OPERATION: As the rotor turns in the direction of the arrow, (Fig. 29.) the volume between the vanes on the inlet side increases, while the volume between the vanes on the outlet side decreases. This change in volume draws fluid into the pump through the inlet port, and discharges it through the outlet port and into the system

Fig. 40 Vane-type Hydraulic Pump (Constant Volume Fixed Displacement This type of pump is normally used on light aircraft, particularly in “POWER- PACK” type hydraulic systems, but is more generally used in fuel and pneumatic systems than hydraulic systems.

JAR 66 CATEGORY B1 MODULE 11.11 HYDRAULIC POWER

engineering

uk