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| 3-8. Piston Pumps Piston pumps are either radial or axial.
a. Radial. In a radial piston pump (Figure 3-14), the pistons are arranged like wheel spokes in a short cylindrical block. A drive shaft, which is inside a circular housing, rotates a cylinder block. The block turns on a stationary pintle that contains the inlet and outlet ports. As a cylinder block turns, centrifugal force slings the pistons, which follow a circular housing. A housing's centerline is offset from a cylinder block's centerline. The amount of eccentricity between the two determines a piston stroke and, therefore, a pump's displacement. Controls can be applied to change a housing's location and thereby vary a pump's delivery from zero to maximum.
Figure 3-15 shows a nine-piston, radial piston pump. When a pump has an uneven number of pistons, no more than one piston is completely blocked by a pintle at one time, which reduces flow pulsations. With an even number of pistons spaced around a cylinder block, two pistons could be blocked by a pintle at the same time. If this happens, three pistons would discharge at one time and four at another time, and pulsations would occur in the flow. A pintle, a cylinder block, the pistons, a rotor, and a drive shaft constitute the main working parts of a pump.
(1) Pintle. A pintle is a round bar that serves as a stationary shaft around which a cylinder block turns. A pintle shaft (Figure 3-16) has four holes bored from one end lengthwise through part of its length. Two holes serve as an intake and two as a discharge. Two slots are cut in a side of the shaft so that each slot connects two of the lengthwise holes. The slots are in-line with the pistons when a cylinder block is assembled on a pintle. One of these slots provides a path for a liquid to pass from the pistons to the discharge holes bored in a pintle. Another slot connects the two inlet holes to the pistons when they are drawing in liquid. The discharge holes are connected through appropriate fittings to a discharge line so that a liquid can be directed into a system. The other pair of holes is connected to an inlet line.
(2) Cylinder Block. A cylinder block (Figure 3-17) is a block of metal with a hole bored through its center to fit the pintle's and cylinder's holes that are bored equal distances apart around its outside edge. The cylinder's holes connect with the hole that receives a pintle. Designs differ; some cylinders appear to be almost solid, while others have spokelike cylinders radiating out from the center. A cylinder's and pintle's holes are accurately machined so that liquid loss around a piston is minimal.
(3) Pistons. Pistons are manufactured in different designs (see Figure 3-18). Diagram A shows a piston with small wheels that roll around the inside curve of a rotor. Diagram B shows a piston in which a conical edge of the top bears directly against a reaction ring of the rotor. In this design, a piston goes back and forth in a cylinder while it rotates about its axis so that the top surface will wear uniformly. Diagram C shows a piston attached to curved plates. The curved plates bear against and slide around the inside surface of a rotor. The pistons' sides are accurately machined to fit the cylinders so that there is a minimum loss of liquid between the walls of a piston and cylinder. No provision is made for using piston rings to help seal against piston leakage.
(4) Rotors. Rotor designs may differ from pump to pump. A rotor consists of a circular ring, machine finished on the inside, against which the pistons bear. A rotor rotates within a slide block, which can be shifted from side to side to control the piston's length of stroke. A slide block has two pairs of machined surfaces on the exterior so that it can slide in tracks in the pump case.
(a) Displacement. Pump displacement depends on the bore and stroke of a piston and the number of pistons. A swash plate's angle (Figure 3-19, diagram B) determines the stroke, which can vary by changing the angle. In a fixed angle's unit, a swash plate is stationary in the housing. In a variable unit's, it is mounted on a yoke, which can turn on pintles. Different controls can be attached to the pintles to vary pump delivery from zero to the maximum. With certain controls, the direction of flow can be reversed by swinging a yoke past center. In the center position, a swash plate is perpendicular to the cylinder's, and there is no piston reciprocation; no oil is pumped.
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