925 Roller type hydraulic pump

The components of this pump (Fig. 9.22 (a and b)) consist of the stationary casing, cam ring and the flow and pressure control valve. The moving parts comprise of a rotor carrier mounted on the drive shaft and six rollers which lodge between taper slots machined around the rotor blank. The drive shaft itself is supported in two lead-bronze bushes, one of which is held in the body and the other in the end cover. A ball bearing at the drive end of the shaft takes the load if it is belt and pulley driven.

The rotor carrier is made from silicon manganese steel which is heat treated to a moderate hardness.

The rotor slots which guide the rollers taper in width towards their base, but their axes instead of being radial have an appreciable trailing angle so as to provide better control over the radial movement of the rollers. The hollow rollers made of case- hardened steel are roughly 10 mm in diameter and there are three standard roller lengths of 13,18 and 23 mm to accommodate three different capacity pumps.

The cam ring is subjected to a combined rolling and sliding action of the rollers under the generated pressure. To minimize wear it is made from heat treated nickel-chromium cast iron. The internal profile of the cam ring is not truly cylindrical, but is made up from a number of arcs which are shaped to maximize the induction of delivery of the fluid as it circulates through the pump.

To improve the fluid intake and discharge flow there are two elongated intake ports and two similar discharge ports at different radii from the shaft axes. The inner ports fill or discharge the space between the rollers and the bottoms of their slots and the outer ports feed or deliver fluid in the space formed between the internal cam ring face and the lobes of the rotor carrier. The inner elongated intake port has a narrow parallel trailing (transition) groove at one end and a tapered leading (timing) groove at the other end. The inner discharge port has only a tapered trailing (timing) groove at one end. These secondary circumferential groove extensions to the main inner ports provide a progressive fluid intake and discharge action as they are either sealed or exposed by the rotor carrier lobes and thereby reduce shock and noise which would result if these ports were suddenly opened or closed, particularly if air has become trapped in the rotor carrier slots.

Operating cycle of roller pump (Fig. 9.22(a and b)) Rotation of the drive shaft immediately causes the centrifugal force acting on the rollers to move them outwards into contact with the internal face of the cam ring. The functioning of the pump can be considered by the various phases of operation as

Sector grid shaft

Fig. 9.21 (a and b) Power assisted steering double ball valve lock limit

Sector grid shaft

Fig. 9.21 (a and b) Power assisted steering double ball valve lock limit

(a) Low pump speed position (b} High pump spaed position

Fig. 9.22(a and b) Power assisted roller type pump and control valve unit

(a) Low pump speed position (b} High pump spaed position

Fig. 9.22(a and b) Power assisted roller type pump and control valve unit an individual roller moves around the internal cam face through positions A, B, C, D, E and F.

Filling phase (Fig. 9.22(a)) As the roller in position A moves to position B and then to position C, the space between the eccentric mounted rotor carrier lobe and cam face increases. Therefore the volume created between adjacent rollers will also become greater. The maximum chamber volume occurs between positions C and D. As a result, the pressure in these chambers will drop and thus induce fluid from the intake passages to enter by way of the outer chamber formed by the rotor lobe and the cam face and by the inner port into the tapered roller slot region. Filling the two regions of the chamber separately considerably speeds up the fluid intake process.

Pressurizationphase (Fig. 9.22(a)) With further rotation of the rotor carrier, the leading edge of the rotor slot just beyond position C is just on the point of closing the intake ports, and the space formed between adjacent rollers at positions C and D starts to decrease. The squeezing action pressurizes the fluid.

Discharge phase (Fig. 9.22(a)) Just beyond roller position D the inner discharge port is uncovered by the trailing edge of the rotor carrier slot. This immediately enables fluid to be pushed out through the inner discharge port. As the rotor continues to rotate, the roller moves from position D to E with a further decrease in radial chamber space so that there is a further rise in fluid pressure. Eventually the roller moves from position E to F. This uncovers the outer discharge port so that an increased amount of fluid is discharged into the outlet passage.

Transition phase (Fig. 9.22(a)) The roller will have completed one revolution as it moves from position F to the starting position at A. During the early part of this movement the leading edge of the rotor slot position F closes both of the discharge ports and at about the same time the trailing edge of the rotor slot position A uncovers the transition groove in readiness for the next filling phase. The radial space between the rotor lobe and internal cam face in this phase will be at a minimum.

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