Hydraulic-Electric Analogies: Hydraulic Power Conversion, Part 3

Hydraulic-Electric Analogies: Hydraulic Power Conversion, Part 3

Power generated by hydraulic systems exhibits current flow traits similar to those of its electrical counterparts.

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6. A side view of an axial piston pump shows the essential parts of the pumping elements. Housing and bearing and shaft seals, etc., are not shown.

This month advances the discussion of a pump into the realm of other commercially viable configurations used in both mobile and industrial applications. In addition, it presents a pump that will also work as a motor. Figures 6 and 7 show the essential internal parts that perform the actual pumping function in the pump. Figure 7 is merely a view of the internal parts as viewed from the shaft end of the pump.

7. This view of the axial piston pump comes from the shaft end of the elements shown in Figure 6.

To understand the logic of the pump function, it’s important to know which elements are stationary and which are moving. If they’re moving, it’s essential to understand the nature of their motion, too. Only the port plate is stationary for the pump elements shown. It neither spins, tilts, nor reciprocates because it is locked to the pump housing by means that are not shown. The plate stands perfectly still as the rotating barrel slides past on a thin, oily, lubricating film.

Role of the Port Plate

The port plate’s function is to serve as the ac-dc converter that allows the alternating flow and pressure inside the pumping cavities to smoothly communicate with the unidirectional flow and pressure in the outside world. It eliminates the need for the check valves shown in  Figures 2 and 4 in Part 1 (see “Hydraulic-Electric Analogies: Hydraulic Power Conversion, Part 1”) and Figure 5 in Part 2 (see “Hydraulic-Electric Analogies: Hydraulic Power Conversion, Part 2”). It is to the pump what the commutator elements are to the electrical dc motor. Later on, more will be said about the port plate.

Of course, the shaft spins about its central axis and is keyed to its prime mover as well as to the barrel. Therefore, as the shaft spins, so does the barrel. The barrel houses the pistons, which are closely fitted to prevent bypass leakage and axial reciprocation within their respective bores. That’s because they are in sliding contact with the smooth and lubricated surface of the swashplate through their swiveling slippers (Fig. 8).

The swashplate can pivot because of the tilt-axis bearing, but it’s important to realize that the swashplate does not spin with the shaft. It can only tilt around its own axis and swashplate tilt bearings. Tilting on an axis is transverse to the shaft centerline, creating an inclined plane for the pistons.

8. An exploded view of the rotating barrel and stationary port plate, along with piston and slipper details, help to visualize the operation of the axial piston pump.

As the shaft spins, also spinning the barrel filled with the reciprocating pistons, the pistons and their attached slippers are forced to “climb the inclined plane”, so to speak. Thus, they are forced into the barrel bores on the side that faces the viewer. The pistons on the viewer’s side of the barrel (Fig. 6, again) are rising as well as moving to the right, expelling fluid during this process.

Piston Action

On the other side of the barrel, however, the pistons are moving “down” the inclined swashplate surface due to barrel rotation. The inclined plane of the swashplate forces those pistons to withdraw from their respective bores, and in doing so, they ingest fluid, creating the inlet side of the machine.

Figure 9 shows a 3D view of the barrel, with all nine pistons at various amounts of stroke, which depends on the amount of shaft rotation. That is, as the shaft rotates about the central shaft axis, it is keyed to the barrel, forcing the barrel to also rotate.

Although they are not shown in Figure 9, several slippers are in sliding contact with the tilted swashplate, forcing the pistons to plunge deeper into their respective bores. Those at the bottom of the barrel have the least penetration into the bores, while those at the top position have the greatest penetration. This means that all of the pistons in the bores on the right side of the barrel are moving into their bores, while those on the left side are being pulled out of their bores.

9. This barrel and port plate view shows the nine pistons in the barrel and various degrees of axial stroke.

Furthermore, the pistons on the right side are expelling fluid, while those on the left side are pulling in fluid (actually, atmospheric pressure acting on the reservoir surface is pushing fluid into the evacuating piston bores). Now, the port plate with its two “kidney ports” is not spinning, but is in sliding and lubricated contact with the flat barrel face.

On top of that, the right hand kidney port is the outlet side and the left-hand kidney port (Figs. 8 and 9, again) is the inlet. This is true whether the machine is functioning as a pump or as a motor.

Pumping vs. Motoring Pressure

When the machine is in a pumping mode, the outlet side is at high pressure while the inlet side is at a much lower pressure. The opposite is true when motoring. The pressure difference between the two sides and their respective external ports (not shown in the figures) could be as much as 350 bar (approximately 5,000 psi), resulting in a huge pressure gradient across the two very narrow crossover spans shown in Figure 10. Because of the viewpoint (i.e., from the port-plate end), the barrel is rotating in the clockwise direction. The pistons on the left side are moving toward the viewer, making it the outlet side as well as the high-pressure side, when operating as a pump.

10. The pistons on the outlet side are moving toward the viewer, expelling fluid; the pistons on the inlet side are moving away from the viewer and taking fluid in. The crossover spans are drawn narrow on purpose to illustrate their sealing functions.

The barrel is spinning along with the spinning shaft, but the port plate is stationary. The narrow crossover spans form a sliding seal between the high and low pressures, requiring very low manufacturing tolerances while allowing for good lubrication. A seal is also formed between the rotating piston chambers and the circumferential zones surrounding the barrel. The seal restricts leakage that travels radially outward. Seal points within the machine are dynamic and must provide the sealing, but not create friction.

Any internal leakage, much of it necessary for lubricating moving parts, results in reduced volumetric efficiency. Mechanical friction, on the other hand, results in reduced mechanical efficiency. Both instances contribute to the overall inefficiency of the machine, whether it be a pump or a motor. Design of the pump or motor requires a balance between internal lubricating leakage past sliding surfaces and internal friction from insufficient clearances.

Crossover Spans

The crossover spans in Figure 10 were deliberately drawn to be too narrow so that the piston at top dead center can still be seen. At the moment the pistons are in the positions shown in the drawing, a dead “hydraulic short circuit” exists between the high- and low-pressure sides. That’s because high-pressure fluid will make its way into the piston chamber (bore), past the crossover span, and out the other side.

It should be apparent that the correct crossover-span width is approximately the diameter of one piston. Actually, it’s just slightly larger to create a better seal. If it’s too wide, though, fluid entrapment will contribute to pulsations (noise) and inefficiency.

Any internal leakage will reduce the machine’s efficiency and is thus deemed undesirable. However, as already stated, some leakage is essential for effective lubrication. To that end, there are other internal leakage paths.

Looking at Figure 8, note the lubricating passage drilled down the center of each piston. This passage provides lubrication to the ball-slipper joint and the slipper-swashplate contact. In addition, necessary clearance exists between the pistons and their bores, each comprising another internal leakage path.

In spite of the mixture of leakage paths, the volumetric efficiency of a modern high-pressure piston pump can reach to 97% at pressures as high as 350 bar. Design of the machine is a delicate balance among effective sealing, effective lubrication, and low friction. Mechanical efficiencies can reach into the mid 90% ranges, leading to power efficiencies in the low 90% range. To reiterate, though, the reciprocating pistons cause ac hydraulics inside the piston bores, but dc flow in the external circuit.

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