Having chosen a suitable cylinder, the next component required is a pump of some description to operate it (Fig. 2.5).
This is the hydraulic component that has probably the greatest number of options from which to choose, including:
• size (displacement and physical dimensions)
• maximum pressure rating
• maximum drive speed
• type of construction
• fluid compatibility
• noise level
• serviceability
• efficiency
• life expectancy
• cost.
Over the years, many different types of pump construction have been developed.
The three main types in use today, however, are gear, vane and piston pumps.
Fig. 2.5 Pump TOP TIP
Be aware that, when measuring the flow rate from a cylinder, because of the difference in area between the full-bore and annulus sides of the cylinder the exhaust flow rate from the full- bore side when retracting will be greater than the ingoing flow rate to the annulus side.
DEFINITION Nearly all pumps used in hydraulic systems can be classified as positive- displacement pumps. This means that very little slippage can occur from outlet to inlet within the pump (unlike a centrifugal pump for example).
For each turn of the shaft a certain volume of fluid will be pushed out, so care must be taken to ensure that the pump outlet is never completely blocked, otherwise damage may occur.
External gear pumps
After the hand pump, probably the simplest hydraulic pump is the external gear pump, which has just two moving parts, namely two intermeshing gear wheels.
One gear wheel (the top one in Fig. 2.6) is driven round by the pump drive shaft and the other wheel (the bottom one in Fig. 2.6) rotates because it is in mesh with the driven gear. On either side of the two gears are closely fitting side plates, which are normally pressurised against the side faces of the gears to minimise the leakage path. Both gears then rotate (normally on plain bearings) inside a housing, which again is fitted closely around the tips of the gear teeth as they rotate.
Fig. 2.6 External gear pump
INLET OUTLET
SHAFT
BEARINGS SIDE PLATES
On the inlet side of the pump the fluid fills the spaces between adjacent gear teeth, the pump housing and the side plates. It is then carried round the top and bottom of the two gears to the outlet port, where the gear teeth mesh, forcing the fluid out of the spaces between the gear teeth, thereby creating a flow of fluid from the outlet port. The amount of flow will be determined by the physical size of the pump components (diameter and width) and also the drive speed of the pump shaft.
Typical characteristics of external gear pumps can be summarised as follows:
• they are more tolerant of poorer fluid contamination levels
• failures tend to be a gradual loss of efficiency rather than a sudden, catastrophic failure
• they are compact and lightweight
• it is simple to configure multiple pumps
• they are readily available
• they are inexpensive
• the gears are side-loaded by out-of-balance pressure forces, thus limiting indirect drive possibilities (e.g. belt drive)
• they are normally not economical to repair
• they are generally noisier than some other types of pumps.
POINT OF INTEREST Most gear pumps use spur gears, although some are available with helical gear teeth, which tend to reduce their noise level.
Figure 2.7 illustrates a double gear pump arrangement where the shaft is extended to drive a second set of gears, thus providing two independent pumps driven by a common shaft. Provided the shaft has sufficient torsional strength, this principle can be extended to three, four or even more sections relatively easily.
External gear pumps are ideal for low- to medium-pressure applications, especially those that do not operate continuously, such as agricultural equipment, fork-lift trucks and man-lifts. Although there are many simpler machines in industrial applications that use gear pumps, they are more commonly found in mobile applications, for which their characteristics are ideally suited.
Internal gear pumps
The internal gear pump operates on a similar principle to that just described but has one external gear and one internal gear, which are separated by a crescent-shaped segment (Fig. 2.8). The inner gear is driven by the pump shaft and meshes with
Fig. 2.7 Double gear pump
CRESCENT SEGMENT
Fig. 2.8 Internal gear pump POINT OF INTEREST
Multiple pumps can be used to separate a hydraulic system into independent circuits.
For example, one circuit on a vehicle could be used for the steering and a second for the auxiliary functions. This ensures that the steering cannot be
‘robbed’ of flow by the other functions.
the outer gear to drive it round within the pump housing, but this time the gears rotate in the same direction. As before, fluid entering the inlet port is carried round in the spaces between the gears to the outlet port area, where the meshing of the gears forces the fluid out of the pump. Although internal and external gear pumps are similar in principle, the characteristics of internal gear pumps tend to differ as follows:
• they are generally much quieter than external gear pumps
• they are available at higher pressure ratings than external gear pumps
• they are normally constructed for continuous, industrial applications
• they are more expensive than external gear pumps.
Other characteristics, such as the possibility of constructing multiple pump configurations, are similar to those of external gear pumps. Internal gear pumps are available for low-, medium- and high-pressure systems, and are commonly used in applications where low noise is an important consideration (e.g. die-casting machines, metal-working machinery and ship-borne systems).
Vane pumps
Vane pumps derive their name from a series of sliding vanes (typically 10 or 12) fitted into a rotor that is driven round by the pump drive shaft (Fig. 2.9). The rotor and vanes rotate within a cam ring, which is approximately elliptical in shape. As with the gear pump, side plates are pressure loaded against the sides of the rotor and vanes to ensure a minimal leakage path.
As the pump is driven up to speed, centrifugal action throws the vanes out of their slots so that the tips of the vanes follow the profile of the cam ring. The vanes slide in and out of their slots twice per revolution. As each vane passes an inlet port, the space between the rotor and the cam ring is increasing, so fluid is drawn in to fill the space. A quarter of a revolution later, the vane is passing an outlet port, where
Fig. 2.9 Balanced vane pump
CAM RING ROTOR VANES
the ring-to-rotor clearance is reducing, so fluid is squeezed out of the port. Exactly the same process then occurs on the second half of the revolution. Therefore, there are two pumping actions for each turn of the shaft. In practice, the two inlet ports and the two outlet ports are connected together within the body of the pump, thus providing single inlet and outlet connections.
As the pump starts to deliver fluid and generate pressure on the outlet port, the outlet pressure is connected via the side plates to the slots underneath the vanes.
This hydraulically biases the vanes outwards, ensuring that the vane tips remain in contact with the cam ring. The reason for using the elliptical cam ring with two inlets and two outlets is to balance the pressures on either side of the rotor. Therefore, unlike gear pumps, the vane pump is pressure balanced and generates no side load on the shaft or bearings.
Smaller sizes of vane pump have a simple construction where the cam ring is normally the central section of the pump body (Fig. 2.10).
Fig. 2.10 Small-series vane pump (Image courtesy of Eaton Corp.)
Larger vane pumps (Fig. 2.11), which are often rated to higher output pressures, tend to contain small vane inserts or pins to reduce the area exposed to the outlet pressure under the vane. This ensures that the vanes are still biased outwards but not with too high a force, which would cause rapid wear on the vane tip. Such pumps also incorporate the ring, rotor, vanes and side plates as a self-contained sub-assembly, which is usually referred to as a pump cartridge. This makes the pump very serviceable, as a worn pump normally requires only a cartridge change.
This is a relatively quick and simple process, which can often be carried out with the pump still in place.
As with gear pumps, double and triple vane pumps are readily available (Fig. 2.12), and with a through-shaft option it is possible to build different combinations of multiple pump assemblies.
WARNING Vane pumps can often be assembled for either right- hand or left-hand rotation. If a pump has been stripped down, therefore, it is important to reassemble it correctly for the drive rotation being used.
Right-hand rotation is defined as clockwise, when looking at the shaft end of the pump.
Typical characteristics of vane pumps are:
• the gradual transition from inlet to outlet pressure provides a relatively low- pulsation, quiet pump
• the internal-pressure-balanced design provides a comparatively long service life
• cartridge-design vane pumps are easily serviced in the field
• the direction of pump rotation can be changed easily
• they are less tolerant of poor inlet conditions or contaminated fluid than are gear pumps
• they are more expensive than gear pumps.
CARTRIDGE
Fig. 2.11 Large-series vane pump (Image courtesy of Eaton Corp.)
Fig. 2.12 Double vane pump (Image courtesy of Eaton Corp.)
WARNING
Pump cartridges in a double pump are normally arranged back to back, so it is important that their drive rotation is assembled accordingly.
Vane pumps are a popular choice for medium-pressure applications in both industrial and mobile applications. They have been used extensively in the power steering of both passenger and commercial vehicles for many years. They are also commonly found on construction and utility vehicles, baling presses and plastics machinery.
Piston pumps
There are several different designs of piston pumps, the axial and bent-axis designs being the two most common.
The axial piston pump consists of a number of sliding pistons (typically nine) fitted into bores in a rotating cylinder block (Fig. 2.13). The spherical front end of each piston has a shoe or slipper swaged over it, which is held in contact with an angled swashplate. At the opposite end of the cylinder block is a stationary valve plate, which incorporates kidney-shaped inlet and outlet ports.
CYLINDER BLOCK
PISTON SHOE
SWASHPLATE PISTON
VALVE PLATE CASE DRAIN
Fig. 2.13 Axial piston pump (Image courtesy of Eaton Corp.)
As the cylinder block and pistons are rotated by the drive shaft, the pistons are reciprocated within their bores, moving in and out once per revolution. As each piston is being retracted from its bore, the connection in the cylinder block is passing the inlet port of the valve plate, thus drawing in fluid to fill the bore behind the piston.
During the second half of the cycle, the pistons are being pushed back into their bores by the angle of the swashplate, and at this time they are connected to the outlet via the kidney port of the valve plate, thus creating flow from the pump.
Unlike gear and vane pumps, the internal leakage of a piston pump is drained into the pump case and then back to the reservoir via a separate case drain connection.
(The internal leakage in gear and vane pumps is simply directed back to the pump inlet port.) The fluid in the casing of a piston pump helps to lubricate all the moving parts, so it is important that when a new pump is installed the case is filled with clean fluid before the pump is first started. It is also important to ensure the pump drain lines are large enough so that excessive pressure is not created in the pump case.
WARNING
Always fill up the cases of piston pumps with clean fluid before they are started for the first time.
Internal leakage will then keep the case topped up when the pump is running, and provide the necessary lubrication.
A second common type of piston pump is the bent-axis piston pump (Fig. 2.14).
This pump works on a similar principle to the one described previously, except that the complete cylinder block is tilted at an angle to the drive shaft. The spherical end on each piston connecting rod is attached to a flange on the drive shaft so that, as the drive shaft and cylinder block are rotated, the pistons are alternately pulled out and pushed into their bores to create the pumping action.
Fig. 2.14 Bent-axis piston pump
A significant advantage of the bent-axis design is that there is no sliding action of the shoes or slippers against a swashplate as in the axial pump. This provides a very compact and robust design capable of generating high pressures.
Summarising the characteristics of piston pumps, therefore:
• they are suitable for high-pressure and high-flow operation
• they are more tolerant of high-water-based fire-resistant fluids than other types of pumps
• they have a long life expectancy provided that operating conditions are well maintained (fluid condition, etc.)
• they can be serviced (but not quite as simply as cartridge-type vane pumps)
• the bent-axis design has no through-drive capability (for multiple pump applications)
• they are expensive.
As mentioned, piston pumps are commonly used when the system pressure requirements are higher than those achievable with gear or vane pumps (typically 250 bar (3600 psi) and higher). However, the good efficiency and durability of piston pumps also make them suitable for applications at lower pressures where continuous operation of the hydraulic system is required.
In many applications the use of a variable-displacement (as opposed to fixed- displacement) pump is beneficial in terms of system efficiency. This is discussed later in this chapter.
POINT OF INTEREST Bent-axis piston pumps provide a large amount of power from a very small component. They are said to have a high power density.