Types of hydraulic pump

Many different methods are used to classify pumps. Terms such as hydrodynamic,positive-displacement, fixed-displacement, variable-displacement, constant-volume,and others are used to describe hydraulic pumps.

Positive-displacement pumps, unlike centrifugal pumps, will provide a definite volume of fluid for each cycle of pump operation, regardless of the resistance offered by the system, provided the capacity of the power unit driving the pump is not exceeded.

If the outlet of a positive-displacement pump were completely closed, the pressure would instantaneously increase to the point at which the pump driver would stall or something in the drive-train would break.
Positive-displacement pump classification can be subdivided into other classifications that include fixed-displacement or variable-displacement. Other terms, such as fixed-delivery,constant-delivery, and constant-volume, may be used to describe this type of pump.

The fixed-displacement pump derivers the same amount of fluid on each cycle. Only changing the speed of the pump can change the output volume. When a pump of this type is used in a hydraulic system, a pressure regulator or relief valve must be installed in the system.

The variable-displacement classification of hydraulic pumps is constructed so that the displacement per cycle can be varied. The displacement is varied through the use of an internal control device. The construction of these devices can vary from an unloading or pressure regulating valve to restricted-flow bypass loops. Some of these devices will be described in the control valve section.

Hydraulic pumps may also be classified according to the specific design used to create the flow of fluids. Practically all hydraulic pumps fall within three design classifications: centrifugal, rotary, and reciprocating. The use of centrifugal pumps in hydraulics is extremely limited.

Hydrodynamic or non-positive-displacement pumps such as centrifugal or turbine designs are used primarily in the transfer of fluids where the only resistance encountered is that created by the weight of the fluid itself and friction.

Most non-positive-displacement pumps (Figure 1) operate by centrifugal force where fluids entering the center of the pump housing are thrown to the outside by a rapidly driven impeller. There is no positive seal between the inlet and outlet ports,and pressure capabilities are a function of rotating speed.

Although they provide smooth continuous flow, the output of a hydrodynamic pumpis reduced as resistance is increased. It is possible to completely block off or deadhead the outlet while the pump is running. For this and other reasons, non-positivedisplacement pumps are seldom used in hydraulic systems.

Hydrostatic or positive-displacement pumps, as their name implies, provide a given amount of fluid for every stroke, revolution, or cycle. Except for leakage, their output is independent of outlet pressure or back pressure from the system. This properly makes them well suited for use in the transmission of power.

ROTARY PUMPS

All rotary pumps have rotating parts, which trap the fluid at the inlet port and force it,through the discharge port, into the system. Gears, screws, lobes, and vanes are com-monly used to move the fluid within the pump. Rotary pumps are classified as positive, fixed-displacement type.

Rotary pumps are designed with very small clearances between their rotating and stationary parts to minimize slippage from the discharge side back to the suction side of the pump. They are designed to operate at relatively moderate speeds, normally below 1,800 rpm. Operation at higher speeds can cause erosion and excessive wear.There are numerous types of rotary pumps and various methods of classification.

They may be classified by the shaft position, the type of driver, their manufacturer’s name, or their service application. However, classification of rotary pumps is generally made according to the type of rotating element. A few of the more common types include the following.

 

Gear Pumps

A gear pump develops flow by carrying fluid between the teeth of two meshed gears.One gear is driven by the drive shaft and turns the other. The pumping chambers formed between the gear teeth are enclosed by the pump’s housing and the side plates.

A partial vacuum is created at the inlet as the gear teeth unmesh. Fluid flows in to fill the space and is carried around the outside of the gears. As the teeth mesh again at the outlet, the fluid is forced out. High pressure at the pump’s outlet imposes an unbalanced load on the gears and their bearing support structure.

Gear pumps are classified as either extemal or internal gear pumps. In extemal gear pumps,the teeth of both gears project outward from their centers . Extemal gear pumps may use spur, herringbone, or helical gear sets to move the fluid.

External Gear Pumps

In this design, the pumping chambers also are formed between the gear teeth. A crescent seal is machined into the pump body between the inlet and outlet where clearance between the teeth is at its greatest. Also in this general family of gear pumps is the lobe or rotor pump. This pump operates on the same principle as the external gear, but has a higher displacement.

Spur Gear Pumps

The spur gear pump, shown in Figure, consists of two meshed gears, which revolve in a housing. The drive gear in the illustration is turned by a drive shaft, which is attached to the power source. The clearances between the gear teeth as they mesh and between the teeth and the pump housing are very small.

the drive gear is turning in a counterclockwise direction and the driven gear is turning in a clockwise direction. As the teeth pass the inlet port, liquid is trapped between the teeth and the housing. This liquid is carried around the housing to the outlet port. As the teeth mesh again, the liquid between the teeth is pushed into the outlet port. This action produces a positive flow of liquid into the system. A shear pin or shear section is incorporated in the drive shaft. This is to protect the power source or reduction gears if the pump fails because of excessive load or the jamming of parts.

Herringbone Gear Pumps

The herringbone gear pump is a modification of the spur gear design.The liquid is pumped in the same manner as in the spur gear pump. However, in the herringbone pump, each set of teeth begins its fluid discharge phase before the preceding set of teeth has completed its discharge phase. This overlapping and the relatively larger space at the center of the gears tend to minimize pulsations and give a steadier flow than the spur gear pump.

Helical Gear Pumps

The helical gear pump is still another modification of the spur gear design.Because of the helical gear design, the overlapping of successive discharges from spaces between the teeth is even greater than it is in the herringbone design. Therefore, the discharge
flow is smoother. As a result, the gears can be designed with a small number of large teeth,thus allowing an increase in capacity without sacrificing smooth flow.

The gear sets in this type of pump are driven by a set of timing gears that help maintain the required close tolerance between mating gears without actual metal-to-metal contact. Metallic contact between the teeth would provide a tighter seal against hydraulic slip, but it would also decrease volume and dramatically increase wear of the teeth. Anti-friction bearings at both ends of the gear shafts maintain proper alignment and radial clearance between gears and minimize the friction loss in the transmitted power. Suitable packing is used to prevent leakage around the shaft.

Internal Gear Pumps

In an internal gear pump, the teeth of one gear project outward from the gear hub, the teeth of the other gear projects inward toward the center of the pump . Internal gear pumps may be either centered or off-centered. illustrates two types of internal gear pumps. Pump A is an example of a centered pump,and pump B is off-centered.

Off-Centered Internal Gear Pumps

In this type of pump, the drive gear is attached directly to the drive shaft of the pump and is placed off-center in relation to the internal gear. The two gears mesh on one side of the pump, between the suction and discharge ports. On the opposite side of the chamber, a crescent-shaped form fitted to a close tolerance fills the space between the two gears.

The rotation of the center gear by the drive shaft causes the outside gear to rotate,since the two gears are meshed. Everything in the chamber rotates except the crescent. This causes liquid to be trapped in the gear spaces as they pass the crescent. The liquid is carried from the suction port to the discharge port, where it is forced out of the pump by the meshing gears. The size of the crescent that separates the internal and external gears determines the volume delivered by the pump. A small crescent allows more volume of liquid per revolution than a larger crescent.

Centered Internal Gear pump

Another design of internal gear pump is illustrated in Figures 6 and 7. This pump consists of a pair of gear-shaped elements, one within the other, located in the pump chamber. The inner gear is connected to the drive shaft of the power source.The operation of this type of internal gear pump is illustrated in Figure 7. To simplify the explanation, the teeth of the inner gear and the space between teeth of the outer gear are numbered. Note that the inner gear has one tooth fewer than the outer gear.

The tooth forms of the two gears are related in such a way that each tooth of the inner gear is always in sliding contact with the surface of the outer gear. Each tooth of the inner gear meshes with the outer gear at just one point during each revolution. In the illustration, this point is at the (X).In view A, tooth 1 of the inner gear is meshed with space 1 of the outer gear. As the gears continue to rotate in a clockwise direction and the teeth approach point X, tooth 6 of the inner gear will mesh with space 7 of the outer gear, tooth 5 with space 6, and so on. During this revolution, tooth 1 will mesh with space 2; and during the following revolution with space 3. As a result, the outer gear will rotate at just six-sevenths the speed of the inner gear.

At one side of the point of mesh, pockets of increasing size are formed as the gears rotate,while on the other side the pockets decrease in size.In Figure 7, the pockets on the righthand side of the drawings are increasing in size toward the bottom of the illustration, while those on the left-hand side are decreasing in size toward the top of the illustration. The intake side of the pump would therefore be on the right and the discharge side on the left.In Figure 7, since the fight-hand side of the drawing was turned over to show the ports,the intake and discharge appear reversed. Actually, A in one drawing covers A in the other.

Lobe Pumps

The lobe pump uses the same principles of operation as the external gear pump. The lobes are considerably larger than gear teeth, but there are only two or three lobes on each rotor. A three-lobed pump is illustrated. The two elements are rotated, one directly driven by the power source, and the other through timing gears. As the elements rotate, liquid is trapped between two lobes of each rotor and the walls of the pump chamber. The trapped liquid is carried from the suction side to the discharge side of the pump chamber. As liquid leaves the suction chamber, the pressure in the suction chamber is lowered and additional liquid is pulled into the chamber from the reservoir.

The lobes are constructed so there is a continuous seal at the points where the two lobes meet at the center of the pump. The lobes of the pump illustrated are fitted with small vanes at the outer edge to improve the seal of the pump. Although these vanes are mechanically held in their slots, they are free to move outward. Centrifugal force keeps the vanes snug against the chamber and the other rotating members.

 

Vane Pumps

Vane-type hydraulic pumps generally have circularly or elliptically shaped interiors and fiat end plates. Figure 9 illustrates a vane pump with a circular interior. A slotted rotor is fixed to a shaft that enters the pump-housing cavity through one of its end plates. A number of small rectangular plates or vanes are set into the slots of the rotor. As the rotor turns, centrifugal force causes the outer edge of each vane to slide along the surface of the housing cavity.

The cavities formed by the vanes, the end plates, the housing, and the rotor enlarge and shrink as the rotor and vane assembly rotates. An inlet port is installed in the housing so fluid may flow into the cavities as they enlarge. An outlet port is provided to allow the fluid to flow out of the cavities, as they become small.

The pump shown in Figure 9 is referred to as an unbalanced pump because all of the pumping action takes place on one side of the rotor. This causes a side load on the rotor. Some vane pumps are constructed with an elliptical housing that forms two separate pumping areas on opposite sides of the rotor.

This cancels the side load. This type of pump is referred to as a balanced vane.Usually, vane pumps are fixed-displacement and pump only in one direction. There are some designs of vane pumps that provide variable flow. Vane pumps are generally restricted to service where pressure demand does not exceed 2,000 psi. Wear rates, vibration, and noise level increase rapidly in vane pumps as pressure demands exceed 2,000 psi.

RECIPROCATING PUMPS

The term reciprocating is defined as back-and-forth motion. In the reciprocating pump it is the back-and-forth motion of pistons inside of cylinders that provides the flow of fluid. Reciprocating pumps, like rotary pumps, operate on the positive principle: that is, each stroke delivers a definite volume of liquid to the system.

One major limitation of reciprocating pumps is the intermittent flow that they produce. The back-and-forth motion generates pulses of volume that create vibration and turbulent flow within the hydraulic system. These systems must include an accumulator downstream from the pump to dampen these pulses.

Hand Pumps

There are two types of manually operated reciprocating pumpsmthe single-action and the double-action. The single-action pump provides flow during every other stroke, while the double-action provides flow during each stroke. Single-action pumps are frequently used in hydraulic jacks.A double-action hand pump is illustrated in Figure 10. This type of pump is used in some applications as a source of emergency hydraulic power or for testing hydraulic systems.

This type of pump consists of a cylinder, a piston containing a built-in check valve (A), a piston rod, an operating handle, and a check valve (B) at the inlet port. When the piston is moved to the left, the force of the liquid in the outlet chamber and spring tension cause valve A to close. This movement causes the piston to force the liquid in the outlet chamber through the outlet port and into the system. This same piston movement causes a low-pressure area in the inlet chamber and the liquid, at atmospheric pressure, in the reservoir acting on check valve B causes its spring to compress. Thus, opening the check valve allows liquid to enter the inlet chamber.

When the piston completes this stroke to the left, the inlet chamber is full of liquid. This eliminates the pressure difference between the inlet chamber and the reservoir, thereby allowing spring tension to close check valve B. When the piston is moved to the right, the force of the confined liquid in the inlet chamber acts on check valve A. This action compresses the spring and opens the valve, allowing the liquid to flow from the intake chamber to the outlet chamber. Because of the area occupied by the piston rod, the outlet chamber cannot contain all the liquid discharged from the inlet chamber. Since liquids do not compress, the extra liquid is forced out of the outlet port and into the system.

Piston Pumps

All piston pumps operate on the principle that a piston reciprocating in a bore will draw in fluid as it is retracted and expel it on the forward stroke. Two basic designs are radial and axial, both are available as fixed- or variable-displacement models. A radial pump has the pistons arranged radially or at 90 degrees to the centerline of the drive shaft .

In an axial configuration , the pistons are parallel to each other and to the axis of the cylinder block. The latter may be further divided into in-line and bent axis types.A further distinction is made between pumps that provide a fixed delivery and those able to vary the flow of the hydraulic fluid. Variable-delivery pumps can be further divided into those able to pump fluid from zero to full delivery in one direction of flow and those able to pump from zero to full delivery in either direction.

Radial Piston Pumps

In a radial pump, the cylinder block rotates on a stationary pintle and inside a circular reaction ring or rotor. As the block rotates, centrifugal force, charging pressure, or some form of mechanical action causes the pistons to follow the inner surface of the ring, which is offset from the centerline of the cylinder block. As the pistons reciprocate in their bores, porting in the pintle permits them to take in fluid as they move outward and discharge it at a higher pressure as they move in.

The size and number of pistons and the length of their stroke determines pump displacement. In some models, moving the reaction ring to increase or decrease the piston travel length or stroke can vary the displacement. Figure 13 illustrates the operation of the radial piston pump. The pump consists of a stationary pintle that acts as a valve and a cylinder block, which revolves around the pintie. The cylinder block also contains the pistons; a rotor that houses the reaction ring against which the piston heads press; and a slide block that is used to control the length of piston stroke.

The slide block does not revolve, but houses and supports the rotor, which does revolve because of the friction set up by the sliding action between the piston heads and the reaction ring. The cylinder block is attached to the drive shaft.Referring to view A of Figure 13, assume that space X in one of the cylinders of the cylinder block contains fluid and that the respective piston of this cylinder is at position 1. When the cylinder block and piston are rotated in a clockwise direction, the piston is forced into its cylinder as it approaches position 2. This action reduces the volumetric size of the cylinder and forces a quantity of fluid out of the cylinder and into the outlet port above the pintle. This pumping action is due to the rotor being offcenter in relation to the center of the cylinder block.

In Figure 13, view B, the piston has reached position 2 and has forced the fluid out of the open end of the cylinder, through the outlet above the pintle and into the system. While the piston moves from position 2 to position 3, the open end of the cylinder passes over the solid part of the pintle; therefore, there is no intake or discharge of fluid. As the piston and cylinder move from position 3 to position 4, centrifugal force causes the piston to move outward against the reaction ring of the rotor.

During this time the open end of the cylinder is open to the intake side of the pintle and fills with fluid. As the piston moves from position 4 to position 1, the open end of the cylinder is against the solid side of the pintle and no intake or discharge of fluid takes place.After the piston has passed the pintle and starts toward position 2, another discharge of fluid takes place. Alternate intake and discharge continues as the rotor revolves about its axisnintake on one side of the pintle and discharge on the other.

Notice in views A and B of Figure 13 that the center point of the rotor is different from the center point of the cylinder block. The difference of these centers produces the pumping action. If the rotor is moved so that its center point is the same as that of the cylinder block, as shown in Figure 13, view C, there is no pumping action. Since the piston does not move back and forth in the cylinders as it rotates within the cylinder block, no pumping can take place.The flow in this pump can be reversed by moving the slide block and rotor to the right so the relation of the centers of the rotor and cylinder block is reversed from the position shown in views A and B of Figure 13. View D shows this arrangement. Fluid enters the cylinder as the piston travels from position 1 to position 2 and is discharged from the cylinder as the piston travels from position 3 to position 4.

In the illustrations, the rotor is shown in the center, the extreme right, or the extreme left as related to the cylinder block. The amount of adjustment in distance between the two centers determines the length of the piston stroke and the amount of fluid flow in and out of the cylinder. Thus, the adjustment determines the displacement of the pump. This adjustment may be controlled in different ways.Manual control by a handwheel is the simplest. The pump illustrated in Figure 13 is controlled in this way. For automatic control of delivery to accommodate varying volume requirements during the operating cycle, a hydraulically controlled cylinder may be used to position the slide block. A gear motor controlled by a push button or a limit switch is sometimes used for this purpose.

 

Swash Plate Design Pumps

In axial piston pumps, the cylinder block and drive shaft are on the same centerline and the pistons reciprocate parallel to the drive shaft. The simplest type of axial piston pump is the swash plate inline design (Figure 15).

The cylinder blow in this pump is turned by the drive shaft. Pistons fitted to bores in the cylinder are connected through piston shoes and a retracting ring, so that the shoes bear against an angled swash plate. As the block turns (Figure 3-16), the piston shoes follow the swash plate, causing the pistons to reciprocate. The ports are arranged in the valve plate so that the pistons pass the inlet as they are being pulled out and pass the outlet as they are being forced forward.

In these pumps the size and number of pistons as well as their stroke length also determine the displacement. The stroke length is controlled by the swash plate angle. In variable-displacement models, the swash plate is installed in a movable yoke (Figure17). By pivoting the yoke on pintles, the swash plate angle and piston stroke can be increased or decreased. Figure 17 shows a compensator control, but the angle can also be controlled manually or by a variety of other means.

Operation of the inline compensator-controlled pump is shown schematically in Figure 17. The control consists of a compensator valve balanced between load pressure and the force of a spring, a piston controlled by the valve to move the yoke, and a yoke return spring. With no outlet pressure, the yoke return spring moves the yoke to the full-delivery position. As pressure builds, it acts against the end of the valve spool. When the pressure is high enough to overcome the valve spring, the spool is displaced and oil enters the yoke piston. The piston is forced by the oil under pressure to decrease the pump displacement.

If the pressure falls off, the spool automatically moves back, oil is discharged from the piston to the inside of the pump casing, and the spring returns the yoke to a greater angle.The compensator adjusts the pump outlet to whatever displacement is required to develop and maintain the preset pressure. This prevents excess power loss by avoiding relief valve operation at full pump volume in holding and clamping applications.

 

Wobble Plate Inline Pumps

A variation of the inline piston design is the wobble plate pump. In a wobble plate design, the cylinder is stationary and the canted plate is tumed by the drive shaft. As the plate tums, it wobbles and pushes against spring-loaded pistons to force them to reciprocate. Separate inlet and outlet check valves are required because the cylinders do not move past the ports.

Bent Axis Pumps

In a bent axis piston pump (Figure 18), the cylinder block turns with the drive shaft,but at an offset angle. The piston rods are attached to the drive shaft flange by ball joints and are forced in and out of their bores as the distance between the drive shaft flange and cylinder block changes (Figure 19). A universal link keys the cylinder block to the drive shaft to maintain alignment and ensure that they turn together. Except to accelerate and decelerate the cylinder block and to overcome resistance of the oil-filled housing, the link does not transmit force.

The displacement of this type of pump varies with the offset angle (Figure 20), the maximum angle being 30 degrees and the minimum zero. Fixed-displacement models (Figure21) are usually available with 23-degree or 30-degree angles. In the variable-displacement construction (Figure22), a yoke with an external control is used to change the angle. With some controls, the yoke can be moved over center to reverse the direction of flow.

Various methods are used to control the displacement of bent-axis pumps. Typical controls are the handwheel, pressure compensator and servo. Figure 23 shows a pressure compensator control for a bent-axis pump. In view A, the system pressure is sufficient to overcome the spring force of the compensator. As a result, the spool lifts,allowing fluid to flow into the stroking cylinder.

Although the holding cylinder also has system pressure applied, the area of the stroking cylinder piston is much greater.Because of the differential pressure, the yoke is forced up to decrease flow. View B shows the yoke moving down as system pressure drops below that required to overcome the compensator spring force.

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