Types of hydraulic pump

There are many ways to categorize pumps. Some words used to refer to hydraulic pumps include hydrodynamic, positive-displacement, fixed-delivery, variable-output and constant-flow among others.

Positive displacement pumps give a definite amount of fluid for each cycle of the pump operation; if this can be done without exceeding the capacity of the power unit running it irrespective of what happens in terms of resistance.

If a positive displacement pump’s outlet were completely closed off, the pressure would suddenly increase to that point where either a pump driver would stall or some element in the drive-train would break.

The classification of positive-displacement pumps can also be categorized into other classifications such as fixed-delivery or constant-volume. For instance, fixed-delivery constant delivery and constant volume are alternative descriptors for this kind of pump.

A fixed-displacement pump delivers the same quantity every time it goes through its cycle. The only way to modify this is by altering the speed at which it is working. When using such a pump within a hydraulic system, there must be a pressure regulator or relief valve installed in the circuit.

Displacement per cycle in hydraulic pumps is altered throughout internal control devices found in any variable-displacement classification. Such devices may take various forms from unloading and pressure regulating valves to restricted flow bypass loops. Some examples will be described later under control valves

Hydraulic liquids’ flow designs determine their rating as per design type . Almost all hydraulic fluids have three major design groups namely centrifugal , rotary ,and reciprocating . Hydraulic systems have very limited applications for centrifugalpumps.

Non-positive-displacement centrifugal designs like hydrodynamic or turbines are usually employed especially in cases where only weight friction constitute resistance against fluid transfer.

Most non-positive – displacement pumps (Fig 1) operate on the principle of centrifugal force where fluids entering around the center ofthe housing are thrown outwards by a rapidly rotating impeller . In fact there is no sealing between inlet and outlet ports, and their pressure capabilities are regulated through rotation speed.

Hydrodynamic pumps cause the flow of water to go down as resistance increases. Even when a pump is running, it can be completely blocked or deadheaded at its outlet. For this reason among others, non-positivedisplacement pumps are rarely used in hydraulic systems.

Positive displacement hydrostatic pumps give out certain quantities of fluid for every stroke, revolution or cycle. No matter the leakage these outputs do not depend on the backpressure from the system. This makes them ideal for power transmission purposes.

ROTARY PUMPS

All rotary pumps have rotating elements that trap the fluid from the inlet port and move it forward to the discharge port in the system. The most commonly used devices include gears, screws, lobes, and vanes for transferring fluid within a pump. Rotary pumps can be classified as positive fixed displacement type.

To avoid slippage from the discharge side back to the suction side of the pump, rotary pumps are made with very small clearances between their rotating and stationary parts. Usually they are designed to operate at speeds no higher than 1800 rpm. Operation at higher speeds can cause erosion and excessive wear.There are many types of rotary pumps and different ways of classifying them.

Shaft position, driver type, manufacturer’s name or service application can be some criteria for classification. Although classification of these machines is often done by looking at what type of rotating element they possess. Some popular examples among many others are as follows.

 

Gear Pumps

Carrying fluid between the teeth of two meshed gears, a gear pump develops flow. One of these gears is driven by the drive shaft and in turn drives another one. The housing of the pump and its side plates enclose pumping chambers formed between the gear teeth.

At the inlet, as the gear teeth unmesh, there is created partial vacuum. This space gets filled with fluid as it comes from outside to be carried along outside of gears. As soon as these teeth mesh again at outlet,fluid is pushed out. The high pressure at which fluid leaves this pump exerts an unbalanced load on their gears and bearing support system.

Gear pumps are classified into external and internal types.Their centres have got the projecting both toothed extemal gear pumps.Extemal gear pumps may use spur, herringbone or helical gear sets for transferring fluid .

External Gear Pumps

The pumping chambers in this design are also made between the gear teeth. The pump body is machined with a crescent seal which is located between inlet and outlet where there is maximum clearance between the teeth. Additionally, another example of gear pumps that belongs to this general family includes lobe or rotor pumps – It has a higher displacement but works on the same principle as external gears.

Spur Gear Pumps

The figure presents the spur gear pump, which consists of two gears that revolve in a housing. As shown in the illustration, the drive gear turns via a drive shaft attached to the engine. These clearances are very slight between teeth of gears as they engage and between teeth and pump housing.

the driven gear is turning clockwise and the driving one counter clock wise. The liquid fills up between teeth and housing each time their tips pass by inlet port. Then it flows around the housing towards outlet port. Upon next meshing of teeth, this fluid pushes back from between them into outlet port. Such action creates a positive flow of liquid through system. A shear pin or shear section is included on the drive shaft. It does protect power source or reduction gears against being overloaded when pump fails due to excessive load or jams on parts

Herringbone Gear Pumps

A herringbone gear pump is offshoots of the spur gear.The modes of liquid pumping are, nonetheless, the same as in a spur gear pump. However, in the case of a herringbone pump, at any given time one set of teeth will already start its fluid discharge phase before another set has completed its discharge phase. This is done to reduce pulsations caused by overlapping and larger space in between gears’ centers as compared to those found on a straight toothed gear type.

Helical Gear Pumps

The helical gear pumps also differ from the spur gears.Therefore, there is even more overlap of successive discharge’s spaces between teeth than in the herringbone design. Therefore, it makes for a smoother flow of discharge.

This allows the number of gear teeth to be lower and tooth size to increase with no loss in smoothness of operation, there by increasing capacity.

In this type of pump, gear sets are driven by timing gears that keep mating gears within close tolerances without actually rubbing against each other. If there were metallic contact between teeth, it would seal hydraulics more tightly but volume would decrease and wear rate on teeth would also increase enormously. Radial clearance and alignment are maintained by anti-friction bearings at both ends of the gear shafts while friction loss in transmitting power is minimized. Leakage around the shaft is avoided using suitable packing.

Internal Gear Pumps

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 either be centered or off-centered. Two types of internal gear pumps are depicted in Fig 1 (a) and (b). Figure (a) is for a centered pump called pump A, while figure (b) represents an off-centered one namely Pump B.

Off-Centered Internal Gear Pumps

In this kind of pump, the drive gear is connected directly to the pump’s drive shaft and positioned off-center within the inner gear. Between the ports of suction and discharge, the gears mesh on one side of the pump. The chamber on the other hand houses a crescent shaped component which fits closely between these two gears.

The outside gear is rotated by center gear rotation as both are engaged. In that chamber, everything rotates except for the crescent that does not. This results in liquid being trapped between gear teeth as they pass through each other while rotating. The sucked fluid moves from suction port via pump internal volume passing discharging space, created by interlocked teeth of these gears in contact with liquid now driven out from tool under operation pressure. Pump’s displacement is determined by size of its crescent which separates internal and external gears: a small hole allows greater flows per revolution than larger one.

Centered Internal Gear pump

The other design of internal gear pumps may be seen in Fig.6 and 7. These include a pair of gears which have teeth that interlock with one another. The inner gear wheel is attached to the drive shaft of the prime mover.This type of internal gear pump is shown in Fig.7 during its operation. In order to explain it in more simple manner, teeth of the inside gear and spaces between teeth on the outside are numbered. It should be noted that there is one tooth less than for the outer gear.

The tooth forms of two gears are such that every tooth on the inside gear will always contact slidingly on the outside surface of the second gear. During a single revolution, each tooth on an inner gear meshes with an outer one only at one point (Figure (X)).In view A, tooth 1 from inside meshed into space 1 from outside. Consequently, as the gears continue rotating clockwise towards point X, space 6 will accommodate with tooth 5 while space 7 shall receive tooth 6.Otherwise put, during this rotation toothing 1 will embrace/slide into space 2; after another revolution it will change to number three.Thus, angular velocity ratio between these two meshes equals six over seven.

As these turn around mesh points at their sides large pockets form while small pockets form at their other sides.On Figure 7, pockets drawn on right-side get larger towards bottom while left side becomes smaller towards top.The right-hand side becomes intake whereas discharge is at the left-hand side.In Figure 7 since ports were shown by turning right hand side of drawing,intake and discharge seems to be wrong position.Actually A in one drawing covers A in another.

Lobe Pumps

The same principles of operation like the external gear pump are used by the lobe pump. The lobes tend to be much bigger than the teeth of a gear but there is only two or three per rotor. The diagram below shows a three-lobed pump. One element is directly driven by the power source while other one goes through timing gears. When the elements rotate, they cause liquid to become trapped between the two lobes of each rotor and also against the walls of the pump chamber. This trapped liquid is then moved from suction side to discharge side of this pump chamber. As liquid leaves the suction chamber, its pressure becomes reduced which causes more liquid to be drawn into that chamber from reservoir.

There is always a continuous seal at center point where two lobes meet for these lobes in construction design. In order to improve sealing capacity of this pump each lobe shown on drawing above has small vanes located next outer edges. Although mechanically held within their slots, these blades are free to slide outwards. These vanes are pressed tightly against rotating members as well as chamber due to centrifugal force acting on them from rotation.

 

Vane Pumps

Their interiors are usually circular or elliptical and with fiat end plates. The vane pump illustrated in fig 9 is one of the circular interior type. The pump housing cavity has a slotted rotor that is fixed to a shaft entering via one of its end plates. Numerous small rectangular vanes or plates are inserted into slots on the rotor. During rotation of the rotor, centrifugal force causes each vane to slide along the surface of housing cavity from outer edge.

The cavities which are formed by vanes, end plates, housing and rotors expand or contract as rotors and vane assemblies rotate. One of its functions is that an inlet port through it allows fluid flow into the cavities when they enlarge. When they become small an outlet port facilitates fluid exit from these cavities.

This however tends to exert a side load on the rotor and this makes such pumps be refuted as unbalanced pumps as shown in figure 9.This cancels out any side load, making it balanced vanes in this type of pump.Name five types vane pumps.Vane pumps have significant limitations because pressure limits at which they can work do not exceed psi.Wear rates, noise level and vibration increase more quickly due to higher pressure demands exceeding 2000 psi in Vane pumps.

RECIPROCATING PUMPS

Reciprocating as a term means moving back and forth. In the case of reciprocating pump, it is the movement of pistons inside cylinders that causes the fluid flowing in the system. Reciprocating pumps like rotary pumps work on the principle of positivity that is; each stroke discharges a given quantity of liquid through the system.

A major drawback of reciprocating pumps is their intermittent flow output. Back and forth motion produces volumes with pulsations which lead to vibration and turbulent flow within hydraulic systems. These must have an accumulator downstream to lessen this impact on them by these pulses.

Hand Pumps

There are two types of hand operated reciprocating pumps – single-action and double-action. The single-action pump gives flow in every other stroke while the double-action reciprocates at each stroke. Hydraulic jacks mostly use single-action pumps.A diagram of a double-action hand pump is shown in Figure 10. In some instances, such a pump used as an emergency hydraulic power source or for testing hydraulic systems.

This sort of pump comprises; a cylinder, a piston with built-in check valve (A), a piston rod, an operating handle and an inlet port check valve (B). When the liquid moves the piston to the left, liquid pressure in the outlet chamber and spring tension closes valve A. The liquid forces in this chamber drive it through the outlet port and into the system during this movement caused by pistons motion. This same movement of pistons creates low pressure area within the inlet chamber; consequently, its spring compresses under effect from atmospheric pressure liquids in reservoir actin on B check valve thereby opening it to allow entry onto chamber.

Upon completion of this stroke of piston towards left side, there is full liquid content inside inlet chamber. Thereby allowing spring tension to close B check valve due to lack of pressure difference between inlet chamber and reservoir caused by that. When moving rightwards, confined fluid force together with that acting on A check value opens it by compressing springs.Accordingly valves open so that fluid flows from intake to outlet chambers.This is because some part of fluid discharged out from downstream cannot be accommodated inside upstream owing to presence of piston rod.As liquids do not compress, further amount overflows via exit port into system.

Piston Pumps

Every piston pump functions on the basis that when a piston reciprocates inside a bore, it takes in fluid as it is drawn back and discharges it upon the forward stroke. There are two main kinds: radial and axial, each of which can come in fixed- or variable-displacement models. A radial pump has its pistons arranged in a radial direction or at 90 degrees to the center line of the driving shaft.

By an axial design , the pistons lies parallel to one another and the cylinder block axis (this may be further subdivided into inline & bent axis types). Another distinction is between pumps with fixed delivery rates and those that allow for variation of hydraulic fluid flow rates. Variable-delivery pumps, in turn, are divided into the ones which are able to pump fluid from zero to full delivery capacity in one direction of flow and those capable of pumping from zero to full output capacity in either direction.

Radial Piston Pumps

A radial pump has a cylinder block that rotates on a stationary pintle, with an interior annular reaction ring or rotor. While rotating, centrifugal force or charging pressure might cause the pistons to follow the inside surface of the ring which is off center from the axis of rotation of the cylinder block. As such, when these pistons move in their bores they can take in fluid as they pull outward and discharge it at an elevated pressure as they push in.

Pump displacement is determined by the size and number of pistons and their stroke length. On some models, displacements can be varied by adjusting piston travel length or stroke through moving reaction rings. The operation of a radial piston pump is shown in Figure 13. It includes a fixed pintle as a valve, and a cylinder block rotating around it. Besides having pistons located therein, this cylinder block also consists of: a rotor with its reaction ring for contacting heads of pistons; as well as slid blocks for controlling piston stroke length.

The block of the slide does not revolve but it revolves inside and supports the rotor through the friction that is set up from the sliding movement between the piston heads and reaction ring. The cylinder block is linked to a driveshaft.Referencing Figure 13 view A, let us suppose that space X in one of the cylinders of the cylinder block is filled with fluid and that this particular cylinder’s piston was in position 1. As both piston and cylinder block are rotated clockwise, when approaching 2nd position, it forces piston into its cylinder reducing volume size of this latter. This action reduces volumetric size of the cylinder and forces out some quantity of fluid out into outlet port above pintle. This pumping action is due to offcenter rotor relative to center of cylinder block.

In Figure 13 view B, at position ‘2’ the piston has forced fluid out through open end of this cylinder via an outlet above pintle into system; during its movement from position ‘2’ to ‘3,’ open end is moving over solid part of pintle so there is no flow either into or out of these ends as there can be no intake or discharge when open end passes over solid side.As centrifugal force moves them from positions ‘3’ to ‘4’, pistons move away from center into external surfaces against reaction rings on rotors.

At this time any fluids fill that are available in pintle enters through open end.The open end falls against solid face of pintle while piston travels between positions ’4′and ’1′ without any backflow thereby preventing any inflow or our flow.Whereas another discharge takes place after passing through pintel by a piston going towards position two.Fluid gets sucked in on one side while blood moves out on other side as rotor turns around its axisnintake.

One should note that in views A and B of figure13, center point for rotor differs from centre point for cylinder block. Pumping action in question arises from difference between these centers. When rotor is shifted so as to have the same center point with that of cylinder block, there would be no pumping action shown in figure13 view C. No back and forth motion of pistons occurs within cylinders thus such a pump fails to work.As illustrated by views A and B on figure 13, this can be done by reversing the position of the slide block and rotor to flow which are reversed.

The fluid moves into the cylinder as its piston moves from position ‘1’ to position ‘2’, which will be, in turn, discharged as it travels from location3′ through 4′.

For the illustrations, rotor is shown at center, far right or far left with respect to cylinder block. The degree of adjustment in distance between these two centers determines the length of piston stroke and amount of fluid flow into and out from the cylinder. By so doing, this adjustment defines the displacement of the pump. There could be different modes for controlling this adjustment.A handwheel can control it directly and manually which is simplest. Such is how the pump in figure thirteen operates.For example, during operation cycle when delivery has to adjust automatically to suit different volume requirements, a slide block may be located by hydraulically controlled cylinder. Sometimes this purpose is met through use of gear motor that is push button or limit switch controlled.

 

Swash Plate Design Pumps

The cylinder block and drive shaft of axial piston pumps are aligned axially and the pistons move in parallel with the drive shaft. The simplest of these types is the swash-plate inline pump, as shown in Figure 15.

The cylinder ring on this pump is rotated by the motor. Pistons put in bores within the cylinder are linked to one another by means of a retracting ring and piston shoes, which bear against an inclined swash plate. As can be seen from Figure 3-16, when tilted rotation takes place then piston shoes trail after it leading to reciprocating action being done by pistons . Accordingly, the valve plate is so ported that as soon as they pass the inlet while being drawn outwards or outlet during their forced motion outwards.

The amount of fluid pumped in this type of pump depends on the size and number of pistons and stroke length. The angle swash plate determines the latter. In models that have variable displacements, a movable yoke holds the swash plate (Fig 17). The pivot point is established by pintles on different sides such that an increase or decrease in piston stroke makes it possible to alter the angle of the swash plate. Figure 17 shows a compensator control, but it could be manually adjusted or through any other means.

This diagram illustrates how an inline compensator-controlled pump works (Figure 17). The valve is made up of a compensator which is balanced against a load pressure as well as spring force, while the yoke return spring moves a piston operated through the valve. When there is no outlet pressure, the yoke return spring pushes back to “full delivery” position. As pressure builds up, its effect is felt at end spool of the valve. When pressure becomes sufficiently high over valve spring resistance, it will displace spool thereby admitting oil into yoke piston. Pressure from oil forces pump displacement down through reducing its volume would cause that movement for any pumping machine subjected to fluid power.

With the pressure decrease, the spool retreats automatically, pushes off oil from the piston into the pump casing and spring retracts yoke to more steep angle. The compensator regulates pump outlet to any displacement it should be to keep up with and maintain a predetermined condition of pressure. It helps prevent an excessive loss of power by eliminating relief valve operations at full pump volume during hold and clamp applications.

 

Wobble Plate Inline Pumps.

Another kind of this type of pump is the wobble plate pump. In this system, the plunger remains stationary in a cylinder while another canted plate is rotated by a drive shaft. As it rotates, it vibrates and hence pounds on springs to force pistons back and forth. For the cylinders not to pass beyond the ports, separate inlet and outlet check valves are required.

Bent Axis Pumps

In this type of piston pump, which has an offset angle between its drive shaft and cylinder block (Figure 18), the cylinder block itself rotates along with the drive shaft. The piston rods are attached to flange on drive shaft by ball joints such that whenever there is a change in distance between flange on drive shaft and cylinder block (Figure 19), they will be forced in or out of their bores. There exists a universal link that connects the cylinder block to the driving-shaft so as to maintain alignment and make them turn together when necessary. This link does not transmit any force except for accelerating/decelerating movement of the cylinder block against resistance from oil filled housing.

The displacement of this kind of pump changes with the angle of deflection (Figure 20), ranging from zero to 30 degrees. The fixed displacement types are available in the market either with an offset angle of 23 degree or 30 degree (Figure21). In variable displacement construction, a yoke externally controlled is utilized to change its angle as shown in Figure22. This implies that if the yoke is moved over center using some controls, then the flow direction will be reversed.

To control the displacement of bent-axis pumps, one can use various techniques. 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 enough to overcome the spring force of the compensator. That causes fluid to enter into the stroking cylinder as spool lifts up.

The stroking cylinder piston has a much larger area unlike the holding cylinder which also has system pressure. The differential pressure, however, forces the yoke upward to decrease flow and reduce any chances of blow off View B shows that system pressure is reducing below that necessary to overcome compensator spring force as the yoke descends.

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