Understanding the Hydraulic Piston Pump: Types, Applications, and ...

Hydraulic piston pumps are a critical component in various industries, playing a pivotal role in the efficient transmission of fluid power. These pumps are utilized in a wide range of applications, including construction machinery, manufacturing equipment, and aerospace systems, due to their ability to convert mechanical energy into hydraulic energy. Understanding the various types of hydraulic piston pumps, their specific applications, and their efficiency is essential for professionals looking to optimize system performance and maintenance. This blog aims to provide an in-depth exploration of the different types of hydraulic piston pumps, examine their key functions, and discuss strategies to maximize their efficiency, ensuring readers understand this vital technology comprehensively.

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What is a Hydraulic Piston Pump?

How does a hydraulic piston pump work?

Within hydraulic piston pumps, the elastic liquid, which is in the form of oil, is sucked into the cylinder chamber and ejected under field conditions by an arrangement of pistons. Such processes occur in constant cycles and are usually accomplished using a revolving cam or a swash plate attached to a reciprocating piston block. In other words, fluid is ejected from the chamber and delivered to the hydraulic system for work when the pistons aadvance When the pistons are pulled back and retracted, more liquid can then be in drawn, forming a pressure.

Main components of piston pump

The main components in the configuration of Piston displacement hydraulic pump comprise of the importance of the others. For example, the cylinder block is the place where the pumps’ pistons, which can retract, allows hydraulic fluid to be moved. Energie-J344F pumppc mp4 also takes part antagonistically in energy transfer among major energy modifying devices of the fluid system. Swash plates or cam arrangements convert rotary drive motion into reciprocating grilling motion to effect induction of the pistons. Besides that, the valve plate is extremely important in controlling the direction in which the fluid will move in and out of the pump. Finally, the rotary shaft is the part that is connected to the driving force and the whole operation begins with the rotation of either the swash plate or the cam by the driving force. The clear comprehension of above constituents is associated with the ability of diagnosis performance and internal hydraulic piston pumps.

Difference between fixed displacement piston pump and variable displacement piston pump

The output of fixed displacement piston pumps doesn’t change and is kept constant in terms of the number of fluids moved in each cycle and internal or external pressure or speed. Because they are easy to use, dependable and low cost, they are the best choice in cases where such flow conditions are required. On the other hand, the displacement piston pumps of variable types facilitate the flow rate modification by altering certain components, such as the swash plate’s stroke angle or stroke length. This flexibility allows the pump to modulate the flowing amount to suit the system needs, thus improving flow efficiency and system performance in variable loads operability. It is thus most common to find these variable displacement pumps being utilized in systems that require and control hydraulic flow and pressure to great levels.

How to Choose the Right Piston Pump for Your Application?

Considerations in the selection of hydraulic pumps

Before settling on a hydraulic pump for your application, some of the crucial factors you may want to look at include:

• Pressure and Flow Requirements: It is necessary to analyze the pressure and the flow rate needed for your particular application in order to ascertain the performance capability of the pump you are choosing.
• Efficiency: Consider the pump’s efficiency as a whole, mainly the power-related and maintenance costs. Look for such units which have an adjustable displacement feature especially if high energy efficiency is needed and wastage is to be minimized.
• Installation and Maintenance: Analyze the labor required to fit the pump and the expected maintenance strain after installation. Choose designs that do not require specialized service or a lot of equipment to avoid production halts and schedule runs.

These factors will ultimately ensure you choose a hydraulic pump that is appropriate for the task at hand and viable within your working conditions and budget.

What are some considerations affected by flow in choosing a piston pump?

In deciding if a piston pump is suited for a specific application, flow rate is an important consideration that holds quite a lot of weight. This is because it represents the pace at which the fluid will be moved by the pump which is essential to ensure that the system is functional and is delivering on its efficiency targets within the required timeframe. In a case where high accuracy is required from a particular system, a pump which is able of adjusting its flow rate such as the variable displacement piston pump is preferred since it can be adjusted to the needs of the system in real time therefore avoiding wastage of energy and reducing component wear. At the same time, stable—if constant flow rates are maintained—system pressure is also ensured, which is beneficial for the hydraulic systems in relation to their reliability and longevity. Hence when acquiring a piston pump, it is imperative to consider how the flow capacity of the pump relates to your operational requirements as this influences the performance and efficiency of the unit.

Pressure as a guiding criterion in the selection of a hydraulic piston pump

The selection of a hydraulic pump teststroke pressure plays an important role as it constitutes how efficient the pump will be in various operating conditions. Also, it is important to note that the working pressure rating of a pump will determine the structural parameters of the designed pumpver high pressure systems are reviewed, they most often call for the use of piston pumps which are quite thermodynamically costly but very functional. Also, the manufacturers prescribe the operational limits that a pump which works within the pressure limitations prescribed will serve the users for a longer duration of time with minimal repairs and other replacements. High-pressure pumps are technologies that are common in that various systems are internal combustion engines and so usage in industries is often under the cultures commonly provides excessive policies of regulators thus wasting energy and millions’ worth of machinery. It is worth adding that there should be available a certain power head pump, the absolute value of which is equal or higher than power head losses within dropper system and required power is only justified in filling all these additional values.

How do Axial Piston Pumps Work?

A XTRA designs of axial piston pumps

As for the axial piston pumps, they construct them with piston series located circumferentially in a cylinder block. The operating shaft is a major element of this construction which rotates and makes the piston shoes to contact a swashplate at an angle. The pistons move along their axial directions as the swashplate angle changes and this facilitates taking in fluid into the pump for pumping in the downward stroke or exerting out fluid in the upper stroke. This configuration enables the automatic pumps to efficiently convert mechanical energy into hydraulic energy. The swashplate mechanism, since the displacement of the pumps is dependent on the swashplate, makes it further possible to regulate the quantity of the fluid pumped out by the angle of the swash plate as well as the output of the hydraulic system. So this is one of the key factors why axial piston pumps are very helpful in applications, where the flow and pressure need to be changed. The omitted pipe also reduces the working pump’s energy consumption, which improves the worked quality of the New: range of sleeve pump.

How does the swash plate mechanism influence an axial piston pump’s performance?

The swash plate mechanism considers the basal performance parameters of an axial piston pump. Variation in the position of the swash plate allows the operators to alter the displacement of the pump directly and as such the pressure as well as the flow of the hydraulic fluid. A larger angle will further extend the stroke of the pistons which in turn displaces more fluid and so the output becomes greater. A lower angle on the other hand increases flow controllability by lowering the displacement and therefore the flow rate which is good for working in slow precision applications. This adjustability only improves the ppump’s performancethrough matching the output with the rsystem’s requirements but also helps reducethe excess hydraulic pressure for energy conservation purposes. Also, the good is because it helps operate the hydraulic pump under varied and rapid pulse of pressure and flow, which would have otherwise been difficult with other pumps.

Benefits Of Using Piston Pumps In Hydraulic Systems

There are numerous advantages for the applicability of axial piston pumps in hydraulic systems. To begin with, their high efficiency is attributed to the ability to vary the amount of power and flow required in a system, which in turn leads to decreased energy and cost in running the system. Second, they exhibit good and long wear resistance in that many of them are made with high quality and established high pressure and prolonged usage materials. Also, their small size makes it possible to use them in the constrained space within the operational system. This design is suitable for many applications, starting with industrial machines and ending with mobile units. Finally, the ability to give a variable flow rate and accurate control improves efficiency and versatility in meeting various operational aspects.

What are the Applications of Hydraulic Piston Pumps?

Hydraulic Piston Pump application in various industrial sectors

Among hydraulic components, hydraulic piston pumps rank quite high in use in many industries, primarily due to the fact that they are efficient and can be adapted in varied ways. In construction, for instance, these pumps are used in high powered machines like diggers and loaders where they provide hydraulic energy to lift, dig or move the load. In industry, for instance, the hydraulic piston pumps are very useful devices in running machines like industrial presses and industrial robots that can undertake delicate tasks at a large scale with the use of power. They are also employed in agriculture in devices such as tractors and harvesters helping in effective functions like ploughing and watering crops where variable loads are required.

Applications of hydraulic piston pumps in mobile equipment

Mobile equipment is very much reliant on hydraulic piston pumps as they provide accuracy, strength, and versatility to the machines. They are also used in trucks and buses where they assist in steering and braked hydraulic systems in cars and other vehicles having them. Heavy trucks, winders, loaders, drill rigs, long wall faces, shuttle cars, and many more mining equipment are powered by these pumps for all hydraulically driven operations such as drilling, cutting, and transporting materials etc. Another field that uses hydraulic piston pumps as components is the aircraft where power is needed to control the flight control system, landing gears, and other vital parts while standing high temperatures bland to pressure extremes which allow for safe and reliable operations.

Why are hydraulic piston pumps effective to use within a hydraulic system?

Hydraulic systems incorporating hydraulic piston pumps are more efficient since they are able to accommodate and output high levels of power while using little energy. These pumps are built with a capability of working with high pressure which means they can produce high force and at the same time offer small space requirements. Through precise manufacturing processes and engineering capabilities, these pumps are able to produce hydraulic fluid efficiently and continuously thereby reducing the pulses and people downtime enhancing operation efficiency. Another advantage is that hydraulic piston pumps are often also made with variable displacement pumps allowing design flow and design pressure to be reached only as required thus also saving energy in the process and minimizing wastage. These features do not only help improve the hydraulic system operation but also help in making them more energy efficient and less harmful to the environment.

How to Ensure the Reliability of Your Hydraulic Piston Pump?

Guidelines concerning the maintenance of hydraulic pumps

Like any other equipment hydraulic piston pumps must be taken care of regularly in order to ensure they remain reliable. The specific types of maintenance that can be undertaken include the following: First, contaminants must be kept out of the hydraulic fluid by ensuring that the fluid is clean, well-kept, and changed regularly depending on the working hours to provide maximum efficiency, and using the correct grade of fluid when this becomes an absolute necessity. Second, check the bushes seals and any other components that may impact the pump adversely and perform under optimum efficiency. Third, the pressure of running a system and the temperature should be checked since abnormally high or low levels of either pressure or temperature can be brought about by wearing out of certain parts or even imbalance in the system. Finally, sealing and end-capping and other components and their, junctions should fit correctly and be free from dirt and other materials so as not to compromise the general system and tidy any pump. Compliance with these high expectations maintenance measures will result in effective running of the operations and a decrease in breakdowns machine time.

What is the significance of proper design in hydraulic systems reliability?

Correct design is fundamental to the reliability of hydraulic systems, as it ensures that components are adequate in size and properly fitted to the system, thus reducing the chances of failure. Robust guarantee also takes care of the selection of materials and components that can withstand the application’s operational and environmental conditions. Further, in a properly designed hydraulic system, the hoses and pipes will be routed so as to minimize pressure loss and avoid leaks and contamination. The systematic layout of the system knows that the filtration and cooling devices are effective in lowering the degree of wear and tear, prolonging the wear-out period of the components, and improving the system’s overall performance. Although the design of a hydraulic system makes such plans only in emergencies, by that time it will be impossible to make any changes in the degree of hydraulic reliability.

Reference sources

1. Piston Pump in Industrial Applications: An In-Depth Analysis – This source provides detailed insights into the efficiency and applications of hydraulic piston pumps in industrial settings.
2. Experimental Test and Feasibility Analysis of Hydraulic Systems – This paper discusses methods for position control in hydraulic systems, offering a technical perspective on their feasibility.
3. Hydraulic Pumps Market Size, Industry Share, Forecast  – This report covers the global market analysis of hydraulic pumps, providing industry trends and forecasts.

These sources should provide a comprehensive view of the feasibility and application of hydraulic piston pumps.

Frequently Asked Questions (FAQs)

Q: What is a piston pump hydraulic in the simplest terms and how does it connect with other hydraulic products?

A: The piston pump hydraulic is a form of a hydraulic pump which has pistons to pump the fluid. Hydraulic Piston pump is the most efficient and reliable in nature, making it the leader in the hydraulics series variable displacement products. Other related products include gear pumps and vane pumps which do more or less the same functions in the hydraulic system of the pumps.

Q: Distinguish the essential characteristics of the piston pump hydraulic？

A: Piston pump hydraulic’s principal components include high pressure generation capability (420 psi maximum), and variable displacement or compact size construction. In addition, it also works effectively regardless of the application. Preferred by many industries’ operating conditions.

Q: Does the piston pump hydraulic work in a closed circuit system?

A: Yes, the use of the piston pump hydraulic is doable in closed circuit systems. The snacks are therefore very efficient in such areas especially in conjunction with other hydraulic pieces of equipment.

Q: What is the maximum RPM for a piston pump hydraulic?

A: The maximum RPM for a piston pump hydraulic may differ for each model, although many do well at up to rpm. However, it is reasonable to check the specifications for the particular product you are interested in.

Q: Are there custom options available for piston pump hydraulics?

A: Of course, there are custom options for piston pump hydraulics. According to your purpose you can choose specific design and or features appropriate for you.

Q: How does the gas and oil selection affect operation of a piston pump hydraulic?

A: Gas and oil selection is very important for the performance of a piston pump hydraulic. Choosing the wrong kind of oil may hamper effective lubrication and increase wear while an incorrect gas pressure may contribute to effective functioning.

*Editor's Note: This article was updated in February .

When a hydraulic pump operates, it performs two functions. First, its mechanical action creates a vacuum at the pump inlet which allows atmospheric pressure to force liquid from the reservoir into the inlet line to the pump. Second, its mechanical action delivers this liquid to the pump outlet and forces it into the hydraulic system.

A pump produces liquid movement or flow: it does not generate pressure. It produces the flow necessary for the development of pressure which is a function of resistance to fluid flow in the system. For example, the pressure of the fluid at the pump outlet is zero for a pump not connected to a system (load). Further, for a pump delivering into a system, the pressure will rise only to the level necessary to overcome the resistance of the load.

Classifications of Hydraulic Pumps

All pumps may be classified as either positive-displacement or non-positive-displacement. Most pumps used in hydraulic systems are positive-displacement.

A non-positive-displacement pump produces a continuous flow. However, because it does not provide a positive internal seal against slippage, its output varies considerably as pressure varies. Centrifugal and propeller pumps are examples of non-positive-displacement pumps.

If the output port of a non-positive-displacement pump were blocked off, the pressure would rise, and output would decrease to zero. Although the pumping element would continue moving, flow would stop because of slippage inside the pump.

In a positive-displacement pump, slippage is negligible compared to the pump's volumetric output flow. If the output port were plugged, pressure would increase instantaneously to the point that the pump's pumping element or its case would fail (probably explode, if the drive shaft did not break first), or the pump's prime mover would stall.

Positive-Displacement Principle

A positive-displacement pump is one that displaces (delivers) the same amount of liquid for each rotating cycle of the pumping element. Constant delivery during each cycle is possible because of the close-tolerance fit between the pumping element and the pump case. That is, the amount of liquid that slips past the pumping element in a positive-displacement pump is minimal and negligible compared to the theoretical maximum possible delivery. The delivery per cycle remains almost constant, regardless of changes in pressure against which the pump is working. Note that if fluid slippage is substantial, the pump is not operating properly and should be repaired or replaced.

The positive-displacement principle is well illustrated in the reciprocating-type pump, the most elementary positive-displacement pump, Figure 1. As the piston extends, the partial vacuum created in the pump chamber draws liquid from the reservoir through the inlet check valve into the chamber. The partial vacuum helps seat firmly the outlet check valve. The volume of liquid drawn into the chamber is known because of the geometry of the pump case, in this example, a cylinder.

As the piston retracts, the inlet check valve reseats, closing the valve, and the force of the piston unseats the outlet check valve, forcing liquid out of the pump and into the system. The same amount of liquid is forced out of the pump during each reciprocating cycle.

All positive-displacement pumps deliver the same volume of liquid each cycle (regardless of whether they are reciprocating or rotating). It is a physical characteristic of the pump and does not depend on driving speed. However, the faster a pump is driven, the more total volume of liquid it will deliver.

Rotary Pumps

In a rotary-type pump, rotary motion carries the liquid from the pump inlet to the pump outlet. Rotary pumps are usually classified according to the type of element that transmits the liquid, so that we speak of a gear-, lobe-, vane-, or piston-type rotary pump.

External-gear pumps can be divided into external and internal-gear types. A typical external-gear pump is shown in Figure 2. These pumps come with a straight spur, helical, or herringbone gears. Straight spur gears are easiest to cut and are the most widely used. Helical and herringbone gears run more quietly, but cost more.

A gear pump produces flow by carrying fluid in between the teeth of two meshing gears. One gear is driven by the drive shaft and turns the idler gear. The chambers formed between adjacent gear teeth are enclosed by the pump housing and side plates (also called wear or pressure plates).

A partial vacuum is created at the pump 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 end, the fluid is forced out.

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Volumetric efficiencies of gear pumps run as high as 93% under optimum conditions. Running clearances between gear faces, gear tooth crests and the housing create an almost constant loss in any pumped volume at a fixed pressure. This means that volumetric efficiency at low speeds and flows is poor, so that gear pumps should be run close to their maximum rated speeds.

Although the loss through the running clearances, or "slip," increases with pressure, this loss is nearly constant as speed and output change. For one pump the loss increases by about 1.5 gpm from zero to 2,000 psi regardless of speed. Change in slip with pressure change has little effect on performance when operated at higher speeds and outputs. External-gear pumps are comparatively immune to contaminants in the oil, which will increase wear rates and lower efficiency, but sudden seizure and failure are not likely to occur.

The lobe pump is a rotary, external-gear pump, Figure 3. It differs from the conventional external-gear pump in the way the "gears" are driven. In a gear pump, one gear drive the other; in a lobe pump, both lobes are driven through suitable drives gears outside of the pump casing chamber.

A screw pump is an axial-flow gear pump, similar in operation to a rotary screw compressor. Three types of screw pumps are the single-screw, two-screw, and three-screw. In the single-screw pump, a spiraled rotor rotates eccentrically in an internal stator. The two-screw pump consists of two parallel intermeshing rotors rotating in a housing machined to close tolerances. The three-screw pump consists of a central-drive rotor with two meshing idler rotors; the rotors turn inside of a housing machined to close tolerances.

Flow through a screw pump is axial and in the direction of the power rotor. The inlet hydraulic fluid that surrounds the rotors is trapped as the rotors rotate. This fluid is pushed uniformly with the rotation of the rotors along the axis and is forced out the other end.

The fluid delivered by a screw pump does not rotate, but moves linearly. The rotors work like endless pistons, which continuously move forward. There are no pulsations even at higher speed. The absence of pulsations and the fact that there is no metal-to-metal contact results in very quiet operation.

Larger pumps are used as low-pressure, large-volume prefill pumps on large presses. Other applications include hydraulic systems on submarines and other uses where noise must be controlled.

Internal-gear pumps, Figure 4, have an internal gear and an external gear. Because these pumps have one or two less teeth in the inner gear than the outer, relative speeds of the inner and outer gears in these designs are low. For example, if the number of teeth in the inner and outer gears were 10 and 11 respectively, the inner gear would turn 11 revolutions, while the outer would turn 10. This low relative speed means a low wear rate. These pumps are small, compact units.

The crescent seal internal-gear pump consists of an inner and outer gear separated by a crescent-shaped seal. The two gears rotate in the same direction, with the inner gear rotating faster than the outer. The hydraulic oil is drawn into the pump at the point where the gear teeth begin to separate and is carried to the outlet in the space between the crescent and the teeth of both tears. The contact point of the gear teeth forms a seal, as does the small tip clearance at the crescent. Although in the past this pump was generally used for low outputs, with pressures below 1,000 psi, a 2-stage, 4,000-psi model has recently become available.

The gerotor internal-gear pump consists of a pair of gears which are always in sliding contact. The internal gear has one more tooth than the gerotor gear. Both gears rotate in the same direction. Oil is drawn into the chamber where the teeth are separating, and is ejected when the teeth start to mesh again. The seal is provided by the sliding contact.

Generally, the internal-gear pump with toothcrest pressure sealing has higher volumetric efficiency at low speeds than the crescent type. Volumetric and overall efficiencies of these pumps are in the same general range as those of external-gear pumps. However, their sensitivity to dirt is somewhat higher.

When pressure is high enough to overcome the compensator spring force, the cam ring shifts to decrease the eccentricity. Adjustment of the compensator spring determines the pressure at which the ring shifts.

Because centrifugal force is required to hold the vanes against the housing and maintain a tight seal at those points, these pumps are not suited for low-speed service. Operation at speeds below 600 rpm is not recommended. If springs or other means are used to hold vanes out against the ring, efficient operation at speeds of 100-200 rpm is possible.

Vane pumps maintain their high efficiency for a long time, because compensation for wear of the vane ends and the housing is automatic. As these surfaces wear, the vanes move further out in their slots to maintain contact with the housing.

Vane pumps, like other types, come in double units. A double pump consists of two pumping units in the same housing. They may be of the same or different sizes. Although they are mounted and driven like single pumps, hydraulically, they are independent. Another variation is the series unit: two pumps of equal capacity are connected in series, so that the output of one feeds the other. This arrangement gives twice the pressure normally available from this pump. Vane pumps have relatively high efficiencies. Their size is small relative to output. Dirt tolerance is relatively good.

Piston Pumps

Inline piston pumps — The simplest type of axial piston pump is the swashplate design in which a cylinder block is turned by the drive shaft. Pistons fitted to bores in the cylinder block are connected through piston shoes and a retracting ring, so that the shoes bear against an angled swashplate. As the block turns, Figure 8, the piston shoes follow the swashplate, causing the pistons to reciprocate. The ports are arranged in the valve plate so that the pistons pass the inlet as they are pulled out and the outlet as they are forced back in. In these pumps, displacement is determined by the size and number of pistons as well as their stroke length, which varies with the swashplate angle.

In variable-displacement models of the inline pump, the swashplate swings in a movable yoke. Pivoting the yoke on a pintle changes the swashplate angle to increase or decrease the piston stroke. The yoke can be positioned with a variety of controls, i.e., manual, servo, compensator, handwheel, etc.

Bent-axis pumps — This pump consists of a drive shaft which rotates the pistons, a cylinder block, and a stationary valving surface facing the cylinder block bores which ports the inlet and outlet flow. The drive shaft axis is angular in relation to the cylinder block axis. Rotation of the drive shaft causes rotation of the pistons and the cylinder block.

Because the plane of rotation of the pistons is at an angle to the valving surface plane, the distance between any one of the pistons and the valving surface continually changes during rotation. Each individual piston moves away from the valving surface during one-half of the shaft revolution and toward the valving surface during the other half.

The valving surface is so ported that its inlet passage is open to the cylinder bores in that part of the revolution where the pistons move away. Its outlet passage is open to the cylinder bores in the part of the revolution where the pistons move toward the valving surface. Therefore, during pump rotation the pistons draw liquid into their respective cylinder bores through the inlet chamber and force it out through the outlet chamber. Bent axis pumps come in fixed and variable displacement configurations, but cannot be reversed.

In radial-piston pumps, the pistons are arranged radially in a cylinder block; they move perpendicularly to the shaft centerline. Two basic types are available: one uses cylindrically shaped pistons, the other ball pistons. They may also be classified according to the porting arrangement: check valve or pintle valve. They are available in fixed and variable displacement, and variable reversible (over-center) displacement.

In pintle-ported radial piston pump, Figure 9, the cylinder block rotates on a stationary pintle and inside a circular reacting 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 as they move in.

The size and number of pistons and the length of their stroke determine pump displacement. Displacement can be varied by moving the reaction ring to increase or decrease piston travel, varying eccentricity. Several controls are available for this purpose.

Plunger pumps are somewhat similar to rotary piston types, in that pumping is the result of pistons reciprocating in cylinder bores. However, the cylinders are fixed in these pumps; they do not rotate around the drive shaft. Pistons may be reciprocated by a crankshaft, by eccentrics on a shaft, or by a wobble plate. When eccentrics are used, return stroke is by springs. Because valving cannot be supplied by covering and uncovering ports as rotation occurs, inlet and outlet check valves may be used in these pumps.

Because of their construction, these pumps offer two features other pumps do not have: one has a more positive sealing between inlet and outlet, permitting higher pressures without excessive leakage of slip. The other is that in many pumps, lubrication of moving parts other than the piston and cylindrical bore may be independent of the liquid being pumped. Therefore, liquids with poor lubricating properties can be pumped. Volumetric and overall efficiencies are close to those of axial and radial piston pumps.

Measuring Hydraulic Pump Performance

Volume of fluid pumped per revolution is calculated from the geometry of the oil-carrying chambers. A pump never quite delivers the calculated, or theoretical, amount of fluid. How close it comes is called volumetric efficiency. Volumetric efficiency is found by comparing the calculated delivery with actual delivery. Volumetric efficiency varies with speed, pressure, and the construction of the pump.

A pump's mechanical efficiency is also less than perfect, because some of the input energy is wasted in friction. Overall efficiency of a hydraulic pump is the product of its volumetric and mechanical efficiencies.

Pumps are generally rated by their maximum operating pressure capability and their output, in gpm or lpm, at a given drive speed, in rpm.

Matching Pump Power with the Load

Comparison of these two pressure signals in the modified compensator section allows the pump to sense both load and flow. This reduces power losses even further, Figure 17. Output flow of the pump varies in relation to the differential pressure of the two orifices. Just as the pressure-compensated pump increased its discharge pressure by the amount required to run the pressure compensator, the load- and flow-sensing pump's discharge pressure typically is between 200 and 250 psi higher than actual load pressure.

Furthermore, a load-sensing pump can follow the load and flow requirements of a single circuit function or multiple simultaneous functions, relating horsepower to maximum load pressure. This consumes the lowest possible horsepower and generates the least heat.

Operator Control

If the variable orifice is a manually operated flow control valve, the system can operate in a load-matched mode at the direction of an operator. As he opens the flow control valve, flow increases proportionally (constant pressure drop across an increasing-diameter orifice), at a pressure slightly above load pressure.

As suggested in Figure 17, wasted power is very low with a load-sensing variable volume pump compensator. Since the control senses pressure drop and not absolute pressure, a relief valve or other means of limiting pressure must be provided.

A load-sensing gear pump, on the other hand, uses a hydrostat in combination with an unloader to vary its volumetric output in response to load and flow requirements. Because load-sensing piston and gear pumps both use a single load-sensing signal to control pump discharge pressure and flow, they are interchangeable in load-sensing circuits. Both types have much in common and offer substantial power savings over systems using fixed-displacement pumps. Both offer reduced power consumption in the running mode - when flow and pressure are required to operate a function. They also conserve power in the standby mode - when the system is idling or in a non-operational mode. Furthermore, they can reduce the required size - and, therefore, cost- of valves, conductors, and filters needed for the circuit.

The load-sensing gear pump illustrated in Figure 19 minimizes power consumption in the running mode by separating total discharge flow according to a remote primary function pressure and a primary flow. This is accomplished through a single load-sensing signal originating from the priority circuit and routed as close as possible to the discharge side of the pump's gears.

Adding an unloader control to the pump circuit, Figure 20, allows the system to conserve power in the standby mode of operation as well as in the running mode. This control must be installed in parallel with the inlet port of the hydrostat and as close as possible to the discharge side of the gears. It must be piloted by the same load-sensing signal as in Figure 19. This signal causes the pump to dump all flow from the outlet to the secondary circuit and at a pressure well below the hydrostat's pressure-drop setting in the standby mode.

The unloader control must operate off the same remote load-sensing signal that controls the hydrostat. Unlike the hydrostat, the unloader poppet of the unloader control is designed with opposing areas having a ratio of at least 2:1. Any line pressure sensed that exceeds 50% of pump discharge pressure will close the unloader control. The ability of the unloader control to unload the pump to near atmospheric discharge pressure is controlled by the poppet or plunger spring force. The unloader control is set to the lowest value to maintain the internal pressure loading of the gear pump. When compared to a standard fixed-displacement gear pump circuit, this control can reduce standby power consumption by 90%.

Dual and Combined Controls

The load-sensing signal can be conditioned by limiting pressure in the remote sensing line or taking it to 0 psig. Doing so causes the hydrostat and the unloader control of the load-sensing gear pump to respond to the conditioned signal according to the discharge pressure. This is accomplished by providing a pilot relief, Figure 21, which causes the hydrostat to act as the main stage of a pilot-operated relief valve. The ability to condition the load-sensing line is patented and makes the load-sensing gear pump useful for functions other than just load sensing.

The combined-control load-sensing gear pump, Figure 22, is intended for large-displacement pumps and bypasses secondary flow to tank. It also is patented, and can be used in the same applications as the dual-control pump. However, because secondary flow must be routed to tank, it cannot be used when the secondary circuit drives a load.