pressure compensated hydraulic pump how it works quotation

A pressure compensator is a device built into some pumps for the purpose of automatically reducing (or stopping) pump flow if system pressure sensed on the pump outlet port, should rise above a pre-set desired maximum pressure (sometimes called the "firing" pressure). The compensator prevents the pump from being overloaded if an overload is placed on the hydraulic system.

A compensator is built into the pump at the factory and usually cannot be added in the field. Any pump built with variable displacement can be controlled with a compensator. These include several types of axial piston pumps and unbalanced (single lobe) vane pumps. Radial piston pumps can sometimes be built with variable displacement but do not lend themselves readily to this action. Most other positive displacement pumps including internal and external gear, balanced (double lobe) vane, gerotor, and screw types cannot be built with variable displacement.

Figure 1 is a schematic of a check valve axial piston pump, variable displacement, controlled with a pressure compensator. The pistons, usually 5, 7, or 9 in number, are stroking inside a piston block which is keyed to and is rotating with the shaft. The left ends of the pistons are attached through swivel joints, to piston shoes which bear against and slide around on the swash plate as the piston block rotates. The swash plate itself does not rotate; it is mounted on a pair of trunnions so it can swivel from neutral (vertical) position to a maximum tilt angle. The angle which the swash plate makes to the vertical causes the pistons to stroke, the length of stroke being proportional to the angle. Normally, at low system pressures, the swash plate remains at its maximum angle, held there by spring force, hydraulic pressure, or by the dynamics of pump construction, and pump flow remains at maximum. The compensator acts by hydraulic pressure obtained internally from the pump outlet port. When pump pressure rises high enough to over-come the adjustable spring behind the compensator piston, the "firing" pressure has been reached, and the compensator piston starts to pull the swash plate back toward neutral, reducing pump displacement and output flow. The spring in the compensator can be adjusted for the desired maximum or "firing" pressure.

Under working conditions, on a moderate system overload, the compensator piston reduces the swash plate angle just enough to prevent the system pressure from exceeding the "firing" pressure adjusted on the compensator. On severe overloads the compensator may swing the swash plate back to neutral (vertical) to reduce pump flow to zero.

Maximum Displacement Stops. Some pumps are available with internal stops to limit the tilt angle of the swash plate. These stops limit the maximum flow and limit the HP consumption of the pump. They may be fixed stops, factory installed and inaccessible from the outside, or they may be externally adjustable with a wrench.

Manual Control Lever. Some pressure compensated pumps, especially hydrostatic transmission pumps, are provided with an external control lever to enable the operator to vary the swash plate angle (and flow) from zero to maximum. On these pumps the pressure compensator is arranged to override the manual lever and to automatically reduce the swash plate angle if a system overload should occur even though the operator control lever is still shifted to maximum displacement position.

Basically the pressure compensator is designed to unload the pump when system pressure reaches the maximum design pressure. When the pump is unloaded in this way, there is little HP consumed and little heat generated even though pressure remains at the maximum level, because there is no flow from the pump.

Variable displacement pumps are usually more expensive than fixed displacement types, but are especially useful in systems where several branch circuits are to be supplied from one pump, and where full pressure may be required simultaneously in more than one branch, and where the pump must be unloaded when none of the branches is ill operation. If individual 4-way valves are used in each branch, each valve must have a closed center spool. The inlet ports on all 4-way valves must be connected in parallel across the pump line. However, if all branch circuits are operated from a bank valve of the parallel type, a pressure compensated variable displacement pump may not be necessary; a fixed displacement pump, gear, vane, or piston, may serve equally well because the bank valve will unload the pump when all valve handles are placed in neutral, but when two or more handles are simultaneously shifted, their branch circuits will automatically be placed in a parallel connection.

As in all hydraulic systems, more pump oil will flow to the branch with the lightest load. Bank valve handles can be modulated to equalize the flow to each branch. When individual 4-way valves are used in each branch, flow control valves may be installed in the branch circuits and adjusted to give the flow desired in each branch.

Figure 2 shows a multiple branch circuit in which a variable displacement pump is used to advantage. Individual 4-way valves, solenoid operated, are used for each branch, and they have closed center porting. Please refer to Design Data Sheet 54 for possible drift problems on a pressure manifold system. A pressure relief valve is usually required even with a pressure compensated pump due to the time interval required for the swash plate to reduce its tilt angle when a sudden overload occurs. The relief valve will help absorb part of the pressure spike generated during this brief interval. It should be adjusted to crack at about 500 PSI higher than the pressure adjustment of the compensator piston spring to prevent oil discharge across it during normal operation.

All hydrostatic transmission systems use a variable displacement pump with pressure compensator, and often combine the compensator with other controls such as the horsepower input limiter, load sensing, flow sensing, or constant flow control.

© 1990 by Womack Machine Supply Co. This company assumes no liability for errors in data nor in safe and/or satisfactory operation of equipment designed from this information.

Pressure compensation is the control of flow by compensating for the changes in load pressure. Most hydraulic systems today use pre-compensation as a means of maintaining consistent flow from an orifice or spool. However, there are applications when post-compensation has advantages over pre-compensation.

The fundamental difference is that with pre-compensation, the pressure drop across the orifice or spools is determined by the compensator. With post-compensation, the pressure drop is determined by the load sense (LS) spring inside the pump.

In post-compensated systems with multiple functions, the pump flow is divided at a fixed ratio. If flow settings exceed the pump output capability, the flow is reduced to each function at a fixed ratio. This is why post-compensation is sometimes referred to as “flow sharing”.

In post-compensated circuits, the pressure drop across each valve is determined by the load sense spring in the pump and all valves or orifices will have the same pressure drop. The load sense differential, sometimes referred to as standby, decreases when the pump cannot satisfy the total demand. All pressure compensators reference the highest load of the various functions.

The benefits include high efficiency under partial load and/or partial speed conditions and all functions slow down together at a fixed ratio when the pump cannot fully satisfy demand.

In the example below, the pump differential, or standby, is 200 PSI. The load sense pump will develop enough pressure to overcome the load and maintain a 200 PSI differential. The pressure drop across the valve or orifice remains fixed and is calculated by: system pressure minus the highest load pressure minus the compensator spring value.

The circuit below is an example of the flow sharing aspect. When another function is operated and the pump cannot fully satisfy the flow demand, the differential decreases. The pressure drop across each valve or orifice is reduced at the same fixed ratio, so the flow is divided, or shared, equally. In this example, each valve is fully open so total pump flow is shared equally between the functions.

So what happens when the functions require different flows and the pump cannot fully satisfy the total flow demand? The pump flow will be divided into the ratio of each function to total flow available. In the example below, the theoretical total flow demand is 42 GPM. The ratio of the function flow demand to total theoretical flow demand multiplied by the maximum pump flow is the resulting actual flow from each valve.

Post-compensation will increase stability and control in systems where demand can exceed the pump’s flow output. Because of its increased efficiency under partial load conditions, the compensator saves horsepower and reduces heat. It will also make the initial movement of actuators more predictable and provide better operator control.

Hydraulic pumps are an incredibly important component within hydraulic systems. IFP Automation offers a variety of pump and hydraulic system products that deliver exceptional functionality and durability. Our partner Parker’s extensive line of hydraulic pumps deliver ideal performance in even the most demanding industrial and mobile applications. In this post, we are going to spend time discussing pressure compensated and load sensing hydraulic pumps.

Do to the surface area of the servo piston and the pressure exerted on that area, a force is generated that pushes the swash plate of the pump to a lower degree of stroke angle.

The pump tries to maintain compensator setting pressure, and will provide whatever flow (up to it’s maximum flow rate) that is necessary to reach that pressure setting.

For more information on how you can make use of hydraulic pump technology in your applications, please contact us here to receive a personalized contact by an IFP Application Engineer:

IFP Automation supplies innovative technology and design solutions to the automation and mobile marketplaces.  Our firm is a technology supplier specializing in the design and supply of automation and motion control products to OEM, integrator, and end user customers. Companies partner with IFP because they like the depth of our product and application knowledge and our commitment to outstanding customer service.

A pressure-compensated flow control valve is designed to provide a constant volume flow rate regardless of the pressure drop across it. By contrast, non-pressure-compensated flow control valves have a variable flow rate that changes when the pressure drop fluctuates.

Pressure-compensated flow control valves are used in a variety of hydraulic applications. They are useful, for example, when it is necessary to maintain a constant speed on a hydraulic cylinder, regardless of the amount of load that the cylinder is under. Because speed is directly proportional to hydraulic fluid flow rate, a hydraulic cylinder’s speed depends on how much fluid is flowing through it.

In a flow control valve without pressure compensation, the flow rate will fluctuate depending on the load on the cylinder. A heavy load on the cylinder will increase the pressure at the valve’s outlet compared to one with a lighter load. By changing the pressure drop across the valve, the flow rate it delivers to the cylinder is altered. Pressure-compensated flow control valves adapt to such pressure changes to maintain a constant flow rate that provides fluid motion at constant speed

Pressure-compensated flow control valves are also useful in maintaining constant rpm of a hydraulic motor independent of load on the motor. Much like the example above, changing loads on the motor will result in a fluctuating pressure drop across the valve ahead of the motor. These fluctuations are compensated for by pressure-compensated flow control valves, which maintain the hydraulic motor’s rpm at a constant level.

It is possible for pressure-compensated flow control valves to compensate pressure fluctuations on either the supply (inlet) or the load (outlet) side of the valve.

Flow control valves that are pressure-compensated normally consist of a variable orifice and a pressure compensator incorporated into one valve body. Flow goes from the supply valve through the inlet and compensated orifice, around the compensated spool, through the variable orifice, and then out the outlet.

By adjusting the pass-through area of the orifice, the desired flow rate is set on the variable orifice. You can make this adjustment either manually using a knob, screw or lever on the valve. Alternatively, by means of electronic signals that are sent to an actuator attached to the variable orifice. Using the pressure compensator, a constant pressure drop is achieved across the variable orifice by modulating the flow of fluid entering the valve. It also provides a constant flow rate across the valve by adjusting the orifice between the inlet flow and the compensator spool.

Variable orifices are made up of valve stems that have a pointed end that can move toward and away from a seat in order to achieve different sizes of openings. Whenever the tip of the stem is in full contact with the seat, the orifice becomes closed, and no fluid can pass. With the stem tip moved away from the seat, the orifice opening becomes larger and more fluid can pass through.

A spool valve with a spring anchors the pressure compensator. Compensation spools consist of a cylindrical barrel with a plunger that slides inside. Plungers have thin and wide sections along their length. As long as the lands and ports are adjacent to each other, they block fluid flow. The wide sections of the barrel are called lands. Spools with narrow, waisted sections allow fluid to pass through them.

By applying a force to the end of the spool attached to the valve housing, a spring keeps the spool attached to it. An additional force is applied to the anchored end of the spool by flowing past the variable orifice in the outlet of the valve. There is a pressure gradient along the line leading from the pressure-compensated flow control valve to the load, such as a hydraulic motor or cylinder.

The fluid that has passed the variable orifice but not yet reached the inlet and compensator is ported to the other end of the spool (the end opposite the end attached to the spring). A force is applied to the spool at this end by the fluid that opposes the force applied by the load pressure and spring pressure. The opposing forces distort the opening of the orifice through which fluid flows from the flow source, modulating the opening of the orifice until the forces at either end of the spool are balanced.

Consequently, fluid flows from the supply, across the compensator spool, and through the variable orifice, while a constant pressure drop across the variable orifice keeps the flow rate constant regardless of changes in pressure between supply and load.

As fluid temperature increases, viscosity increases as well, affecting flow rate. A temperature-sensitive element is incorporated in some pressure-compensated flow control valves, which adapts the position of the compensator when fluid temperatures and viscosity vary. This ensures a constant flow rate regardless of fluid temperature and viscosity. By using a sharp-edged orifice design for the variable orifice, some designs are also able to minimize the variations in flow rate due to changes in viscosity.

An integral part of a pressure-compensated flow control valve is the pressure compensator. A valve without it would have a variable flow rate when pressure across the valve varies. If more fluid is forced through the valve as a result of a higher pressure drop, the flow rate will be higher; if the pressure drop is lower, the flow rate will be lower.

By automatically adjusting the volume flow rate from the flow supply to the variable orifice, the pressure compensator keeps the internal pressure drop across the variable orifice constant, regardless of the change in pressure drop between the inlet and outlet. With a constant internal pressure drop across the variable orifice, the valve always produces a constant volumetric flow rate regardless of the pressure differences between the valve inlet and outlet. T

his decreases the incoming input process on the inlet port to the lowest operation working pressure for the valve to output accurate flow rates. After regulation, this lowered pressure is applied to the proportional valve orifice, thereby allowing for consistent flow rates even with fluctuating input pressures. So long as the incoming pressure does not drop below the minimum required pressure, accurate proportional flow is maintained to the system.

Kelly Pneumatics offers its Pressure Compensated Proportional Valve for projects that require a pressure-compensated flow control valve. There is a mechanical pressure regulator built into the unit, which lowers the incoming input process on the inlet port to the lowest operating working pressure recommended for the valve to output accurate flow rates. By lowering the pressure after regulation, a consistent flow rate can be achieved despite fluctuating input pressures. A proportional flow to the system is maintained so long as the incoming pressure does not drop below the minimum required pressure.

A hydraulic pump is a mechanical device that converts mechanical power into hydraulic energy. It generates flow with enough power to overcome pressure induced by the load.

A hydraulic pump performs two functions when it operates. Firstly, its mechanical action creates a vacuum at the pump inlet, subsequently allowing atmospheric pressure to force liquid from the reservoir and then pumping it through to the inlet line of the pump. Secondly, its mechanical action delivers this liquid to the pump outlet and forces it into the hydraulic system.

The three most common hydraulic pump designs are: vane pump, gear pump and radial piston pump. All are well suited to common hydraulic uses, however the piston design is recommended for higher pressures.

Most pumps used in hydraulic systems are positive-displacement pumps. This means that they displace (deliver) the same amount of liquid for each rotating cycle of the pumping element. The delivery per cycle remains almost constant, regardless of changes in pressure.

Positive-displacement pumps are grouped into fixed or variable displacement. A fixed displacement pump’s output remains constant during each pumping cycle and at a given pump speed. Altering the geometry of the displacement chamber changes the variable displacement pump’s output.

Fixed displacement pumps (or screw pumps) make little noise, so they are perfect for use in for example theatres and opera houses. Variable displacement pumps, on the other hand, are particularly well suited in circuits using hydraulic motors and where variable speeds or the ability to reverse is needed.

Applications commonly using a piston pump include: marine auxiliary power, machine tools, mobile and construction equipment, metal forming and oil field equipment.

As the name suggests, a piston pump operates through pistons that move back and forth in the cylinders connected to the hydraulic pump. A piston pump also has excellent sealing capabilities.

A hydraulic piston pump can operate at large volumetric levels thanks to low oil leakage. Some plungers require valves at the suction and pressure ports, whilst others require them with the input and output channels. Valves (and their sealing properties) at the end of the piston pumps will further enhance the performance at higher pressures.

The axial piston pump is possibly the most widely used variable displacement pump. It’s used in everything from heavy industrial to mobile applications. Different compensation techniques will continuously alter the pump’s fluid discharge per revolution. And moreover, also alter the system pressure based on load requirements, maximum pressure cut-off settings and ratio control. This implies significant power savings.

Two principles characterise the axial piston pump. Firstly the swash plate or bent axis design and secondly the system parameters. System parameters include the decision on whether or not the pump is used in an open or closed circuit.

The return line in a closed loop circuit is under constant pressure. This must be considered when designing an axial piston pump that is used in a closed loop circuit. It is also very important that a variable displacement volume pump is installed and operates alongside the axial piston pump in the systems. Axial piston pumps can interchange between a pump and a motor in some fixed displacement configurations.

The swivel angle determines the displacement volume of the bent axis pump. The pistons in the cylinder bore moves when the shaft rotates. The swash plate, in the swash plate design, sustain the turning pistons. Moreover, the angle of the swash plate decides the piston stroke.

The bent axis principle, fixed or adjustable displacement, exist in two different designs. The first design is the Thoma-principle with maximum 25 degrees angle, designed by the German engineer Hans Thoma and patented in 1935. The second design goes under the name Wahlmark-principle, named after Gunnar Axel Wahlmark (patent 1960). The latter features spherical-shaped pistons in one piece with the piston rod and piston rings. And moreover a maximum 40 degrees between the driveshaft centre-line and pistons.

In general, the largest displacements are approximately one litre per revolution. However if necessary, a two-litre swept volume pump can be built. Often variable-displacement pumps are used, so that the oil flow can be adjusted carefully. These pumps generally operate with a working pressure of up to 350–420 bars in continuous work

Radial piston pumps are used especially for high pressure and relatively small flows. Pressures of up to 650 bar are normal. The plungers are connected to a floating ring. A control lever moves the floating ring horizontally by a control lever and thus causes an eccentricity in the centre of rotation of the plungers. The amount of eccentricity is controlled to vary the discharge. Moreover, shifting the eccentricity to the opposite side seamlessly reverses the suction and discharge.

Radial piston pumps are the only pumps that work continuously under high pressure for long periods of time. Examples of applications include: presses, machines for processing plastic and machine tools.

A vane pump uses the back and forth movement of rectangle-shaped vanes inside slots to move fluids. They are sometimes also referred to as sliding vane pumps.

The simplest vane pump consists of a circular rotor, rotating inside of a larger circular cavity. The centres of the two circles are offset, causing eccentricity. Vanes slide into and out of the rotor and seal on all edges. This creates vane chambers that do the pumping work.

A vacuum is generated when the vanes travel further than the suction port of the pump. This is how the oil is drawn into the pumping chamber. The oil travels through the ports and is then forced out of the discharge port of the pump. Direction of the oil flow may alter, dependent on the rotation of the pump. This is the case for many rotary pumps.

Vane pumps operate most efficiently with low viscosity oils, such as water and petrol. Higher viscosity fluids on the other hand, may cause issues for the vane’s rotation, preventing them from moving easily in the slots.

Gear pumps are one of the most common types of pumps for hydraulic fluid power applications. Here at Hydraulics Online, we offer a wide range of high-powered hydraulic gear pumps suitable for industrial, commercial and domestic use. We provide a reliable pump model, whatever the specifications of your hydraulic system. And we furthermore ensure that it operates as efficiently as possible.

Johannes Kepler invented the gear pump around year 1600. Fluid carried between the teeth of two meshing gears produces the flow. The pump housing and side plates, also called wear or pressure plates, enclose the chambers, which are formed between adjacent gear teeth. The pump suction creates a partial vacuum. Thereafter fluid flows in to fill the space and is carried around the discharge of the gears. Next the fluid is forced out as the teeth mesh (at the discharge end).

Some gear pumps are quite noisy. However, modern designs incorporating split gears, helical gear teeth and higher precision/quality tooth profiles are much quieter. On top of this, they can mesh and un-mesh more smoothly. Subsequently this reduces pressure ripples and related detrimental problems.

Catastrophic breakdowns are easier to prevent with hydraulic gear pumps. This is because the gears gradually wear down the housing and/or main bushings. Therefore reducing the volumetric efficiency of the pump gradually until it is all but useless. This often happens long before wear causes the unit to seize or break down.

Can hydraulic gear pumps be reversed? Yes, most pumps can be reversed by taking the pump apart and flipping the center section. This is why most gear pumps are symmetrical.

External gear pumps use two external spur gears. Internal gear pumps use an external and an internal spur gear. Moreover, the spur gear teeth face inwards for internal gear pumps. Gear pumps are positive displacement (or fixed displacement). In other words, they pump a constant amount of fluid for each revolution. Some gear pumps are interchangeable and function both as a motor and a pump.

The petrochemical industry uses gear pumps to move: diesel oil, pitch, lube oil, crude oil and other fluids. The chemical industry also uses them for materials such as: plastics, acids, sodium silicate, mixed chemicals and other media. Finally, these pumps are also used to transport: ink, paint, resins and adhesives and in the food industry.

Mathematical calculations are key to any type of hydraulic motor or pump design, but are especially interesting in the gerotor design. The inner rotor has N teeth, where N > 2.  The outer rotor must have N + 1 teeth (= one more tooth than the inner rotor) in order for the design to work.

If you’d like to access more information about an introduction to hydraulic pumps, we highly recommend that you visit LunchBox Sessions’ website and make use of the fabulous resources available there.

Some of the sessions are free; you don’t need to sign up. However, if you are serious about expanding your knowledge of hydraulic systems, we recommend you consider signing up for a subscription to the entire LunchBox Session service.  This way, you get full access to all the interactive training materials, tests and simulations for just $29 per month.  Students are entitled to a 60% discount off this price when they share their college or university ID.

Because there is always a pump inefficiency that makes the input power higher than the hydraulic output power. Because no useful work is being done, that means all of the input power becomes heat. In other words, all the power being put into the hydraulic system by the electric motor is being wasted because it"s just being used to generate high pressure fluid and send it back to tank, with no useful output work being done (eg, no cylinders delivering mechanical output power). RE: Pressure Compensated Pump - Heat Generated During Stand-by

So what you are now saying is that total input energy in standby mode is wasted, generating heat. Therefore you do not need to know electric motor efficiency when the pump is in standby mode.

I thought your original question was only about hydraulic system heat generation. Very little electric motor heat makes its way into the hydraulic system.

Right, I"m saying input power to the hydraulic system, meaning motor output power, meaning pump input power (all the same thing) is all wasted and becomes heat generated. Pump inefficiency is heat generated. Case drain flow flowing from high pressure to low pressure is heat generated. There is no useful work being performed. All input power becomes heat.

Simple math example: Let"s say input power to the hydraulic system is 10 HP (input power to hydr system = motor output shaft power = pump input shaft power). Let"s say pump efficiency is 50%. So 5 HP goes to heat generated by the pump. The other 5 HP goes to pressurizing the case drain flow, which then just flows from high pressure (pump case) to zero pressure (tank), which creates 5 HP of heat in the process. Total heat generated = 10 HP = input power to the hydraulic system = motor output shaft power = pump input shaft power. I"m trying to calculate heat generated, knowing compensator setting, case drain flow, but not knowing pump efficiency. RE: Pressure Compensated Pump - Heat Generated During Stand-by

Pump efficiency is 0% when there is no flow. The pump is not doing any useful work. It is a little conservative to say it all goes into heat, because some energy creates vibration, noise, and there may be a small amount of flow. But for what you need to know, efficiency can be considered 0%.

What is the miles per gallon of a car that just sits in park with the engine running? 0 mpg. RE: Pressure Compensated Pump - Heat Generated During Stand-by

Ted, but what is the efficiency when the pump is in stand-by? That"s what I"m trying to get at, so I can calculate input power, which I believe equals heat generated.

Pump overall efficiency can vary widely for a pump, and it depends on discharge pressure. Every time we do an HPU, I look at the pump curves for the pump we"re going to use, and I do the calculations based on the curves. I can tell you that pump overall efficiency for the pumps we deal with can vary between 20-25% at very low discharge pressure, up to about 85% at "sweet spot" discharge pressure (for lack of a better term). 90% is very generous. 96% must be a very expensive well-made pump, because I have not seen anywhere close to that overall efficiency in any of the pumps curves I"ve dealt with. Unless you"re talking about volumetric efficiency only? In which case, yes, 90%-96%, even up to 99%. But overall efficiency varies widely, and it depends on discharge pressure.

Gibson, pump efficiency cannot be 0% in stand-by, because then input power would be infinite. Pump input power = pump output power / pump overall efficiency.

Ted, I think you and I don"t agree on whether input power = heat generated for a pump in stand-by. I believe it does. I believe that all input power goes to heat generated. My simple example above demonstrates why I think so. Am I correct to say that you believe input power =/ heat generated? If that"s a fair characterization, how would you calculate heat generated for a pump in stand-by? As I mentioned before, there is a manufacturer called Duplomatic that publishes what they call "input power at full cut-off" (meaning pump has compensated, de-stroked to no system flow). If you Google "Duplomatic VPPL pump", the first or second search return is a PDF. In that PDF, go down to the pump curves and you can see "input power at full cut-off". I say this is the heat generated. Seeing these curves, if you were using or analyzing this pump, how would you calculate heat generated for this pump "at full cut-off", meaning "in stand-by"? RE: Pressure Compensated Pump - Heat Generated During Stand-by

I may not be using the right terminology. By "stand-by", I mean the pump discharge has reached the compensator setting, and the pump has de-stroked. Maybe the better definition of "stand-by" means pump at low discharge pressure, and de-stroked. But I"m talking about high discharge pressure and de-stroked. Sorry for any confusion. RE: Pressure Compensated Pump - Heat Generated During Stand-by

Depending on the pump size, the charts show cut-off flow rates from 1.0 to about 1.5 l/min at 210 bar. So what power loss do you have for QxP for your size pump?

Ok, you say 0.9 x input power is hydraulic heat. I agree so far. The other 0.1 x input power is.....also heat. That"s the pump inefficiency, the other 10%. It manifests as hydraulic heat. If you have it, see IFPS hydraulic specialist study manual, page 3-56, review question 3.9.2.1. The pump inefficiency contributes to the heat generated. So all 100% of input power turns into heat.

So if you look at the Duplomatic "input power at full cut-off" curves, the input power is much higher than the QxP, where Q = drain flow rate and P = cut-off pressure. If you calculate QxP / "input power at full cut-off", you get an efficiency of like 53% for their -016 pump, and it goes down to like 23% for their -046 pump. Assuming this is a valid way of looking at it??

The 90% efficiency at 210 bar only applies to the pump operating at full displacement, not at full cut-off. If you try to take the pump efficiency as 90% at full cut-off, they give you the input power at full cut-off, so you can back-calculate the QxP. QxP = input power at full cut-off x efficiency. What would this calculated QxP mean? It can"t be "drain flow x cut-off pressure", because "drain flow x cut-off pressure" is way too low.

To answer your question, I don"t know what power loss I have at full cut-off for the pumps we use. That"s what I"m trying to figure out. As I said before, the pumps we use almost exclusively here at my company don"t publish any data of what"s going on at full cut-off, and couldn"t help me with my request. RE: Pressure Compensated Pump - Heat Generated During Stand-by

I don"t understand the charted increase in input power as the pump moves to shut-off. Just spinning the pump at little or no output flow should be a low power condition, even at 210 bar.

I know! That"s why i"m thinking the pump"s efficiency must be very low in this condition. If it"s taking 3.5 HP (2.6 kW) input at 1800 RPM to spin the -046 pump at cut-off, the pump must be very inefficient and there must be a lot of heat generated in this condition! That"s why I mentioned the 20% efficiency number in my original post, which I had calculated for their -046 pump using Q = their case drain flow, P = 2500 psi cut-off pressure, and their graphed input power. Since I imagine their pumps aren"t significantly different in performance than other pumps, I"m hoping to extrapolate their numbers at cut-off to other pump brands, to get the heat generated when the pump is sitting there pressure compensated for 45 minutes or an hour at a time while a cylinder is holding force against a workpiece.

(I think the input power isn"t increasing as the pump moves to cut-off. I think what those curves are saying is: for the range of different cut-off pressure settings that you might set this pump at, from ~15 bar to 210 bar, here is your input power at any of those pressure settings.) RE: Pressure Compensated Pump - Heat Generated During Stand-by

If nobody has any other comments or insights, I"m next going to post my question in the Fluid Power sub-forum here. Thank you for discussing this with me. It"s nice to discuss and get the thoughts of others in hydraulics engineering. RE: Pressure Compensated Pump - Heat Generated During Stand-by

I think you"re trying to calculate the impossible as there is so little work #(fluid flow) being done by the pump that efficiency would be in single figures.

I also think your quoted duty is a little odd - 45 minutes sitting at 210 bar with no fluid flow? Why don"t you use an accumulator to hold pressure and turn the pump off to stop the hydraulic fluid boiling?

Duplomatic, a pump manufacturer that we don"t use, publishes the input power to the pump with the pump in the pressure compensated condition. This input power to the pump is equal to the heat being generated, which I have to dissipate through the oil cooler. I think efficiency when the pump has compensated to full cut-off is in the double digits, but from 50% down to possibly the high teens. At least that"s what I calculate from the Duplomatic data, using their stated input power, Q = their stated case drain flow rate, and P = cut-off pressure. I"m trying to use the Duplomatic back-calculated efficiency, the case drain flow rate for the pumps we use, the pressure cut-off valve for the application, and use these pieces of information to calculate the input power = heat generated to size and select an oil cooler.

My company"s chief engineer and owner has designed a hydraulic circuit for a recurring customer for several of that customer"s units. When they want more of the units, I have to use the previous designs as a go-by, and not stray too far from them or the boss will ask me what the heck I"m doing. We as staff engineers don"t always have the luxury of designing fancy systems or knowing exactly what the customer"s duty cycle is. Our customers are always looking for the up-front cheapest simplest solution. And almost to a man, they can"t articulate or write out how they are operating the system, or maybe the salesman just doesn"t ask. I don"t like to be pushy or step on toes, so I just go along with the information provided and work within those confines. I don"t know when the customer is turning off the pump. Maybe he leaves it running the whole work day. I don"t know. He probably doesn"t even know, because his workers might each do it differently. I have to plan for the worst case, which is that he leaves the pump on, and the pump has compensated, while he is holding cylinder force against the workpiece and waiting for the plastic to cure. RE: Pressure Compensated Pump - Heat Generated During Stand-by

Case drain flow delivers heat to the reservoir. Energy other than hydraulic loss heat goes into heating the pump. Until the pump temp exceeds the case drain flow temp, no pump component heat conducts to the case drain flow fluid. Unless the pump is submerged in the reservoir, the only way for pump component heat to enter the system is via fluid flow of the case drain and that will not happen until pump component temp exceeds fluid temp.

Why do you want to know how much heat is generated in cut-off mode? Is system temp rising to an unacceptable level? Are you wanting to change the heat rejection capacity of the system?

I don"t know the full history because I"m fairly new to the company, but apparently a past system (either customer"s original or one my company provided) had gotten too hot according to the customer. On any new system we do for them, we are using a small dedicated kidney loop pump (through-drive off the main system pump) to take fluid from the reservoir, pump it through the oil cooler and filter, and right back into the reservoir. Since doing all new systems this way, the customer has not reported any overheating issues.

When I get handed one of their new systems to design and spec all the components (using an older system hydraulic schematic and BOM as a template), I want to go ahead and calculate the heat I need to reject, and size and select the appropriate oil cooler. Let"s just say the full engineering record on the older projects isn"t always possible to come by, and leave it at that. So I need to do my diligence on the oil cooler sizing and spec"ing.

What you explained makes sense. However, if you have it, please look at the IFPS hydraulic specialist study manual, page 3-56, review question 3.9.2.1. I can try to upload the page here if you want.

According to what"s on that page, the "pump component heat" as you say contributes to the heat generated, regardless of temperatures. They don"t take into account temperature differences. They just say, calculate all the power losses from pressure drops and inefficiencies, add them up, and that"s your heat generated that you need to reject. RE: Pressure Compensated Pump - Heat Generated During Stand-by

Go about this another way. Do you know the efficiency of the driving motor? I assume it is electric. Measure the input electric power when the pump is in stand-by mode. Calculated the electric motor output shaft power which is equal to the pump shaft input power. That would be your worst case wasted power.

Ted, that"s a good idea. But by the time we test, all the components including oil cooler have been bought and installed. They wouldn"t let me measure V and A to the motor with the rest of the system installed and almost ready to be shipped out to the customer, and then run back and do the calculations and buy the oil cooler. But that would at least be a data point that I could use for future applications. I was just hoping for a way to calculate it up front using case drain flow rate, cut-off pressure, and pump efficiency at cut-off. But again, that is a good idea, I like it, and I will try to have V and A measured the next time we do final testing on a unit before shipping it out. RE: Pressure Compensated Pump - Heat Generated During Stand-by

LittleInch, customer is not having overheating issues anymore, because every new unit we do for them has the kidney loop pump sending reservoir fluid through an oil cooler and back into the reservoir. This has presumably solved any overheating problem like the problem they had on a past system. They may very well be leaving the motor & pump on for a very long time in the pressure compensated state. They are holding a certain target pressure in a cylinder against a workpiece, and if that pressure drops off, a pressure-reducing valve opens and the pump goes on stroke. So they need that motor on and that pump standing by in the pressure compensated state, ready to deliver high pressure at a second"s notice.

So let me ask you, how would you calculate heat generated, for ultimately sizing an oil cooler, for an application like I have here? You have to assume the motor & pump stay on for long periods of time, with the pump in the pressure compensated state. All you have for cooling is the kidney loop pump circulating fluid from reservoir, through cooler, back to reservoir. How would you size the oil cooler? RE: Pressure Compensated Pump - Heat Generated During Stand-by

Either take the figure from a similar vendor like your duplomatic vendor or use the last known power figure from your current pumps. Or measure it even as a test as it"s going out the door for next time.

Don"t have a power figure from current pumps. Don"t have any existing data points. All I have is Duplomatic and their generosity in providing comprehensive data. We never use Duplomatic pumps. I just stumbled across their data while searching this topic one day.

"Kidney loop" is the function it performs: offline circulation of fluid for filtration, cooling, or both. In our case, we use gear pumps. RE: Pressure Compensated Pump - Heat Generated During Stand-by

It seems that if you sized your reservoir correctly for the rated horsepower of the pump you shouldn"t have any capacity issues when operating in compensation mode. RE: Pressure Compensated Pump - Heat Generated During Stand-by

The size of the reservoir is normally determined by the heat dissipation requirements. External coolers allow for a smaller reservoir. RE: Pressure Compensated Pump - Heat Generated During Stand-by

It"s Fluidyne (A)A10VSO Series 31. Mostly 18, 28, 45, or 71 cc. Almost always with only the "DR" pressure compensator control. Very similar to the Rexroth pumps of the same model. RE: Pressure Compensated Pump - Heat Generated During Stand-by

It seems that if you sized your reservoir correctly for the rated horsepower of the pump you shouldn"t have any capacity issues when operating in compensation mode.

Sizing a reservoir for all of the heat dissipation, even in the compensation mode data I"ve seen from Duplomatic pumps, would require a very large volume, many multiples of what we normally use in these systems. That would go over like a lead zeppelin here at my company. In the first couple oil cooler sizing calculations I did when I got here, I included reservoir cooling, per the formulas in the IFPS study manual and the lightning reference book. I used a reservoir size somewhere in the range of 2.5 - 5 times the pump max flowrate, which is the range where most of my company"s past reservoir sizes fall. It turned out to be such a small percentage of the heat generated that I just ignored it in future calculations. RE: Pressure Compensated Pump - Heat Generated During Stand-by

Fair enough. The best data I have at this time is the Duplomatic data I keep referencing. As Ted (hydtools) suggested, I"ll try to measure volts and amps to the motor on future HPUs with a pressure compensated pump that we commission test before shipping out. RE: Pressure Compensated Pump - Heat Generated During Stand-by

When I was involved in the design of portable power units, we used 50% of system power as the design heat load. The reservoir was sized to 1x the flow rate and roto-molded plastic.

Spoke to the lead tech here and he says he can measure motor volts and amps, and he will involve me in that when the next HPU with pressure compensated pump is being tested. RE: Pressure Compensated Pump - Heat Generated During Stand-by

Motor output HP calculated = 15.2 HPSo the motor is outputting 15.2 HP. This is also the pump input power. Since none of this power is used for useful work, this all goes into hydraulic system heat. Therefore the hydraulic system must be capable of rejecting 15.2 HP when the pump is in the pressure-compensated condition for long periods of time.

For the Duplomatic pumps that I"ve mentioned in this thread, I looked at their similar sized pump. Its "input power at full cut-off" for the cut-off pressure we were at in the test is about 2.5 kW, or 3.4 HP. So apparently it is much more efficient in the pressure-compensated state than the Fluidyne we use. RE: Pressure Compensated Pump - Heat Generated During Stand-by

I just found that the performance curves for a similar Bosch Rexroth pump, their (A)A10VS, show about 3-5 HP at zero flow (pressure-compensated condition) for their 45 cc pump. (Hard to determine exact HP value from their pitiful graphs; they should be ashamed of publishing graphs like that). Anyway, this 3-5 HP is on par with what Duplomatic shows for their 46 cc pump (about 3.4 HP).

The Fluidyne is much cheaper than the Rexroth, so perhaps its efficiency is much worse than the Rexroth at pressure compensation, and therefore requires much more motor power. In which case my calculation might be correct.

Add an air to oil cooler rated for 3 to 4hp heat rejection; take into consideration ambient temperatures. Get the FluiDyne pump with a through shaft, add a small gear pump to circulate oil in a kidney cooling loop. Test it.

customer is not having overheating issues anymore, because every new unit we do for them has the kidney loop pump sending reservoir fluid through an oil cooler and back into the reservoir. This has presumably solved any overheating problem like the problem they had on a past system.

Just want to be able to calculate motor power (= heat load) accurately in the design phase from now on. 3-5 HP motor power is what Duplomatic and Rexroth publish for their ~45cc pumps in the pressure-compensated state. The motor power I calculated for the Fluidyne was much higher, but there were some assumptions. I calculated the motor side of things, nothing on the pump side. The pump side of things seems to be too difficult to quantify. Too difficult to calculate a motor power from the pump"s parameters in the pressure-compensated state. Even a Fluidyne technical rep couldn"t tell me what motor power it takes to run their pumps in the pressure-compensated state. I think it"s better to use the motor side, as you suggested earlier:

Go about this another way. Do you know the efficiency of the driving motor? I assume it is electric. Measure the input electric power when the pump is in stand-by mode. Calculated the electric motor output shaft power which is equal to the pump shaft input power. That would be your worst case wasted power.

Although the motor side also comes with its uncertainties, like efficiency and power factor. But at least I found representative data on those 2 factors in some pretty authoritative literature, the Dept of Energy document I mentioned. RE: Pressure Compensated Pump - Heat Generated During Stand-by

I think you"re trying to calculate the impossible as there is so little work #(fluid flow) being done by the pump that efficiency would be in single figures.

Continental Hydraulics LPV series of pumps, are variable displacement axial piston pumps with variable swash block, suitable for applications with open loop circuits and intermediate pressures. Available in 5 nominal displacements. These Pressure compensated pumps automatically adjust the output flow rate to maintain the set pressure. The maximum output flow can be limited via maximum volume adjustment screw. SAE J&44 2-Bold Mounting Flange Available with four difference types of control compensator options designed to meet your application or requirements.

Multiple compensator options including: Pressure Compensator, Remote pressure, load sensing and a D03 (CEETOP 03) valve mount permitting pressure control of circuits from dual pressure to Proportional pressure.

Like the proven Continental HPVR piston pump, the swash block and saddle design allows consistent control and prolonged operating life. Saddle bearing can also be easily serviced.

Piston pumps are durable and relatively simple devices. A basic piston pump is made up of a piston, a chamber, and two valves. The pump operates by driving the piston down into the chamber, thereby compressing the media inside. In a hand pump, this is usually air. Once the pressure of the air exceeds that of the outlet valve spring, the compressed media goes through the open outlet valve. When the piston is drawn back up, it opens the inlet valve and closes the outlet valve, thereby utilizing suction to draw in new media for compression.

Although somewhat expensive, piston pumps are among the most efficient types of pumps. They have an excellent pressure rating (as high as 10,000 psi), but their design makes them susceptible to contaminants. They provide an excellent solution for many high-pressure hydraulic oil pumping applications.

Axial piston pumps are positive displacement pumps that use multiple cylinders grouped around a central axis. The group of cylinders, usually containing an odd number, is called a cylinder block. The pistons within each cylinder are attached to a swashplate. The swashplate is also known as a cam or wobble plate and attaches to a rotating shaft. As the shaft turns, the angle of the swashplate changes, which drives the pistons in and out of their respective cylinders.

Since the swashplate is at an angle to the axis of rotation, the pistons must reciprocate axially as they orbit around the cylinder block axis. The axial motion of the pistons is sinusoidal. As a piston rises, it moves toward the valve plate. At this point in the rotation, the fluid trapped between the buried end of the piston and the valve plate is expelled to the pump"s discharge port through one of the valve plate"s semi-circular ports. As the piston moves back toward the valve plate, the fluid is pushed through the discharge port of the valve plate.

Axial piston pumps can be designed as variable displacement piston pumps, making them very useful for controlling the speeds of hydraulic motors and cylinders. In this design, a swashplate is used to vary the depth to which each piston extends into its cylinder as the pump rotates, affecting the volume of discharge. A pressure compensator piston is used in some designs to maintain a constant discharge pressure under varying loads. Cheaper pressure washers sometimes use fixed-rate designs.

In a typical pressure-compensated pump, the swashplate angle adjusts through the action of a valve using pressure feedback to make sure that the pump output flow is precisely enough to maintain a designated pressure. If the load flow increases, the pressure momentarily decreases, but the pressure-compensation valve senses the decrease and then increases the swashplate angle to increase the pump’s output flow, restoring the desired pressure.

Axial piston pumps can contain most of the necessary circuit controls intrinsically by controlling the swash-plate angle, to regulate flow and pressure. They are very reliable and can allow the rest of the hydraulic system to which they’re attached to be very simple and inexpensive.

They are used to power the hydraulic systems of jet aircrafts, being gear-driven off of the turbine engine"s main shaft, and are often used for automotive air conditioning compressors for cabin cooling. The design of these pumps meets the limited weight and space requirement in the vehicle"s engine bay and reduces vibrations.

Pressure washers also use these pumps, and axial reciprocating motors are used to power many machines. They operate on the same principles as axial piston pumps, except that the circulating fluid is provided under substantial pressure and the piston housing rotates and provides shaft power to another machine. A typical use of an axial reciprocating motor is powering small earthmoving machines such as skid loader machines.

This guide provides a basic understanding of axial piston pumps. To find out more about other types of pumps, read our guide here. For more information on related products, consult our other product guides or visit the Thomas Supplier Discovery Platform to locate potential sources or view details on specific products.

Sudenga Industries is a leading manufacturer of durable ag equipment for grain, feed and seed handling applications. Products can be found in farm and commercial agriculture installations as well as industrial material handling applications worldwide. Sudenga was founded in 1888 in the northwest corner of Iowa where it is still located today.

A hydraulic valve is a mechanical device that regulates the flow of the hydraulic fluid in a hydraulic system. Hydraulic systems are typically high pressure systems, ranging from 200 Bar averaging 700 Bar upwards. This means they have to be constructed from materials that can withstand these high pressures. The methods of controlling these valves are also vast. They can be controlled physically and mechanically with electrical actuation, hydraulics, and pneumatics.

In Bernoulli’s tapered tube depicted below, varying the diameter of the pipe from d1 to d2 will increase the velocity of the fluid running through it (V1 < V2) whether the pipe is inclined or not. Increasing the tube’s velocity means the flow rate has also been increased, as shown before. So any mechanism that will vary the cross-sectional area across the valve will effectively vary the flow rate.

It will consist of a variable orifice and a mechanism that compensates for pressure loss. The fluid will flow a path as illustrated above. It enters through an inlet whose size is varied by the pressure compensator. In this example, the pressure compensator is a compensator spool. The compensator spool is spring loaded such that the resultant force from the spring, the hydraulic load and the incoming fluid will position it to open the inlet to just the right size to maintain a constant volumetric flow rate even with pressure drops in the system.

A variation of pressure compensated flow control valve is a temperature compensated flow control valve. This variation comes because sometimes the temperature of operation may rise such that set tolerances in orifices will become inaccurate. Temperature compensators are installed to cater to these variations.

This is the most basic method of fluid flow control. It consists of a drilled hole in what acts as a passage on an otherwise blocked fluid passage. When employed for flow control, it is usually put in series with the hydraulic pump.

A common adjustable flow control valve is a priority valve. Priority valves will switch flow to only a required outlet at a given time. For instance, if the pressure in a system drops to a certain extent, the priority valve will block other outlets just to supply the crucial outlet. It achieves this by a spring load that responds depending on the pressure applied.

In a hydraulic system, these valves are used to maintain or adjust the flow rate of the hydraulic fluid. They usually have a means to adjust the flow rate. This is usually an opening or port that is able to change the flow area and by altering that flow area, it then affects the flow rate.

A typical example would be controlling the speed of extending or retracting a hydraulic cylinder. It can sometimes be on hydraulic motors or any other hydraulic actuator. The speed of operation is directly related to the flow rate of the hydraulic fluid.

There is weight flow rate, measured in lb/sec generally used to compute power mainly in imperial units. There is also a mass flow rate measured in kg/sec, usually used to compute forces of inertia when decelerating or accelerating.

Since the flow control valves basically regulate the amount of fluid that passes through the valve per unit time, all flow control valves can be used for all types of flow rates. They vary by the mechanism that is employed to alter the flow rate. For example:

Ball valves use a mechanism of a ball that has holes in it. Anytime the holes align an input and an output, hydraulic fluid will flow in that path. As such, there are several ball valve configurations. These configurations vary with the number of inputs and outputs are linked. They can be two way, three way, or four way ball valves.

Ball valve mechanisms are used as “Switches” to shut out or open the flow. They can also be used as throttle valves when you turn them only partially but they are not well recommended for throttling.

The needle pin valve is used to control flow rates with high accuracy in low pressure applications. They are used to control flow rate, especially in pressure compensated flow control.

Needle valves work with a plunger that sits on a tapered orifice to shut off the flow. Opening and closing the flow is achieved by lifting the plunger.

(A) Is the handle that is fixed to the plunger, which can also be called a stem (F). As the handle is turned, the plunger will move up and down the threads (C) while the Locknut (B) will stop it from fully unscrewing. When the plunger comes down, the tapered end or stem (I) will sit on the valve seat and that fully seals the (H) orifice. G is an inlet port, (D) is the Bonnet, and (E) is the valve housing.

The needle pin can sometimes use an electric or pneumatic actuator to turn the plunger. These can also be remote controlled, especially in a closed-loop circuit with feedback.

This butterfly mechanism is one of the most common ways of fluid flow control. It incorporates a flipped disk to either open or close a pathway. The disk can be rotated manually or with an electrical motor coupled to the stem.

Butterfly valves are a very affordable means of flow control. They are also lightweight and the disk material comes in vast materials to cater to different hydraulic fluid properties. They can be used to shutout flow as well as to throttle flow.

Pressure-control valves as their name suggests are used to regulate the fluid pressure in a hydraulic system. This is done either by making sure the system pressure does not exceed a certain set point.

They keep the system pressure below a set level. They can be used to sustain upstream or downstream pressure from the valve. They also serve as protection to the equipment against pressure spikes or pulses.

Many fluid power systems work within a set pressure limit. These limits or ranges are a function of the generated forces required to do the work by the actuator. If these are left unmonitored, there can be excessive damage to the equipment. Relief valves chip in to help safeguard to prevent machine damage and operator damage.

The pressure at which a relief valve will start to allow flow to pass is called the cracking pressure. The difference between the current pressure in the system and the cracking pressure is called pressure differential or pressure override.

The direct acting relief valve will have a poppet ball that is directly exposed to the pressure in the system on one end. On the other end it is connected to a spring that pushes it against the system pressure. When a direct acting valve is a normally closed one, the force exerted by the spring will be greater than that of the system.

The spring can also be adjusted in length and thus the cracking pressure can be adjusted on these valves. When the system pressure surges that it becomes greater than the force from the spring, the fluid will unseat the ball such that it opens the flow to let excess fluid flow out until the system pressure is in the accepted range.

For applications where huge flows need to be relieved but using a small pressure override or small pressure differential, pilot operated relief valves are used.

The valve works in 2 different stages. The first is called the pilot stage. This is where a small relief valve (depicted as a rod with piston above) is moved so that it actuates the main relief valve (depicted with spring above).

The main relief valve acts as normally closed, especially if the system’s pressure is below that of the force exerted by the main valve spring. It must be noted that the main relief valve is exposed to pressure at both its ends with the front having less surface area in contact with the fluid than the back.

This difference in the surface area will mean that a rise in the system fluid pressure will be multiplied on the smaller surface area (pressure is inversely proportional to the area). This allows the main relief valve to open and empty excess fluids into the tank and thus subside the otherwise surging pressures.

In hydraulic systems, for regulating secondary lower pressure, pressure reducing valves are used. These valves are normally open and they are also two way valves that shut off the flow when subjected to unwanted downstream