What Is Hydraulic Pumps? How It Works? - SKengineers

 

WHAT IS HYDRAULIC PUMPS? HOW IT WORKS?

A hydraulic pump converts the mechanical energy from the prime mover into hydraulic energy for use by the system. Hydraulic energy is the combination of pressure and flow required by the actuators to perform useful work. It is important to understand that hydraulic energy is both pressure and flow combined, because one without the other cannot achieve work. Pressure would just consist of trapped fluid and flow would have no energy to move fluid alone.

A hydraulic pump pushes on fluid, and in this regard, fluid can be considered a solid as it is transmitted throughout the machine and then pushes on actuators to eventually move loads. Motion control professionals will have me point out that oil is compressible, but that’s a discussion for another blog. The point is that a pump could be pushing on sand, ball bearings or any other solid medium capable of taking the shape of its container, and the result would still be the transmission of force.

Transmission of force is really the name of the game with hydraulics, and is the basis for Cosford’s Law, which states that “pressure makes it go, flow is just the rate in which you can create pressure.” For fluid to be moving, pressure must absolutely be highest at the pump; always. This flies in the face of the fallacy that pressure is resistance to flow. Pressure will rise as high as it needs to be to overcome downstream resistance, but if it didn’t start at the pump, fluid would move backwards.

Pressure in hydraulics is the result of Newton’s Third Law of Motion, that every action has an equal and opposite reaction. The opposing force can be a loaded cylinder or a flow control, and the pump doesn’t care which. It will continue to push the fluid as pressure rises to overcome resistance, even if it results in something blowing up or the prime mover being overloaded.

Flow from a pump is a function of displacement (volume) and speed. A larger pump can push more fluid at once, or by spinning a pump faster, it will push on fluid more often. Just like in the world of electrons, where power is a combination of voltage and amperage, power in hydraulics is a combination of pressure and flow. By doubling pressure while leaving flow the same, horsepower is doubled. Also, by doubling flow while leaving pressure the same, horsepower is also doubled.

Understanding the operation of a hydraulic pump will do volumes (no pun intended) for your understanding of fluid power. When you realize all energy starts at the pump, you can better design or troubleshoot any system.

TYPES OF HYDRAULIC PUMPS -

GEAR PUMPS -

For truck-mounted hydraulic systems, the most common design in use is the gear pump. This design is characterized as having fewer moving parts, being easy to service, more tolerant of contamination than other designs and relatively inexpensive. Gear pumps are fixed displacement, also called positive displacement, pumps. This means the same volume of flow is produced with each rotation of the pump’s shaft. Gear pumps are rated in terms of the pump’s maximum pressure rating, cubic inch displacement and maximum input speed limitation.

Generally, gear pumps are used in open center hydraulic systems. Gear pumps trap oil in the areas between the teeth of the pump’s two gears and the body of the pump, transport it around the circumference of the gear cavity and then force it through the outlet port as the gears mesh. Behind the brass alloy thrust plates, or wear plates, a small amount of pressurized oil pushes the plates tightly against the gear ends to improve pump efficiency.  

FEATURES -

Most common design

Fewer moving parts, easy to service, more tolerant of contaminates, relatively inexpensive

Fixed, also called positive, displacement pumps

Rated in terms of max pressure rating, cubic inch displacement, max input speed limitation

Used in open centre hydraulic systems

Transports oil around circumference of gear cavity and forces it through outlet port

Encompasses thrust plates that push against gear ends with small amount of pressurized oil to improve pump efficiency.

PISTON PUMPS -

When high operating pressures are required, piston pumps are often used. Piston pumps will traditionally withstand higher pressures than gear pumps with comparable displacements; however, there is a higher initial cost associated with piston pumps as well as a lower resistance to contamination and increased complexity. This complexity falls to the equipment designer and service technician to understand in order to ensure the piston pump is working correctly with its additional moving parts, stricter filtration requirements and closer tolerances. Piston pumps are often used with truck-mounted cranes, but are also found within other applications such as snow and ice control where it may be desirable to vary system flow without varying engine speed.

A cylinder block containing pistons that move in and out is housed within a piston pump. It’s the movement of these pistons that draw oil from the supply port and then force it through the outlet. The angle of the swash plate, which the slipper end of the piston rides against, determines the length of the piston’s stroke. While the swash plate remains stationary, the cylinder block, encompassing the pistons, rotates with the pump’s input shaft. The pump displacement is then determined by the total volume of the pump’s cylinders. Fixed and variable displacement designs are both available.

FEATURES -

Withstand higher pressures

Higher initial cost, lower resistance to contamination and increased complexity

Additional moving parts, stricter filtration requirements and closer tolerances

Truck-mounted cranes

Good when desirable to vary system flow without varying engine speed

Fixed and variable displacement designs available

Encompasses cylinder block containing pistons that move in and out – this movement draws oil from the supply port and forces through the outlet

Angle of swash plate determines the length of the piston’s stroke

Swash plate remains stationary.

Displacement determined by total volume of pump cylinders.

FIXED DISPLACEMENT -

With a fixed displacement piston pump, the swash plate is nonadjustable. Its proportional output flow to input shaft speed is like that of a gear pump and like a gear pump, the fixed displacement piston pump is used within open centre hydraulic systems.

VARIABLE DISPLACEMENT

As previously mentioned, piston pumps are also used within applications like snow and ice control where it may be desirable to vary system flow without varying engine speed. This is where the variable displacement piston pump comes into play – when the hydraulic flow requirements will vary based on operating conditions. Unlike the fixed displacement design, the swash plate is not fixed and its angle can be adjusted by a pressure signal from the directional valve via a compensator.

Should more flow be required, the swash plate angle changes, increasing the pump displacement by creating a longer piston stroke. Contrary to a fixed displacement piston pump, the variable displacement is used in a closed centre system. With a closed centre system, the swash plate angle within the variable displacement pump decreases as the flow requirement diminishes so that there is no excess flow or loss of hydraulic horsepower.

Variable displacement piston pumps can be flow compensated, pressure compensated or both flow and pressure compensated.

Flow Compensated –

As flow requirements change, the swash plate angle is adjusted to maintain a constant margin of pressure.

Pressure Compensated –

Regardless of changes in system pressure, a specified flow is maintained through adjusting the swash plate angle.

Flow and Pressure Compensated Combined –

 These systems with flow and pressure compensation combined are often called a load-sensing system, which is common for snow and ice control vehicles.

VANE PUMPS -

Vane pumps were, at one time, commonly used on utility vehicles such as aerial buckets and ladders. Today, the vane pump is not commonly found on these mobile (truck-mounted) hydraulic systems as gear pumps are more widely accepted and available.

Within a vane pump, as the input shaft rotates it causes oil to be picked up between the vanes of the pump which is then transported to the pump’s outlet side. This is similar to how gear pumps work, but there is one set of vanes – versus a pair of gears – on a rotating cartridge in the pump housing. As the area between the vanes decreases on the outlet side and increases on the inlet side of the pump, oil is drawn in through the supply port and expelled through the outlet as the vane cartridge rotates due to the change in area.

FEATURES –

Used on utility vehicles, but not as common today with gear pumps more widely accepted and available

Input shaft rotates, causing oil to be picked up between the vanes of the pump which is then transported to pump outlet side as area between vanes decreases on outlet side and increases on inlet side to draw oil through supply port and expel though outlet as vane cartridge rotates.

 Screw pumps -

Screw pumps (fixed displacement) consist of two Archimedes' screws that intermesh and are enclosed within the same chamber. These pumps are used for high flows at relatively low pressure (max 100 bars (10,000 kPa)).[clarification needed] They were used on board ships where a constant pressure hydraulic system extended through the whole ship, especially to control ball valves[clarification needed] but also to help drive the steering gear and other systems. The advantage of the screw pumps is the low sound level of these pumps; however, the efficiency is not high.

The major problem of screw pumps is that the hydraulic reaction force is transmitted in a direction that's axially opposed to the direction of the flow.

There are two ways to overcome this problem:

put a thrust bearing beneath each rotor;

create a hydraulic balance by directing a hydraulic force to a piston under the rotor.

Types of screw pumps –

single end

double end

single rotor

multi rotor timed

multi rotor untimed.

Bent axis pumps

Bent axis pumps, axial piston pumps and motors using the bent axis principle, fixed or adjustable displacement, exists in two different basic designs. The Thomas-principle (engineer Hans Thomas, Germany, patent 1935) with max 25 degrees angle and the Wahlmark-principle (Gunnar Axel Wahlmark, patent 1960) with spherical-shaped pistons in one piece with the piston rod, piston rings, and maximum 40 degrees between the driveshaft center-line and pistons (Volvo Hydraulics Co.). These have the best efficiency of all pumps. Although in general, the largest displacements are approximately one litre per revolution, 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 can in general work with a working pressure of up to 350–420 bars in continuous work.

Inline axial piston pumps -

axial piston pump, swashplate principle -

By using different compensation techniques, the variable displacement type of these pumps can continuously alter fluid discharge per revolution and system pressure based on load requirements, maximum pressure cut-off settings, horsepower/ratio control, and even fully electro proportional systems, requiring no other input than electrical signals. This makes them potentially hugely power saving compared to other constant flow pumps in systems where prime mover/diesel/electric motor rotational speed is constant and required fluid flow is non-constant.

Rotary vane pumps -

Fixed displacement vane pump -

A rotary vane pump is a positive-displacement pump that consists of vanes mounted to a rotor that rotates inside a cavity. In some cases these vanes can have variable length and/or be tensioned to maintain contact with the walls as the pump rotates A critical element in vane pump design is how the vanes are pushed into contact with the pump housing, and how the vane tips are machined at this very point. Several type of "lip" designs are used, and the main objective is to provide a tight seal between the inside of the housing and the vane, and at the same time to minimize wear and metal-to-metal contact. Forcing the vane out of the rotating centre and towards the pump housing is accomplished using spring-loaded vanes, or more traditionally, vanes loaded hydrodynamically (via the pressurized system fluid).

Radial piston pump -

A radial piston pump is a form of hydraulic pump. The working pistons extend in a radial direction symmetrically around the drive shaft, in contrast to the axial piston pump.

CLUTCH PUMPS -

A clutch pump is a small displacement gear pump equipped with a belt-driven, electromagnetic clutch, much like that found on a car’s air conditioner compressor. It is engaged when the operator turns on a switch inside the truck cab. Clutch pumps are frequently used where a transmission power take-off aperture is not provided or is not easily accessible. Common applications include aerial bucket trucks, wreckers and hay spikes. As a general rule clutch pumps cannot be used where pump output flows are in excess of 15 GPM as the engine drive belt is subject to slipping under higher loads.

FEATURES -

Small displacement pumps

Belt driven

Aerial bucket trucks, wreckers and hay spikes

Limited to 15 GPM applications

DUMP PUMPS -

Of the different types of hydraulic pumps, the dump pump is the most recognizable. This type of pump is commonly used in dumping applications from dump trailers to tandem axle dump trucks. The dump pump is specifically designed for one application – dump trucks – and is not suitable for other common trailer applications such as live floor and ejector trailers.

What separates this pump from the traditional gear pump is its built-in pressure relief assembly and an integral three-position, three-way directional control valve. The dump pump is unsuited for continuous-duty applications because of its narrow, internal paths and the subsequent likelihood of excessive heat generation.

Dump pumps are often direct mounted to the power take-off; however, it is vital that the direct-coupled pumps be rigidly supported with an installer-supplied bracket to the transmission case with the pump’s weight at 70 lbs. With a dump pump, either a two- or three-line installation must be selected (two-line and three-line refer to the number of hoses used to plumb the pump); however, a dump pump can easily be converted from a two- to three-line installation. This is accomplished by inserting an inexpensive sleeve into the pump’s inlet port and uncapping the return port.

Many dump bodies can function adequately with a two-line installation if not left operating too long in neutral. When left operating in neutral for too long however, the most common dump pump failure occurs due to high temperatures. To prevent this failure, a three-line installation can be selected – which also provides additional benefits.

FEATURES -

Dump pump most recognizable

Specifically designed for dump trucks

Displacement of slightly more than six cubic inches, pressure relief assembly and integral three-position, three-way directional control valve

Not suited for continuous-duty applications

Often direct coupled to PTO, need installer-supplied bracket to support

Two- and three-line installations available (two-line can be converted to three-line)

REFUSE PUMPS -

Pumps for refuse equipment include both dry valve and Live Pak pumps. Both conserve fuel while in the OFF mode, but have the ability to provide full flow when work is required. While both have designs based on that of standard gear pumps, the dry valve and Like Pak pumps incorporate additional, special valving.

DRY VALVE PUMPS -

Primarily used on refuse equipment, dry valve pumps are large displacement, front crankshaft-driven pumps. The dry valve pump encompasses a plunger-type valve in the pump inlet port. This special plunger-type valve restricts flow in the OFF mode and allows full flow in the ON mode. As a result, the horsepower draw is lowered, which saves fuel when the hydraulic system is not in use.

In the closed position, the dry valve allows just enough oil to pass through to maintain lubrication of the pump. This oil is then returned to the reservoir through a bleed valve and small return line. A bleed valve that is fully functioning is critical to the life of this type of pump, as pump failure induced by cavitation will result if the bleed valve becomes clogged by contaminates. Muncie Power Products also offer a butterfly-style dry valve, which eliminates the bleed valve requirement and allows for improved system efficiency.

It’s important to note that with the dry valve, wear plates and shaft seals differ from standard gear pumps. Trying to fit a standard gear pump to a dry valve likely will result in premature pump failure.

How do hydraulic pumps work?

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.

Advantages of Pump -

These are some advantages of Pump -

As there is no drive seal so there is no leakage in the pump.

There are very less frictional losses.

The construction of the pump is Simple.

Almost no noise.

Minimum wear as compared to others.

Disadvantages of Pump -

These are some disadvantages of Pump -

Produce cavitation.

Corrosion.

Cannot be able to work at high speed.

Applications of Pump -

The main applications of the pump are -

As we already discussed Pumping Water from one place to another place.

Aquarium and pond filtering

This is also used for Water cooling and fuel injection in automobiles

Pumping oil or gas and operating cooling towers in the energy industry.

Uses in waste-water recycling, pulp, and paper, chemical industry, etc.

So this is all about Pump, I hope you like my article, by the way, feel free to post your doubts on the comment section I will love to answer those.