Hydraulic pumps

Hydraulic pumps are used in literally every single hydraulic power transmission system. A hydraulic pump is the device that converts mechanical energy into hydraulic energy, which is a combination of pressure and flow. A hydraulic pump can be any device that you can input force into to create pressure, which in turn creates flow.

Image courtesy of Hydro Ludec

Most hydraulic pumps have a mechanical input from an internal combustion engine or electric motor. These prime movers input their mechanical power to the hydraulic pump in a rotational fashion. The input shaft of the pump will be connected to gears, vanes or pistons of the hydraulic pump, where they will rotate or reciprocate to transfer pressure (force) to the hydraulic fluid. As long as the force (pressure) created by the pump is high enough, flow will occur at a rate dictated by the displacement volume of the pump and the speed at which it rotates.

These pumps, also called positive displacement pumps, have a small clearance between rotating and stationary parts. A specific amount of fluid is delivered to the system for each revolution. Positive-displacement pumps can be further divided into two categories: fixed- and variable-displacement. Fixeddisplacement pumps provide a single, specific volume displacement per revolution. In variable-displacement pumps, displacement per cycle can vary from zero to maximum volumetric capacity. Some of the more widely used types of positive-displacement pumps are gear, piston and vane.

Gear pumps can be either internal or external styles. External gear pumps are one of the most popular types used in modern hydraulic systems. Gear pumps produce flow by using the teeth of two meshing gears to move the fluid. Their simple construction ensures limited purchase costs and servicing. They feature decent mechanical and volumetric efficiency, compact dimensions and low weight/power ratio. Of the three common types of positive displacement pumps, gear pumps are the least efficient; their appeal is low cost and simple design.

External gear pumps can be equipped with straight spur (the most common type), helical or herringbone gears. In operation, the drive gear and driven gear rotate, creating a partial vacuum at the pump inlet (where gear teeth unmesh) that draws fluid into gear teeth. Gear teeth mesh at the outlet, forcing fluid out of the pump.

Image courtesy of Oilgear

Internal gear pumps contain one internal and one external gear. They pump fluid in the same manner as external spur gear pumps. In the basic design, the internal gear, which drives the outer gear, has one tooth less than the outer gear. As they mesh, the teeth create sliding seal points. Because their transition zone from low to high pressure (the area over the crescent) is relatively long, internal gear pumps can offer lower noise levels than some other types of pumps.

Gears are made of special steel and are often case hardened and quench hardened. Then gears are ground and fine finished. Proper tooth profile design and geometric proportions can reduce pulsation and noise levels during pump operation.

Piston pumps supply high flows at high speed. Two types of piston pumps— axial and radial—are manufactured in both fixed- and variable-displacement versions. Axial-piston pumps contain one or more pistons that convert rotary shaft motion into axial reciprocating motion. An angled cam (or wobble plate) rotates, causing pistons to reciprocate and take fluid in as they move toward the thin part of the plate. Fluid is expelled as pistons approach the thick end. In the bent-axis design, both pistons and shaft rotate, making a wobble plate unnecessary. Bent-axis pumps use the drive shaft to rotate pistons.

With the longer sealing paths along the piston walls, piston pump efficiencies tend to be higher than other types of pumps. In addition, variabledisplacement pumps can provide savings by only providing the pumping necessary for the function, saving additional energy and costs.

Radial-piston pumps (fixed-displacement) are used especially for high pressure and relatively small flows. Pressures of up to 10,000 psi are common. Variable-displacement is not possible, but sometimes the pump is designed in such a way that the plungers can be switched off one by one, so that a sort of variable-displacement pump is obtained.

Radial-piston pumps are characterized by a radial piston arrangement within a cylinder block. As pistons reciprocate, they convert rotary shaft motion into radial motion. One version has cylindrical pistons, while another uses ballshaped pistons. Another classification refers to porting: Check-valve radial-piston pumps use a rotating cam to reciprocate pistons; pintle-valve pumps have a rotating cylinder block, and piston heads contact an eccentric stationary reaction ring.

Rotary vane pumps (fixed and simple adjustable displacement) generally have higher efficiencies and lower noise levels than gear pumps. They can be used for mid pressures of 2,500 psi.

Image courtesy of Continental Hydraulics

Some types of vane pumps can change the center of the vane body, so that a simple adjustable pump is obtained. These adjustable vane pumps are constant pressure or constant power pumps. Displacement is increased until the required pressure or power is reached and subsequently the displacement or swept volume is decreased until equilibrium is reached.

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 types 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 center and toward the pump housing is accomplished using spring-loaded vanes, or more traditionally, vanes loaded hydrodynamically (by the pressurized system fluid).

COMMON PUMP SELECTION CONSIDERATIONS

Hydraulic pumps have just one role; transform incoming mechanical energy into hydraulic energy. All pumps provide flow and pressure (pressure starts at the pump, not the resistance as is commonly mistaken). With one simple task, there is surprising variability in the pump market. This FAQ should elucidate the intricacies of pump selection for you.

Q: Do pumps only provide flow?

A: No! This is a common myth in the fluid power world. Energy can only move from an area of higher potential to lower potential. If pressure started at the restriction, as many people believe, fluid would travel backwards to the pump.

Cosford’s Law states that “pressure makes it go, and flow is the rate in which you can create force.” The pump is your hydraulic system’s source for both flow AND pressure.

Q: Will installing a larger pump provide my hydraulic system with more force?

A: Your pump’s size represents its displacement. A higher pump displacement will provide you with more flow, all things being equal. Force is dictated by pressure, not flow. A larger pump will provide you with nothing more than extra speed so long as your prime mover (motor) can handle the extra horsepower.

If you are looking for extra force from the same prime mover, downsizing to a smaller pump while increasing your pressure will help you achieve more force, so long as all the components used can withstand the increase in pressure.

Q: Should I install a gear, vane or piston pump?

A: Your choice of gear, vane or piston pump depends on your ideal trade-off between budget and performance. Pumps are increasingly more expensive gear to vane, and from vane to piston. Each pump has its strengths and weaknesses, as spelled out below.

Gear pumps are inexpensive but are often noisy and inefficient. Gears pumps are highly resistant to contamination and readily available from nearly every manufacturer.

Vane pumps are modestly priced but not inexpensive. Vane pumps are more efficient than gear pumps but much quieter. Vane pumps are also reliable and easily repaired.

Piston pumps are the most efficient, most expensive and with the highest pressure capacity. They are often loud, sometimes bulky, but if your budget can afford them, they should be at the top of your shopping list.