Guide to Choosing the Right Hydraulic Pump
If you’re in the market for a new hydraulic pump, you may be a little overwhelmed by all of the different options in front of you. Between pump types, styles, fluids and all the specifications you need to follow, it can be a difficult task. Different applications have different needs and understanding how the design characteristics of hydraulic pumps influence each other can help you better fit the pump to the job.
Fortunately, we’ve compiled a guide to get you ready for a hydraulic pump purchase. Keep reading to learn more about different types and how to choose a hydraulic pump.
Types of Hydraulic Pumps
Hydraulic pumps use the principle of fluid displacement to convert mechanical energy into hydraulic energy, imparting it onto the gears of the pump. In general, the pump inlet creates a vacuum, using atmospheric pressure to pull the liquid in, out of the reservoir. Then the pump’s mechanical action, which varies by design, pushes the liquid through to the outlet and the rest of the hydraulic system. The pump does not generate pressure but creates flow.
Pumps can use positive displacement or non-positive displacement methods to generate this energy, which affects the rate of flow.
- In a positive-displacement pump, a fixed amount of fluid is trapped and displaced. The volume of liquid stays constant, and pressure does not increase. A positive displacement pump works on low-pressure applications, and they are sometimes called hydrostatic pumps.
- A non-positive displacement pump changes the velocity of the fluid as needed to create a continuous flow. This means that the pressure can vary with each cycle, but the maximum pressure is much higher than that of a positive displacement pump. Some pumps that follow this design are centrifugal and propeller pumps.
Most hydraulic pumps use positive displacement and include the following.
Gear pumps include internal and external types. In either configuration, two gears mesh together to carry fluid in between the teeth. These gears are usually straight spur, herringbone or helical styles. The drive shaft powers one gear, while the other idles and moves by linking up to the other. Since the chambers between the teeth are sealed, a vacuum forms when they come apart and pulls in the fluid from the inlet.
A gear pump tends to have high-efficiency levels when running at its maximum speed. There are several different design types when it comes to gear pumps.
- Internal gear pump: An internal gear pump, sometimes called a gerotor pump, is made up of two gears, one smaller than the other, usually with one or two fewer teeth in it. The outer gear has teeth pointing inward, while the inner gear has teeth pointing outward, so they mesh for part of the rotation. The flow of liquid moves through space inside of the gears. They create very little pulsation but lack high-pressure capabilities.
- External gear pump: In an external gear pump, fluid moves around the outside of two interlocked gears. Liquid moves between the pump housing and the gears, which form a vacuum to pull liquid in from the inlet port. These pumps offer higher speeds and relatively quiet operation.
- Axial-flow gear pump: An axial-flow gear pump, also called a screw pump, uses one, two or three screws to generate an axial flow. In two- and three-screw pumps, the rotors use intermeshing threads to carry the liquid. A single-screw pump moves it around between the rotor and the pump housing. This design offers very quiet operation since there is no pulsation or contact of metal parts.
In a vane pump, small vanes move in and out of a central rotor, usually offset for eccentricity, to create chambers that transfer the liquid through an oval- or crescent-shaped opening. The vanes extend outward past the edge of the central rotor and create the chambers when they push against the housing wall and extend with the natural curve of the space. Liquid is forced through the outlet when the space closes. A vane pump can follow fixed or variable displacement but has a relatively complex design compared to some other pump models.
A piston pump uses rotary power to create flow and is typically available in axial and radial types, which can offer fixed or variable displacement.
- Inline axial pumps: In this design, pistons are arranged in a circle in a cylinder block, with an angled swashplate on one side and inlet and outlet ports on the other. As the block rotates, the pistons meet the angle of the swashplate, causing them to move in and out and creating space in the chamber. This action occurs near the inlet valve, which creates a vacuum to pull in liquid. By the outlet valve, the pistons are forced back in, pushing the liquid out. The variable-displacement model allows the swashplate to change its angle and, by extension, the size of the piston stroke and the space it creates.
- Radial-piston pumps: Radial piston pumps have several different configuration options, including a change in piston shapes, variable and fixed displacement choices and valve types. For the most part, a radial pump is designed with pistons placed like wheel spokes around an eccentrically placed cam mounted on the drive shaft. As the drive shaft rotates, it moves the cam so the pistons push inward as it passes them. Springs on the pistons allow them to extend out as the cam moves further away. Each piston has an inlet and outlet port leading to its chamber, where valves control the intake and release of fluid.
- Bent-axis pumps: In the bent-axis design, two parts of the pump meet at an angle. On one side, the drive shaft turns a cylinder block with pistons attached. These pistons match up to bores in the other side. As the block with the pistons rotates, the distance between the pistons and the valving surface changes, altering the amount of space in the chamber. On the other side of the mechanism, these chambers take in liquid as the pistons move out and force it through the outlet as the pistons push in. The angle of the two parts partially determines the degree of displacement.
Considerations and Features
Between power, noise, maximum pressure and other factors, choosing a hydraulic pump involves a lot of different considerations. You’ll have to think about your needs and how you’ll operate the pump. Here are some features you should consider when purchasing one:
1. Hydraulic Fluid Viscosity
Fluid viscosity refers to the thickness of the liquid in your pump. The viscosity of your fluid can influence how well a given hydraulic pump will perform and which one you should use. Most pumps will have a maximum kinematic viscosity rating associated with them, and you’ll want to stick to this number. A fluid with too low a viscosity can limit the pump’s efficiency and increase wear. A fluid that is too viscous can also decrease efficiency and cause mechanical problems.
2. Fluid Type
The fluid used in your hydraulic system should also match the pump’s specifications. Most pumps will work well with standard hydraulic fluid, which is usually based in mineral oil. It has inherently good lubrication properties with a higher boiling point than water. Other types of fluid that you may need for specific applications include:
- Biodegradable: Many environmentally-sensitive applications use biodegradable hydraulic fluids to reduce risk in the event of spills. These fluids include those derived from vegetables, like soybean, sunflower and rapeseed oil. They are typically high in lubricity and are inherently anticorrosive, though they do oxidize quickly and can be contaminated by water. There are a few other types of biodegradable fluids that address some of these issues but may have more specific system requirements. Farm equipment and marine work are examples of jobs that may require biodegradable fluids.
- Phosphate ester: This synthetic fluid offers high thermal stability, lubrication and antiwear properties. Its primary use is in high-temperature applications where there is a risk of fire. Between high ignition temperatures, minimal oxidization and low heats for combustion, phosphate esters are hard to burn and can even self-extinguish. They are less viscous, however, and can be chemically aggressive on some seals and coatings. This kind of system can be expensive to maintain.
- Water glycol: Water glycol fluids are another fire-resistant option in applications where this is a concern. Water glycol is made up of water, a high molecular weight polyglycol, ethylene or diethylene glycol and an additive package. It is generally 38-45% water. The additives can offer characteristics like corrosion resistance, oxidation resistance and antiwear.
These fluids are not interchangeable with every pump, and finding one that works with your fluid needs is vital to choosing the best industrial hydraulic pump for your operation.
3. Flow Rate
Flow rate is calculated with:
- Pump speed in revolutions per minute (RPM)
- Pump efficiency in a percentage
- Displacement value
Pumps typically have maximum flow ratings in gallons per minute or liters per minute, which tell you how much they can move and may determine its ability to meet your needs.
4. Power Curves/Torque Ratings
When looking at the power of a pump, we combine torque and rotational speed. Torque is a critical component of determining your power needs. The easiest method of choosing needed torque is by comparing a new machine with an existing one that performs a comparable job. Consider how much more work is going to be needed of the new machine and multiply it by the old one’s torque rating. If an old machine is not available, you’ll have to calculate the torque.
Power curves can help you select the right pump by showing you a visual representation of how the power is affected by other specifications. It shows you how much power the pump demands at certain flow rates.
Remember that gasoline engines differ from electric motors because of the internal combustion engine’s torque-speed curve. Hydraulic pumps running on gasoline-powered engines need a higher power capacity than they would with an electric motor.
Operating speed looks at the revolutions per minute that the driveshaft makes. Different designs may offer higher or lower operating speeds.
6. Max Operating Pressure
Pumps usually have a maximum operating pressure listed in bars or pressure per square inch (PSI). Remember that a pump doesn’t create pressure. Any pressure is created by a load on the fluid. Loads placed on the fluid show up as pressure on the outlet, which is the maximum for the pump. This rating identifies where a pump can effectively withstand pressure without leaking or damaging the parts. Maximum operating pressure can vary widely across different pump designs.
7. Fixed Displacement Vs. Variable Displacement
Many types of hydraulic pumps are available in both fixed displacement and variable displacement configurations. These models differ in the amount of fluid that they displace.
- For a fixed displacement model, each cycle results in moving the same amount of fluid.
- A variable displacement pump is a little more complicated but can change factors such as flow rate and outlet pressure.
Variable displacement pumps are good for a wider variety of tools and projects, but fixed-displacement pumps work well in applications where they will be performing the same task repeatedly.
8. Maintenance Time and Costs
You’ll also want to factor in the maintenance considerations when purchasing a new hydraulic pump. Some are costlier than others to maintain, and staying on top of maintenance can help you extend your pump’s life and improve its performance.
Variable displacement pumps are generally more expensive to maintain due to their complexity. Some pumps have design properties that make them more adaptable to wear. Vane pumps, for example, are quite reliable since the vanes simply extend out as much as they need to if the casing starts to wear. External gear pumps are also known for being durable, but internal ones can be costly. Axial and bent-axis piston pumps are a little more complex to fix but tend to have long lifespans.
RG Group for Your Hydraulics Needs
Choosing a hydraulic pump is no easy task. It requires you to know a lot about your system’s demands and how different characteristics will interact with each other. Whether you know exactly what you need or may still need a little help, the experts at RG Group are ready to assist.
We’ve been in the business of hydraulics for over 70 years and take pride in our ability to support your company. Our staff is full of highly trained and certified technicians and engineers to help bring your hydraulics to life. We’ve worked in a wide variety of industries, from agriculture to power generation to transportation. Our line of services includes full assembly, testing and systems integration.
For more information on selecting the best industrial hydraulic pump for your business needs, contact one of our representatives today, or take a look at our selection of high-quality Parker hydraulic pumps. Whatever you need to get your hydraulic power up and running, RG Group can help.