How to Select a Hydraulic Pump and Motor

When designing a hydraulic system, choosing the right motor and pump is crucial for ensuring optimal performance. The goal behind building an efficient hydraulic solution is to select products that match the overall efficiency of the performance you are looking to achieve.

Motors and pumps are both key components in any hydraulic system, and it’s important that they work together in tandem. To achieve this, there’s a process that should be followed when selecting a hydraulic motor and pump.

In this article, we will ensure that you have a clear understanding of the differences between a hydraulic pump and motor, the parts they play in a system, and how they should be paired.

So, what comes first? The motor or the pump?

The design and performance of a hydraulic system will change depending on the motor you select, so this should be the first step in the process. This is because the motor selection should be based on the system’s load requirements first, before considering a pump that will work to drive the fluid power into the motor.

How to choose a hydraulic motor

Now that you know the motor comes first in the process, how do you choose the right one for your application?

There are many different hydraulic motors available, all boasting varying operating specifications, features and performance profiles. This is the data that you will use to determine which is the right fit for your hydraulic system – finding the one that best meets the performance expectations of your application.

Hydraulic motors produce a rotary actuation when pressure is applied from a fluid. This can either be a bidirectional or unidirectional actuation, depending on the design of the system.

What to consider when choosing a motor

Operating specifications vary between motors, and they all offer different performance indicators and capabilities which should be considered when looking to purchase.

The main operating specifications of motors to look out for, are:

• Operating torque defines the torque that the motor is capable of delivering, i.e. the amount of twisting action it can offer.
• Operating pressure is the pressure at which the fluid will be fed to the motor, which affects the motors’ torque and power.
• Operating temperature dictates the temperature range of the fluid which the motor can handle. These will usually be defined as minimum and maximum temperatures.
• Operating speed indicates the speed at which the motor rotates, in revolutions per minute (RPM).
• Power is dependent on the flow and pressure of fluid being delivered to the motor, but each motor will have different power capabilities.
• Displacement is used to rate motors and is measured in cubic inches per revolution (CIR), or cubic centimetres per revolution (CCR) which indicates the volume of fluid it takes to rotate the shaft once.

All of these specifications will vary greatly between products, so it’s important to match them up with the expectations of your system to choose the best hydraulic motor that will guarantee performance.

Types of hydraulic motor

There are many different types of hydraulic motor available, all designed to suit different systems and deliver varying performances. Typically, you will find each motor placed into one of two classifications: high speed, low torque (HSLT) or low speed, high torque (LSHT).

The motor you choose should have the capabilities and capacity to generate the output required for the application’s needs. For best practice, consider choosing a motor that delivers higher maximum ratings than required if the budget allows, to prevent it from being overworked and extend its lifespan.

Gear motors

Gear motors are simple in design, consisting of a matched gear set enclosed in robust housing. The gears (internal or external) generate torque when the hydraulic fluid enters the motor.

They are typically classified as LSHT motors, and offer a wide range of speeds and temperature ranges, with medium pressure capabilities. Small, compact and lightweight, gear motors are widely used in mobile hydraulics.

Vane motors

The large majority of vane motors are classified as HSLT units. They feature a vane that causes the output shaft to rotate once hydraulic fluid enters the motor, with medium pressure capabilities.

Used across a wide range of industrial and mobile applications, vane motors offer good operating efficiencies and generally produce less noise than other types of motors.

Piston motors

Piston motors are generally found in two different types: axial and radial.

Axial piston motors generate output torque with the use of an axially-mounted piston and are typically classified as HSLT units.

In radial piston motors, multiple pistons are mounted perpendicular to the shaft and can deliver high torques at low speeds.

Choosing a hydraulic pump for your motor

After selecting your hydraulic motor, you will have a good understanding of the rating of the motor and therefore the power output you are looking for in a pump.

For best practice, it’s recommended to choose a hydraulic pump with similar ratings as the motor. To prevent the motor from being overloaded, the fitting of motor mounted pressure relief valves is advised.

{When designing the motor pump unit there should be at least 20% more power available than the system requires (fudge factor/safety factor)}.

From gear pumps and vane pumps to piston pumps, there are many different options to consider. Whilst the motor chosen should assist in the requirements of your pump, other considerations also include:

• Hydraulic fluid viscosity – how thick is the hydraulic liquid that will be pressurised by the pump? Pumps will have a maximum kinematic viscosity rating which you should adhere to maximise efficiency.
• Flow rate – hydraulic pumps have a maximum flow rate depending on how much fluid they can move.
• Maximum operating pressure – this will indicate how much pressure the pump can effectively handle without it causing damage to the component.
• Fixed displacement or variable displacement – choose the configuration that meets the demands of your application, whether you require the same amount of fluid moving (fixed), or changes to flow rate and outlet pressure (variable).

 

Above all that, you will need to correctly size your motor and pump to suit the application. For this, many different mathematical formulas and equations should be factored into the process.

Designing a hydraulic system is no easy task, and is certainly not something to go into with limited knowledge of how the components work in conjunction with one another. For a hydraulic solution that meets all of your requirements, we highly recommend seeking the help of a professional.

Primary Fluid Power has over 40 years of hydraulic experience, with a team of technical experts ready to find the perfect solution for you. Whether you need some advice on which components you need or a bespoke build – we can help.