Rightfully Size a Hydraulic Pump and Motor

To have an ideal hydraulic system, the right size of the hydraulic pump has to be met with the right motor size. The key to having a hydraulic pump that performs up to expectation is to match the pump and the motor to a specific performance rating. When sizing the motor and pump component of the hydraulic system, the prime size consideration is based on the torque, speed, and power of the hydraulic pump.

Knowing how to size a hydraulic pump and motor can help increase operational efficiency and reduce energy consumption. This article sets out to help you determine the right sizing for your hydraulic pump and motor.

Factors to Consider when Sizing Hydraulic Pumps and Motors

Before you start sizing your pump and motor, you need to consider the requirements of your application. Here are the main factors to keep in mind:

Flow Rate: The flow rate is the amount of fluid that the pump can move per unit of time. It’s usually measured in gallons per minute (GPM) or liters per minute (LPM). You need to make sure that your pump can handle the required flow rate for your application.

Pressure: Pressure is the force that the fluid exerts on the walls of the hydraulic system. It’s usually measured in pounds per square inch (PSI) or bar. You need to make sure that your pump can generate the required pressure for your application.

Power Requirements: The power requirement is the amount of energy that the pump needs to generate to move the fluid. It’s usually measured in horsepower (HP) or kilowatts (kW). You need to make sure that your pump can generate the required power for your application.

Operating Conditions: You also need to consider the operating conditions of your application, such as the temperature and viscosity of the fluid, as well as any special requirements, such as explosion-proof or marine-grade pumps.

Calculating the Correct Size for Hydraulic Pumps and Motors

Once you’ve determined the requirements for your application, you can start calculating the correct size for your pump and motor. Here are some of the methods you can use:

Basic Formulas: There are some basic formulas you can use to calculate the required flow rate, pressure, and power for your application. For example, to calculate the required flow rate, you can use the formula:

Flow rate = (Cylinder area x Stroke length x Number of cycles per minute) / 231

Sizing Charts and Tables: Many manufacturers provide sizing charts and tables that you can use to find the correct size for your pump and motor. These charts and tables take into account the factors we discussed earlier, as well as other variables such as the speed and torque requirements of your application.

Examples of Sizing Calculations: To give you an idea of how to apply these methods, let’s take a look at some examples of sizing calculations. For example, if you need a pump for a hydraulic press that requires a pressure of 2,000 PSI and a flow rate of 10 GPM, you can use the formula for pump power:

Pump power = (Pressure x Flow rate) / 1714

Using this formula, we can calculate that the required pump power for this application is:

Pump power = (2000 PSI x 10 GPM) / 1714 = 11.67 HP

Once you have determined the required pump power, you can look for a pump that can generate this power while meeting the other requirements of your application.

Hydraulic pump motor power calculation

The ideal-size electric motor is one with the most suitable horsepower (Hp). You need to determine the HP motor rating that is suitable to power your pump. Sizing a hydraulic motor for a pump can be calculated based on the level of pressure of the hydraulic pump.

Sizing hydraulic motor for high-pressure hydraulic pump

How to rightfully size a hydraulic pump and motor - High pressure hydraulic pump

The formula for calculating the required motor Hp for a high-pressure hydraulic pump is quite straightforward. This HP is generally gotten by finding the product of flow capacity in gallons per minute (GPM) and the pressure in pounds power square inch. You then need to divide the result by 1714 times the efficiency per inch (PSI) of the pump.

The horsepower equation is expressed thematically as:

Horsepower HP = Q x P / 1714 x e
HP is power rating in horsepower,
Q is flow in GPM,
P is pressure in psi,
And e is the pump’s mechanical efficiency

The above formula can be used to calculate the sizing of a motor to be used in terms of rating for a high-pressure hydraulic pump

Sizing hydraulic motor for low-pressure pumps

How to rightfully size a hydraulic pump and motor - Gerotor hydraulic pump

Low-pressure pumps generally indicate a low efficiency that is lower than 85%. Overall efficiency is the summation of internal mechanical friction and volume efficiency. When using a low-pressure pump calculations should be made taking low pressure into account.

When sizing a hydraulic motor for low-pressure pumps, it’s important to consider the lower efficiency of the pump, which can affect the overall system performance. To calculate the required motor size, you need to determine the torque and speed requirements of your application.

The formula for calculating the torque required is:

Torque (lb-ft) = (horsepower x 5252) / speed (RPM)

Once you have determined the required torque and speed, you can select a motor that meets these requirements while considering other factors such as mounting type, displacement, and speed range.

For example, if you have a low-pressure pump with a flow rate of 5 GPM and a pressure of 500 PSI, you can use the formula for pump power to determine the required horsepower:

Pump power = (500 PSI x 5 GPM) / 1714 = 1.46 HP

Assuming a motor efficiency of 90%, the required input horsepower to the motor would be:

Input horsepower = 1.46 HP / 0.9 = 1.62 HP

Using the torque formula, we can calculate the required torque at a speed of 1000 RPM:

Torque (lb-ft) = (1.62 HP x 5252) / 1000 RPM = 8.52 lb-ft

By considering these factors and calculations, you can size your hydraulic motor for low-pressure pumps and ensure optimal performance and efficiency in your hydraulic system.

Sizing a motor for different pressure hydraulic pumps

If the system involves different pressures at different parts of the cycle like a 2-stage pressure displacement pump then you can find the root mean square (RMS) power and go for a smaller motor.

Factors that drive motor sizing

3 major factors are worth considering when it comes to sizing a motor to drive a centrifugal pump. They are:

1. The power demands of the pump

The horsepower rating of the motor should be greater than that of the pump. For example, if the pump requires a power of 6.5HP, the motors should be rated at least 7HP. This consideration will ensure that the pump is operated as required.

2. Typical operation of the pump

Some pumps operate at the rated design condition of the system. However some system operates under variable conditions, therefore you need a motor that can work under various conditions.

3. Frequency of operation

If a pump will be operating on a constant frequency drive, the sizing of the motor may just become a matter of preference. If the pump will be operated at variable frequency drive then the motor’s Hp should be more than that of the pumps’ NOL BHP.

Hydraulic pump efficiency

How to rightfully size a hydraulic pump and motor - Hydraulic pump motor

The efficiency of a hydraulic pump is the ability of a pump to convert one form of energy to another. An ideal pump is a 100% efficient pump whereby the amount of Hp going into the pump is the same as that leaving it.

To calculate the efficiency of a hydraulic pump, simply use the formula:

e = H x Q / 367 x P1
Q = flow in cubic meters per hour m3/h
H = pump pressure
P1 = motor power consumption in KW

Common Mistakes to Avoid When Sizing Hydraulic Pumps and Motors

When sizing hydraulic pumps and motors, there are some common mistakes that you should avoid:

  1. Oversizing or under-sizing: Oversizing your pump and motor can lead to unnecessary costs and reduced efficiency, while under-sizing can result in poor performance and premature wear and tear.
  2. Ignoring operating conditions: Neglecting the operating conditions of your application can lead to pump and motor failure or reduced performance.
  3. Using incorrect formulas: Using incorrect formulas or sizing charts can result in incorrect calculations and improper sizing.
  4. Failing to account for system losses: System losses, such as friction and leakage, can reduce the actual flow rate and pressure in your hydraulic system. Failing to account for these losses can lead to under-sizing your pump and motor.


Why is it important to bleed the air from a hydraulic system?

Air bleeding in hydraulic pumps involves filling voids and spaces with clean hydraulic fluid before starting the system. If the pump absorbs enormous gulps of air along with the oil, it can cause foaming in the tank, leading to the erratic running of the pump. Bleeding helps to ensure that pockets of air entering the hydraulic cylinder chamber does not impair the function of the cylinders and prevent system failure.

How to improve pump efficiency?

There are several ways to improve hydraulic pump efficiency. Use only specified hydraulic motors that match the rating of the hydraulic pump. When your pump is not in use, consider shutting it down to avoid unnecessary workload. With that, you will be able to save energy. Reduce usage of oversized pumps for low-duty requirements. This can cause your hydraulic system to lose efficiency. Only use what the system demands. You can also consider using multiple pumps as an alternative energy solution to variable frequency drives.

Where are hydraulic motors used?

Hydraulic motors are employed in industrial applications such as mixers, conveyors, and augers, as well as rolling mills, where their ruggedness and heat resistance make them an excellent choice. Hydraulic motors can be used in each application that an electric motor can. They are also well suited to mobile machinery and other tracked vehicles like excavators, bulldozers, and bridge layers or drill rigs. 


Choosing the appropriate motor rating is necessary to ensure that the complete pumping system operates reliably and efficiently for a long time. Understanding and following the fundamental rules outlined above will help you choose the correct motor power every time. Also sizing your motor and hydraulic pump correctly can save a sizable amount of money and energy.

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