rightfully size a hydraulic pump and motor

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 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.

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

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

The horsepower equation is expressed thematically as:

Horsepower HP = Q x P / 1714 x e
Where: 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

Low-pressure pumps

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 calculation should be made taking low pressure into account.How to rightfully size a hydraulic pump and motor - Gerotor hydraulic pump

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 at variable conditions, therefore you need a motor that can work at 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 motor calculations

How to calculate the hydraulic motor flow rate

The flow rate Q of a hydraulic motor can be gotten using the formula:

F = D x / 231
Where; Q is the flow rate in gallons per minute,
D is displacement in cubic meters
is the speed in rpm

Hydraulic pump efficiency

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

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

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


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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