# Specifying a hydraulic pump and motor for a class project

#### koii123

I am doing a project of hydraulic vehicle. My professor ask our team to install a hydrualic pump and motor in the vehicle. We are considering to purchase a set of hydraulic pump and motor, and we have the following requirements:

1. We have an existing electric motor of 22kW, 60Nm (continous) and 3500rpm (continous)
2. For the hydraulic motor, we would like to achieve:
high torque (300Nm) --> low speed (at least 300-500rpm, can be higher)
low torque (50Nm) --> high speed (at least 500-1000rpm, can be higher)
In both cases, the speed (rpm) should be as high as possible.

Does anyone know how to do the calculation? So that we can select a suitable pump and motor combination for our design. :rofl:

I really don't know how to do it, please kindly help me!!

Thank you very much!!!

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#### anorlunda

Mentor
Gold Member
You begin by matching the electric power, 22 Kw.

Then survey different hydraulic pump/motor combinations of about rating or less. There are things other than RPM and torque to consider. There is power efficiency, operating temperature, starting torque and more.

#### Baluncore

As an educational project you should not take any risks. For that reason you will restrict pressures to about 2000 psi =13.8 MPa.
From that you can work out the flow rate. Power = flow rate * pressure.
22 kW / 13.8 MPa = 0.0015942 m3/sec =1.5942 litre/sec.

At 3500 RPM you have 3500 / 60 = 58.3 revs per second.
A positive displacement pump capacity will be 1.5942 litre / 58.3 revs = 27.3 cc per rev.
Now you can select a pump based on capacity.

But you must decide how you will control the speed or acceleration of the vehicle. Will you use a variable flow pump and/or bent axis variable speed motors?

Will you run your hydraulic wheel motors in parallel, which has 4 times the flow and equal torque (like a differential), or in series, which will give equal speed ( like four wheel drive) ? Will you switch between various combinations? That will decide the capacity of the motors you use.

Hydraulic transmissions are not used in road motor vehicles because they are inefficient when compared to mechanical gearboxes. Some hydraulic 4WD off-road or earth-moving vehicles may be economic. Assume 60% hydraulic efficiency which means you must dispose of 0.4 * 22kW = 8.8kW. You will need a large oil cooler to keep the fluid temperature down.

"Specifying a hydraulic pump and motor for a class project"

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