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sizing of gear motors/pumps and horse power sizing of petrol engine

sizing of gear motors/pumps and horse power sizing of petrol engine

sizing of gear motors/pumps and horse power sizing of petrol engine

hi all,
I'm trying to build a ride on train, it will pull anywhere from 1 - 3 tonnes, however it's the initial pull which is the load and once it's going the load drops off considerable and depending on track gradients the load will vary. I have had no support from any hydraulic companies here.
I'm trying to keep the petrol motor to about 11-13 horse power
top speed 20 km/hr
can anyone help me in my endeavour

RE: sizing of gear motors/pumps and horse power sizing of petrol engine

An OOM estimate of the motive force required could be created considering the principles provided following [model the train as a block on a hill, with friction]:


Power could then be calculated through considering P=Fs [where F is the force calculated above, and s is the speed].

If this does not assist, and perhaps even it does, I propose you consider consulting a qualified professional.


RE: sizing of gear motors/pumps and horse power sizing of petrol engine

There is more to this than meets the eye...

You don't have much power, so your acceleration will be slow, but that may be ideal for you.

You will need to know what your gear ratios are and you will (ideally) need a variable speed motor. Otherwise your engine will be at max RPM all of the time.

Do you know the power and torque curve for the engine you are looking at?

What will you do with the motor when the train is up to speed? If you leave it engaged, the engine will act a break when you back off the gas.

Things are more straight forward when you have constant torque, this application has anything but constant torque, so it can get expensive to make it a smooth ride.

As you point out, it's the initial load when accelerating from zero that will be the highest and it will drop to almost nothing when the train is up to speed. So you need to figure out if you want to change the drive ratios in the transmission...or clutch the drive out some how when you want to freewheel.

There will be lots of people that can help you out with this, but the key to success is sharing the information about what you want to do and how you want to achieve your goal.

Let's start with how you imagine this will work...

Good luck


Adrian Wright CEng MIMechE
Engineering Specialist
Hydraulic Systems
Caterpillar (UK) Ltd

RE: sizing of gear motors/pumps and horse power sizing of petrol engine

hypothetically, if i was to join all the hoses between pump and 2 motors in series and i have a hose connected between the inlet and outlet which can be opened (in other words, i have got rid of flow controls etc) then the train will free wheel. i would use a flow control which would drain back to tank when stopped or lower speeds.
what i am trying to do is to get young people involved in the hobby, cheaply. An older gentlemen did one here, however has passed away and took the knowledge with him.
to your questions
slow acceleration is fine. as the public doesn't want to go fast
most engines varies between 3400 - 4000 rpm max, however i don't need max rpm, only if required
when the train is up to speed you generally leave it until you need to slow down or stop (your right it acts like a brake)
this elderly gentlemen used small gear motors (looked aluminium) and it still runs today in another state, the size of the motors were probably a tad larger than a cricket ball in diameter.
What i need is the size of pump to run 2 motors in series using preferably an 8 - 10 hp motor.
i can place a picture if anyone would like to see what i'm trying to finish at.

RE: sizing of gear motors/pumps and horse power sizing of petrol engine

I would suggest you start with the peak torque capacity of the engine. Size the pump so that at maximum planned hydraulic pressure the torque demand of the pump does not exceed the torque capacity of the engine. This would tell you how large a pump could be used without stalling or lugging the engine. Search pump torque as it is proportional to the product of pressure times displacement.
Engine peak torque will be at some speed. Figure running the engine higher than that speed so that as the hydraulic system loads the engine, the engine speed drops toward the higher engine torque. Knowing the operating speed and pump displacement will allow you to calculate pump flow output.
Size the motors to produce enough torque to get the train moving.


RE: sizing of gear motors/pumps and horse power sizing of petrol engine

Torque = psi x cu.in./2π in pound-inches


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