Sizing Hydraulic Motor

[Herman1170]

I am looking for some help sizing a hydraulic motor for a snowblower. I work as a mechanic, mostly farm equipment, so I am not up to snuff on hydraulic motors. I am looking to put a hydraulic motor on the auger of a 3 point tractor mounted snowblower. This is what I have to work with: 16 GPM, 2100 PSI, needs to run 175 RPM, and the snowblower is rated for a 40 HP tractor. What size hydraulic motor do I need? Thanks in advance for the help, really appreciate it.

 

[Oldhydroman]

Very first question: 16 GPM? is that US gallons per minute or Imperial gallons per minute.

Do you want to use the full 16 GPM and let the pressure go up and down according to the load on the motor? or do you need to make sure that you don't use the full 16 GPM so that the pump outlet pressure can remain high enough for the hydraulic system to still be able to work something else?

Are you thinking of this being some sort of accessory being plugged into the tractor's auxilliary circuits or are you contemplating something more substantial/efficient?

Motors are sized according to their "displacement" which is typically expressed as cubic inches per revolution (in³/rev) or cubic centimeters per revolution (cc/rev or cm³/rev). A big motor will give you lots of torque for the available pressure but need lots of flow for the required speed. A small motor gives lots of speed for the available flow but doesn't give much torque at the available pressure.

Assuming you meant 16 imperial gallons/minute (72 Litres/min) and you wanted to use all of this flow to get a shaft speed of 175 rpm then you would be looking for a motor with a displacement of about 400 cc/rev. Such a motor would deliver about 6.3 Nm of torque per bar. Your supply is limited to 2100 psi (144 bar) so your maximum (running) output torque would be in the region of 830 Nm.

A typical selection for this duty would be an orbital motor such as a Sauer Danfoss OMS400. If you are on a budget there are many lookalikes for this sort of motor. There may be other issues which you need to consider such as anti-cavitation valves, speed control valves, pressure limiting valves etc. This is the biggest motor that will still give you 175 rpm for the 16 [imperial] GPM you have available. If the maximum torque output is too much then you need to reduce the pressure or fit a smaller motor (and then reduce the flow otherwise it will turn too quickly).

DOL

 

[Herman1170]

Thanks for sharing knowledge and the quick lesson on hydraulic motors! The answer to your questions, I will be using one of the auxiliary circuits. I think it is rated 16 GPM imperial, but the book does not specify. I do not want to use the whole 16 GPM since tractor has power steering, power brakes, and the snowblower has a hydraulic cylinder to turn the spout. Budget: $400-500. I know I am limited on what I have to work with, so is this conversion even possible?

 

[Oldhydroman]

The conversion might just about be possible if you are very circumspect with your choice of components.

You should be able to get a suitable motor for about $130 but this will be a budget Taiwanese copy of the real thing. At this end of the market there isn't much difference in the price of the motor whatever displacement you go for. A smaller motor will take less fluid from the system but will deliver less torque and will stall more easily. Your friendly local budget hydraulic motor supplier might even let you try a motor out and then let you swap it for a different displacement unit if your first guess was too far out to be usable. Within any particular frame size each motor mounting arrangement is identical; as the displacement increases so does the motor length but the shaft size and front flange remain the same.

A couple of barrel type throttle-check valves will give you maximum speed limitation (one valve for each direction of rotation - use them to throttle the flow going IN to the motor) and these will cost you about $30 each. You will then need some hoses and couplings to match the tractor connections (I'm presuming you are using one of the auxiliary spools on the tractor's own hydraulic system for the forward/stop/reverse control). You probably haven't got enough money in the budget for niceties like anti-cavitation protection, but I dare say the auger doesn't have a huge inertia, it isn't going super fast and even the cheap motor is fairly robust and will manage without.

Then the rest of the money can be soaked up on the mechanical mounting for the motor, the shaft coupling, sundry hydraulic fittings and hose clamps, and, of course, your time.

 

[kcj]

Really basic here, but a starting point is to calculate the power being delievered: 16 gpm at 2100 psi is only 15-20 hp. Will that do what you want?