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Discussion Starter · #1 ·
This is the question I have. I am building a small front loader tractor that I plan on runing 2 hydraulic motors in series coupled to chevy 14 bolt axles for 4 wheel drive. I am planing on running 4.11 with 33" tires on the back and 3.55 with 28" tire on the front. the front tires will spin 49 rpm faster than the back. I have a chevy 235 for an engine and a 4 speed manual. The hydraulic motors put out 1,335 lb torque would you use the 4 speed trany for more torque off of the hydraulic motor or just run the motor off the diff. ? I would like to keep this as short and simple as posible but the trany is only 20" long would there be much benifit for a small tractor in running the trans?
 

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1300 foot pounds of torque is a large hydraulic motor! 1300 inch pounds is pretty tiny.

Two motors the same size in series will result in no differential action. Both motors will get the same amount of oil and both sets of tires will turn at the same speed. Using the motors in parallel isn't much better. Hydraulic fluid will follow the path of least resistance and all the fluid will go to the axle with the least traction and that differential will turn the tire with the least traction 4 times as fast as is normal. Either change the size of one of the motors ( there will still be no differential action for turning corners) or get a pressure compensated flow divider and run them in parallel, which will cut your speed to about half.

You already have a huge rpm reduction and torque multiplication using just one motor, the addition of a transmission won't help the rest of the drive train. A third differential (limited slip) to drive the front and rear axles would yield even more reduction, and using one or both motors to drive it may be a better plan. Both motors in series would give you high range and in parallel, low range. A suitable control valve would give you the option.

Just remember that all that torque needs weight to get the power to the ground without spinning the tires. If your tractor weighs less than a truck with the same rear end and engine and the truck will spin the tires in first gear, the tractor with one of those motors will too.

Figure out the torque at the output of the transmission in first gear by multiplying the engine torque by the first gear ratio and compare it to the torque rating of your motor. This project has a lot of math involved to balance engine, pump and motor performance levels, as well as differential requirements.

Big horsepower or torque numbers are not necessarily a good thing without the weight. Define "small", as it pertains to your project. My MF1655 has a 20 hp engine, weighs 2400+ lb and can spin the 2 chained up back tires pretty much at will at considerably less than full throttle when I'm using the FEL with a 54" bucket and the front wheels off the ground.
 

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why not just use the the whole drive train axles,transmission, transfer case then then you can get by with just one hydraulic motor and some cash in your pocket for something else
 

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i would love to find something like a ford f-150 4wd with the straight 6 to make a tractor out of, or something with solid axles front and rear 6 or 4 cylinders. narrow the axles and make my own frame
 

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Discussion Starter · #5 ·
I am thinking about the size of a 20 hp Kubota I am figuring on a weight of about 2,000 lb with a small 240lb operator. the hydraulic motors are 45.6 cu in and put out 15,750 in lb at 150 rpm with 33 inch tires will give me a speed of 3.7mph with the 3.55 diff I have. I think I will stay with conecting the motors in series with a flow controle valve in the front so I am not so limited on tire size. I am planing on running 2 double pumps one for the drive one for the pto and one for loader and the last for steering. they will be direct conected to the engine front and back
 

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I hope you realize that you need about a 30 gallon reservoir for oil with all those motors. You might be able to get away with less, 'cause you sure won't be working them very hard, except maybe for the PTO.

A problem with series circuits, and this shouldn't have a major effect with your project, is that the pressures for the motors are additive. If you need 500 psi for each motor, the system needs 1000 psi to function.

It will be interesting finding the right combination of double pumps. The drive will need 29.6 gpm, not sure about the PTO, the FEL about 6 gpm and power steering about 2 gpm. Higher than noted volumes on the last 2 can lead to operational issues. If you're going to use a stock p/s unit, use a regular p/s pump.

I make the speed at 3.52 mph for your 28" tires with 3.55 gears, and 3.58 mph for the 33" tires with 4.11 gears. That's really close. You could probably get them perfect with tire pressures. Nicely matched.
 
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