I may be getting in over my head on the hydraulics. The numbers below are just for the drive pump and motor, I did not factor in any losses or consider a charge pump. Using some equations I found online, I put the numbers into a spreadsheet and came up with the following:
First I calculated drawbar pull, with 8000 gross combined weight (3500 tractor + 4500 trailer worst case), 30 degree slope, and .1 rolling resistance = 4800 lbs drawbar pull required
This leads to hydraulic motor torque of 5300 in-lbs (450 ft-lbs) after factoring total gear reduction (13.8 ratio) and tire size (31").
Picking a speed of 4 mph for the worst case going up a hill, results in hydraulic motor speed of 600 rpm.
So far, everything looks reasonable to me. Moving on to sizing the motor and pump.
To determine motor displacement; the motor torque, the operating pressure differential accross the ports of the hyd motor, and the hyd motor hydraulic/mechanical efficiency need to be known. I do not know an appropriate number for the operating pressure differential, I would assume that most of the pressure is converted to rotary motion, so I used 2800psi differential (in 3000psi system). I am sure I am wrong here. With motor torque of 450 ft-lbs and .92 eff, this results in 13.15 in^3/rev displacement.
Hyd motor GPM using displacement, shaft speed, and efficiency results in 37 GPM.
Hyd pump displacement using the required GPM, pump shaft speed of 2000 rpm, and .92 efficiency is 4.65 in^3/rev.
Then lastly, the hp required by prime mover, assuming 3000 psi system, the above pump displacement and pump shaft speed, with 0.85 efficency results in 83 hp. This does not take into account any other hyd pumps.
Can someone enlighten me on the pressure differential for the hyd motor ports?
Am I making some gross mis-assumptions?
I will try another vendor sizing guide and see if the numbers are the same.