In comparing hyd motors for cutting decks,
We can make assumptions that are logical, so here goes. Smaller length blades have to turn faster to achieve a good blade tip velocity. Lets assume that the hyd motor on the 48 in brush hog is 3 cu in. If we push 8 GPM's through that motor, it will turn at 616 rpm, and a torque of 1433 in lbs.
You can relate that to a 48 in 3pt brush hog, where the 540 PTO speed is turning a 48 in blade at 540 rpm if the gear box is a 1 : 1 ratio, and a higher rpm if the gear box is say, 1 : 1.5. That will give a higher brush hog shaft rpm of about 810 rpm, and is within the range of brush hog cutting operations. Some tractors have three PTO rpm's.
Back to the hyd brush cutter.
The brush hog hyd motor will turn faster if you increase the GPM's to the same 3 cu in motor.
The other way to increase the rpm is to decrease the size of the hyd motor to a smaller cu in displacement, say 2.5 cu in. That will allow 8 GPM's to turn the hyd motor at 737 rpm. If you decrease the cu in of the hyd motor, the torque will decrease also, keeping the pressure at 3000 psi .
If you kept the same 3 cu in hyd motor and put it on a 60 in deck, with 19 GPM"s flowing, the shaft speed would be 1463 rpm, a little high for a 60 in brush hog, and the blade tip speed would definitely increase. In order to get the rpm of the brush hog back to a lower rpm, you have to increase the cu in. So now if we have 19 GPM's flowing through a hyd motor of 8 cu in, the rpm of the shaft would be 549, and the torque will be 3852 in lbs, at 3000 psi.
Remember the torque on the 48 in deck would be 1433 in lbs. at 616 shaft rpm. Pressure of 3000 psi.
In general, if you decrease the cu in displacement of a hyd motor, the rpm will decrease, and the torque will increase. Therefore the larger hyd machines will have large displacement motors, and the rpm will be lower
