Woodlandfarms:
Some random (as will become obvious) thoughts about your project.
There may not be a hydraulic motor that will tolerate the radial loads and turn fast enough for your needs. Char Lynn and others make 5.0 cu in/rev and larger motors with tapered roller bearings that will tolerate 5,000# or so of radial load, typically at a point about 1" from the motor face. But that translates into about 5,000 in# of torque perpendicular to the shaft. A 40", for example, diameter disk on the shaft will place 5,000 in# on the shaft if a 250# load is placed on one edge of the disk. That could easily occur if the disk were dropped on a stump such that the edge of the disk supported the weight of the entire mechanism.
And even if these motors would tolerate the side load (to say nothing of the shock loads), I don't think there are any smaller than about 5 cu in/rev. For example, 10 gpm of hydraulic flow will only turn 5 cu in/rev motor about 500 rpm.
Most brush cutters turn about 1,000 rpm, so you would need 20gpm of flow to get the motor to 1,000 rpm (which is probably about its maximum rated speed).
The direct drive brush cutters I have seen use a specially built spindle housing that holds the spindle with tapered roller bearings, and the hydraulic motor bolts onto the housing with its shaft connected to the spindle with a solid coupling.
If you need a motor less than 5 gpm to get the rpm's you need, none of those motors that I am aware of tolerate more that a few hundred inch pounds of torque perpendicular to the shaft. Therefore, you would certainly need to have a spindle housing and spindle made to use such a motor.
I did something similar and used a Bush Hog Squealer 40 hp gearbox with a roughly 2:1 ratio, a 4.9 cu in Char Lynn tapered roller bearing motor, and a Prince 11 gpm pto pump. The motor turns the gearbox input shaft at about the same 540 rpm as the PTO did, and the output shaft turns the same roughly 1,000 rpm as it did when directly driven from the pto. And the gearbox output shaft is equipped to tolerate the shock and side loads inherent in brush cutting.
Most of the brush cutter gearboxes use a standard Society of Agricultural engineers tapered spline output shaft (there are two or three different standard sizes) and hubs to fit those shafts are available so that you can weld your disk to the hub. The gearbox solves your shaft load problem and your rpm problem for between $150-$200.
