A bush hog, also known as a rotary mower, is attached to a tractor's three-point hitch, and you supply power to it through a rotating shaft connected to the tractor's Power Take Off (PTO). If you run over a stump or other immobile object while mowing a field, you risk wreaking havoc on the gearbox that sits on top of the bush hog, or you may break or dull one of the blades underneath. Diagnosing and repairing such damage is fairly straight-forward.
Locate and inspect the shearing pin on the rotating shaft that leads from the PTO to the gearbox. The pin is a safety device, designed to break if your bush hog's blades encounter an immobile object. If you find that it has been severed, remove fragments of the old pin. Align the shaft so the holes line up at the shearing pin's location, and replace the pin. Odds are good that this is all you'll need to do.
Raise the bush hog using the tractor's hydraulic lift and look underneath. Remove any debris that may have gotten caught up in the blade. Inspect the blades, determining whether they remain sharp and intact. Unbolt and replace the blades as necessary.
Unbolt the covering of the gearbox, which translates the energy of the rotating PTO shaft to the whirring blades underneath. Look inside the gearbox. It should be clean and well-oiled, and the gears should be intact and meshing smoothly. In most cases, a damaged gearbox will require a potentially expensive visit to a repair shop.