Bush hog gear box rating

Hello everyone

If a gear box had a 90 hp rating, would there be a minimum required HP to use it?

Kerry

You'd need enough power to make up for friction loses in the bearings and gears and the "drag" of the oil.

In other words not much power would be needed to operate it, but somehow I don't quite think that's what you are asking!

I doubt the manufacutor would be worried about damage from too low a horsepower. Usually the rating is given as a maximum. If you go higher, it will void the warranty and possibly damage the equipment.

How low are you looking to go?

The min hp is what you need to swing the blades to use it. Its on what, a 6 or 7 ft hog? That can need 30 hp in stemmy weeds to 60-70 hp in long thin grasses, all depending on speed of course.

I wanted to run a 90 hp rated bush hog with a 45 hp tractor (39 pto). I wanted to make sure that the tractor had enough HP to handle this (6' rotary cutter)

You should be fine. We run a 6' mower with a 135 MF which is a little less hp than you mention and it handles it fine in most conditions. As with anything, you have to adjust your speed depending on what you are doing.

36 hp is about normal for a 6 foot in NORMAL cuttings.. but if your in saw grass that is 12 foot high, 150 hp would be needed.

The rating is set up for maximum horsepower of the pulling tractor. In other words, if you have a90HP gearbox, you don't want to pull it with a150HP tractor. 40-45HP should turn it fine, from my experience.

What is the make/model of your tractor?

Not an issue if 1st gear is slow enough.
All you need is enough tractor to carry the bush hog without the front wheels getting light.
Wait for a while to bush hog. Right now the grass is full of birds nests, young animals too small to escape and newborn fawns.

When used properly, the horsepower rating of the gear box in meaningless for practical purposes. Some equipment sellers post the max. power the box is rated for before breaking point, and a few others rate how much power the box can be run on a sustained basis and still last as long as it was designed for. A cheaper brush cutter will have a box with a shorter projected lifespan than an expensive one the same size.

The idea of too big a tractor hurting a less rated gear box escapes me. They all have shear pins or slip clutchs. Most makes say nothing in the warranty about being voided with too big a tractor. They DO void the warranty if the wrong shear pin is used, OR the slip clutch gets tightened up too much. I.e., if the box is rated at 50 horsepower, and the slip clutch is designed to slip beyond that, a 100 horse tractor won't hurt it a bit. Under normal circumstances it takes X amount of power to run X size of cutter in H high brush - regardless of how much power potential the tractor has.

King Kutter, for example, sells a 6 foot cutter with a choice of a 40 horse, 60 horse, or 80 horse gear-box. All use the same amount of power, but the bigger the box, the longer it's going to last under heavy use.

Now, having enough power is a different issue. Figure somewhere at least 6 PTO horsepower per foot to be in the ball-park for all-around work. Many of the warrantees DO state that warranty is void if the unit it damaged from lugging (being underpowered or run too slow).

NC:

Thirty nine PTO HP is a bit marginal for a 6' rotary cutter but you should be able to handle it in most conditions so long as the cover is not too thick or you plan to mow too fast.

I use 6' cutters with both a Ford 860 (43 PTO HP gasoline) and a MF 533 (45 PTO HP diesel). Both handle the cutters well in heavy cover and on very steep ground.

Dean