How to Choose 75 HP Rotary Cutter Gearbox?

Author: Ruby

Jul. 21, 2025

Rotary Cutter - Sizing of Brush hog and Gear Box HP Help

Oct 5, / Sizing of Brush hog and Gear Box HP Help #1

bmartin47

New member

Joined: Aug 12,

Messages: 3

Location: Oklahoma

Tractor: Kubota L series

I am currently trying to upsize my tractor to a 50 HP. I am also looking at buying a 6 or 7 foot king kutter heavy brush hog but I am concern that the larger brush hog will need a larger tractor. I am also not sure I under the gear box sizing in HP. Can someone explan what the gear box sizing is used for. Thanks Oct 5, / Sizing of Brush hog and Gear Box HP Help #2

dex

Elite Member

Joined: Aug 22,

Messages: 3,494

Location: N. of Charleston WV

Tractor: Kubota L-1 HST,FEL, 3x3 remotes, TNT. BX 54 mmm

When looking at the gear box rating of a mower it is the input HP that the gearbox can handle. So you can run a 60HP gear box with a
30 PTO HP tractor. But you wouldn't want to try to run a 60HP gearbox with a 75 PTO HP tractor. Oct 6, / Sizing of Brush hog and Gear Box HP Help #3

Transit

Veteran Member

Joined: May 13,

Messages: 1,303

I think what you are asking is what is the Hp rating for the hogs gear box? Ok, the PTO Hp is about 15% less than the engine HP. A 50 Hp tractor will have a PTO output of about 42.5 Hp and so a 50 Hp or larger gear box well do fine.
Next are the TPH lift capacity and the hitch category 1 or 2?
A 6 to 7 foot hog will weigh about 800 to 1,000 pounds, not only will the hitch need to lift the hog, you do not want the tractor to be tipsy or too light on the front wheels. Ballast weight in the FEL will fix that. Oct 6, / Sizing of Brush hog and Gear Box HP Help #4

greg_g

Super Member

Joined: Dec 18,

Messages: 6,126

Location: Western Kentucky

Tractor: JD Cab, 300X loader with 4-in-1 bucket

Can someone explan what the gear box sizing is used for. Thanks

There are typically 3 general grades of rotary cutter; standard duty, medium duty, heavy duty. I suppose someone might come back with an example of a light duty rotary cutter, but to me that's simply an oxymoron. Anyway. Each grade is progressively stronger - both in terms of construction and input torque capability. The rougher the stuff is you're going to cut, the higher grade of RC you want. And this is usually commensurate with the size of tractor that you'll be working the rough stuff with. So standard duty RCs will typically have transmissions rated in the 40-60hp range, mediums in the 60-100 range, heavy 100+. I just bought a 60" Kodiak a few weeks ago. The SD60 (standard) had a 40hp rated tranny, the MD60 had a 90, and the HD60 had a 110 (or so said the salesman). My larger tractor has 45 PTO hp, so I bought the MD60.

Bottom line: determine the PTO horsepower of the tractor you intend to buy. Select a rotary cutter rated for MORE than that number. Any less, and it likely won't be able to handle the excess input torque.

//greg// Last edited: Oct 6, Oct 6, / Sizing of Brush hog and Gear Box HP Help #5

sweettractors

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Joined: Jul 19,

Messages: 7,088

Location: Central Kentucky

Tractor: JD CHA-JD CHA

Theoretically, you can run a 40 HP gearbox behind a 60 hp tractor, IF, you use the correct shear bolts or the slip clutch is adjusted correctly. Ken Sweet Oct 6, / Sizing of Brush hog and Gear Box HP Help #6