Rotary cutter recommended PTO confusion

[biodegraded]

I don't understand why recommended PTO values vary so widely between "light", "medium" and "heavy duty rotary cutters. This seems to be the case for all the major names. As a common, typical example, consider Bush Hog. The minimum recommended PTO ratings for the BH16, BH26, BH326 are, respectively, 30, 45 and 60.

The blades have the same "1/2 x 4 inch uplift", which I assume works out to the same mass profiles, and they spin at the same rate. Unless the gearboxes are wildly different in energy consumption, it should take essentially the same umph to drive all of them. The rest of the widgets, frame and wall gauges, etc, has nothing to do with what it takes to drive the blades.

So why do the recommended minimum PTOs vary by as much as 100%?

 

[MtnViewRanch]

Take a look at what they are rated to cut, that might be what the difference is. Grass vs a steady diet of 3" material is bound to make a difference. I imagine that the gear boxes are different, not that they would take a bunch more power to spin, just need bigger ones to hold up to the bigger stuff. Drivelines are different, I'm sure.

Without looking specs up, this is what I would say is the reasoning. :confused3:

I'm sure someone will answer that actually knows.

 

[brown40]

I am curious about this too; working through whether to repair a JD MX5 (medium duty) in great shape, or abandon it, and go as heavy as I can find.

I understand you've got to be able to turn the blades, and manage the weight on the back, but getting one started doesn't take much, and bushhogs with recommended even 50% higher than my PTO minimum output still are only at half the hitch capacity.

If the driveline protection is in place to slip appropriately, why couldn't you run a heavy duty instead of a medium duty? I see a nice unit, 280# heavier, cat 5 drive, but its asking for 50hp at the pto, and I have 30.

Any experiences with this?

 

[blueriver]

My understanding ... You can run a 100 hp rated gearbox on any size tractor ... As long as the tractor can turn it . If it's on a 50 hp you won't damage it cause it can take up to a 100 hp .. on the other hand put the same gearbox on a 150 hp tractor and well your out cutting away and think to yourself I'm going to cut those large saplings ... The tractor won't have a problem cause it's 150 hp .. overtime that 100 hp gearbox ain't gonna take the punishment ... As I said that's my understanding and I'm sure someone will correct me if that's not the case.

 

[boggen]

metal that the decks are made out of. thickness of the metal is one thing, but also the hardness of the metal. not all metals are made the same. a 1/4" of X type of metal. could be equivalent of 1/2" thickness of Y type of metal. metal has a bunch of extra specifications to it, that are beyond my own knowledge. but *shrugs*.

metal of the decks = rocks putting dents all through the deck, for say a light duty unit, vs a medium duty unit, a heavy duty deck metal going to take more punishment, without getting dents from rocks being picked up and through into the deck.

width of the cut = more width = more HP to spin and cut through material, and to keep the blades spinning.

some folks use rotatory cutters, not to go out into the pasture and around fields to cut that are 1 to 6 plus feet high, but rather sharpen the blades, much like a finishing mower deck, to mow the yard/lawn around the house, mowing the 1" to 6" grass for a yard/lawn, that has very fey if any rocks, or tree limbs = very little HP to run the rotatory cutter. vs cutting the tall weeds.

MPH (how fast you are going), more so geared tractors that have say 3 to 4 gears to choose from, has a great effect on how rotatory cutters do there job in cutting stuff up. if your traveling to fast, tractor bogs down to possibly stalling out, and not cutting weeds. going to slow (low gear), and takes a lot more time to use the rotatory cutter. generally the faster you travel MPH=miles per hour, the more abuse all equipment will take, including tractor, more so on rough terrain, and everything is bouncing, chattering away. in this respect you will more likely want something stronger and/or more flexiable, to be able to handle the abuse.

type of welds used to put stuff together has a difference of how well a deck can handle stuff long term. over all frame work of the deck also plays a roll in it.

location, location, location, i would imagine different areas of the world, and countries, even state to state, to county to county, to property to property is different. as in cutting down small trees, to how many rocks are in the area to be mowed, to finding that unknown tree limb that fell of tree and running over it, to finding unknown junk while using the rotatory cutter (ya don't know till the rotatory cutter hits it), and maybe not even then.

commercial use (use rotatory cutter for 100's of hours per year) vs a home owner that may put on 2 to 40 hours on it a year. does a home owner really need something that can handle running over a tire when mowing ditches along the road like road crews might need? or dealing with running over a stump? and not twisting the frame work, blades, etc...?

by default majority of all rotatory cutters come with a "slip clutch" but that is not always the case, and some come with a "shear pin" in the PTO shaft.

shock loading, "blades hitting a hard object like a large rock", that vibration from hitting, goes right up the gear box, through slip clutch... PTO shafts need to be able to handle that to some degree, or they will flex like a rubber band and bust / crack / come apart.

===========

The rest of the widgets, frame and wall gauges, etc, has nothing to do with what it takes to drive the blades.

your saying nothing else matters? i would have to disagree. it is a system of parts put together, it is not the individual parts that makes it all up and works it, but the overall system of parts and how they are put together and how they are used.

 

[LD1]

That is why they only call them "recommendations"

Buy the cutter sized based on what you intend to cut, not what size your tractor is.

If you only have 30 PTO HP, but plan on cutting lots of brush and saplings and rose bushes, Dont get the BH16.

And I think alot of the recommendations are because of cutter weight also.

It takes a 60hp tractor (or close to it) to handle a BH326 without adding alot of weight up front, or having to wory about driving up a hill and doing a wheelie. The BH326 is almost 1500lbs And the BH26 is over 1000lbs. Thats more than alot of 30HP tractors will handle without lots of weight up front.

My 306 cutter calls for 40HP minimum. And have been just fine for the last 3 years using less than that.

 

[motownbrowne]

I agree, it's just a recommendation. If you can carry the heavy duty model and can go slow enough to use it effectively (low enough gears or a hydro) then it doesn't matter what the recommended PTO HP is. Unless of course you have too much HP, which can damage things.

I have just under 40 PTO hp, but my tractor is hydrostatic. As such, I can infinitely vary my ground speed while maintaining 540 PTO RPM. Thus, even with considerably less than recommended HP, I can still do the job without bogging down. It'll just take longer. I wouldn't hesitate to hook up to any medium or heavy duty cutter in the size you're talking about.

Also, boggen, I don't mean to be rude, but you really answered a totally different question than what was asked. The different duty ratings obviously translate to different machines and different uses, but the question asked was if there's any reason one couldn't use a cutter with a higher recommended HP than what the tractor puts out. The cutters he's talking about are all the same width, so the will perform similarly on his tractor, regardless of PTO HP, although, obviously the heavier ones will throw the tractor around more.

 

[biodegraded]

Thanks for the replies. They've provided what I wanted to know. Tractor on.

 

[Deerherd]

Don't over buy on a cutter, especially with a smaller tractor. Sure you can run a heavier model, but why? More weight = reduced stability, increased fuel consumption and wasted time. I wouldn't trade those things off because I was thinking I may want to chop down saplings some day.

For the majority on here, a good light to medium duty would be the best bet. A quality one with good maintenance will last for years. You need to pick your cutter by what kind of person you are, not some chart.

I look at a Rotary cutter as an implement to cut my clover, crop stubble and anything else that has grown in the past year on ground I have already worked. If it's much older/bigger than that I rip it out with my root rake and save the blades and deck of my cutter. Even if you DO have a cutter big enough to cut down those saplings, you now have created a dangerous area for tires, pets, farm animals etc and should take care of it. What have you saved?

Now there are others on here that beat the bejesus out of their cutters forging into the unknown. Decks look like they have been hit by shrapnel from land mines exploding every 50 feet. Save the heavy duty for the Commercial and road maintenance types that have to deal with the unknown all the time.

 

[LD1]

I agree that a good medium duty cutter is good for most here. IF you know your land, and know that you wont be hitting the unknown "God only knows what", then you will be fine. And when you do come to a sapling that is just a bit much, break out the saw or loppers. In a business, yea, heavy duty needed.

But, a heavy cutter IMO can make things more stable on slopes IF you are going sideways. Thats alot of weight and really low. And shifts the CG back toward the rear tires and away from the single point (pivot pin) of the front axle.

So heavier is better if sideways. But not so much if going UP a slope.

Heavier is also better ballast if doing heavy lifting with a FEL.

BUT, all of these things add up to about 2x's the cost.