HP difference between finish and rotary mower

jimg · Jun 21, 2004

Hi,
This w/e I mowed an extremely overgrown paddock for my neighbor. The grass was very heavy, 4'+ tall, lodged in places w/ some small bushes in one corner. The land was mostly flat w/ a small steep hill at one end. I did this w/ my TC24 (18.5 PTO hp) and a 5' LP finish mower (FDR 1660). I figured it would be a struggle for the tractor but to my surprise it went rather quickly w/ no bogging down. This got me wondering about hp requirements. Does a 3 blade finish mower require more or less hp than a single blade rotary mower of the same width? How do mfgers come up w/ hp requirements? Based on the w/e's experience my tractor could handle a 5' mower. However, the rule of thumb (5 hp/1' of mower width) says it should barely be able to handle a 4' mower. Maybe the recommendation is based on cutting heavier cover like small trees?

Keith_TN65 · Jun 21, 2004

Jim,

I've never heard of the 5hp/1foot rule. If that was the case then my TN65 with 47 PTO HP should be able to handle a 9' rotary cutter /forums/images/graemlins/confused.gif I don't think so. My dealer seems to think a 6 foot rotary cutter is all I should put behind it. I've always figured most of the requirements for rotary cutters is the weight. Remember, while a 5 foot finish mower is only 3 feet long and 5 foot wide, (Wider than it is long) a rotary cutter is typically longer than it is wide putting more weight further back. I've used a 6 foot cutter on my John Deere 790 to mow a field of grass and that was a chore that I won't do again. Even with 200 pounds of weight on the front, you could still lift the front end of the tractor off the ground by pushing down on the back of the cutter. It had little problem running the unit, just couldn't move it.

Now, if you look at the horsepower ratings for semi mounted or drag type rotary cutters, they are considerably less than those that are mounted because the weight of the cutter is now supported by the cutter itself.

I'm also guessing that it requires less HP to spin three small blades than a stump jumper with two hunks of edged steel hanging from it. That is just a guess though.

There we go, as clear as mud! /forums/images/graemlins/tongue.gif

Keith

slowrev · Jun 21, 2004

I am not sure of the HP ratio for Rotary vs. finish mowers, but the finish mowers I have used do work better with less HP than a rotary cutter. I guess it is partially because of the more massive hardware you are spinning and also the uses that each are usually put to. For example a finish mower will not usually last long if you cut 4 foot high brush, but this is what a rotary cutter (brush hog) is designed for.

nomad · Jun 21, 2004

Without knowing HP magnitudes for each mower, we can find the HP ratio between finish mower and rotary mower. I'll not give here exact formulation. With a simple formulation:

HP ~ total weight of rotating blades x speed (rpm)

HP_fm (hp of finish mower) divided by HP_rm (hp of rotary mower) is equal to (speed of fm / speed of rm) x (blade weight of fm / blade weight of rm).

Speed of FM is about 3000 rpm and speed of RM is about 1500 rpm. So, speed ratio is 2.

Then,
HP_fm / HP_rm = 2 x (total_blade_weight_FM / total_blade_weight_RM)

Assume there are 3 blade units in each of FM and RM so that our comparison can be meaningful. I assume total blade weight of FM is half of total blade weight of RM. Then, HP ratio = HP_fm / HP_rm = 1. Meaning that HPs of FM and RM are more or less same.

jimg · Jun 21, 2004

Thanx for all of the inputs. I hadnt considered the weight of the stump jumper or the heavier blades. OK, Im all set now...thanx again.

slowrev: I did this as a favor for my neighbor on short notice and wouldnt ordinarily use a finish mower for a job like this. I also knew there wasnt junk laying around in the tall grass.

Keith: As for the 5hp/1' rule of thumb: If you search around the older threads you can find it.

RoyJackson · Jun 21, 2004

"I'm also guessing that it requires less HP to spin three small blades than a stump jumper with two hunks of edged steel hanging from it. That is just a guess though"

There is a lot more inertia in those big cutter blades compared to finishing mower blades. I've never come close to stalling the tractor using my RFM (when engaging the PTO) but I sure have come close to stalling occasionally when spinning that cutter up.
Also, the brush to be knocked down can be considerably thicker as individual stalks as well as grouping as compared to lawn type grass. That means more work...and more horsepower requirements.

And, as others mentioned, those cutters stick waaay out back of the tractor (attached pic shows the cutter rigged to my old 670...the cutter is as long as the tractor). Even with the loader installed, when I'm lifting the cutter you can feel the strain on the tractor. When I had my little 670, I used to keep the top link fairly long so the tail wheel was always on the ground thus bearing some of the weight. I still do this with the 790 in some spots...the weight of the cutter suspended up can definitely have a "tail wagging the dog" effect when you make a turn. Leaving the tail wheel in contact with the ground lessens that considerably.

jimg · Jun 21, 2004

Thanx...not sure if Im planning on getting a rotary mower but I appreciate the real world experience you related. BTW whats the PTO hp rating of the 670? In the pic you posted whats the width of the mower?

RoyJackson · Jun 21, 2004

( Thanx...not sure if Im planning on getting a rotary mower but I appreciate the real world experience you related. BTW whats the PTO hp rating of the 670? In the pic you posted whats the width of the mower? )

The cutter is a 60" Land Pride. Cost just over a thousand bucks. Great unit! Per the Mfr's manual, this cutter need a 20 PTO HP tractor.

The 670 has 16 or 17 PTO HP. It handled the cutter with little difficulty while operating. Had to go pretty slow, but since there was a bunch of junk under the brush, slow was fine.

The pic shows what I was cutting. This was taken while I still had the 670.

I've a 790 now. This machine has 24 HP at the PTO. Although the 790 has a bigger engine, the 3PH capacity isn't that much greater then the 670's. The 790 does handle the cutter better...less bogging down. And when the tractor does bog, the 790 recovers faster. However, I bought the 790 for more PTO HP for a chipper. The 670 did the cutting quite well and I wouldn't have replaced it just for brush cutting.

jimg · Jun 21, 2004

Thanx Roy
It looks like you were cutting some pretty heavy brush and the 670 handled it pretty nicely. I assume this tractor had a manual transmission?

How important would you say it is for the cutter to be wider than the tractor? My TC24 is right at 48" (maybe an 1" more...cant recall right now). I was thinking that it might be good as I could mow right along side fence lines. I dont think these mowers can be offset...right?

Soundguy · Jun 21, 2004

( TN65 with 47 PTO HP should be able to handle a 9' rotary cutter I don't think so. My dealer seems to think a 6 foot rotary cutter is all I should )

I think I would get a new dealer.. My dealer told me my NH 1920 will pull a 6' cutter...

47 pto hp is pretty decent.. I would think you could at least get an 8' back there..

Soundguy

RoyJackson · Jun 21, 2004

How important would you say it is for the cutter to be wider than the tractor? My TC24 is right at 48" (maybe an 1" more...cant recall right now). I was thinking that it might be good as I could mow right along side fence lines. I dont think these mowers can be offset...right?

Yep, that cutter should be wider...however, if you're running a lower HP tractor (say, the Kubota B7500) spacing the rear tires out for stability might be more important then the cutter or mower width. Personally, I think it's more important for the finishing mower to be wider to get close to the house, fence and such. Also, if your tractor's tire mash down the grass, a wider mower might lift the grass up a bit. My cutter work is on a powerline right of way, so I'm not overly concerned if the tires mash down the brush a bit.

If the tractor can handle a 60" cutter, that is the best all around size IMHO. My 790 could sure handle a larger finishing mower (got a 60" RFM), but the 60" rotary cutter is a great fit!

There are mowers and cutters that can be offset, but these are commercial units and pretty expensive. Probably cost prohibitive for residential or small farm work.

The 670 is a manual transmission (4 speeds plus reverse, high/low ranges) as is the 790. I just prefer gears.

jimg · Jun 22, 2004

Thanx!

Keith_TN65 · Jun 22, 2004

Soundguy,

I think he is more concerned about the weight. I'd like to get a 84" (7') to at least be a little wider then the tires, but the dealer says the TN65 won't handle it. But if I wanted to go with a pull type, he said it would handle a 10' cutter. Too big and too pricey for me.

I used it once with my old 6 foot King Kutter and you couldn't even tell it was back there. Unfortunately, that old cutter needs replaced.

Keith

Keith_TN65 · Jun 22, 2004

( Keith: As for the 5hp/1' rule of thumb: If you search around the older threads you can find it. )

I've since discovered that. /forums/images/graemlins/blush.gif Not the first time I've leaped with out looking /forums/images/graemlins/crazy.gif, probably won't be the last. /forums/images/graemlins/smirk.gif

Keith

kemartin3 · Jun 22, 2004

Keith,

Have you looked at a bat-wing style cutter. My neighbor has a 10' one that he uses with a tractor no bigger than yours. I haven't seem him do it, but it also appears to be able to have one side flex up/down some while it is cutting. He used it to mow the edge of our shared road and the half that was hanging off of the road followed the bank as it changed from downhill to flat to uphill. I would have sworn he had used his sickle mower until one of the other neighbors told me "nope, used the bat." And because they don't stick so far out behind the machine (10' wide, but only about as long as a 5' or 6' cutter), the leverage isn't so bad.

Kevin

Keith_TN65 · Jun 22, 2004

Kevin,

I don't think I really need any thing that big. Also, as soon as you go multiple gearboxes, the price increases dramatically. Too rich for my pocket book /forums/images/graemlins/frown.gif . I probably end up just getting a 6 or 7 foot 26 series from Land Pride.

Keith

AndyMA · Jun 22, 2004

Keith

I went to the Bush Hog website and checked on minimum hp required for various rotary cutters.

Model 285 5' medium duty 30 min hp Model 305 5' heavy duty 30 hp
Model 286 6' medium duty 40 min hp Model 306 6' heavy duty 40 hp
Model 287 7' medium duty 50 min hp Model 307 7' heavy duty 60 hp lift 40 pull

At least Bush Hog's specs show a difference in hp requirements for medium and heavy duty 7' units

Andy

RedDog · Jun 22, 2004

( ( TN65 with 47 PTO HP should be able to handle a 9' rotary cutter I don't think so. My dealer seems to think a 6 foot rotary cutter is all I should )

I think I would get a new dealer.. My dealer told me my NH 1920 will pull a 6' cutter...

47 pto hp is pretty decent.. I would think you could at least get an 8' back there..

Soundguy )

I think he should be able to do way better than that!!

What is it with these "high horse power ratings" to run a mower? /forums/images/graemlins/confused.gif Do they need these high HP ratings because people are "scalping" their grass?

I have a 12' Bush-hog cutter that I mow my parents field once or twice a year. I do not have it set real low, but my old Massey Ferguson 65 (gas) handle it with ease! /forums/images/graemlins/cool.gif I mowed in high range without any problems. I'm thinking that Massey was around 40 PTO HP.

Now if the field was a little smoother,,,,, I would be in "road gear" with the Kioti /forums/images/graemlins/grin.gif

I'm actually thinking of selling my Landpride 7' finish mower, because I like the 12' Bush-hog so well,,, not to mention the less time it takes to mow.

RedDog

jimg · Jun 23, 2004

Good question...HOW do they come up w/ these hp recommendations??? Id be interested to see what the list consists of. Its not limited to just mowers either.

Keith_TN65 · Jun 23, 2004

Thanks for the info Andy. Land Pride's literature also shows different minimum horsepower requirements for Standard, Medium, and Heavy duty cutters.

One thing I find interesting is that the minimum HP requirement for the 7' Land Pride medium duty 3 point rotary cutter is 60, while the minimum HP requirement for the 7' drag medium duty is only 30.

Keith