# Tractor HP vs implement rating



## DHeraud (Sep 12, 2014)

Hey guys,

So I have a 50HP MF65, and I'm working on 1.10m (approx. 42in) wide beds. Now I didn't realize this but not many implements are made to be that narrow while also being rated to work on tractors with this kind of horsepower.

So, my question is, can someone explain to me what the effects of using lower rated implements (say, a 20HP PTO rated rototiller) on a 50HP tractor?

Is the implement going to break right out of the gate, is it going to wear prematurely?...

Thanks!


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## Hoodoo Valley (Nov 14, 2006)

Well, you have I believe a category 2 hitch, whereas you're talking about what would amount to a category 1 implement. That aside, if you were to manage a hook up, then the implement would be severely overpowered at the recommended pto rpms......but...... so long as you ran your tractor at a lower rpm, much lower, then you could essentially closely provide the recommended horse power, as engine horse power is connected directly to a certain rpm, but on the flip side, the implement would be operating very slowly because the pto rpms would be diminished greatly. You might be able to add some sort of slip clutch to the pto or the shaft, that could prevent damage too. I'm not sure what kind of ground you have, but I'd be inclined to think that barring larger rocks or hard pan, something like that, that if your did hit the implement with the full horse power of your tractor, and just traveled at a very reasonable ground speed, that you could easily avoid damage or premature wear to the tiller. It would be just like a small car (Ford focus) with a huge engine in it without beefing up the rest of the car. If you were reasonable with it, you can avoid damage, but push your luck and........ kablewy! This is my take on it.


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## DHeraud (Sep 12, 2014)

tractor beam said:


> Well, you have I believe a category 2 hitch, whereas you're talking about what would amount to a category 1 implement. That aside, if you were to manage a hook up, then the implement would be severely overpowered at the recommended pto rpms......but...... so long as you ran your tractor at a lower rpm, much lower, then you could essentially closely provide the recommended horse power, as engine horse power is connected directly to a certain rpm, but on the flip side, the implement would be operating very slowly because the pto rpms would be diminished greatly. You might be able to add some sort of slip clutch to the pto or the shaft, that could prevent damage too. I'm not sure what kind of ground you have, but I'd be inclined to think that barring larger rocks or hard pan, something like that, that if your did hit the implement with the full horse power of your tractor, and just traveled at a very reasonable ground speed, that you could easily avoid damage or premature wear to the tiller. It would be just like a small car (Ford focus) with a huge engine in it without beefing up the rest of the car. If you were reasonable with it, you can avoid damage, but push your luck and........ kablewy! This is my take on it.


Thanks for the input! The MF65 actually has a Cat-I hitch so that's no issue.

I was under the impression that 540rpm was 540rpm, regardless of how many HP's the engine is putting out... no?


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## Fredneck (May 25, 2012)

tractor beam said:


> It would be just like a small car (Ford focus) with a huge engine in it without beefing up the rest of the car. If you were reasonable with it, you can avoid damage, but push your luck and........ kablewy! This is my take on it.


it's funny that u used ford in your explanation. my favorite example of this was a real-life ford product. in the 1980s (iirc), they produced a version of the mustang which, until they realized the problem, was available with an optional engine that had enough horsepower to actually twist the unibody if u jumped on the gas too hard


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## Hoodoo Valley (Nov 14, 2006)

Fredneck said:


> it's funny that u used ford in your explanation. my favorite example of this was a real-life ford product. in the 1980s (iirc), they produced a version of the mustang which, until they realized the problem, was available with an optional engine that had enough horsepower to actually twist the unibody if u jumped on the gas too hard


I could imagine you out there welding up cracks..........


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## Fredneck (May 25, 2012)

lol - happily, i was not one of the victims of that fine bit of engineering expertise. i had already stopped buying ford cars when those came out


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## Hoodoo Valley (Nov 14, 2006)

Fredneck said:


> lol - happily, i was not one of the victims of that fine bit of engineering expertise. i had already stopped buying ford cars when those came out


I've got a 1994 Toyota truck that I've put over a quarter million miles on and it still uses no oil in between oil changes. It has the 4 cyl so no need to worry about twisting the frame, breaking drivelines etc.


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## Country Boy (Mar 18, 2010)

DHeraud said:


> Thanks for the input! The MF65 actually has a Cat-I hitch so that's no issue.
> 
> I was under the impression that 540rpm was 540rpm, regardless of how many HP's the engine is putting out... no?


The PTO speed varies with engine RPM. Your Tach might have a line that says "PTO" or "540 PTO" by it, indicating how fast to run the engine to get the PTO shaft to spin at the correct speed. All my IH tractors with the exception of the H have that marking on the Tach (the H doesn't have a Tach, its too old!). Running the larger tractor will work, but as TB said, you'll want to either slow the unit down or make sure you are using the correct shear pins for it. You don't want to be running 3x the horsepower on it and then hit a big rock. Some tillers can be narrowed by removing the outer tines. Perhaps you can look into doing that with a tiller more suited to that tractor's power? It would give you the option of widening the tiller out down the road if you need to.


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## OldCoastie (May 9, 2015)

Don't you need to be more concerned about PTO horsepower output? I have a MF65 diesel and some of the specs I've read IRT the PTO hp rate it at around 30 hp.


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## Dave41A (May 4, 2011)

I see this is an old thread but I'll add my experience for what it's worth.

In short, horsepower ratings of implements reflect the *maximum* amount of horsepower that the implement is expected to draw or use. Typically a shear pin or other limiting device is in place to prevent the device from drawing more power.

As an example, a tiller that is spinning in air (lifted out of the ground) will draw practically zero horsepower. Lowering it and engaging the soil will increase this horsepower draw to some non-zero value. Since the tractor's governor opens up to maintain the 540 PTO rpm, in this case RPM is constant so the horsepower drawn by the implement is reflected in increased PTO torque alone (hp = rpm x torque in ft-lbs x 0.000190 or use this converter: http://www.1728.org/mtrtrq.htm). 

So a tractor at rest spinning the tiller is producing no real horsepower other than that small amount to overcome the frictional torque of the attachment. Engaging the soil will increase the PTO torque and the horsepower supplied (or hp drawn by the attachment, which is the same). Digging in deeper (or hitting a rock) will increase it further until one of two things happens:
a) the shear pin fails or (b) the engine bogs down and stalls.

If the implement is too large, the engine will bog down first. If the engine is over-sized, the shear pin --which is sized to break at a certain torque level--will break first. If they are approximately equally matched, then your guess is as good as mine.

Hope that makes sense.

Good luck,
Dave41A


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