Too much HP on pto?

[shui5612]

Alright so I have a question concerning pto's.

Being somewhat new to tractors and only owning one PTO attachment, a 5 foot Harley Rake, it has a recommended PTO hp range of 18-32.

My current tractor has 16pto hp, and having used the rake this summer it did a fair job. I had to slow down abit over roots or rocks, and the weight on the 3PH is about at the maximum lift capacity of my tractor (2012 yanmar SC2450).

Fast forward to today I am looking at a tractor with ~49-50HP PTO.

Is it possible to run that implement and say reduce the engine rpm/pto rpm so that I could avoid damage, or is having too much HP a bad thing? What I'm saying is; most people size their implements to fit their tractors power rating/size but is having "extra" HP for a given implement a bad idea (if it's over the recommended range)

I have run the Harley rake in light sand to level it out while reducing the rpm on my tractor because it managed to keep up easily.

P.s. I understand that if you put too much HP on the attachment it might "explode" but I'm wondering if running the attachment at reduced rpm therefore HP) is safe seeing how rpm does not affect the performance of the implement as much as it would on say a pto generator.

Thanks again.

 

[k0ua]

if you had 10,000 PTO horsepower available to run your harley rake it would not matter except in the following way. If for some reason the harley rake stopped turning for some reason and the Shear bolt or slip clutch failed to work for some reason, something has to give. Rake, shear bolt, PTO shaft, but for darn sure the 10,000 PTO horsepower machine is not going to bog down even one RPM. You see where I am going with this?

If the shaft is turning at 540 RPM with a potential of 1 million horsepower or 10 horsepower the implement is not going to know it. You are just not going to bog down the 1 million horsepower power source no matter what happens to the implement. On the 10 horsepower power source it is liable to bog down and the RPM drop or even stall completely the 10 horse PTO. Does this make sense.?

In your case, going to the 40 horse machine, all that is going to happen is the shear bolt is going to break a little easier if implement becomes bound up with a big rock or something. If the shear bolt fails to break, then something else is liable to break. Maybe twist up the PTO shaft or something else. But as long as the implement is operating normally nothing bad will happen by hooking it to a much larger power source. 540 RPM is still 540 RPM, the bigger tractor can just deliver much more torque into a load.

 

[RedNeckRacin]

x2. :thumbsup: better to have too much then to not have enough. Just leave the implement protection devices in place and you will be fine. The shear bolts/clutch may not like it but treat your equipment well and you should have no problems.

 

[SPYDERLK]

540 RPM is still 540 RPM, the bigger tractor can just deliver much more torque into a load.

... And, to reiterate, that torque is limited by safety devices that are incorporated into the implement hookup.

 

[shui5612]

Wow thanks a lot everyone,

Yes it makes sense now; all of the safety devices will stay on but they will become the limiting factors ( as intended) instead of being limited by them and the tractor.

Thanks again;
Ian

 

[MHarryE]

Torque is limited by the slip clutch or shear bolt for a single catastrophic overload, but other than those obstructions for which the machine provides protection, fatigue loading limits machine life. Design equations for different components (PTO driveline, shafts, bearings) all differ for calculating life but in general, think of life being reduced by the square of the power transmitted. For example running a PTO driven implement operating at 16 hp will last 4 times longer than one operating at 32 hp. Then again there are some components where the life calculation involves the cube off the load. In this case operating at 16 hp will provide 8 times the life. This is the reason manufacturers place a power rating on their implements. Reducing rpm will reduce your tractor's available power but it may also significantly reduce productivity of the implement. You will do better to take it easy - run the engine to provide 540 rpm at the PTO but drive slower. The shear bolt or slip clutch is still there to protect against a blockage and implement life will be improved.

 

[newbury]

The shear bolt helps but make sure you have a PTO shaft designed for for your HP. You may be running a shaft designed for the HP of your present tractor,
Horsepower Rating and Length shows a series 1 PTO shaft rated for 16 HP, and a series 5 rated for 47 HP.
So if you have a series 1 shaft it might be damaged when you apply just 32 HP to it continually.
It shouldn't damage the implement but when the shaft goes and your out in the field it is not pleasant.

 

[Egon]

The slip clutch adjust may be more difficult with the power increase and may tend to too high an adjustment.

Shear pin remains the same so no problem.

 

[SPYDERLK]

The shear bolt helps but make sure you have a PTO shaft designed for for your HP. You may be running a shaft designed for the HP of your present tractor,
Horsepower Rating and Length shows a series 1 PTO shaft rated for 16 HP, and a series 5 rated for 47 HP.
So if you have a series 1 shaft it might be damaged when you apply just 32 HP to it continually.
It shouldn't damage the implement but when the shaft goes and your out in the field it is not pleasant.

If his current shaft works with the implement overtorque safety system it will work likewise on the higher power tractor.

 

[k0ua]

If his current shaft works with the implement overtorque safety system it will work likewise on the higher power tractor.

Exactly. The only reason to increase the strength of the PTO shaft is if the load that the implement puts on the shaft increases. As an example consider a 4 foot tiller and lets say a hypothetical 8 foot tiller. The 8 foot tiller would put considerable more load on the PTO shaft because the resistance of the tines penetrating the ground and its much larger gearbox resistance would require more torque to be delivered than the smaller lighter duty shaft was designed for. But in this case, his implement load stays the same, only the potential torque delivered by the power source is more. The load is not more.