PTO HP versus PTO Torque

check · Aug 22, 2018

I see a lot of discussion and details in tractor brochures about PTO horsepower, and also PQ horse power requirements for various implements. But isn稚 torque what is needed for many PTO operated implements? As an example, my Kubota BX 25 has 17.5 PTO horsepower, and BL 25 old one apparently has only 19 PTO horsepower. Very small upgrade in horsepower, but what about the torque? Will the much bigger L2501 handle a lot heavier implements that require a lot more torque to run? Or is it basically a waste to upgrade to this bigger tractor if PTO power is all that matters to me?

ericm979 · Aug 22, 2018

Horsepower and torque are directly related. HP = Torque x RPM (with an additional factor to translate the units)
At a fixed 540 rpm the torque increase from 17.5 to 19 hp will be proportional.

CalG · Aug 22, 2018

ericm979 said:
Horsepower and torque are directly related. HP = Torque x RPM (with an additional factor to translate the units)
At a fixed 540 rpm the torque increase from 17.5 to 19 hp will be proportional.

To continue the thought....

PTO Speed in RPM can tell one a lot on this subject. Compare the HP capabilities of a 1000 rpm pto vs a 540 rpm unit.

The 1000 rpm pto can carry just about double the power of the 540 rpm assembly using the same u-joints and shaft sizes.

IceStationZebra · Aug 23, 2018

Torque is measured, power is calculated. You can not separate the two. So as Eric said, assuming the same RPM 17.5 vs 19 is directly proportional. power[HP] = (RPM x Torque[lb-ft])/5252

As far as whether it is worth the extra 1.5hp you also have to consider that you need some of the engine power to move the tractor and implement, assuming it isn't a stationary application. It would also be interesting to see if either has any torque rise as the engine bogs down, very useful and important on ag tractors.

Texasmark · Aug 25, 2018

check said:
I see a lot of discussion and details in tractor brochures about PTO horsepower, and also PQ horse power requirements for various implements. But isnç¨š torque what is needed for many PTO operated implements? As an example, my Kubota BX 25 has 17.5 PTO horsepower, and BL 25 old one apparently has only 19 PTO horsepower. Very small upgrade in horsepower, but what about the torque? Will the much bigger L2501 handle a lot heavier implements that require a lot more torque to run? Or is it basically a waste to upgrade to this bigger tractor if PTO power is all that matters to me?

HP in short is (torque in ft-lbs x rpms)/5252. PTO rpms are usually 540. Input your PTO rated hp and solve for torque for your answer.

Torque is the twisting "moment of inertia", meaning the radial pressure applied to an immovable object. Think of it as stored energy, like a 10' pole mounted to a shaft with a 100# box of concrete on the end of it. 1000 ft-lbs "ready" to turn the shaft. Lots of potential energy to get the shaft to turn but until it moves, it's just energy. Once it rotates, it's no longer torque, it's HP.

On a tractor's PTO a lot of times you can modify the required torque to keep the PTO rpms up, aka PTO hp at the rated value by reducing the "resistance to rotation" aka load on the shaft to be turned. On a rotary mower this means raise it, or don't take such large bites...like only run with half the deck cutting.

On whacking large limbs, you have 2 things working for you: Kinetic energy stored in the rotating blades and PTO torq to keep them in the extended position. If you don't have the HP for big jobs you can get the energy up by backing off and letting your rpms come back up to spec, putting your kinetic energy in the blade tips back where it was and going back after your target.