Torque versus Horsepower

hazmat · Oct 22, 2002

Low torque, high RPM can be converted to low RPM high torque with the proper gear box.

True, but you actually lose torque & power due to driveline friction etc. Better to start with the appropriate motor for the application.

Ever play with the lego sets as a kid? I remember using every gear I could find to make a hand cranked windmill. well I geared it up so much that I wasn't able to turn it. The friction force was greater than my 10 year old hand could produce.

Anonymous Poster · Oct 23, 2002

Is this torque or horse power

Trev · Oct 23, 2002

Diesels give torque at low rpm, modern engines give less torque but rev higher. A small, high revving engine with low torque can have the same HP as a high torque, low revving engine, but don't expect it to pull a plow at 540 rpm...

Well, if you geared it down low enough (say, about one inch per hour forward speed) it might pull a plow, no? /w3tcompact/icons/smile.gif

Seriously, I keep coming back to the feeling that there is something fundamental in what I am missing here. When I feel that way, generally I try to go back to basics and start from the beginning ... hoping to spot where I'm going wrong. In this case, that approach may or may not work:

Let's say we start with the idea that a given amount of "energy" is needed to do a given amount of work. You can dole the energy out slowly, and accomplish the job slowly, or you can dole it out fast and get the job done quickly. But the amount of energy needed will be a constant. (Let's forget for the moment about incidental losses in the drivetrain, gears, and so on.)

Some jobs need to be done quickly, like drag racing.. while some other hypothetical job isn't time-critical (you leave for vacation and when you return it's completed.)

Hmm.. I think this isn't leading in a useful direction. All of the above is probably either obvious or simply incorrect. /w3tcompact/icons/smile.gif

Let's try again:

An engine needs to output energy in a way that's useful for the given application. For pulling stumps, all you need to do is find the right gearing and almost any engine can do it. For drag racing, you need to avoid the wasted time of shifting ... so what you want is an engine that can maintain its energy output over a wide RPM range.

Could it be that "horsepower" really reflects how wide this RPM range is for a given energy output? For example, if we need 50 foot pounds of torque, an engine that can generate this much between 3000 and 4000 RPM would be seen as having a lot less horsepower than one that can generate it between 1000 and 10000 RPM?

Okay, I've displayed enough ignorance for one post. /w3tcompact/icons/blush.gif

Thanks to everyone who's replied!

Trev · Oct 23, 2002

Is this torque or horse power

Actually, that's an excellent question! We keep thinking in terms of twisting force exerted on driveshafts and wheels, but here is a form of energy that doesn't lend itself to that kind of analysis at all. /w3tcompact/icons/smile.gif

BigDave · Oct 23, 2002

O.K. it took two different Physics text books /w3tcompact/icons/clever.gif, and the great inputs from my fellow TBNers who contributed to this post, but I think I got it. /w3tcompact/icons/grin.gif

In plain simple english,

Torque is the amount of force that a rotating shaft can apply to an object.
Horsepower is the speed at which it can supply that force.

Remember from the original article:

HP = (Torque * RPM) / 5050

Rearrange it so that it makes sense to tractor folks:

Torque = (5050 * HP) / RPM

If the rpms are higher, the horsepower must be higher to maintain the same amount of torque. Lower the RPM's, and you lower the horsepower requirement.

This is why manufacturers don't specify HP alone, they always specfy HP at a particular RPM. My tractor is rated at 33HP at 2800RPM, but that's car talk. What really matters is what comes out of the PTO. It delivers 27 PTO HP at 540 rpm, thus from the equation it makes about 250 lbs of torque at the PTO, or to clarify, 250 pounds of force in the direction of rotation.

ddl · Oct 28, 2002

Torque is rotational force, t=Fl, where t= torque. time is not part of the equation. we increase torque by increasing the size of shaft or raduis of the gear. If we apply a torque and rotate a shaft a distance, work has been accomplished. Horsepower is a measure of work per time. thus we have HP= (torque*radians)/time and since we are dealing with rotational work, as the size of shafts or gears change horsepower developed at that point change. engine horsepower vs PTO vs wheel horsepower.
Dan L

RobS · Oct 28, 2002

Here's a slightly different way to look at it Bob. Not sure it will help, but what the heck...

Grab your handy dandy torque wrench. Put your biggest socket on it and find a matching large nut on your tractor. Make like you're tightening the nut and you'll see torque in action. The key is that you can apply a lot of torque without even moving the nut. If it's like the blade bolts on my bush hog, you can apply enormous amounts of torque without budging a thing. Now consider all that work you thought you did trying to budge a frozen nut... lots of torque but no movement. Your torque wrench was moving at ZERO RPM! Thus, no horsepower was developed and in spite of your sweating and swearing, us engineer types would say you did no work! Again, lots of torque but no power as you didn't actually move anything.

Now take the same example but this time the torque wrench is moving. Voila, you've got some horsepower. Not much, but a little.

Now to engines. Engines don't like to operate at zero rpm. They need a little speed just to keep running. If you hooked your engine up to a torque wrench with a slip clutch you could easily measure the torque (someone else mentioned the Prony Brake method). The engine speed while it's doing that work is what gives you the horsepower. The torque/power curves you see are generated on a dyno at max output conditions. Rarely would any of us operate our tractors/vehicles like that.

Hope this helps, it is a confusing subject /w3tcompact/icons/smile.gif

Trev · Oct 28, 2002

Hi Rob,

Now take the same example but this time the torque wrench is moving. Voila, you've got some horsepower. Not much, but a little.

Actually, that did help!

In a way it's kind of like earlier posters explained, but I was still stuck in a dead-end train of thought at that point so it didn't sink in.

The concept was a hard one for me.. and still needs a tad more thought.. but this nudge in the right direction helped.. so thanks again.

Bob

Anonymous Poster · Oct 31, 2002

These explanations all appear to be technically correct, and make a great deal of sense, but you have to wonder sometimes how they apply to the real world. Looking at the specs for the new John Deere 4x10 series tractors shows some strange numbers. If you compare the 4310 thru 4710, all the engines are rated at 2600RPM, and of course the PTO is rated at 540RPM, so you would expect to find a linear relationship between engine torque and PTO horsepower using the gear transmissions in each case:

4310 - 73.1 ft lb, 27 PTO HP (3 cylinder)
4410 - 81.0 ft lb, 29 PTO HP
4510 - 73.7 ft lb, 33 PTO HP (4 cylinder)
4610 - 78.7 ft lb, 37 PTO HP
4710 - 86.3 ft lb, 41 PTO HP

The 4410 is the torque champ (for the money), but sure doesn't show the PTO HP you would expect. It's hard to believe that its gear transmission is that much less efficient than a very similar transmission in the larger tractors.