Help me understand HP vs Torque

[Thenior]

I suck at mechanics....

I am working on a project using small DC motors, and I'm trying to understand how all this Torque/HP stuff works so I can work on a diagram that indicates if I have X Torque at X RPM and it weighs X, it will move at roughly X speed.

My understanding with torque is that it's a rotation energy, if you will. 1 ft/lb of Torque means that if you have a circle with a 1ft radius, 1lb of force is being applied on the outside edge.

Horsepower is lifting 33,000lbs 1ft per minute.

To me, these just seem like interchangeable units of measurements, much like inches and feet. But I know a sports car has high HP/low torque, and a tow truck has low(er)HP/high torque, in a very generalized sense. I don't understand why that is the case. Wouldn't more HP equal more Torque, and vice versa?



More specifically, I have motor that has 8.84 oz-in torque @ 17,000RPM's. Using this formula, I have Horsepower = 0.0460416666372 (lb ft) * 17000 / 5252, or around 3/20HP.

That would imply my motor (assuming perfect efficiency and traction) could move around 4950lbs 1ft per minute. Obviously my motor isn't powerful enough to do that, so I assume it's implying when geared correctly (since at 17,000RPM's it would move much faster than 1ft/minute). Does this mean that theoretically I could move 10lbs at 6.625 MPH ( (4950/10) * (60/5280) ), 5280 being how many feet there are in a mile?

A little direction would be super....

[Money!]

Thenior - I cannot help. But I thank you for asking, because I would like a little illumination on the subject as well.

Request seconded!

[Spidey]

Torque and Horsepower are related…

Torque is the ability to “twist” and can be measured several ways.

Horsepower is “developed” at any given rpm, whereas torque can be rated without any movement at all.

Example: An engine that is all torque will need many more gears to get something up to a high rate of speed, than an engine that develops a lot of horsepower.

An old adage from back in my drag racing days…torque gets you off the line, but horsepower gets you thru the traps.

I’ll leave all of the boring formulas to you, but that is the jist of it.

[Duper]

Read HERE on wiki.

Good stuff there.

[Thenior]

I feel stupid looking at that article. I have to keep rereading it... thanks for the link, looks like the right path.

[Ferno]

in a nutshell:

horsepower is the work done via RPM.

Torque is the rotational force from zero RPM up to maximum RPM.

Reference material

[snoopy]

Yeah, so I think it's easier for me to visualize in linear space, rather than rotational space so:

Force (or torque) is a measure of how "hard" you push. The heavier an object, the harder you have to push to make it move.
Work is force * distance: So, work is a measure of how much useful stuff (movement) you do with all that pushing.
Power (Watts or HP) is a measure of how quickly you can do work.

Here's the tricky part: when a motor/engine/etc is rated for power, it's rated as a peak power. So, at different speeds, the device will produce different amounts of power. That's where gear reduction gets in: Gear reduction's aim to let the motor run at it's optimal speed (usually high speed - which also implies low torque) while getting an output at the desired speed. Gear reduction essentially trades speed for torque.

So, you know how hemi engines are supposed to be about torque? Basically they designed the engine so it produces its peak power at a lower speed than more traditional engines.

[Thenior]

Thanks snoopy, that helped. That's where I was getting hung up - I kept seeing stats like ft-lbs indicating it can move with X force, but I was always asking myself, yeah ok but how fast? Then as Ferno said, the HP is how fast it's spinning at that Torque.

Thanks for the help everyone, I knew there was some basic elements I was missing.

[fliptw]

torque is technically force expressed thru angular momentum, eg work over radians per second.

Horsepower is force expressed thru rotational momentum,eg work over rotations per minute.

the horsepower formula that duper posted assumes you already know what the torque is.