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View Full Version : Torque wrenches use Newtons (correct) and Foot Pounds (Wrong)

wolf_meister
10-26-2003, 01:29 AM
Okay, I've noticed that on every torque wrench I've ever seen the readout is in Newtons (which is a unit of force and torque is a force). However, they also read out in foot pounds which is a unit of ENERGY and that is wrong. POUNDS are a unit of force so why do they say foot-pounds? To me this makes as much sense as having a measuring cup that measures cubic centimeters and square inches.

sailor
10-26-2003, 01:45 AM
You got it all wrong. Torque is not a force but force multipled by the length of the arm. Foot*pounds is correct. Newtons is not.

commasense
10-26-2003, 01:57 AM
There seems to be widespread confusion between the foot-pound (the unit of work required to raise a mass of one pound a distance of one foot) and the proper SAE unit of torque, which is the pound-foot. A pound-foot is the torque exerted by a pound of mass at a distance of a foot from the axis of torsion.

This confusion apparently extends to many Web-based conversion tables, and the makers of torque wrenches. I have two, from different manufacturers, both of which incorrectly read "foot-pounds." (Until just a few days ago, when I read an item about the pound-foot in the current issue of Road & Track, the confusion also extended to me.)

Here's (http://science.howstuffworks.com/fpte3.htm) a site that gets it right.

scr4
10-26-2003, 02:02 AM
Originally posted by commasense
There seems to be widespread confusion between the foot-pound (the unit of work required to raise a mass of one pound a distance of one foot) and the proper SAE unit of torque, which is the pound-foot.
Why is this distinction important? It's the same unit; same exact dimensions and value.

Earthling
10-26-2003, 02:03 AM
You'd better check those torque wrenches again. The metric measurement should say N-m, for Newton-meters. That's what it says on my wrench.

sailor
10-26-2003, 02:42 AM
Originally posted by scr4
Why is this distinction important? It's the same unit; same exact dimensions and value. No, not really. When you exert a force of 1 lb for a distance of 1 ft you have done 1 lb*ft of work. It is a unit of work or energy.

OTOH, torque is not work or energy. It is the force multiplied by the length of the arm. To obtain the work done you cannot multiply the force by the length of the arm but you have to multiply the force by the distance traveled which is the arc of circumference traveled.

If I am turning a wheel with a torque of 1 ft*lb, for every complete turn I have done 2*pi() lb*ft of work.

Different concepts even if the dimensions are the same.

racer72
10-26-2003, 06:09 AM
At my job everything is measured in inch/pounds. In fact last week I got me a brand new 15 in/lb cordless screwgun in my tool kit. No more dragging air hoses around the airplane!!!

scr4
10-26-2003, 06:39 AM
Originally posted by sailor
Different concepts even if the dimensions are the same.
OK, I agree they are different concepts, and I realize that SI uses different units for the two (Joule vs. Newton-meter). But I'm still not sure if it's important to distinguish between "pound-foot" and "foot-pound" - I can't think of any other compound units where the order matters.

10-26-2003, 06:43 AM
It should be N-m, not just N.

The difference between ft-lbf versus lbf-ft seems IMO to be one of parlance. Equation-wise, they have the same dimensions. And in some professions, like mine, parlance wars are the equivalent of "Enterprise versus Star Destroyer".

sailor
10-26-2003, 07:21 AM
Originally posted by scr4
OK, I agree they are different concepts, and I realize that SI uses different units for the two (Joule vs. Newton-meter). But I'm still not sure if it's important to distinguish between "pound-foot" and "foot-pound" - I can't think of any other compound units where the order matters. Conventions exist for a reason. It is only a matter of being clear about whether you are talking about torque or work. As long as it is clear to everybody you can say it whichever way you want but following conventions makes it easier for everybody to understand and avoid mistakes.

aerodave
10-26-2003, 07:36 AM
Originally posted by sailor
Different concepts even if the dimensions are the same.

But the N-m is the proper metric unit of both work and torque. Different concept, same dimensions...but same unit as well.

And yes, the OP should notice that his torque wrench, unless very poorly designed, has N-m on it. Force times Length

The difference between work and torque (even using the same dimenstions) becomes clear when working in vector math. For the unfamiliar, vectors are quantities having both magnitude and direction.

Torque is the cross product of force and the length of the moment arm. The result is the torque vector, which is perpendicular to both.
T = F x r

Work is the dot product of force and the distance over which it is applied. The result is a scalar. That is, it has magnitude, but no direction.
W = F • d

10-26-2003, 07:39 AM
There seems to be widespread confusion between the foot-pound (the unit of work required to raise a mass of one pound a distance of one foot) and the proper SAE unit of torque, which is the pound-foot. A pound-foot is the torque exerted by a pound of mass at a distance of a foot from the axis of torsion.Nitpick: don't you need to specify g? Torque and Energy should be force x distance not mass x distance iirc.

Anyway, after some googling, it appears that:

1. They do in fact have the same units.
2. But the convention is to use J or foot-pounds for energy and Nm or pound-feet for torque, to try and avoid confusion.
3. While it's normal to gloss over this, torque is defined by physicists as a vector, (while energy is definately a scalar) so they can't be compared.

j666
10-26-2003, 08:20 AM
I don't know why I opened this post, but I am really glad I did; I wish my physics instructor had explained this as neatly as Shade.

So this is why my pruning sheers and bolt cutters have such long handles and why I hold my hammer at the far end rather than at the forte, right?

If I finally understand torque, it's why levers work, right?

wolf_meister
10-26-2003, 12:20 PM
Thanks to all for the quick replies.

Saltire
10-26-2003, 12:37 PM
Nitpick: don't you need to specify g? Torque and Energy should be force x distance not mass x distance iirc. Actually, no. Though most people tend to think of the pound as the American system's unit of mass, it is actually the unit of force, analogous to S.I.'s newton. Mass in the American system is measured in slugs.

Almost no one needs to know this, since almost any application where the distinction is important would use S.I. units.

10-26-2003, 12:44 PM
OK, fair enough. But I think "a mass of one pound" is not the clearest way to express "a mass weighing one pound" as opposed to "a pound-mass."

j66: Thanks. Yes, that's why levers work :)

Achernar
10-26-2003, 12:48 PM
1. They do in fact have the same units.
2. But the convention is to use J or foot-pounds for energy and Nm or pound-feet for torque, to try and avoid confusion.
3. While it's normal to gloss over this, torque is defined by physicists as a vector, (while energy is definately a scalar) so they can't be compared.Torque is actually a pseudovector, but so what? Velocity is a vector and the speed of light is a scalar, but they can still be compared. And they still use the same units.

For a magnetic dipole mu in a magnetic field, the torque is given by:
tau = mu &times; Band the interaction energy is given by:
U = -mu &middot; BHow could they possibly not have the same units?

swansont
10-26-2003, 01:23 PM
Originally posted by audilover
Torque is the cross product of force and the length of the moment arm. The result is the torque vector, which is perpendicular to both.
T = F x r

T = r x F

The resultant vector ends up in the opposite direction if you do it backwards.

j666
10-26-2003, 01:48 PM
Psuedo vectors?!!

aerodave
10-26-2003, 01:58 PM
Originally posted by swansont
T = r x F

The resultant vector ends up in the opposite direction if you do it backwards.

You are correct. Thanks for the friendly reminder. I was spouting from memory, and wasn't thinking about the non-commutativity of vector multiplication.

Then again, it would have been more conventional to write t ("tau" if it doesn't format right on everyone's machines) instead of T, as well.

But I think my point was still clear.

Chronos
10-26-2003, 02:21 PM
Nitpick: don't you need to specify g? Torque and Energy should be force x distance not mass x distance iirc.No, you don't need g. Pounds are inherently a measure of force already; it's when you're using pounds as a unit of mass that you need to specify g.

And if you really want to nitpick, torque is considered by physicists to be a pseudovector, not a vector. Basically, whenever you need to use a right-hand rule to define the direction of a quantity (such as the cross-product of any two true vectors), it's a pseudovector.If I finally understand torque, it's why levers work, right?Correct. Or in more detail, the rotational form of Newton's 2nd law is why levers work. If your lever isn't accelerating, or has a relatively low moment of inertia compared to the things at its end (usually a good approximation), then it has approximately zero net torque on it. Which means that the torque you're putting on one end of the lever is equal in magnitude to the torque your load is putting on the other end of the lever. Torque is force times distance from the fulcrum, so if you're (say) further from the fulcrum, you need less force to get the same torque.

Newton meter
10-26-2003, 02:54 PM
It's a little late, but I just checked both of my torque wrenches, and they say Newton meter.

10-26-2003, 04:43 PM
Torque is actually a pseudovector, but so what? Velocity is a vector and the speed of light is a scalar, but they can still be compared. And they still use the same units.DOH! Good point, never mind. Sorry, guys.

sailor
10-26-2003, 06:36 PM
Originally posted by Newton meter
It's a little late, but I just checked both of my torque wrenches, and they say Newton meter. They call you by name?:p

KP
10-26-2003, 08:46 PM
Torque is actually a pseudovector, but so what? Velocity is a vector and the speed of light is a scalar, but they can still be compared. And they still use the same units.

Um... no. As you noted, a velocity is a vector, and the speed of light -or any speed- is a scalar. You can't directly compare them. Velocity is a vector that MUST have a speed and a heading.

In some cases you can trivially compare the speed component of a velocity vector with a scalar speed, but in other cases, that is not just useless but will earn you (and "earn" it you will) a world of hurt.

"Captain, we fired our lasers at the enemy ship at 300,000 km."
"So it will be destroyed in roughly a second, then? Good."
"Actually... no, sir."
"No? Has someone changed the value of C? Explain yourself, Lieutenant!"
"Oh, our laser bursts still travel at roughly 3x10^5 km/sec. But they weren't aimed at the enemy ship. Due to a sign change in heading by Gunnery mate Bassackward who won't believe in the difference between a vector and a scalar, we fired on our flagship instead."
"Our Flagship? Dear god! What do we do now?"
"Given the disparity in firepower between us and the flagship, and the fact that her main guns are powering up, I'd say... die, probably"