Torque... We've been wrong all these years! 11

[kthree]

For many years we've rated our gear reducers in 'inch pounds'. Lately I've been noticing, mostly automotive and implement ads, torque being rated in 'pound inches' or 'pound feet'. Today I grabbed my Machinery's Handbook and there it is, "...pound-feet, pound-inches, kilogram-meters, etc." Wow, We've been wrong all these years!

 

[JStephen]

The commutative property of multiplication at work.

 

[dhengr]

Ah... But they changed the sign convention at the same time, so if you rotate it 180̊ the old way still works. I learned and have used inch-pounds, etc. for so long that pound-inches seems awkward to me. I just assume the other guy doesn’t know any better.

 

[TheTick]

kilogram-meter is an abomination

 

[LiftDivergence]

I am more accustomed to seeing units labeled "in-lbf" than "lbf-in" if that makes any difference. As far a I'm concerned they're interchangeable.

Keep em' Flying

"I intend to live forever, or die trying" - Groucho Marx

 

[dgallup]

I've always considered anyone who insisted it made a difference was a tool.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[redpicker]

For those who aren't aware, the issue arises from the fact that Energy and Torque have the same fundamental units, Force and Distance.

It could be confusing if one was dealing with Energy in Ft-Lbs and Torque in Ft-Lbs.

For example, 1HP is defined as 550 FT-Lbs per Second. Now, if you have an engine that can produce 1 HP, it has the power to lift an object with a weight of 550 pounds a distance of 1 foot in 1 second, or lift an object with a weight of 1 pound 550 feet in one second.

On the other hand, if you have a 550 Lb weight on the end of a 1 foot arm, it produces a torque of 550 Ft-Lb. It does it for one second if you leave it there for a second; it does it for an hour if you leave it for an hour. Noting is moving, there is no energy expended.

To keep the different uses of these fundamental units separate, the SAE (I think it was SAE) decided that energy would be described by Distance-Force, and torque by Force-Distance. The guys torqing nuts, bolts, etc..., however, didn't get the memo.

So Ft-Lbs describes the energy produced by an engine. Divided by time, it describes the power.
Lb-Ft describes the torque the engine can produce. When the RPM is defined, it also describes the power.

Ft-Lbs can also describe the tightening torque use on the head bolts. An engine that can produce 400 Lb-Ft or torque at 3800 RPM (280 HP) may need the head bolts tightened to 65 Ft-Lbs.

rp

 

[kthree]

Thanks All. As it was explained to me way back in the dark ages by one of the Old Timers, "it is always easier to find a 1" long lever." K.

 

[dhengr]

Dgallup:
By ‘tool,’ do you mean a torque wrench?

 

[drawoh]

I am more accustomed to seeing units labeled "in-lbf" than "lbf-in" if that makes any difference. As far a I'm concerned they're interchangeable.

Continuing with TheTick's point, what does that letter "f" tell us that the "lb" part did not?

--
JHG

 

[LiftDivergence]

The "f" in "lbf" refers to the word "force". This makes a distinction between pounds force and pounds mass. Simply putting "lb" is ambiguous, although it is generally easy to figure out from context. However, I always make a point to include the "f".

Keep em' Flying

"I intend to live forever, or die trying" - Groucho Marx

 

[dvd]

I learned torque as lb-ft (actually lbf-ft) and work/energy as ft-lbf, but I can say it either way in conversation depending on the audience.

 

[JStephen]

Well, hopefully, you're a little more on top of your calculations so you don't have to tell torque from work by the order of the units.

One motivation for keeping the f on lbf is so you can figure out along the way if you need to throw a gc factor in there or not.

 

[3DDave]

When it comes to lb vs lbm vs lbf I let them all just slug it out. Usually, the slug wins.

 

[GrandpaDave]

Now don’t forget these “guys”… The blob is the inch version of the slug (1 blob = 1 lbf•s2/in = 12 slugs)
or equivalent to 175.126 kg. This unit is also called slinch (a portmanteau of the words slug and inch).
Slang terms include slugette, and a snail. Metric units include the "glug" in the centimetre-gram-second
system, and the "mug", "par", or "MTE" in the metre-kilogram-second system.

I cheated because I looked these up on the Net. In the past I worked with a loads engineer, RIP, who used
to call the inch version “sligs”… I like that better than any of the above.

PS: I also was raised on inch-pounds as torque. I've been wrong for 60+ years!

 

[kingnero]

kilogram-meter is an abomination

True. we use Nm (newton-meter) over here...

 

[Tmoose]

1958 Edsel shop manual - "tighten all the (292 V8 connecting rod) nuts to 45-50 lbs. ft. torque."

 

[drawoh]

Tmoose,

In college, we were told by an instructor never to place an "s" after "lb". "lb" is Latin.

"lbi"?

--
JHG

 

[Duwe6]

Until we hold some serious motivational seminars at NASA, including hot irons and bastinados, lbf vs lb is pretty stupid. We are trapped at the bottom of a 1G gravity-well. It would be real nice to have to engineer things for locations other than the surface of the Earth, but can't see it happening any time in my lifetime.

Unless i get to 'motivate' some folks.

 

[3DDave]

Any work that involves accelerations besides 1G cause a problem because one-lbm will not exert one-lbf.