85 replies to this topic

[ciaoduc1]

I've been trying to keep up with the see-saw battle in the shift strategy thread about which way the expression is written. In fact, I just looked through two of my favorite magazines and discovered they are in disagreement also (the car mag says lb/ft and the motorcycle mag says ft/lb).

It seems to me that it doesn't really matter though as far as the bolt (or crankshaft or tire) is concerned. If I torque a bolt using 50 pounds of force at the end of a foot long bar, I have applied 50 pounds per foot (50lb/ft) of torque. However if I apply 1 pound at the end of a 50 foot long bar I have applied 1 pound per 50 foot (lb/50ft). Isn't the end result the same?

The only difference I see is not in the application of torque to bolt, but the distances traveled at the end of the bars if we actually want to move the bolt. If I want to turn the bolt one revolution I will have to travel 6.28...feet with the short bar while I will have to travel 314...ft with the long bar.

Have I missed something?

[Ursus]

Actually it's neither. It should really be lb-ft or ft-lb as torque is force times lever arm.

You have missed nothing but dug too deep, as the order doesn't mean one iota more than you make of it.

[david_martin]

Quote

Originally posted by ciaoduc1
Have I missed something?

Yes - the unit is the product of force and lever arm length, not the the quotient. The product is commutative, so it can be expressed either as pound force feet or feet pound force - both are perfect valid.

[Engineguy]

Every time I type it as ft-lb (or lb-ft) I hear a little voice in my head that says, "It should be ft*lb." I guess that's from years of programming and spreadsheet formulas. The dash looks like a minus sign... seems like a product should be represented by a multiplication operator... ft*lb, lb*ft, ftxlb, lbxft. :

[McGuire]

Technically speaking, torque is referenced in pound feet while work is recorded in foot pounds. Or at least that is the formal distinction. (In the SI system the distinction is newton-meters vs. joule.)

From Units of Measurement, By Russ Rowlett (great resource on all sorts of arcane weights and measures, fascinating stuff)
pound foot
a traditional unit of torque. Torque is the tendency of a force to cause a rotation; it is the product of the force and the distance from the center of rotation to the point where the force is applied. Thus it can be measured in pounds of force times feet of distance. One pound foot is equal to approximately 1.355 818 newton meter (N·m) in SI units. Algebraically, torque has the same units as work or energy, but it is a different physical concept. To stress the difference, scientists and engineers traditionally measure torque in pound feet (or newton meters) and work or energy in foot pounds (or joules).

However, you can find this convention ignored in some very distinguished publications that otherwise aspire to technical precision. In enthusiast publications you can find both ways used to describe an engine's torque, and in either case with every conceivable form of punctuation used betweem them: lb/ft; lb-ft; lb. ft.; lb ft. It depends entirely on the magazine's style sheet.

There are some gray areas as well, especially if you take the approach that torque should be referred to only as work if work is indeed being performed. For example, in the torque specifications for tightening fasteners: if the fastener is moving, work is being perfomed. So one could presume the "ft lb" usage would be in order. However, usually the fastener is not moving as the final tightening force is noted so there the technnically correct term is "lb ft." However, you will find that in virtually all service manuals the convention is foot first. They always call it torque, but the term they use to quanitfy it is the one for work. Interesting, eh?

[McGuire]

Quote

Originally posted by david_martin
Yes - the unit is the product of force and lever arm length, not the the quotient. The product is commutative, so it can be expressed either as pound force feet or feet pound force - both are perfect valid.

The usage employed by ciaoduc1 (lb/ft) is perfectly acceptable. While you have assumed that is a division sign between the terms, actually that mark is a virgule.

The term virgule comes from the Latin virgula, meaning basically "little penis." This typographical mark is employed, according to Webster's, as "a punctuation mark used to separate related items of information." For example: dates (2/15/2005) and measures (km/hr; lb/gal/hr) or as a substitute for "or" ("and/or") "per," "by means of," or "with respect to." This mark is also known as a separatrix, or in plain English, a slash. Yes, it is also used as a division sign when math is depicted in single-line format, but that is not the specific usage here.

Personally, I don't like to use virgules except with more than two terms, but the style can often be found in SAE papers and FAA publications, so there you are. But my preference does not often matter as in practice, we are at the mercy of the publisher. Their copy editors and proof readers typically make the call, according to the style sheet they use.

Interestingly, word people (as opposed to engineering people) will tend to prefer lb/ft to lb-ft. As a very learned editor once said to me, "What the hell is poundfoot? There is no such thing as a poundfoot. There is a foot or a pound, or a foot and a pound, but no poundfoot. These two nouns will NOT be joined with hyphenation in MY magazine. It's an abomination."

[Paul Ranson]

Using lb/ft when you mean lb ft is technically illiterate. I'd like to see an SAE or FAA document that gets this wrong.

Look at http://physics.nist....P811/sec06.html for a guide, the principles are the same for other systems. See http://physics.nist..../appenB8.html#H for an example.

Paul

[Greg Locock]

Well, to those of us that spent 3 years drinking beer and having fun while paying as little attention as feasible to the lecturers, there is an enormous difference between ft/lb and ft.lb

There's this handy little technique called dimensional analysis, and things won't work if you treat those punctuation marks as arithmetical operators. Since we (engineers) read those as arithmetical operators, we make sure they are right. So it is ft.lbf or ft*lbf or ft lbf , not ft/lb or ft-lb

ft/lb is inherently dangerous, since a bridge designer would use a distributed load measured in lbf/ft of span, or would look up the weight of a steel section and get a figure of lb/ft

Frankly, we don't much care if the technical illiterates (including most journalists) get it wrong, their opinion doesn't matter anyway.

Interstingly the SAE used to get it wrong, and used to use ft-lb up until 1976 at least.

[djellison]

Nm

Problem solved.

[McGuire]

Quote

Originally posted by Greg Locock
Well, to those of us that spent 3 years drinking beer and having fun while paying as little attention as feasible to the lecturers, there is an enormous difference between ft/lb and ft.lb

There's this handy little technique called dimensional analysis, and things won't work if you treat those punctuation marks as arithmetical operators. Since we (engineers) read those as arithmetical operators, we make sure they are right. So it is ft.lbf or ft*lbf or ft lbf , not ft/lb or ft-lb

ft/lb is inherently dangerous, since a bridge designer would use a distributed load measured in lbf/ft of span, or would look up the weight of a steel section and get a figure of lb/ft

Frankly, we don't much care if the technical illiterates (including most journalists) get it wrong, their opinion doesn't matter anyway.

Interstingly the SAE used to get it wrong, and used to use ft-lb up until 1976 at least.

The trouble here is virtually no one adheres to those conventions. Not one of the posts in this thread (except for djellison's) fails to violate one of the varius Pedantic Commandments. Including yours and mine, no offense. I can show you SAE documents much newer than 1976 that do not obey your instructions, and from the FAA and NASA. How about last week? My favorite book at the moment is Maurice Olley's posthumous memoirs and papers, collected and published by the Millikens. There you can find the grave sins you speak of. Was the man who invented all our concepts of chassis dynamics "technically illiterate?" Er, who do we think we are?

Here is the deal: In the normal context of formal, casual and even most technical writing, typesetter's marks such as the virgule and en dash are not arithmetical operators, nor are they intended to be, nor did anyone ever say they were. People don't ordinarily write and speak in Mathematical Notation. No, they speak and write in a natural language, in this case English.

In English a virgule does not indicate division, any more than a hyphen indicates subtraction. How far would that assumption get someone trying to negotiate a DOS command line? That's not what those symbols mean in DOS, and it's not what they mean in English either.

[Greg Locock]

I don't think I hold Olney to account for failing to adhere to an improved convention that didn't exist when he was alive! Standards change. I'm not too sure when people started trying to align engineering units with Dimensional Analysis, but I wouldn't mind betting that SI was the first system of units to embrace it (fairly) consistently.

If you want to base an argument on dimensional analysis (and there has been some discussion confusing (?) work and torque on another thread) then it helps to get it right. Eliminating sources of confusion, such as ft/lb is one step in that direction. It is not essential, just helpful. I'll happily work in traditional units, but I bet I make more mistakes and take longer.

[AS110]

Quote

Originally posted by djellison
Nm

Problem solved.

Yep,Newton Metres is the officail term - all previous posts are rambles of yanks and fuddy duddy poms.

[Bob Riebe]

Quote

Originally posted by McGuire


The usage employed by ciaoduc1 (lb/ft) is perfectly acceptable. While you have assumed that is a division sign between the terms, actually that mark is a virgule.

The term virgule comes from the Latin virgula, meaning basically "little penis." This typographical mark is employed, according to Webster's, as "a punctuation mark used to separate related items of information." For example: dates (2/15/2005) and measures (km/hr; lb/gal/hr) or as a substitute for "or" ("and/or") "per," "by means of," or "with respect to." This mark is also known as a separatrix, or in plain English, a slash. Yes, it is also used as a division sign when math is depicted in single-line format, but that is not the specific usage here.

Personally, I don't like to use virgules except with more than two terms, but the style can often be found in SAE papers and FAA publications, so there you are. But my preference does not often matter as in practice, we are at the mercy of the publisher. Their copy editors and proof readers typically make the call, according to the style sheet they use.

Interestingly, word people (as opposed to engineering people) will tend to prefer lb/ft to lb-ft. As a very learned editor once said to me, "What the hell is poundfoot? There is no such thing as a poundfoot. There is a foot or a pound, or a foot and a pound, but no poundfoot. These two nouns will NOT be joined with hyphenation in MY magazine. It's an abomination."

[I] BRAVO for HIM!

[red300zx99]

Quote

Originally posted by McGuire

The term virgule comes from the Latin virgula, meaning basically "little penis." This typographical mark is employed, according to Webster's, as "a punctuation mark used to separate related items of information." For example: dates (2/15/2005) and measures (km/hr; lb/gal/hr) or as a substitute for "or" ("and/or") "per ," "by means of," or "with respect to." This mark is also known as a separatrix, or in plain English, a slash. Yes, it is also used as a division sign when math is depicted in single-line format, but that is not the specific usage here.

Maybe splitting hairs again, but wouldn't per imply division?

[McGuire]

Quote

Originally posted by red300zx99
Maybe splitting hairs again, but wouldn't per imply division?

Yes, that's certainly reasonable, especially if you are regarding the mark arithmetically rather than grammatically.

This entire subject is ironic to me as for years I was an absolute fanatical stickler for the Pedantic Commandments... for example lb.f ft. or lb.f ft./min. were the ONLY acceptable constructions as far as I was concerned, and I was a genuine pain in the ass anywhere an alternative version was used. It is not incredibly wise to be too much of a pain in the ass to the people who sign your checks, and as the years wore on I got better at choosing my battles. Today distinctions such as these don't mean nearly as much to me as they used to. I can use whatever version is in effect as long as it is not totally ridiculous. As djellison noted, the whole issue is fading with the use of extended SI anyway.

In the USA the biggest enthusiast periodical house is Primedia. Here is one of its conventions for describing torque in lb ft: 350 TQ. What do you think of that?

[McGuire]

Quote

Originally posted by AS110


Yep,Newton Metres is the officail term - all previous posts are rambles of yanks and fuddy duddy poms.

That's us in a nutshell.

[Spunout]

What follows now is a great debate whether it should be Newton Metre, newton metre or Newton metre

[gbaker]

Quote

Originally posted by Spunout
What follows now is a great debate whether it should be Newton Metre, newton metre or Newton metre

Neither. It's Newton meter.

[Wuzak]

Quote

Originally posted by gbaker


Neither. It's Newton meter.

Only in some regions.....

Here meter is a measuring device, and metre is a unit of measurement.....


Anyway, it has to be Newton metre, since Newton is a name of a person, and is afforded the respect of having the first letter capitalised, whilst metre isn't a person or place name, and thus doesn't have the capitalisation!

[desmo]

I can just see those seppo engineers accidently specing measuring devices on rotating bits.

[gbaker]

Quote

Originally posted by Wuzak
...since Newton is a name of a person, and is afforded the respect of having the first letter capitalised...

It's "capitalized".

[McGuire]

Like the newton (SI unit of force) many, many measures are named after famous men and women of science: the ampere, watt, volt, clausius, fermi, hertz, joule, curie, pascal, gauss, tesla, coulumb...Georg S. Ohm had two named after him, the ohm and the mho (the reciprocal of the ohm, now known as the siemens). There is even an einstein, a bohr and a galileo. Bet we can think of some more...

Units of measure are common nouns, so lower case is called for; however, when we refer to the historical figure his or her name is still capitalized, naturally, like any proper name. Indexes and scales etc. (Mach index, Beaufort, Celsius, Kelvin) are capitalized as well.

cheers, warmest regards, etc,
the copy editing dept.

[Wuzak]

Quote

Originally posted by gbaker


It's "capitalized".

I can never remember whether we Australians are supposed to use -ize or -ise.

[Wuzak]

Quote

Originally posted by McGuire
Like the newton (SI unit of force) many, many measures are named after famous men and women of science: the ampere, watt, volt, clausius, fermi, hertz, joule, curie, pascal, gauss, tesla, coulumb...Georg S. Ohm had two named after him, the ohm and the mho (the reciprocal of the ohm, now known as the siemens). There is even an einstein, a bohr and a galileo. Bet we can think of some more...

Units of measure are common nouns, so lower case is called for; however, when we refer to the historical figure his or her name is still capitalized, naturally, like any proper name. Indexes and scales etc. (Mach index, Beaufort, Celsius, Kelvin) are capitalized as well.

cheers, warmest regards, etc,
the copy editing dept.

Perhaps.....


Then again most (all?) units in the SI system named after people are capitalised (just realised that if McGuire, an American, uses -ize, I'm probably supposed to use -ise ) in their abbreviations.

The newton is always N, never n, the Pascal is Pa, the Hertz is Hz, Joule is J. Watt is W, Volt is V, the Ampere is A.

[Paul Ranson]

A newton must be 'N' since 'n' is 'nano'.

'~ize' is English English, although '~ise' is allowed. Words like 'advertise' are always 'ise' since the 'ise' isn't a suffix, and with 'compromise' for instance the suffix is 'mise'...

Paul

[ciaoduc1]

What does "lb" stand for anyway?

[McGuire]

Quote

Originally posted by ciaoduc1
What does "lb" stand for anyway?

pound, but the abbreviation used is "lb"...which is short for the Latin word libra (as in balance or scale) so it will not be confused with the British unit of currency.

[McGuire]

Quote

Originally posted by Wuzak


Perhaps.....


Then again most (all?) units in the SI system named after people are capitalised (just realised that if McGuire, an American, uses -ize, I'm probably supposed to use -ise ) in their abbreviations.

The newton is always N, never n, the Pascal is Pa, the Hertz is Hz, Joule is J. Watt is W, Volt is V, the Ampere is A.

Well, the newton is N, but the pascal is Pa, the hertz is Hz, joule is J, etc....the letter symbol is upper case, but the word is uncapitalized.

Not correcting you, just pointing out the apple-perfect forms. If I am ever reduced to correcting folks on their spelling and punctuation here, please shoot me.

[McGuire]

For those who REALLY enjoy tedious semantical arguments, may I present the actual term

"torque horsepower"

which is employed in the aircraft industry. In piston/propeller aircraft (note the proper use of the virgule eh) a propeller's efficiency is essentially determined by the ratio of thrust horsepower to torque horsepower. Thrust horsepower is that which acts in a plane 90 degrees perpendicular to the propeller's plane of rotation; that is, the actual net thrust produced by the propeller as expressed in horsepower, while torque horsepower is that which acts directly in the propeller's plane of rotation. Highest kudos to the first person who can correctly calculate an engine's torque horsepower from its net brake horsepower.

[desmo]

Couldn't prop efficiency be more simply expressed as a lift/drag ratio? (Hoping I got the virgule right here )

[McGuire]

Quote

Originally posted by McGuire


short for the Latin word libra (as in balance or scale) so it will not be confused with the British unit of currency.

You know, the blindfolded chick with the balance and the wardrobe malfunction?

[McGuire]

Quote

Originally posted by desmo
Couldn't prop efficiency be more simply expressed as a lift/drag ratio? (Hoping I got the virgule right here )

I believe there are probably an unlimited number of ways to express it more simply than any description which requires the term "torque horsepower."

[Greg Locock]

That's funny because ship designers face much the same problem, and they settled on 'shaft horsepower' for the same concept.

However just to confuse things the transmission efficiency of a propshaft (and potentially gearbox) is not 100%, so the shaft horsepower at the prop does not equal the shaft horsepower of the engine.

The Royal Navy, at least in the 1920s, used DHP to represent the shaft power at the screw, but my reference never explicitly states what the D stands for.

They also use thrust horsepower. THP = DHP * prop efficiency

[michaelab]

Quote

Originally posted by Paul Ranson
'~ize' is English English, although '~ise' is allowed. Words like 'advertise' are always 'ise' since the 'ise' isn't a suffix, and with 'compromise' for instance the suffix is 'mise'...

Hello Paul - didn't realise (or should that be realize ;) ) that you hung around these parts too. Never knew about the "suffix rule" in relation to ise/ize. Is that covered in Ms Truss's famous book?

Michael.

[Paul Ranson]

Michael,

I've not read her book but I do find etymology interesting, it's one of the few good reasons for encouraging correct spelling, IMO.

AFAIK if you're forming a verb from a noun then '~ize' or '~ise' are equivalent, depending on whether you like the French influence of if the 'z' key works on your keyboard. '~ize' isn't an Americanism (izm?) But if the noun has an 's' then the verb should keep it, 'television'->'televise' for example. And with something like 'surprise' the verb is the noun and the etymology is 'sur~'-'~prise' so a 'z' would make no sense.

Aren't dictionaries great?

Paul

[McGuire]

Quote

Originally posted by Greg Locock
That's funny because ship designers face much the same problem, and they settled on 'shaft horsepower' for the same concept.

However just to confuse things the transmission efficiency of a propshaft (and potentially gearbox) is not 100%, so the shaft horsepower at the prop does not equal the shaft horsepower of the engine.

The Royal Navy, at least in the 1920s, used DHP to represent the shaft power at the screw, but my reference never explicitly states what the D stands for.

They also use thrust horsepower. THP = DHP * prop efficiency

Quite right, marine shaft horsepower is the parallel to torque horsepower in aircraft piston engines, and closely related to ESHP (equivalent shaft horsepower) in turboprop engines...except that in the latter, exhaust thrust is included. At 375 mph, one pound of thrust equals one thrust hp.

In marine engines there are also ratings such as ahp, bhp, mhp... these are essentially load-hour or duty ratings. ahp means the engine is intended to operate continuously at 80 to 100% of its rated power...or at full rated power 80 to 100% of the time....but at the moment, I forget which exactly.

Interesting thing about the power ratings of piston aircraft engines...these figures are usually net at the prop flange, to include power losses through the (integral) gear reduction unit. However, the power@ rpm referenced is crankshaft speed rather than prop speed. So the TO power rating of the RR Merlin in one version is 1320 hp@3000 rpm with power as measured at the prop flange; however with .477 reduction the prop speed at that point is 1431 rpm. (Some aviation engines actually drove the prop off the camshaft.)

There are endless variations on the theme of horsepower ratings...in agricultural, mining, etc. equipment you will find drawbar horsepower and PTO horsepower, with the former typically around 80 to 85% of the latter.

[soubriquet]

The rule I was taught was that only two words need the z in "ize": size and capsize. All others may take an s.

My understanding is that the z is an archaic usage where British usage diverged from North America in the nineteenth century, and replaced the z with an s.

Cheers
S

[D-Type]

'My name is Duncan, I am a pedant'

lb/ft would be a spring constant, i.e. force applied (in pounds) divided by compression (in feet)

As others have said torque is the product so it has to be lb ft, lb-ft, lb.ft, ft lb, ft-lb or ft.lb

In full I would write foot-pound or newton-metre as the two words are connected so I would write ft-lb, but Nm as the SI rules say I should

~ize and ~ise is for the grammatarians to sortout, not for engineers

[Bob Riebe]

Quote

Originally posted by soubriquet
The rule I was taught was that only two words need the z in "ize": size and capsize. All others may take an s.

My understanding is that the z is an archaic usage where British usage diverged from North America in the nineteenth century, and replaced the z with an s.

Cheers
S

I am afraid you do not win the prize.

Bob

[soubriquet]

Matthew excelled only at mediocrity....... Nope never won a prize.

-----------------------------------------------------------------------------------
"When athletes nowadays talk of pressure they only reveal what they don't know of life. They've never had a Messerschmitt up their arse. That's pressure." Keith Miller.

[McGuire]

If we wished to get really pedantic about it, we could assert that 6.2832 ft lb = one lb ft.

[Dmitriy_Guller]

Quote

Originally posted by McGuire
If we wished to get really pedantic about it, we could assert that 6.2832 ft lb = one lb ft.

Only if we wished to be really wrong about it too.

[McGuire]

Quote

Originally posted by Dmitriy_Guller

Only if we wished to be really wrong about it too.

How so?

[Dmitriy_Guller]

Quote

Originally posted by McGuire


How so?

Well, in math, we try to avoid situations where we set one number equal to another. For example, we don't want to have 2 equal to 1, nor do we want to have 2 pi equal to 1, as in this case. You may still use those tricks, such as when you really want to prove something and conventional math doesn't get it done, but unfortunately this method of proof is not yet widely accepted in the mathematical community.

[McGuire]

Quote

Originally posted by Dmitriy_Guller

Well, in math, we try to avoid situations where we set one number equal to another. For example, we don't want to have 2 equal to 1, nor do we want to have 2 pi equal to 1, as in this case. You may still use those tricks, such as when you really want to prove something and conventional math doesn't get it done, but unfortunately this method of proof is not yet widely accepted in the mathematical community.

What about 2 pints = 1 quart? Is that right out then?

[Greg Locock]

Well, if you'll accept the tedious notation what you are really saying is

2*pint=1*quart

and the conversion ratio is pint/quart=2

in your other example you had

1*ft*lb=6.28*lb*ft

which is obviously nonsense, since lb/lb=1 and ft/ft=1

The reason why the 2pi gets involved is that you are converting from circular to linear. Physics works in radians, not revolutions.

[McGuire]

Quote

Originally posted by Greg Locock
Well, if you'll accept the tedious notation what you are really saying is

2*pint=1*quart

and the conversion ratio is pint/quart=2

in your other example you had

1*ft*lb=6.28*lb*ft

which is obviously nonsense, since lb/lb=1 and ft/ft=1

Nonsense indeed. Non sequitor: one foot may be one foot, and one pound may be one pound, but that doesn't make a pound a foot eh. You can assign them equal numerical values, but that doesn't mean they are equivalent properties. So one lb ft or one ft lb need not identify the same property or quantity, and there is nothing in mathematics, logic or English which says they must.

Traditionally, torque is regarded in lb ft while work is reported in ft lb, to distinguish the fact they are NOT the same properties. No use popping off at me, sport. All this was invented weeks, perhaps months before I was born.

[Paul Ranson]

Your assertion up thread would still be meaningless.

What happens if the torque rotates twice? Suddenly 12.56xx ft lb = one lb ft...

Paul

[Dmitriy_Guller]

Quote

Originally posted by McGuire


Nonsense indeed. Non sequitor: one foot may be one foot, and one pound may be one pound, but that doesn't make a pound a foot eh. You can assign them equal numerical values, but that doesn't mean they are equivalent properties. So one lb ft or one ft lb need not identify the same property or quantity, and there is nothing in mathematics, logic or English which says they must.

McGuire, can you please leave mathematics and logic to professionals? You're not having a lot of success applying those lately. As a professional in math and logic, I know for a fact that lb.ft and ft.lb do indeed identify the same quantity. Mathematically ft and lb are (nonzero) constants (regardless of what they physically represent), and so is their product, either lb.ft or ft.lb. The only difference between lb.ft and ft.lb is the convention, mathematically those quantities are identical, as multiplication is commutative for constants. You can perform the same operations on constants as you can on numbers. That's why your statement that "6.2832 ft lb = one lb ft" is completely nonsensical: after you divide by ft and lb (which you most certainly are allowed to do), you're left with 6.2832 = 1. McGuire, are you for real?

[Greg Locock]

McGuire - sorry, I thought you were genuinely trying to understand where the error was. Obviously not.