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MG MGA - lb/ft and ft/lb
|What is the difference in ft/lb of torque and lb/ft of torque? I noticed in Road and Track they show engine torque as lb/ft, but when tightening a bolt, we use ft/lbs????|
I'm probably the only one that doesn't know the answer to this, so be kind!
|There is no difference. Though it is often written with the slash which would imply division, torque is actually force times moment arm so it doesn't matter if it is pounds times feet, or feet times pounds.|
|It sure does sound weard to hear it on TV when they pound feet, rather than foot pounds of torque...|
|I found an article that says I am wrong. It would explain why they say pound-feet when talking about engine power. You learn something every day. http://findarticles.com/p/articles/mi_qa3726/is_200409/ai_n9458018|
|Seems to me it goes this way - it's been a long time, but I still have to reconstruct it everytime it comes up, or I write it:|
Torque is a force applied at a distance from a pivot axis, so it is force @ distance, written as lbs-ft, oz-inch, or whatever. This would apply to any torque, whether engines or bolts.
The use of the slash, by convention taken as "per" or "divided by", is incorrect.
It is also incorrect to use the term ft-lbs for torque; but, this is correctly used as a unit of work done, as in you move a 40lb weight a distance of 100ft. Even though in English it's awkward to say "100 feet you move the 40lb", the convention of distance-load has been adopted precisely to differentiate work done from torque created.
As I recall, this convention was officially adopted maybe in the 60s, because of confusion in the usage, especially across countries.
Torque = force @ distance, written F-D
Work = distance a load is moved, written D-L
So, your wife says "get out of my way you lazy ass (work = ft-lb), or I will get my knickers in a twist and twist off your nubbies (both in oz-in, unless you really blew it)" Hence the expression, "if I drag home another car (mile-ton), she'll get really torqued (lazy ass applying force to locked door due to twisted knickers = torque everywhere, but figure out your own units for this case!)"
|It bothers my ears to hear something that sounds backwards, but if it is correct, I will have to get my ears re-worked, so it doesn't bother me as much.|
Meanwhile, I am downstairs going, working on my MGA green.
|I probably knew all that at one time, but it has been 40 years since I had any physics courses. I guess the problem is that torque wrenches predate the convention. Mechanics usually measure torque in ft-lbs and measure work in dollars. But in England, I guess they measure work in pounds, so I can see where the confusion would come in.|
Over here my kids measure it all in cms and grammes now! It's only us oldies doing it the old fashioned way.
|As one old enough to have been taught in Imperial, cgs, mks and SI at the same time (which probably accounts for my terminal confusion), can I point out it should be lbp (pound-force)-ft. In the same way it is Newton-metres and not Kg-metres.|
|Torque in England was always lb-ft, US was ft-lb, hence the convention.|
Malcolm is correct, but it would be lbf-ft.
This thread was discussed on 18/05/2007
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