Torque calculation

[oldodge41]

 I had a couple bolts, not Mopar related, that were to torque to 320 ft-lb. Since I typically don't work with such high torque numbers I do not have a torque wrench that can be set that high. While researching torque wrenches that will accommodate that setting I came across some internet postings that recommend a simple math calculation and a lever of a specified length to achieve proper torque spec. Growing up we never had a torque wrench and just tightened things until they felt tight or as tight as you could if there was no feeling of fastener failure. Not the proper way to do it, I know, but it worked for years and years. 

The postings I found said to take the torque spec divided by your weight times 12 and that is how many inches of lever such as a pipe or breaker bar to use when applying your full weight to the fastener to be torqued. Some guys claimed this method was more accurate for them than a random uncalibrated torque wrench.

So for example:

320/225=1.42 X 12 = 17.04" 

 

Anyone ever used this method? Anyone ever used this method and then verified with a calibrated torque wrench?  

I will now sit back and let the bashing commence.

[Eneto-55]

I doubt if I'm so accurate anymore now, but back when I was overhauling my engine I had access to a torque wrench that had probably been recently calibrated, because Dad borrowed it from the mechanics at the dealership where he worked at the parts service desk.  I got to where I could use a ratchet or break over bar (depending on the target ft lbs), and be pretty much right on.  I think that we all develop a sense of how tight any given bolt should be, depending on its size, and that seems to be a pretty good judge of what the book will actually say.  (It assumes that the bolt was sized to its target tightness/load, as any decent engineer would do.)

[TodFitch]

I use basically that method when tightening the rear axle nut. Spec says 142 ft-lbs minimum and my torque wrench does not go that far. But I know what I weigh and can measure the length of the wrench. Torque is simply force (my weight in this case) times distance (length of my wrench’s handle).

[Ulu]

I have used this method while standing on a scale. By reading the weight on the scale versus my known weight I can do this without trying to use a short lever.

 

If I weigh 200 pounds, and the scale says 100, I am laying 100 lbs on the lever. With a neat 4’ lever I generate 400 ft-lb, and I am standing on both feet Instead of lifting myself off the ground. If I only want 200, I push down until the scale says 150. Then I’m pushing 50 pounds on a 4 foot lever.

 

Of course I used easy numbers In this example, so you can do your calculations in your head.

 

But say I need 213 ft-lb with a 4’ bar.

213/4=53.25  and 200-53.25 = 146.75

 

Push until the scale says 146.75 lbs

 

If it hits 146.8 you are only off 4x of the Total discrepancy of 0.05 so you will be over only 0.20 ft-lb over from 213  so it’s 213.2 ft-lb

 

For clarification, I only weigh 170, but I used 200 to make the calculations easier to follow.

 

Also my scale does not read in hundredths of a pound, so I can only read to the nearest 1/10.

 

If you wanna make the calculations easier there are things you can do. If you only weigh 197 pounds, just stuff 3 pounds of sockets in your coveralls and you will weigh 200.

 

If you only have a 42 inch bar,  it’s better to work in fractions of feet for these calculations so figure that your bar is 42/12= 3.5 feet.

Edited August 3, 2022 by Ulu

[Sniper]

That is the exact definition of ft/lb.

 

1 pound hanging exactly 1 foot from the centerpoint of the rachet square drive is 1 ft/lb.  100 pounds hanging at one foot is 100 ft/lbs.  50 pounds at 2 feet is still 100 ft/lbs.

 

The accuracy will depend on your measuring tape and your weigh scale.

 

A torque wrench just makes it faster and easier.  I bought my SIL a Brownline digital torque wrench for his birthday.  There is a promo code for it somewhere, got it for $100.  Not made in China.  Not made in USA either, but I can't afford one of those.  It beeps, i like clicker types myself. 

 

 

[Ulu]

I don’t have anything that goes over 250 foot pounds. But I have a quarter inch, 3/8 inch, & 1/2” torque wrenches.

 

I have three-quarter inch drive sockets and bars, but I will put my jack handle tube on there to get what I need.

 

And while I’m thinking about it, the handle from an old worn out floor jack can make a great cheater bar. Stronger than iron pipe of the same weight.