when using an extension on a torque wrench what is the right formula for figuring out the torque to set it to?

i have seen two different ones and the both come up with different answers.

heres ones:
M1 = M2 x L1 / L2

Where:

M1 is the torque setting of the wrench.

M2 is the actual torque applied to the nut

L1 is the normal length of the wrench

L2 is the extended length of the wrench



and the other:

ACTUAL = INDICATED x (WRENCH LENGTH + EXT. LENGTH) / WRENCH LENGTH

i have also heard people sometime dont even use the formula and just set the wrench to whatever the need to torque the bolt to.

 

yes the formula is only needed when your adding length to the the wrench (crows foot, handle extension)not extensions. The trick i always use is to put the crows foot on at a 90 degree angle witch does not require the formula because no length is added.

i cant find my book right now but this one lookes correct.
http://www.belknaptools.com/extcalc.asp


waht are you torquing?

 

12 volt is correct, the torque setting does not change when using an extension.

 

Well I've allways added 5% per 3" extention to factor in the flex of the steel, but it's a rough guestimation that my family and shop teacher spoke of. Scribe a perfectly straight line onto one of thoose cheapo extentions that everybody buys mechanics for gifts and you'll get the exaggerated effect. Use a nice straight edge and a engraving tool... If you exert that much force and can still notice the steel twisting, you have to feel there's an effect on the torque output to the fastener...

~Nate

 

From the above thread it seems there are two issues being discussed:

A. Adaptors that by their nature change the effective length of the torque wrench (to increase applied torque or when using crowsfoot/flare adaptors) Conversion formula needed here (except when adaptor is perpendicular (90 deg) to the axis of the torque wrench) When possible mount crowfoot or flare adaptors at 90 deg. More on this below.

2. Extensions in the classic sense (extends in length on axis with the drive of the torque wrench; can be thought of as making the fastener taller. This extension will always "lose" torque (spring twist) but this is a non issue. No conversion formula needed.

The simple formula for the first case above assumes the length increase is directly in line with the axis of the handle; if there is any angle (like to get around an obstacle) it gets more complicated. If the angle is 90 deg no conversoin is required - 30 pound/ft indicated would equal 30 pound/ft at the business end of the adaptor no matter how long it is. Neat, huh?

 

The Formula is for cheater pipes ect giving extra length , using a small 2/3" extension to the socket will barely affect the torque , Using a 6" extra will vary the torque more as the length can cause added flex Easy way to prove it as torque with the extension then retorque without

 

I must respectfully disagree (bad venge)

A pipe cheater, while multiplying the amount of force a user is able to apply to the wrench, will not change the indicated torque. It will however change the point of applied force from the handle pivot (on a beam type t/wrench) which will under-torque the fastener slightly. On a clicker (without a handle pivot) a cheater pipe will not affect the indicated torque at all; it will still click at the correct torque. It will just require less force at the end of the longer handle to reach the same torque at the fastener.

You are sort of correct in saying that a 2-3" adaptor (such as a crowfoot) would not affect the reading significantly but I would add that is true only if the torque wrench is very long in relation to the adaptor. For example if the length of the wrench (from point of applied force to the center of the drive) was 9" (weenie for sure) and the adaptor added 3" then an indicated torque of 75 pound/in would give an actual 100 pound/in at the fastener (a significant difference 25%). If the wrench was 20" and you used a 2" adaptor and were aiming for 100 pounds/ft you would stop at 90.9 (still a fairly significant 10% error)

Use adaptor = use the formula

 

What I'm talking about is a 2/3" extension between the wrench and the socket , There is goin to be a small amount of twisting/torsional stress on the extension as well as between the joints of the wrench and the extension ,and the extension and the socket ,as well as the bolt/socket... Using the longer extension the amount of twisting will increase as will the amount of twisting increase with the amount of torque applied ...
Also remember that these will vary depending on the size 1/4" ,3/8" , 1/2" or 3/4" of the drive