Wednesday, January 8, 2014

Calculating Torque Values For AN Fuel Fittings

Staying inside during the latest artic blast (which incidentally left a broken water line at DOG Aviation in its wake) and avoiding the nasty temperatures outside, a little time was spent calculating the torque settings that will be used on the AN fittings for the fuel lines. The standard torque range for the AN flared fittings used for 1/4" aluminum tubing is 50 to 65 inch pounds and the torque range for the 3/8" tubing is 110 to 130 inch pounds.

As a side note, if a builder does not have a quality torque wrench (can’t imagine that) a fish scale can be used to measure the torque. For example - if a wrench is one foot long, pulling the end of the wrench with a fish scale until the scale reads one pound applies a force of one foot pound to the item being tightened … which equates to 12 inch pounds.

Huston we have a problem … unfortunately, sockets can not be slipped over the fuel fittings to torque them. Solution, use a crowfoot attachment on the end of the torque wrench and calculate the proper torque setting for the torque wrench.
An 11/16" crowfoot wrench for 3/8" AN flared fittings attached onto the end of the torque wrench … this essentially makes the torque wrench longer.

Calculate proper torque? What’s the deal? Well, the crowfoot wrench essentially extends the length of the torque wrench … so unless compensated for, the resulting torque will be higher than the desired torque because the extended length creates more leverage which throws off the tool's calibration. The corection formula is simple ... once the extra length is determined.

The crowfoot wrench attachments were placed onto the torque wrench and the offset measurements were made from the center of the torque wrench drive to the center of the AN fittings. In my case, the 3/8" fittings require an 11/16" crowfoot wrench and the offset was measured at 1.05". The 1/4" fittings utilize a 9/16" crowfoot and the offset measured .9".
The 3/8" fittings require an 11/16" crowfoot and the offset here from center of the fitting to center of the torque wrench drive measures 1.05".

Armed with the offset measurements, the formula for calculating the torque setting when using an extender is: Torque wrench setting = DT x L / E + L
DT= desired torque applied to object requiring torquing
L = length measured from center of torque wrench drive to palm of hand (mine is 8")
E = extension offset length of crowfoot from center of drive to center of AN fitting
TWS= torque wrench setting - depending on the type of torque wrench you own either the value you set on an adjustable wrench’s scale or the value you are looking for on the needle reading.

Want my torque values to be in the center of the specified range … so my desired torque value for 1/4" AN fittings is 58 inch pounds and for 3/8" fittings it is 120 inch pounds. Using the above formula with the offsets obtained for the crows foot wrenches, the torque wrench will need to be set to 52 inch pounds for the 1/4" fittings and 106 inch pounds for the 3/8" fittings.

What if the crowfoot has to be at an angle … say, 45 degrees? No problem. I won’t go into the vector math (available on the Internet at lots of sources) just use the above formula with an additional step … multiply the extension length offset by .71 then add that result to the length of the torque wrench.
The formula is: TWS= DT x L / (E  x .71) + L
In my case, using the crowfoot at 45 degrees results in needing to set the torque wrench to 54 inch pounds for 1/4" AN fittings and 110 inch pounds for the 3/8" AN fittings.

Believe it or not, using the crows foot at 90 degrees is the same as not having an extension on the torque wrench so set the wrench to the desired value and twist away. To be factual, there is a very small offset value but that is likely to be within the margin of error for the torque wrench anyway.
No torque correction necessary if the crowfoot wrench is 90 degrees to the torque wrench.

I was planning to put all this to practice in the shop today but while walking over to the shop this afternoon, discovered water dripping from the back door threshold. An investigation reveled a broken water line at DOG Aviation’s host property caused by the record breaking cold temperatures. Needless to say, motivation for working in the shop was quickly doused, so to speak, by a need begin moping. Fortunately, the running water was caught fairly early (I think) and because of where the pipe is located, there appears to be no lasting damage … except for the water pipe. Bummer!