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Sanity Check -- AN6 Torque
For clarification, AN6's get torqued 160-190 inch pounds, right? 180 inch pounds just didn't feel that tight .. for an engine mount .. if someone could verify, just a sanity check :p :confused: :( :o
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Yep, that's where I got my numbers .. I just expected to have to really muscle these bolts .. guess I'm underestimating how strong these parts are.
Thanks! |
What you and many people are doing is over-estimating the amount of pre-load a bolt needs, in order to have that maximum strength available when really needed.
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my seat of the pants observation is that people tend to over torque small bolts and under torque large bolts when relying on feel rather than a torque wrench
erich |
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A separate but related fun-fact: Critical bolts subjected to cyclic loads (think about the studs holding the cylinders on) are often specified to be torqued to a value which maximizes their fatigue life. Torqued below or above the spec and their fatigue life will likely be lower. |
It's the leftover military mentality for me .. give me a hammer and a direction and by God it will get done .. so if the bolt still turns then keep on turnin' :D
Of course, I'm following the torque specs to the letter (just so it's clear to readers). I just expected to have to enlist the large torque wrench and some muscle and it's simply not the case. Good information, thanks all! |
Be careful of units. 400-500 in/lbs is about 40 ft/lbs. The average lug nut on a car is tightened to 80 ft/lbs and SUVs/pick up trucks somewhere in the 140 ft/lbs range. So, that 400-500 inch/lbs can be over torqued if you're not careful.
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What is torque . . . really ??
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And - 99% of the aircraft applications for fasteners are loaded for shear, not tension, that is why careful attention is paid to the hole sizes and why threads are not allowed in the joints. Other terrestrial things we are familiar with are primarily tension applications. That is where the bolt tension holds something and takes additional load, or the bolt tension clamps parts together and the tension/force prevents the parts from sliding. Engine parts are one example, all heavy duty machinery, off highway machines all are built to those standards. This allows hole sizes to be larger for assembly. Sliding causes fretting and won't last very long. Another difference is dry vs oiled threads for torquing. Dry threads result in a wide variation in actual tension of the bolt. Like plus/minus 20%. All (nearly?) the aviation fasteners are dry torqued. So NEVER build an engine without oiling the threads before torque. [ok so the engine mount frame is tension loaded to the firewall and appears to be an exception, but it isn't.] |
Another thing to worry about: most "clicker" torque wrenches have high inaccuracy near the ends of their range, even when freshly calibrated.
I find these little doodads to be superior in convenience, usability and accuracy: ACDelco ARM602-3 3/8? Digital Torque Adapter (3-59 ft-lbs) with Audible Alert Throw one of those and a set of adapters in your tool bag and you'll be able to retorque things accurately with just duct tape and a broom handle. :D |
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