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TORQUE WRENCHES
I use a cheap-o 'Draper' bendy pointer one and have found it to be acceptably accurate to plus or minus 1 lb ft, throughout its entire range of 10 - 150 lb ft. My confidence comes from having taken it to work (Oxford Airport) and tested it on their 'Acu-Torque' machine.
However expensive a torque wrench you buy, if you don't get it tested you might as well use it as a hammer. The type that adjusts by winding-in the handle is notoriously 'dodgy' if not tested. Your local garage "might" have a tester, but any local airport with a maintenance facility will have a proper one, which itself should have an up-to date test certificate label.
I've noticed on several peoples build sites that they have stripped the threads on bolts while torque-ing them up. If you put any substance that has lubricating properties on the threads, such as Copper Slip or 'Loctite', you MUST use a lower torque value (multiply original torque value by 0.7).
Check-out these websites for more info:
http://www.norbar.com/pdf_files/torquevalueguide.pdf
http://www.norbar.com/faq.htm
http://www.norbar.com/twec/index.html
USING A TORQUE WRENCH
The importance of correct torque application cannot be overemphasised. Under-torque can result in unnecessary wear of nuts and bolts, as well as the parts they secure. Over-torque can cause failure of a bolt or nut from overstressing the threaded areas. Uneven or additional loads that are applied to the assembly may result in wear or premature failure. The following are a few simple, but important procedures, that should be followed to ensure that correct torque is applied.
- Calibrate the torque wrench before it is used for the first time, following purchase. Thereafter, at least once a year, or immediately after it has been abused or dropped, to ensure continued accuracy.
- Be sure the bolt and nut threads are clean and dry, unless otherwise specified by the manufacturer.
- Run the nut down to near contact with the washer or bearing surface and check the friction drag torque required to turn the nut. Whenever possible, apply the torque to the nut and not the bolt. This will reduce rotation of the bolt in the hole and reduce wear.
- Add the friction drag torque to the desired torque. This is referred to as “final torque,” which should register on the indicator or setting for a snap-over type torque wrench.
- Apply a smooth even pull when applying torque pressure, by holding the torque wrench at the centre of the handle and smoothly apply the required force at right angles to the wrench. If chattering or a jerking motion occurs during final torque, back off the nut and re-torque.
- NOTE: Many applications of bolts in aircraft/engines require stretch checks prior to reuse. This requirement is due primarily to bolt stretching caused by over-torqueing.
- When installing a castle nut, start alignment with the cotter pin hole at the minimum recommended torque plus friction drag torque.
- NOTE: Do not exceed the maximum torque plus the friction drag. If the hole and nut castellation do not align, change washer or nut and try again. Exceeding the maximum recommended torque is not recommended.
- When torque is applied to bolt heads or cap-screws, apply the recommended torque plus friction drag torque.
- If special adapters are used which will change the effective length of the torque wrench, the final torque indication or wrench setting must be adjusted accordingly - see web site below.
http://www.norbar.com/twec/index.html
Extracted from FAA AC43.13
Tony Cummings