BMW torque tips, wrench, nuts, bolts, tightening, methods
This should be the last word on torque
I was a fastener Procurement Agent with Boeing – yup, I ran the Nut Desk. I sat on a panel in a 7 year study – yup, 7 years – which studied not only the run-on torque, but also the force needed to break the nut loose from the bolt. One of the large nuts on the 6 bolts holding the fin on a 767 backed off, which is nick-named a ‘spinner’ by aircraft maintenance folks. It was more common than desired to find spinners during one of the major maintenance checks on planes. Often enough to initiate a study. Most of the major manufacturers and specialty houses participated in the testing.
Yes, lube is important. The majority of fasteners used on aircraft in critical torque installations have a dry lube on them. There has been a lot of research on the type and amount to achieve consistent run-on torque to spec. The installation guns are pre-set for each size/type of fastener – not only for the torque – but the speed the gun turns, which generates heat. Some coatings are metals like silver, zinc and cadmium (now replaced). There are many, many bolts/nuts which have a cycle limit spec’d – or non reusable.
Here are some things discovered to affect both Run-on Torque and Break-away Torque:
Temperature: of the parts, the gun, the bolts, the nuts, the room (at time of installation).
Speed of run-on to torque.
Number of prior cycles.
Amount and type of lube – and how applied (sprayed, dipped, chemical, electrical)
Length and size of bolt and nut.
Type of metal or alloy for both bolt and nut.
Pitch of threads for both nut and bolt.
Type of thread: J-thread, cold rolled thread, hot rolled thread, cut thread (speed of cutting and metallurgy)
It was found the exact same spec bolt or nut made by two different manufactures had different results.
It was found that when all of the above were exactly the same for multiple tests – it had different results.
It was found that if a test jig that measured the torque was vertical or horizontal it got different results.
It was found a full moon, low tide, Monday, height and weight of person testing, and leap years got different results.
There was such a huge amount of test data to be correlated Boeing dedicated one of their Super Cray-2 computers to analyze the variables. It ran for 6 weeks.
The results were – there were no definitive results. It was determined there were so many variables that affected the outcome they could only agree to specified ‘norms’ and create an industry standard accordingly. The rest was up to God.
A 747 is about 6 million parts flying in close formation, mostly held together by fasteners. You should learn to pray.