Originally Posted by
Blknight.aus
galling is essentially the removal of metal when 2 pieces of metal are forced to slide along each other. its usually a microscopic effect and is generaly more noticable when a corse hard material (like say a steel file) is dragged across a softer metal (like say aluminium) if you try this you will notice that the alluminium gets stuck in the file, ITs the same deal with threads on a bolt. This is usually bought about by not cleaning out the threads of the nut/bolt/stud or when there is defects in the production of either. At the microscopic level its not a big deal when it becomes noticable it is. First off because where theres thread damage you have a potential fail point (stripping the threads entirely or jambing and snapping the fixture) but worse it has the potential to skew your torque setting.
nice segway to rundown torque.
Rundown torque is the amount of torque required to have a threaded fastner draw the components the fastner is supposed to hold into contact BEFORE you start torquing it up. In theory if you know the rundown value you add that to the final desired torque.
ITs a pain in the ass when you do have to deal with it and its part of the reason that the phrase "torque in sequence and in stages" came about. In day to day use if you use the correct tensioning pattern and do the bolts up a little bit at a time then run over them all with the torque wrench a second time you will usually eliminate rundown torque issues. HOWEVER. That assumes a perfect world scenario If you have a warped piece that needs to be pulled flat, if you have a dodgy thread, some crap in the hole, suffer galling on the way in have thermal difference issues, have a badly made nut or bolt, are using the wrong nut/bolt or any other of the plethora of things that can pooch the motion of the fixture (nut/bolt/stud) that can add some big numbers to the rundown torque.
Heres where it gets dangerous. If you try to measure the rundown torque and you find it and like your supposed to you add the rundown torque figure to the desired torque figure you can wind up over stressing the fixture or the component and it'll fail.
My advice.
If you encounter rundown torque while assembling a component (bolts that you cant wind in by hand) find out why and sort that problem out. IF theres a good reason for it (its a bloody big bolt, its a nyloc nut, its holding something heavy that wants to sit on the threads) mount the thing in place and put in some dummy bolts wind them down to your initial torquing value (remember in sequence in STAGES) then remove and replace them one by one with the correct fastener.
Where possable the use of guide dowels to locate the component first and a little brain one about the tensioning sequence will eliminate about 90% of the problems you find with rundown torque thats not caused by thread abuse.
