Originally Posted by
halfspeed
The 3T ARX Team stem has a maximum torque specification of 5 Nm for all bolts. I know this because it is stamped on my stem.
Is that maximum to protect the stem, the bolts, the faceplate and steerer clamp, or the bars and steerer? If the latter, has 3T conducted extensive testing to assure that every steerer and bar can handle that clamping force or is there an industry standard tolerance? If the bolts, then do you know that 5 Nm is the appropriate torque for your bike's bars and steerer?
Is that maximum an actual hard limit or is there some upper tolerance built in? If so, is that tolerance wider than the manufacturing tolerances of the types of affordable torque wrenches commonly sold for working on bicycle parts? Does it account for calibration drift in those tools over six months? A year? Five years?
If I decide to install different bars, am I doing it right if I tighten each face plate bolt to 5 Nm starting in the upper left corner and proceding sequentially?
OK, first know that I do not work for 3T or any other bicycle or bicycle-related company.
When you design a bolted connection, I think 2 things are important: for the situation that we're discussing, it would be generating enough clamping force and leaving the bolts in tension when you're done. I don't know how they would determine the clamping force. You can calculate a theoretical value, but you would need to verify that by physical testing as parts have tolerances, surface finishes can vary a bit, etc.
Once you know what clamping force you need, how many screws will you use to get the clamping force? Often you can use a small number of screws, but your clamping force would not be evenly distributed. The screw manufacturers can tell you the strength of a given screw size (and head design) and what torque you need to develop realize that full strength.
The key to this is knowing the required clamping force. Deciding how to configure the bolted connection comes aafter that.
Your first defense against a bolted connection failure is properly torqued bolts. Note that I'm talking about usage at this point and not assembly. The bolts are tensioned or preloaded, depending upon what terms you like, at assembly. It is important to slightly lubricate the bolts before assembly. If you don't, the torque wrench (or human touch) won't give you a true reading. The next line of defense is a threadlocking material like LOCTITE. LOCTITE is not a glue. Threads are not perfectly round or have perfect peaks and valleys. Therefore they don't mesh 100%. Threadlocking compounds fill the voids and prevent rotation.
So, the torque value that you get from the screw manufacture does have a safety factor. I don't know offhand how much. The max stress that a bolt should ever see (and be reused) is at the point of yielding. Going beyond the Yield Stress resulting in permanent deformation. Regular carbon steels yield at arould 35,000psi and breaks at about 120,000psi. There are various compositions of titanium alloys. The yield stresses range from about 25,000psi up to about 120,000psi. Usually you get higher strength if you pay more. The money can go into any number of things: improved composition control, improved annealing or additional heat treating processes, etc.
Regarding assembling a stem and handlebars, I would put all 4 bolts in and lightly tighten in and X pattern. The next step would be to repeat the X pattern and torque to the appropriate setting. If this is what you meant by "sequentially", then we are in agreement.
So, what happened to the bolts in the origional situation? I would suspect either poor quality or the bolts OR misformed or mismatched threads that worked to bind up the bolt with the tapped hole.