Quote:
Originally Posted by blktoptrvl
When you torque reused bolts what target do you use? Do you still got to 42 ft lbs + 90 degree angle?
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Disclaimer - don't try this at home.
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No, you can't do that. That procedure, if repeated enough times, will almost certainly break the bolt.
The Y = "yield" in TTY suggests that when installed the first time, if done properly with careful surface preparation, light oil on the threads and washer (or whatever exactly is specified for this application), the stress on the bolt will exceed the yield stress, resulting in plastic deformation (yield, permanent stretch).
My Bentley, page 310-2, says the spec is 56 Nm (41 ft-lb) + 90*. In addition to saying the fasteners should be new, it says the tightening should be done with the car on the ground and loaded, bouncing the suspension a few times before final tightening. So for those purists who want to follow specs to the letter, there it is. I can't imagine a professional mechanic getting on the ground to do this, just like (I hear) they don't actually use new bolts, and (I hear) dealers don't even stock them.
My Bentley, page 020-4, referencing DIN 267, give a general torque value for M10 10.9 as 66 Nm. They intend that as a default, safe value for bolts with otherwise unspecified torque values. So if you were to tighten just by torque value (and no angle spec), using 66 Nm should be possible without plastically deforming the bolt. And a little more should be possible on re-use, due to the work hardening.
The plastic deformation "work hardens" the bolt, meaning that its elastic range has now been extended to include the plastic strain.
So when you apply 56 Nm, and then add 90 degrees, at some point in that 90 degrees, the bolt "should" begin plastic deformation. You might feel this as a fairly sudden change where pulling further on the wrench is not actually getting much more difficult. Once done with the 90 degrees, the bolt (and maybe nut too, since these are not TTY bolts, they are just regular bolts used in a TTY application) will have plastically deformed, imparting a permanent stretch to the bolt (and maybe deforming threads too).
Ideally, when re-using these, you should theoretically be able to tighten it back up to this exact bolt stretch multiple times with no problem at all. And all this would be in the elastic region, with no further plastic deformation. The bolt would be performing exactly as intended.
So that is the goal = tighten it up with the exact same stretch, or "strain" to use the proper term. Not easy to do. And certainly less easy than telling other people to just use new ones.
Following the same 56 Nm + 90* will definitely not do this, since the 90* will be pushing further into the plastic region each time it is done, until finally the ultimate tensile stress is reached and the bolt will snap.
If you knew the torque applied the first time, when you did the 56 Nm + 90*, perhaps 70 Nm, you could apply that pretty safely. Maybe back off a little.
If you feel plastic deformation (when angle increases without a linear increase in torque required), you've gone too far, so stop. So this is what I do, torque to the torque value, but ready to feel for plastic deformation and stop.
If you want to follow the instructions, but modify them, as
crystalworks suggested, with 75* rather than 90*, I'll say something like that makes sense, but I don't know if the answer is 75* or 30*. This would be TTA rather than TTY, and should be the most reliable way to do it, if you knew the correct angle.
To answer that experimentally, you could start with a new bolt, install with the spec of 56 Nm + 90*, and measure the actual torque when you reach 90*. (Ideally there would be some time spent here to let things settle out). Then back off to a low torque value, torque to 56 Nm, and measure the angle required to get back to the final torque you just measured. In theory, with no change in surface conditions, that angle should be repeatable. But it's not perfectly repeatable, so at this point, it is easier/safer/more profitable to say replace them.
It would be great if someone some day would do this experiment with a new bolt.
Also, if any of the purists who use these things only once would like to send me their old throw-aways, I'd be happy to test and post any findings.