Quote:
Originally Posted by dradernh
Thank you very much - I think that was very clear.
If you know of any bolts on our cars that are clamped in this fashion and can offer examples, whether accurate for those applications or not, I think that might further our understanding of this subject.
|
The rear caliper bolts are a good example, they are M12x1.5 32mm long. About 16mm of the bolt seems to be exposed between the head and thread engagement when in use. The 60 degree turn of the bolt in the 50Nm +60 degree tightening spec will stretch the bolt 0.25mm, or about 1.5% of the exposed shank (this is the strain value). 8.8 spec bolts will yield at 800MPa of stress at about 1% strain so from a bolt calculator the required torque will be in the range of 120 to 200Nm, depending on thread and bolt head friction / lubrication.
Another example is the M12 aluminium bolts attaching the gearbox to the transmission. A typical high strength aluminium alloy will yield at about 400 MPa and 2% strain. The tightening spec on these bolts is 25Nm +130 degrees. I think about 20mm of the shank is exposed, so when tightened to 130 degrees beyond jointing torque a stretch of about 2.7% is induced in the exposed shank, again into the bolt plastic / stretch range. The torque required to do that in this case is in the range of 60 to 100Nm.
Stretch bolts are now used by BMW in many areas, they provide accurate clamping without having to deal with the wide range torque required due to thread and friction variability.