Quote:
Originally Posted by Lovel
Thanks for posting, enjoyed it but oh dear where do I begin, torque to yield  |
Torque to yield is where a bolt is tightened beyond the yield point of the material.
When tightened to just below the yield point it is deformed elastically, so when removed it returns to it's original length and in most cases can be re-used.
When tightened past its yield point it is deformed plastically so when removed does not return to its original length and must not be re-used.
Head and other bolts need to supply a consistent, repeatable clamping force, this can be achieved by applying a specific torque to the bolt, but depending on if the bolt is lubricated and other factors the clamping force at a specific torque setting can vary, substantially in some cases, but amazingly around 85% of the torque needed is used to over come friction in the threads and at the bolt head to cyl head interface.
So for example, for a dry bolt torqued to 100Nm, 85Nm is used to overcome friction and only 15Nm applies the desired designed clamping force, however, lubricate the threads and the bolt / head interface and you reduce friction, on applying 100Nm torque in this situation the lubrication will reduce the torque needed to overcome friction to say 60Nm, the remaining 40Nm now applies the clamping force which will exceed the design spec.
Torque to yield with torque angle tightening overcomes many of these variables, at the initial small torque say 20Nm friction isn't a factor and it gives an equal start point to all the head bolts, then tighten by angular means.
So an engine manufacturer will say to a bolt manufacturer, I need a bolt of Y dimensions to produce a clamping force of X, the tightening instructions could say 15Nm, then turn 90 degrees x 3 for the clamp force, which will be consistent day in day out and friction variables due to lubrication, or lack of, variations in thread tolerances etc won't affect the clamp force or the integrity of the assembly, which could occur by simply using a torque wrench with the same variables as above.