[Vwdiesel] FW: retorque.
jhsg at sasktel.net
Wed Aug 17 02:37:50 EDT 2005
What is so special about STRETCH bolts ? ---NOTHING ---all head-bolts
stretch ---BUT the
factory found that --when a bolt hits solid resistance ? The stretch is
pitch on thread. and they posted the optimum.
The clamping force is according to thread pitch, so finer threads need less
torque to get the same amount of clamping force. (finer pitch, is a lower
"ramp" to drive up, so less force is needed to attain the same bolt clamping
force) I guess I'm saying that bolt stretch is sort of related to thread
pitch in a round about way, but not directly related really to the actual
mechanism at work in a torque to yeild fastener.
The stretch bolts are just that, but of an alloy designed to stretch more
consistently than a regular bolt.
You may notice the reduced diameter after the threads, That is not
coincidental. Measure it sometime. It's pretty consistent across the
various bolts for a reason- the elastic yield point of the alloy at that
diameter gives the desired amount of clamping force the bolt is designed to
make. How much it is "torqued" or turned isn't nearly as critical as a
standard bolt, because it is designed to essentially constructively fail
over a small margin of elongation.- if you don't get the full 1/4 turn or go
a little over, no biggie, because after the second torquing, you are passing
beyond the elastic limit of the bolt, and it is mostly stretching. The
second torquing is just taking up the slack that may have come as a result
of gasket squish etc. and the yeild point of the bolt keeps the clamping
force farly consistent. This isn't coincidental- you can hammer the motor
together with some pretty accelerated methods at the factory, and still have
a stretchy torque to yield fastener provide the correct head gasket squish
(torque if you will), making assembly faster, easier, and more repeatable
with big nasty quick power tools. That's why you see them in most
applications that use automated assembly these days.
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