On Sun, 21 Oct 2018 21:02:44 +0200, Emanuel Berg
wrote:
I take it allen screws (or "socket head cap
screws") and hex bolts have the same standard
torque values?
Sorta. The maximum torque is normally a function of the threads and
material(s). However, for smaller hex head bolts, the head can be
broken off before the threads are stripped. I wouldn't worry about
that too much because for allen head cap screws, the head is stronger
than the threads and for the sizes normally found on a bicycle, the
hex bolt heads are also stronger than the theads.
However, the maximum torque is for the bolt, not the material the bolt
is threaded into. For example, if you were to torque a bolt to the
maximum torque allowed, in an aluminum block, it would strip the
threads out of the aluminum long before the bolt threads fail
(depending on how many threads are in the aluminum block). Look up
the recommended bolt torque in the brake and shifter data sheet.
"How much Torque does it take to Break a Bicycle Bolt?"
https://www.youtube.com/watch?v=vPFtEj4JZvA (7:36)
What maximum torque means is how much torque can be applied before the
threads strip. What recommended torque means is how much torque can
be applied for 0.2% bolt elongation.
If I read the charts [1] correctly, if you find
an allen screw on your bike that takes a 4mm
allen key, then its an M5, and assuming "clean,
dry threads" with 8.8 "relative strength
marking", the maximum torque is 6Nm?
Ummm... try this instead:
https://www.amesweb.info/Screws/Metric_Bolt_Grades_Strength.aspx
A 5mm type 8.8 bolt is rated at 7.6Nm maximum breaking torque. That
means exactly what it says. Torque it to 7.6Nm and it will break.
I rode my bike some 40km yesterday, and noticed
a couple of things. One was that the left
shifter and hand brake was just a bit (~1mm)
too high, while the right one, while at the
same level, was just right.
Later, I was done fixing that, I checked with
the torque wrench, and both screws fell just
a little short of the prescribed 6 and 10Nm for
M5 and M6, respectively.
Offhand, if the bolt was loose AFTER applying maximum torque, you've
probably stripped the aluminum threads in the brake and shifter body.
Hint: Stay away from the maximum torque.
[1] http://user.it.uu.se/~embe8573/allen-torque/
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558