Josh,
Did you happen to get a look at the broken bolt looking across the face
of the break? Good possibility it was a fatigue failure which occurs at
stresses well below the breaking strength, in which case you can
sometimes see the "beach marks" as the failure works its way across the
section.
Neil Gallagher
Weatherly 35-1
Glen Cove, NY
On 2/24/2021 8:47 PM, Josh Muckley via CnC-List wrote:
Dennis,
Your number of 7500 lbs confirms my number of 7200 lbs. That just
seems incredibly high considering the OEM decided that 2 bolts would
be required. It also seems ridiculous that one of them broke. I am
at a complete loss as to how that would have happened.
Josh
On Tue, Feb 23, 2021, 12:00 Dennis C. via CnC-List
<cnc-list@cnc-list.com <mailto:cnc-list@cnc-list.com>> wrote:
I'm guessing the thread degradation doesn't have much effect on
the strength as long as the shaft is not damaged. Having said
that, the shear strength of a M12 10.9 hardness bolt is about 7500
lbs.
Did I win? What's my prize? Can it be shipped or do I need to
pick it up/sail it home?
Dennis C.
Touche' 35-1 #83
Mandeville, LA
On Tue, Feb 23, 2021 at 9:42 AM Josh Muckley via CnC-List
<cnc-list@cnc-list.com <mailto:cnc-list@cnc-list.com>> wrote:
Engineering discussion for the group.
Here's the background. For about 9 years I have known about
having 1 of the 2 bolts broken on the forward starboard engine
mount. These bolts hold a piece of 1/2 inch plate steel which
was bent at a 90° angle to the vertical side of the engine.
This creates a horizontal shelf. The shelf then rests directly
on the vibration dampening mount which is then bolted to the
boat. Periodically, I have made attempts to extract the bolt
but only a few days ago was I finally successful. The bolt
was a M12x1.75. I don't know what hardness the OEM called
for. Years ago when I discovered the broken bolt I was quick
to replace the remaining good one with one having a hardness
of 10.9. For a fresh start, I now intend to replace both of
them with new and again a hardness of 10.9. In removing the
old bolt, my drill hole was off center and ended up cutting
some of the threads. I approximate not more than 1/4 of the
thread integrity was compromised.
https://drive.google.com/folderview?id=15GCOH3qKIWXl1lmjsbZqsf-ECGCrl1fN
Here comes the question(s). Just for kicks, I decide to
attempt to figure out what the sheer strength of the engine
mounting bolts are individually. I came up with an answer but
would like hear what the group comes up with.
I also now have a bunch of other questions tumbling around
like: How much weight would it take to break one? How much
margin to failure did I have while operating for the last 9
years? What could have possibly caused this bolt to break
prior to my ownership? Theories and ponderings are welcome.
With 1/4 of the threads degraded on the newly available bolt
hole, I plan to only torque the bolt to ~50% of max (90
ft-lbs) which would be ~45 ft-lbs. I'll also make sure that
the lock washer is fully collapsed but I'm confident that that
will happen well before final torque is reached. In sheer, I
believe this bolt is providing as much or more strength simply
by being there than anything to do with its torque. All
though, I have heard the engineering theory that (when built
appropriately) the bolts apply the pressure which creates the
friction to prevent the pieces from moving in sheer. Under
this argument the 2 pieces of metal don't actually ride on the
bolt shank specifically, and the surface area where the 2
pieces touch is of particular importance. Looking at the
engine castings, it would appear that maximizing surface area
contact was not considered. As such, I can only fall back on
my previous thought that the bolt shank is in fact carrying
the load in sheer and torque is more or less irrelevant except
for the purpose of keeping the bolt tight. I'm also aware of
various thread repair options but in the awkward location I am
fearful that I would cause more damage than good by attempting
any of them. Particularly if the value added by the second
bolt truly is its presence not its torque.
All thoughts and insights are encouraged.
All the best,
Josh Muckley
S/V Sea Hawk
1989 C&C 37+
Solomons, MD
Thanks to all of the subscribers that contributed to the list
to help with the costs involved. If you want to show your
support to the list - use PayPal to send contribution --
https://www.paypal.me/stumurray Thanks - Stu
--
Dennis C.
Touche' 35-1 #83
Mandeville, LA
Thanks to all of the subscribers that contributed to the list to
help with the costs involved. If you want to show your support to
the list - use PayPal to send contribution --
https://www.paypal.me/stumurray Thanks - Stu
Thanks to all of the subscribers that contributed to the list to help with the
costs involved. If you want to show your support to the list - use PayPal to
send contribution -- https://www.paypal.me/stumurray Thanks - Stu
Thanks to all of the subscribers that contributed to the list to help with the
costs involved. If you want to show your support to the list - use PayPal to
send contribution -- https://www.paypal.me/stumurray Thanks - Stu
