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> 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
