M6 8.8 bolt fatigue failures (HELP!!!!) 4

[trikeflyer]

I am trying to improve a bolted connection (aluminum plate bolted to threaded aluminum case) which currently uses three M6 8.8 allen head cap screws. I have had all three bolts fail in fatigue, presumably due to vibration. I suspect that the problem may be that the bearing strength of the aluminum is lower than the tensile strength of the bolt, resulting in creep of the aluminum and loss of pre-load. Unfortunately, I don't have the option of using a different size of bolt or more of them.

I am looking for suggestions. I was considering replacing the 8.8 fasteners with 10.9 hex head cap crews and a stainless steel fender washer. I realize that the 10.9 fastener will require a higher torque to reach a working pre-load. The purpose of the washer was to spread this pre-load over a greater area in order not to exceed the bearing strength of the aluminum. I'm no expert (Obviously!), so I'd really appreciate an expert opinion.

Thanks for suggestions and direction!.

Dave C.

 

[hydtools]

That is a really ugly idea. Bondo comes to mind.
Better yet call the maker to verify what that fit should be. Best, compared to the Bondo idea, would be to have a machine shop machine the plate for a close slip fit over the dowelling diameter of the cranckcase.
Since you have another plate, try it for fit, too.

Ted

 

[hydtools]

Another idea would be to remove material from the face of the dowelling diameter on the crankcase. Since it would not be a precision contact surface, you could file it away leaving some dowelling length for the location and shear function while allowing full contact of the bolt ears.

Ted

 

[trikeflyer]

Ted, funny you should mention Bondo. I had a report from one pilot who had the top bolt location fail twice. He filled it with JB Weld and clamped it with a 12.9 grade screw and hasn't had a problem since. Unfortunately in spite of my prior claim I do find that a bit too ugly to try.

I'm a little nervous about machining the counter-bore/dowling for a slip fit as the manufacturer indicated to me that the interference fit was a design feature. Unfortunately he bought the design from another company, and I'm not sure how much of the lore has been lost, or how much design expertise he has. Certainly someone went to a lot of effort to incorporate the feature in the design.

Instead of taking on this part of the design myself, I'm thinking that I will build the motor up with the other plate and the 10.9 fasteners. Rather than using Loctite I will check the torque at short intervals to see if the plate will settle into position.

I will also monitor it for fretting wear. If I find any, I will tear it down, inspect it, and see where the contact surfaces are and attempt to make small changes and repeat the process. WD-40 sprayed along the clamped joint seems to be a really good fretting wear detector, if it starts seeping out black, there's fretting.

 

[desertfox]

Hi trikeflyer

Reading these last posts it seems to me that the interference fit is important and if that intereference fit is no longer there for whatever reason then just bolting it together is not the answer because now those bolts are taking all the load which in the original design they weren't.
Also my calculation method is only valid if there is no interference fit.
I'll post later bit short on time now but I have a bit more to say.

desertfox

 

[hydtools]

Looking closer at the last pictures there appears to be contact of the outer edge of the dowelling diameter on the cranckcase and a fillet radius in the corner of the matching diameter on the plate. Relieving this contact may be enough. File the edge on the cranckcase to a camfer to clear the radius in the plate.

I still think the ears need to be brought tight together.

Ted

 

[trikeflyer]

Chamfering the edge of the crankcase seems like a good idea. I also think that your earlier idea of removing some material from the face of the crankcase dowling is good. Looking at the plate it appears there may be some contact in this area, and that would only prevent the ears from coming together and full engagement of the dowling into the counter-bore.

On the other battle fronts: I seem to have isolated and corrected the carburation issue, and I have ordered a dynamic propeller balancer (Balance Master) to help eliminate vibration from any slight propeller imbalance.

I think this one is just about nailed down. It's just a matter of building it, running it and watching it. Thanks again everyone for all the detailed work and analysis on this. Your help has been unbelievably valuable.

I've been a bit remiss in awarding stars, here. Ted and Mike get one each for really delving into the issues beyond my original question and getting me to understand how this plate *really* connects to the motor. And of course corypad gets two stars for so clearly and definitively answering my original fastener question and follow-ups.

Dave C.

 

[1oldwelder]

I realize this has nothing to do with the original question, but have you thought of applying a layer of machinist dye, (dykem blue) to the contact surfaces prior to assembly. Then during the next dis assembly, the wear points will be easily seen? Just a thought.

 

[trikeflyer]

Thanks; yes, I think that might have even been suggested before. It's just a question of how to best approach the analysis: If I use the WD-40 test to look for fretting on the assembly then that would defeat the dyekem. I think that I may need to allow the assembly to settle and re-torque a couple of times to really see where it settles out, and I'd really like to monitor it with the WD-40 during this process. In any case I think that before I am done I will have tried a couple of different approaches, and using dykem should likely be in one or more of them.

Dave C.