Just a point of Engineering, as this site is about free comment on that subject: the stress raiser of the root of the thread is at least 3. To machine this away (with a large radius at the ends) so you have a smooth, unblemished surface of the bolt through the solid part of the rod will eliminate the stress raiser in this region. All you need is to take of 0.001 in below the root diameter of the thread to make the bolts at least 3 times stronger. The tool radius should be at least the depth of thread. That is Engineering. Your choice if you want to just make something or do "the Engineering".
I am sure you'lloyds be torquing the nuts core try on final assembly, so the bolts will be appropriately pre-stressed, so they can take the fatigue of the oscillating loads within the big-end. Just help them resist fatigue by using the best steel, and design practice.
It is the old adage, "spoil the ship for a ha'porth o' tar"...
Just my opinion....
I'll not debate it further.
sometimes over-react to many people who just say "I think it is OK" without offering their reasoning.
It's the design of Brian Rupnow .I am just trying to be sure that wherever you got your design from, you are sure that the bolt design is adequate for your application.
Hi again Bob,
Sorry to be a nuisance, but I just had to re-check everything I could on this one... (Worrying "Does my brain in" sometimes!) I.E. the meaning of "D & T FOR #5-40 SHCS". I reckon "D & T" means "Drill and Tap"?:
And I just looked up "SHCS" - (I had guessed it referred to "High Strength Carbon Steel" - but guessing is not right, so I checked.).
It actually means "Socket Head Cap Screw" - which I GUESS would likely mean "regular Alloy steel" - Not "Stainless".
You may note from the table that for:
I.E. from which I conclude your stainless screws are likely to fail, as they are only about 1/4 of the strength of the Specified parts. (here I must assume that the fatigue strength of the alloy steel is not exceeding 75% of the yield strength, but that the stress loading from the running engine may be as high as 70% of the yield strength (= 903psi: because I don't know any better! And is a lot more than the yield strength of the larger Stainless screw) - But that is only my interpretation of the problem, and justification for being a nuisance to you. And it does depend on the pre-load on the bolts, quality of parallelism of the end flat surfaces (Flippin' excellent I think from what I can see of your work!), washers under nuts, etc... including the "actual stress oscillation loading" of the bolts from the con-rod when running.
- a #5 screw in Alloy steel has a yield strength of 1290psi.
- a #6 screw in Stainless has a yield strength of 363psi.
Also, I GUESS that the fatigue resistance of Alloy steel is better that stainless, compounded with the larger root diameter of the "40" thread compared to a UNC... or whatever, so the specified bolts may be stronger than my guess? And this is then compromised by the smaller root diameter of the "40" thread versus the UNC of the table... so too much "guess work" here!
Make of this what you will... this is only my advice with the best of intentions.
(These problems awaken brain stuff I haven't used in years... but please tell me to "go away" if I am too interfering.).
Best wishes for a successful model,
I just spoke with the people I buy my socket head capscrews from. A standard hex head bolt is grade 2 or grade 5. A socket head capscrew is grade 12.9.---Brian
Minh Thanh: I am not sure of your comment saying that you think the Stainless bolts are OK? (Post #226). Do you have some better information than I have been studying? (I am not perfect, nor always right, so if you can teach us something then that is a good thing).
Sorry if I made my feedback a bit too complicated? - I do prattle-on a bit...
sorry to barge in on your build Bob, but on the discussion of tensile strength of bolts and shcs's. i guess limit what im asking to just bolts for discussion, but i recall being told once when i was installing a hitch on my truck DO NOT USE grade 8 bolts or higher to use grade (i believe it was) 5 bolts. they described the reasoning for this was something to the effect that a grade 5 would stretch before sheering or snapping vs a grade 8 would just shear off. even though grade 8 was considered harder and stronger. so in my mind i related that to knife making where a rock hard blade will shatter vs a blade that has been tempered or drawn to a light straw and maybe even down to a light purple on the spine to soften up the non cutting part so it wouldnt crack. so with all that said (and im hoping steamchick and or anyone else) can throw in some teaching wisdom on when to consider using a softer bolt vs a harder one. maybe a "how do you know when you should use a grade 5 vs grade 8" aside from the obvious answer of "because the plans said so" but what if you dont have plans or the original bolt to go by and have to do a best guess.
I wouldn't be at all surprised if grade 5 stretched a bit more before snapping than grade 8.
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