Another Rupnow 1 by 1

Home Model Engine Machinist Forum

Help Support Home Model Engine Machinist Forum:

Joined
Apr 2, 2020
Messages
2,579
Reaction score
785
Location
Sunderland , UK.
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'll 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.
K2
 
Last edited:

CFLBob

Well-Known Member
HMEM Supporting Member
Joined
Feb 10, 2018
Messages
952
Reaction score
289
Location
Central Florida
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.
K2

I hope you don't think I was debating, ridiculing or doing anything less than studying what you said.

You and I don't know each other, and to make matters worse, there's that quote about Americans and Brits being two people divided by a common language so that anything we say may be understood in an unintended way - especially typed text with no voice inflections.

For future reference, or for anyone like to come across this, there's only one person on this planet I make fun of and that's me.

When I build something to someone else's plans, I strive to live by that famous MIL SPEC, MIL-TFP-41C "Make It Like The F***ing Plans For Once."
 
Joined
Apr 2, 2020
Messages
2,579
Reaction score
785
Location
Sunderland , UK.
Hi Bob, I'm sorry, I wasn't taking offence at what you are doing. I appreciate you have (previously) stated that you are basically an Electronics Engineer, but enjoying making models (exemplary work I might add! - I am learning from you as much as anything!). Therefore you are asking for comment, feedback and advice about what you are doing. As such I am offering my "mechanical Engineering" expertise - for what it is worth.(Only 50 years in this...). But I am not the expert on everything, although I have worked with more than a few and learned some of the questions and pitfalls of Engineering life. I apologise as seeming to be a "tactless old what's-it sometimes", but (Based on a lifetime of "I told you so" experiences) sometimes over-react to many people who just say "I think it is OK" without offering their reasoning. Often they are correct, but just as often they didn't look at the problem I (or others) have queried, and then make errors of judgment as a result. 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. In am not a metallurgist. But - to put my message simply:
Con-rod Big-end bolts are usually designed by the original designer to be strong enough, and stressed to be fatigue resistant for the application to have some long durability.
If you use a different material, size or whatever, then it only seems reasonable (to me, anyway). that some simple correlation is conducted to see if the proposed alternative is adequate to the original design.
Whether or not you are the original designer, you have made the rods for bolts with 13% more CSA for tensile stress - which in itself reduces the stress accordingly, but also have selected a bolt with what I think is a lower strength than what I GUESS the original designer may have chosen? So if the resulting stress on the bolt is relatively higher (in relevant terms) compared to the strength of the material, then you are at risk of reducing the lifetime before failure (due to fatigue).
N.B. Off the web: 18-8 stainless is typically 0.2% proof stress >205 MPa. Here's a table of strength of parts (not as complete as to include your parts):
1652076089696.png

but it shows that "Stainless" is only about 1/4 of the strength of "Alloy" steel screws. As the specification you have is so vague (#5-40 SHCS), I would have certainly gone for the "best available" material, rather something unknown, for this application.
It is "not hard" to do some checks: (I, or others, can do them for you or check what you have done if you wish). All one needs is the actual details of the original con-rod big-end bolt design versus your selected alternative. The internet has all the "text book stuff" to make this relatively easy to do.
I just found this...
Connecting Rod Bolts Calculation with VDI 2230 standards (excelcalcs.com)
But I haven't downloaded it yet (pointless unless I have all the data to input anyway!).
Just for some "Odd" background: having worked on engines since the 1960s, through to the 1990s, I have only ever experienced the "nuisance" of manufacturers' instructions to change the con-rod bolts for NEW parts every time the big-end is assembled. (The exception being any engine that had not run, and had not been fully torqued). There is a table in the link that shows the various methods ("Qualities") of tightening, and the "factor" to use in the calculation spreadsheet. This may give you some appreciation of how highly critical the industry consider the design and installation of the big-end bolt.
Recent experience (within this website) has had 2 threads discussing strength of crankshafts (because of failures at VERY early life), so I am advising "what I know" so maybe you will not experience a running failure within minutes of first start-up - or ever.
Sorry, if my English is a bit blunt: I reiterate that "good Engineering" is necessary on some components, and I am trying to advise you to "check, check, check". It will help avert any failure.
My best wishes for a successful model, I do think you are presenting some excellent work.
K2
 
Joined
Apr 2, 2020
Messages
2,579
Reaction score
785
Location
Sunderland , UK.
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:
  • a #5 screw in Alloy steel has a yield strength of 1290psi.
  • a #6 screw in Stainless has a yield strength of 363psi.
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.
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,
K2
 

minh-thanh

Well-Known Member
Joined
Dec 29, 2016
Messages
1,218
Reaction score
747
Location
Viet Nam
K2 !

sometimes over-react to many people who just say "I think it is OK" without offering their reasoning.

Yes . that's me: a guy who can only say: " It's fine "
Maybe I'm not too bothered about complicated calculations or actually I don't know or have forgotten most of them..

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.
It's the design of Brian Rupnow .
 

CFLBob

Well-Known Member
HMEM Supporting Member
Joined
Feb 10, 2018
Messages
952
Reaction score
289
Location
Central Florida
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:
  • a #5 screw in Alloy steel has a yield strength of 1290psi.
  • a #6 screw in Stainless has a yield strength of 363psi.
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.
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,
K2

I was dimly aware that stainless has a lower yield strength than "alloy" steel, although I'd guess in both cases the question is "which alloy?" Likewise, I'm aware that hex bolts come in different strengths and quality standards, shown by marks on the head of the bolt. If I had gone to buy screws, I probably would have bought "alloy black finished steel SHCS" like these:
not that I know what that alloy is either. After spending a month on one part, I wanted to be done so I used what I had. I can pick up those screws at any time.

The engineering question is whether 3.5x better yield strength matters. It's such a big difference that I'd guess it would, but I have no idea what the loads are. Maybe a much smaller screw, like #0-80 would be strong enough. Since we come from very different fields, I can't offer an example for comparison, but I think any designer has had a manager ask/tell them to cut cost and that always comes down to "how strong does it have to be?" What's the cost/benefit ratio?

I live in an area where stainless is practically a necessity - I've been told our oceanfront gets the saltiest air in North America. You can walk around houses and know which side gets the sea breezes just by noting how corroded the metal around windows is. That's why around half of my hardware bin is stainless.

Strangely, I actually wrote this reply almost nine hours ago and forgot to hit the "Post Reply" button as life interrupted, which it sometimes does, with a larger than normal bunch of chores and distractions.
 

Brian Rupnow

Design Engineer
Project of the Month Winner
Joined
May 23, 2008
Messages
14,081
Reaction score
7,258
Location
Barrie, Ontario, Canada
Bob--All I have ever used for rod bolts are "over the counter" black oxide finished socket head capscrews from a local company. I don't really know what grade of steel they are, but I don't think it is anything special.---Brian
 
Joined
Apr 2, 2020
Messages
2,579
Reaction score
785
Location
Sunderland , UK.
Bob, Brian, I don't know what is "standard" from US shops, but in the UK we can get "unknown grade" steel bolts over the counter, - which are probably 80 ton steel, not the 100 or 120 ton alloy steel of "reputable specified parts". but who knows what comes from the more Eastern countries? possibly even Mild steel (40 ton?). But the "Stainless" in one manufacturer's table is quoted as much worse (akin to 30 ton steel?), so I should change to a "better" bolt if I were making this engine. Failures are such a nuisance!. Is the crankcase closed and oily? - Which would tend to keep Mr. Rusty at bay...
K2
 
Joined
Apr 2, 2020
Messages
2,579
Reaction score
785
Location
Sunderland , UK.
Hi Bob, That's what I was guessing, so "proper" steel bolts will be better than Stainless, I think?
In Electronic terms, if the design showed 3A resistors or diodes, you may try 1A, but at the risk that something would blow. You would tell me to use the proper parts..... I reckon?
Vheers,
K2
 

CFLBob

Well-Known Member
HMEM Supporting Member
Joined
Feb 10, 2018
Messages
952
Reaction score
289
Location
Central Florida
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

An answer on Quora says, "12.9 grade steel means that a component manufactured form this grade of steel has 12X100 N/mm^2 tensile strength and 9 is a multiplier means 90% of 1200 = 1080 N/mm^2 is the yield strength of the grade. "

McMaster-Carr says that translates to 170,000 PSI. "Alloy Steel Thread-Locking Socket Head Screws. With a tensile strength of 170, 000 psi, these alloy steel screws are among the strongest we carry. They are stronger than Grade 8 steel screws and are nearly two and a half times stronger than stainless steel thread-locking screws. They have a thread locker to prevent loosening from vibration. "
 
Joined
Apr 2, 2020
Messages
2,579
Reaction score
785
Location
Sunderland , UK.
Hi Bob,
I'm glad you checked. If I had been wrong, well, it's happened before.
But there is nothing like doing the check yourself, "to be sure to be sure" - as the Irish are oft to quote.
If Brian is using grade 12.9 bolts, then that's the top grade (Alloy steel) on the table I sent in post #224.
So:
  • Brian's 12.9 grade #5 x 40 bolts have a yield strength of 1345lbs (UNRF thread?).
  • Your #6 x 32 (UNRC?) Stainless bolts (Grade "3.5"?) only manage 363 lbs Yield strength. = 27% of the strength of Brian's design.
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...
K2
 

minh-thanh

Well-Known Member
Joined
Dec 29, 2016
Messages
1,218
Reaction score
747
Location
Viet Nam
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...
K2


Sorry, I don't have any research to prove it.
Perhaps relying on experience is not a good way either...
Maybe I'm wrong, but with that engine, if it runs for a long time maybe one or more parts will be damaged or worn out...before those bolts break.
Still inference based on experience and nothing to prove.
Maybe I should limit the my comments based on my little experience.
Teach someone: I don't think I have enough experience, knowledge, expertise, .... to be able to teach someone .
 

werowance

Well-Known Member
HMEM Supporting Member
Joined
Sep 2, 2011
Messages
1,283
Reaction score
335
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.
 

CFLBob

Well-Known Member
HMEM Supporting Member
Joined
Feb 10, 2018
Messages
952
Reaction score
289
Location
Central Florida
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 think that's an excellent point. All metals are treated with various combinations of heating physical treatment. That's one reason for the image we all have in our minds of the guy pounding on red hot metal with a hammer. "Hammer forged" is harder. Unlike most guys in my line of work, I took a class in materials science as an undergrad. A vivid demonstration I'll never forget was in cold working. The lab instructor put a sample of some metal into a tester to test hardness. This one dropped a known weight metal ball from an exact height onto the test piece to measure the height it would bounce back to.

The vivid part was he repeated that drop, with nothing changed about the test piece. It bounced higher the second time - I mean so much obviously higher there was no need to look at photographs to compare them - showing that first drop work hardened the metal.

I think I recall the only metal that doesn't tend to get more brittle or break more easily when it gets hardened is cartridge brass.

I wouldn't be at all surprised if grade 5 stretched a bit more before snapping than grade 8.
 

ajoeiam

Well-Known Member
Joined
Aug 14, 2020
Messages
597
Reaction score
173
Location
blank (like some others I've noticed)
snip

I wouldn't be at all surprised if grade 5 stretched a bit more before snapping than grade 8.

Exactly!
Then Gr 10 and 12
Then look at L9 hardware - - - tough like Gr 10 but stretches more (think for use of equipment frames).

The whys and whens can consume a lot of time.
I do tend to do the monkey see monkey do routine (looking at what has been specd and then doing similar.
 
Top