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Stud grip length vs joint strength
3

Stud grip length vs joint strength

Stud grip length vs joint strength

(OP)
Guys, I have a very basic question: in a bolted joint, does using a threaded stud beyond a certain length gain me anything in strength? Specifically, I have an aluminum racing engine block with threaded stud holes 2.7" deep for 1/2-13 SAE studs holding the aluminum heads. ARP doesn't make such a stud off-the-shelf, and I wonder if all that threaded length is necessary. The block mfgr is out of business so can't ask him directly. Thanks-

RE: Stud grip length vs joint strength

You get to a point where the holding strength of the thread is greater than the strength of the stud. A little extra length to cater for inaccuracies in thread fit or fatigue of the aluminium can't hurt, but much more is no benefit.

Others here might have a formula for calculating that, but the difficulty will be knowing the strength of the grade of aluminium used.

Regards
Pat
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RE: Stud grip length vs joint strength

2
The rule of thumb for aluminum is that the engaged thread length should be 2.5-3X the thread diameter. So a 10mm thread diameter would typically have 25-30mm of engagement in the block. For steel it's 1-1.5X and iron is typically 1.5-2X.

More threads won't buy you anything because the block threads will shear before the threads on a high tensile stud due to the fact the first ten threads carry most of the tensile load.

RE: Stud grip length vs joint strength

(OP)
Thanks, guys. The cast aluminum grade is unknown- done in CA in the mid '80s. I'm not enthusiastic about helicoiling 20 big. perfectly good holes just on a whim. I'll go with ARP's stud length recommendation.  

RE: Stud grip length vs joint strength

"More threads won't buy you anything because the block threads will shear before the threads on a high tensile stud due to the fact the first ten threads carry most of the tensile load."  I.e., a domino type failure; the first ten threads fail, followed by the 11th, etc.

RE: Stud grip length vs joint strength

Jsgarage,

For an optimum thread engagement length with a steel stud into an aluminum substrate, you should have enough threads engaged so that the pull-out strength of the stud into the aluminum is equal to (or greater) than the tensile strength of the stud.  The pull-out strength of the stud is based on the root shear strength of the aluminum it's threaded into.  The root shear area is basically the dimension across the base of the thread crest section times the total helical length of the engaged threads.  The simple shear strength is just then load/area.  

If you wish to be more accurate in your analysis you should also apply the appropriate knock-down factors (Kt) for stress concentrations.

For your example, assuming your block is cast from A356-t6 aluminum, then it has a shear strength of about 17ksi.  If your tapped holes are 1/2-13 UNC x 2.7 inch, then the aluminum threads would have a simple pull-out strength of just over 32,000 lbs if all the threads are loaded.  And if you assume that your 1/2-13 UNC ARP stud has a tensile strength of 180ksi, then its simple tensile strength (P/A) is somewhere around 23,000 lbf.  If you want to equalize the strengths of your stud and head, then you can safely reduce your thread engagement to about 1.94 inches.

Hope that helps.
Terry

RE: Stud grip length vs joint strength

As ever, I tend to agree with TB.  It is notable that if you add "a bit to be on the safe side" to his 1.94 inches it is not a lot different to the original 2.7 inches. I would be inclined to stick with the original length - quite possibly the makers of the block knew what they were doing.
 On a different point - I know from personal experience that it is very difficult to have a stud made that is anywhere near as strong, tough and unbreakable as an ARP (or some other good brand) stud would be.
  Maybe you can find a good-quality bolt that will do the job. I did this - the really extra good bolts I used were $7 each when a normal very good bolt was about 50 cents at the time.  

RE: Stud grip length vs joint strength

The problem with theoretical and actual thread load length however is that the load is not spread uniformly across the threads as I mentions above. As a result the highest loaded threads strip and then the load transfers to the next threads and they also strip. In a perfect world if the load was spread relatively uniformly across the thread length, then you could always get increasing strength with longer threads but in reality this does not happen.

In regards to bolts vs. studs there are other issues besides clamp force. Bolts impart much more block distortion in typical thin wall castings compared to studs which generate primarily a tensile loading. In soft alloys like aluminum, bolts can easily gall and damage the threads in addition to causing inaccurate tightening torques and variations in clamp loads.

RE: Stud grip length vs joint strength

TrackRat,

I agree with your points about load distribution within the threads.  I was just trying to simplify things.

Generally, when you exceed a thread engagement beyond about 5P, the thread pitch tolerance accumulation (especially with a UNC thread) pretty much guarantees that you will not have equal load sharing in the thread engagement.  With a 1/2-13 UNC thread engaged over 1.94 inches, that means around 25 thread pitches.  Not only will those 25 threads not load share, you'd be lucky to get the stud to thread all the way down 1.94 inches without seizing up, due to the pitch errors.

As a rule of thumb, you should never have a thread engagement beyond about 1.5 or 2.0D.  That would mean about 13P max for this application.  If you need to equalize the root shear and tensile strengths of the base material and stud, then you should use a larger diameter thread on the end of the stud going into the aluminum.

Regards,
Terry  

RE: Stud grip length vs joint strength

Terry-

I think we're on the same page here. I was trying to clarify that increasing the thread length beyond 2.5-3X in aluminum was not going to increase grip strength because the load is not evenly distributed. In this particular case going beyond 3X 1/2", i.e. 1.5" of thread, does nothing to increase thread strength. I'm surprised to see designers who use 2.7" deep threads when this does not make for a better design and can in fact cause block distortion or issues in other areas.

RE: Stud grip length vs joint strength

Rather than try to calculate and speculate on the thread length of studs - what is the stud diameter/thread length on some well-known alloy-block engines like the Chev LS series V-8's and the Rover V-8's etc. Probably a good indication of what is needed.    

RE: Stud grip length vs joint strength

There's no guessing required. That was the point in the above discussion. The info. supplied has been tested and confirmed. That's why I find it odd that some designers would use thread lenths much longer than usable. Doing so increase costs, machine time, tooling wear, etc. It makes me wonder if they bothered to research the subject or if they just drew the deepest thread they could fit in the space assuming more is better? <LOL>

RE: Stud grip length vs joint strength

TR - My intention was that it would be interesting to compare calculated and actual thread lengths used by manufacturers.
  And yes, - I am a great believer in "(a bit) more is better".  Proper engineering calculation and design and then being slightly generous in material thickness, bolt diam., thread length etc.
 Unless absolute minimum lightness etc. is required (as in F1 for example) "a bit more" probably is better.
 I recently saw on TV the front wheels and suspension come off an F1 car when the drivers braked - I bet the designer thought to himself "I should have used a bit of extra material on those suspension arms". I am sure the calculations said they would be strong enough.   

RE: Stud grip length vs joint strength

A simple tensile tests tells you what the grip strength is for various thread depths and materials. It's very easy to conduct. That's why I was surprised that some designers haven't bothered to investigate this information. The 2.5-3X depth in aluminum gives you an extra safety margin, so going beyond that is a waste of time and energy. They are probably operating on the principle that the thread load is uniformly distributed and thus more thread length = better, but of course this is not true.

RE: Stud grip length vs joint strength

This al sounds a bit variable to me.

The claim that only so many threads contribute because of accuracy of the threads must vary according to the accuracy of various thread forming processes and also with the stress strain and elongation at yield points for the materials and for temperature changes and various co efficients of thermal expansion and rate of application of load and of fatigue strength and on any cold flow effects and even on the extra bit of load sharing that comes from gap filling compounds like Loctite.

Regards
Pat
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RE: Stud grip length vs joint strength

YvesLLewelyn,

If you read my post of June 21st, I gave a design approach for determining the optimum number of engaged threads with a steel stud and aluminum substrate.  While my suggested analytical approach was somewhat simplified (for the sake of brevity), it certainly was not rote speculation.  And even a simplified analytical approach is certainly better than copying some other engine's fastener installation.  Whose loading conditions, fatigue life, materials, factors of safety, tolerances, stress concentration factors, etc. you likely know nothing about.

I hope that Jsgarage considers the suggestions presented to him and takes the time to gain a greater understanding of the engineering principles involved in threaded fastener joints.  That's the purpose of this website.

Regards,
Terry

RE: Stud grip length vs joint strength

That's precisely why a simple tensile test or two eliminates any speculation regarding the variables. What's the saying: "one test is more useful than a 1000 opinions"?

RE: Stud grip length vs joint strength

A test is only as good as the variable it controls and how well it replicates in use conditions.

I think that is a valid reason to allow some safety factor.

That factor in my opinion should be determined by the extent and importance of the variables.

I do agree, do tests, but also consider what extra to allow.

Regards
Pat
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RE: Stud grip length vs joint strength

Of course. That's the reason why you use 2.5-3X thread depth in aluminum so that you have an extra margin of safety beyond the minimum thread depth possible for maimum strength of the base material.

RE: Stud grip length vs joint strength

I don't disagree with any of the calculated values or the logic behind them.
 
However - the original problem was a practical one. Jsgarage asked if he needed to use the original length 2.7" (68mm) threaded length.  If the original engine used a thread length of 68mm - I personally would continue to use 68mm - you could obtain suitable studs somewhere.
 If I had an engine rebuilt by someone who told me that he used 40mm long studs instead of the original 68mm because the "calculations" said they would be OK - I don't think I would be very happy.  The original maker of the engine may have discovered (for whatever reason) that they needed to use 68mm of thread length.
 There would also be the awkward problem with a shorter stud that they couldn't be securely tightened against the bottom of their holes - meaning they would be a problem to tighten/untighten easily.    

RE: Stud grip length vs joint strength

Yves-

The original question was would he gain any thread strength by using a longer thread length than necessary. He asked this because no one currently offers the stud he needs with 2.7" of thread. He's asking can he use a shorter stud/thread that may be available. The answer is obviously yes as discussed above. Loctite can be used if he desires to fix the stud in the block at the correct height, though it's not necessary but acceptable in most applications.

My concern would be determining the correct tensioning of the new studs with many unknowns regarding the original stud/joint clamp force, old stud tensile and elasticity, the block material and tensile, thermal growth, existing thread fatigue, etc.

RE: Stud grip length vs joint strength

For all practical purposes, nearly 3 inches of thread length to install a stud is laughable at best.  I have to agree with TrackRat, 2.5-3X is considered normal in cast Al.  I have spent more than a fair piece of time in the machine shop over the past 28 years, and if you need all of that thread length to keep the thing from coming unglued, there are deeper design problems.  

Helicoils are definitely worth considering if you think that you'll ever be servicing that block again in the near future, especially since you mentioned that the OEM is no longer in business.

Just my 2 cents.

RE: Stud grip length vs joint strength

http://members.fortunecity.com/alicat1/ccfwa-c.jpg
http://studiotimecapsule.com/yahoo_site_admin/assets/images/P1010015.251172249_std.JPG

This coventry climax engine tends to crack its aluminum block, with cracks originating at the end of the stud, in the long stroke version when raced with mods requiring high revs.

I don't have info on stud length, but the bosses seem to end a  little beyond the main bearing boss, at which point the casting also get pretty thin, pretty quick. I envisioned it as a double or triple shot of stress concentration at a point without the benefit of bolt preload's innoculation against fatigue.

RE: Stud grip length vs joint strength

I would agree with all of the good advice above, but presented with an unknown aluminum block to head fastening system, there are a few more constrains then just the tensile strength of the threads.

First, determine if the head gaskets you intend to use are expecting torque to yield fasteners (TTY) as in the case of most metal laminate steel (MLS) gaskets. MLS gaskets typically use TTY fasteners in AL to AL engines, as the thermal expansion is better handled when tension is normalized throughout the operating thermal cycle. TTY tension will be different than on a non-TTY fastener.

If you're using shouldered studs (pre-tensioned), they will tension differently than bolts or non-shouldered studs (pre-compressed), as the threads will be pre- tensioned between the shoulder and the threads in the block before the heads are torqued on. This more evenly spreads the tension across the threads, loading the bottom threads more effectively.

Extending the threads deeper in the hole then is necessary for adequate tension, may also help reduce block distortion, as the goal is to vector the head stud tension to the crank bed-plate when ever possible.

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