Connecting rod shape 3

[jriordan]

I cannot find any analysis of a I-shaped connecting rod vs. a H-shaped. The H-shaped rods seemed to be used in the more expensive applications, but I cannot figure out why it should be preferable.

 

[EnglishMuffin]

patprimmer: So we both seem to agree that the guy on that site has his definitions back to front. Since you and he are both Aussies, it appears unlikely that this is a "definition inversion" peculiar to your continent, unless the terms "left" and "right" somehow become interchanged in the southern hemisphere. His error might cast doubt on some of his other assertions, such as the statement that most rod failures occur at the bolts, although that one seems to me likely to be true.

 

[Dman93]

H beam vs I beam, I have been building racing engines of all type for some years now. What I have seen as the cause of rod deflection are 1: elongation ,occurs when the revs are sustained at high numbers. this is evidenced by carefull claying of the piston and dry assembly then cranking engine over ,dissassemble carefully and checking pisto to head clearance. documenting this clearance then running on a dyno dissassembly and claying again. with forged I beams there is almost allways a narrowing of the clearance gap.
Theory for this is that the pistons on the exhaust stroke are flung to the end of their stroke unrestrained and then jerked back putting large strain on the rod .Solution stronger rods that are lighter and lighter mass at the end of the rod (pistons)
2: compression failures almost always due to detonation or Pre ignition caused by too much compression and too much spark lead. Or just plain too much fuel load IE nitro or turbo boost too high.

 

[EnglishMuffin]

I would have thought that the greatest tensile stresses would occur on the intake stroke, since only in that case are the inertia loads not counteracted by gas forces, although there wouldn't be much in it between the intake and exhaust strokes.

 

[MaxRaceSoftware]

I would have thought that the greatest tensile stresses would occur on the intake stroke
-----------------------------------

on a normally-aspirated engine ,
if the headers are working correctly,

you should/want a negative pressure wave to
be centered on the overlap period

so the rod should be in more tension
during overlap period
in normally-aspirated engines with headers working



Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3

 

[EnglishMuffin]

Yeah - I guess I'm wrong - wasn't thinking about overlap - the maximum tensile stress occurs at TDC between the intake and exhaust strokes. So I suppose if a tensile failure were to occur in a normally aspirated or supercharged engine, it would probably be just before the intake stroke began. What is the situation on a turbocharged engine?

 

[patprimmer]

EnglishMuffin

The only two orientational differences I have seen between Aus and USA are:-

1) Here the sun travels from right to left, whereas in the USA it travels from left to right. Singapore is quite confusing in this regard though.

2) You guys drive on the wrong side of the road, as you are probably very well aware, being of British origin.

Oh yes, I almost forgot, we also have the Gay Mardigra, but then again, you guys have San Francisco.

I guess turbos are a bit easier on rods re tensile as there is no effective exhaust scavanging, and there is always some residual back pressure in the exhaust port, as well as boost pressure in the inlet, but TDC overlap is still probably the highest tensile load.

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[EnglishMuffin]

When I was in Australia, the orientation of the moon crescent looked different as well.

 

[Azmio]

Gentlemen,
I just want to add about the effect of buckling due to the combustion pressure when comparing between I and H conrod.
If we simulate both the I and H for buckling, we will notice that the H conrod is stronger due to more cross section area at the critical points.
Overall, it is still difficult to predict which one is better. For turbocharged application, we have to consider a lot about the buckling effect. On the other hand, for NA application like the F1 where it revs up to 19000rpm, definitely centrifugal effect and repeated compression and elongation effect will have to be considered. Not to mention about critical fastener attaching both the big and small ends.
In addition to that, the effect of bore and stroke will also have to be considered. The Honda F1 turbocharged engine has longer conrod if compared to the new F1 conrod. This will yield different result entirely.

AO

 

[EnglishMuffin]

If flange buckling of the H rod ever turnesd out to truly be a problem - and it is not clear that it ever would - one might be able to control it somewhat by leaving one or two small linking portions between the webs at intervals. Of course, that would probably entail machining with a ball nose end mill rather than a form cutter, which is a negative.

 

[MaxRaceSoftware]

for a little more controversy ??

why do majority of Hi-Perf rods use bolts rather than studs ?

especially in aluminum rods where you would think they would be beneficial in protecting aluminum threads
by using studs rather than bolts .

Answer=> ? .... the stud has a too much of a
"lever-effect" and transmits distortion to the journal hole
(any motion of the stud distorts the journal hole ?)

any other opinions / insight ??


Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3

 

[enginesrus]

Bolts studs?

Most big end bolts are through bolts with nuts on the end.
Why wouldn't Al rods be counter bored and use the same? Much stronger than threading the Al. It would be a bit difficult to aquire any stretch threading into Al, especially the minimal amount on the big end.

 

[EnglishMuffin]

Through bolts sounds better to me. But if there is a difference between non-through bolts and studs, it would appear to me that it would probably have something to do with the fact that studs are usually designed to be tightened until their threads jam against the end of the mating female thread. If this is not done, I can't see much difference between studs and non-through bolts, but perhaps I don't fully understand your "lever effect". In my view, the reason for using non-through bolts rather than studs has to do with the following. I remember once being told that for the best fatigue performance, studs should be designed so that when fully tightened they contact the bottom of the hole instead of being tightened to the end of the thread in the above mentioned manner, but this seems to be rarely done, and using a bolt is the next best thing, since it never reaches the end of its thread when tightened. Although stud bottom contact is tougher to do from a manufacturing standpoint, it should be possible to achieve it by having a short relieved portion of thread at the end of the stud so that it can contact the bottom of the pilot drilled hole in a repeatable manner. I also believe that to help maintain preload one should use the longest bolts or studs practicable, especially with aluminum rods.

 

[patprimmer]

The problem with through bolts in high performance rods is that the rod must be notched to make room for the head of the bolt. This creates a weak point in the rod.

One solution is to make the bolts longer so the head is further from the bore of the big end, but this makes the rod and the bolt heavier, and increases the room taken for the crank to rotate at a given stroke. Many high performance engines are pushing the limits re RPM and capacity, so extra weight and bulk are a significant dissadvantage.

Most good high performance rods use a pair of tube dowels to locate the cap on the rod, and to reduce flex at the parting line, thereby reducing fatigue due to bending of the bolt, stud, cap screw.

The above applies to both steel and aluminium rods to varying degrees.

Extra considerations re aluminium are:-

The extra bulk is even more critical as aluminium rods are already bulky

The notch is more critical, as aluminium (to the best of my knowledge) is more notch sensitive than steel.

The longer the bolt, the greater the dimensional change difference, due different co-efficients of expansion between aluminium and steel.

I would think the ideal design would be a stud, mounted in the rod as described by EnglishMuffin, with tube dowels, and possibly a very small pilot hole right through to avoid trapping air under the stud, as this might expel and blow out the stud retaining adhesive (locktite).

This would allow significant strength to be retained in an area where (from anecdotal evidence), rods are weakest , while reducing the weight and bulk of aluminium rods and the bulk and to a lesser extent, the weight of steel rods.

The reduced bulk would be in the area that often interfers with the cam lobes in a typical American V8, and obviously the cam lobe is an area where interference is difficult to measure and correct for.

The linking portion on the "H" beams sounds like a good idea to me in high compressive load situations.

The cross sectional area of the "H" beam is much lower near the outer fibre which is resisting bending.


Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[EnglishMuffin]

The rods of the venerable Meyer-Drake Offenhauser had through bolts nested in between local webs, with no notch -just a small counterbore, and didn't look as though they had more bulk than that Ferrari rod, but whether the design would pass muster after an FEA I don't know - as you say, somewhat weak in the critical area. The rods also appeared to be tubular, but how on earth they were made I don't know. The bolts themselves doubled as locators.

 

[MulletMobile]

Does anyone mind reviewing some formulas for figuring out the loading on the rod at a given rpm? I remember seeing a thread regarding that here some time ago, but I am no mathematician. The reason I ask is I am trying to figure out the load I would place on a rod I wish to use in a buildup of a budget racing engine. I dont want to spend money on billet rods if I dont need to. I know what rpm I want to rev to and I know the piston weight I want to use and the crank stroke. If someone can help me work out the formulas I would be glad to create a little VB script that makes it a plug and chug operation to calculate.

 

[tdirs1]

If you take a look on our website centralvwaudi.com and click on the tech button youll see the i beam rods i had made by farndon engineering,and theres a picture of the new rod next to a std rod.I had them made as i bent all the rods in both directions basically shortening the rod by 2mm.

 

[aduetto]

re: EnglishMuffin (Mechanical) Jan 20, 2004
[...] appear to me that it would probably have something to do with the fact that studs are usually designed to be tightened until their threads jam against the end of the mating female thread. If this is not done, I can't see..."

Hmmm, I wonder about your installation procedure. I suggest you look at the ARP site (

http://www.arp-bolts.com/index.html),

go to Product, then the Main or Head section, then the How To Install section. They say to finger tighten only.

Some time ago in Precision Engine (formerly Precision Machining) magazine they said to back off the stud 1/4 to 1/2 turn.

 

[EnglishMuffin]

I agree that sounds like a good procedure, but how often is it actually followed in practice, and can one be sure that the stud won't turn during the nut tightening operation? After all, most garage mechanic's tool chests include a stud extractor - a staple of all the major tool manufacturers.

 

[Azmio]

Stud bolt? With the use of the stud bolt, the weak point is shifted to the nut thread or the bolt thread where the two are engaged between one and another.

I guess it is much better to use bolt engaged straight to the conrod upper end. This way the bolt would have longer engaged thread if compared to the nut engaged to the stud bolt.

 

[EnglishMuffin]

The length of the engaged thread is not of overriding importance, since the first two or three engaged threads generally carry the majority of the load. You can demonstrate this if you do a detailed FEA. With a correctly designed stud, stud bolt, or through bolt, tension failure will usually occur in the male thread close to the mating member.