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A question about connecting rods
2

A question about connecting rods

A question about connecting rods

(OP)
Ran across a discussion about the merits of I beam vs H beam con rods. It was a "which is best" question.
I would lean towards the I beam for the proper application of beam loading. I would like to see what you folks have say about this.  

RE: A question about connecting rods

The I design is easier to forge ('I' being the case where the middle of the 'I' can be seen when viewing along the pin axis, and H being the case where there is a small slot in the rod when viewed perpendicular to the pin axis).  I don't think the 'I' generally enjoys any important advantage in regard to buckling.

 

RE: A question about connecting rods

Considering the plane of angular acceleration, the I-beam seems better.  OTOH, the "width" of the rod is constrained by the bore spacing and the need to share space with the crank cheeks and other rod & main bearings.  Thus perhaps making the plane of the rod beam longitudinal (i.e. "H-beam" design) improves the buckling strength in the inherently weaker (because the depth is constrained to be narrower) plane.  Though I haven't done the bending/buckling stress analysis, my gut says a well engineered H-beam rod is stronger for this reason (considering that the flanges can be made as wide necessary for the required strength in the lateral plane).
BTW, I have a real world experience of rods being "squished" (shortened) very slightly, under the stress of knock, without any visible bending.  I did a compression stress analysis that indicated this was indeed possible with cylinder pressures exceeding 2000 psi and the connecting rods in question.  The plastic strain was approximately 0.020" in the worst rod, IIRC.

RE: A question about connecting rods

I thought I beam was easier to forge and H beam was easier to machine from billet.

I am of the unproven belief that the long thin ribs of the H beam are more prone to deflection under compression of the rod than is the centre of the I which is restrained by the shorter thicker ribs typical of I beams

Regards
Pat
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RE: A question about connecting rods

Good points Pat.  I'd like to see the FEA on both designs.

RE: A question about connecting rods

(OP)
I don't think the bending stresses have been considered.
I beams as in say a large gantry crane have the load situated in such a way that the flanges on the top and bottom are in compression and tension. So how accurate is FEA?  Does it take into consideration the common equations for I beam vs H beam design? What is FEA based on?  Old strain gauge measurements?  Brittle Lacquer cracks?  
I'm curious.

RE: A question about connecting rods

Not being a stress engineer myself, I perhaps naiively assumed that the latest methods (FEA) are an improved model over the macro empirical methods that were taught when I was in uni.  Of course, as with anything, garbage in = garbage out.

RE: A question about connecting rods

I suppose another important question to ask is whether buckling of a rod is a common failure mode.  Seems like the only bent rods I've ever seen had a stuck piston or a flooded cylinder to blame. If you were to make a rod as light as you possibly could, perhaps the question would get more important.  
 

RE: A question about connecting rods

Hydraulic lock is the main cause of bent rods, but once you get into high boost, reasonably high compression and exotic fuels that require very low air:fuel, like less than 5:1. This can get close to hydraulic lock and bent rods can happen if suitable rods are not used.  

Regards
Pat
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RE: A question about connecting rods

In Charles Fayette Taylor's "Internal Engine" series from MIT press (Volume 2) there's a section on preferred con rod design.  For the most part it is technically based on a collection of sources from various ( widely varying) industries from the 30s to the 80s. The bibliography is 122 pages.

Anyhow, his recommended (not mandatory) orientation is H-beam, like a Carillo rod.  One of the explanations offered is heavy unsymmetrical loading can cause greater bending moments than inertial "whip".

The design details other than beam profile are more more important for durability, I'd say.
Most of the cracks in connecting rods I saw originated from the terrible profile ( a notch, really) to accept the rod bolt head in many stock rods.  H beam rods almost invariably thread the rod and use a bolt coming up thru the cap.  The resulting detail where the beam joins the big end is much more nicely shaped.

RE: A question about connecting rods

Like most real life applications, the "which is best" question evolves down to the actual application or end use of the rod.  Obviously using a H-Beam Carillo rod in a slightly modified engine is not the "which is best" use of the rod for this application.  You really have to allow for the actual end use application to determine the answer to your question don't you?  Another major issue to consider is the weight of the connecting rod as this has a significant impact on the application.  You are going to see the machined H-Beam rod to outweigh the traditional I-Beam rod.  Talking about weight, the difference between a standard rod bolt on the Carillo rod and their SPS-CARR bolt can be as much as 28 grams per bolt.  This is significant as well.

Larry

Larry Coyle
Managing Partner
Cylinder Head Engineering, LLC
CNC Porting
De Soto, KS 66018

RE: A question about connecting rods

Apart from the head, I would have thought a bolt pretty much weighed the same as the metal it replaced, and all steel bolts of similar dimension were pretty much the same weight.

I have seen I beam rods that have cap screws from the bottom as well. It certainly does improve the strength at the shoulder which is a common weak spot on rods with through bolts.

To compare a Carrillo with an OEM I beam is not exactly a real comparison. To compare rods of equal quality and weight to see which is strongest is more relevant. Strength for bulk might also be an issue as is fatigue strength vs weight as it affects the material choice. The balance of these properties  obviously need to be tied to the individual application.

Regards
Pat
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RE: A question about connecting rods

(OP)
In my search, it appears, there has been no honest study other than FEA, on connecting rod bending forces. As far as compairing H vs I beam.
Does anyone have any photos of F1 con rods?  

RE: A question about connecting rods

I don't have photos, but I do have some nice drawings by Tony Matthews of the conrods in Ferrari's F1 in 2000.

To my unpractised eye they look almost identical to a road car's.

"there has been no honest study other than FEA, on connecting rod bending forces."

I don't know what to make of that sentence. I've seen straingauged conrods going into engines, I rather imagine that the engineer involved discovered what he needed.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: A question about connecting rods

I've seen piles of straingaged rods too, but most of the time the rod forces were of interest for reasons other than rod life.
 

RE: A question about connecting rods

patprimmer
You mention comparing a Carillo vs OEM I beam, how many OEM H beam rods are you aware of?  The original question was comparing H beam to I beam.  Someone mentioned Carillo and I do have direct knowledge of this product and others.

You statement regarding I beam rods using cap screws is the best solution when you can use it as limited by individual application rules.

Larry

Larry Coyle
Managing Partner
Cylinder Head Engineering, LLC
CNC Porting
De Soto, KS 66018

RE: A question about connecting rods

(OP)
Straingauged rods?  How many where H beams?
I personaly only know of one application for a H beam rod in a old airplane engine, and it was chosen I think for the clearance space it provided for the 2 close articulated rods, and not so much because it was the strongest ie best as far as strength.
If you compair an H beam vs I beam con rod, looking at them straight on, that is in line with crankshaft centerline axis. H beam rods are always wider, so before jumping on the H beam band waggon, a fair comparison would be to have an I beam of the same width and flange cross section thickness. The I beam always wins, must be why all the OEMs' In large industrial engines win.  

RE: A question about connecting rods

(OP)
Wow I wish there was an edit function.
I had someone at the door when I posted that last one. And looks like I really goofed up.
"Large industrial engines use them"

RE: A question about connecting rods

hmm... I was sure i'd posted this before, but it's not here:

The cost of a forged rod (I-beam) will make it more attractive than a fully-machined rod (H-beam).

(of course) none of the strain gaged rods were h-beam.  nobody uses them in production.
 

RE: A question about connecting rods

  In trying to evaluate the yield difference between forged and billet rods we learned the forged won by >10% delta.

  In a real world I looked at a lot of broken rods and after a discussion with a con rod manufacturer, the conclusion is that the rod should be a tapered tube. That reminds me of Myer Drake's welded tubular rods of the Offy days.

   The Carrillo rode are both forged and fully machined that cures one of the weaknesses in  forged metal.
That being inclusions at the surface being driven into the part.
Although I believe VAR cures part of that, The fully machined component has a distinct advantage.
   I guess my point is besides the 'I' factor of the cross section there are apples and oranges in the way it is manufactured.

Here is an starting point http://www.autosteel.org/AM/Template.cfm?Section=Technical_Documents&CONTENTID=23463&TEMPLATE=/CM/ContentDisplay.cfm
 

I don't know anything but the people that do.

RE: A question about connecting rods

SAE paper 2009-01-0816 is quite fun, talking about strain gauges and conrods.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: A question about connecting rods

(OP)
I have been following this H vs I con rod deal on another site. It does seem there is no good data for H rods like Ivymike mentions. And it does seem to be more of its a cool looking part so it must be good stuff type of attitude.
I would like to see some real data compairing them, not FEA, but compression and tension bench tests, bending loads etc. Cost is not an issue on some engines, especially huge industrial engines, and top of the line engines like the VW W-16, if there is a superior rod design the manufactures would latch on to it. There are outfits like GMPP that puts H beam rods in some engines, so its not a cost issue I'm looking for its a what rod is the true best?  I lean towards the I beam, it is the much stronger design, for the loads it is asked to take. And especially at high rpms. I've seen some discussions saying that not all F1 engines use I beam, but the ones I have pictures of are I beam type, and that makes alot of sense for the rpms that are encountered.   

RE: A question about connecting rods

You'll be disappointed to learn that F1 conrods are designed using FEA, not people's opinions. I used to work with the FEA guy who designed Toyota's F1 conrods.

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: A question about connecting rods

(OP)
Most all FEA I have seen is for either compression or tension. I wish we could see some FEA showing the bending.
Of both types of rods. During engine operation the rod experiences cyclic bending. Simple proof of how a rod can bend is the typical hydraulic lock bend.
H beams would have to carry tensil and compression loads in those very narrow edges of the H flanges. And if the FEA is really up to snuff, it should show them as very highly stressed members.  

RE: A question about connecting rods

I think it is pretty obvious that if the FEA shows excessive compressive loads on one side and excessive tension loads on the other that the rod will bend.

I for one am not in the least surprised that F1 uses FEA to design stressed components.

Regards
Pat
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RE: A question about connecting rods

Basic engineering principles will tell you that an "I" section is most structurally efficient for a 4-stroke connecting rod beam area.  The reason some high performance rods use an "H" section beam, is purely due to economics.  

High performance rods (steel or Ti) are typically hot forged blanks, that are subsequently 100% machined on all surfaces to remove the metallurgically unsound outer surface layer that is contaminated during the hot forging process.  Machining an "H" beam configuration rod can be done with larger cutters than an "I" beam rod.  Thus it is quicker and less expensive.

The small difference in strength, weight and stiffness between an "H" beam rod and an "I" beam rod is generally not an issue, except in ultra-critical applications like F1.  Where cost is no object.

Regards,
Terry

RE: A question about connecting rods

(OP)
tbuelna, H beam with larger cutters?  Most all I have seen are very small slots as compaired to I beams.

I beam is the standard connecting rod style used by most all OEM's.  So are we all talking the same I vs H beam here?

RE: A question about connecting rods

in my racing experiences forged i beam rods fail at high rpm, (pulled aprt) h beam rods handle the higher stresses, oems forge i beams as everyone stated because they are cheaper, jmo

RE: A question about connecting rods

dicer
I have been to Fred Carrillo's shop and the cutter that cuts the sides is about 4" in diameter and although they are forged blanks they still are in the rough shape of the finished product.

I don't know anything but the people that do.

RE: A question about connecting rods

dicer,

From a purely structural standpoint, the I beam rod is slightly preferable to the H beam rod.  Most production automotive gas engine rods are now made from powdered metal (PM) blanks, due to cost.  PM blanks give adequate strength, have very little scrap, and can be produced very close to the net finished shape.  A PM rod blank can just as easily be produced in either an H beam or an I beam configuration, but they are universally I beams.

As thundair noted, old man Carrillo made the H beam racing rod ubiquitous.  But as I noted in my previous post, Carrillo does this for manufacturing cost reasons.  Carrillo makes dozens of different rods, but they all come from only 3 or 4 forging sizes.  Which is one of the reasons Carrillo must 100% machine his rods.  Carrillo is a smart business man and a good machinist, so he quickly figured out that removing all of that extra metal from the forged blanks was accomplished more quickly and cost-effectively with large cutters.  Can you imagine how long it would take to machine the I beam web pockets in a typical alloy steel rod with a little 1/2 inch diameter ball end mill?

Here's a current titanium F1 rod:



Regards,
Terry

RE: A question about connecting rods

Wow

That is a completely different look to the 2000 Ferrari ones.

 

Cheers

Greg Locock

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RE: A question about connecting rods

(OP)
tbuelna, thank you for the photo. I know lots of people will say the I beam is easier than the H beam to manufacture. I agree with what you said. I think the H beam is just one of those bench racer/ computer arm chair racer favorites. It is not the best design. And the highspeed overlap stress argument is a nonissue, reason, because they are not compairing a properly designed small end on the I beam. And the same end on the H is always much more encompassed that most I beam rods.

RE: A question about connecting rods

dicer,

If bending was the big issue, an I beam would be far superior to an H beam, since the I beam puts the material away from the neutral bending axis where is does the most good.  But a rod beam is more a compression/tension member, due to the fact that the load transfer joints at each end of the rod have a rotational DOF in the bending plane.  So the only bending that can be produced in the rod beam is from friction moments at the joints and inertias due to the rod mass whipping about.  Of course, one must also account for compressive buckling if the rod beam length is long and the cross-section limited.

One tangential benefit of the H beam is that I believe it gives a more uniformly stiff back-up structure for the upper rod bearing shell.

Look closely at this photo and you'll see a big old honkin' H beam rod:

Regards,
Terry

 

RE: A question about connecting rods

tbuelna,  

I think there were a couple of I beam master rods used on radials, but in general, no one sorted out how to do the transition required for the slave rods ( which were invariably I beams).   I believe Liston, Taylor and Ricardo discuss this engineering dilemma in some detail.  It is funny you picked a radial as an example, as the master rod I think does see significant bending stresses due to its role in restraining the motion of the slave rods and their lower link not being in line with the center of the crank.   

Charles

RE: A question about connecting rods

xlch58,

Wright used H beam master rods.  Pratt & Whitney used I beam master rods.

The big Pratt radials were far better engines.  But I don't think it had to do with the rod beams.

Pratt R2800 master rod:


Regards,
Terry

RE: A question about connecting rods

I think this thread is starting to form a fairly recognizable pattern exemplified by the corny joke, mummy mummy, why am I running around and around in circles, shut up kid or I will nail your other foot to the floor.

Regards
Pat
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RE: A question about connecting rods

(OP)
Thank you for bringing up the old ac radials. I already knew about the wright using the H beam. It was most likely chosen for the clearance it offered the articulated rods, it was an easier rod to make than the R2800.
Pat well then lets see some hard data compairing these 2 designs. Yes H will work. Yes H will buckle under pressure.
Bending loads???? Have you ever seen a hydraulic locked rod? High pressures in a cylinder will cause bending. And so true about the radial engine master rod feeling some bending loads.
Take note of the material surrounding the wrist pin hole.
H vs I beam.
 

RE: A question about connecting rods

No one has produced any data here yet. I suspect there is no back to back data in the public domain.

Most comparisons of H vs I are a racing H vs a production I so not a fair comparison.

I thought the radial engine, thread going in circles pun was an obvious joke.

I have seen a bent rod from hydraulic lock. It was a production I beam and was very bent. I have no doubt a H beam would have been equally bent so that does not really prove anything.

Re the production radials, one uses H beam for the master only and one uses I beam. Also totally non conclusive.

This really is going nowhere unless someone comes up with hard back to back data, and even then, it might depend more on the quality of the individual pieces rather than the basic I vs H design. This has all been repeated several times in this thread alone so I really have finished with it.

Regards
Pat
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RE: A question about connecting rods

(OP)
The prove anything part, is the fact that a connecting rod does have a bending moment, and the excess pressure of the hydraulic lock helps prove as an exageration if you will of what is and can be happening in the cylinder as far as pressure and its effect on bending a connecting rod, a proof of forces other than just inertial effects on the rod. And shows what kind of strength is needed in high boost applications, or NOS usage in the extream, or even in those hotrod engines like top A fuel.

The usage of the two designs in the old radial engines is conclusive as we disscused why, the H was chosen.
It simply needed the cut outs to accomidate the articulated rods. It is the most effcient way to do the job and quickly, but note the articulated rods are of I beam design. The engineers most likely figured the cut outs for like the PW rod would weeken that area, and decided the H beam would sufice and be a one shot wheel cutter machining job. It was a cost cutting thing not a this is the best structure for the application thing.
Also as a side note they chose forged steel for the crankcase vs the forged aluminum that the Pratt & Whitney used. Both engine designs are exemplary, and have proven very reliable and longlasting.  

RE: A question about connecting rods

what's better a ruler or a tape measure?
It depends on the application. :)

RE: A question about connecting rods

(OP)
what's better a ruler or a tape measure?
It depends on the application. :)


Niether one.
And this post has nothing to do with the topic.
The application has nothing to do with a design that is stronger than the other.  

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