A question about connecting rods 2

[dicer]

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.

 

[patprimmer]

F1 uses I beams also, at least as far as I know.

Regards
Pat
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[ivymike]

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.

 

[thundair]

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/Templat...NTENTID=23463&TEMPLATE=/CM/ContentDisplay.cfm

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

 

[GregLocock]

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

Cheers

Greg Locock

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[dicer]

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.

 

[GregLocock]

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

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[dicer]

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.

 

[patprimmer]

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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[tbuelna]

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

 

[dicer]

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?

 

[bcs5274]

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

 

[thundair]

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.

 

[tbuelna]

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

 

[GregLocock]

Wow

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



Cheers

Greg Locock

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[dicer]

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.

 

[tbuelna]

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

 

[xlch58]

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

 

[tbuelna]

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

 

[patprimmer]

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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[dicer]

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.