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.

 

[glenn120A]

Most of the questions that are being asked have been answered by catalogs from manufacturers over the years. Yes they have thier own agendas but i have found that discussions with thier tech lines will result in answers closer to the truth about what they want to sell and why. Most of the better rod manufacturers will make custom rods to your specs.

 

[Azmio]

Englishmuffin,
For high revving engine, we want the crankcase and the oil scrapper to be as close as possible to the conrod locus. This will ensure compact design as well as low friction loss from the splashing oil.
Therefore, with the nut instead of bolt head, I wont be able to keep the oil scrapper very close to the conrod locus. The closer the oil scrapper is to the conrod locus, the more horsepower I would get out of the engine.
With the stud bolt, I need to use a special nut to enable it to be closer to the oil scrapper. With this combination, the weak point is shifted away from the main conrod structure.
As for the FEA, yes we did. We did it over titanium bolts (not the stud bolt though). The factor of safety is very close to 1.0. It's just enough for one Sunday afternoon race. To join the 18000rpm NA engine club, the conrod, piston and crankshaft need to be extremely lighweight, stud bolt for the conrod does not belong there.

AO

 

[76GMC1500]

"A tubular connecting rod construction can be used to provide acceptable stiffness in axial, bending, as well as torsional load cases."

With my very limited engineering backround, I have just completed my first semester of college, I have to disagree. I would think that the majority of the forces put on a connecting rod are bending forces as a result of the piston pushing downwards on the rod and the rod being at an angle due to rotation of the crankshaft. I would think that you would want to concentrate a majority of the metal towards the outside of the rod hence the use of I-beam rods. A tubular rod would have very little metal towards the outside with most of its mass concentrated in the center. This means the tubular rod would be more prone to bending, especially with small rod/stroke ratios. Tubular connecting rods are used, though. Our ship's service diesel generator, about 450kw@910rpm for an idea of size, has solid round rods. An ideal application for tublar rods would be in slow speed 2 stroke diesels as they have an articulated rod. The piston rod moves only vertically and has no side loads on the rod. For the most part they are tubular. The actual connecting rod which joins the piston rod and the crankshaft is of an I-beam style.

Here is a link to the best example of an articulated rod I can find.

http://www.haskey.com/johnh/railfair99/16_porter.jpg

 

[Azmio]

76GMC
Interesting. We did several classical calculation comparison between I,H and cross shape conrod. The H appeared to be strongest. The engineer in charge did inform me about the hollow rectangular shape conrod. This will yield better result. My respond to his suggestion was, how can we manufacture the hollow conrod.
If you can manufacture for me a tubular conrod while at same time having the small and big ends as strong as the conventional conrod, you should get your process patented.
As for my choice of conrod for high revving engine, my first choice will still be H type conrod. With the I shape conrod, the thick outer metal is in neutral position to resist bending movement. As for the H type conrod, I have two thick structure opposing the bending movement.
Anyone can disagree, but as a designer I am free to choose the design. The moment of truth comes during the WOT durability test.

AO

 

[EnglishMuffin]

The offenhauser ones looked as though they might have been drilled out, with a plug put in from inside the big end bore. But I could be wrong.

 

[76GMC1500]

I usually see H-beam rods used in forced induction applications like drag racing, while I-beams are used in high RPM naturally aspirated applications such as NASCAR. The reason being is that the H-beam weighs more and is a bit overkill when you're only making slightly more than 2hp per ci like a NASCAR.

 

[Azmio]

76CGM,
I dont think it is an overkill unless you overdesign the thickness of the H conrod for the required application. Many racing bikes and cars that rev at very high rpm (some at double the speed of NASCAR max rpm) use H type and there is no clear rule for forced induction or NA engines.
It is the preference of the engineers, some use I type and get better result and some use H type and get better result. No matter which one we choose, it is the details of the thickness, radius, length, tolerance, manufacturing process, development history, FEA calculations, R&D, etc. that will determine the final result. In racing, the final result is the one that matters.

AO

 

[cnova]

This is somewhat related to similar posts on the Maximum Piston speed forum. In a previous position with one of the former Japanese engine suppliers to CART, we used H-beam con rods. We were working with a leading Euro connecting rod manufacturer and at the time our revs were relatively low (compared to our final revs)and they recommended the H section due to our anticipated progress. Our rod bolts were from an American manufacturer of rod bolts, but we didn't use nuts but instead threaded the rod so that we had more threads (~2.5 x d) of engagement in order to obtain more consistent measurements of rod bolt stretch.
We used 2 ø3.0mm by 6.0mm dowel pins (parallel to bolt centerline) for locating the cap relative to the rod instead of hollow dowels, concentric to the rod bolts.
Prior to this, we were building our own rods and ours looked like they were chopped out of a block of wood (they were designed and built by passenger car engineers who had never experienced what race engineering is about).