Off topic connecting rods

[snowman]

Wayne has banished my connecting rod posts to the off topic forum. So here it begins.

The posts will remain a purely technical discussion on connecting rods, no opinions of what rod is better, just technical topics relating to every aspect of a connecting rod and how they work, or don't.

The first posts will be of the connecting rod in tension, that’s when the piston is trying to separate itself from the crankshaft by going in the other direction.

I will try to show the stress on the rod and where it originates. That is the forces that try to deform the rod and the weak points during the tension condition. It will be technical but non analytical, ie no specific calculations, just food for the gut.

It will take about 1 or 2 months to complete, no specific schedule, so be patient.

The goal is to arrive at a non controversial summary of the important features of connecting rods. The information should allow everyone who reads it to be able to select the best connecting rod for his application.

[Dottore]

Troll alert!

[Moneyguy1]

Egos? Here? Nonsense!!!

[snowman]

Well my ego is now where it belongs. Banned wasn't technically correct, but close nuff. I will the rod thing here and if an edited version of the result will fit in the engine forum I will put it there.

However my ego is larger than your ego, so there.

[snowman]

A little home experiment to start the series. Cut several strips of newspaper, about 1/2 inch wide and 6 inches long. Use good clean paper without defects. Place a pen mark in the middle of it. Now wrap the paper half way around a large marker or a 1/2 inch drill bit on the smooth part. Hold both ends of the paper together with your fingers, making sure the mark is on the outside, opposite edge of the pen or drill bit. Now pull on the pen or bit until the paper breaks. Do this again for several times. Note where the paper broke. Was it at the mark or about half way around the pen?

My results showed that the paper usually breaks about half way around the pen or bit. Anyone else see something different? Also note how strong the paper strap is, for a piece of paper. If you were to use 0.0005" copper shim stock, a very weak metal, you probably couldn't break it with your hands.

I will be drawing a diagram shortly to clarify this test if you are not clear on it now.

The point of this experiment is to demonstrate where the weak points are in a strap, and eventually where they are in a connecting rod.

[snowman]

You contest the largeness of my eqo? THats another thread.

[beepbeep]

Hello.

I belive that best way to find out connecting rod strength is by using FEM. There are nice plugins for Solidworks. If you don't have access to it there are shareware tools around. Model your connecting rod. Place the restraints, chose material. Do a FEM analysis, check out the loads and flush all this Origami crap.

Doing rod strength analysis by cutting paper is like curing a cancer with herbs.

I have basic knowledge of CAD but managed to model an adapter plate and do FEM analysis of it in matter of few days.



P.S.

Snowman, you have been overrun by time/progress. I understand that you, being retired , feel great urge to teach the world & dog all your knowledge of physics, politics, engineering and whatnot but your ego is in terrible disproportion to your actual skills/knowledge. I find many of your posts inflamatoric, self-congratulatory and often plain wrong. Just in case you wonder why people react negativly on some of your posts.

[snowman]

There is no substitute for having some gut feel. Some cannot understand the computer model, no matter what it says. The only real way to REALLY understand a specific item, say a connecting rod is to break it down into its parts, get a feel for what is going on and then to go to the computer analysis. First of the same specific parts, and then of the entire rod.

One example is the connecting rod bolts and their attachment to the rod. Using the case of the threaded rod cap one can look at the stress on the rod bolt. But then look at the stress on the cap. That stress is distributed to threads in the cap. If the cap is asymetrical around the bolt, those stresses may cause a bending of the rod bolt. Bending of a rod bolt is bad. Why? Because one side of the bolt is now stressed more than the other. If the bolt is already at max stress, the bending will cause excessive stress on one side of the bolt and it will now break. The exact geometry of the rod is critical. The use of rod pins to prevent side forces on rod bolts is critical. All these subtle facts are not clear with just an overall stress analysis.

The simple, non exact, demonstrations I will present will help show where the critical areas of a connecting rod are and give some insight to why they are critical. Using your stress analysis, show why the paper, ie rod, will break at the sides and not the middle. Show that the rod will not deform (on the bottom side) under the tension condition, and explain why not. It can do nothing but add to my explanation of what is going on.

GO with it untill I am finished and then let me have it, but not before.

[beepbeep]

You don't need "the feel"´when doing the computer model. You chose appropriate material (or define your own) and let the computer do the crunching. CosmosWorks will not only do the FEM for you, it will even calculate approximate deformation of material. It's just a matter of picking the right colour (red?) for loads which exceed elastic area of chosen material. Then look for the red areas on FEM model. If you find that stresses are too high (and it's actually prudent to have substantial margin on this), re-model the part until stresses are evenly distributed.

Bending of the rod-bolt? No problem, model the bolt and let FEM do the rest...it will calculate bending and loads on the rod-bolt >in the piston< as well.

Want to take this one step further? No problem, calculate piston accelleration at highest revs you might expirience. Knowing piston accelleration and mass will give you approximate forces applied to rod. Most of aftermarked rod manufacturers have CAD-drawings of rod readily available at their home-pages. Having rod drawings will allow you to easily model the rods in software and apply approximate loads that you expect in your application.

That being said, rod manufacturers usually did this number-crunching already and can give you good recomendation from the scratch.

Cheap FEM software is a gift from heaven. This sort of calculations were impossible to do for a layman just ten years ago. It was only aerospace industry with high powered mainframes that could afford the tools. It's now being possible thanx to influx of hi-powered PC's ... indirectly, we have Doom, Quake and Windows bloatware to thank to, which pushed hardware vendors to produce ever faster home computers.

I modelled a solid valve tappet that we planned to use in our project. Reddish hue was used to indicate stress limit which I found appropriate. The small rectangular part is approximate calculated footprint of camshaft (it's actually riding on oil-film so it's not 100% accurate).

Unfortunately, manufacturing the part prooved to be more complicated/expensive than actually modelling it

[snowman]

I have seen the results of Carrillo modeling, but where are they available on the internet?

[beepbeep]

I'll try to find the web-page where I found the drawings. Otherwise, it might be possible to modell them out of physical example as they are quite straight-forward design devoid of anny excessive 3-D structures.

Drawings I saw where 2D outlines but they can be usable as outline for extrude feature. After that, you could just use CSG to cut out the H-beam pattern and chamfer/fillet to model the round parts around big-end.

[IROC]

Originally posted by beepbeep
You don't need "the feel"´when doing the computer model. You chose appropriate material (or define your own) and let the computer do the crunching.

While I do agree with you, the computer models and analysis applications are only as good as the people using them. Additionally, there are circumstances where the part is just too difficult to model (defining boundary conditions, etc.).

We had a critical piece of Shuttle payload hardware a few years ago (it's failure would have probably resulting in losing a Shuttle), so we modeled and analyzed the crap out of it. And then we tested it. NASTRAN said it would fail at approximately 55k lbs of load. We all sat around taking bets on when it would actually let go. We watched as the tensile testing machine (at a NASA center) went past 50k, 60k, 70, 80k...the machine's maximum capability was 120k pounds and as we approached that, our test fixture failed. But the part that was supposedly only good for around 55k was fine.

Computers and FEM analyses are really indispensible these days, though. They're better than hand analyses (and waaaay better than gut feel), but ther are not perfect.

OK, back to Jack's ego.

Mike

[David]

I'm with Goran on this one, just model it and run a finite element analysis. I'd do the analysis myself but until I hear of and see a Carrillo or Pauter rod failure I'm not concerned about it. Do some searching for an F1 rod if you really want to know the optimum design, I doubt they look like Carrillo or Pauter.

[jluetjen]

Snowman;
I'm all for you doing this as a project. Hey -- I did similar projects on cam design and FEA'd a 914 trailing arm -- and I'm a freakin salesman with no mechanical technical training (two business degrees degrees though ).

I thought that this is what the Pelican Forum was all about. I bet you that you could even keep it on the "Engine Rebuilding Forum" if you were to start out by modelling a 911 rod. Comparing a 2.2 rod to a 2.7 versus a 3.6 rod I think would be valuable excercise.

Just be open to ideas from the rest of the forum, and we can all learn something as opposed to spouting "butt dyno" results for muffler "updates"!

PS: I think that care should be taken in the expected outcome -- some general learning about con-rods would be great. But given the depth of the subject, I doubt that any vendor specific conclusions could be drawn. But it would be interesting to have some idea of the forces and how they are managed through the rod. Another interesting comparison would be I-Beam versus H-Beam?

[snowman]

I will accept all the help I can get. Lets start with a strap of various thicknesses around a round rod, in tension. Lets get someone to model it with a computer and see where the weak points are. Thats exactly what I want to do.

Once we see the weak points of a strap around a round rod, ie crankshaft, lets modifiy it and see what does what to whatever. Thats the next point I want to make.

But first we have to establish where a strap would break and why it breaks where it does. I suggest that the strap is supported by the crank all the way around until it is tangent with the crank and at that point the maximum stress is applied, the first point where the strap is no longer supported by the crank.

THis point is very improtant. If we want to prevent the rod from breaking, we would add extra support at this point and never take away any support here.

Anyone who can contrubute a computer simulating of this stress is most welcome, otherwise we have to go with the paper "simulation"/

Gut feel is developed by experience and or simulation of a specific condition that is easily understood.

[jluetjen]

Snowman, I can understand the value of starting with a simple model -- like your strap. But I'm not really sure what you'll learn that will be applicable to con-rods since con-rods have a more sophisticated shape in order to withstand compresson as well as tension forces, not to mention the centrifical forces. My understanding is that the most common failure locations are near the rod bolts, or at the small end where the rod necks down. But then again I haven't seen that many rod failures first hand.

[snowman]

I think that my journey thru the many models of a connection rod will result in a good understanding of what is and in not important in a connecting rod. I do not thing everyone has such an understanding at this time. You are correct, the smallest part is the most vunerable, sort of makes sense, but there those of us that do not understand this, and thus this presentation. Overall its very complicated, but broken down to its basic parts it becomes comprehensible.

Just this strap part will show a lot of info. SO if no one has a computer model of the strap part I will commence with my simple model.

[snowman]

[dtw]

Holy crap, graphic picture. Only a matter of time before the mods remove that.

Look, you're not telling any of the male Pelicaners anything they don't know. You can hold it and pull it within reason - as indicated in your diagram. But as soon as you introduce sharp strips of paper and drill bits - you sick - well of COURSE it is going to break - no need to fire up expensive modeling programs...

BTW, does it really get all narrow near the balls like that when you get old?? Oh, and who is "Mark"? With all your Christian family values talk here on the board, I kinda figured you for a hetero..

[jluetjen]

Jeez DTW, that takes piercing to a whole new level! I'm squirming in my seat just thinking about it.

Anyhow -- now to get back on track...