CON ROD JOURNAL SIZE 3

[thundair]

I need help understanding the criteria of sizing con rod journals.
Is there something I am missing besides the dynamic loads?

All the ones I have checked seem large and the numbers prove that the added weight is detriminal to the rest of the crank points.

 

[EngJW]

Loads are the sum of (1) centrifugal force from rod big end, (2)inertia force from reciprocating parts, (3)pressure in combustion chamber acting on piston top. Loads vary in size and direction at every crank angle. However, there are two worst cases that can give you good results: maximum inertia force at high rpm and closed throttle, maximum gas load at low rpm (lugging) where there is minimal inertia load to offset it.

Having the force, you would next need to know the allowable unit loading (psi) of the bearing material, and that will give you bearing area. There are other things like l/d ratio, overlap between main and rod journals, stroke, and how much room you have in the crankcase. You don't want to make the journal so large that the rod bolts hit something.

 

[sreid]

I'm not certain that this relates to your question but I've always assumed that cranks are designed for deflection and torsional resonances more than for stresses.

 

[patprimmer]

Certainly main to big end overlap is an issue.

Fillet radius between the journal and the crank arm also has significant effect, and if sufficient, it can allow for a smaller journal if the journal meets other criteria like bearing area.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Greenlight]

Don't forget about the additional loads that occur during detonation or preignition. Cylinder pressures can triple during these conditions.

Also, you have to design for infinite life, so check the S-N curves for the particular crankshaft material being used. This will usually reduce the material strength to 50% of a new crank after 1 million or so cycles.

You also have to consider any residual stresses caused by the casting/forging process that may remain in the material. There is also variation in material properties from one batch to another.

And then there is the grain direction in the material, especially in a "twist" forging. As you probably know material strengths differ in different grain directions.

One more thing, the safety factor (usually 2 or more) to account for the things we didn't consider in the calculations.

What about when the local machine shop crank grinder turns the crank to 0.030" under and does not include the nice fillet radius that you designed?

Then some young kid (or old kid like me) decides he/she is going to "Hot Rod" the engine and doubles the HP with the newly turned crank.

After all of this, suddenly the cranks are not so overdesigned.

 

[icrman]

"All the ones I have checked seem large and the numbers prove that the added weight is detriminal to the rest of the crank points."
Qouted from original post.

I'm a bit slow today. Weight is detriminal? I tried the dictionary on line and could not find what it meant.

Anyway if detrimental is what is meant. I don't see what is meant about the rest of the crank points???

A crankshaft is a trade off like everything else in engineering design.
Your concern for weight is taken care of by drilling lightening holes in journals. Take a look at a crank shaft from an old Allison aircraft engine. And besides weight is only a consideration with high rpms. If you are only running in the 100 or so rpm range then weight is not a big deal, but delivering the HP is a big deal. So in this case a many ton crankshaft is a good thing.
Also in your average highspeed production automotive engine with a balanced crankshaft, that is heavy enough to take the torsional loads, I don't see a problem with unit weight being detrimental? It plays almost nothing as far as contributing to bearing loads. It may add some extra pounds to the car, and thus fuel mileage is reduced.
The main thing of larger diameters in journal size is surface speed. And extra frictional losses. But then large size means large bearing area thus reduced loading.

 

[EngJW]

Drilling out the crankpins could remove the oil passage between the main and rod journals. That would certainly be "detriminal" to lubrication. Aircraft engines have some elaborate and expensive ways for getting around this.

 

[thundair]

I guess I will have to type my threads in Word before posting a spelling error
I rely on spell check to make up for my old and feeble mind.

I am working on a 8.2L offshore racing engine and was looking at how small the journals are. The rods are also narrow 17mm that is all for about 75 HP per cyclinder. The lead on the job is confident in the calcs and so I guess I'll move on and trust the numbers...

 

[patprimmer]

thundair

A free program called ispell can be used in a text box like these posts.

You can get it from

http://www.iespell.com

There are many sources of "racing" cranks for big block American V8s.

Many of the top end models have lightweight features, including hollowed out journals.

When determining min journal diameter, a major factor is journal overlap, as many crank failures are due to cracks propagating from the big end journal rad to the main journal rad. The width of the crank arm, the journal overlap and especially the size of the rad are the main influencing factors.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.