Anybody have a link to the picture....

[MSLiechty]

of the difference in connecting rod sizes on teh CTD, PSD, and D-Max. My employer has a cheby and I want to show him the error of his ways.



Michael

 

[Turbo Tim 1]

This is out of ToolManTimTaylors gallery.



#ad

 

[B KIRK]

Here's another:



#ad




Brad

 

[SHobbs]

Makes me glad I have the mighty Cummins under the hood. Anyone see the change in the furd connecting rod. Was the change for the new 06 p. s or are they trying duplicate quality?

 

[HeavyHauler]

And here's another!

 

[fkovalski]

It is always neat to see those photos... . Why though is the Cummins rod at crank journal on an angle?

 

[Turbo Tim 1]

Looks like someones got a little pee pee envy, that last picture is a little distorted, in the first two the Ford rod is bigger in that one it's smaller. Looks like a little manipulation.

 

[RankRam]

It is always neat to see those photos... . Why though is the Cummins rod at crank journal on an angle?

Being an I-6 the Cummins crank journal is at an angle... ... ... because by it's design it has to be.

 

[RankRam]

Looks like someones got a little pee pee envy, that last picture is a little distorted, in the first two the Ford rod is bigger in that one it's smaller. Looks like a little manipulation.

LOL you have a good eye, that pic is a photo shop and is of course phony. I used to have that pic in my gallery and one of our astute members informed me of this and I deleted it.



I believe it was the first "pics" of the 3rd gen con rods.

 

[fkovalski]

Being an I-6 the Cummins crank journal is at an angle... ... ... because by it's design it has to be.

I kinda' figured that it had something to do with the inline design, but still (theoretically) don't kmow why this would dictate the angle? Isthere is some sort of clearance issue as it rotates? I don't see this being a problem. I would think that access to the rod bolts at the crank end would be accessible at BDC. Is the angle by design because it provides a stronger connection?- I could see that being the reson.



Thanks- frank.

 

[rbattelle]

I kinda' figured that it had something to do with the inline design, but still (theoretically) don't kmow why this would dictate the angle?

With all due respect, it has nothing to do with the engine being inline. It also does not make for any stronger connection.



The reason for the angle is simply so that the rod can be removed through the cylinder. The massive crank journal diameter precludes removal of the rod unless the cap is angled. Obviously the Ford 6. 0 suffers from the same problem. Most very large industrial engines are offset this way too... as far as I've read.



-Ryan

 

[fkovalski]

The reason for the angle is simply so that the rod can be removed through the cylinder... .

I'll buy that reasoning. I take it that you mean that the "rod can be removed through the cylinder" meaning upward towards the top of (the removed) head to get it out keeping the crankshaft in place.

 

[HeavyHauler]

Looks like someones got a little pee pee envy, that last picture is a little distorted, in the first two the Ford rod is bigger in that one it's smaller. Looks like a little manipulation.

LOL, you know, I've never looked at that thing really close before today and I was thinking how funny that Ford rod looked. :-laf I'm glad someone cleared this up. I bet I've seen that picture posted a 100 times.

 

[DIESELMAN]

With all due respect, ... . It also does not make for any stronger connection.

-Ryan

With all due respect to your all due respect, It does make a stronger connection. It places the bolts(screws) equally in shear and tension (on combustion stroke), and loads the split connection preventing "elongation" of the journal bearing surface on each combustion cycle which causes the cap to become "upset" and when it settles, and were talking miniscule amounts, keeps the bearing out of metal to metal contact with the journal, preventing scoring and oil shear at the edges of the bearing surface. This maintains bearing integrity and even wear over tremendous hours of use and extreme loads.



This design also allows(requires) the use of threaded holes and cap screws instead of drilled holes/bolts/nuts, this allows more accurate and stronger connection, more threads are engaged with a threaded bore instead of a nut on conventional rod bolts/nuts. Again stronger.



And, the reason it is done on the Ford 6. 0 is the 6. 0 rods are cast(Powdered Metal), not forged/machined, with very little if any finish machining other than the journal surfaces. Since they are cast the weights of the rods is the same for any rods pulled from the production line, machining(decking) the surfaces for a nut/bolt changes the weight and each would be different so instead the holes for the cap screws are cast in and the threads are rolled or forged in, not changing the amount of metal, and to do this you need to angle split to get enough thread area while remaining light in weight. This makes for lower cost and higher production.



Besides, it looks cool.

 

[rbattelle]

With all due respect to your all due respect, It does make a stronger connection. It places the bolts(screws) equally in shear and tension (on combustion stroke), and loads the split connection preventing "elongation" of the journal bearing surface on each combustion cycle which causes the cap to become "upset" and when it settles, and were talking miniscule amounts, keeps the bearing out of metal to metal contact with the journal, preventing scoring and oil shear at the edges of the bearing surface. This maintains bearing integrity and even wear over tremendous hours of use and extreme loads.

I've heard this argument before, but I'm not convinced. During any stroke, force can only travel parallel to the rod (right up through the center), since you've got a pin at both ends. Furthermore, it's always compressive, never tensile. I think that places near-0 stress on the cap itself during all 4 strokes, since the crank is never actually pulling the piston down (it only pushes up the piston or is pushed down by the piston, both of which put the rod in compression). I don't see how the bolts are equally in shear and tension during combustion if the force is always parallel to and centered on the rod.



I could be missing something... wouldn't be the first time!!



On Edit:

Yup, I'm missing something alright. This statement:

Furthermore, it's always compressive, never tensile.

Is wrong. Rod is in tension during the intake stroke (assuming little or no boost). So the cap does see tension, so the offset will certainly come into play in the resultant stress on the cap screws. But since they'll be in a mix of tension and shear, wouldn't that make the assembly somewhat "weaker" (for lack of a better word) than traditional caps with the screws parallel to the rod, since maximum shear stress is lower than ultimate stress?



-Ryan

 

[Hohn]

Furthermore, it's always compressive, never tensile. I think that places near-0 stress on the cap itself during all 4 strokes, since

-Ryan

With all due respect to the due respect you rendered to those whom respect is due:



The rod DOES see tensile stress. The cams on these engines DO have a slight amount of overlap in them as the piston transitions from exhaust to intake strokes.



At this transition point there is almost no compressive stress on the rod at all, but there is the tension caused by the intertia of the piston with nothing above it to resist.



In gasser racing engines, it's somewhat common for a rod to fail in tension. Usually, though, the piston will fail at the pin bore (the top of the piston separates from the skirt) before the rod does.



On a CTD, the pistons are strong enough around the pin bores to where this isn't an issue.



Suffice it to say, there are times when a connecting rod is in tension-- and it can be a LOT of tension in an engine with such heavy pistons.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~



Now, the offset rod caps. The offset generally makes the caps stronger. As Ryan rightly pointed out, a connecting rod can only transmit load at its connecting points. Thus, the focal points of the stresses are the imaginary centers of the rod journal and the pin journal. We'll ignore torsional forces, as rods generally aren't designed to withstand that. Bottom line: a rod will ALWAYS experience all of it's load (whether compressive or tensile) along the centerline of the bores on either end.



The offset end means that the cap fasteners do not see the entire tensile load, as they would in a straight cap design. If a rod with a straight cap has 20ksi tension applied, each fastener sees 10ksi of tension, and no shear-- plain and simple.



With an offset cap, the amount of tension is a function of the angle of the cap, and the total rod tension. Say you have the cap at 45° angle. Then the caps fasteners will only see about 70% of the rod tension. The rest acts in a shear direction. It's a geometric vector. Since the Cosine of 45 is . 707, then 70% of the tension gfoes to the bolts at a 45° angle. If the cap is at a 60° angle, then it's just 50%. So on etc...



By angling the cap, you just swap tension for shear.



Also, a capSCREW design is not inherently stronger. It's just stronger in this case because of the space limitations. A capscrew gives you more threaded area.



Assuming you had more space to work with, a through bolt and nut is MUCH stronger overall.



jmo

 

[rbattelle]

The offset end means that the cap fasteners do not see the entire tensile load, as they would in a straight cap design. If a rod with a straight cap has 20ksi tension applied, each fastener sees 10ksi of tension, and no shear-- plain and simple.

With all the respect being thrown around here it's hard to get any work done! :-laf



Anyway, I understand now the operating conditions that cause tensile stress in the rod. And I agree that the offset design is potentially somewhat stronger than a standard design. But, if we agree that the cap screws see a shear stress in an offset design, doesn't that make the screws themselves slightly more prone to failure since a steel beam cannot handle as much shear load as tensile (or compressive) load? What I'm saying is, I'd rather my bolts saw only tensile (or compressive) loading rather than any shear load.



Of course, the question is purely academic, since in reality the caps on these rods have features to take the shear load off the screws (hard to describe the features with words... anyone who's ever seen one in person knows what I mean).



A look over chapter 11 of The Internal Combustion Engine in Theory and Practice, by Charles Fayette Taylor, basically verifies all this discussion. Except that he depicts a rod with an offset cap and never makes any comment as to its being stronger than the standard cap. It seems very strange that he wouldn't make that comment if it were true... he's pretty thorough. He simply points out that it's done to facilitate removal of the rod. He does comment specifically (as does Hohn) that inertia loading at TDC on the exhaust stroke cannot be ignored, especially in high-speed engines.



Also of interest (not to our engines) is that 2-stroke engines are commonly assumed to have no tensile load on the rod, since gas pressure during the power stroke always overcomes inertia at TDC. He comments, however, that such an assumption is dangerous because it doesn't account for the possibility of mis-fire.



-Ryan

 

[Vaughn MacKenzie]

Just a quick comment about the pics. . .



Pic 1 is a Cummins 24-valve rod, 7. 3L PowerStroke and GM 6. 2L



Pic 2 is Cummins HPCR, 6. 0 PSD and DuraMax



Notice the two Cummins rods look a bit different. They started the fractured cap technique with the HPCR.

 

[DIESELMAN]

I think everyone agrees on a piece of the reasoning, everyone is just much clearer than I was.



I do still think it stronger, it doesn't just swap tension for shear, but loads that shear from the rapidly decelerating rod into a greater area, and that makes it stronger. More threads, stronger connection, no additional mass.



And I know I am right on this point, it looks cool.



(If this was TheDieselStop we'd be booted for discussing facts, hmmm. )

 

[fkovalski]

(If this was TheDieselStop we'd be booted for discussing facts, hmmm. )

:-laf :-laf :-laf

... They started the fractured cap technique with the HPCR.

Not familiar with this term. Would you elaborate further...