Acceptable conrod cap deformation 1

[nchanga]

Hi all,
Does anyone know what would be acceptable conrod cap deformation. The big end details are 57mm ID, 30mm wide, 7500 rpm, max force on the cap 35kN. Thanks! Derek

 

[ivymike]

do you mean permanent deformation, or dynamic deformation while running? If the former, I'm inclined to say "zero."

 

[nchanga]

Sorry I wasn't clear-- elastic deformation at TDC due to the tensile load in the rod.

 

[ivymike]

then it's a harder question to answer. The conrod big end bearing is notoriously picky about deformation and many times even careful analysis doesn't predict future problems. There are some "rule of thumb" guidelines for picking L/D for the bearing (unfortunately I can't remember at the moment what they are) and then the best advice I know is to make the big end as rigid as possible. That must of course be weighed against the effects on engine inertia and internal clearances to the block.

In other words, if I gave you a number you should disregard it.

 

[Tmoose]

By inspection FORD's philosophy for a little while was make a cap resembling a hose clamp and let it conform to the journal.

http://fuelpumpsusa.com/AUTOPARTS_PICTURES/ford/f2tz-6200-a_single/ford_f2tz-6200-a_single_4.jpg

 

[MikeHalloran]

I don't know about rods, but Ford did do something like that for rockers back in the sixties. They designed a rocker that was very light but would surely fail, ran it on a dyno cycle until it broke, made the next one a little stronger at the fracture, and repeated until they couldn't break it on the dyno anymore. I think that was for a '429 Cammer'. There are advantages to making your own tooling.



Mike Halloran
Pembroke Pines, FL, USA

 

[Tmoose]

Here is a link to a Mahle presentation about con rod optimization.

http://www.mahle.com/C125708F0068F6...78E200452808/$FILE/11 Conrod Optimization.pdf

It seems to say 1 or 2 thou of big end distortion is OK, depending upon direction.
But the rod was optimized with this in mind.
• Ideal for Range Extender/Hybrid Applications Ideal for Range
Extender/HybridApplications
• Light Weight Engine Applications

Sounds like engines limited to 3600 rpm or so, and unlikely to ever be allowed under the care, custody, and control of a child,
or someone who acts like a child once in a while.

 

[nchanga]

Thanks, My gut feeling was around 1 thou as the recommended big end bearing clearance is 1.8-2.5 thou.
My FEA gave 10 times this which was a shock so I've either done it wrong or I need to beef up the cap a bit

 

[MikeHalloran]

Yeah, that doesn't sound good. I'm guessing that if you even double the bearing clearance dynamically, you will have bearing life problems. It would probably be a good idea to recalc the hydrodynamics with the extra clearance.

... or social- engineer your way into Federal-Mogul and ask a real expert.



Mike Halloran
Pembroke Pines, FL, USA

 

[pontiacjack]

I can't answer. But I believe that the designers of race bearing inserts could help you- since they build-in appropriate amount of eccentricity to race inserts, to prevent the clearance from going to zero at the rod's parting line. I haven't seen eccentricities available of much more than about .001" (per side); so apparently "expected" rod/cap deformation is in that same ballpark.

 

[nchanga]

Thanks Mike and Jack,
It sounds like 1 thou would be ok. I'm still hoping that someone who works in this area will pitch in. cheers, Derek

 

[Tmoose]

Hi Nchanga,

How are you modeling the interface between crank journal and bearing insert?

I think to some extent the rod, especially the cap, will conform to the shape of the journal when highly loaded.

here's some almost useless info about the contours built into rod bearing inserts other than one purpose is to reduce problems when the bore distorts. Sounds like Within 3/8" of the parting line their may be LOTS of extra clearance, which might be helpful in starting the hydrodynamic oil wedge anyhow.

http://www.stealth316.com/misc/clevite-77-rod-main-bearings.pdf

http://www.motorheadheaven.com/it_bearinginstall.asp

 

[nchanga]

Hi all,

We found the mistake. The deformation was measured from the fixed piston pin so included the stretching of the beam. Measured across the big end diameter, the deformation was 0.7 thou.

Thanks for the links Tmoose. From reading them I can see that I'll need to do some testing of these titanium rods to see how they wear and adjust the eccentricity of the bearings to suit.

cheers, derek

 

[Tmoose]

Hi nchanga,

Is the big end geometry similar to a successful steel rod? If so, is there room to beef it up a bit?
I'd be concerned that Titanium's lower modulus would automatically make life harder for the rod bolt unless the "foot print" of the parting face is larger

 

[nchanga]

The surface between the cap and rod are bigger than the steel rod and serrated (cap will be cut off by EDM.)

 

[Tmoose]

When seating ball and roller bearings on the cylindrical seats, against abutting shoulders, it is often specified that the bearing nut initially be tightened to a value ~3X greater than the installation torque to really "seat" the ring/race and prevent positional changes due to emebedment.
Bearing races are normally finished very flat and quite smooth. Often bearing seat features are ground, so are likely pretty flat and smooth as well.

I'd wonder about the potential for embedment of the EDM'ed parting faces.
Is there clearance provided at the tips and roots of the serrations?
Since it is unlikely that the final installation preload is just 1/3 of the capabilities of the bolt, I'd be tempted to do a few experiments with multiple tightening cycles or subjecting the rod and cap to clamp loads far greater than the bolts/screws will be able to provide.

FWIW Ford flat head V8 main bearings had a single, curved serration.

http://www.precisionenginetech.com/wp-content/uploads/2010/09/PT256-500x475.jpg

Looks like maybe it is created as a single diameter, so it is up to the bolts to keep the cap from skewing.