Second flexplate failure in 4 years 1

[tommj]

Dodge 3.6L v6, flex plate centers break out. The steel center is like glass. The one shown replaced the first one that failed in the same manner 4 years ago and was supposed to be an improved part. All are MOPAR OEM parts, current failure at 174k mi. The part is cheap but costs $600 in labor to get to. What are they doing wrong? Should this be annealed or otherwise heat treated to improve longevity? This didn't happen 20 years ago....

 

[3DDave]

This appears to be a fatigue failure and fatigue failure is largely** unrelated to material yield strength; it typically occurs with loads (causing stresses) far below the the yield strength of the material. Because it's not related to yield failure of the material making the part weaker or more malleable doesn't help.

You can identify fatigue failure because there is nearly no yielded material - the fragmented parts fit neatly together and look nearly brittle.

The flex plate takes the thrust load from the torque convertor and there isn't much to be done about that in terms of reducing the load.

One could lower the bending stress on the part by getting a thicker part. The change is roughly to the 3rd power of the thickness of the material - make it 2X thicker and the bending stress, which seems like the main culprit, will drop by a factor of 8X.

To get 2X the life would require 2^(1/3) change or about 25% increase in thickness. If it is 0.190 inch thick now then a part 0.240 inch thick should double the life.

Probably someone ran the numbers and decided that saving a pound or two off the weight of the part would lower the cost and improve the gas mileage and figured the cars would be off the road by the time they hit 100k, depending on location - from what I know in New York state 4 years is when you can peel the structural elements from underneath with fingers. But they knew people wanted a punchier engine because they could feel that on the test drive, so it loads up the torque convertor more which increases the thrust against the flex plate which doesn't last as long.

**It's imprecise to say that, but fatigue failure is mainly due to the amount of flexing, however small, that the part sees - the deformation of the crystals in the metal allows tiny defects to progress. Spring rate is more influential but that is sufficiently constant for plain carbon steels.

 

[GregLocock]

Hmm, a thick flex plate won't flex. Now to be honest I'm not sure how important the flexing of the flex plate is, I suppose it is there to accomodate the angular misalignment of the TC through back to the bellhousing to the engine block.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[BrianPetersen]

As far as I know, flex plate failure is not an endemic problem with the Pentastar or any of the transmissions it connects to, and that suggests a problem with this specific vehicle. Misalignment due to something machined wrongly?

Which transmission, which vehicle?

 

[3DDave]

It would certainly be worth looking to see if the crank surface runout was too much, if the alignment pins were both there, if the mating holes in the bell housing were still a tight fit, that the runout on the torque convertor was also within spec.

More than that is checking to see if the alignment pins and alignment holes are in the correct location - not sure how to do that on the vehicle for the pins.

 

[tommj]

This IS a big problem with these vehicles even with the prior engine design. Do a Google search for dodge caravan flexplate failure and it will return a ton of results. The first flexplate broke and was replaced (out of warranty) trans failed 2 years after that, now the second flex plate 2 years after the trans. 2011 dodge caravan 3.6l

 

[enginesrus]

What is the material hardness?

 

[TugboatEng]

The machining tolerances of the crankshaft flange, line bore, and bellhousing flange are all very tight and I doubt the stack up could lead to issues. However, the pads on the torque converter are "as welded". If you are experiencing repeat failures I would be highly suspect of the converter. Considering using a dial indicator to measurement run out near the converter bolts and use shims if you find it excessive. I would not expect the flex plate to be flat before being bolted to the converter as it has been warped by stamping and welding of the ring gear.

Increasing the thickness of the flex plate increases the strain on the flex plate for a given deflection. It also increases the pressure on the thrust bearing. If you want a thicker/stronger flexplate you need to build it up out of thinner leaves.

 

[enginesrus]

There could be runout that is constantly flexing it.

 

[3DDave]

The required tolerances are tight. Meeting them isn't a guarantee.

The torque convertor floats according to the internal pressure, there is no fixed deflection except runout - the thicker the plate the less deflection due to the internal pressure. The pressure on the thrust bearing is from the movement of the torque convertor, unless someone has wedged all the torque convertor internal clearance out of the picture - it's suppose to allow around 0.150+/- of travel for that internal clearance.

 

[3DDave]

Since the data points are inconsistent it's not possible to graph this.

Like if it was 80,000 miles first, 174,000 miles second, that I can handle but the first is -4 years and the current is 174,000 miles.

But if it was evenly split, that's maybe 500 Million cycles in 90,000 miles, which is a lot of cycles.