Thread: Another TDV6 Crankshaft Bearing Failure

Just came across this link on the Disco 3 UK forum http://www.4x4community.co.za/forum/showth...ost3745430

The guy had the cracked crank tested.
This is the text without pics - click on the link above to get everything

"Crank examination results are in. I asked an expert in lubrication and broken rotating machinery to look at the front piece of my broken TDV6 crank, as well as some main and big end bearing shells and a piston and rings.

The main conclusion is that the crank break started in two areas (within the same main crack), and probably took a couple of months to go from start to complete break.

The break happened at the junction between the second big end and the second main (the first main is at the front ie pulley side of the engine).

I have attached photos.

In the first, the origin of one crack is shown: in the material very close to the thinnest part of the crank, in the area where the oil drilling from the second main to the second big end passes close to the surface of the crank. In the second picture of the same page, is a closeup of where the crack probably grew very quickly just after it started.

In the second photo, the second crack origin is shown. Beneath the surface of the metal, and a bit further from the oil drilling.

Both of these photos show areas of very high stress within the crank.

The third photo is a close up of the first crack. The initial part happened in mostly hardened metal, and therefore travelled very quickly.

The fourth photo is a close-up of the oil drilling, showing the poor surface finish, next to the radius in the big end journal, which is polished and more like what the surface of the drilling should look like.

Conclusions: The crank does not have much material in this particular area, and this is exacerbated by the poor placement of the oil drilling. It is difficult to see exactly why the drilling is placed as it is, perhaps there was bad access (by the CNC machine) to the area, compromising the placement. The material in this area is also hardened quite deeply, meaning that the steel is brittle and cracks will grow quickly.

This must all be seen in the following context: all engineering parts are a compromise, in some way. Generally, parts cannot be made as strong as possible, because they also need to be lighter, smaller, and even manufacturable (for instance, it may not have been possible to drill the oil gallery optimally in terms of stress). Not to mention cost.....So there will be areas of stress which are higher and lower than the average. This may be tolerable, if the overall stress conditions of the crank remain below whatever the limits are. If they are not, for whatever reason, then failure is likely to occur in the high stress areas.

However, in this case, it looks as if the crank has a particularly poor design in this area (there may be others too, this is just the one which failed first). There are ways to reduce the effect of this poor design, for instance having a torsional vibration damper that is working properly. But really, the design needs to be changed, specifically, the oil channel needs to be re positioned, and ideally, polished.

The analyst made the point that he has seen flimsier cranks, which don't break (eg Subaru). So its not a matter of a blatantly weak crank. More one which was inaccurately designed in the beginning, as often happens with designs: then there is a reaction which addresses the problem with a redesign, and it stops happening.

The analyst also confirmed a point which was made earlier in this thread (Forest Fab I think?) that a billet crank would not be better. A forged crank is the best way to go in terms of basic material strength. You don't get better, for cranks. I think there is probably a bit of leeway on that statement. Maybe other perspectives. But I'm not going to run out and look for a billet crank.

I can only hope that LR has made some changes in the crack area to the latest cranks. This is where comparing two 3 D models (old and new) would be really, really valuable. I'd be able to see where changes were made. But I guess thats a pipe dream.

Other things: there was a little corrosion on the top ring of the piston he inspected. Should not happen, and is due to buildup of nitric and sulphuric acid in the diesel. Maybe an indicator of too long oil change intervals (not the case with me, they were firstly 12000km in early life, then 10000km later), or poor oil.....While talking about oil, my oil analysis showed that the oil was up to spec in terms of viscosity, so it doesn't seem that there was fuel in the oil. The crank broke 5000km after the last oil change. Magnatec 5W30 A5.

The front main bearing was quite badly scored, indicating that the front part of the crank, forward of the break, probably wasn't running true for a while, because of the advanced cracking. By "for a while", he meant months, not seconds.

There was some carbon accumulation on the piston sidewall near the crown, but not much. Pretty good for 353000km.

I also checked my TVD. It looks slightly damaged: certainly not visibly trashed, like some I have seen on the web. But bad enough for me to be happy with my decision to change it.

So, as I expected. No hard and fast definite findings, but much more insight.

I'm hoping that:

1. LR has made appropriate changes to the oil channel design for the second main to second big end drilling, both in the geometry, as well as the surface finish
2. That heat treatment/hardening in the problem area is not too deep in later cranks
3. That the new TVD takes the torsional damping back to what it should be
4. That there are other changes to the crank which will reduce stress concentrations
5. That my new oil choice (dunno yet) is better in reducing sulphuric and nitric acid buildup'

So maybe a design issue combined with some maintenance/usage issues.

Garry

Originally Posted by garrycol

Just came across this link on the Disco 3 UK forum http://www.4x4community.co.za/forum/showth...ost3745430


The analyst made the point that he has seen flimsier cranks, which don't break (eg Subaru).

Garry

Subaru factory cranks are dealing with 400+kw and 6-700NM in many cars in AU, no issues at all, and they are revving them to 8-9krpm