Titanium rods

[grh222]

I have the opportunity to buy some ex factory 935 rods. Do they have a fatigue life? If so, is there some sort of guideline?

Thanks

Graham

[KobaltBlau]

This is a very interesting question. Titanium as a material does not have a fatigue life like aluminum does. However, there are many flaws which could make one or more of the 935 rods unusable, and replacing a single rod may be very difficult.

If I were in your position, I would first try to determine whether I was building an engine for which these rods would be useful. They are quite lightweight, but this is of course more useful with a higher RPM engine. I'm not sure if there are many 3.2 935 rods floating around, but you should determine if they are in fact for the shorter or longer crank as this may change whether you can use them.

The 935 rods also used titanium rod bolts. I have talked with respected folks on both sides of the issue of reusing 935 rod bolts, but I personally would not reuse rod bolts that I had not purchased myself, i.e. of unknown history.

The key thing I would want to do, however, is send the rods to a respected engine builder who has experience with 935 rods. I would give the seller some sort of deposit contingent on an inspection of the rods, and ask the engine builder to inspect them in detail. if they don't pass muster, the deposit could be refunded, but if they do you could pay the balance and the engine builder would send the rods to you. This "trusted third party" approach may be especially useful on this expensive but very critical item.

Good Luck,

[andrew15]

FYI - I'm pretty sure that ARP makes a rod bolt set for Ti rods.

AM

[grh222]

Thanks for the info. I'll get the centre distances first.

Graham

[Jim Sims]

"Titanium as a material does not have a fatigue life like aluminum does." Only if the the stress level is below the endurance limit S'subn will titanium have "infinite life" otherwise it will fatigue.

[Steve@Rennsport]

Originally posted by Jim Sims
"Titanium as a material does not have a fatigue life like aluminum does." Only if the the stress level is below the endurance limit S'subn will titanium have "infinite life" otherwise it will fatigue.

Very true as I've seen plenty of failed 935 Ti rods,......

Personally, unless the Ti rods were brand new, I would not install them in anything. 935's run well past 8000 RPM making 700+ HP and most used ones can be failure prone.

Remember, the consequences of rod failure in 911's is decidedly NOT inexpensive.

[grh222]

Hi Steve, thanks for reply. I think the case is closed!

Graham

[KobaltBlau]

Originally posted by Jim Sims
"Titanium as a material does not have a fatigue life like aluminum does." Only if the the stress level is below the endurance limit S'subn will titanium have "infinite life" otherwise it will fatigue.

Yes Jim, I should have been more specific. you said what I meant.

Originally posted by Steve@Rennsport
Very true as I've seen plenty of failed 935 Ti rods,......

How do they tend to fail, and in what kind of situation? (pure curiousity)

[Steve@Rennsport]

Andy:

They fatigue from too many revs for too long and tend to break around the shoulders where the bolts sit. Sometimes the small ends break, as well.

I think if one bought a new set of Ti rods (Ruf, Pankel, etc) and did not run exteme RPM's, they would last a long time. Remember, these are somewhat tricky to setup and totally unforgiving of errors.

[KobaltBlau]

Thank you Steve, your experience is always enlightening, in this case particularly the fatigue situation. As you said above, 935 rods don't tend to be babied

Cheers,

[kenikh]

Originally posted by Steve@Rennsport
Andy:
I think if one bought a new set of Ti rods (Ruf, Pankel, etc) and did not run exteme RPM's, they would last a long time.

What is extreme? 700 HP at 8500 RPM? 230 HP at 8000 RPM? It seems that 906 rods live forever and those motors spin HARD, but don't make the HP and torque numbers of the monster 935. I'd like to hear an opinion on what Ti rods would mean for a high spinning motor of more pedestrian HP and Torque values.

[chris_seven]

It is not true to say that aluminium alloys have a fatigue life.

The only materials that really exhibit this beaviour are ferritic or martensitic steels. (Certain manganese bronzes can also behave in the same manner but these alloys are not too common)

The 'Endurance Limit' of a material is the stress range that can be repeatedly applied without causing failure.

In steels the crystal structure allows dislocations to be pinned and this reduces the formation of the slip band intrusions and extrusions that cause fatigue crack initiation.

Neither Aluminium or Titanium have a crystal structre that can behave in this manner.

The Endurance Limit of Aluminium and Titanium is often quoted as the stress range which will allow a life of 10^7 (10 Million) cycles, however, even if a component manufactured from these alloys is stressed at below this level failure will eventually occur.

The problem with old titanium parts is that 95% of the components life will be taken up in accumulating the surface damage needed to cause crack initiation. Once the crack initiates it will rapidly propogate and cause failure.

The only way to 'life' these types of component is to estimate the number of significant fatigue cycles per hour of use, apply a conservative 'Safety Factor' and then throw them away.

There isn't really any inspection technique that can measure how much of the life has been used.

[tadd]

Chris_seven:
Ok, now I am just a chemist and tend to deal with much smaller subunits, but isn't that what surface treatments are for - like shot peening? Put the surface under compression greater than what will cause dislocations and bam, no more surface imperfections and no more faults to lead to failure. Ti doesn't like notches, so rods, half shafts, heck - even crankshafts should be near perfect applications. Toss in a surface coating like Mo or even anodizing and short of another failure mode a rod should live forever if kept below its yeild. Now if you are saying that Ti work hardens like copper, then I learned something new.

Not all materials are perfect for everything, but for non-contact parts I would think Ti is the schiznet. Heck, even the high strength steels start getting notch sensitve.

tadd

[chris_seven]

Tadd,

Most Alpha/Beta Titanium alloy have a relatively high work hardening exponent, but not as high as Austenitic Stainless steel.

It is this fator that means you have to take great care in the type and form of cutting tool used for machining. If not machined correctly they will easily gall or suffer other surface problems.

Shot peening is generally beneficial to the fatigue life of high strength titanium components but needs to be carried out with some degree of expertise and using the same parameters as you would for a steel may not be the best thing to do.

It is quite possible that careful shot peening may eliminate the slip band intrusions and extrusions that initiate fatigue cracks and effectively re-life the component but I would like to do a little more research before I am 100% confident that this is true.

Anodising, unfortunately isn't a great idea for components that suffer fatigue loading.

As with most hard surface layers anodising does reduce the fatigue life of a component even if only by a few percent.

Titanium is also quite good at picking up atomic hydrogen and if the voltages in the cell used for anodising aren't well controlled nascent hydrogen will enter the lattice and reduce the materials ductility and significantly increase notch sensitivity.

Using titanium for cranks is an altogether differnt issue. Although Titanium has good strength and low density its E and G Modulii are around 50% of a steel component. This means for a given forcing function (Inertia and Gas Torques) the deflection of a given crank would be about double that of steel and this may be a real issue.

The use of a bearing on a Titanium journal wouldn't be great either as the rubbing that inevitably occurs on starting and stopping would almost certainly cause damage.

It is common practice to coat Titanium Couplings that have to opearte with rubbing seals with Chromium Oxide but I just don't know how well this would work with a journal bearing.

I gusee a specialist company such as Poeton may have a view.

[MBEngineering]

HI just for your info, if you do bye the rods do not for get you will have to find some bigend bearings, bigend bolts and a std crank to fit every thing together????

regards mike

[tadd]

Chris_seven:
Thanks for the reply... Informative. I assumed, for the sake of the response, that who so ever was making the things would know what they were doing, ie use whatever mil spec call out for shot peening Ti.

I will say, from shopping for Ti rods, that nobody anodizes and all do a plasma moly coating over the peening (no polishing anymore like the early factory rods). Currently, I am sitting on the fence about getting out the wallet for a set of rods from Crower. I have a buddy with a mud toy who used a set and loves them. They are twice as expensive as a steel paulter or carillo, but half as much as a paulter Ti and seem to do all the right stuff engineering wise (as best as I can tell from my limited experience/knowlage base).

It's only money right?

tadd

[chris_seven]

I guess that the guys making new rods will do the peening properly I was just concerned about trying to reclaim used rods.

The price you are talking about sounds interesting as we are in the process of trying to build a high spec 2.0 litre and I was choking a bit on the price.

[MBEngineering]

HI have a word with josh saddler at autofarm as thay built a 2.0lt with S/H ti rod last year, and it is now up and running.
www.autofarm.co.uk

regards mike

[}{arlequin]

factory teams replace things for a reason. they've outlived their useful lifespan. maybe for a concour street motor which will be driven leisurley on sundays, but if if you plan on spinning it up (why else go lightweight?) then i wouldn't use them.