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Originally Posted by kenikh
Anyeone with their noodle wrapped around these items, I am itching for your thoughts...
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What a huge subject and very complex.
Tribology is a subject that when I studied Metallurgy was more of a 'Black Art' than a Science but things have improved over the last few years
To say that wear surfaces must have a differential hardness isn't specifically true and is too simple an approach to cover all cases.
In general it is necessary to consider both adhesive wear and abrasive wear for many automotive applications.
The Aluminium to Aluminium analogy is quite correct but is due to the high surface energy associated with this metal which, as has been said, makes it very prone to gall. This is an ideal example of adhesive wear.
This mechanism is more akin to Diffusion Bonding and generally occurs when sliding speeds are in the 'stick-slip' range which result in aggressive wear. At higher speeds it may be less of a problem.
Fretting is a classic example of this type of wear.
The wear regime most closely associated with cams and followers is abrasive wear and it is common practice to harden both surfaces to avoid this type of problem.
It is quite clear that the cam/rocker interface on a 911 is influenced by sliding wear and a good test for any combination of material/lubrication and surface treatment would be a pin on disc machine as results obtained could probably be directly applied.
The other significant variable that can and does have an impact on valve train wear is the type of oil being used and the level of anti-scuff and anti wear additives.
The contact stresses on high performance cam/follower interfaces can be as high as 850MPa at idle and 400-450 MPa at 8-9000rpm.
The most successful additives adsorb onto the surface of a material and provide very good boundary lubrication. (We generally use Joe Gibbs XP4)
I guess it is useful to look at some of the basics first and to see what is wrong with the standard technology and the problems that occur.
I believe that early (1965 and 66) engines had forged steel rockers with hard chrome wear surface.
I would imagine that these rockers would be manufactured from a steel similar to 4140 or 4340 which would have been heat treated to a reasonable level of strength and tempered so that it had good notch impact and hence fatigue properties.
Cams would have been manufactured from cast blanks with the lobes having been chilled during the casting process so that they transformed to a white cast iron which is relatively hard and has good wear properties.
The hard chroming and subsequent hard chrome plating would have been quite expensive but I don’t believe that there is any significant problem with these parts on a correctly lubricated engine. hard chrome can be prone to pitting as it wears but generally lasts quite well.
The valve train issue where tensioner failure caused damage is a Red Herring as these rockers are completely adequate until some other component fails.
The hard chrome has excellent scuff resistance and good sliding wear characteristics. It is also slightly porous and retains a reasonable oil film.
Apart from cost and quality control issues with plating quality I think these rockers are very good and may be worth reproducing for high performance engines.
It would be good to know the precise chemistry of the forged rocker arm material and its hardness so its detailed mechanical properties could be estimated.
The Investment Cast Rocker (Lost Wax Process) which replaced the forged rocker was, in my view, a backward step.
Investment casting is a very cost effective way to produce small automobile components as this process allows casting to very close tolerances and would reduce machining costs but notch toughness and fatigue resistance of cast materials will generally be inferior to a forged part.
This is not due to any change in density, which would be insignificant, but due to the coarse nature of the ‘prior austenite’ grain structure of the casting which would be retained even after heat treatment.
The life of Investment cast rockers and standard cams seems to be quite acceptable so for road engines why bother changing.
With high revving race engines I have never been confident that the Investment cast rocker has good enough fatigue life. I have often said that I find it difficult to understand how the brittle fracture characteristics can be well enough managed so the rocker will break at a very specific force but have an adequate fatigue life for race engines but I am always assured that I am wrong.
It would be very good to obtain the basic material specification and know more about its heat treated condition.
I would imagine that cast rockers were initially manufactured from a material such as 45L, which is a 0.45% carbon steel.
I also believe that the wear face would have been locally induction hardened to give wear resistance. (I would think that this surface would be around 650 Vickers).
Obviously once an induction hardened rocker has been ground the wear resistant surface will have been removed. Re-Heat treating the entire rocker will not reproduce the wear resistant surface unless the rocker is aggressively quenched and effectively through hardened.
This level of hardness would of course make the rocker far too brittle and fatigue properties would be very poor.
I suspect that the rockers are hardened to a much lower level so they retain some toughness but are more prone to wear than original parts.
It would be much better to locally harden refurbished cast rockers and there is no reason why they couldn’t be induction hardened again (other than cost) or hard chromed or have some other surface layer applied. Plasma Nitriding would be a reasonable idea but it is probably more cost effective to just buy new parts.
If I were buying new race cams today, I would tend to use a steel blank and finish with either a gas nitrided or plasma nitrided surface. This would give the cam excellent scuff resistance, good wear characteristics and good fatigue life (although this is probably less important as cams rarely fail in fatigue. There would be a small amount of growth during this process but this would be limited to about 10 microns – (less than half a thou)
Nitrided layers are generally quite low in friction as the slightly porous nature of the converted layer holds an oil film quite well.
Titanium Nitride is often used in conjunction with nitrided surfaces as it will further reduce the coefficient of friction but the layer is limited to about 3 microns depth and on its own would be of limited value and I would think it is a bit more than needed.
Cubic Boron is a very hard and diamond like substance as boron is very similar to Carbon. It is very hard, very wear resistant and quite brittle. I tend to think of Cubic Boron as being used for the manufacture of cutting tools for lathes and millers rather than a surface treatment .
Boronising, however, is a common surface treatment. Boron as with Carbon and Nitrogen is an interstitial alloy with iron.
This means that iron can be Carburised, Nitrided or Boronised by diffucing these atoms into the surface of steel components.
Boronisng is used as an alternative to nitriding as it can be applied to a wider range of steels than is possible with nitriding and can even be applied to cast irons. The process tends to be more expensive than nitriding and is not as commonly used.
Diamond Like Carbon (DLC) is currently very fashionable and offers surfaces with very low friction and good wear properties but isn’t cheap. It also produces very even surfaces which are not good at retaining an oil film and there is a significant amount of work being carried out to ‘laser profile’ DLC surfaces so that they have ‘pits’ or patterns laser etched to help them contain an oil film.
As regards rockers I would tend to favour a forged rocker rather than a casting (but that’s probably me being old fashioned) I would tend to use a nitriding steel again and harden and temper to a reasonable condition.
With regard to the wear face I would plasma nitride as this process would allow the rocker to be treated locally just on the wear face.
Running two well lubricated nitride surfaces together would not cause any problems. Wear would be roughly even on both surfaces and much better than the current standard arrangement.
I would also be interested to know why Schrick recommend chrome plated rockers – I will try to find out.
The above quick look at some of the surface technology being used has only really considered wear.
It is now fashionable to worry about engine friction and here surface finish has become vitally important.
Modern techniques such as isoptropic sueprfinishing have become very fashionable and I am not sure how we ever managed before it was invented.
In F1 when engines were revving to 20 000rpm and until rule changes were expected to go to 24000rpm friction became extremely important.
I was involved with friction testing a couple of 3.0 litre V10 engnes a few years ago and at 20 000rpm torque was typically 300Nm.
When the spark was turned out the engine friction measured was 100Nm, as the inertia of the engine was around 0.012 kgm^2 it slowed quite quickly!
Obviously at this level it was worth trying to reduce friction as thermodynamic development was becoming more and more difficult and the gains were clealry very beneficial.
A 250 BHP motor at 8000rpm has much less to gain in this respect. It is an intersting excercise and ultimately must be beneficial but the cost/complexity may not be worthwhile.
Sorry to bang on but quite a complex set of questions.