enginehistory.org

[jjuutinen]

I have been wondering why Merlin´s valve rocker cam follower pads were chromed, not nitrided? If I have not totally misunderstood several references, nitriding would have been much more suitable method for this application. My theory is cost since isn´t nitriding a considerably more expensive process than chroming?

Jukka

[wallan]

I have no specific idea why R-R chose hard-chroming (important that it is hard-chrome) instead of nitriding, on the follower pads, but I can tell you why a 1930’s materials engineer would probably have chosen it.
Gas nitriding was a recently invented process, was relatively unstable, and gave erratic results for a number of years. R-R are/were a conservative company, and wouldn’t have been keen to innovate. Nitriding requires expensive steels that contain as alloying elements some or all of the following: aluminium, chromium, vanadium, and molybdenum. (the nitrogen atom is so small that without these alloys, it diffuses too deep into the metal: plain carbon steels that are nitrided have less than half the hardness of hard-chrome, for this reason) For a pad on the end of a rocker, it wouldn’t be economic: nitriding the crankshaft would.
Nitriding is an expensive business to start up, and really needs a long production run. When it was designed, nobody realised they would make over 150,000 Merlins, so the mass production of parts wasn’t thought about.
Hard-chrome requires low deposition temperature: make a mess of the nitriding temperature, and you have to start again, but you wouldn’t know unless you tested each part.
Hard-chroming is a simple and cheap process, and the only real disadvantage it has is environmental, with chromic acids as waste. That’s why new methods have been invented, that reduce this waste. (nobody bothered back then) It would be easy to build up the hard-chrome surface before final machining, and as nitriding is only about 15-20 per cent harder than hard-chrome, (1150 against 850-1000 Hv) it wouldn’t have been worthwhile. Plus, hard-chrome, unusually, combines hardness with toughness, and can be applied very thickly: nitriding layer is usually only about 1 millimetre thick. (I know it’s time-dependent)
Next, hard-chrome is about 100 times better at metal-to-metal sliding wear resistance: I’m sure that were there to be a breakdown in the oil layer, during high-speed manoeuvres, that would be a bonus. It also has great abrasion resistance, and given the amount of debris carried around in the pre-detergent oils, this would be another advantage.
So, if I was at R-R, designing engines, back in the 1930’s, I would choose hard-chroming because it was cheap, quick, effective, and was what we had always used. Hope this helps.

PS Just thought: R-R probably had a hard-chrome capability, to reclaim crankshafts, and would use what they had.

[joder]

In my experience, nitriding is considerably less than a millimeter thick - perhaps 10 to 15% of that.


John Oder

[wallan]

The gentleman is quite correct that in modern times, with multigrade detergent oils, efficient filtration systems, and lead-indium and aluminium based shell bearings, that the nitriding layer, applied to current engines, is quite thin. However, as I said, nitriding layer depth is time dependent. I should also have said that the layer can be up to 1mm thick, but this would take days, instead of hours to apply, as compared to applying hard-chrome. (The rate is exponential. With aluminium as an alloy in the steel, a 0.1mm nitrided layer can be applied in a few hours: a 0.7mm layer takes over 100 hours) Another major factor is that, back then, to avoid spalling, due to the abrupt change in composition from hard skin to soft core, chromium would have been needed to allow a deeper layer, requiring an even longer heat treatment. If core strength is even more important, aluminium is omitted as an alloy, which drops the hardness down to less than 850Hv, putting nitriding back on the same level as hard-chrome. (Aluminium-free alloy steels are normally used in aircraft engines, to give this core strength: it wouldn’t make sense to specify a different grade of steel for the rocker pads, when R-R were buying large quantities of Aluminium-free steel) Finally, we are talking about the rocker pads, which suffer from a wiping action, as opposed to a rotational load. In that situation, a thick coat would be needed to prevent breakthrough, due to the high probability that there were a great deal of relatively large particles in the oil. (I’m referring to the 1930’s) Hope this clears it up.

PS One other thing I didn’t mention is that alloy steels are much more difficult to manufacture with, and cost a lot more than plain carbon steels. (They can do weird things when being worked)

PPS I’m also taking about aluminium as an alloying element deliberately added to the steel, as opposed to being used as a de-oxidiser.

[jjuutinen]

Thanks for the replies! Now, a follow up question. Why didn´t RR use rollers? After all, Allison used them, DB used them and Junkers used them for rockers.

Jukka

[awright]

I was in the Experimental Shop in Hillington in early 1955 as graduate apprentice. I was asked to work up a design scheme to put roller rockers on the Merlin camshafts. I got the scheme designed and a set of rollers were made and tested but the imminent end of piston engines at Rolls and the cost of changing to this obviously better design was not worthwhile. It would have made a great improvement in overhaul life as rocker wear was the limiting factor for engine blocks by that time. I was sent to Conway design not long after this experiment so not able to say whether any further steps were taken. Good question though.