Resurfacing vintage crankshafts and nitrided component surfaces

Another fascinating pathway followed was ‘cold welding’, where, under vacuum, in the absence of heat, the crystal structure of a metal will merge with an adjacent, similar crystal structure. It is very counterintuitive, and only emerged in the 60’s in association with spacecraft, out of a dreadful experience in the early Gemini program, where a door could not be shut after a spacewalk. In this case, the door hinges had started to ‘cold weld’. Another example was the failure of a solar ‘umbrella’ to open as the stays cold welded together. So great care is taken in modern spacecraft to ensure no direct contact between similar metals. In a crankshaft application, its all about grain flow, determined in the original forging process. So the theory is that cold welding does not disturb the original grain flow. In theory, you could centrifugally cast identical crankshaft alloy into sleeves, creating a linear grain flow in the sleeve, which would be ideal in a ‘journal surface’. This machined sleeve could then be clamped to a prepared, cleaned, old journal, and the lot put in a vacuum chamber. After a few weeks, the join between the new sleeve and the old journal surface might be indistinguishable. I can’t think of a better way to preserve the original grain flow in the crankshaft while building up the journal surface to ‘as new’. Only way to find out is to try. Another option is fine laser engraving of the new journal surface for optimum oil holding properties. 

Some analyses of one sectioned Cheetah crank were done pre Covid and determined high embedded stresses in the surface of the journal. By going back to old Aircraft Engineering periodicals an article emerged showing AS factory practice was to place a finish machined crankshaft in a jig, camped in bearings, while the crankshaft was run by an electric motor, in facsimile of an installed crankshaft. The purpose was ‘work hardening’ of the journal surface, prior to the advent of nitriding, which only came into use in the WW2 period. So the ‘embedded stresses’ in the surface were purposeful work hardening. Its the combination of yellowing old books and modern forensic techniques that explains a lot. It seemed to work, as most Cheetah crankshafts you find are within limits, while they must have done a lot of hours in largely training aircraft. 

For the record, a 1936 Air Ministry standard for Acceptance Tests for aircooled and liquid cooled engines was obtained, as well as AS factory procedures for resolving various issues with Cheetah components. So enough data to satisfy an inspector of the day in regard to this engine type. 

I have a copy of AP1526 Vol II Schedule of Fits and Clearances, Cheetah IX & X. It’s a lot of pages and no time to scan. If your friend is on FB or Messenger or Whatsapp then its easy to snap and send a particular page, but not 50…

Hi, would anyone have any Tolerances, or acceptance standards for the Cheetah 9 or 10 engines. 

for Non Destructive testing , DPI and MPI Inspection ? 

I would appreciate any help, for a friend who is rebuilding one  

Good for the Christmas stocking ! Yes please !

P & P
I find I have a copy of TN.135, dated 12th march 1945, “The 12 hour nitriding of crankshafts, airscrew shafts, connecting rods and minor components.” I do not have a copy of the other instructions I listed above.

EB and AC, very interesting information indeed…EB, I am wondering if it is possible to procure a copy of AID AP4089 engine inspection/testing information…

P & P
I find I have a copy of T.N.109 issue 2, dated 3rd June 1942, which is a special instruction issued by Bristol for “the 24 hour nitriding of crankshafts, airscrew shafts and minor components.”

I will PM you with the details.

Further instructions for other parts that are listed on the document are:

T.N.43. Nitriding Valves and the preliminary Copper and Nickel Plating.
T.N.44. Nitriding Steel Cylinder Barrels.
T.N.86. Nitriding Poppet Valves by the method involving reverse etching.
T.N.94. Nitriding Cylinder Sleeves.

You guys might want to d/l some of these books…

http://freebookstock.blogspot.com/search/label/ASM%20Hand%20Book%20pdf%20Collection

Cheetah into Air Compressor

The engine that was converted into an air compressor reveals its ingenious conversion secrets. Each piston was drilled to accept a bolted disc on top of the piston crown, to radically increase the compression. Each exhaust valve was replaced with a one way diaphragm type valve, which actually made for a quite efficient, high capacity air compressor. It is a wonderful example of turning a ‘sword into a ploughshear’. Unfortunately, a new set of pistons are required !

Not twins

A number of Cheetah IX and X engines and remains lifted from the mud have been disassembled to generate enough material for a test and control engine. The Armstrong Siddeley technical manuals say the guts of the IX and X are the same. But in respect of the crankshaft and reduction gearbox they are most definitely not. Forward of the throws the prop shaft is quite different and the various bits and bobs that fit to it are different. Of course the accessory drives at the rear are different between the two so probably only the crankcase and cylinders offer any opportunity for mutual donor material. Below is a picture of the respective, marked crankshafts and reduction cases.

This is a truly interesting thread!
Have you had any discussions with the folk at CAMS or TVAL in NZ, or is WW1 stuff too old to be relevant.

Thank you for your interest JonL. No, no discussions with NZ. There’s a little bit of wild sea between Melbourne, Australia and NZ, filled with sharks and probably house sized octopuses. (I understand that a Greek derived plural word does not use ‘i’, ie octupi, for those who might be concerned by what the Fonz would call ‘Incorrectomundo’ spelling.) I figure that if anything useful comes out of the trialing, then it can be useful to folks all over.

Working within an ‘advanced manufacturing precinct’ at a University in Melbourne some early issues arising with laser sintering are :

Size of $50,000 standard head of sintering machine cannot easily fit between throws of radial crankshaft.
Requirement to rotate crankshaft, at speed, with in a jig, to allow 360 degree sintering means high possibility of injury to $50,000 sintering head and loss of teeth and fingers of machine operator.

Just working through these issues at the moment.

One non airworthy crankshaft is being x-rayed and tested for pre-existing stresses, cracks etc. By cutting off one of the throws (it can be glued back later and sold on ebay as a Dambusters relic) it will be straightforward to sinter. This will then be sectioned to determine any new stresses or damage caused by the sintering process on the particular nickel chromium steel alloy of the crankshaft.

Out of this process some new ideas have emerged on how to deal with this issue, that are completely different to those that we started with.
It’s not until you start chopping the wood that new ideas start to emerge on how to chop it better.

This is a truly interesting thread!
Have you had any discussions with the folk at CAMS or TVAL in NZ, or is WW1 stuff too old to be relevant.

I understood that the lead (tetra-ethyl lead (TEL)) was added to assist in lubrication of the valves as well as an anti-knock agent. Anon.

Yes, can’t just fill’er up like I do my lawn mower. It specifies 87 octane in the books, which I can get at the local servo, but I don’t know what sort of chewing gum they put in the fuel these days instead of TEL. Luckily for me the only petrol refinery that makes aviation fuels in Australia is down the road, so I will wander down to annoy some guy in a white labcoat.

By the way, I took a handheld XRF into my front garden to test if decades of TEL emissions from the road in front were getting into my carrots. Not a super busy road, and the garden was OK. But soil from the old front fence, after being scraped of lead paint over the decades, was at the upper limit. Most exiting was the veranda where I scraped the paint off, in my bare chested, no respirator, early home renovation days. Soil was off the dial !

Out of curiosity I tested some old industrial infrastructure, say like a hangar, where rain would carry particles of paint down, and the soil at the footings would glow, if lead could glow. Don’t eat carrots grown next to a hangar. Wear a respirator when scraping old paint. Wear a respirator when blasting vintage engine cylinders. Lead apparently causes cerius menatal redardition.

I don’t know about older (WWII era) air cooled engines, but in the TVO-435 engines I used to work on, the barrels didn’t time expire. If they made it to overhaul, they were inspected dimensionally, NDT’d for cracks and returned to service.

Got about 30 barrels which will be dry ice blasted and dye penetrant tested, hopefully end up with 21 that will last the distance.

Engine Test Protocols.

I have copies of original Packard Merlin engine Acceptance tests, which ran for up to 10 hours. The most wonderful Newnes Publishers have revealed within their 1938 booklets the engine acceptance tests used by Bristols for their air cooled radials. A new type design might be subject to a 100 hours test, with strip down in between. A production engine test would run for a similar duration to the Packard. I figure that we should find a protocol somewhere in between. It would be useful to find a copy of an original Cheetah factory acceptance chart.

I am also after NOS copper-lead steel backed main bearings for Cheetah, please.
I have boxes of P&W 1830 if anybody wants any.

Simon says sintering’s sinteresting

At the Avalon Airshow recently held in Victoria, Australia, a gaggle of brave industry reps stayed behind to expose themselves to the army of i-diots like me who descend on the trade stands on public days, looking for bowls of free lollies or misplaced VIP trade passes. In between I was able to talk to some very interesting folk about processes used in modern aerospace to address the issues tabled here for worn vintage aerospace components.

In tracking these things over the last decade I get a sense that we are very close indeed to getting these processes accepted as mainstream. It is not the processes themselves that are suspect anymore, just the applied research, to build a body of data around their application. Today, heart pacemakers are mainstream, but it took many surgeons listening to Mahler as they cut open many chests to build a statistical data set on efficacy and safety.

It was interesting to see laser sintered steel alloy components edging their way from non load bearing components to critical load bearing components. Here are some photos of a guide rail bracket for underwing projectiles, worn in use, laser sintered, then machined back to specification. It was exciting to see that this was an accepted repair, in demanding service use. There were other parts there in the process of gaining acceptance, waiting on a statistical date set to build up. We are close.

So I think we should try laser sintering too, and start building a statistical data set on vintage aviation components.
So know we have three Cheetahs : HVOF, Laser Sintered and Standard Control.

I understood that the lead (tetra-ethyl lead (TEL)) was added to assist in lubrication of the valves as well as an anti-knock agent.

Anon.

I don’t know about older (WWII era) air cooled engines, but in the TVO-435 engines I used to work on, the barrels didn’t time expire. If they made it to overhaul, they were inspected dimensionally, NDT’d for cracks and returned to service.

They didn’t normally last that long though. Heat cycles would kill them and they’d crack somewhere and lose compression. Heat cycles are the enemy, probably more so than heat itself. The heating and cooling is more stressful than heating and keeping it hot. Go to your local gliding club and talk to the engineer about the tow plane engine and how they try to keep it from cracking cylinders! Lots of heat cycles going up and down all day.

Will lead free fuel have an impact on the valve seating in the heads? I know that was an issue with older cars when unleaded came out. At least Avgas doesn’t have ethanol in it (yet!!).