Ping Dave Plowman

Yet the most successful gun in the world designed by Mikhail Kalashnikov was one built to agricultural tolerances.

There is a reason why it is successful, and reliable. And why others are called 'Widow Makers".

In the 1970s, a works colleague had a Hillman Hunter estate. He did various bits of maintenance on it, including stripping down and refurbishing the automatic gearbox (in the works mechanical workshop while leaving the car in the carpark for a few days). He reckoned that Hillman deliberately designed their vehicles to work with loose tolerance in order to allow them to be easily serviced and maintained in the far distant parts of the British Empire.

Remember the story of Rolls allowing a US company(s) to make their V12 engine, as used in the Spitfire etc, under licence? The US ones never made the specs, power wise. According to Rolls, because of not being able to work to the close tolerance needed.

Of course it's the sort of story every maker criticises a rival with. The smallest hypodermic needle being sent back with one threaded through it. And so on.

Certain types of treatment to crankshafts mean they can't be reground. And aluminium cylinder bore are coated, so can't be re-bored either. But if that treatment extends the life dramatically, the engine may well never need a major overhaul anyway.

Hillman didn't make the auto box.

I remember working for Rolls Royce in the finishing shop during the

1980's when their rear light reflectors were rejected by the Japanese because they weren't polished enough, yet we were expected to except their crap at the time.

In message snipped-for-privacy@davenoise.co.uk>, "Dave Plowman (News)" snipped-for-privacy@davenoise.co.uk> writes

But he still refurbished it - and he did a fair bit of other maintenance and overhauling. At least some of the rest of the car must have been made by Hillman (a Chrysler company, IIRC)!

It was the other way round. Rolls-Royce made parts to a slack tolerance and fitted them to the required tolerance. Fitting was done by skilled craftsmen "Fitters" and would entail hand scraping, lapping, honing, filing or a final light machining. The car makers told Rolls-Royce that if they were to have any hope of supplying the demand they would have to adopt car makers interchangeable fits.

Car makers every part fitted every other part to much tighter tolerance. If very tight tolerance was needed there would be selective fit. Such as pistons having 4 or 5 grades.

How many treatments? Most are a very thin layer, such as nitriding. Shot peening being another.

Both can be done after a reground, but I've always been sceptical how they increase the life for the crankshaft.

Shot peening introduces a compressive layer at the surface. As compressive stresses do not crack, resistance to fatigue cracking is greatly improved. The stress is additive, a tensile stress that would normally cause cracking can be reduced below the endurance limit. If stress is below endurance limit of steel it will never crack. Other materials with alloys such as titanium, nickel or aluminium have no endurance limit, they will crack with very small stresses applied often enough (Mr Geller's stainless steel tea spoons). To double the cyclic life the stress only has to reduce by 12.3%. For instance, 500 Mpa tensile stress with 61 MPa compressive stress due to shot peen.

All rotating jet engine parts, shafts, discs and drums are shot peened. Everything in a jet engine is going to crack sometime. The art and cost is removing and replacing them at 2/3 of the life that the worst min spec part will fail at.

All journal bearing surfaces should be hardened and polished. This prevents scuffing on startup before oil film develops.

The treatment that is pure hogwash is dunking in cryogenic Nitrogen. The only aerospace parts that are processed using cryogenic heat treatment are steel ball and roller bearing tracks. They use an acetone and solid CO2 mixture at -75°C to quench the red hot bearing ring, the whole heating and quench process is done in an inert atmosphere to prevent oxidation. This is to obtain the required hardness and not life. As each flight for a large civil jet engine is around £500 in overhaul costs, one more flight life would pay for a huge amount of liquid nitrogen dunked parts. But as it doesn't give any life improvement at all they don't bother. Dunking parts in liquid nitrogen is worse than using water. Water makes steam which insulates the hot part, nitrogen will boil and bubble just the same with room temperature parts. That's why quenching is normally done in oil.

Meanwhile jet engine makers do platinum plate some parts, for corrosion protection, paint would burn off.

Yet if you go to the science museum in London (I'm assuming these exhibits are still there) you can see an actual Merlin engine cut-way juxtaposed with a Messerschmitt from a 109. The build quality of the former knocks the latter into a cocked hat. It's astonishing that RR were able to turn out that kind of quality in wartime. Check it out sometime; it's a work of art in its own right. Actually it may have been the Imperial War Museum...

Fettling. And R-R was not a volume car maker but a specialist marque noted above all else for quality. So a rather spurious comparison there.

I never stated how Rolls achieved the tight tolerances needed. Only that they did.

I am aware of the principles, but generally crankshafts are designed to be fairly bullet-proof, and normal motoring should not cause cracking. If, on the other hand, we were boy racers, then such treatment may well improve the life of the crank.

I am aware of crack propagation and increased stress at a fracture but not of a level of stress that can endure a near infinite number of cycles of stress.

Does suggest that titanium alloys also have a distinct limit.