Napier Sabre engine in Washington

So why strip each engine after initial build ? Welcome to the world of adverse tolerance stack up.

Now you will have the tolerance variation on everything, dimensions, materials, coatings, treatments and assembly. Each one of these has numerous sub categories I.e dimensions includes length, diameter, squareness, parallelism, flatness, concentricity, and more… Just one part, with one parameter at top acceptable tolerance is not a problem but get a number of these at the top limits and you maybe lucky and spot the assembly won’t go together but it could assemble with gaps critical for function being significantly compromised so will wear very rapidly or even crack.

When a complex assembly is designed it’s assumed all parts are in the middle of the tolerance range. Although you’re supposed to consider permutations of tolerance, in reality it’s massively time consuming on technical resources.

A great explanation, thank you.

…The old saying is .. “you’ve got to have a crack”…..We could all worry about the “would be” and the “may be” until it becomes time for someone to have actually done something !!

…I’m sure the Napier people and the various aeroplane manufacturers experimenting with completely new engine designs, factored in the degree of these “tolerance risks” to still enable them to forge ahead to combat any potential foe (with possibly an even more effective fighting machine) all the while being confronted with these multifarious situations…. and the ongoing battle that was seen as being far from over.

So why strip each engine after initial build ? Welcome to the world of adverse tolerance stack up.

Even if you try very hard and are very skilled, no two parts you produce will ever be exactly the same. So manufactures define an acceptable band of inaccuracies known as a tolerance. Now you will have the tolerance variation on everything, dimensions, materials, coatings, treatments and assembly. Each one of these has numerous sub categories I.e dimensions includes length, diameter, squareness, parallelism, flatness, concentricity, and more… Just one part, with one parameter at top acceptable tolerance is not a problem but get a number of these at the top limits and you maybe lucky and spot the assembly won’t go together but it could assemble with gaps critical for function being significantly compromised so will wear very rapidly or even crack.

When a complex assembly is designed it’s assumed all parts are in the middle of the tolerance range. Although you’re supposed to consider permutations of tolerance, in reality it’s massively time consuming on technical resources. So when you go into volume production you start to run the very real risk of an adverse tolerance build which goes un-spotted but will kill the assembly in a fraction of it’s intended life.

Add to this the fact that tolerancing of sleeve valves was very non intuitive I.e one of Bristols sleeve valves secret sauce’s and a 100% test & strip of production is about the only way you can address the probability of an adverse build occurring on the assembly line..

The build and functional tolerances are found on the drawings and cannot be determined from the measurement of a single part……. Those who believe you create something like a Sabre from measurements taken from parts beware. In particular remember that there’s massive amount of data on the drawing on things such as coatings, forging, surface engineering, etc which cannot be determined from a non distructive examination of the part and what works on other engines may not work. Get this wrong and the engine might appear fine initially only to very suddenly catastrophic failure during its very early service life.

I would imagine that the visual inspection was key to identifying components that may not have exhibited abnormal wear during the tests, but may have shown indicators of developing defects during service. These indicators could have been from any number of issues including a series of components at one end of the tolerance range.

Regarding the reverse engineering of a Sabre, there are a great deal of resources required to do so, not only measuring existing parts. The problem is locating enough of the scattered documentation to make clear sense of exactly what has to be done for each component. There are many ways of approaching this type of project, all with various pros and cons; none are easy and each one is far more complicated than what can be posted to forums. While it’s important for us to publicly share our goals, basic forum and Facebook posts can never go into the detail required to actually complete the task.

I understand tolerances but I don’t understand how the problem of ‘tolerance stack-up’ will be solved by stripping every engine; if building an engine once risks ‘tolerance stack-up’ why does stripping it down and building it again not risk ‘tolerance stack-up’?

There are ways to avoid tolerance stack-up and these would be well known to aero-engine designers before the war; measurement of the critical dimensions of sub-assemblies, crush-washers, shims or selective-fitting.

So why strip each engine after initial build ? Welcome to the world of adverse tolerance stack up.

Even if you try very hard and are very skilled, no two parts you produce will ever be exactly the same. So manufactures define an acceptable band of inaccuracies known as a tolerance. Now you will have the tolerance variation on everything, dimensions, materials, coatings, treatments and assembly. Each one of these has numerous sub categories I.e dimensions includes length, diameter, squareness, parallelism, flatness, concentricity, and more… Just one part, with one parameter at top acceptable tolerance is not a problem but get a number of these at the top limits and you maybe lucky and spot the assembly won’t go together but it could assemble with gaps critical for function being significantly compromised so will wear very rapidly or even crack.

When a complex assembly is designed it’s assumed all parts are in the middle of the tolerance range. Although you’re supposed to consider permutations of tolerance, in reality it’s massively time consuming on technical resources. So when you go into volume production you start to run the very real risk of an adverse tolerance build which goes un-spotted but will kill the assembly in a fraction of it’s intended life.

Add to this the fact that tolerancing of sleeve valves was very non intuitive I.e one of Bristols sleeve valves secret sauce’s and a 100% test & strip of production is about the only way you can address the probability of an adverse build occurring on the assembly line..

The build and functional tolerances are found on the drawings and cannot be determined from the measurement of a single part……. Those who believe you create something like a Sabre from measurements taken from parts beware. In particular remember that there’s massive amount of data on the drawing on things such as coatings, forging, surface engineering, etc which cannot be determined from a non distructive examination of the part and what works on other engines may not work. Get this wrong and the engine might appear fine initially only to very suddenly catastrophic failure during its very early service life.

Kermit’s already got two “almost NOS” Sabres. The Smithsonian example probably wouldn’t benefit him much.

[QUOTE=Alloy;2293808]OZFURYFAN,

A few feelers are out for trading our Sabre for an engine in rebuildable condition, it’s a decision that takes a great deal of decision making from any museum. Rebuilding our Sabre to appear “new” is doable, but before we commit the time, we will need to be closer to a trade.[/QUOTE
I think Kermit Weeks will also be very interested in this engine to power his Tempest which is coming along pretty well.

The key point in the film is that tear down and re assembly is done by Inspectors rather than usual line staff, a prudent control measure, and the tear down is partial…

Yes, a partial strip-down, because if the line-staff can’t be trusted and the inspectors have to completely strip and rebuild every engine, including detailed measurements and inspection of parts…

…why employ the line-staff at all? 🙂

I’ll try and watch that YouTube series tonight.

By that, are we led to think it is a zero houred power plant?

That was exactly what I was thinking when I saw this engine.
It shouldn´t be a big deal for Kermit Weeks to swap this engine with a perfectly restored static example for the museum. I think the museum´s example should be far easier to restore to running condition. Would be a win-win situation for both of them.

Attached are excerpts from the Packard Merlin engine logbook for a Merlin manufactured in 1944 for supply to an Australian built Mosquito. These come from the AARG Moorabbin National Aviation Museum library in Australia. Though it does not show documentary evidence of a tear down it does demonstrate the systematic care taken by the Packard company in testing the engine :

1. The pictured test is a “Final” of 2 hrs 45 min duration, implying other tests preceding, that have taken 6 hrs 11 min. Does this imply an intermediate ‘tear down’ inspection after Initial tests?
2. In the Final test, the engine is run on 65, 91, and 100 Octane fuel, between 2650 and 3000 RPM. An incredible amount of data was captured in this Final test, oil pressures and flow, air pressure and flow, ambient air conditions.

[ATTACH=CONFIG]244029[/ATTACH]

[ATTACH=CONFIG]244030[/ATTACH]

I would not be surprised if Napiers, being Napiers, did not have in a place a similar regime.

Tear down

I seriously doubt that every new Napier Sabre was stripped and rebuilt as a matter of routine after ‘running-in’; although it could identify, presumably, a very small number of engines with problems it would not make economic sense and, to me, would introduce another potential source of problems.

I wish we had some statistics to get a better handle on this.

CD, there is a superb, contemporary, 4 part serial on youtube by Hall Scott which deals with ‘tear down’ inspection after initial test run :

https://www.youtube.com/watch?v=Wdpg16jeKYU

It is worth watching the whole presentation to appreciate the care taken in the construction of these engines, used in PT and Fairmile boats, landing craft and other applications. I once had a 700 HP Hall Scott V12 Defender, which is now in a Fairmile restoration project, and it was a magnificently built engine. I originally bought it as a Mother’s Day present for the missus, at an auction that happened to be conducted on the day…she took it silently and I lived in ‘icing conditions’ until it was sold on to fund a family holiday. Wimmin!:)

The key point in the film is that tear down and re assembly is done by Inspectors rather than usual line staff, a prudent control measure, and the tear down is partial, seeking to identify obvious points that may develop into field failure.

I can think of quite a few issues that would warrant a ‘tear down’ after an initial test :

1. Within the thousands of components and human interactions assembling these into a finished product, it is statistically probable that incorrectly dimensioned parts may be incorporated, eg one oversize crankshaft bearing shell half out of a set, that might allow the crankshaft to turn but would cause obvious wear. This is a personal experience with one auto engine.
2. Under wartime production pressure, there would not be the opportunity to ‘lap in’ components that today’s engine builder, dealing with a rare and expensive engine, might invest the time in to ensure no issues with correct clearances between articulating parts. This is the ‘running in ‘ period, when this is accomplished by ‘careful’ field operation. With shipping space at a premium, say to Guadalcanal, sailors dying to transfer goods across the Pacific, soldiers desperate for food and ammunition, why would you risk shipping a unserviceable engine?
3. How much would German manufacturers have given, to have the luxury of a tear down, to discover all the bits of rags shoved down fuel lines by slave labourers in their factories?

OZFURYFAN,

A few feelers are out for trading our Sabre for an engine in rebuildable condition, it’s a decision that takes a great deal of decision making from any museum. Rebuilding our Sabre to appear “new” is doable, but before we commit the time, we will need to be closer to a trade.

So Ian, have you asked the Smithsonian if they will do a straight swap for your engine?? Surely thats a fair deal if your engine is made to appear the same externally:)))))))

It is also a possibility that this article was written during the early operational years of the Sabre when some of the “bugs” were being worked out. One note that hasn’t been discussed much was that the components were measured and logged upon disassembly, this could have been the procedure for the Sabre I, when sleeve issues persisted. My guess would be that the disassembly was likely a part of all early engines, but reduced as the production increased and the sleeve and cooling problems were resolved.

Unfortunately, these are just my personal thoughts at this point.

I think we’re all on the same page here.

I seriously doubt that every new Napier Sabre was stripped and rebuilt as a matter of routine after ‘running-in’; although it could identify, presumably, a very small number of engines with problems it would not make economic sense and, to me, would introduce another potential source of problems.

I wish we had some statistics to get a better handle on this.

Yes. Engines wear out with time and give less power than when new. They have overhaul cycles just as aircraft do. Refurbishment is supposed to return them to as new, with the worst-worn parts replaced, but generally would only result in a partial recovery. And, as described, a refurbished engine would not only be less powerful/thirstier than a new one but also less reliable. I’m told this is not true of modern engines, and own a laptop bought refurbished many years ago which is still going strong – but that’s not wartime 1940’s British built technology.

This is also true of aircraft, the Russians complained that many of the fighters they were sent were decidedly second-hand on arrival. Many spare Hurricane Mk.Is were refurbished and converted into Mk.IIs for sending to Russia.

A refurbished engine is one that has already served its time on aircraft in service and has gone back to be reworked…

Gone back to be reworked, why? Because some of the parts are worn?

It rose to around 30 hours after Bristol Engines were told by the Govt. to tell Napier where they were going wrong. Aparently something to do with the sleeve valves.

Napier had problems producing the sleeves to the required accuracy; five inch bore, high-tensile, steel cylinders that were ground cylindrical within an accuracy of two thousandths of an inch!

Bristol were brought in to solve the problem, somewhat reluctantly I gather, since Bristol would be giving away a hard-won commercial secret, but fortunately a problem Bristol had already solved on their own sleeve-valve engine, the coincidentally five inch bore, Bristol Taurus radial engine.

Ironically, Bristol had had similar problems initially and had only solved them by an fortunate accident. Originally sleeves were first ground with a worn tool and then finished with a new tool but this had only led to warped sleeves. Accidentally, a worker misunderstood the instructions and first ground with the new tool and then finished with the worn tool; remarkably this mistake produced almost perfect sleeves every time!

A refurbished engine is one that has already served its time on aircraft in service and has gone back to be reworked. That’s a different thing from looking at a new engine in the inspection stage.

Bristol had a lot of trouble getting their sleeve valve engines to work – some consider it Fedden’s greatest mistake. So there was a lot of hard-earned proprietary knowledge that they had, and yes it took high-level pressure to get it to share it with one of their major competitors.

My uncle, who was an engine fitter on Typhoons, told me that the early Sabres had a life before failure of around 12 hours.
It rose to around 30 hours after Bristol Engines were told my the Govt. to tell Napier where they were going wrong. Aparently something to do with the sleeve valves. So an engine that had done 5-6 hours on the test bed would be about half-life on the early models!!