Is this true..?

[Mick B1Participant @mickb1]

"The careful text-books measure

(Let all who build beware!)

The load, the shock, the pressure

Material can bear.

So, when the buckled girder

Lets down the grinding span,

The blame of loss, or murder,

Is laid upon the man.

Not of the Stuff – the Man!"

Kipling – The Hymn of Breaking Strain.

https://www.tumblr.com/headpoet/4558025921/the-hymn-of-breaking-strain

[richardandtracyParticipant @richardandtracy]

Do you ever fly in an aeroplane? If so, why?

Regards

Richard.

[Neil LickfoldParticipant @neillickfold44316]

The 1.5D or 2.D rule work well for similar strength materials. But a steel capscrew in an aluminium housing that is frequently being reassembled, the threads in the Al housing will last much longer with a 3d thread engaugement . On those we replace the worn threads with 2D helicoils and they are ok, the ones that had the 2d thread in the 1st place. Irony is to get the 2d helicoils in takes the same room as tapping for 3D threads. The 3D threaded ones have not failed after 4 years of service. The 2D threaded ones failed in less than a year. For some materials fine pitch series threads are stronger than the course pitch series.

Neil

[vintagengineerParticipant @vintagengineer]

RR used to specify helicoils for all aluminium threads.

Posted by Neil Lickfold on 29/06/2018 23:57:26:

The 1.5D or 2.D rule work well for similar strength materials. But a steel capscrew in an aluminium housing that is frequently being reassembled, the threads in the Al housing will last much longer with a 3d thread engaugement . On those we replace the worn threads with 2D helicoils and they are ok, the ones that had the 2d thread in the 1st place. Irony is to get the 2d helicoils in takes the same room as tapping for 3D threads. The 3D threaded ones have not failed after 4 years of service. The 2D threaded ones failed in less than a year. For some materials fine pitch series threads are stronger than the course pitch series.

Neil

[Pete RimmerParticipant @peterimmer30576]

Posted by Hopper on 29/06/2018 12:26:37:

Nuts are about 1 x diameter thick, usually. Some cylinder head nuts etc about 1.5. I was taught 1.5 x diam. is considered max strength but 2 x is a bit of a safety margin.

Also, was taught that tapping into aluminium, use a coarse thread as fine will pull out easier. Some textbook theory is at variance with this but over many years I have seen BMW motorbikes with countless stripped fine threads in cast aluminium on drain plugs, filler plugs, cylinder studs and the like. But Harley-Davidson, who use coarse threads in aluminium, seem to last 80 years and more without stripping out.

Good quality aluminium will hold fine threads no problem. I re-built a bike engine to allow for 3x the power via a turbo and I drilled & tapped the cases for 12mm x 1.25 pitch head bolts which is extra fine (originally M10 fine) and the main bearing bolts (from M9 fine) . They were torqued to 75lbs/ft in 2007 and that engine's still going strong today.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Pete Rimmer on 30/06/2018 07:07:50:

Posted by Hopper on 29/06/2018 12:26:37:

… over many years I have seen BMW motorbikes with countless stripped fine threads in cast aluminium on drain plugs, filler plugs, cylinder studs and the like. But Harley-Davidson, who use coarse threads in aluminium, seem to last 80 years and more without stripping out.

Good quality aluminium will hold fine threads no problem. I re-built a bike engine to allow for 3x the power via a turbo and I drilled & tapped the cases for 12mm x 1.25 pitch …

.

Just a thought:

There are two ways of looking at the coarseness/fineness of a thread

In relative terms 12mm x 1.25 pitch is considered 'fine' .. but

In absolute terms 1.25 pitch is quite coarse

MichaelG.

[Anonymous]

Posted by SillyOldDuffer on 29/06/2018 11:26:57:

Considerably more care is taken specifying and making threads for safety critical applications. Aircraft are interesting because they also need to minimise weight, leading to components being more stressed than in ordinary applications. Many aircraft have crashed because the wrong bolt was substituted during maintenance.

There has been at least one fatal crash, involving a Stampe, due to a replacement part having a cut thread rather than rolled.

Andrew

[Anonymous]

Posted by richardandtracy on 29/06/2018 16:06:12:

Steel fasteners into alumium or brass are more difficult. Steel fasteners into wire thread inserts in aluminium or brass are more difficult still. A good rule of thumb for steel into 6082-T6 is 3D engagement makes the screw break first for grade 12.9 or weaker.

Some years ago on a power inverter I had a need to use M4 12.9 SHCS into an aluminium liquid cooled heatsink where that was limited thread depth. In addition the screws held down a 3-phase bridge that had spring loaded ceramic plates that held the active devices. Total clamping force was around 9000N. To prevent bottoming out at the start of engagement, and spring compression, the screws were only engaged by just over a turn.

Naturally I was worried about stripping the threads in the aluminium heatsink. So I did some tests. I drilled and tapped some holes in a scrap piece of aluminium less than 2D thick. I stopped the test after the screw head broke off in the first hole tested. That hole was drilled for a nominal 50% thread engagement.

Andrew

[richardandtracyParticipant @richardandtracy]

Andrew, that latter can happen. As a stress engineer I always work to the minimum specification strength (S in aerospace terms). In practice the material is rarely this weak. I've seen aluminium with an 'S' strength of 295 N/mm^2 have a test certificate strength of 420N/mm^2, which is around the Aerospace 'B' value band for the alloy where you are 90% confident that 95% of samples will exceed that strength.

Regards

Richard.

[vintagengineerParticipant @vintagengineer]

Everyone is refering to aluminium, when it doesn't actually exist. They are all alloys of some description, I have casting cast in an alloy that can then be hardened to increase it's strength.

[richardandtracyParticipant @richardandtracy]

Just been reminded of a question one of our old draughtsmen asked in 1988.

'Do you want it made of Metal or Aluminium?'

Thought it may raise a chuckle.

Regards

Richard.

[Anonymous]

Posted by vintagengineer on 30/06/2018 17:56:14:

Everyone is refering to aluminium, when it doesn't actually exist. They are all alloys of some descripion.

Oh dear, I made an assumption that people would have the wit to realise I using the term alumimium as a shortcut to mean a common alloy. Clearly I was wrong.

Mind you, 1050A is around 99+% pure aluminium. And it's absolutely horrible to machine, like fudge.

ANdrew

[vintagengineerParticipant @vintagengineer]

If you asked me to make something in Aluminium I would make it from an old saucepan. If you asked me to make something from alloy I would ask what the end product would used for and choose a suitable alloy. Putting a thread in aluminium is like putting a thread in toffee! Use the correct thread for material you are using and the loading you are going to subject to.

Posted by Andrew Johnston on 30/06/2018 19:00:26:

Posted by vintagengineer on 30/06/2018 17:56:14:

Everyone is refering to aluminium, when it doesn't actually exist. They are all alloys of some descripion.

Oh dear, I made an assumption that people would have the wit to realise I using the term alumimium as a shortcut to mean a common alloy. Clearly I was wrong.

Mind you, 1050A is around 99+% pure aluminium. And it's absolutely horrible to machine, like fudge.

ANdrew

[JonParticipant @jon]

Thread depth is dependant on the material tapped in to.

Vintage i can show you various aluminium 6082 T6 with M30 X 0.75mm pitch that will withstand min of 3000 psi = tonnes of pressure.
Likewise can also show numerous M28 X 1mm pitch thread X 10 full threads holding 345 bar, pulsed to 370bar in test.

On the other hand have stripped blind hole threads at M4 x 0.5 x 12mm long in 7075.

Quite right Richard only the other week bought some 1 1/2" dia supposed 6082 T6. Though specs were not sent i could have got them from supplier.
Upon initial turning it was apparent there was horizontal side on through the length soft spots more akin to unhardened annealed 6082 and a totally different colour.
No good for a pressure bearing component, trust is with the rolling mills to produce to spec.

[Author]

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