Is this true..?

Have a look at the table here. Given that lifting gear suppliers are not normally known for living life on the edge, we can probably take it as reliable. **LINK**

The M10 eye screw is 17mm long (1.7D) and rated for 230kg, whilst the M20 is 30mm long (1.5D) and rated for 1200 kg.

Brian

As a full time stress engineer for a number of years, I will number among the sort-of camp. If the materials are similar, and the tapped hole is of a material not less than 60% the strength of the screw, it is true. 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. Wire thread inserts make the 3D reduce to 2.5D.

Regards

Richard.

Tapped holes for fixing studs are usually made considerably deeper than needed to carry the tightened up tensile load to ensure stability. With only 1 or 2 diameters of thread depth the stud will be quite unstable if even a tiny bit loose. The thread joint will also be much weaker in bending than the stud proper. So unless the stud is hard bottomed, flat end on flat end, into its hole any sideways knock is likely to distort the thread so the stud sits cockeyed or even tear the thread right out.

With 3 or 4 d of thread its much more stable in the hole. Odds are the stud will bend if clonked leaving the thread unharmed.

Extreme example is probably the chemically fixed anchor bolts where up to 10 d of engagement may be demanded.

Clive.

"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.

http://headpoet.tumblr.com/post/4558025921/the-hymn-of-breaking-strain

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

Regards

Richard.

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

RR used to specify helicoils for all aluminium threads.

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.

.

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.

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

Andrew

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

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.

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.

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.

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

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.

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.