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Thread Depth vs Strength

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Hugh Gilhespie12/01/2011 22:30:18
130 forum posts
45 photos
Hi, Does anyone know if there is a way of estimating how the strength of a threaded fixing is affected by how far it is threaded into a matching thread. I am guessing that based on standard nut thicknesses, maximum strength is obtained when the engaged depth is about the same as the thread diameter, e.g, say 6 mm for an M6 bolt. I want to make some screwed fixings with a very limited depth for engagement and I wonder how effective they will be?
Terryd12/01/2011 23:05:15
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1946 forum posts
179 photos
Hi Hugh,
 
It's probably a load of baloney but I read somewhere that some reputable engineering professor claimed that due to elastic deformation under load only 2 or 3 threads are responsible for supporting the load in any thread system.  I can't refute or confirm, perhaps others can!  An interesting question.
 
Regards
 
Terry

Edited By Terryd on 12/01/2011 23:15:47

Andrew Johnston12/01/2011 23:14:33
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7061 forum posts
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It's all laid out in Machinery's Handbook, including various formulae to calculate exactly what you require.
 
The factors at work are essentially the tensile strength of the bolt versus the shear strength of the threads. So the materials used and their properties are as important as the thread engagement.
 
I have a vague recollection that once the first 2 or 3 threads are fully engaged further engagement becomes less important, as the loads are not carried uniformly over multiple threads. In plain English, the bolt stretches!
 
Regards,
 
Andrew
 
Blast: too slow on the keyboard!

Edited By Andrew Johnston on 12/01/2011 23:15:26

Terryd12/01/2011 23:28:03
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1946 forum posts
179 photos
Sorry Andrew,
 
Will PM you soon, Happy New Year,
 
Regards
 
Terry
Nicholas Farr13/01/2011 00:55:18
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3988 forum posts
1799 photos
Hi, FWIW, standard metric nut depths are close to the core diameter of the the thread designation, up to 12mm, thereafter it becomes slightly less. i.e. a 6mm thread has a 5mm deep nut, whereas a 30mm thread has a 24mm deep nut. Respective core diameters are; 4.7732 and 25.7060. (Zeus)
 
In my experence of usage, quality bolts and studs are not prone to fail on the threads, while being under load or overloaded, even in situations where there hasn't been a full thread in the nut. It is more often than not, that it is the portion of the bolt/stud outside the nut which fails.
 
However, steel studs in aluminium and cast iron normally have a little more depth of thread.
 
Regards Nick.
Chris Trice13/01/2011 02:32:55
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1376 forum posts
10 photos
I think the material of the male and female thread makes a lot of difference. For example, ever wondered why a high tensile bolt can have a mild steel nut? It's because it's easier to shear the threads away from a bolt core than it is to shear them away inside a nut made from the same material because the threads are in compression inside a nut, therefore high tensile nuts would be overkill and cost more. The relative shear strength of the threads of a high tensile bolt and mild steel nut are pretty much the same. Just food for thought. If the depth available is shallow, you could up the diameter of the screw fitting while keeping the thread pitch the same or find a thread form equivalent that balances a good number of threads in the available space to what suits the parent material. That way you effectively up the thread engagement area for no increase in depth.

Edited By Chris Trice on 13/01/2011 02:34:54

Mongo13/01/2011 02:36:11
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7 forum posts
42 photos
There are charts that give the shear diameter for threads. This is used to calculate the required number of threads for engagement. As to just a few threads holding the load, well the bolt and threads will yield until enough threads will pick up the loads w/o further yielding. The issue become exacerbated when assembling a threaded pressure connection since there is some deformation and therefore thread contact/engagement diameter can vary.
Stackerjack13/01/2011 08:12:08
14 forum posts
I also read somewhere, that if you reduce the actual thread (not the complete diameter) of a bolt by 50%, the thread is still 90% as strong.
I suppose it's because you are only removing the tops of the Vs of the thread, which is only a small ammount of metal.
Jack
John Slater 113/01/2011 09:21:18
3 forum posts
This case of a thread stripping is a complex stress system that is not well understood.
 
Factors involved include the material of the nut and male thread and how they each respond to loads, the quality of the thread both male and female( ie how well they fit together and the surface finish) and the nature of the load applied.
 
In the UK, professionally guidance would be sought from British Standards as they are supposed to embody whatis considered by the great and the good to be "best practise".
 
Have a look at BS3580 "Guide to design considerations on The strength of Screw Threads" for further information, particularly appendix A where equations are given to enable a "length oif engagement" to be determined. You should be able to access a copy through a town library or local technical college library.
 
Sorry if this reads as a "cop out" but if you are designing something critical it could have serious consequences.
 
Practical rules of thumb generally indicate that a length of engagement equivalent to the thread diameter would give failure in tension rather than stripping, however don't forget that chamfers on the male and female threads reduce the effective engagement.
 
The late Professor Chaddock published some data on bolted joints through the Design Council that might be useful too.
 
Best wishes
John
Ian S C13/01/2011 11:50:14
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7468 forum posts
230 photos
Just a wee bit OT, I resently got some fluteless taps (they are BA, a thread I don't usually use), and looking at my Sutton taps and tapping book, under Threadflo taps, found them quoted as machine only, and for example, 1BA, 1750rpm for aluminium, or 1400rpm for brass. The drill size is 4.9mm (normal tap 4.5mm tapping drill), so I thought what the heck, drilled a bit of aluminium, and started the tap by hand fitted in the drill chuck, then finished it of with the tap wrench, it went through much easier than a normal tap, and gives a beautiful smooth thread, and for brass or aluminium, a much stronger thread.  Going to try the 6BA tap in brass tomorrow.  Ian S C
 
Used CRC( WD_40) as lub

Edited By Ian S C on 13/01/2011 11:52:18

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