choice of screwing length according to material

[francesco1990]

I ask that I am new and above all very inexperienced with respect to the people who are enrolled in this forum :
yesterday I had the written test for the industrial design exam, and among the various questions there was one that I couldn't really give an answer. and like me anke other

"explain how the screw length is chosen according to the type of material of the pieces to be connected. "

I have studied and also found in various researches, methods that concern the choice of a big step rather than a fine step to connect pieces that do not ensure good hold of threads, but regarding the choice of screwing length according to the type of material ... I do not know exactly where to put the hands *_*

I hope you can help me with your experience :d

 

[davide75]

Well to me they taught that regarding steel the screw length must be 1.5-2 times the screw thread diameter.for aluminum cast iron 2.5 times the diameter of the screw lock.

 

[francesco1990]

Well to me they taught that regarding steel the screw length must be 1.5-2 times the screw thread diameter.for aluminum cast iron 2.5 times the diameter of the screw lock.

Can you tell me anke about that?
cmq is already a good starting point because now I remember for steel something on the internet I had seen it

 

[zeigs]

That's not what I would call a trivial calculation.
for steel screws two good empirical methods: look which thickness has the corresponding commercial nut or 1-1.5 times the nominal diameter

if the material of the mothervite is different, the general discourse is complicated: In practice, it is necessary to calculate the contact pressure on the flank of the screw and to make it less than that permissible from the material, considering that it should not yield with an adequate safety coefficient (g.niemann, elements of machines vol 1, p 211-213, is one of the various text books used during the courses, but of construction of machines!). in principle the fine-step screws require a greater thread depth because they have a lower bearing surface (textual words)

considers that however the distribution of tensions is not linear and always breaks the first thread in socket

Hi.

 

[francesco1990]

If instead the material of the mothervite is different, the theoretical discourse is complicated: In practice, it is necessary to calculate the contact pressure on the flank of the screw and to ensure that it is lower than that permissible from the material, considering that it should not yield with an adequate safety coefficient

but by chance the motivation lies in the fact that if the screw screw tightening length increases, does the stem part that is stretched by the prevented mothervith to advance in its tightening movement? because if that were then reasoning works... If the tightening length increases the pressure exerted by the flanks of the screw threads on those of the screw (due to the tendency of the ironed stem to want to return to its natural condition), then we must pay attention to this particular. . :

 

[zeigs]

we are not talking about self-tapping screws, we are talking about cylindrical screws: the mothervite does not oppose the warning, has the same form (in the limits of dimensional tolerances). increasing the tightening length (with a certain limit) decreases the contact pressure because at equal strength the support surface (the side of the intake threads) increases.
I say within certain limits because being not linear the distribution of pressure (just because the part not in the grip is elongated, and the stretching decreases as you enter into the threads in socket) after a certain length I no longer have the decrease of the pressure.
in theoretical line there are cases where only a thread is enough in socket, but obviously in that case the screw is clearly oversized compared to the load it has to support.
if instead you refer to the increase of tension that you have tightening the screw, that is the preload and should be considered before dimensionaling the tightening depth

pay attention to this every time a screw has to take in a blind hole, or where you can not use a commercial nut (which is therefore already well sized to bear the maximum load of the screw)

 

[francesco1990]

we are not talking about self-tapping screws, we are talking about cylindrical screws: the mothervite does not oppose the warning, has the same form (in the limits of dimensional tolerances). increasing the tightening length (with a certain limit) decreases the contact pressure because at equal strength the support surface (the side of the intake threads) increases.

Thanks I understood the concept :d
However I was referring to the fact that when for example a nut is used to connect two pieces for example through bolt, at some point in screwing the nut comes in contact cn one of the two pieces... can not + move towards the head of the screw, then tends to iron the threaded stem of the screw itself :d

 

[zeigs]

is called preloading the screw

 

[francesco1990]

is called preloading the screw

Well, but I had predicted at the beginning that I am inexperienced with respect to you: I am only at the first year of university and the first subject in this area... :d

 

[Pastorman75]

I enter with two general considerations concerning:

1) the theory of the distribution of the efforts on the coupling screw/magazine.
2) production technology and respectable tolerances.


1) considered that once under load (or under pre-load+load) a screw-modrevite coupling "downloads" such load by finding it among the threads in a decreasing non-linear way according to the rigidity of the constituent materials screws and screws.
for example for steel screws it is has that 1/3 of all the load if it takes the 1st thread while at the 6th thread remained only 1% of the overall load.
distribution is not linear between these two values but assumes a form that depends on the type of thread, and other factors.
therefore for steel/steel, the fillets in addition to the 6th are practically useless.
for materials with different young module, these proportions change in the sense that more elastic materials distribute the load on multiple threads (you get to 7-8 also 10 threads).


2) Too long threads risk to "grip" their seats once you try to insert a screw that has length equal to the thread due to the sum of errors on the step between screw and screw.
For example:

mothervite error on the step: 0.01 mm/turn (10 microns per step)
screw error on the step: 0.02 mm/turn (20 microns)

ammuniting that the error is equiverse (it is not said) then after 20 threads I accumulated a possible "interference" of 0.2 mm overall that are often enough to cancel the "natural game" that exists between lives and mothers.

Considering that for considerations at point 1 it is useless to make too long screws, and idem for point 2, I by experience, on steel/steel never make more than 10-12 threads.

greetings

 

[gerod]

@francesco1990: when you write you use Italian words not those of SMS!!! learn to write in Italian so as not to see you "bubbled" at the exams because you write "xché instead of why, etc."
It's a good practice in this forum so I ask you to align yourself.
by-pass the technical aspect because it seems to me that it has been widely discussed.
the choice of length comes from analytical considerations, it is not randomly thrown there!
Hi.

 

[francesco1990]

pasteman75 I understood for that little ke I have in my knowledge ke therefore the factors ke must be taken into consideration for the choice of the screwing length are:

1- the materials of which both the screw and the screw are made because materials with different characteristics of rigidity distribute the load in a different way; therefore depending on these characteristics the number of threads on which the load will be distributed

2-longer screw lengths (with certain limits) lead to a greater distribution of the pressures that the screw threads go to exercise on those of their mother-in-law; threads are therefore less stressed

3- Long tightening lengths can lead the screw to "be stuck" in the mothervite due to inevitable errors on the ke step are obtained in normal processing to produce these elements

 

[francesco1990]

@francesco1990: when you write you use Italian words not those of SMS!!! learn to write in Italian so as not to see you "bubbled" at the exams because you write "xché instead of why, etc."
It's a good practice in this forum so I ask you to align yourself.
by-pass the technical aspect because it seems to me that it has been widely discussed.
the choice of length comes from analytical considerations, it is not randomly thrown there!
Hi.

Yes. Sorry. and yet I must say that I have read several times the messages I have written; it will be too much habit (not sure good ) by now.

 

[PaoloColombani]

Well to me they taught that regarding steel the screw length must be 1.5-2 times the diameter of the screw thread.

This is the most used empirical criterion in the field of mechanical construction, because it is generally assumed that the material of the mothervite is more tenacious than the material on which the thread is made.

There is an analytical method that is based on the determination of the yield strength of the fillet, in practice it is considered the cutting force distributed on the resistant section of the thread. I don't remember the details however for bolts it turns out that the screw yielding voltage at the traction equals the thread yielding load in the screw when the thread is about 0.5 times the nominal diameter. safely assumes a thread height of 0.8 times the nominal diameter. so much of the commercial bolts are sized, in fact if you take a commercial bolt with a nominal diameter of 10 mm in most cases you will find that its nut is 8 mm high. all this because it is considered that the bolt material is always the same, both for the screw and for the nut. In case of different materials I think it will be enough to make a relationship between the different stresses of yielding the two materials and multiply it for the safety factor 0.8 and for the nominal diameter, since the dependence relationship is a simple equation of first degree.

 

[Pastorman75]

pasteman75 I understood for that little ke I have in my knowledge ke therefore the factors ke must be taken into consideration for the choice of the screwing length are:

1- the materials of which both the screw and the screw are made because materials with different characteristics of rigidity distribute the load in a different way; therefore depending on these characteristics the number of threads on which the load will be distributed

2-longer screw lengths (with certain limits) lead to a greater distribution of the pressures that the screw threads go to exercise on those of their mother-in-law; threads are therefore less stressed

3- Long tightening lengths can lead the screw to "be stuck" in the mothervite due to inevitable errors on the ke step are obtained in normal processing to produce these elements

1 and 3 yes.

2 no, for example steel/steel 7 threads or 15 threads does not affect the distribution of tensions if not negligible (<1%)

 

[francesco1990]

ok I understand the concept then