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Max Moment for screws

Jacoolo

Jacoolo

New member
Hello all

Yeah, it is not Pro\E related. But where I can count on good and fast answer than in this forum among experienced people for mechanic industry.

So, my problem is that I wnat to know max moment for screw used in assembling plastic parts.

I have M3 steel screw which is used to assemble two plastic parts(ABS + 2mm thickness)

Can any one hlep me with that or show right links?

I tried to count it myself but the value I obtained are rather strange. It is the range between 0.006 - 0.6 Nm.
 
You mean to say that you have M3 thread in a plastic part of 2 mm thick and you're going to pull on that ? Then the answer you calculated is probably right.


Alex
 
The complete design concept is unclear. What 'loads' will the joint see I service? This design appears to be cheap but is it practical? Will the joint need to be dismantled at anytime in its life. i.e. this could be a cover that needs to be removed during servicing. If the joint is to be opened and reclosed during its lifetime you may find that when it is torqued up the second time the threads 'strip'.


Usually when plastic components are joined in this manner one component either has a metal insert moulded into it or has a larger thicker plug of material at that point such that a self tapping coarse thread screw can be used. Both of these joints usually withstand being reused many times before the threads fail


CAD2448
 
Hello guys

well this connections does not take any loads. It is just for sure closure.

So, let me show You my algorithm:

I took a ABS material:

*tensile strength - 37MPa

so what I did first was simple equation

t = Ms/Wo, where t < kt,

kt(torsion) = 0.5k(tensile), so I took kt = 8MPa

for M3 Ms was 0.6 Nm

but it was simple equation for head of screw

what about a thread, that was much more complicated:

View attachment 3514

kd = 0.2k

so from this equation I count F(force)

and from next one max Ms = 0.006Nm

View attachment 3515

and You know what? Honestly, for me this is to many equation as for such simple case like max torsion Torque:))))

I am waiting for Your opinion
 
If you say a "sure closure" then even that implies a load
smiley1.gif
, even if the only purpose is to compensate for tolerances and deformations.


As mentioned in the other reply it is not good to treat plastic as if it were metal. The same rules simply don't apply.


If you want to join plastic parts then you'd better look at alternative ways. If you stick to screws then there are special types for that, having a different type of (high) thread. Buth there are also rivets, inserts, ...


Without seeing what you actually want to join, and some indication of the use (join forever, open occasionally, open frequently, ...),it's difficult to indicate any direction.


Alex
 
muadib3d,

If you are self tapping the screw threads into one part and holding a second part, then I suggest you use a special screw type called "plastite". It is a special thread shape that is made on a screw that is more like a triangle than a cylinder. Works great, and does not require a larger diameter screw boss...

take a look:
<a href="http://www.dsm.com/en_US/html/dep/screw_design.htm" target="_blank" target="_blank">
http://www.dsm.com/en_US/html/dep/screw_design.htm</a>

Cheers...

M





Edited by: magneplanar
 
thx magneplanar

I will take a look on this closely latery

but after qiuck view - indeed it looks like the say as those ones we use here
 
Base rule when connecting plastic : forget everything you learned about metal connections.


Alex
 
maybe it sounds naive - for sure it does - but AHA-D could You describe some basic rules that You can share here with me and other collegues

first time it all looks simple but "deeper in the forest, more trees you meet", so now when I would be concerned once again in this topic I would use simplified equations from the pdf mentioned in my previous thread
 
ok two cents more to this thread

here are these three conditions one must face to check the funcionality of thread

View attachment 3568

great thanks for Artur for time and patience he found for me

I hope he would find this thread sometime
 
Plastic has low strength. Fasteners all tend to concentrate force on small sections and surfaces, making connections fail. You also have general deformation when loading a fastener. When you drive a screw through a sheet of plastic and start pulling you actually widen the top of the hole, so the amount of material supposed to hold the screw gets less while pulling. Since plastic is so soft it is easy to damage thread, cut "new" thread with the screw, ruining the fixture. Threads in plastic parts have to be made afterwards or with costly addons to the mold. Typical plastic connections turn the properties of plastic into advantages. You can clip a cover over a box because the entire cover gives away and then you have all the circumference to hold the load. You can clip a PCB into a plastic unit and make it sit firm without using a single screw. You can melt and glue plasics together. If you need fasteners then all you do is create a place where a metal fastener can cut into the plastic material, giving it enough material to hold and enough room not to split the model when screwing.


These are general guidelines, it is not evident to summarize years of experience in a few lines.


Alex
 
I am not expecting this from you. This your experience YOu earned in many years.

I also have a hope to earned similar experience too.

thanks for that
 
AHA-D,



had some trouble using fasteners?
smiley4.gif






Muadib3d,

we use almost every method there is for assembling plastic...each has
benefits. I would venture a guess to say that nearly half of our
products using plastic also use a fastener. In most cases they are
meant to be installed once and never removed. There are benefits to making a plastic to plastic assembly method. In some cases a fastener is just as good and easier to deal with.

as AHA-D described there are benefits to plastic to plastic assembly. A properly designed screw boss with the right screw with last the life of the product as well.

cheers,

M
 
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