Maximum tightening torque for metric self tapping screws

[navneet.dhote]

Hi all,

Can someone help me to find out the maximum tightening torque for metric self tapping screws.
Actually I am using M3, M4 and M5 ST screws for fixing the plastic parts together.
Now I am going to prepare assembly guideline document for each SKU, So there I need to mention the tightening torque of each screw which is used in assembly. But I am not getting the direct values of torque.

I got the analytical calculations to find out the require tightening torque when clamping force is known but I am interested in maximum tightening torque which can be applied on screw.


Have a great time...


Thanks,
Navy

[jboggs]

I think you are going to have to do some experimentation yourself. Most screw torque data I am familiar with assumes the material is some kind of metal. There might be some tables for different types of plastic but I'm not aware of them. Max torque in steel and max torque in plastic are going to be two very different numbers.

[Kelly_Bramble]

Well, the link below should give you some guidance. You will need material characteristic data on the screws as well as the interacting material.

When calculating for maximum pullout you will need to include the applied loading to the assembly and transferred to the scew and the effective max torque pullout applied during installtion of the screw.

https://www.engineersedge.com/hardwa...esign_9998.htm

[PinkertonD]

for fixing the plastic parts together.

I am with JB, trial and error is going to be the best bet here. There are way too many unknowns to use a Table. The depth of engagement will play a big role and most likely not be included in Tables. The hardness of the plastic on the day of fixing will also play a role, hot days cold days. If using air tools, air pressure regulation.

The thing I suggest you look at is strip-ratio. Try for about 1:3 as in tightening torque is about 1/3rd the torque that strips. I have spent probably half my life Engineering in plastics and learned long ago that Tables just don't cut it for this kind of stuff.

[navneet.dhote]

Thank you very much to all of you!

I also think that trial and error method is the best one to deal with a stuff mentioned above.
But if we don’t have the tooling parts available with us (Parts may be in design stage) and we have to give predicted values of tightening torques then I think the only way will be analytical calculations, right?

Well, I am working on analytical calculation side. If I got the correct way to solve an issue then I will share it with you by posting the same.

Have a great time....


Regards,
Navy

[navneet.dhote]

Hi all,

As per Reference given by PinkertonD, I have done the sample analytical calculations for M3 ST screw. I also working on making an excel worksheet which will provide the direct value of tightening torque based on the input given by user.
You please go through the attachment, so that if any mistake in the calculation can be worked out.

Your suggestions are of great use to me.


Thanks, Have a great time
-Navy

[Kelly_Bramble]

Hi all,

As per Reference given by PinkertonD, I have done the sample analytical calculations for M3 ST screw. I also working on making an excel worksheet which will provide the direct value of tightening torque based on the input given by user.
You please go through the attachment, so that if any mistake in the calculation can be worked out.

Your suggestions are of great use to me.


Thanks, Have a great time
-Navy

OK, your numbers look like they where plugged-in alright. Your recommended torque of 1.6-3.2 Nm seems reasonable.

Be aware that I just took a quick look and challenged half of your calculations...

[navneet.dhote]

Hi all,

Here I have come up with excel sheet, which will directly gives the values for tightening torque of self tapping screws.

Also the attached pdf having updated analytical calculations of Required tightening torque of M3 Self tapping screw.


Regards,
Navy

Recommended tightening torque for M3 ST Screw_updated.pdf

[Kelly_Bramble]

Reattached excel workbook for navneet.dhote

[ATTACH]549[/ATTACH