Calculating torque required to tighten a screw!
Calculating torque required to tighten a screw!
(OP)
I am trying to find which preset torque screw driver to use for my application.
I have a M2 pan head screw, retex abs plastic enclosure and aluminium back cover. The alu cover has M2 threaded blind holes and clearance holes in my abs plastic. I have found numerous formulas in google to calculate the torque required to tighten a bolt using a torque wrench.
No success with how to calculate the torque for a screw using a torque screwdriver. Some formulas don't take into account the material the screw head will be sitting on (in my case a s.s screw rests on ABS). So how will i know what is the right amount of torque I should apply without damaging the plastic?
Any formulas ? Any suggestions? or is this going to be just trial and error?
I have a M2 pan head screw, retex abs plastic enclosure and aluminium back cover. The alu cover has M2 threaded blind holes and clearance holes in my abs plastic. I have found numerous formulas in google to calculate the torque required to tighten a bolt using a torque wrench.
No success with how to calculate the torque for a screw using a torque screwdriver. Some formulas don't take into account the material the screw head will be sitting on (in my case a s.s screw rests on ABS). So how will i know what is the right amount of torque I should apply without damaging the plastic?
Any formulas ? Any suggestions? or is this going to be just trial and error?





RE: Calculating torque required to tighten a screw!
A rough guess would be 2.5 inch pounds or less. I do not know if they make a plastic driver insert that would break off when that torque value is achieved.
If the head of the screw is slotted, you might be able to tighten it with plastic strips as thick as the slot and have them break off when the torque is reached to make the screw seat. Interesting problem.
RE: Calculating torque required to tighten a screw!
Only way is to experiment.
RE: Calculating torque required to tighten a screw!
Not really trial and erro, more experiments to gather data.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Calculating torque required to tighten a screw!
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Calculating torque required to tighten a screw!
This works out fine for the vast majority of commercial joints. You can do it with a torque wrench, but as was mentioned earlier, you run into the static torque, dynamic torque issue.
RE: Calculating torque required to tighten a screw!
Regards,
Cockroach
RE: Calculating torque required to tighten a screw!
RE: Calculating torque required to tighten a screw!
Regards,
Cockroach
RE: Calculating torque required to tighten a screw!
(see PDFs on the page top right)
You will need to find out the "Under-head coefficient of friction" between your M2 screw and the ABS, as the tables provided are only for metals. If you don't find a number then you should look on the metal-metal friction coefficients tables and chose one that is the worst case. If you implement a formula in excel then you can play with it and see how it affects your final torque.
There is a part that says: "Normally a figure of 90% of the yield strength is acceptable but may be varied to suit the application." to determine the max tensile stress (clamping force) in the screw. If you plan to screw/unscrew very often then you should use max 85% of the screw yield strength.
If you are using M2 I guess you are not building a structural part, in any case plastics can soften with temperature or deform with the clamping force loosening the screw, so it may help using a washer.
Cheers,
Roserio
RE: Calculating torque required to tighten a screw!
No its not a structural part. They are something similar to PDA's or old big bulk nokia phones. I can't get a washer in there because it has a 4.5mm spotface and not many companies in Australia have a 4mm M2 flat, don't have the time to get it from the states.