Calculating torque required to tighten a screw! 1

[var10]

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?

 

[dinjin]

Torque is torque whether a wrench or screw driver. Torque values are normally for tightening similar materials. You do have a special case. You need enough torque to overcome the thread resistance. Experiment is the right approach.
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.

 

[var10]

I have ordered a 1 inch pound driver. The problem is like you said I am trying to tighten aluminium & abs with a stainless steel screw. The ABS plastic is clearance hole with no threads.

Only way is to experiment.

 

[LittleInch]

There was a reply on something similar ( a much higher torque / size) a few days ago which basically said experiment to failure for a number of screws (say 10), take an average torque value at failure and then use 75% of that as your max. Be warned though that these are generally "static" torque values. Once you use a driver or other rotating device, there could be considerable additional torque in this instance coming from the momemtum of the driver when the screw suddenly reaches the end making it difficult to set correctly. So it would probably need to be a two stage process - go for say 50% of your break load to get your screw in and reasonably tight and then go round and tighten them all up gently. Also if you're using rechargable drivers, the amount of charge can make a big difference as well....

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

 

[LittleInch]

See

http://www.eng-tips.com/viewthread.cfm?qid=343505

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Screwman1]

When we are setting up a seating torque that is not clamp load critical (screw is the weakest component in the joint and we won't yield anything else in the joint first), we take a sample (I would like to see 30-40 pieces, all virgin parts for each test) and then run them down to failure using a power tool at the same RPM as the end user, and pull off the peak torque with an in-line torque transducer. We plot out the distribution of the failure torques, find the mean and +- 3SD and suggest 65 - 75% of the mean as the seating torque (watching that the seating torque tolerance doesn't land in -3SD of the failure torques.
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.

 

[Cockroach]

Provide the details, your case will be worked out as an example to this forum. You have a little bit of a specialized case.

Regards,
Cockroach

 

[bigTomHanks]

As you already know the torque is dependent on the weakest material and should be calculated based on that. For cases in which a self-locking feature is used for smaller fasteners, the "torque amount" is left to the individual with sufficient training to know how to tell when something is tight enough without causing damage.

 

[Cockroach]

I have a screw catalog that lists 4.5 inch pounds for the seating torque of a course M2 Button Head cap screw. So you must be in that neighborhood. Search the web for similar or close examples in the same size screw. Bet you find the same answer.

Regards,
Cockroach

 

[RDDV]

This website may help:

http://www.torqueleader.com/torqueleader-support/torque/

(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

 

[var10]

Thanks everyone, especially RDDV. All your comments helped me get to the result few steps closer. Cockroach yes almost close to what you said. I get around 3 inch pounds. But a lot of constants are based on assumptions and the screw is 304 grade ss with an yield strength of 215MPa. I couldn't find information on ABS ABSFR PS PSFR plastic specially the under head friction coeffiecient (used 0.2).

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.