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Screw Torque not achieved due to plating issue

December 20, 2009

I am Guruprasad representing a leading electronic fastener manufacturer in India. Sizes ranging from M1 to M4 (Self Tapping Screws) catering to Mobile phone industry, laptops, computers, & all other electronic applications.

Currently we are supplying screws to one of the Mobile phone manufacturer where we face the spinning issue. Spinning issue is nothing but the screw is rotating continuously without achieving the required torque while tightening in the auto-line.

We did some extensive analysis in the dimension part, heat treatment part & found O.K. Further we analysed plating & found that the plating chemicals plays a major role. The application is Zinc Plating + Black Passivation + Top Coat (Sealant). Out of these we feel Top coat is playing a major role which is assisting in achieving the require torque. Added to that we feel the spinning issue is coming due insufficient friction between the bearing surface (seating surface of bottom face of the screw head portion)of the screw & the steel plate over which the screw is seating. Further the friction between the thread & the mating component (Self tapping done on plastic (ABS + PC)) is not sufficient to achieve the torque. By trying different top coats having good friction factor (CoF) we found the problems are reducing.

We seek expert advise to resolve the issues.

Anandharao Guruprasad
Buyer - Chennai, Tamilnadu, India

Hi, Anandharao. I admit to being confused. The reason one measures the torque when tightening a screw is not to keep it from loosening, but because that specific torque is supposed to correspond to a particular tension in the screw, sufficient to assure that the screw is maintaining good joint tightness, but is not under so much tension that it is likely to snap. Measuring the torque is an indirect way to measure the tension.

So, do you know what the proper tension is? Do you get to the proper tension?

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
December 21, 2009