Ed, 

Is the torque spec correct for the fastener being used?   Years ago had an issue with the twelve 1/4-20 cap screws on a group transmitter covers, the cover had two 'O' rings and was torqued to a ft-lb spec when the correct spec was in in-lb, more than 3/4 of the bolts had necked to less than 2/3 their original diameter and in some instances snapped off, the cover screws when discovered were finger tight, we thought the problem was wrong material and elongation, so they changed bolts and then snapped them off, someone with a slight mechanical background picked it up. 

Pat Goodyear 
Control Technician
Diablo Canyon Power Plant 
         

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ed Popielarski
Sent: Wednesday, May 15, 2013 8:57 AM
To: [log in to unmask]
Subject: [TN] Elastic/plastic deformation of thermoplastic transistor housing

I have run across an interesting phenomenon and was hoping someone has done exhaustive research on it in Technet-ville.

I have a TO-3PF style package (plastic power transistor with plastic tab) that has a specific torque requirement and thread-locker material (microsphere type). When the subassembly is first assembled in a fixture to align the transistors to the heatsink and hold the nut in rotational place then torqued, they are verifiably tightened correctly. Once they sit for a day or so, they are no longer tight and the screw/nut spins in situ together. To my amateur eye, it seems the thermoplastic is exhibiting elastic deformation at this point. If re-torqued again, it stays put, which seems to possibly have crossed the threshold from elastic deformation ("bending" the atomic structure) to plastic deformation ("shearing" the atomic structure).

Is there any insight I can harvest before my 1:00 meeting today?

Ed Popielarski
Engineering Manager

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