Hi Phil,

I have to disagree with your "more torque to loosen" theory. When we do torque testing the torque meter will read something like 24 in/lbs to torque. Then when we loosen the screw to do another test it only takes about 8 to 10 in/lbs to break it loose. We've also tested this using a recording torque wrench and got the same results.

Now tell me about this "Square Cone Screw"? I went on the internet and could only find "Square Cone Washer". The washer sounds really interesting. We have a problem on occasion of breaking the plastic shoulder washers we use to install TO-220's. I wonder if this washer would help. Have you used them? Does using just a flat washer help?

Thanks,

KennyB

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Phil Nutting
Sent: Friday, May 06, 2005 5:19 AM
To: [log in to unmask]
Subject: Re: [TN] Torque


Oops, sorry for the blank post.  My finger went too fast.

It takes more torque to loosen the screw because you have to overcome the clamping forces that are being applied to the parts in question.

For plastic bodied parts (TO-247, TO-220, etc.) with holes in the bodies you should be using either a Belleville washer and screw or a flat washer and a square-cone screw.  The washer is needed to distribute the load forces of the screw head.  You also need to determine just how much torque is necessary to obtain the desired clamping force.  Keep in mind that if there is a hole in the device for mounting, you can never fully clamp the device for maximum thermal transfer under the device die because the screw cannot go through the middle of the die.

Hope this helps.

Phil

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