Using a flat washer can help immensely.  The square cone screw is in the
SEM screw family.  It has a "conical" shaped permanently attached washer
on the screw.  It is designed for temperature cycled applications, like
heat sinks, where repeated temperature cycles can cause normal hardware
to loosen.  It needs a metal surface to bind against, otherwise it will
cut into the soft plastic case of the device.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bloomquist, Ken
Sent: Friday, May 06, 2005 10:25 AM
To: [log in to unmask]
Subject: Re: [TN] Torque

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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