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May 2005

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Subject:
From:
James TerVeen <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, James TerVeen <[log in to unmask]>
Date:
Fri, 6 May 2005 11:08:23 -0400
Content-Type:
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text/plain (173 lines)
I have to agree with the less for to untorque.   I have never had an
instance where the force to remove was larger than the torque to tighten
to.   In fact  someone tried to come up with a procedure that the force
used to untorque was larger and when we did a large number of experiments
it was always less using the same torque wrench to torque them down. .....




                      "Dehoyos, Ramon"
                      <ramon.dehoyos@NG        To:       [log in to unmask]
                      C.COM>                   cc:
                      Sent by: TechNet         Subject:  Re: [TN] Torque
                      <[log in to unmask]>


                      05/06/05 10:57 AM
                      Please respond to
                      TechNet E-Mail
                      Forum; Please
                      respond to
                      "Dehoyos, Ramon"






        It has to do also with how the wrench is used, the angle of pull,
hand placement on the tool. Yes the materials have also a big significance
on the resistance both ways. But in general it takes more force to
untorque.
        Regards,
        Ramon

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Roach, David
Sent: Friday, May 06, 2005 10:41 AM
To: [log in to unmask]
Subject: Re: [TN] Torque


My limited experience with torque testing has been similar to Ken's.  We
were tightening some #4-40 screws (holding transistors to a heat sink) to 7
to 9 in-lb.  We measured this using a free-standing load cell connected to
a
max-hold digital torque meter, connected to a Mountz hand-operated torque
driver.  Loosening torque was typically 4 to 6 in-lb, but not as consistent
as the tightening torque.  I suspect the ratio between tightening and
loosening torques would be highly dependent on several variables, including
the thread pitch, materials, surface roughness of the contact area, etc.


Dave Roach

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bloomquist, Ken
Sent: Friday, May 06, 2005 9:25 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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