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May 2005

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Subject:
From:
"Dehoyos, Ramon" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Dehoyos, Ramon
Date:
Fri, 6 May 2005 14:59:09 -0400
Content-Type:
text/plain
Parts/Attachments:
text/plain (280 lines)
        Some soft materials will significantly flow out under torque pressure. With those materials the torque has to be repeated after sometime. With hard metals that factor is negligible. 
        Ramon

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Phil Nutting
Sent: Friday, May 06, 2005 11:49 AM
To: [log in to unmask]
Subject: Re: [TN] Torque


Dave,

This is a good application for the "Square Cone Screw".

For "un-torquing" hardware, we need to keep in mind that the screw is
under tension and there is a certain amount of lubrication on new parts
(not necessarily oil, just clean parts).  For hardware that has been set
for some time there will be some amount of oxidization which will impead
the loosening of the screw.

In one assembly we believe our inability to loosen flat head screws that
were in countersunk holes in FR4 was because the board had been
submerged in oil and the weave had expanded slightly.  I believe it may
also be because the exposed fibers of the FR4 dug into the screw and
acted like a one way lock.

Another thing to remember is that the torque of the screw does not
equate to the clamping pressure on the device.  The Power Devices web
site FAQ suggests that after 20 pounds of clamping force there is little
to be gained in the way of thermal transfer by adding more force.

Phil

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Roach, David
Sent: Friday, May 06, 2005 11:30 AM
To: [log in to unmask]
Subject: Re: [TN] Torque

In our case, we have a few different transistors that all take #4-40
screws.
The transistor manufacturers specify a torque to get the right heat
transfer
to the heat sink.  On some of our transistors, they specify 7 to 9 in-oz
of
torque.  On others, they specify 4 to 6 in-oz.  I don't understand the
reason for the difference, though it probably has something to do with
the
transistor package material and design.  In some cases, we have had
problems
with screws "loosening" when the PCB goes through wave solder, so we
have to
re-torque them afterwards.  One screw in particular that "loosens" goes
through the bottom of the PCB, through the center of a large bridge
rectifier, and into a large heat sink, about an inch cube of aluminum.
I
suspect that the FR4 relaxes slightly, and possibly the bridge rectifier
case as well, so the screw is loose after wave solder.

Generally, there are industry standards for tightening torques for
different
screw sizes.  These probably vary depending on the grade (strength) of
the
fasteners.  There should be a table in the Machinery's Handbook
(http://www.amazon.com/exec/obidos/ASIN/0831127007/qid=1115393316/sr=2-1
/ref
=pd_bbs_b_2_1/002-3342572-5276022) or Mark's Standard Handbook for
Mechanical Engineers
(http://www.amazon.com/exec/obidos/ASIN/0070049971/qid=1115393358/sr=2-1
/ref
=pd_bbs_b_2_1/002-3342572-5276022).  I don't have either of these handy
right now so I can't look up the tables.


Dave Roach
Trane GCC
4833 White Bear Parkway
St. Paul, Minnesota  55110
651-407-4294

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

Folks,
  I appreciate all the input the TORQUE inquiry has generated.
I will be a waiting on the consensus of HIGHR or LOWER torque for
LOSSENINF.

Is that a table for recommended torque give dimensions of the hardware
working with.

Victor,


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Blomberg, Rainer
(FL51)
Sent: Friday, May 06, 2005 9:59 AM
To: [log in to unmask]
Subject: Re: [TN] Torque

Ramon,
You must be working with old rusty bolts to get more torque to remove.
This
does not happen with clean, freshly torqued hardware.  Think of the
screw/thread as two ramps sliding together and you can visualize the
physics
involved.  Loosening involves overcoming friction under the head and
against
the threads.  When loosening, you are unloading the forces holding the
threads together...this is less friction, less torque than driving the
threads together.  I will try your experiment, but my experience is less
torque to loosen.

----Rainer

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dehoyos, Ramon
Sent: Friday, May 06, 2005 10:23 AM
To: [log in to unmask]
Subject: Re: [TN] Torque

        Hi Victor:
                        The untorqueing force will be higher. When the
hardware is torqued, it starts from being loose and as it is  tighten it
gets hot due to friction and the heat makes the molecules from both
surfaces
soft so it slides. Once the hardware is cold, the initial force to break
the
cold bond has to be higher. This is ignoring the oxidation between the
adjacent materials due to time and moisture. Some torque tools work in
either direction. Find one of them and set the tool to a torque in the
midrange and tighten a bolt. Reverse the tool and try to release it and
you
will see that it takes more torque. Make sure that your hand is placed
in
the same area on the tool and the pull is perpendicular to the bar.
        Regards,
        Ramon

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Victor G. Hernandez
Sent: Thursday, May 05, 2005 5:25 PM
To: [log in to unmask]
Subject: [TN] Torque

Fellow TechNetters:

   Can someone share their experience with the topic of TORQUE.   How is
the torque valve of hardware differ from the
value of Un-torqueing that same hardware.   Is the torque valve higher
than the un torque value or vice versa.

   Power FETs can be damaged when the plastic package exhibits a hole
where
the bolt is inserted.
Not the typical TO220 or TO66 plastic style.

Victor,

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