I suspected that was the case, but never verified it. That's why I put the
word "loosens" in quotes.
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 Graham Collins
Sent: Friday, May 06, 2005 10:51 AM
To: [log in to unmask]
Subject: Re: [TN] Torque
Hi Dave
Your screws aren't loosening, but your material thickness is
changing... when the board goes over the wave the material will subside
away from the pressure point, in this case the screw head. This is
because the TG of the PWB material is less than the wave temperature.
Thus in effect the screw becomes loose.
At a previous employer we had many complaints about hardware falling
off into the in-circuit tester before this was figured out.
We hand tighten before wave and then torque all hardware after wave
here, if the hardware will be exposed. Another plus of doing partial
wave...
regards,
Graham Collins
Process Engineer,
L-3 Communications Electronic Systems Inc.
Halifax
(902) 873-2000 ext 6215
>>> [log in to unmask] 05/06/05 12:30PM >>>
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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