There are several possible explanations. Vibration and shock can cause the
clamping screw to rotate and thus loosen
Thermal cycling and differential expansion between the wire and clamp is
another possible cause. If the wire expands faster than the clamp as the
temperature goes up, the clamping force may increase to the point where the
wire is deformed. When the connection cools down again, the clamping force
is reduced. The clamping screw would then appear to have loosened, even
though it had not rotated.
Cold flow is another possible cause. I have had personal experience here. I
used ordinary power "zip cord" for connections to a speaker that had steel
binding posts. To prevent the fine wires of the zip cord from splaying out
as I tightened the binding post, I soldered the wires together and then put
the soldered end into the binding post. A few months later, the speaker
became intermittent. I retightened the binding post, but again a few months
later the problem returned. The solder was cold flowing under the pressure
of the binding post. I then put a ring terminal on the end of the wire, and
had no further problems.
Seems to me there are several keys to a successful design: 1. Make the
clamp from materials with a thermal coefficient of expansion that closely
matches the wire. 2. Design the thread so that there is significant
friction between it and the screw so that the screw will not rotate under
shock/vibration/thermal cycling. 3. Design the clamp to have a bit of
spring action to maintain clamping force when there are thermal variations.
4. Keep the clamping area large enough so that the wire is not near the
limit of deformation.
Don Borowski
Schweitzer Engineering Labs
Pullman, WA
USA
"Brewster, Alan"
<Alan.Brewster@NO
VELLUS.COM> To
Sent by: [email protected]
[email protected] cc
Subject
08/15/2006 02:22 Failure of electrical terminations
PM
Greetings Listers,
I was looking at EN60204-1, Clause 14.1.1, ‘All connections…shall be
secured against accidental loosening.’, and came to wonder if there are any
published scientific studies on the causes of loosening, in particular of
screwed terminals. It occurred to me that throughout my career I have seen
plenty of terminals that have backed off. Some expensive terminal blocks
failed and some that were cheap ones, worked. I have heard a number of
theories, some even plausible, but have never seen a good, solid, written,
technical explanation as to why it is possible for clamping force to
change.
Can anyone out there help me with an answer to this question?
Regards,
Alan Brewster, MIET
Staff Engineer,
Product Safety and Environmental Department
Novellus Systems, Inc.
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