At 06:37 AM 5/7/2005, Kristinn Andersen wrote:
>Hi:
>
>
>
>I am working on an extensible vertical consisting of a 6m/18ft outer
>aluminum tube (40mm/1.5" diameter) and an inner aluminum tube that can be
>extended out of it with a motor mechanism. For certain reasons the inner
>tube has to be substantially slimmer than the outer one, probably around
>25mm/1" diameter. So, to make a reasonably tight fit I will make a
>narrowing "collar" at the top of the outer tube, where the inner one will
>emerge upwards. Furthermore, at the bottom of the inner tube I will fasten
>a cylinder that almost fills the outer tube, providing mechanical stability
>there. Sort of like a piston inside the outer tube. Thus, the mechanical
>connections or interfaces between the outer tube and the inner tube are at
>the top of the outer tube and at the bottom of the inner tube. At these key
>points I need to ensure:
>
>
>
>a) Good electrical contact (so that antenna currents can continue from
>the outer tube to the extended inner tube;
>
>b) Minimal mechanical friction as the inner tube is pushed up and
>retracted back down.
>
>
>
>Are there any suggestions on this? What metal should I use that slides with
>low friction against the aluminum tube wall, does not corrode at the contact
>and provides good conductivity?
>
>
What about using something like Delrin (or UHMW Polyethylene (aka "poor
man's teflon")) for the bushings. Inexpensive, easy to machine, etc.
Does it need to make contact during retraction and extension, or only when
fully extended.
Beryllium-Copper finger stock should work for a rubbing contact. Readily
available in a wide variety of configurations.
If you only need contact when fully extended, you could do something like a
pin jack/plug that engages (with a wiping contact) when you're at the
movement limit.
What sort of environment is the aluminum in? Can you have the aluminum
tubing treated so that it has a non-corrodable, conductive finish (i.e.
Iridite, Alodine: Chromium conversion finishes) (so all you have to worry
about is "dirt")
Can you arrange for the tubing to rotate as it
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