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 ensu

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?

73 - Kris, TF3KX (http://www.simnet.is/net/tf3kx)

Linear bearings
Check out E Bay
Art
wrote in message
oups.com...
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 ensu

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?

73 - Kris, TF3KX (http://www.simnet.is/net/tf3kx)

You might try using "NoAlox" or "Penetrox" - or some other namebrand
compound between the tubes. It contains particles for 'cutting-through' the
aluminum oxide insulating film that would normally form - and - would
inhibit the oxidation of the aluminum. Its also a grease - encourages
smooth action. (put your gloves on.)
available at in the electrical depts at home centers like lowes - or at
electrical supply companies.

Years ago I built a hand operated telescoping vertical.

No HF current inside the tubes therefore no need for electrical contact at
the bottom of the inner tube: a plastic sleeve/ring is ok, adjust shape and
size for mechanical stability and limited friction, water is a good
lubricant for plastic...so let rain enter the bottom tube... and drain off .

A good, low friction and corrosion resistant contact at top of the outer
tube can be made of pieces of graphite ( like electric motor brushes )
pressed by stainless springs. You don't need to have a 360° contact, 4
pieces equally spaced would be OK.

If you use regular commercial aluminum you can't avoid corrosion as friction
when moving the tubes will remove any surface protective treatment. You need
a special "professional" alloy: check with a specialist. This is specially
critical for the bottom tube as you cannot clean the inside.

Also check that your bottom tube is an "extruded" tube with a perfectly
smooth internal surface without ridge.

Hope that can help, 73

F8ND

a écrit dans le message de news:
...
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 ensu

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?

73 - Kris, TF3KX (http://www.simnet.is/net/tf3kx)