Criteria:
Will be used to lubricate aluminum tube in plastic channel. It will be inserted/removed repeatedly.
Can’t be too messy…e.g. graphite is right out.
Shouldn’t attract grit like gravel dust. In fact, gravel dust will probably be the main operating environment.
Easy to clean off and re-apply when necessary.
Preferably, water/moisture resistant.
Oh, rats. From the thread title I was expecting something… well, you’re talking about inserting and removing something repeatedly, but it’s just aluminium tubing. Robot sex? Is this on their behalf, or are you a robot yourself? I always assumed they just put their hands together, or shared USB sticks, but I’m not a robot, I don’t know how they, you know.
Silicone spray.
Nothing will be all that great in a gravel dust environment. Graphite may be your best bet. I don’t know why you consider that messy in a gravel dust environment. Just apply some, wipe off the excess. Anything that’s not dry will collect gravel dust and do the opposite of lubricating. There are some graphite film lubricants which are graphite embedded in a binder of some kind. Look on this site of Dow lubricantsfor something suitable. I think some form of Molykote will be intended for abrasive environments like that.
A dry bicycle chain lube is waxy and designed for dusty applications.
I seem to recall some kind of silicon-coated tape that was very slippery. We used it on the sealer elements for making plastic bags, so it was also very heat-tolerant. Can you get enough clearance for 50 mil tape?
I ended up with a ‘dry’ Teflon spray from Home Depot: http://www.blastergroup.com.au/dry-teflon-lube.html
Worked great, for the record. It’s pretty cheap, too, at about $4.50 a can, so it should last a while.
Alternately, can you put some sort of flexible housing (rubber or whatever) around the tube-channel assembly to protect it against the dust?
Teflon spray and install a felt wiper.