Re: [ATM] Public Terrestrial Telescope: Focuser Materials & Lubricants?

["Robert Mohr" <rmohr@tiwcorp.com>]

Hi David,

It's not cheap, but Nylatron is a nylon that is designed to be a bearing. 
Machines real easy. It's nylon mixed with molybdenum Disulfide, and is used 
as a replacement for brass or bronze bushings.  

Bob Mohr


> Hi again Folks:
> 
> Continuing with my public terrestrial telescope project, I'm now in the
> process of deciding what materials and lubricants to use for the focuser
> mechanism. To clarify, I'll machine the main focuser tube from stainless
> steel. If I make it a helical focuser, it will screw into and out-of a large
> threaded female nut. If I make it a slide-focuser or a rack-and-pinion, it
> will slide into and out-of of a female bushing.
> 
> If I make the nut or bushing from brass (a traditional choice), it will
> require a lubricant like a grease, but there's a problem with using
> something that has to be greased because this telescope will be mounted
> permanently outdoors. As you know, on a commercially-made rack-and-pinion
> focuser, there's always a little grease on the exposed parts of the
> rack-gear. In a telescope that's stored indoors or an observatory dome,
> that's o.k. But in an outdoor environment, any exposed-and-greased parts
> will attract dirt and dust that will transfer to the rest of the mechanism
> as the focuser it used, and that's not good. A few months of exposure to the
> elements will render the grease LESS like grease, and more like
> valve-grinding compound.
> 
> A bushing or nut made of a self-lubricating material would be ideal, and
> Delrin (aka acetal) immediately comes to mind. But when I do a Google search
> on the properties of Delrin, I find some references to the effect that it
> shrinks with age. In particular, I found some comments by radio-controlled
> model car enthusiasts complaining that some of the Delrin gears in their
> cars had been press-fit onto aluminum hubs, and that, as the Delrin aged, it
> shrank and split. Assuming that what they say is true, that would eliminate
> Delrin.
> 
> Of course there's Teflon. But Teflon is expensive, it's very soft, and when
> stressed, it permanetly deforms, so I'm guessing that a Teflon bushing would
> gradually loosen with time.
> 
> Then there's HDPE (high density polyethylene). As a "poor man's Teflon",
> HDPE has good self-lubricating properties, and it's tougher than Teflon.
> Might HDPE be the best choice? Might there be something even better" What do
> you guys think. All suggestions will be appreciated.
> 
> Dave Sleeter/Moreno Valley, CA
> 
> 
> 
> 
> 
> 
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Robert Mohr
Director of Technical Services
TIW Technology, Inc
rmohr@tiwcorp.com
See our web site at www.tiwcorp.com
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