Re: Geared Dobsonian question

Bob Luffel (bobl@gr.hp.com)
Mon, 6 Mar 95 17:46:38 MST

> For teflon, the coefficient of sliding friction is very near the static
> coefficient of friction. Most materials exibhit a sliding coefficient of
> friction which is much less than the static coefficient of friction.
>Thus with
> ordinary materials you must push hard to get it to move, and once it starts
> moving, it continues to slide using a much smaller force. So if you push with
> a constant force, after it starts to move it quickly accerates because you are
> pushing too hard. Thus it is hard to make very small adjustments by sliding
> with ordinary materials (a very bad condition for a telescope mount). Teflon
> on textured plastic laminates, on the other hand, has a sliding coefficient of
> friction very near its static coefficient of friction. So this sudden jerk of
> acceleration is much reduced. The sliding coefficient of friction of teflon on
> textured plastic laminates also increases with velocity. For ordinary
> materials this is nearly constant with velocity in the ranges under
> consideration. This increase in the sliding coefficient of frictin creates a
> nice self-damping effect making the motion very smooth.
>
> Nylon does not exhibit these effects and should not be used.

Another material that comes close to teflon in these regards (static vs dynamic coeff of friction) is Delrin AF (has some teflon in it). It may be more bondable, too. Still, Teflon is the king.

>
> Once you have rollers, there are problems (which may not be insurmountable) in
> using one of the rollers for the drive in slow motion control system.
>
> 1, the rollers (and pads) are connected to the ground board, so the
>location of
> the slow motion control would probably be fixed, with respect to the ground
> board, which would be inconvient when pointing the telescope in different
> directions. There are structural and practical reasons why the rollers (and
> pads) are connected to the ground board, and not the rocker box, so this
>is not
> simple to change. It wouldn't be too hard to do this for a small, light
> telescope, but for a big, heavy telescope where any ground board flex is
> significant and adding any height to the base is a major problem, then it is
> difficult.
>

Another advantage of a solid bearing (teflon pad) is more immunity to debris (dirt) with regards to smoothness (a roller could roll over the debris, giving a gritty feel to the motion) whereas the teflon pads tend to wipe the debris off the leading edge and bear over an area contact versus a rollers line contact. Also, you would want to use sealed bearings to keep debris out of the bearings (if you used ball bearings versus a solid bushing type). A teflon pad will last virtually forever (unless they come off).

And, finally, the solid teflon pads (located above the ground board feet) offer a better coupling of the scope loads versus a roller/shaft. A roller, of course, can work - but the line contact and mounting considerations put it at a disadvantage. A teflon pad is such a good, elegant solution.

Bob