Big dob - bearings for az motion

[mdstuart]

My big dob az motion is relatively stiff so I thought, despite prevailing wisdom to try a lazy susan bearing rather than Teflon on laminate.

So here is the very cheap bearing I used.

Before, using three rubber bands attached to a corner of the mirror box I had to stretch them 6 inches to get the box to turn. Now it's one inch!

It might need a bit of braking now but it feels smooth but I have yet to check it at high power when observing though.

Still need to work on the alt motion.

Mark 

Edited May 26, 2021 by mdstuart

[Piero]

My dobson has 4 square teflon pads sliding under a sheet of FRP.

The 4th teflon really helps (see pivot bolt in the photo). Each teflon was sanded at the edge. The three teflon pads above the feet have a vertex pointed towards the central pivot. This also reduces friction.

Friction can also be reduced by reducing the size of the pads as I'm sure you already know. It might also be worth trying to slide a solid soap bar on the laminate where the pads slide. This works well for reducing friction on the altitude bearings.

 

Edited May 26, 2021 by Piero

[Piero]

I just realised that your media is actually a video and not a photo.

It seems to me that your telescope has insufficient azimuth friction. When assembled, a modest breeze could be sufficient to rotate it horizontally. Of course, it is down to one's tastes, but I'm not sure that is practical..

[mdstuart]

Piero you may be right. I will try it out and may add some braking. 

Ultimately I can go back to ptfe maybe with soap etc as you suggest.

It's interesting to me to try things like this. Plus it might be interesting to others as well.

Mark

[Rusted]

The problem with non-Teflon bearings is balancing static V dynamic friction.
A great deal of research was done back in the 80s to achieve the correct "buttery" movement.

The central PTFE pad probably helps to overcome cupping or convexity of the bearing board.
Being so close to the pivot its contribution to friction levels must be very low indeed.
Yet it can help  to unload the active PTFE pads depending on bearing board curvature and the central pad's own thickness.

If the bearing board becomes convex or concave [or is simply warped] then the pads would [arguably] only contact on one narrow edge.
It being all but impossible to achieve perfect flatness in typical Dobsonian building materials.

A fisherman's spring balance is a handy tool to check friction levels measured at the eyepiece.
Outside the overhead, Dobsonian "Black Hole" the friction levels should remain stable in all planes.

 

[planetman83]

Try to add a piece of teflon at the center and tighten the central bolt really tight.

[mdstuart]

So much advice, thank you.

I will report back.

Mark

[faulksy]

as said mark. 4th bearing pad at pivot is a must. as a bearing surface. i used hammerite paint on all ptfe contact points. the dimmpled effect works brilliantly

[Peter Drew]

• I gave up on PTFE as a primary bearing for large Dobsonians years ago after my 30" eventually needed a rugby scrum to turn it in azimuth.  I now use 6  stainless and shielded ballrace bearings set vertically running against a stainless steel disc.  3 of the bearings have adjustable PTFE pressure pads to control the ease of movement, a similar principle is applied to the altitude axis.  The ability to adjust the motion makes it possible to dial in the movement to make the effort identical in both axes, a requirement for good Dobsonian operation.   🙂

Edited May 27, 2021 by Peter Drew
typo

[mdstuart]

Peter - Do you have a pic of how you used the PTFE pads to control the ease of movement with your bearings approach.

Mine now still needs three rubber bands stretched by once inch on the corner of the mirror box to start motion ( not sure how many newtons per square meter that is :)) so I might not need control but I would like to see how you did it.

Mark

 

 

[mdstuart]

So I now have anodised aluminium strips on my alt bearings. Thank you to Stephen my neighbour for the cutting and drilling. It's running on sash window wheels and it is better in terms of motion than before.

Will test whilst observing and report back.