Advice on teflon pad placement for alt bearing

[Bart]

Hello Ladies and Gentlemen

I am armed with some teflon and ebony star for the alt bearing on my lightbridge. Can anyone recommend the optimum placement for the teflon pads please. I'm guessing the alt bearing should rest 'on' the pads as opposed to being wedged between them. Any adviec very welcome, especially pics. They say they speak a thousand words.

Many thanks in advance

Barry

[jimmyjamjoejoe]

I believe the common arrangement is 2 pads per side, spaced at 90 degrees from each other (45 degrees from the vertical).

[Carbon]

How thick are your teflon pads? The reason I'm asking is that once you apply ebony star to your alt bearings that changes the bearing diameter and depending on the thickness of the teflon pads you might have to enlarge the cradle diameter where the felt pads were.

[Bart]

thanks jimmy

Carbon, the teflon pad is 3mm (or 1/8" from scopestuff) and the ebony star strip is about 1mm I suppose. Thanks.

Barry

[Carbon]

After you remove the felt strips you can see if their thicknesses work simply by holding them in their places, as suggested, at 4:30 and 7:30 positions or 90° apart on the cradle. At those positions I'm thinking the spacing should be ok.

[Carbon]

Just to add to my comment above, it'll be a lot easier to do this with the bearing wheels removed from the OTA.

[Carbon]

You know, on second thought, I remember I used 1/8" teflon pads and pebbled formica (ebony star substitute) and they were a bit too thick. I had to cut into the cradle.

Here's a photo from one of the mods I did earlier this year with the modified cradle:

[Moonshane]

90 degrees will provide a mount with tough and I believe jerky movement. the optimum according to Kriege and Berry (and I trust them) is about 68-70 degrees (i.e. about 35 degrees either side of the centre of rotation). I have followed this method with every dob base I have ever made. less than this and mount will not have enough friction, more and it will be stiff. don't forget that you will rarely see a rocker side with a full semi-circular arc. they are usually just a partial semi-circle.

the radius of the arcs on the rocker sides should be the radius of the side bearings (finished) plus the thickness of the teflon pads.35 degrees each side of the centre of rotation).

[Bart]

thank you all very much for your comments, very helpful. Got a wedding this weekend, so take this job on next week. Cheers all

Barry

[Roel]

the optimum according to Kriege and Berry (and I trust them) is about 68-70 degrees (i.e. about 35 degrees either side of the centre of rotation). I have followed this method with every dob base I have ever made. less than this and mount will not have enough friction, more and it will be stiff.

I totally agree. I use this method for my 65 pound heavy 16" Dob and the friction is just right. Tracking is easy, even at 300x, but the scope does not tip backwards whe removing the eyepiece.

[Astrobits]

There are three things that control the ease of moving the Dob in altitude.

1. the relative friction of the two surfaces, (Teflon and Ebony Star or other materials where necessary).

2. the diameter of the altitude bearing and

3. the angle of placement of the two pads.

When I imported some 8" and 10" dobs from Taiwan many years ago the balance between these requirements was so poor that simply removing a small, 20mm fl Plossl eyepiece caused the dob to tip up and putting in a larger, heavier, 40mm fl plossl, eyepiece caused it to drop. It made it impossible to change eyepieces while observing an object. I replaced the Teflon successfully with a block of High Density Polyethylene. The altitude bearing was not, of course, Ebony Star but was a smooth plastic moulding. (The USA dealers in these dobs chose to increase the friction by using a spring to increase the pressure between the pads and altitude bearing.)

Nigel