Altitude and Azmuth Bearings up

 There are many ways to make the altitude bearing. We want the bearing to be stiff enough that the telescope
won't teeter totter when we change the eyepiece. At the star party we will be guiding the telescope after every
third person so we don't need a bearing lock.  Some people add a screen door spring from the center of the
bearing to the rotating base to increase the friction.

'Better Dobsonian Bearings' by Martin Lewis from Sky and Telescope, October, 2003.

bngalt02.gif (5157 bytes) Here is a design that will work for most Dobsonians. We cut a plywood disk 8 inches in diameter from 3/4 inch plywood and slit it with the saw. From the sheet metal shop we find some scrap sheet metal less than 3/4 inch wide and wrap it around the disk and into the slot. Stainless would be nice but galvanized will do. Careful of that sharp edge on the sheet metal. Dull the sheet metal edge with sandpaper.  For more about this type of bearing see
bngalt03.jpg (53349 bytes) We found this 4 inch plastic pipe cap at the hardware store. It is almost 5 inches on the outside. Two of these would work for the side bearings on a small telescope.
bngalt04.jpg (179724 bytes) We found these 9 inch OD pipe flanges at the junk yard. They worked well for years but eventually the telescope got too heavy for us and we had them machined down. They are still much stronger the necessary.
bngalt01.jpg (68257 bytes) We make the tube box 1/8 inch oversize so we can slip the tube through the box for fine adjustments later. The tube box is roughly a hollow cube made of 3/4 inch plywood. The altitude bearings must be mounted near the eyepiece end of the tube box. If the altitude bearings were centered in the sides of the tube box the box would hit the web of the base structure when the tube is tilted down. In this picture we have placed a bolt on the ground below the bearing center as a balance point. To find the final balance point we have placed the diagonal and eyepiece assembly in the tube.
We slide the tube through the box until the tube is balanced.
If you plan to add a heavy camera later the cradle boards must be taller.
Dsc00012aa.jpg (155066 bytes) We need to know where the tube box will be on the telescope tube so we will know where to mount the cradle boards and how tall to make the base structure. One deck screw in the center tacks the tube box to the tube temporarily. Then we measure from the surface of the bearing to the ground at the mirror end of the tube and add an inch for later adjustments and we have the height of the cradle above the top of the rotating base. To build the base we also need to know the space between the sides of the base.
bngalt05.jpg (128282 bytes) Here we see how the Teflon pads will be screwed or nailed to the cradle boards with tiny finishing nails.  If Teflon is not available there are slip pads made for use under furniture at the hardware store. The Teflon pads overhang on the inside about 1/16 inch to keep the tube box from scraping on the cradle boards. When deciding the distance between the inside of the base structure we add 3/16 inch for clearance.  Since you have already built the tube box add the tube box width to the 3/16 inch clearance plus twice the thickness of the cradle board plywood to find the space between the base structure sides. Now knowing the space between the sides of the structure we can decide the size of the fixed and rotating base and cut them from plywood.
bngalt06.jpg (147619 bytes) We decided we wanted the tube to tip down to horizontal so we "V"ed the web of the structure. After building the structure on the rotating base we can tack the cradle boards to the base structure with deck screws and test the clearances with the rotating base and the web. When we are done we can fill any extra screw holes with latex caulk.

Later we will try to get the friction of the base bearing to be about the same as that of the altitude bearing. 

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bngbtm01.jpg (140667 bytes) The three pads of Teflon are nailed down with finishing nails. The finishing nails are driven down below the Teflon surface slightly. The Teflon pads are on the bottom so that dirt that might collect will fall away from the bearing surface. The taffy colored disk is an antique computer hard disk. In place of this disk you can use plywood that is laminated to Formica. This is available at Home Depot. I greased the pads with a bit of silicon grease. The farther the pads are from the center bolt the greater the friction. The friction of the altitude and the azimuth bearing should be about equal.
bngbtm02.jpg (67834 bytes) Note that the 'center' bolt is not in the center. This is because the stationary base is supposed to have three short legs to form a tripod. Two legs on the right and one on the left. This telescope uses four wheels but if that is not satisfactory you can still slip three bricks under the bottom to lift the wheels off the ground. The wheels make it possible to quickly pull the scope outside for a quick look at a comet on a cold night without missing the TV weather. During assembly the lag bolt is pulled snug then loosened a quarter turn.