Traditionally a dobsonian telescope has formica-teflon bearings for
moving smoothly in altitude and azimuth. This works well, but for tiny
movements: putting an on object exactly in the midpoint of the field of
view, especially when the object is somewhere near the zenith, this
kind of bearing can still be too sticky. Also problems can occur when
using a poncet mount: when the azimuth bearings have to much friction,
it will be possible that the drive sector slides over the driveroller
when moving the telescope in azimuth. With my 12 inch f/6 dobsonian I
have experienced both problems and this is how I solved them.
The altitude bearingFor altitude bearings I use two 20 cm diameter PVC lids, each sliding over two teflon pads. To make the bearing run smoother I swapped two teflonpads, one on each side of the rockerbox, for rollerskate bearings. I used two of these bearings (against each other, on one shaft) per side. To mount the bearings an eight mm diameter metal shaft is needed. With a drill and a chisel I made holes at the positions from where the teflonpads were removed. The holes must be deep and wide enough for the bearings to be able to move freely. It's important, that the bearings are as far above the wood of the rockerbox as the thickness of the remaining teflonpads. The shafts are mounted in the wood with a good quality, waterproof two component glue. Be sure that the PVC (or other bearing-material) has about equal pressure on all four bearings and is in touch with the whole bearing and not only with one of the edges. After the job, the friction is about half of the friction the bearing has when using teflon exclusively. Instead of rollerskate bearings other ball- or needle bearings can be used of course.The azimuth bearingTo reduce the friction of the azimuth bearings, more then one option has been tried out by atm-ers. One of the most used options is mounting a very thin extra teflon pad around the rotationpoint of the rockerbox. As a result, the pressure on the three other pads is less: the thicker the central pad, the less pressure on the other pads and the less the friction. The central pad must not be too thick, because in that case the rockerbox will lose contact with one or more of the other pads. Some atm-ers make the height of the central pad adjustable, which in turn makes the pressure on the other pads adjustable.This is not the way I solved the friction problem. I didn't use a central pad but swapped two of the three teflonpads for ball bearings. They type of ball bearings I used are called "ball transfer's". They are used on conveyer lines to allow packages to be moved across the line as well as down it. They are also used to roll have furniture to another place of the room instead of breaking one's back lifting it up. But the one's used for this purpose are often of lesser quality. The bearing consists of one big ball, 10-20 mm (I used 15 mm) diameter, in a can filled with small balls (see photo). The can encloses the big ball, except for the top part of course. These 'multi-axis' bearings, if of good quality, run very smooth and can move without problems in every direction. This makes them more usefull for this purpose then rollerskate bearings or other types of bearings with only one axis. Well, the way to mount these bearings is rather straightforward: remove two of the three teflonpads and drill holes of the right size. It's important that the ball is as high above the surface of the baseboard as the remaining teflonpad. After the job is done, the resulting friction will be about one-third of the original friction. I used this method in my (two-sector) poncet mount, with very satisfying results (see photo).
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Email: Jan van Gastel |
