The Long Radius Rack and Pinion Sector Drive in Right Ascension
No Machining whatsoever is required to build this Right Ascension Drive


Each photograph can be enlarged with a regular left click anywhere on the picture; then one may save it to one’s own local directory (save to your hard drive) with a right click on the enlarged picture to call up a menu and selecting “save picture as” from this menu. Use the dialog box to select where you will keep the picture and click “save”. (This works for Windows users; I am not familiar with other systems, sorry)


This picture is a view from the north showing the long radius sector installed on the polar axle of the mounting’s latitude adjustable polar axle rocker box. The long narrow slot in the middle of the sector allows it to flex so that one can clamp it tighter or looser, as is necessary. The rectangular assembly just adjacent the rim of the sector, below it, is the panel holding the mechanical parts of the driving mechanism proper, that transmits motion to the sector itself.







The next photograph (above, right) shows the entire mounting more from the side, showing the general overall design of the mounting proper, with the driving sector and its mechanism mounted on the north face of the polar axle rocker box (bearing cage). This polar axle rocker/bearing cage can be tilted over a range of approximately 35 degrees, providing adjustment for use across a wide range of latitudes. The design of the mounting was configured to allow the front, or north face of the P.A. rocker assembly to be quite long (tall) from top to bottom, to allow a generous radius for the sector. The sector pictured here has a radius of 17.58”. The mounting can accommodate a sector with a radius as long as 22.85” The longer the radius of the sector, the smoother and more accurate the driving action will be.

These next four photographs (first two photographs, here- please remember that you can enlarge and save any of these pictures) show the action of the driving mechanism in operation. A threaded rod drives a nut (held captive from rotating) along its length, and pulls an assembly attached to it that includes an angle-aluminum “runway”, or “rack” that is held in contact with the rim of the sector, driving it with its motion through contact with the sector. Although not pictured, liners to enhance friction between the aluminum angle runway and the rim of the sector will be used to increase the grip of the runway on the sector. Even without these high friction linings, the runway still easily drives the sector without slipping (in tests). In these first two illustrations (a wide view and then a close-up), the sector is set all the way over to its westernmost position limit for the beginning of an approximately 140 minute run of driving in right ascension.

In these next two pictures, the sector has passed through the midpoint of its total travel capacity, and then to its furthest eastern limit, for a total run of about 140 minutes. The sector and drive assembly will now have to be reset all the way back over to the right (west) end to make another 140 minute run. Please note that the rim of the sector stays centered above the point between the runway's two primary supporting roller bearings for taking its downward thrust here as the runway rolls across them.

Here is a close-up of the driving assembly. The threaded driving rod can be seen below the rectangular assembly with its aluminum angle “runway” that engages the rim of the sector. The clear plastic cross-piece that holds the runway, and its outrigger support below, is not actually yet connected to the nut. No machining is required to mount any of the six bearings on this panel that carry the threaded rod and support the runway in its travel. We'll show you how this is done a little further on.


For this picture we have removed the runway assembly to reveal the arrangement of the bearings that take the thrust of the sector when it is riding against the runway. The runway is always in contact with at least three of the four bearings throughout its approximately 140 minute tracking run. The threaded rod that carries the long nut is a section of ordinary cadmium plated hardware store "all thread" rod. For this drive I chose one-half inch diameter 13 thread rod, which yields, for a one RPM output motor, a radius of 17.58" for the sector. A section of one quarter inch diameter 20 thread rod will give a longer uninterrupted run in Right Ascension and will yield a shorter radius sector (11.427"). I opted for the greater traction and tracking accuracy of a longer sector. The dimensional errors (manufacturing tolerances) for threaded rod remain about the same regardless of what size threaded rod one uses, so as the radius goes up, the error (periodic or overall) caused by dimensional inaccuracy is reduced. There is room on the north side, front face of this mounting for driving with a one-half inch diameter 10 thread (Acme thread) rod. I have some of this special order rod on hand and may try a longer sector later; however, the run in Right Ascension will of course be shorter.

This photograph shows one of the number 1621-DCSR12 precision ground sealed ball bearings that carry the threaded rod on one end (east end) with the one of the runway's end support roller bearings just adjacent to it. The retaining rings for the bearings carrying the threaded rod are not yet in place. No machining was required to mount these small bearings, rather, their mounting seats were constructed out of small wooden blocks with the correct sized “U” shape cut in them, and then clad with appropriate sized pieces of aluminum angle to retain them (take thrust). Excessive clearance in the mountings can be adjusted for by shimming. Their removable retaining caps have not yet been fabricated.

Here is a view of the west end of the driving assembly showing the threaded rod slung in its carrier bearing on this end. Retaining rings (nuts) are not yet installed. As you will note, this bearing is just adjacent the runway's eastern most roller bearing, exactly as on the west end. The location of everything is bisymmetrical on the mounting panel.


In this close-up of the east end of the panel, we have removed both the threaded rod's carrier bearing (and the threaded rod) and the runway's roller bearing also, to show you how the bearings are mounted. The threaded rod's carrier bearing is mounted in a little flanged seat made from one half inch plywood, with a carefully cut "U" shape to match the bearing's diameter closely. This little plywood "U" is then clad with appropriately sized pieces of thin angle aluminum for retaining flanges. The runway's small roller bearing is mounted on a little hub cut from a section of appropriately sized wooden dowell rod. In this case the little hub required shimming up with an appropriate number of turns of smooth scotch tape for a perfect fit. The little "O" ring around it is made out of illustration board. It is to stand the bearing out slightly from the panel so that the outside race will not rub against the surface of the panel. The little wooden hub which carries this bearing (with its metric sized inside race inside diameter) is both glued to the panel, and then secured with a sheet metal screw.. Using sheet metal screws to secure wooden parts together is both more convenient and results in a much stronger fastening. Only one diameter hole needs to be drilled, and the screw does not taper. Sheet metal screws have threads all the way up to the head, so the threads just under the hread add holding power to the fastening, whereas wood screws rely mostly on just the head of the screw for holding the topmost piece fastened. One can use really long ones for enormous holding power (for larger jobs, naturally, than the one shown here). Always glue the components in place and wait for them to set before drilling and using sheet metal screws. A friend of mine tried to use them to hold parts in place when he glued them together; this was a disaster for him, as the threads run the entire length of a sheet metal screw- the pieces were forced apart, rather than held together. Of course one may drill the hole in the upper piece to be fastened to the major diameter of the screw, and use the screw like a wood screw in this fashion- but the extra holding power of the topmost threads is lost this way (but there are some occasions where this is expedient). I just do not use "wood screws"; their design is all wrong in my opinion.

Here is a view showing the western half of the drive panel from a low angle with its threaded rod and bearings installed to help convey the spatial arrangement of its components. Please note the extra long "connector" hex nut, the kind that is sold with threaded rod, that will be used to drive the runway assembly. The longer the nut, the less periodic error will remain in the system. If one has space, one may even gang two of these nuts together to reduce error even more. One of our fellow amateur telescope makers and list members has shown me his advanced technique to reduce periodic error in a threaded rod/long nut drive to nearly zero.

Here is a close-up of the two central, primary support roller bearings for the sector's angle aluminum runway. Any old salvage bearings one may have in his or her collection of odd bearings may be used here, if they are at least approximately the right size for this application- it is not necessary to narrowly specify a particular bearing to use. One need not have any concern over what size the inside race's inside diameter is- we simply tailor the little mounting hub to fit the odd lot (odd size) bearing, whatever that bearing's dimensions might be.

In this view we have slipped the bearings off to show the little wooden hubs that they are mounted on. If, as in my example, the inside races of these bearings are larger than any available wooden dowell rod, one simply takes a careful measure of the inside diameter of the inside race, and sets a drafting compass to its radius, and then draws a circle on a pieice of plywood and carefully cuts it out with a sabre saw, which after a little sanding (or a few rounds of tape wrap if it comes out a little undersized) to fit becomes the little hub the bearing is to be mounted on. The little hub is first glued into place, and after the glue (or epoxy- I use slow setting epoxy so that I can position the little hub precisely so that the bearing's rim comes to exactly where it is needed to share support of the runway with the other bearing) has set one then drills the holes for the two sheet metal screws that are required to add support for the considerable downward thrust of the runway on these bearings. Please note, again, that little "O" rings, cut out from ordinary poster board with a hobby knife, are in place at the base of these little hubs to make the bearings stand out slightly from the panel so that their outside races do not rub against it.

Making a little wooden hub to mount the odd lot or odd sized bearing on which is to be used as a roller is a simple and easy way to avoid the expense of machining to mount a bearing (to be used as a roller). A close inspection of the photographs of my large mounting in this and the other articles will reveal that its wheels are actually precision ground ball bearings, and that they are mounted exactly as the roller bearings for the sector drive's angle aluminum runway. They are just larger, and required larger diameter and thicker hubs. These larger hubs (for the mounting's "wheels", bearings) are made of two circular thicknesses of plywood laminated together, and glued (epoxied) in place and then further secured with larger sheet metal screws, one above the other, just as for the roller bearings for the drive's runway.

I have not published a tedious list of dimensions or specifications, etc. in this article, because I understand that we amateur telescope makers are artists as well as technicians- and if we've ever undertaken to construct an instrument from scratch, we certainly will, all of us, have the necessary conceptual skills to work out the precise geometry of our own version of a thing, such as my new Long Radius Rack and Pinion Sector Drive. As many of you are aware, I have a certain fascination with the sector drive, and actually owned a United States design patent at one time for a version of a sector drive patterned after the one in the article I wrote as published in Sky and Telecope many years ago, "Experiments with a Toothless Sector" (November, 1987 issue). I think my United States patent number for this patent was 4,904,071 (I think- if memory serves me correctly). I prefer to share ideas now, instead of seeking to hoard them with patents.

The principle advantage of a sector is of course that one may have one with a very long radius for accuracy and stability in a much more compact space than for a complete disc or gear of the same radius. I invite your further inquires- please email me at your convenience


© 2000 David Anthony Harbour

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