Bearings for ATMs.


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This page is a catalogue of bearings that I wish to give away to my fellow amateur telescope makers.

Before we get to the bearings, I would like to show some who may not be aware of it a rather elegant way to mount a large polar axle for a fork or yoke mounting without having to sleeve the axle inside two large, very expensive bearings. With this alternative method, one may arrange five small, inexpensive bearings for the polar axle to roll on. This method illustrates a single application of the rules of "kinematics" in instrument design.


Here is a side view of the undercarriage of my large fork mounting with its latitude adjustable polar axle bearing cage rocker box. In this picture we see the 4.5" O.D. polar axle protruding from the unit. The steel axle was turned and faced from ordinary plumbing "black pipe", and is not sleeved inside of any large bearings. Rather, it is carried on four #1638DC English measure precision ground sealed ball bearings acting as rollers, with a fifth and different sized bearing taking thrust on its rim on the south end.

In this close up of the north face of the mounting, we have removed the polar axle to show the two #1638DC bearings that support it underneath here on the north face of the polar axle latitude rocker bearing cage. These two little bearings are arranged underneath the axle and located on lines radiating from its centerline at an included angle of 90 degrees (i.e., 45 degrees to either side).


In this view we are looking straight down the centerline of the polar axle's bearing cage, and if we look closely, we can see the hex cap bolts that carry two more #1638DC ball bearings at the lower end of this cage, and just barely make out the rims of the bearings protruding from their mounts. Note that the two bearings are carried above the polar axle down here on the south end- of course: the axle is levered up against them by its payload. Support is required above the axle on the south end, and below it on the north end. The bearings are mounted at the same included angle of 90 degrees here on the south end, only above instead of below the polar axle. No machining is required to mount these English measure ball bearings- their inside race inside diameter is .75", and one simply carries them on four hex cap bolts of the same diameter.

We can also see the rim of another bearing protruding into a rectangular cutout on the south end of the bearing cage. It is mounted at ninety degrees to the south face of the cage, and engages the rim of the polar axle to take the southwards thrust of the polar axle. The next photographs show how this bearing is mounted.






Almost any old salvage bearing can be used to carry the thrust of the polar axle. The pictures make it clear how the thrust bearing is mounted- one may cut out and laminate together, out of plywood, a circular mounting arbor for the bearing and mount it on a small, strongly buttressed shelf, and attach this shelf to the south end of the polar axle rocker in a location to take the rim of the polar axle against the bearing's rim.


The Bearing Catalogue.


A Note About Requests:

I would like each of you who might have a need for a bearing or two to be as sure as you can be that you will use what you request, as to request and receive bearings for something only very tentatively planned might prevent another who is keen to forge ahead with a project from having them. I would just like for all of you to be pretty sure that you are going to use what you request. I cannot think of any more equitable way of distributing them except on the basis of order of request (excepting in the case of matched sets- if a fellow ATM has a need for a matched set, I will prefer to award the matched set to that person rather than break it up). In the case of multiple requests for a matched set, I think I should apply the same rule as for single bearings and award the set to the first in line with a request. I will also consider calling two different bearings a "matched set" for the purposes of carrying a polar axle sleeved inside them conventionally, if an amateur requires two bearings for that purpose. I will suggest these combinations in the text of the catalogue.

After confirming your requests, I will package your bearing(s) and have it (them) weighed for shipping and notify you what the charges will be, and you can send me a grocery store money order. "Collect" doesn't work very well for me, because UPS tells me that I still have to pre-pay, and then they reimburse me. I know you guys have to go to the grocery store pretty regularly, and a grocer's money order is usually only around 25 cents or so.

Without further ado, then, here is the illustrated catalogue of bearings with a description and suggested best uses for each bearing (or set of bearings), and pertinent dimensions. Please note that the dimensions are not adequately precise for critical machining specifications. My cheapie calipers are not too trustworthy. And, in any case, I can show any one of you how to mount any of these bearings without recourse to any machining whatsoever, including an application requiring an axle to be sleeved inside of any of these bearings.

In my nomenclature for the dimensions of the bearings, "OD" will mean the outside diameter of the outside race of the bearing. "ID" will mean the inside diameter of the inside race of the bearing. "L" will mean the length of the bearing along its longitudinal axis. Where the inside race is longer than the outside race, that dimension will be designated "L2".



Catalogue: #1. OD=7.09" ID=3.346" L=1.613"

This is the largest bearing I have available to give away. I have a larger tapered roller bearing, but it is in use in a large "lazy Susan" platform I built to carry my swing away computer table. I cannot sit erect at a regular workstation and so made a special table that lets me use a more comfortable chair (when I can sit at all). The big ball bearing pictured here is tapered to take thrust. This bearing, brand name "SKF", is in excellent condition and I do not know why it was replaced. It runs true and smooth. I believe it came out of a natural gas pumping unit, a large piston compressor to force natural gas through gas lines. The best use of this single bearing in the collection, along with the next bearing listed in this catalogue, would be for a conventionally sleeved polar axle for an equatorial mounting. I would award this bearing, and the next listed one, to an individual who is planning to build a good sized mounting and would like to mount a polar axle this way, instead of by the method of "kinematics", as I've described above. Failing to receive a request for both bearings for a purpose such as this from an individual, these bearings will be available separately.


Catalogue: #2: OD=5.16" ID=2.952" L=.982"

I can't believe I missed photographing this bearing by itself. It is the next largest bearing I have available (except for the set of four bearings with split inner races, which have a very slightly larger inside race inside diameter). However, click on the picture of the entire collection to link to the full screen JPEG and you can see it very nearly centered on the table, near the front of the table. It is the fourth one from the left, in front (the very shiny one). This large ball bearing is not tapered to take thrust, but will, of course, accept some thrust without any serious compromise of wear or function. It would be the ideal choice for mounting a polar axle conventionally, with the cat. #1 bearing, (above). In the case of using these two bearings this way, contrary to conventional practice, I would recommend using the larger bearing on the north end of the polar axle, as it has a wider inside diameter for its inside race than this bearing. This fine bearing is in excellent shape, is tight, and runs smooth as silk. This is a Harvester International bearing. If no one wants these as a pair, they will be available separately.






Catalogue #3: OD=4.328" ID=2.356" L=.863", and Catalogue #4: OD=4.71" ID=3.063" L=. 688" L2=.965"

The Cat. #3 bearing is a fine Fafnir ball bearing in excellent condition, and is tapered to take thrust. The second unit, catalogue #4 bearing, is a tapered roller bearing, designed to take thrust also, and will disassemble from the front of the taper (from the large end of the taper) as is common with tapered roller bearings. One will not want to use a bearing such as this one for an equatorial mounting for lower latitudes, as the bearing's inside race will want to run downhill (towards the north) out of the tapered outer race. This unit would be okay for a mounting for higher latitudes. These are not recommended as a pair. Those ATMS working in lower latitudes should avoid tapered bearings for thrust, as they will usually be working in reverse, i.e., their taper will be greater than one's latitude, and the bearing will want to run downhill towards the front, out of its taper, defeating its design purpose. The two pictures to the right depict the front and back side of the catalogue #3 Fafnir ball bearing, to show that it also is tapered for thrust. Good design for carrying a polar axle sleeved inside two bearings is for only one of them to provide for thrust, and one for lateral support. Both of these nice medium sized bearings are in top shape and run smooth and true.

Late Addition, Special Note: With an especially clear head today, I see that I made a mistake about the left-hand photograph for this catalogue entry (above) that shows the two bearings with the tapered roller bearing on the right. The ball bearing on the left in this photograph is actually the bearing described in catalogue entry #2, the Harvester International bearing; it appears that I did photograph that ball bearing at least almost by itself, after all. So the only mistake is in the left hand photograph. The two right hand photographs of the Fafnir ball bearing and the dimensions given for it are correct. Apologies to all for my wooden headedness. My only excuse is that I am over fifty.


Catalogue #7: OD=5.118" ID=2.95" L= .983" L2=1.153"

This is a matched set of four fine, rather large ball bearings. These bearings are peculiar in that they have a split inner race for disassembly/assembly. The second photograph shows one of them broken down into its four components. Naturally enough, in any application in which they will be used, their two part inner races will have to be very securely held in place with a strong restraining assembly as the load of the balls is pressed hard against the inside races of each of these bearings. When assembled, the inside race's end is flush with the outside race on one end, but protrudes nearly an eighth of an inch on the other end. These four bearings run good and true, very smooth, and show no signs of wear or corrosion. The alloy is strange in appearance, with a warm, golden hue to it. They may be aircraft engine bearings. The best application I can think of for four of these would be to support a very large split ring mounting as rollers for the horseshoe's rim, each pair mounted on a rocker bar, "bogey", who's pivot centers are on lines radiating at 45 degrees each from the center line of the polar axis of this hypothetical split ring mounting. I have not seen anyone mount four bearings, two pairs on rockers ("bogeys") to give a more evenly distributed support for a large split ring mounting than using only two rollers, but it is a well know principle in engineering for reducing the load on contact points by increasing the number of contact points. Putting them on rockers will allow them to automatically find truly equal support loads. Another potential application for all four of these bearings would be to use them as rollers to support north and south axle extensions, "kinematically", as rollers, for an English yoke or cross axis two pier mounting, with a fifth bearing running against the rim of the axle extension at the south end of such a mounting. Failing a request for all four of these fine bearings, I would next consider awarding them in sets of two each. Failing a request for two sets of two, I will break up the set (seems a shame to do it, though). One of you guys needs to plan a big split ring mount, or a big English yoke mount!






Catalogue #8: OD=4.33" ID=3.145" L= .63" L-2= .695" and Catalogue #9: OD=4.134" ID=2.952" L= .629" L2= .792"

Catalogue #8 is a matched set of two bearings and catalogue #9 is a matched set of three bearings (when I took the group photograph I'd not found the third one of these so it didn't get in the picture). They are similar, but differing slightly in size. The other two pictures show one of the bearings disassembled. These are light duty roller bearings without taper for thrust. The inside of these bearings do not have a keeper shoulder or flange at either end and so they slip apart in either direction. This will somewhat limit their utility; however, for a light duty application it should be possible to mount them with keeping tabs for both inside and outside races. I can think of a couple of applications where the lack of keeping assemblies for the races would not be a handicap. One application that would be suitable for these bearings would be as idlers, (guide rollers), for a steel tape measure/threaded rod/disc drive in right ascension. Inasmuch as in this application they would be mounted on the sloped north face of a mounting, the outer races would tend to stay in place (and in any case one could fabricate keeping tabs for the races). Their large diameter suits them for this application- a steel tape measure flexes less where it rounds a larger guiding idler bearing than where it rounds a smaller one. Another ATM has suggested that larger diameter light duty bearings like these would have an application in field de-rotators. For a very successful steel tape measure drive such as the kind I build, one would require four of these bearings. If anyone is committed to building this kind of drive, I will award four of these bearings to him or her. Otherwise, I will be glad to part the sets up and give out the bearings individually.


Catalogue #10: OD=3.346" ID=1.768" L= .746" and Catalogue #11: OD=2.834" ID=1.179" L= .746"

These two different ball bearings are pictured, here, together, just to save film. I do not consider them a set in any way. The larger bearing is a Fafnir, and the smaller one is a Hoffman, made in England. I have a total of two of the larger (cat. #10) units, the other one being an SKF brand with a metal shield on one side. I have a total of three of the smaller (cat. #11) units, the other two of these smaller bearings being SKF brand with shields on one side. One of the smaller units is reserved for a fellow ATM for a south end thrust bearing. Therefore, two sets of two each of these units are available as sets. These bearing sets would be fine for declination axes, or a small polar axle. The set of larger bearings would be the preferred choice for a polar axle installation. None are tapered for thrust. And for those who prefer a split ring mounting, they would be ideal for a medium to large mounting as the roller bearings for the rim of the split ring. Far better than roller-skate wheels. In fact, I have never understood why ATMs use roller skate wheels for a split ring mount when for about $20 one can purchase two of the versatile precision ground #1638DC bearings and just mount them on ordinary hex cap bolts for the two rollers for a split ring mounting.


Catalogue #12: OD=2.834" ID=1.377" L= .667"

Though only two bearings are pictured, I have a set of three of these units, with identical dimensions. Two are SKF brand and one is a New Departure. Again, I have reserved one for a south end polar axle rim running thrust bearing for a prior request. Therefore, a set of two is available. They are not tapered for thrust preference. Again, available separately if not requested as a set. Either of these sets of two bearings would be useful in the same way as the other two just previously listed sets: declination bearings, rollers for a split ring mounting, or even for carrying a smaller polar axle for a fork mount. It is possible to use any of these bearings for any of these applications without requiring any machining to mount them.


Catalogue #13: OD=3.936" ID=2.164" L= .825"

Here is a matched set of two fine, sealed, large NSK bearings. I bought these in their original boxes bearing their brand name label. One of them slipped out and dropped to the concrete floor. Didn't find a mark on it after I dropped it. I suppose they are new and unused, since I bought them in their original boxes. They are tight and run true and smooth, but with a little resistance (constant), I suppose this latter because of their large radius seals. It occurs to me that probably the very best application for this pair would be for the rollers for the rim of a quite large split ring mounting. They are not bias tapered for thrust. Would sure like to see someone build a large split ring mount with these. If anyone (or more) of you builds a nice, large mount with any of these bearings I've provided from this catalogue, I will feature your scope on my site (Blueomino gives me unlimited space and something (?) they call "unlimited bandwidth").


Catalogue #14: OD=3.264" ID=1.746" L=1.00"

Here is a very nice little tapered roller bearing, "L & S" brand, made in USA. This little unit would make a very nice south end thrust bearing for a polar axle that is sleeved inside of two bearings conventionally, for a mounting intended for latitudes above 30 degrees.


Catalogue #15: OD=2.44" ID=1.118" L= .628"

I have three of these little roller bearings. They are finely made, not a blemish on them, brand new and sealed in their plastic bags (I unwrapped one of the, naturally). These little roller bearings are not tapered for thrust. Their lack of a keeping shoulder on their inside race will limit their utility, but they would work very well for rollers for supporting a runway as in the kind of rack and pinion sector drive that I build. See my article about this drive right here on my web site. One of them is hiding from me, but I am sure that I will find it before I receive a request for them. They will be preferentially awarded as a set. New with their boxes bearing their brand name of "NTN".

This essentially completes the catalogue of useful bearings I have that I would like to give away. Now, I expect you guys to clean me out! Come and get 'em!


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