rack and pinion

The telescope I've referred to in recent postings has a pair of knobs joined by an axis whose center is a pinion that engages a rack on a tube that holds the eyepiece. This enables one to focus the telescope. The rack seems to be well-oiled but still moves somewhat unevenly in some places as one turns the knobs.

When I look at it so that I can see all the teeth below me: ||||||||||| it seems that they have not all worn evenly, that some are apparently thicker than others, and that some of the valleys between them are more filled than others. I don't know if the latter condition indicates there is more oil in some than others or whether it indicates the presence of more dirt, which contributes to the uneven motion. The person who loaned it to me told me it would be ok for me to oil it and maybe that also means it is ok for me to clean it (e.g. with a toothbrush). But before I do anything, I'd like to be sure of what kinds of conditions contribute to this kind of uneven motion in a rack and pinion. It mostly turns evenly but in some places it seems more prone to offer some resistance.

Leaving aside whether it would be permissible to make repairs on the borrowed scope, I don't know if it is possible to replace the rack. It seems to be riveted in place. I wouldn't know how to select a replacement rack or two replacement rivets. Maybe with suitable machines, I could make my own rack. That is something I would keep in mind for the future, when I have some machines and want to modify or build my own telescope.

Continuing with the hypothetical, suppose I want to be really fanatical about figuring out what exactly is wrong with the rack. Since eyeballing it suggests some irregularities, suppose I want to measure all of the heights of the teeth and all of the widths of the peaks and of the valleys. Is there any convenient way to do this? One way that occurrs to me is to take the rack, clean it, ink it and press it against a piece of paper to make a print of the rack. Then I can photocopy the inked page with enlargement and measure it. There would still be a lot of measurements to make, but they wouldn't be so small and easy to mess up. Also, if the print were faint in some places, that would confirm my impression that it was badly worn in some places.

There is one other issue related to the rack and pinion. There seems to be no barrier between the rack and pinion and the interior of the telescope tube. It occurs to me that this might let vapors from the oil diffuse into the tube and possibly also coat the inner surface of the telescope lens at the other end. That sounds undesirable and makes me wonder whether someone might have made a mistake by oiling the rack and pinion in the first place. If so, the source of the mistake might have been that someone noticed the uneven motion and resistance to turning, didn't realize the role that wear might have played in it, and instead tried to solve the problem by oiling it without realizing that this might be bad for the optics.

On the other hand, I don't know anything about scopes or racks and pinions, so my 'pinions aren't worth anything. If someone is better informed, please inform me. Thanks.

Reply to
Allan Adler
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SWAG: Try cleaning the rack with a tooth brush and lube it with way oil (rack & tube both).

Reply to
Nick Hull

Accepted lube is RT-44, using way oil or any other substitute is going to guarantee problems with lube migration, or shiny stains where you don't want them.

IF you replace the rack, you might be tempted to use the nylon rack, bad idea, I've done it and it didn't take long to switch back to the brass. Rivetted sounds like an import, 0-80 screws are what I use. It ain't rocket science, pretty straight forward job.

Helical focuser is also an option, and I've made a couple of those too, each has it's benefits and drawbacks. Personal preference there, neither has a clear advantage over the other unless you're putting a camera on the focuser, in which case the helical is out.

Reply to
Nobody

Allan Adler wrote in news: snipped-for-privacy@nestle.csail.mit.edu:

You might want to investigate the pinion. There may be nothing wrong with the rack at all. If the center hole on the pinion is offset, or at an angle to the teeth, this would also explain the uneven wear. Even a mis- alignment of the two bores the pinion rides in could cause this.

Reply to
Anthony

Anthony writes:

I ruled out a problem with the pinion on the grounds that the resistance wasn't repetitive as one turned the knobs completely a few times. I don't know if that is a valid argument.

The pinion and the knobs form a single piece. So, unless it was made incorrectly, I don't have any reason to suspect the pinion itself. I'm not sure what you mean by "at an angle to the teeth". One meaning might be that it crosses the teeth. With that meaning, it might not work at all, so I'm guessing you mean that the planes containing the teeth of the pinion and of the rack are parallel but the peaks of the pinion are not parallel to those of the rack. To put it another way, I think you're saying that the axis of the knobs might not be parallel to the bottom line of the rack valley it passes through, so that it dips to one side. That is conceivable but again it would seem that it would result in consistent problems all along the length of the rack, not just in one place.

On the other hand, there still might be a problem such as you describe. I unscrewed the "box" that covers the pinion. It is not perfectly rectangular, since it has to fit against the cylindrical tube and also it has cuts to allow the axis of the knobs to pass. I turned the box over, so that these cuts were all on top and the box was open, and noticed that there was a flat rectangular piece of metal inside of nearly the same length and width of the box. The rectangular piece had a rectangular hole in it. When I opened it, it was jammed in so as to run diagonally in the box from one edge on the bottom to the opposite edge on the top. There were two more identical pieces of metal sitting flat on the cylinder and the axis of the knobs, enclosing the little towers that the screws screwed into and the pinion. The bottom of the box has four "pedestals", one in each corner. I don't know whether the flat piece of metal was correctly placed. Possibly someone else opened it up to oil the pinion and put the flat piece back in incorrectly and possibly it is really supposed to be in the diagonal position in which I found it. If the three flat pieces are removed entirely, then the axis of the knobs wobbles badly. I think the three flat pieces are intended to sit flat on the four pedestals. When they are stacked that way, they fit in the box but rise to block off most of the hole the knob axis passes through. So, I think the flat pieces are used to clamp the knob axis in a fixed position. The flat pieces also rise above the bottom of the circular arcs that are intended to fit the cylinder body. I think that when the box is screwed back onto the cylinder, the flat pieces are forced down to the bottom of the circular arcs, forcing them to bend to conform more to the cylindrical shape. This has the effect of pushing the other two, still straight, edges of the flat pieces to clamp the knob axis even more tightly. This arrangement makes so much sense to me in terms of the operation of the rack and pinion that I find it hard to believe that the diagonal arrangement I found could possibly be correct, but maybe there is something I don't know, such as the possibility that the diagonal arrangement eliminates some lateral movement.

I didn't find that correcting it, even if it was wrong, made any difference in the resistance, but this is all kind of subjective. Unfortunately, I can't see the pinion engaging the rack, so it is hard to know what the problem is exactly, except by trying something that has the effect of curing it and concluding that what I thought I was correcting was actually what was wrong. But I do have one idea: I can count how many teeth there are on the pinion and how many are on the rack and how much of a turn I have to make of the knobs to move a given number of teeth along the rack and how many teeth there are between the visible part of the rack and the part that actually engages the teeth. That means I may be able to wait until I find some resistance and then turn the knobs the appropriate amount to move the bad part of the rack to where I can see it.

After I've gotten that crackpot idea out of my system, I'll probably just clean and re-oil it, as was suggested by others. But I'm still concerned about the possibility that oil fumes might be contaminating the telescope tube and coating the inside surface of the telescope lens.

Reply to
Allan Adler

It was made in Japan according to the specifications of Meade. I didn't see a date anywhere, but I know that it is at least 10 or 11 years old and probably more.

The "root". That's a good name for the bottom of teeth.

Like on the lens?

Is there any convenient way to try to figure out, without damaging the telescope or screwing up its optical alignment, whether any lubrication has migrated into the telescope tube? If it has, is there a good way to clean up the mess inside? Maybe if I don't see any chromatic aberration or haloes, or something like that, when I look at the sky, all is well, but maybe the effects are subtler than that.

I don't actually know that they are rivets. I only know that when I look at the rack (not brass, by the way, but some grey metal), the brass things I see holding it in place don't look like screws. I can't see into the tube very well but the one "rivet" I can see in it seems to have a kind of circular hole, rather than a polygonal one, so I don't believe it was intended to be unscrewed with any kind of allen wrench or other special tool.

Reply to
Allan Adler

Allan, what if you ask for a digital camera for Christmas?

GWE

Allan Adler wrote:

Reply to
Grant Erwin

If it's not repetitive, then the pinion is probably reasonably concentric to the shaft, and the rack is the problem. In the older scopes, wear on the rack would take a very long time, but the newer ones sometimes have plastic rack, or soft brass, which will wear quickly.

RT-44 is used because it does not outgass, it stays where you put it. Oil fumes on the mirrors are a legitimate concern, not fatal, but cause for cleaning and collumating more often than they should be.

If the rack is worn, there is nothing that will even things out but a new rack. I would be more concerned if there was slop in the drawtube/focuser body fit. This is also compensated for by the flat spring in the "box". The spring should press the pinion into the rack, and also to one side of the slots the shaft runs in.

Reply to
Nobody

Ok, newer import. But Meade supports their products pretty well, and it might be possible to just buy a new drawtube, with the rack already installed.

The biggest problem wouldn't be coating the lens, but causing shiny places on the interior of the tube. It's generally noticable as a loss of contrast, more when looking at a bright object, the moon for example. If it's on the lens/mirror, it *may* cause haloes, or other abberations, but chances of it getting there are pretty slim, especially if the instrument hasn't been relubed with the wrong thing.

I clean, relube, recollumate my three reflectors about three times a year, and there isn't too much you can do that isn't field adjustable to correct. If it's a refractor, they're almost impossible to screw up as long as you don't take the lens out of the cell. Once it comes out, unless you mark everything to make sure it goes back in exactly as it came out, all bets are off. Still, all it takes is marking so you know what came out when. Especially the front and back faces of the lens, in a non cemented lens, they can be reversed easily, and then comes the bitch of trying to figure out which one is reversed. A dot of water soluble marker can save hours of frustration.

Could be a zinc alloy, not unheard of. Saves machining at the cost of a longer life. If they were screws, the heads would be instantly seen as such. Play in the rivets is also not unheard of, and sometimes can be frustrating when you're trying to focus that 6mm eyepiece. The higher the magnification, the worse small problems become. (Which is probably why my favorite EP is a 28mm plossl.)

Reply to
Nobody

Alan:

A couple of simple things you can do to "diagnose" the gear mesh. First thoroughly clean both rack and pinion. Do not polish, but be sure they are clean. Then, do a contact pattern check. This is simply applying some sort of compound to the entire rack and repeatedly running the rack end to end several times and reviewing the "pattern" left behind. Obviously, don't use too much or anything that will get into the optics. Any good grease will do. (On a side note the professional gear guys use colored componds similar to jewelere's rougue.)

What you should see ( in a correctly functioning gear mesh) is a uniform pattern on both the rack and pinion through out the entire stroke (all the way round the pinion and full length of the rack). The pattern should be centered on both the rack and pinion and should be as wide as the narrowest member (either the rack or the pinion).

You probably will not see a uniform pattern based on what you wrote. If you see the pattern move back and forth across the gears, then you have a misalignment of the pinion shaft (not 90 degrees to the rack). If you see the pattern move deeper and then shallower in the gears then either the rack is warped, or the pinion is ecentric, or you have excessive wear. Use a dial indicator to determine whether you have an alignment problem and deal with it accordingly if you find one. Alsi check to see if the top of either the pinion teeth or the rack teeth is contacting in the root of the mating gear.

Rack wear is simple to check. Use a caliper and measure the pitch between any two teeth near either end of the rack (just like thread pitch). If two adjacent teeth are too close for you to measure then simply measure between any convenient number ot teeth (say three or four). Then repeat this measurement several times along the length of the rack paying special attention to the teeth in the middle of the rack. Shorter measurements indicate wear.

The pinion can be checked for wear similarly. A helpful trick is to get two short pieces of wire the same diameter (No. 12 copper wire is good enough) and lay one wire in the root of one tooth pair and the other wire in the root of another tooth pair as close to 180 degrees as possible from the first pair. Measure the distance "over the wires". Now repeat this procedure with sucessive tooth root "pairs" all around the pinion You should get the same measurement every time (or real close). Smaller measurements indicate worn teeth.

Hope this helps.

Reply to
Kelly Jones

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What I would do to examine the rack is to check it on an optical comparator (after a through cleaning, of course). Mount it so you see the rack teeth in profile. Put some clear plastic film under the clips on the display, select the largest magnification you can manage with the optical comparator, and trace the outline of the profile on the plastic film. Now, move to one of the worst teeth, adjust so the bottom of the tooth profile is the same, and trace this one, The area between the two profiles should give you a good idea how badly worn it is.

To *make* one, the ideal way (with HSM tooling) would be a horizontal milling machine, a proper gear tooth profile mill (for the rack gear -- which is either #8 or #1 -- I forget which end is correct, but when you buy it you can look that up in the catalog. One end of the set is for 135 tooth through rack, and the other end is 12-13 teeth.

Obviously, you first have to know what the size of the rack teeth are -- both diametrical pitch (or "module" for metric gears, IIRC), and the pressure angle. And if the teeth are cut at an angle instead of straight across the gear (more like "/////////" than "||||||||") you will need to angle the blank, and to calculate a correction factor to your feed using a bit of trig to get the tooth spacing to come out right.

Note that the angled teeth are a lot smoother in the feed, but you will need better bearings on the pinion, as it will have a side thrust which the straight tooth will not.

Now -- if you want to avoid the side thrust, but benefit from the smoothness, then a herringbone gear would be ideal -- but more difficult to make. It would look somewhat like this:

/ / / / / / / / / / / / / \ \ \ \ \ \ \ \ \ \ \ \ \

The typical lube used in the focusing rings of cameras (multi-start threads) is typically a quite thick grease, and has fairly minimal vapor -- unless it is kept in the direct midsummer sun. :-)

Well -- you have most of what I know about the subject above.

Good Luck, DoN.

Reply to
DoN. Nichols
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A double helical -- a tube threaded inside and outside -- one left-hand thread and the other right hand thread, with an inner tube connected to the camera and the outer tube connected to the telescope, and some kind of keyway to prevent rotation of the inner tube relative to the outer tube and you are fine. This is pretty much the way the focusing ring on SLRs is done. (Some older lenses on folding cameras had the front element on a single thread -- but that did not need to move as far to focus through the desired range. Moving the entire lens to focus takes more motion.

Good Luck, DoN.

Reply to
DoN. Nichols

Nope. Focus ring is a nut, with a retainer to keep it from moving, the male thread is on the lens barrel. Usually 4 to 6 lead though, never single lead. Which means there are a lot of ways that it will go together, only one of which is right.

Suppose a helical focuser for a scope could be made that way, but most of them don't have a real thread as such, it's usually a radiused groove, and engages a ball indent for rapid focusing. I make mine like a very shallow acme thread, the groove just wide enough to take the ball, and deep enough so the ball almost touches the bottom. Parfocal eyepieces eliminate the need for the rapid, but add quite a bit to the cost. There are focusers available in just about any configuration anyone would want, if their wallet is heavy enough.

Reply to
Nobody

That may be the case in the eyepiece focusing on a telescope, and for *some* camera lenses, but the longer the focal length, and the more desire for close focusing, the more likely the camera lens is to be with double helical. I have disassembled lenses of this sort, so I am sure that they exist -- even as short as 135mm focal length on a 35mm camera. The focus ring is tied to the ambisexual ring, with a female thread attached to the camera body, and a male thread attached to the lens cell itself, so you can move the lens cell a long distance without having to have a threaded sleeve the whole length of the travel.

Yes, multi-lead makes for faster focusing, and is amost always found in SLRs at least -- and even in the very fine thread on the front element of a Zeiss 75mm f3.5 Tessar on a folding 120/620 roll film camera (16 shots per roll, with that short a focal length -- vertical format by default. That was my first camera with a reasonable quality lens.

The ball and groove one would work well enough for something light, like an eyepiece, but when you hang the weight of a camera body

-- especially if it has a motor drive as part of it for semi-remote shooting -- you need something more sturdy, like a real thread.

Enjoy, DoN.

Reply to
DoN. Nichols

OK, Grant: Can I have a digital camera for Christmas? (How's that for grantsmanship?)

Reply to
Allan Adler

That seems like a simple enough test.

As a matter of fact, there seems to be, if I understand you correctly to mean on the same surface that the rack is mounted to. It is mostly near the rack but "slop" is a good description of it.

When you say "to one side", I think you mean either towards or away from the eyepiece. This is an interesting point. As I mentioned, all three flat pieces could simply sit on the 4 pedestals in the box, which would lead to a symmetric pressure on the pinion axis. You seem to be saying that is undesirable. On the other hand, when I opened the box, there was asymmetry in the placement of the flat pieces: the one furthest into the box was placed diagonally with one short edge at the "top" of the box and the other short edge at the "bottom", where top and bottom refer to distance from the drawtube/focuser body. The other two were placed as flat as possible, given the asymmetric placement of the first one. So, you seem to be saying that the way I found it was correct. Thanks!

Reply to
Allan Adler

I'll look into that.

You also clean the inside of the telescope tube?

Is there a good book that discusses all the things you can do to maintain a scope in good condition?

It's a refractor. I noticed that, where the lens is, there are 6 screws holding a metal piece that I figure probably clamps the lens in a certain position. The 6 screws are arranged in three pairs, with the three pairs placed 120 degrees from each other. I noticed that the screws are not all screwed in all the way. Some seem to be and others stand quite far away. I considered tightening them but changed my mind when it occurred to me that it might be like that for a reason. I don't know what the reason might be but one possibility might be that it has something to do with correctly aligning the lens with the eyepiece and the axis of the scope. In that case, I really shouldn't mess with it.

Optics is a very interesting subject but on every occasion that I have had an opportunity to do any experiments with optical equipment, I wound up spending all my time in thankless and usually fruitless effforts to correctly align something by turning triples of thumbscrews. If I messed with the lens, I might have a chance to go through that all over again.

On the other hand, it's probably a good thing to know how to do right, if I believed I could ever get it right...

Reply to
Allan Adler

It's a refractor, and it's borrowed. Two good reasons not to mess with it. The latter because it's very irritating to find that something you've loaned out and mentioned that it would be OK to oil is "rebuilt" without your blessing. Especially if the rebuilding goes awry.

The former because I'm about 99% certain that what you are describing (right down to the zinc rack) is a cheesoid "christmas and birthday telescope". The outside of the box prominently tells you it's 200x or

400x, it's got a wobby tripod which may have a low quality "polar" mount, the aperture is a whopping 3 inches or so, etc. These things have probably driven thousands of people away from astronomy.

They never work very well, and you should be able to get your own at a junk shop, flea market, recycling center, goodwill or tag sale for very little money. Don't buy one new. You can waste as much time and effort as you would like tearing that apart, without annoying anyone. Don't expect a very satisfying viewing experience.

If you actually want a telescope to look at the sky with, get a reflector with a "dobsonian" mount. A physics teaching acquaintance has

2 or 3 of the cheesy refractors (people donate them when they discover that they can't actually see all the stuff they thought they'd see), and one reflector (bought). There's just no comparison, the reflector is that much better - and not really much more expensive if bought new. You can also go the traditional route and grind your own mirror, if you like, or buy mirrors and build the rest of the scope (pretty easy). The mount is easy to build, as well.
Reply to
Ecnerwal

As far as cleaning out the spiderwebs, but all I do is run a dry towel through it for that. Mine are all reflectors. I do have a wide field made from a Kodak Aero-Tessar, but that lens is so heavy that I seldom use the instrument, Very difficult to balance it on the mount. The tube is some 20 inches long, not counting the focuser, and balances 3 inches behind the objective lens.

Amateur Astronomer's Handbook, J.B.Sidgwick, Dover press 0-486-24034-7 Also Amateur telescope Making, Ingalls, but it's long out of print, might find a copy at a library. Scientific American Publishing, my copy is copyright 1935.

Yes. Antagonistic screws, one will move the lens mount, the threads are in the mount, the other is the locking screw, the threads are in the part that is mounted to the tube. They should be snug, but not tight, excess pressure from them can distort the lens, or in really bad cases, crack it. Also a sign of a little better quality instrument, BTW.

Even with the reflectors, collumination is a process of three interactive adjustments, the degree of perfection being dependent on your patience. I have cheaters methods I use in the field, probably not giving perfect collumation, but good enough for casual stargazing. Sidgwick covers it fairly well, but he also goes into making micrometers and other attachments that would require a higher degree of perfection than I do.

Reply to
Nobody

Ok. On some of the focusers, I don't know if Meade ever used it, there are flat springs opposite the rack, and they furnish friction to keep it from moving unless driven by the rack/pinion. If these are properly set, they will hold the drawtube in a sort of balance with the pressure of the rack, but will seem to be allowing movement, which in use will not be the case. The old 6" Edmonds has two felt strips opposite the rack, some have other methods, even going so far as to having eliminated the rack for a friction drive, and having ball bearings instead of springs or felt. The main thing is that with the eyepiece installed, it should be centered and with the optical axis also centered and parallel to the optical axis of the objective lens. If it was a cheaper focuser, and has the felt, chances are that the felt has compressed and is no longer holding it firmly. Quicky repairs can be made by replacing the felt, but I prefer to use the flocking from the film slit of a 35mm cartridge instead of felt. It seems to not compress as quickly, and sometimes gives a little more "solid" feel to the focuser.

It should be holding it so the shaft can't move up or down or towards the drawtube easily. If it's too loose, it acts like backlash, or excess play when changing the directon you're moving the drawtube. There are as many variations in how it was accomplished as there are makers of telescopes, every one of them believing they have a "better idea." My own "better idea" would be a planetary drive fine focus knob, but due to the costs of making such an animal, maybe it wouldn't be worth it. I probably have ten or fifteen focusers in different stages of "maybe this will work" in the basement, but still have to come up with the "perfect" idea. It's pretty hard to improve on something that people have been working on for several hundred years.

Reply to
Nobody

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