Torque spec for the screw that holds the clutch assembly together

[rpdayton]

I get my 130SLT out about once a year, mess with it for a day or two, get frustrated, and put it back away.  This year I took it with us camping in the mountains and never unpacked it because it just isn't worth the hassle.  Anyway. This year I'm ready to just put the whole thing on the curb and post it on facebook.  Before I do that though, thought I'd ask here and on other forums for a little help.

Inside the cover on the mount is a nut - M6 threads at a guess - that screws onto a threaded shaft that appears to be the pivot rod for the Alt axis.  This nut is extremely loose (freely moves) and has been since we got the scope.  When the scope move in Alt, it causes the nut to come unscrewed a little bit, which in turn lets the dovetail and tube to flop around, ruining any alignments or tracking. I can screw it in by hand and it will work for a little bit, but even making Alt moves for aligning it causes it to loosen.  A few years ago, I spent a 4-day weekend doing a total of 90 alignments (45 3-star, 45 2-star.) Only 16 of them succeeded, and of those 16 only 3 times was it then able to goto the moon and have it visible in the eyepiece (I gave up trying to goto planets prior to that until I figured it out. I figure if it can't find the moon, there's no sense trying for anything harder to see.)

What I want to do is get a locknut (with a little nylon ring to prevent it unscrewing) or a jamb nut to hold this nut in place.  But I don't want to overtorque it and mess it up any more, or damage the gear train, or cause the motor to struggle.

I've asked Celestron about it, but they dismissed it as 'you need to keep practicing alignments because it's so hard to do' which is why I'm hitting up several forums looking for the answer.

So 2 questions... 1 - Would a locknut work to keep from coming unscrewed? and  2- How tight should it be? lbf-in, nm, whatever, units are not a problem. 

 

Thanks.

[michael8554]

Very rarely do manufacturers specify torque settings for OTA and mount fixings.

Try a  locknut, spring washer, or Loktite on the threads.

Michael

[Louis D]

1 hour ago, rpdayton said:

This year I'm ready to just put the whole thing on the curb and post it on facebook.

Perhaps you could try mounting the OTA on a manual alt-az mount and simply push it to objects using a RACI, RDF, QuikFinder, etc as guides.  There are various planetarium apps to help you find your way around the sky.

[rpdayton]

2 hours ago, michael8554 said:

Try a  locknut, spring washer, or Loktite on the threads.

 

Right, but my concern is how tigh to I make it, or what force spring nut would work?  What do other users use to keep it from coming unscrewed?

[rpdayton]

2 hours ago, Louis D said:

try mounting the OTA on a manual alt-az mount and simply push it to objects

That would work, but for as much as we spent on this thing I'd like to try at least one more time to see if I can get it working as advertised :/

[StevieDvd]

It would seem a simple test to try using a Nyloc nut and tighening until the dovetail/tube does not move freely. Then see if that stays put and aligns properly. If it does not align well try a loosen of the nut first, if that is the same or worse try a tightening adjustment.

If it works okay then monitor over time.

 

[Ricochet]

1 hour ago, rpdayton said:

Right, but my concern is how tigh to I make it, or what force spring nut would work?  What do other users use to keep it from coming unscrewed?

Presumably, they don’t use anything because normally it doesn’t unscrew. If the nut is unscrewing when the mount is moved in alt then the friction between the mount and the nut must be higher than the friction between the threads of the nut and the threaded rod. What you need to do is to decrease the friction between the nut and mount and/or increase the friction between the nut and threaded rod.

Starting with decreasing the friction between mount and nut, I would first look at the nut itself. Perhaps on the face that touches the mount the surface is not smooth and needs sanding down. Also, with nuts generally there is one side where the corners are slightly rounded and one where they are not. I would turn the nut so the side with the rounded corners is facing the mount. I would also see if there is the possibility to fit some washer(s) between the mount and nut.

With regards to increasing the friction between nut and rod there are two options. The first is to put loctite (I’d go with blue) on the threaded rod and then do the nut up to the correct position. The second, as you have already thought of, is to use a nyloc nut either instead of the original nut, or as a locknut behind the original nut. Using a spring washer will not help as it will increase the friction between the mount and nut and so help the nut unscrew.

There is not going to be any set torque that anyone can give you for this adjustment. A torque setting is for when you have a static joint, but yours must be able to move for the mount to work. What you will need to do is to judge when the nut is in the correct position on the threaded rod to hold the dovetail in the correct position but not so tight that the movement of the mount is impeded and the motors strained.

[Peter Drew]

I think experimentation is the key.  As for GOTO performance, the Moon is probably the hardest thing for the system to point to accurately as the Moon's motion is very complex.  Fortunately it is usually bright enough to be found unaided.   🙂

[rpdayton]

1 hour ago, Peter Drew said:

As for GOTO performance, the Moon is probably the hardest thing for the system to point to accurately

Most of the time it would miss it by >30° in Alt, often times it wouldn't even end in the correct hemisphere of the sky.  I gave up trying to goto specific stars almost immediately, as it will rarely goto even one of the alignment stars it just aligned to.  I started using the moon only as a sanity check... if the moon's high in the east, and the scope points at the ground after a goto, my assumption is that it isn't going to the correct location.  In hindsight, I assumed it would be able to find the moon's location very easily; I will stick to stars. (That would also explain celestron's tech support telling me to absolutely not use planets for alignment even though the manual says that's acceptable.)  My second reason for choosing it is to easily ensure it would track an object after a successful alignment, but that is something I've never been able to get working.

[rpdayton]

2 hours ago, Ricochet said:

... A torque setting is for when you have a static joint, but yours must be able to move for the mount to work. 

My experience in such things has been in installing and setting up CNC machines; I have learned that these scopes work completely differently.  I am used to using spring washers that are able to apply constant force against a moving component, hence my assumption this connection would work similarly.

[michael8554]

"What do other users use to keep it from coming unscrewed?"

An image would help, as it's not clear from the descriptions whether this nut is expected to be tight enough to lock against the surface it's adjacent to.

Or whether it's supposed to lock with a clearance to allow Alt to rotate with minimum slop.

Michael

Edited September 20, 2023 by michael8554

[Cosmic Geoff]

I have a SLT mount, but I would appreciate a photo (preferably annotated) of the faulty area in case I can help.

There is a slip clutch and on one occasion I removed the dovetail clamp screw so I could access the large nut and adjust it.  I have not had to touch it since.

IIRC there are various preparations, e.g. Loctite, used to discourage nuts and bolts from unscrewing.

Edited September 20, 2023 by Cosmic Geoff

[rpdayton]

Thanks all for the suggestions; I will get a couple pictures this evening.

[Louis D]

16 hours ago, rpdayton said:

My experience in such things has been in installing and setting up CNC machines; I have learned that these scopes work completely differently.  I am used to using spring washers that are able to apply constant force against a moving component, hence my assumption this connection would work similarly.

Just compare the price difference to understand why the difference in mechanical sophistication.

[rpdayton]

Hopefully they post correctly.  Not sure if they will get compressed or pixielated.

But, a picture of the tripod and mount, just in case there are different versions.  I'm sure there are, Celestron told me I should be using the 'Index Marks.'  I don't have index marks on mine, and they indicated that not all scopes have them.  And then gave me instructions on how to use them (facepalm.)  But just in case there is some different model or body or something.

A picture of the back cover removed; the screw in question is just above and to the left of the center of the picture.  I was holding the cover in my other hand to keep tension off the wires, and my aim is not so good one-handed.

And a close-up of the screw in question.

 

And just to answer these ahead of time: 

I have leveled the tripod using a machine level in the dovetail and checked every 90 and 120 degrees.  I was confirming the bubble level wasn't grossly out-of-square to the tripod.  I figured 2.5 microns per meter was a good sanity check lol. (They told me I was making it *too* level, whatever that means.)

Location, date, time, time zone, dst all accurate, verified before every alignment attempt.

The tube is not out of balance; i have it balanced with the tail (mirror end) heavy by ~2.5 ounces - which isn't hard, more than about 6oz and it drifts wildly in alt. I have several balance points laid out on the tube for evypieces, eyepieces+barlow, eyepieces+zoom, and camera mount+camera.

I do not use planets for 2- or 3- star alignments, nor do I use polaris.  I use alignment stars >30 degrees above the horizon, and generally try for at least 120 degrees apart, although I will be honest, after an hour or so of failed alignments I will sometimes lose patience and go for closer to 90 degrees than 120.

As was explained above, using the moon as a goto target to confirm alignment is a bad idea; I never considered it may be too complex an orbit to locate it accurately. I started using it a few years ago when it was fairly close to whichever alignment star I had used.  However, I have also found it often times won't even goto one of the alignment stars!

I will point out, the times it does align, if it misses its target in a goto move, it will always be pretty close in Azimuth, but it will always be off in Alt.  Celestron told me that is just a matter of practicing alignments.

I have actually used 2 different collimators, I confirmed the second one using a concentricity gage at work; the beam moved less than 10cm at 30 meters in one revolution; overkill I know, but I went through a phase where I was convinced I wasn't doing something right, and just wanted to rule out anything I could have wrong.  I don't actually know if there is a spec for checking the concentricity, but I am guessing that is close enough?

I have NOT adjusted backlash.  I asked Celestron about that; everything I read says 'don't adjust it too high or it could cause alignment and tracking issues (which is what I have already) and wanted to know if there was a way to see if it was already set too high.  That was also when I asked them about the torque on the nut.  No answer.

I primarily try 2- and 3- star alignments.  Celestron told me there are 'lmany ways to align and alot of people have their own methods that they swear by." but they never gave any details about these other methods.

I think that's all the relevant info.

Thanks again for all the help!

[michael8554]

I'm not an owner of this mount model, so here's my interpretation of the image:

A is is non-locking nut.

B is the outer race of a ball bearing

C is either the outer cage of that bearing.

OR a separate loose cage washer that retains the balls

D may be the inner ball bearing race

Or a tapered cup.

 

IMO the bearing is only halfway into the mount, the area B should be inside the mount.

Which is why the nut is half off when tightened.

D also shows that the Nut isn't central over the bearing.

 

So two courses of action:

1) the bearing needs to be drifted all the way in.

The nut should be a fully tightened lock-nut

OR

2) the bearing needs to be drifted all the way in.

If C is a separate cage, then the locknut should be tightened enough to minimise play, but allow free rotation of the shaft.

Michael

Edited September 21, 2023 by michael8554

[Cosmic Geoff]

Sorry I can't help as I have never exposed that part of my mount.

[rpdayton]

2 hours ago, michael8554 said:

A is is non-locking nut.

B is the outer race of a ball bearing

C is either the outer cage of that bearing.

OR a separate loose cage washer that retains the balls

D may be the inner ball bearing race

Or a tapered cup.

A is the nut that gets loose over time.

B is the outer race

C is a keeper that keeps the actual ball bearings equally spaced.

D is the inner race.  

For this picture, the nut is snugged up against the inner race.  With it like this, the mount still moves freely in Alt.  If I tighten it any further, it immediately causes the mount to become very hard to move in Alt. I don't like using it that tight; I'm afraid it will damage the gears or motor. If I raise or lower it in Alt a few times, the nut will again be loose.  I didn't go through the exersize of doing that for these pictures; I didn't feel like getting ut the controller and power supply.  I haven't tried checking clearance between the inner race and the keeper, but visually it looks like they clear by about .005-,010" when the nut is snugged against the inner race.  Sorry for changing units, my shim stock is all inch standard.  Call it 100-200 microns.

The bearing appears to be seated all the way; I've not tried taking this part of the assembly completely apart to try to force it in any farther because I'm sure at that point nothing will move at all, and I'm sure the pivot rod is pressed into the the inner race and I don't have a puller that small.

 

As for not being centered, I haven't checked that, or even noticed it really until you mentioned it. I'm sure that's just caused by the weird camera angle. The threads are part of the pivot rod (or whatever it's called), and I wouldn't think the threads would be off-centered to the rest of the rod.  I'm not even sure how to make a threaded rod with offset threads short of a second op... The face of the rod has a bit of pucker which likely means they used a thread roller, which won't do off-center threads if used in the same machine that cut the OD.  Hmmm...I'm thinking I should completely remove the nut and check for concentricity between the rod threads and OD, and the nut's ID threads to the hex.

[AstroKeith]

The bearing is not seated fully in the housing, you can even see it is tilted. The nut is way off centre, indicating that the shaft is also not seated in the inner race. 

Take the nut off and you will see the shaft is a bigger diameter than the thread, and may be stopping the bearing going in.

Its a serious mess

[doublevodka]

13 hours ago, rpdayton said:

 

 

Okay so here's my 2p worth on this, I've previously stripped my Nexstar GT mount which looks very similar. The bearing above is definitely not seated correctly as others have said. I believe mine also had a large washer under the nut and a nyloc-type nut wouldn't hurt either. Your alignment is definitely going to be all over the place as that axis is at least a few degrees out.

So firstly what I would personally do is remove the nut and pull out the alt axis assembly, cover the bearing with a scrap piece of wood, and tap the wood with a hammer (gently) until it is properly seated. Find a washer with the right size hole and a locking nut and reassemble it.

No torque spec to give you, but definitely don't go too tight, it will lock up the axis.

As for alignment, once this is done have a look here - https://www.nexstarsite.com/ some good guides that will be more helpful than Celestron  

[doublevodka]

slight edit to above, found this picture which shows just the bearing (fully seated) and a nyloc nut, so defo change the nut

 

[Mandy D]

In agreement with others, the bearing is not properly seated. When it is correctly positioned on the shaft and in the outer seat, you should not be able to see the outside of the outer raceway. It should be up to the shoulder on both the socket it sits in and the shaft it carries. It should, then, be possible to tighten the securing nut as much as you can without it shearing and it will not make the bearing any tighter. The tightness of the centre nut is only critical on taper roller bearings, where it is always left loose and secured with a split pin, which this is not.

Have a look at the attached photo which shows the end shield from the drive end of a tiny generator with a stub shaft. The bearing is properly seated in the outer case, but the shaft needs to be pressed in until the bearing is seated against the shoulder on the shaft. Then, you can fit a nut on the opposite end of the shaft to secure it. In this, case it is a tight press fit, so does not need a nut.

[rpdayton]

Sorry for sporadic responses; I'm in training at work this week, so bouncing back and forth between this forum and our latest cadcam system.

Suggestions I'm receiving:

Tighten it down really tight and it won't impact motion, and also Don't over tighten it or it won't move.

Currently it is just finger tight against the race, and it is already making it harder to move in Alt. I can go a little tighter by hand and it becomes very difficult to move the alt axis. I think if I actually put a wrench on it it will sieze up the entire assembly.  When I first found the loose nut, I gave it about a quarter turn past where it is now, and it became very difficult to move in alt, atleast 2 lbs at the end of the tube.  I tried it tightened down to about 1 lb, and had better luck, but it didn't last long.  (Last time I balanced the tube in the dovetail with the nut appx as tight as it is now, it will move in alt with about 6oz at either end of the tube.)

 

The bearing needs to be seated differently. I'm a little nervous about banging on it even with a piece of wood, it seems like as delicate as it is that would cause some damage if not extremely careful.  Followed by, I can't get it to move any deeper just squeezing it by hand; it feels like it seats pretty securely where it is.  If I need to completely disassemble it, would it be better to use a shoulder screw that has a diameter close to the ID of the bearing, and try drawing it in using a nut?  Just a thought; I know I asked for help and shouldn't dismiss any suggestions, but I'm nervous about damaging anything.

 

Someone (@doublevodka) posted a picture showing a similar mount with a nyloc nut instead of the one that mine has; is there a specific nut that is used for this, or is something from home depot good enough?  And also, several replies suggesting using loctite instead of a locknut; is that a common fix for this?  Again - I just want to make sure I'm not causing any damage going through this.  Loctite seems like a good way to make a bad situation even worse.

[Mandy D]

I think the best thing you can do is to remove the bearing completely and have a look at the whole picture. If you take it to an engineering workshop, someone will be able to remove it for you. It will probably be very tight. Bearings are frequently pressed in and to fit them on to a shaft they usually need to be heated using an induction heater to expand them, then they just slip on, shrink and grip the shaft. If the bearing is being removed, you should replace it with a new one as a matter of course. Personally, I would only use SKF bearings as the quality is known. Avoid cheap Chinese ones and anything bought from dubious sources on the internet that are badged as SKF.

If you post your location (general area) someone might be able to help you with this in a practical sense rather than back and forth with questions on the internet.

[AstroKeith]

1 hour ago, rpdayton said:

Sorry for sporadic responses; I'm in training at work this week, so bouncing back and forth between this forum and our latest cadcam system.

Suggestions I'm receiving:

Tighten it down really tight and it won't impact motion, and also Don't over tighten it or it won't move.

Currently it is just finger tight against the race, and it is already making it harder to move in Alt. I can go a little tighter by hand and it becomes very difficult to move the alt axis. I think if I actually put a wrench on it it will sieze up the entire assembly.  When I first found the loose nut, I gave it about a quarter turn past where it is now, and it became very difficult to move in alt, atleast 2 lbs at the end of the tube.  I tried it tightened down to about 1 lb, and had better luck, but it didn't last long.  (Last time I balanced the tube in the dovetail with the nut appx as tight as it is now, it will move in alt with about 6oz at either end of the tube.)

 

The bearing needs to be seated differently. I'm a little nervous about banging on it even with a piece of wood, it seems like as delicate as it is that would cause some damage if not extremely careful.  Followed by, I can't get it to move any deeper just squeezing it by hand; it feels like it seats pretty securely where it is.  If I need to completely disassemble it, would it be better to use a shoulder screw that has a diameter close to the ID of the bearing, and try drawing it in using a nut?  Just a thought; I know I asked for help and shouldn't dismiss any suggestions, but I'm nervous about damaging anything.

 

Someone (@doublevodka) posted a picture showing a similar mount with a nyloc nut instead of the one that mine has; is there a specific nut that is used for this, or is something from home depot good enough?  And also, several replies suggesting using loctite instead of a locknut; is that a common fix for this?  Again - I just want to make sure I'm not causing any damage going through this.  Loctite seems like a good way to make a bad situation even worse.

We need to be very clear.

The bearing must be seated fully and the shaft returned to centred.

At the moment the shaft is not centred, which means the gears on the other side are not meshed correctly which will seriously damage them.

The bearing has got stuck half way in because it is going in at an angle. It needs to be either removed and reinserted straight, or the bearing outer tapped where it is 'proud' until straight. Then push straight in. If you dont feel up to it, then find a local engineer. It's a simple job.