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Tal 2 mount DEC bearing overhaul- please help


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Hi, I bought a Tal-2m recently 🤦‍♂️ 

But I think it has some issues with its equatorial mount- particularly the dec axis which seems very rough in motion like it has ball bearings that have suffered a shock and dented their races.

So I’d like to open it up and have a look to maybe replacing them if they are standard bearings

I found Astro Baby’s excellent detailed tear down of the Tal-1 mount but the 2 seems a different beast even though it looks very similar.

I read of someone on here in the past replacing the RA bearings. Wondering if anyone ever came across information or ever took the DEC apart before? 

There are 2 grub screws that appear to lock the OTA mounting plate to the DEC shaft but I’ve undone those and it doesn’t want to budge and I don’t want to damage it anymore. It’s hard to say if that plate is threaded onto the shaft or just a press fit but it’s solid.

Any experienced help much appreciated

BTW the RA bearings feel ok though there is a definite detent when the DEC shaft is moved to be in line with the pier mount- wondering if it’s intentional to aid in putting the mount into its box or perhaps also damage 🤔

Mark

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Keep the scope fixed on the ground and use them to reposition The Earth?

I'm having to use my removed hat to mop up the drool. I'm reminded of my favourite quote from an old episode of bargain hunt; "I've always wanted one of those....... what is it?"

The shims arrived from China in record time so I spent this weekend rebuilding the mount again. The DEC shaft required 5x 0.3mm shims so 1.5mm total to get the bearings nicely snugged up without

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I made a bit of progress- the OTA mounting plate does screw onto the shaft after all and came loose while I was attacking the other end. Under it is a threaded collar that I assume holds the top bearing in place. Whilst the machining and design of this mount seem very robust they had an unfortunate habit of using pointed grub screws to lock threads- distorting the threads in the process and making dismantling for service a bit of a pain! I gave up unscrewing that collar after only managing about one turn before it felt like it was galling up. I had to grip it in a vice which is making a bit of a mess and will require paint touch up and re-forming of the steel notches. I tried drilling out the messed up threads where the grub screws bit and WD40, but no luck- I’ve given up for now. I think I’ll have to fabricate a special tool capable of a lot of torque to get this free.

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At the other end- the counterbalance weight end, there are more threads locked with grub screws 🤦‍♂️ It looks like the part the weight bar screws into is in turn only a short cap threaded onto the main shaft and locking the shaft to the inner race of the lower bearing and preventing its removal. I failed to loosen this part but in the process loosened the OTA mounting plate 👍😉

I think I’ll tackle this part next time with a bit of heat from a blow torch, gripping the other end in the vice. I don’t want to damage the threads though...

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I know this isn’t very interesting to most but I couldn’t find this info anywhere so maybe it’ll be of use to someone down the line.

Made progress today- I made a spanner for the upper bearing housing using the heads of 2 m4 Allen screws to engage the notches in the housing.

Alas I still couldn’t unscrew it...

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Then it clicked- the threads weren’t galling up at all. In fact I probably didn’t need the special spanner at all though nice to have. You have to remove the lower threaded cap as it locks the shaft and the upper bearing into the housing 🤦‍♂️ A little heat and it came off easily

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  • 1 year later...
Posted (edited)

Resurrecting this 1.5year old thread (maybe it should have been in the mounts section mods?) as I found a thread with an excellent disassembly guide by @AndyH in pdf form here: 

oh boy do I wish I had found that earlier (maybe I hadn't discovered search 🤦‍♂️) as it turned out I removed that steel bearing housing in the DEC casting for nothing- it's not meant to be removed at all! So don't do what I did- follow Andy's guide! I couldn't see the bearing under the flange of the DEC shaft but it would have pulled out without any of the hassle I got myself into. doh

Anyway I now have a need for this mount so finally getting round to looking at it again- haven't touched it since back then...

I think at some stage this head must have been dropped or maybe rust got to the bearings- the DEC bearings are really bad and notchy but looking closer at the RA axis after removing the motor, clutch and worm gear, it too was far from smooth with distinct notches where it would lock up like it had a ball detent (what I originally thought it was but why would it have a ball detent on the RA??) The worm bearings seem OK but I will order all new bearings for the RA and Dec axes and rebuild it.

Plan is to upgrade it to stepper motor drive like I did with my Tal-1m but maybe this time with a final belt drive stage to the worm to minimise backlash and try it for AP which I want to get into this year :) According to some it should be good for 20kg load! If I can get the PE low enough...

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Another vid of the bearings as the last one needs some weird codec

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From Andy's pdf the bearings required should be:

Upper (large) DEC: 1x  61908 or 6908 (62x40x12mm)

Lower (small) DEC: 1x 6904 (37x20x9mm)

Upper (small) RA: 1x 6005 (47x25x12mm)

Lower (large) RA: 1x 6006 (55x30x13)

Worm: 2x 16002 (32x15x8mm)

Mine are all open but I'll likely replace them with sealed type (2RS or just dust 2ZZ)

 

Mark

Edited by markse68
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So out with the old and in with the new

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It's lovely and smooth on both axes now

But I ran into a bit of a problem- well 2 as it goes. I don't know if the design originally used odd sized bearings and then later on they switched to standard sizes without updating the housings and shafts but both axes have what I think is a bit of a design flaw. Both axle shafts have shoulders that should sit on the inner races of the bearings. The housings have opposite shoulders that should sit on the outer races of the bearings. Threaded parts then tighten the assembly to prevent end float. But in my example the housing shoulders are 2mm further apart than the shaft shoulders once the bearing widths have been considered. So this means that the threaded part tightens onto the inner race pre-loading the bearings. Unfortunately rather heavy weight parts hang off those threaded tightening parts on long levers. This means that you either have to tighten the threaded parts so tight that the bearings bind and will be ruined in very short order, or you snug them up enough to take out the play in the bearings but then the heavy parts are free to wiggle the threaded joint which naturally has working clearances and is no match for the bending moments applied to them, giving a very wobbly feeling mount! I'm guessing now that the mount hadn't been dropped as I suspected to cause the stiff notchy bearings, but rather had been overtightened at the factory!

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Not sure if its clear in this video but both the threaded joints can be felt to move when I apply pressure to the weight bar.

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I guess the best thing to do would be to shim the inner race shoulders so that the inner race could be clamped against it whilst not overloading the bearings but that's going to be a lot of work... Or maybe threadlocking the threaded joints would provide enough gap filling to stop the rocking- but again there are quite some loads applied to the joints by the heavy parts.

Bit disappointing really 

Mark

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Edit- just ordered some shims- slow boat from China but 10 for £2.39 seemed reasonable. Oh yea, ebay is really great for bearings if you watch out for fakes (ask the seller)- the ones I bought (old stock) were a fraction of the cost from Simplybearings etc.

Edited by markse68
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Nice work Mark. I'd agree shims seem a mechanically better solution given the loads involved. Threadlock probably wouldn't hold up well for very long and if it crumbles and ends up in the bearing race it may just increase wear instead?

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2 hours ago, DaveL59 said:

Nice work Mark. I'd agree shims seem a mechanically better solution given the loads involved. Threadlock probably wouldn't hold up well for very long and if it crumbles and ends up in the bearing race it may just increase wear instead?

I think shims AND  thread lock should do it Dave  😉 The bearings I’ve bought as replacements are all sealed types so no worries with detritus getting in and ruining them. Bit stiffer but I don’t think that’ll be a real problem. 

I noticed the EQ5 uses tapered roller bearings at the load carrying ends but i don’t think I can get them in the same sizes and the shims i’m waiting for probably wouldn’t work with them anyway.  

I didn’t realise the worm gear is exactly the same as the one in the Tal-1M mount i stepperised so I’ll just put a quick connect in both and i can swap my stepper gearbox between them easily rather than build another- don’t use the 1 much at the moment anyway.

Mark

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  • 2 weeks later...

The shims arrived from China in record time so I spent this weekend rebuilding the mount again.

The DEC shaft required 5x 0.3mm shims so 1.5mm total to get the bearings nicely snugged up without too much pre-load.

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I put 2 short M3 screws in the holes for the locking grub screws to provide stops in the vice so I could really torque the screw joint up- I thread locked it too so hopefully it'll be a lot more solid. 

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Whilst the mount was apart I took the opportunity to drill and tap an M5 hole for mounting a polar alignment cam- the little ASI120 I just bought. Still need to design a mount for it.

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I decided as I'm going to use this for AP and will try to use plate solving, I don't need the DEC setting circle and it's a bit vulnerable sticking out at the bottom of the DEC shaft like that so best it goes. Trouble is without it the weight bar attachment looks kind of ugly so I designed a beauty collar that I'll get printed to fill the gap.

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The RA required 9x 0.3mm shims- 2.7mm total!

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It was a bit more tricky than the DEC as the shaft is short and recessed inside the motor housing so I had to make a special tool to engage with 2 of the holes in the worm-gear driven plate. Worked very well to allow decent torquing of the DEC axis housing onto the RA shaft, preloading the bearings.

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I really think something odd was going on at the factory when they built my mount. There's a 3mm thick threaded washer that is meant to screw onto the RA shaft before the DEC housing but with it in place the threaded end of the RA shaft is way too short for comfort- you can see that the locking grub screws didn't even reach the threaded part but just dented the end of the shaft. 🤔 

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It must have been designed originally for slimmer bearings but the new ones I bought are exactly the same dimensions as the old Russian ones I took out. Anyway I decided it would be much better to leave that 3mm washer out of the assembly- the additional 3mm of thread inside the DEC housing would be much stronger. Trouble is with that gone the RA setting circle wouldn't fit and I had planned to keep that one. So I cut a perspex ring to fill the ugly gap and 8mm thickness worked just right- might even act as stiffening support for heavy loads like the plastic rings in the Vixen GP mounts.

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So it's all back together again now with lovely smooth motion on both axes and feeling very solid now :)

Mark

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And stepper drive fitted.  Might see about fitting a stepper drive that can be used with a guide cam at some point. Stronger springs fitted for the clutch- might need even stronger though.

Mark

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Thanks Dave :) Not finished yet but getting there.  I was hoping that I’ll be able to use it for some photography with reasonable exposure length without guiding- it has a reasonably large RA worm gear and the stepper should run at a pretty constant speed. The more I read up on it though the less convinced I am that it’ll work- but it’s fun trying ;)

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Of course having no Dec drive, i’ll be relying on as perfect polar alignment as i can get- so I bought this little ASI120 in the classies and just bought a cheap 35mm lens which should work i hope with Sharpcap to achieve that. The fast c-mount lens should help too. A copy of the Polemaster really. Just need to design and print a mounting plate for it. I found this nifty little quick release from smallrig 👍

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Mark

(wish i could change the title of this thread to “Pimping a Tal-2m”)

Edited by markse68
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  • 4 weeks later...

That was a long wait but the parts finally arrived from the printers so i could finish this. Firstly a mount to fit the QR and polar cam to the Tal

then a “beauty” ring to fill the void left after removing the dec axis setting circle. It’s not very pretty but better than the big gap i think.

Trouble is in the weeks of waiting I decided i really need a motorised dec axis and my mind has wondered off onto another project so it’s all a bit superfluous now. Oh well- I’ll at least experiment with polar alignment in sharp cap with this mount.

Mark

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I'm having to use my removed hat to mop up the drool.
I'm reminded of my favourite quote from an old episode of bargain hunt; "I've always wanted one of those....... what is it?"

  • Haha 2
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Harmonic drive units with integrated motor- ex google robotics i believe! Found them v cheap on ebay and couldn’t resist. Bit of a project though 😬🤞

Mark

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13 minutes ago, markse68 said:

Thanks Glob. I’m hoping i haven’t bitten off more than i can chew with the new project- couple of these arrived last weekend

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What are you planning to drive with those Mark, the rudder on a large-scale model of the titanic? 😉 

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Lol 😂 They should make a v nice mount i’m hoping Dave but there’s only one way to find out 

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I already bought a dovetail for it so I’m committed now 

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