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AZ Gti worm bearing rough


JimothyC
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Took my AZ GTi out of a spin when I was camping a couple of weeks back. I was lucky enough to have a couple of clear nights on the Hartland Peninsula in Devon during a new moon. The sky was spectacular being only Bortle class 2, unfortunately the mount was not. I'd just upgraded the wedge to one of the Williams Optics numbers, which made polar alignment a lot easier and more reliable than the Skywatcher Star Adventurer one I'd been using previously. But once aligned and pointed at my target the mount refused to play ball, PHD was reporting huge declination issues, which I would have thought wouldn't really be an issue with good polar alignment.  PHD wasn't even able to complete calibration as the star wasn't moving enough. 

Suspecting backlash in the Dec axis, I followed one of the many guides on the internet to adjust the backlash between the 2 45 tooth brass gears which connect the motor gearbox to the worm gear.  Once apart the backlash didn't seem all that bad to be honest, but I thought it was worth a try adjusting. Whilst I had the brass gear off of the end of the worm gear I was able to rotate the worm shaft freely, it was then that I noticed that one or both of the bearings that support it were feeling very rough. I would have expected this to turn smoothly and with minimal resistance, but this wasn't the case. Which leads me to the following questions. 

Has anyone else who's had one of these apart had a similar experience? Is this normal levels of quality for a cheap Skywatcher mount and I'm expecting too much? Has anyone had any joy replacing the worm gear bearings? Can you get hold of spare parts for these from Skywatcher (I suppose I should ask them that)?

I've had the mount for a while, so it's well out of warranty. But as I'm rather invested in the accessories for it now I'm tempted to just get another one. But I would rather repair it and would like to avoid the same problems with a new mount. This mounts was one of the early ones, which I got in a package with a 127 Mak. Has the quality of these improved, or is it still pretty hit and miss? It might be 3 or 4 years old now, but it's not had a lot of use, certainly not enough to wear out a bearing. 

If anyone can help with one or more of my questions, I would be most grateful. Thanks. 

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Just a quick follow up on this if anyone is interested. I spoke to the UK Distributor Optical Vision who offered to order a replacement motor and worm drive unit from Skywatcher. The cost for both axis was going to be around £150 including delivery and it would take at least 4 months. Give that a new unit is £250, I think I'm going to have a go at replacing the bearings myself and if I break the unit catastrophically I'll order a new one. I'll try and document the process, in case it goes well. 

I'd like to mention that Optical Vision were extremely helpful and prompt in their responses, so thanks to them. 

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  • 2 weeks later...

I've finally completed refurbishing my AZ-GTi with new bearings. Sorry to say that I didn't really take any additional photo's over and above what's already available on here and CN, so I won't bother posting them. 

I did however measure all the bearings, so if anyone wants to have a go at the same job, they'll know what to get. I thought as I was taking it apart, I'd replace all the bearings in the mount with higher quality ones and clean and regrease all the parts with high quality PTFE grease. 

                    Inner mm    Outer mm     Width mm    in washers mm    Qty    Price £ GBP    Total    Item
Worm Gear            4            11            4                            4    £6.01    £24.04       https://simplybearings.co.uk/shop/p6710/SKF-619/42Z-Metal-Shielded-Deep-Groove-Ball-Bearing-4x11x4mm/product_info.html
RA Axis Thrust        45            65            3            5                1    £8.90    £8.90       https://simplybearings.co.uk/shop/p9634/AXK4565-Major-Branded-Needle-Roller-Cage-with-2-AS4565-Washers-45x65x5mm/product_info.html
RA Axis Spindle        17            26            5                            1    £12.18    £12.18       https://simplybearings.co.uk/shop/p366616/NSK-6803ZZ-Metal-Shielded-Deep-Groove-Ball-Bearing-17x26x5mm/product_info.html
Dec Spindle Outer    17            30            2            4                1    £4.13    £4.13       https://simplybearings.co.uk/shop/p9628/AXK1730-Major-Branded-Needle-Roller-Cage-with-2-AS1730-Washers-17x30x4mm/product_info.html
Dec Spindle Inner    20            35            2            4                1    £4.38    £4.38       https://simplybearings.co.uk/shop/p9629/AXK2035-Major-Branded-Needle-Roller-Cage-with-2-AS2035-Washers-20x35x4mm/product_info.html
                                
                                
Total ex Vat                            £53.63    
In Vat                                    £64.36    
 

Apologies for the table formatting. 

It all went back together pretty easily. The only issue I encountered was making sure you don't tighten each of the axis to much. I found after reassembly that the RA/AZ axis was quite stiff, so I disassembled and loosened off the retaining collar on this shaft a little and this freed up things nicely.

The mount seems to been working fine now, but I've been rather cloud restricted to try testing its guiding performance. But the axis seem to move much more freely now and it's much easier to balance. 

Edited by JimothyC
Table formatting
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  • 2 weeks later...
On 30/07/2021 at 15:37, JimothyC said:

Just a quick follow up on this if anyone is interested. I spoke to the UK Distributor Optical Vision who offered to order a replacement motor and worm drive unit from Skywatcher. The cost for both axis was going to be around £150 including delivery and it would take at least 4 months. Give that a new unit is £250, I think I'm going to have a go at replacing the bearings myself and if I break the unit catastrophically I'll order a new one. I'll try and document the process, in case it goes well. 

I'd like to mention that Optical Vision were extremely helpful and prompt in their responses, so thanks to them. 

@JimothyCWere the replacements from SkyWatcher (i.e. the same as those already in the AZ-GTI?

When I tried to move the motor to reduce the backlash on mine, I noticed that the spindle on one of them moved slightly side-to-side which meant that the gear engagement was always slightly loose.

I'd be interested if I can get a better quality replacement, but if I still have to rely on the luck of the draw in terms of SW quality then it wouldn't be worth it.

   

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All the replacement bearings were purchased form a 3rd party supplier. Skywatcher wanted far to much for a replacement motor and worm unit. 

Which side is the movement coming from when adjusting the backlash? If it's the worm side, it could be the bearing. If it's the motor side when there maybe some movement in the planetary gear head on the motor. These can be carefully dismantled and cleaned and regreased with tweezers and a keen eye, but I'm not sure if that will help. You might also want to check the screws which attach the gear head to the motor are done up properly. 

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1 hour ago, JimothyC said:

All the replacement bearings were purchased form a 3rd party supplier.

My apologies, I meant were the replacement motors quoted at £150 ones that would come from Skywatcher? As you say, way too expensive, but if you knew they were going to be better quality it might be able to turn a sow's ear into a silk purse 😉

 

1 hour ago, JimothyC said:

Which side is the movement coming from when adjusting the backlash? If it's the worm side, it could be the bearing. If it's the motor side when there maybe some movement in the planetary gear head on the motor.

It is the motor side, and the thought of opening and fiddling with internal gears fills me with dread - I don't have the right sized hammer, for a start 🤣

 

Your helpful list of bearings has been bookmarked, though, thanks 👍

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3 hours ago, adyj1 said:

My apologies, I meant were the replacement motors quoted at £150 ones that would come from Skywatcher? As you say, way too expensive, but if you knew they were going to be better quality it might be able to turn a sow's ear into a silk purse 😉

Sorry didn't read your post properly. Yep these would have come from Skywatcher via the UK distributor. So would have been the same or whatever spec they're using now. 

I suppose it would be technically possible to source an alternative as long as you get the correct specs. I think you're looking for a 45:1 reduction or at least that's what a random PDF I found on the internet told me. There are websites full of the things, such as https://www.robotshop.com/uk/planetary-gearmotors.html but it would be an expensive experiment. I'm not entirely convinced what is sold as a premium component here isn't the same thing as a Chinese factory would be buying for pennies anyway. 

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  • 3 weeks later...
On 08/08/2021 at 14:17, JimothyC said:

I've finally completed refurbishing my AZ-GTi with new bearings. Sorry to say that I didn't really take any additional photo's over and above what's already available on here and CN, so I won't bother posting them. 

I did however measure all the bearings, so if anyone wants to have a go at the same job, they'll know what to get. I thought as I was taking it apart, I'd replace all the bearings in the mount with higher quality ones and clean and regrease all the parts with high quality PTFE grease. 

                    Inner mm    Outer mm     Width mm    in washers mm    Qty    Price £ GBP    Total    Item
Worm Gear            4            11            4                            4    £6.01    £24.04       https://simplybearings.co.uk/shop/p6710/SKF-619/42Z-Metal-Shielded-Deep-Groove-Ball-Bearing-4x11x4mm/product_info.html
RA Axis Thrust        45            65            3            5                1    £8.90    £8.90       https://simplybearings.co.uk/shop/p9634/AXK4565-Major-Branded-Needle-Roller-Cage-with-2-AS4565-Washers-45x65x5mm/product_info.html
RA Axis Spindle        17            26            5                            1    £12.18    £12.18       https://simplybearings.co.uk/shop/p366616/NSK-6803ZZ-Metal-Shielded-Deep-Groove-Ball-Bearing-17x26x5mm/product_info.html
Dec Spindle Outer    17            30            2            4                1    £4.13    £4.13       https://simplybearings.co.uk/shop/p9628/AXK1730-Major-Branded-Needle-Roller-Cage-with-2-AS1730-Washers-17x30x4mm/product_info.html
Dec Spindle Inner    20            35            2            4                1    £4.38    £4.38       https://simplybearings.co.uk/shop/p9629/AXK2035-Major-Branded-Needle-Roller-Cage-with-2-AS2035-Washers-20x35x4mm/product_info.html
                                
                                
Total ex Vat                            £53.63    
In Vat                                    £64.36    
 

Apologies for the table formatting. 

It all went back together pretty easily. The only issue I encountered was making sure you don't tighten each of the axis to much. I found after reassembly that the RA/AZ axis was quite stiff, so I disassembled and loosened off the retaining collar on this shaft a little and this freed up things nicely.

The mount seems to been working fine now, but I've been rather cloud restricted to try testing its guiding performance. But the axis seem to move much more freely now and it's much easier to balance. 

Really useful post but I think the first link for the SKF-619 is possibly incorrect, as this is a small ball race for a thin shaft and not a worm gear???
 
I replaced the main bearings but couldn’t remove the Dec inner spindle bearing, so I just greased it with Lithium grease and left it :)

 

I removed all obvious backlash my ensuring the motor gears were fully meshed and fixed the motors hard up to the main gears (instead of relying on the springs to press the worm drive to the gear).

That all said, I couldn’t calibrate the DEC on my Asiair pro :) It sort of works in RA mode only but still get star trails for 60 sec subs with my Redact51. I was really hoping the the Skywatcher GTI in EQ mode would be the perfect portable setup, but so far it’s been terrible. I got far better results with my IOptron Skyguider Pro, ie several minutes without trails ??

Edited by odyseus10
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Yep that's definitely the right one. You have 1 that sits at either end of the worm gear shaft. 
 

I don't have a better picture than this, but they sit roughly where the red boxes are, inside the motor/worm assembly. 

I've not had a chance to check it's performance in EQ mode with guiding, but it's performed better during some Alt/Az planetary imaging. Going away camping next week, so hopefully get a change to try it out then. 

azgti-bearing.jpg

Edited by JimothyC
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On 07/09/2021 at 14:01, JimothyC said:

Yep that's definitely the right one. You have 1 that sits at either end of the worm gear shaft. 
 

I don't have a better picture than this, but they sit roughly where the red boxes are, inside the motor/worm assembly. 

I've not had a chance to check it's performance in EQ mode with guiding, but it's performed better during some Alt/Az planetary imaging. Going away camping next week, so hopefully get a change to try it out then. 

azgti-bearing.jpg

I right I can see what you mean now :) Many Thanks. I didn’t want to force off the middle bearing on the RA which was stuck fast. I actually pressed each worm gear against the main axis gear drive and bolted in place ie they were not sprung anymore.

I think I sort of got it working with my ASIair Pro but I had to set the aggressions in the guiding to very low percentages. I still think this mount is the best option for a portable system, hence I am keen to perfect it :) 

I decided to but the Williams Optics wedge (I had one of these before and was excellent) as the skywatcher wedge flexes too much and it very difficult to align and get decent polar alignment :(

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  • 1 year later...

Hello, did you manage to replace the RA or ALT worm gear? I saw you provided a link but the link isn’t for the worm gear . A replacement motor was going to be too expensive 

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