Help with dec backlash (video)

[Brent]

I have an old problem with massive dec backlash that PHD2 is struggling with and I'm trying to get it sorted now.

I have the motor off and have some play in the gear. After checking that the gear grub screw was tightened down on the shaft (it was), I have realised that the gear shaft turns with the gear and so the play appears to be within the motor itself 🙁

If I was to open the motor up is it likely that there would there be some way of tightening the shaft? Or has the motor had it?

 

Edited May 6, 2020 by Brent

[Brent]

1 hour ago, KevS said:

Is there a "grub screw" holding the spur gear to the motor shaft? There is normally a "flat" that it sits on. If the bearings are shot on the motor you will probably be able to move the gear and the shaft vertically. I would expect some apparent radial play between magnets, but not vertical or lateral play.

 

K

 

Oh, yes thanks, forgot to say that the grub screw is sat on the flat of the shaft and tight, no detectable movement at all between them. The shaft will also move in and out of the motor housing maybe by about a mm or two, so if I understand what you're saying that would indicate that the bearings have had it?

[Brent]

12 hours ago, KevS said:

If the motor shaft is moving longitudinally it probably indicates that  the motor shaft is slightly loose and moving through the bearings, or the bearings are moving in the housing(s); that's really nothing to worry about short term as there is no vertical or lateral play, as when the shaft is "loaded" it will all probably take it up. Although it may be worth your while investigating, if possible what is happening. If you were to reassemble the mount, ensuring that there is the absolute minimum play/backlash in each pair of spur gears all the way through the system and then carefully adjust the worm and wheel mesh you will probably solve it. Also make sure that all of the spur gear grub screws are located on the "flats" of their relative shafts.  When adjusting the worm and wheel backlash, a 1/16 of a turn on the adjustment screws can make the difference between the mount binding and running well. 

K

Thanks for that. I'd got it back together and was outside when you replied, sorry. Everything else feels good and tight so I didn't bother messing with the worm and wheel mesh as I think I managed to sort that a couple of years back, I think the issue is the motor shaft now. Everything was back to 'normal', at least I hadn't made anything worse.

Just as a test as I haven't done it properly before, I guided in one direction and dithered in RA only. Not too happy to dither long term like that but it seemed to work to keep the dec more under control. I haven't stacked anything to see if that will be noticeable yet. An EQ6 Pro purchase may be closer than I thought though!

[1parsec]

Does the motor have an integral gearbox attached ? Can you post another photo of it.

 

 

 

[Brent]

On 07/05/2020 at 12:13, 1parsec said:

Does the motor have an integral gearbox attached ? Can you post another photo of it.

 

 

 

Sorry, been snowed under with work and didn't see this.
The only photo I have at the moment is this, don't know if it will be of any help.
I'd removed the three bolts form the right hand side to try to slide the motor out but the ferrite core (?) won't go through the housing without cutting the wires.
I didn't want to do that there and then but will do if necessary and resolder after. Thanks.

 

Edited May 10, 2020 by Brent

[1parsec]

That looks like a DC servo motor with geared head fitted on the front of the motor for reduction.

I would say the play in the gear shown in your video is the natural backlash you find on the output shaft of a gearbox of this type.
It could be one of the internal gears has worn on it's shaft allowing more clearance between the internal gears and increasing the backlash. If that's the case, it is going to be quite an involved fix.

The thing to try first would be to have a small imbalance in the DEC axis so the 'weight' of the scope keeps the gears pressed in one direction which will take up the backlash.

 

 

[Brent]

46 minutes ago, 1parsec said:

It could be one of the internal gears has worn on it's shaft allowing more clearance between the internal gears and increasing the backlash. If that's the case, it is going to be quite an involved fix.

Thanks. I now think this may be the biggest issue then as until PHD2 started to warn of this I had very minimal dec backlash which wasn't a problem to PHD2.
 

46 minutes ago, 1parsec said:

The thing to try first would be to have a small imbalance in the DEC axis so the 'weight' of the scope keeps the gears pressed in one direction which will take up the backlash.

Thanks for the suggestion, I've done this for a long time and last time out also tried guiding in one direction only and RA dithering only. It didn't help the calibration of course but did seem to stop the constant struggle back and forth during guiding.
I was hoping to tackle this at source (which might be beyond me by what you've said), so I will keep an eye out for a replacement drive for the time being if I can get one cheap enough. They seem to be getting rarer to find now though, so failing that an EQ6-R Pro may be on the cards.

 

 

Edited May 11, 2020 by Brent