Dual speed focuser renovation

[Giles_B]

I've just taken delivery of a second hand Altair Astro Lightwave 66ED-R. The fine focus on the rack and pinion focuser is not performing as it should: the fine focus knob turns the focuser at the same speed as the main focus knob.

I've now dismantled the focuser to try to work out the source of the problem. There are nicks and marks on the bearing cage where it has been adjusted or opened with the wrong tool. Opening the bearing cage, it is about half full of bearings, about ten I'd say from memory.

My understanding is that the fine focus works by the bearings having a gearing effect. My guess is that either some of the bearings have been lost and this is causing a lack of transmission to the fine focus mechanism, some bearings have been added or perhaps the wrong type of bearing has been inserted.

I'm new to the mechanics of focusers, and wondered if anyone had any insights into the problem? Ideally, I'd like to find out how many bearings should go into the focuser, what size of bearings are needed, should any need replacing, and where I could source them. Of course you might also tell me that I am barking up the wrong tree or put me onto another component that could be causing the issue.

I have set up a ticket with Altair, but their tech support can be very slow to reply, so posting here in the hope of working this out a bit sooner!

Edited June 11 by Giles_B
Answering my own questions, so feels more like a project!

[Giles_B]

So... I have a part answer to my own question inasmuch as I found Astrofriend's blog which details stripping down a fine focuser mechanism. I haven't gone through it yet, other than to note that -

1. yes, the grease in my focuser is also gluey

2. no, I don't think I have the right number of tiny bearings in my focuser (so I now have a selection on order)

3. my understanding of the purpose of the two sets of the bearings was slightly off beam

The blog is linked below, and I'll come back to this thread with some pics once I'm done.

https://www.astrofriend.eu/astronomy/projects/project-focus-fine-tuner/project-fine-tune-focuser.html

[inFINNity Deck]

Hi Giles,

see fig.3 on my webpage about my SkyWatcher Esprit 150ED: https://www.dehilster.info/astronomy/skywatcher_esprit_150ed.php

The tiny balls, 18 in my FeatherTouch focuser, should fill the whole bearing.

The purpose of the large balls is to create friction between the central thin pin and the conic outer bearing cage. This friction is created by tightening the large brass ring-nut, which pulls the three balls down in the conic cage, and to avoid that this causes too much friction between the gear housing and that nut a bearing with small balls is placed between them.

As I wanted zero slippage I replaced the whole friction planetary gear by teethed planetary gear from a electro-motor gearbox (see above page).

Nicolàs

[Giles_B]

@inFINNity Deck

Nicolàs, many thanks for sharing this, it is very helpful - I hadn't realised the mechanism for increasing the friction between the large bearing and the outer cage and was a bit perplexed that the three balls were rotating freely - clearly some of the small bearings have escaped at some stage as the cage is only 1/3 full, and there is insufficient friction, until the bearing cage is made quite tight, then all of a sudden there is too much friction to turn the darn thing!

Hopefully by replacing some of the bearings in the cage and regreasing everything with some Aeroshell, I can get it working again.

[inFINNity Deck]

Hi Giles,

that cage should indeed be properly filled as otherwise the pressure of the ring-nut does not get properly and evenly distributed. And replace all balls, so that they all have the same size. Then when tightening do it in steps and test whether it holds enough load. When overtightened the central pin may get dented at which stage the focuser gets beyond repair (unless you can source another pin).

Nicolàs

[Giles_B]

15 minutes ago, inFINNity Deck said:

When overtightened the central pin may get dented at which stage the focuser gets beyond repair (unless you can source another pin).

Ah - this might be a problem. The central pin is very flattened at the business end - I assumed this was by design, but the cage had been tightened very firmly, so from your description this sounds like deformation. I presume with the flattening the pin won't distribute the friction evenly, or probably won't even turn.

So it looks like I'll have to scour the web for another pin....

On the other hand the mechanism looks very similar on your Espirit, the internal parts are almost identical to the ones I have. I wonder if I bought a retrofit micro focus kit like this lacerta kit, perhaps even the internals would be the same and I could replace all or part. However the cost sort of cancels out the benefit of having bought the scope second hand. So I will look for a pin. Again thanks, it would probably have stumped me why my fix didn't work if you hadn't mentioned this.

[Giles_B]

 

 

Actually a quick search shows lots of bearing dowel pins out there, and only a few pounds for a dozen.. I just need to work out the size...

2mm x 20mm dowel pins now ordered - had to go with Amazon, not a big enough order from other sources, even so I will have 19 spares in case I overtighten!

 

Edited June 11 by Giles_B

[inFINNity Deck]

7 hours ago, Giles_B said:

 

 

If this photo is the pin in your focuser, then that is not the original one. The pin should be smooth, only having a smooth thinner section where the large balls run against. The original pins are made of hardened steel, not sure if the dowels you found are made of it as well. If not, they will be too soft.

cheers,

Nicolàs

[Giles_B]

Thankyou again - the pins on order are stainless steel, so maybe not hard enough.

So a quick search this morning found an engineering supplier with hardened pins - https://springfasteners.co.uk/product/solid-dowel-pin-din-6325-2mm-x-20mm/

The bearings (also Amazon I'm afraid) arrive on Saturday , so once all these parts arrive I'll let you know the outcome.