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Rigid or flexible focus motor couplings....


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Hi all,

I have a flexible motor couple on my Pegasus Motor focuser on my scope, but I notice now that Pegasus and some other motor focus suppliers are using rigid motor couplers.... My question is why, is there a performance or other reason for the move to rigid couplers...?

Thanks

Stuart đŸ‘đŸŒ

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  • 1 year later...

Good question!! I am quite interested if anyone can give an answer. I have had to move to rigid coupler between my ZWO EAF and my Esprit100 because flexible couplers delivered with EAF are too much big and dont fit in the knob axis.

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Although these couplers are cheaper than what we used on our high precision grinding machines (we achieve micron and sometimes sub micron on some machines) for encoder couplings the principle is the same and there should be no difference in rotational position but the slight flex should allow for some small amount of error in alignment of the two spindles.

The coupling should not flex at all in a radial manner but can flex slightly side to side or up and down to allow for this misalignment.

So my opinion is that a flexible one would be better than a fixed one unless you can guarantee absolute perfect alignment of the two spindles which in most situations you cannot as they are just not manufactured to a high enough tolerance on most amateur equipment.

Steve

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I think it will depend on the coupler. The Sesto senso ones are slightly flexible in the direction of the drive but there is virtually no rotational flexibility. I have certainly not had repeatability issues with this and the Baader Steeltrack.

I guess the advantage is that if the alignment is not perfect it will not put excessive load on the focuser or motor.

Edit. Beaten to it...

Edited by Clarkey
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Having thought about it a bit more I will add that my statement does maybe also depend on the focusser being rigidly attached to the scope with no movement between the two whatsoever.
So a focusser say on a L shaped bracket could have some flex, in a back to front direction, not much but maybe enough to lose one or two pulses of the stepper motor. 
I have two sesto senso focusers and I like how they attach rigidly to both my Esprit 100 stock focusser and RC6 upgraded Baader Steeltrack and I think in this instance the flexible one is the way to go..

But, maybe on one with a L shaped bracket if it is not absolutely solid and it could flex back and forth ever so slightly there is a situation I can see a solid coupling being better, but I would get the alignment as near perfect as you can, so maybe even locking the coupling to both spindles before fully tightening to the L shaped bracket.
The problem being is if there is any movement back to front then the coupling sort of unwinds, or winds up depending on direction of movement, like a spring , all very slight but it would maybe again loose a pulse or two of movement, developing some slight backlash as the coupling winds up and winds back on change of direction.

Steve

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I had a flexible on my Tak FSQ85 and moved to a solid coupling, as long as it is fitted correctly, no issues at all. đŸ‘đŸŒ
 

fit the coupler to the focuser shaft, and tighten down, then fit motor bracket loosely, fit motor to bracket loosely, then slide the motor shaft into the coupler and tighten bracket down, then the important bit, with motor loose on bracket, run the motor slowly to centre the shaft in the coupler, this will move the motor on the bracket as needed, then tighten motor on bracket, run it again and make sure motor shaft turns in coupler with no radial movement, then tighten grub screw on motor shaft
then finally run it 360 degrees in both directions to check for and forced movement  in the motor and bracket assembly, which would indicate radial movement

..simples
😀

PLEASE NOTE, the above instruction were for fitting the motor to the coarse focus shaft and I don’t recommend fitting a solid coupler to the fine focus shaft
just thought I would mention that
as some focusers use this side of the focuser
.my Pegasus does not..đŸ‘đŸŒ

Edited by Stuart1971
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56 minutes ago, Stuart1971 said:

I had a flexible on my Tak FSQ85 and moved to a solid coupling, as long as it is fitted correctly, no issues at all. đŸ‘đŸŒ
 

fit the coupler to the focuser shaft, and tighten down, then fit motor bracket loosely, fit motor to bracket loosely, then slide the motor shaft into the coupler and tighten bracket down, then the important bit, with motor loose on bracket, run the motor slowly to centre the shaft in the coupler, this will move the motor on the bracket as needed, then tighten motor on bracket, run it again and make sure motor shaft turns in coupler with no radial movement, then tighten grub screw on motor shaft
then finally run it 360 degrees in both directions to check for and forced movement  in the motor and bracket assembly, which would indicate radial movement

..simples
😀

Yes I would agree, pretty much along the lines I was thinking in an earlier post..
I think maybe its another case of horses for courses. If you have a bracket with the stepper on and can easily adjust it as you suggest then a solid one probably is slightly better but I am not sure it would be as easy on a sesto senso where the coupler is inside and only can be adjusted through a slot and the collar of the focusser then attaches directly to where the rough focusing knob was attached.

And then there is the question are we over thinking this anyway.
If the motor is attached directly to the fine focussing spindle (not a belt) does it really matter to 2 or 3 pulses ????
To my eyes even really zooming in when I get everything in focus I can move 10 or more steps either side and I cannot tell any real difference in focus. 
Maybe that depends on the focal length of the scope and maybe some are more critical than my Esprit 100 ???

Steve

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1 minute ago, teoria_del_big_bang said:

Yes I would agree, pretty much along the lines I was thinking in an earlier post..
I think maybe its another case of horses for courses. If you have a bracket with the stepper on and can easily adjust it as you suggest then a solid one probably is slightly better but I am not sure it would be as easy on a sesto senso where the coupler is inside and only can be adjusted through a slot and the collar of the focusser then attaches directly to where the rough focusing knob was attached.

And then there is the question are we over thinking this anyway.
If the motor is attached directly to the fine focussing spindle (not a belt) does it really matter to 2 or 3 pulses ????
To my eyes even really zooming in when I get everything in focus I can move 10 or more steps either side and I cannot tell any real difference in focus. 
Maybe that depends on the focal length of the scope and maybe some are more critical than my Esprit 100 ???

Steve

Well, I have to be honest, I would never use a solid coupler on the fine focus shaft, my advice was just for using the motor on the coarse focus side of the focuser, as my Pegasus is fitted
.too much flex in the fine focus shaft and that could be easily damaged
.😼

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7 minutes ago, Stuart1971 said:

Well, I have to be honest, I would never use a solid coupler on the fine focus shaft, my advice was just for using the motor on the coarse focus side of the focuser, as my Pegasus is fitted
.too much flex in the fine focus shaft and that could be easily damaged
.😼

Ah well I guess we have two completely different setups.
Apart from a DSD focusser on my WO73, which was a timing belt, I only have had 2  sesto senso focusers which as far as I am aware can only attache to the fine focusser spindle, well that's what they are designed to do anyway.
So I guess because they are on the fine focuser then it takes a lot of pulses to move a short way and so a few pulses here and there really do not affect focus to any noticeable extent.
I guess if the two spindles cannot be lined up accurately then maybe a solid one s not a great idea.

Now then, this shows how forgetful I am but just went to check my setups.
My first 
 sesto senso was the version 1 and just looked that actually has a solid coupling, which I thought was odd because in my mind I was sure it was a flexible one. But what confused me is the 2nd purchase for another scope, which was a  sesto senso 2 that does have a flexible coupling, so they seemed to have changed the other way from a solid one to a flexible one.

But after saying that I cannot remember having much of an issue lining up the version 1 with the two spindles.

Steve
 

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