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Crayford 10:1 adjustment

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#1 speedster

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Posted 06 December 2018 - 07:59 PM

Refurbing a William Optic 80 and the 2 speed Crayford style focuser has me stumped.  The 1:1 knobs work fine.  The 10:1 knob hits random dead spots and quits turning the shaft.  No pattern.  It just turns without engaging the balls.  With the slightest turn of the 1:1 knob, the 10:1 engages and all is well until the next random dead spot.  I mean "slightest" turn - maybe a degree or two.

 

I took it apart and found the balls virtually dry.  Sprayed lithium and then packed lithium.  That helped overall but did not solve the problem.  Adjusted the nut both ways and the final adjustment helped overall but also did not solve the problem. 

 

Any ideas on this?


Edited by speedster, 07 December 2018 - 01:59 AM.


#2 speedster

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Posted 06 December 2018 - 08:25 PM

The 10:1 knob only slips when racing the tube "in".  Racking out, it always works fine.  When it slips racking in, it still works racking out.



#3 mikerepp

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Posted 07 December 2018 - 04:02 PM

Sounds like it just needs a little more tension.  Try adjusting the tension screw a little and see if that makes a difference.



#4 Eddgie

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Posted 08 December 2018 - 10:43 AM

If the coarse knob drives properly it is most likely not the tension screw on the focuser bottom. In this case it is far more likely that the preload on the gear drive is not sufficiently tight.

 

This page may not be exactly the same as your focuser, but pretty much all of generic Crayford 2 speeds (including the Moonlight) use some form of pre-load to ensure that the fine speed of the focuser has enough friction to engage.   If it is too tight, the focuser will be stiff, and if it is too loose, it will slip in low speed.

 

http://polluxchung.c...o_crayford_fix/

 

Again, this assumes the the focuser works well in 1:1.

 

The step relevant to the low speed pre-load is step 11.  

 

Now, if you are using a very heavy load (like Binoviewer or other very heavy load) and the 1:1 knob also slips, that requires that you adjust the tension on the focuser drive shaft support bearing, but again, if the scope is only slipping in low speed and the load is a normal load, you probably need more pre-load.

 

In some focusers like the Moonlight, there is a spring between the nut and the drive and if this spring is too weak for the load (again, binoviewer is the best example) it sometimes has to be replaced with a heavier spring (the Moonlight Refractor Focusers use a heavier spring than the Newtonian focusers because they have to support the weight of the diagonal when the scope is pointing at zenith.  Newtonian rarely gets to a point where the draw tube is vertical).

 

Anyway, if the focuser appears to work properly in 1:1 and is slipping in 10:1, then I would suggest that there is a pre-load issue.


Edited by Eddgie, 08 December 2018 - 10:45 AM.


#5 Eddgie

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Posted 08 December 2018 - 11:07 AM

And a caution.. As long as the drive works properly in 1:1, do not add more pressure to the pinion support bearing!!

 

Adding pressure to this bearing will do nothing to fix a slipping 10:1 and in fact, additional pressure on the knob shaft bearing will make the problem worse.

 

If you have to increase the friction of the pinion shaft support to control your load to the point that the 10:1 gets rubbery, your focuser is not sufficient for your load and in this case, consider a rack and pinion or Feathertouch rail type Crayford. Both of these can carry very heavy loads without having to resort to excessive pinion bearing pressure.  Jacking up the pressure on the pinion support is the path to bearing failure and focuser tube damage.


Edited by Eddgie, 08 December 2018 - 11:08 AM.


#6 Eddgie

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Posted 08 December 2018 - 11:17 AM

If there is a spring between the preload nut and the back plate, it is possible that even with the spring fully compressed it does not have sufficient tension.

Make sure you got all shims and parts facing out in the right direction. 

 

Again, if you are driving properly in 1:1, the problem has to be in the Pre-laod area, or you assembled something incorrectly.   



#7 Jon Isaacs

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Posted 08 December 2018 - 11:29 AM

I have worked on a number of Crayfords with 2 speeds but never come across this problem. 

 

It reminds me of a Crayford that had the bearings that supported the drawtube coated in plastic. I could take it apart, reassemble it and it would work for a while. Then it would quit, I take it apart, reassemble it..  I discovered the plastic coating and then I found each bearing had a flat spot. When the flat spots lined up, it quit working.

Your symptoms certainly suggest a flat spot but normally that results in a rough focuser.

Here's something to look at:

 

The outer bearing of the two speed is secured to the focuser with a set screw. If that is not tight, the two speed will not work properly, that spins.

Jon




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