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Zhumell 2" single speed Crayford focuser problems

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#1 Scanning4Comets

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Posted 25 June 2013 - 04:35 PM

Hi there,

I currently have a Zhumell 2" single speed Crayford focuser on my 10" dob. It is an upgrade from the one I originally had on the 10" F/4.7 Skywatcher one that was originally on there.

I don't know if this has anything to do with how the focuser works, but mine is mounted sideways so that the focusing knobs are in a different orientation than normal. See my pic attached:

I am now using heavier eyepieces, (a 14mm ES 100 and a 9mm ES 100), and I find that the only way I can use the focuser is if I have the tension knob fairly tight so the focuser won't "slip". The only thing about this is that the focusing becomes somewhat "stiff" and no longer has any smooth action.

Is there anything I can do to fix this? or am I looking at getting yet another focuser which is better?


PS: I was thinking of getting a 2" GSO Linear Bearing focuser to replace it.....Is that one a lot better?

http://agenaastro.co...eflectors-du...

...Or do I just need this below? :question:

http://agenaastro.co...pgrade-kit.html

Cheers,

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#2 csrlice12

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Posted 25 June 2013 - 05:59 PM

Yes, the linear focus were designed for large eyepieces, cameras, etc. They will handle a lot more weight than the standard crayford. They have a steel shank attached to the focuser tube that the friction bearing rides on. This allws for more tightening and somehow smoother motion.

#3 tag1260

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Posted 25 June 2013 - 06:32 PM

The two speed conversion kit won't help you with that particular problem. All it does is give you the two speed option. It doesn't make it any smoother. There a web page out there about taking your focuser apart and re-working it to make it smoother but I don't have it handy at this moment.

#4 tag1260

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Posted 25 June 2013 - 06:36 PM

Here it is.

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

#5 Jon Isaacs

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Posted 25 June 2013 - 08:48 PM

I am now using heavier eyepieces, (a 14mm ES 100 and a 9mm ES 100), and I find that the only way I can use the focuser is if I have the tension knob fairly tight so the focuser won't "slip". The only thing about this is that the focusing becomes somewhat "stiff" and no longer has any smooth action.



If you feel up to it, I would remove the pinion shaft, remove the drawtube and carefully clean the surfaces. I would also check the upper bearings to make sure they are free and the screws holding them in place are tight.

The way I do it is to place the scope in a horizontal position so no small parts can fall down the tube. Then I remove one of the focuser wheels, loosen up the "tension" adjustment and side the focuser shaft (pinion shaft) free. There are two plastic bushings to watch for.

The two speed conversion will also go a long way to resolve this problem, it is after all an 11:1 gear reduction unit so only a small fraction of the current force is required for focus. In some ways, this is the most important benefit of a two speed, the light force means a light touch is possible and so it is easier to focus without disturbing the scope and causing vibration.

Jon

#6 tag1260

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Posted 25 June 2013 - 09:05 PM

Jon,
If it's rough and too tight, will the two speed really help the problem? I thought the two speed was only for fine movement on focus. I've never had one apart so is it really a gear reduction or something else?

Thanks

#7 Starman81

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Posted 25 June 2013 - 09:15 PM

No problems with a GSO focuser fitting the mounting holes on a Synta dob?

#8 Jon Isaacs

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Posted 25 June 2013 - 09:43 PM

Jon,
If it's rough and too tight, will the two speed really help the problem? I thought the two speed was only for fine movement on focus. I've never had one apart so is it really a gear reduction or something else?

Thanks


If a careful cleaning it doesn't smooth it out, then a two speed will not help. In that case, one of the bearings may be damaged.

Generally I find Crayfords are smooth and effective if they are clean. When they are clean, less force is required so they are much less likely to slip. Roughness can result from too much force required because they are slipping...

The micro-focuser is actually an 11:1 planetary gear unit that uses balls under pressure as gears. Like the Crayford focuser, they depend on friction for the driving force so they can be quite finicky.

Jon

#9 Scanning4Comets

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Posted 26 June 2013 - 01:38 AM

I guess I should have worded my original post differently. It's not like the focuser "slips" all of the time. The only time it really slips is if I rack it all of the way out, or all of the way in, and it hits and the focuser knobs just keep on turning.

What I am really trying to say is that I *do* get "smooth" focusing motion, but I kind of have to tighten the one screw a bit to keep things secure. However, I find that I then need to *loosen* the top thumb screw to a point that the motion almost becomes "loose" and not as secure.

I hope you all know what I mean here.

I'm going to go with some of your advice and try to tweak it a bit before I go and spend $$$ buying a new one.

Thanks for all of your thoughts. I find it really nice that I can just come on here and find the advice and help I need at times. :waytogo:

Cheers,

#10 hottr6

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Posted 26 June 2013 - 07:42 AM

I don't know if this has anything to do with how the focuser works, but mine is mounted sideways so that the focusing knobs are in a different orientation than normal.

If the Zhumells are anything like the GSO (and I think it is a copy), the focuser can rotate on the curved base plate by loosening 1-2 Allen grub screws.

#11 Scanning4Comets

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Posted 26 June 2013 - 07:46 AM

If the Zhumells are anything like the GSO (and I think it is a copy), the focuser can rotate on the curved base plate by loosening 1-2 Allen grub screws.


Correct! I used the grub screws and rotated the focuser 90 degrees from where it originally was.

In any event, I took it off last night, as it can be removed by the two Allen screws and I messed around with the screws on it. There is another small Allen screw located right in between the tension thumb screw and the focus lock thumb screw. I tightened that a little bit and put it back together. I re-rotated the focuser again and now it seems much better.

Time out in the field will tell.

Cheers,

#12 Starman1

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Posted 26 June 2013 - 06:20 PM

Glad to see you learned how to rotate the focuser.
If you put the knobs at a 45 degree angle, both knobs will be accessible no matter where the scope is pointed.

As for adjusting the focuser, the lower screw (the one closest to the tube) of the 2 silver adjustment screws is a photographic lock screw and inhibits about a centimeter of focuser out travel. You should remove it and store it in your tool box. You won't need or use it.
The top screw (nearest the eyepiece) is the tension screw. It is a little coarse-threaded, which means a *tiny* amount of tightening or loosening makes a big difference in how the focuser moves.
To select the optimum tension:
--put your heaviest eyepiece in the focuser.
--loosen the tension screw until the weight of the eyepiece causes the focuser to fall in.
--gradually tighten the tension screw 1/64 turn at a time while trying to lift the eyepiece with the focus knob. When the eyepiece is finally lifted, check to see if this is the correct tension by tapping the eyepiece. If the eyepiece moves in, tighten the screw 1/64 more turn. The minimum tension that allows the focuser's knob to lift the eyepiece and which holds the eyepiece in place well enough that tapping it with your finger doesn't move the eyepiece is the correct tension. It should feel smooth and easy to turn the focus knobs at that point.

Resist the temptation to tighten the focuser more--you don't need it.
When adjusted just right, the one-speed version of this focuser can be just as smooth as the best focusers out there.
Definitely read the websites that talk about how to overhaul the focuser.
But usually, all it needs is adjustment.

#13 Scanning4Comets

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Posted 26 June 2013 - 10:47 PM

That's some great info you have posted here Don. It's weird that you suggested that I put the focuser at a 45 degree angle, as this is exactly what did this morning!

I'll try the steps you mentioned here.

Thanks for the info.

Cheers,

#14 csrlice12

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Posted 27 June 2013 - 07:23 AM

You collimated the scope before you removed the focuser, right (or at least know the scope was collimated)? Not a disaster if you didn't, It'll just require a bit more to collimate.

#15 Scanning4Comets

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Posted 28 June 2013 - 05:13 AM

My scope has held it's collimation for months. I do check it every now and then, but I hardly find that an issue. All I have to do when putting the focuser back is make sure it sits completely FLUSH like it was when I had it in there.

I'll check on it right now.

#16 Scanning4Comets

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Posted 28 June 2013 - 05:16 AM

Just checked on it, and it is dead center right where I left it. The focuser is way better now since I took it off and tweaked it a little like I said in my previous 2 posts above. It now rides smoother once I tightened up the small Allen Grub screw between the two thumb screws.

I'm gonna order the focuser upgrade now to make it a dual speed!






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