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Anatomy of the Crayford dual speed focuser knob (and how to adjust it)

johninderby

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johninderby

johninderby

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If your dual speed crayford feels "lumpy" or just plain rough when you use it it's most likely the planetary arrangement in the dual speed focusing knob that's not properly adjusted.

For SkyWatcher and William Optics (most others are very similar) remove the dual speed knob assembly by undoing the two grub screws right next to the focuser knob on the bottom of the focuser. When you've pulled the dual speed knob off, on the inside of the knob assembly there is a locking ring that adjusts the "play" of the ball bearings inside. Adjust this one way or the other until the knob turns smoothly. That's really all there is to it in most cases.

Photo 1:

Disassembled with all the bits spread out in line. Starting from the right there is the locking ring and the grooved washer (the small ball bearings fit in-between them). Next the main housing, then the brass shaft unit that all the other bits fasten into or onto. The three large ball bearings fit into the large holes in the shaft and the locking ring and the small ball bearings with the grooved washer fit into a recess on the right hand end of the main housing and the locking ring screws down on top of the small bearings. The small steel pin (next to the brass shaft) fits into the left hand end of the brass shaft and into the centre of the large ball bearings and the small fine focus knob fits onto the outer end. Next the large knob and its rubber sleeve.

Photo 2:

The assembled inner workings.

Photo 3:

SkyWatcher knob on the left and William Optics on the right. Spot the difference in the inner workings....no neither could I :D

You shouldn't have to completely disassemble the knob, but if you do make sure you don't drop the small bearings !!! :) I replaced the standard grease with a high quality Teflon grease and this did seem to smooth out the feel a little bit.

John

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Robfal

Robfal

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Very fitting, I dis-assembled and rebuilt it last night because of the lumpy feel; however I couldn't move the cover washer tensioner. I also levelled the flat on the focuser tube to get a better fiction face whilst I was at it.

Rob

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  • 2 months later...
John

John

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Lucky I found this thread and many thanks to John - I've just got one of these focusers and it does need some adjustment I think.

When I loosened the bracket that retains the pinion (it's held in place by 4 chrome hex bolts) two very small rubber O-rings dropped out - they are about 4mm in diameter - any idea where they go back as I did not see where they dropped from ?.

Thanks for such a useful post :mad:

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johninderby

johninderby

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When I loosened the bracket that retains the pinion (it's held in place by 4 chrome hex bolts) two very small rubber O-rings dropped out - they are about 4mm in diameter - any idea where they go back as I did not see where they dropped from ?.

:mad:

I'm not sure where the O-rings fit as no O-rings have fallen out when I've adjusted a focuser. What focuser is it exactly? GSO, Skywatcher, WO etc.

John

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John

John

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I'm not sure where the O-rings fit as no O-rings have fallen out when I've adjusted a focuser. What focuser is it exactly? GSO, Skywatcher, WO etc.

John

It's the Skywatcher dual speed for their refractors.

The O-rings fell out from this area (SCT focuser in the pic but it looks identical):

post-12764-133877465277_thumb.jpg

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johninderby

johninderby

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I checked a Revelation newt focuser which is the same as the SkyWatcher. I didn't want to take it right apart, but after loosening the four silver hex bolts so I could see into it a little bit It looks like there are small O-rings around the shafts of the silver bolts where they enter the main body of the focuser. I could only see the two at what would be the the top of your photo, but I presume it would be the same for all four.

John

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John

John

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I checked a Revelation newt focuser which is the same as the SkyWatcher. I didn't want to take it right apart, but after loosening the four silver hex bolts so I could see into it a little bit It looks like there are small O-rings around the shafts of the silver bolts where they enter the main body of the focuser. I could only see the two at what would be the the top of your photo, but I presume it would be the same for all four.

John

Thanks very much John :mad:

I'll check this out - there could easily be a further couple of O-rings laying somewhere on my dining room carpet !.

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  • 3 years later...
lpor

lpor

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I know this is an old post but would really like to see the photos if still available to see how this is built.

Just got a new SW 80mm ED pro and the focuser doesn't work on the dual knob side keeps turning but no movement of the focus tube.

When this happens the other sides single knob doesn't turn which confuses me

as I would have expected it to be a single shaft.

If I start focusing it with the single knob side I can get it to work for a bit but then stops again like it is slipping.

The single knob side works but feels very rough.

It means I have no fine focus which defeats the purpose of the thing.

I have contacted the retailer who has agreed to have a look but would like to know what the issue may be.

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lensman57

lensman57

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I know this is an old post but would really like to see the photos if still available to see how this is built.

Just got a new SW 80mm ED pro and the focuser doesn't work on the dual knob side keeps turning but no movement of the focus tube.

When this happens the other sides single knob doesn't turn which confuses me

as I would have expected it to be a single shaft.

If I start focusing it with the single knob side I can get it to work for a bit but then stops again like it is slipping.

The single knob side works but feels very rough.

It means I have no fine focus which defeats the purpose of the thing.

I have contacted the retailer who has agreed to have a look but would like to know what the issue may be.

Hi,

I bought a SW 150 PDS a couple of weeks ago and the focuser shaft was just loose, there is a little grub screw hidden in the right hand wheel that locks the clutch, yours might be loose as mine was. Try tigthening the grub screw, it may work for you.

A.G

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Peter Drew

Peter Drew

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The dual speed Crayford type focusers do have two shafts. The fine focus knob has a much smaller shaft that runs in the inner space between the "planetary" balls. The difference between this and the larger coarse focus knob, the shaft of which is driven by the outer contact of the balls is what gives the 10:1 reduction.  :smiley:

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lpor

lpor

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Thanks A.G had a closer look and found the grub screw hidden below a hole on the  single knob side. It was tight OK but I loosened it pulled the knob out about 1 mm and retightened.

 What do you know focuser now works on both sides and fine focus. Surprised at the poor quality control from Skywatcher, maybe they should drop the Pro from the scope name.

Liam

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  • 7 months later...

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