SW Dobs

[Vartigy]

G'day g'day.

I'm a proud new owner of a skywatcher Dob 8" collapsible.

After giving it a thrashing last night, I've come up with a few issues, wondering if anyone else has run into the same troubles.

- Rotation in the horizontal plane. Jerky at best. The nut/bolt config is loose but secure. Loose enough to rattle about. I'm wondering whether the teflon pads contacting between the plates aren't all that good. Has anyone else come up with an aftermarket solution to this?

- The Black Screws right next to the locking screws on the truss arms. Anyone know what they are for? I'm guessing they hinder/help the free movement of the truss up and down.

cheers

[erick (Eric)]

About your "horizontal plane" rotation - call it the Azimuth (Az for short) movement. Rip it apart and see what is happening. Probably three teflon bearings are attached to the ground board. They may be held in place with staples. Are the staples well under the surface and not scratching the upper board? Is there any central support around the pivot bolt?

Best solution is to buy the pivot kit and three nice big teflon bearings from these folks:-

http://www.astrosystems.biz/pivot.htm

You get four altitude ones as well which I don't think you need in the SW collapsible, but can sell to someone here, I'm sure.

Also you put a layer of Ebony Star laminate under the upper board for the teflon bearings to contact. All solved!

OK, USD/AUD exchange rate too high? Ebony Star too expensive?

First steps then:- check the three teflon bearings. Staples well below surface? No sharp edges that might be catching on the upper laminate? Now give that upper laminate surface a good clean, then a good polish with a silicone car polish. Some swear by a spray of Mr Sheen! I reckon the car polish is best. Next add the "Milk Bottle washer" solution which is the cheap version of the pivot kit above. Read up in here:-

http://www.iceinspace.com.au/63-116-0-0-1-0.html

You should see much improvement.

[Tallstock (Peter)]

Hi Aaron,
From what I have read, all dobs seem to have a az mount problem.
Many suggested solutions but only one has worked for me.
The shop didn't have an answer and the importer still hasn't got back to me.

Here is a copy of my recent post;
---------------------------------
"Sticky" SWFLEX8 dob az mount.
Success!
Went to Bunnings at Tuggerah and found a 305mm "lazy susan" for $35. Sold under the brand name of "Prestige". Installed (no mods needed) swivel assembly today and can't believe how smooth the action is.
Website http://www.itwproline.com.au/HSSwive...s/default.aspx for all those other dob users having trouble with "stiction" --- sounds so much better than "sticky" or jerky.
----------------------------------

The "black screws" are possibly some sort of tension guide but I haven't touched mine and there is nothing in the documentation to suggest you need to.

Hope this helps.
Peter

[erick (Eric)]

Originally Posted by Tallstock

... found a 305mm "lazy susan" .....

Worth trying, but don't throw the teflon bearings away just yet. GSO went with roller bearings for a while, then went back? Reports I have seen suggest the roller bearings have too little friction to stay put where you want the scope to stay. A bit of wind and the scope can apparently make a good impression of a windmill? But I haven't had my hands on one with roller bearings so this is all hearsay. Do you see any of these reported problems, Peter? If not, it sounds like a good solution if the "lazy susan" is not too sloppy.

[JethroB76 (Jeff)]

The Lightbridges have a roller bearing Az, but have a tensioner so it doesn't move too easily.

[Vartigy]

Thanks for the responses.

Yeh I checked out the teflon pads before I put the base together. The staples seem to be well subsurface.
First pass, when constructing the base, I found myself holding the white larger washer, looking at the completed base wondering "Did Ikea have a hand in it's construction?" Then I looked at the diagram again and noticed it goes between the plates. Mind you, the base rotated quite nicely without the washer.

I'll try a little polish or sheen on the underside and see how that helps.

[Tallstock (Peter)]

I thought that creating a smooth surface would be the answer but the "smoother" I made the teflon ( used Helmar H4000) the worse the problem became.
I found this interesting reading;
13/12/2007 GSO Dob --- Review by Scott Tannehill on IIS website
" Smoothness is not a good choice because two smooth surfaces create a slight vacuum when pushed together. "
This was news to me and contrary to some long held beliefs.

When I got to use a much bigger dob, with a lazy susan, at the last Mangrove Mountain Pony Club meeting that convinced me that the swivel mount was the only answer.

JethroB76 has mentioned that the more expensive Lightbridge has a roller bearing so it looks like GSO and SW have tried to cut costs.


Erick, after struggling with the stiction and a SW wobbly centre axis bolt (no longer needed), the fact that the az mount now runs super smooth is such a relief. The gusty breeze last night was not a problem. I read somewhere that a wedge of foam rubber between the bases could act as a tensioner but I didn't find it necessary.
Peter

[JethroB76 (Jeff)]

The roller bearing style can work well, but many still don't like it.

Most big premium dobs (eg SDM) are still made with ebony star laminate and teflon. And in this case no, you don't want two perfectly smooth surfaces; you want the teflon to ride across a slightly bumpy (hence the ES laminate) surface

[bmitchell82 (Brendan)]

I was wondering would it be any chop to run powdered Graphite? that stuff we used to use in our chairs at home stopped them squeeking and made them easy to spin around really really really reeeaaallyyy ... spew. fast

[Spanrz (Brett)]

I have one of the later GSO (Bintel) Dobs. It has a large neddle roller bearing assembly for the Az.

Reading all your comments and various websites, these teflon pads seem to work, just seem a little sticky.
This needle roller bearing design has no issues with sticking or jerky issues.

It has a needle roller assembly in a plastic cage, sandwiched between 2 thin sheetmetal plates.
To control Az stiffness, you tighten the central screw, compressing the roller assembly.

http://farm4.static.flickr.com/3324/...924064498b.jpg

[Screwdriverone (Chris)]

Hi Aaron,

The Az bearing should have the two metal plates with the black "wheel" with the silver bearings built into it sandwiched between the two metal plates. Mine is beautifully smooth to rotate. Makes a bit of noise but its very smooth to turn with no jerkiness. Take the top off and check the bearings are all straight and also make sure your central bolt is tight but not too tight. The more you tighten this, the more stiction you get.

As for the black screws on the collapse assembly, these seem to be a vacuum type friction system for the poles so you can adjust the tension it requires to lift and lower the secondary cage. Loosen these too much and you may risk "dropping" the secondary too fast when you loosed the t bar screws.

Its a bit more serious on mine as the secondary cage is quite heavy, but I have almost pinched my hand once or twice.

I hope this helps.

Cheers

Chris

[Vartigy]

Thanks Chris. Will pull apart the tension handle, take a look at it. Seemed alright when I first put it all together.
Think it's probably more of a point of finding the equilibrium where it moves but stays still also. Bitta practice should do the trick.

And yeh, those black screws, played around with them. I did notice significant differences in how the truss extends and packs up again when I unwound them.

[Maxim (Drew)]

I got a 12" SW dob about a month or so ago and when I first assembled the base I had it a little too tight which made it stiff and jerky to turn. Once I loosened it slightly it's been nice and smooth. I don't know if this helps any though...