Q: angular separation of alt. bearing Teflon pads?

[mental4astro (Alexander)]

Hi all,

I've being asking questions and for assistance all the way through the rebuild of Odessius, and it ain't stopping now.

The 'Good Book' states that the angular separation of the altitude bearing Teflon pads as being 70*. However, I've seen many, if not all contemporary DIY dobs as having this angle less than this. The scope I've modelled mine on has it as being 60*, and I've seen others as little as 40*.

As I'm now at the rocker box making stage, this angular difference can mean the rocker box is as much as 100mm shorter. The depth of the bearings will be 25mm.

What would the effects of a reduced angle be? Should the Teflon pads be smaller?

Help will be appreciated.

Mental.

[Starkler (Geoff)]

Originally Posted by mental4astro

What would the effects of a reduced angle be? Should the Teflon pads be smaller?

Help will be appreciated.

Mental.

Reduced angle would reduce the effective pressure in kg/cm^2 on the pads. This can be calculated with trigonometry. I'd be surprised if the "good book" didnt cover this.

[GrahamL]

Shouldn't matter much with your scope Alex .. as you still have all the lisence of an atm to make it work. ... If you need 60 to make it fit someplace go it..!.. Slide that finder up/down the truss pole is a
good balance trick to .


Imo your at the point now where all the many good words we have
in print to help us along .. arn't as relevant as you'd think ... finish that sucker mate !!

[Starkler (Geoff)]

Originally Posted by mental4astro

Hi all,

As I'm now at the rocker box making stage, this angular difference can mean the rocker box is as much as 100mm shorter. The depth of the bearings will be 25mm.

I think its one of those things that has to be worked out experimentally to match alt movement to azimuth. I would err to the side of making the rocker box slightly taller to accommodate wider spacing of the alt pads, which can be moved closer later if need be. If so inclined, you could lop off the extra un-needed height too at that stage (but probably wouldnt).

[mental4astro (Alexander)]

Thanks Chris and Geoff.

I'll make it to accomodate the 70, and see what happens at 60 then. If all is good, then I can always lop of the extra. Mind you, it is only to lose a few grams of weight, maybe 0.5%. Not worth the effort and does not change the height location of the scope, only shortens the rocker box.

Time to finish!

[tnott]

Changing the angle a bit won't make much difference, but it's good to have some margin of error. As other have said, you can always lop off the top of the rocker later.

[Barrykgerdes]

The teflon pads should subtend the smallest angle to the rotational axis consistent with stability. The bearing performance of the pads is best if the downward force acts at rightangles to the pad.

If you make them at too wide an angle the downward force tends to be a sheer and this will increase the stickiness of the pads which will tend to make the movement a little jerky.

I would use a nominal 60 degrees as being about the best compromise.

Barry

[mental4astro (Alexander)]

Thank you for your thoughts. I've wieghed up the points on offer, and as I'm using 800mm diameter alt. bearings, I don't forsee problems with 60*. Done! Smaller diameter bearings, even of the size recommended by the 'Good Book' would have seen the pads sit inside the position of these. No more lost sleep.

Many thanks again for your council, everyone.