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OzDave

Mechanics of Dome Casters/Rollers

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I'm in the planning stage for my own observatory and have been looking at Roll-off designs, but am now having a think about a possible DIY dome. My design would be based on a geodesic.

At present I am thinking about how the dome rotation would operate. Having looked at several online dome builds, most people seem to sit the dome on a circular base, which either rolls on casters mounted on the walls somehow, or the base actually has the casters which roll on the top of the wall in some way. For the sake of discussion, let's imagine that there are two circular rings, one on which the dome rests and which has 6-8 casters on the underside that roll on an identical circular ring attached to the building walls.

When I exert a tangential force on the dome in an attempt to rotate it, it seems to me that the dome will actually want to move sideways (not rotate), which several online sources seem to confirm. People seem to solve this issue by making the castors roll on some kind of track, or they fit guide casters to keep everything in the right place, essentially forcing rotation.

So, because rotating is not the natural tendancy of the dome, it seems to me that the caster wheels are continually being forced to steer round a curve when actually they just want to go in a straight line tangential to the circular rings. Thus, I would expect them to kind of skid a bit as they operate, a bit like a bicycle might do if it had to follow a circular path but I wasn't allowed to turn the handle bars at all. (Actually, the bicycle probably wouldn't be able to follow the path in that case as there would be no guide force to make it skid). This leads me to believe that flat bottom wheels would be a bad choice because they'd create too much friction and make it hard to turn the dome. Much better wheels would be rounded ones (donut-shaped) that have minimal surface area in contact with the rolling surface so that skidding friction is kept as low as possible. Is my thinking logical here, or am I just not on the right track at all?

Assuming I'm understanding what is going on well, it seems to me that the best thing I could use would actually be spherical ball bearing castors (think like mouse ball) that could rotate however they needed to help the dome rotate with minimal friction.

I'd be looking to automate the dome rotation later, so having something that moves easily would be fairly important given that I'd probably go for a friction drive wheel.

Does anyone have any experience here or a good understanding of the mechanics involved to give some good advice on how to make the dome easy to rotate?

David

Edited by OzDave

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I used ball transfer units (your mouse ball) for my dome.

I have 12 of them on 30 deg spacing...i then have three wheels which run around the vertical edge of the dome to keep it centered...

Peter...

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I used ball transfer units (your mouse ball) for my dome.

I have 12 of them on 30 deg spacing...i then have three wheels which run around the vertical edge of the dome to keep it centered...

How did that work out for you? Is the result easy to rotate? Do you know roughly how much your dome weighs?

David

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When I exert a tangential force on the dome in an attempt to rotate it, it seems to me that the dome will actually want to move sideways (not rotate), which several online sources seem to confirm. People seem to solve this issue by making the castors roll on some kind of track, or they fit guide casters to keep everything in the right place, essentially forcing rotation.

I think the force you refer to would be absorbed by the bearing the caster is seated in, rather than in the caster-rail interface. The more casters you have along the diameter, the smaller the outwards force on each one, so the less will be the tendency to "push outwards", so the smoother the rotation, and the less the force needed to rotate the dome. Although this last factor will be countered to some extent by the extra friction from each caster/bearing. You could also try mounting the casters at a slight outwards angle, though the extra difficulty involved could well exceed any gains in smoothness.

You can use these sorts of bearings,

340067_l.jpg

but the forces will still be the same, just exerted on different parts of the dome, wall and rails.

Edited by pete_l

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It works Quite well i should have put a metal ring onthe bottom of the dome as i used ply and the weight of the dome has caused the btu's to wear a grove in the marine ply..

Sorry I haven't go an actual weight but it is 19mm marine ply for the structure covered in upvc shiplap... 12 sided hybrid with a slide-off as i prefer widefield its wide enough to only need tweaking every 2 hours or so...

Peter...

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I think the force you refer to would be absorbed by the bearing the caster is seated in, rather than in the caster-rail interface. The more casters you have along the diameter, the smaller the outwards force on each one, so the less will be the tendency to "push outwards", so the smoother the rotation, and the less the force needed to rotate the dome. Although this last factor will be countered to some extent by the extra friction from each caster/bearing.

I've read one analyis online that claims more casters make for a harder to rotate dome, but that obviously you need a minimum of say 5-6 or it won't rotate at all. Assuming that analysis is correct, maybe it gets harder due to the extra friciton of extra caster as you say?

About the tengential force being absorbed, I'm not sure about this. Let's say I push exactly where there is a wheel causing that wheel to want to start moving in a straight line. The next castor ahead of the one I pushed at will have a little bit of side force applied, the next one again will have even more side force, and the one at 90 degrees around the perimeter of the dome from where I pushed will have mostly all sideward force. Are you saying that the side force gets absorbed by the bearing in the caster and the remaining vector component of the force in the direction the wheel is facing will cause movement, just with a lesser force than what I applied at "source"?

Do the wheels skid round like I suggested in my original post? If so, this would mean flat bottom wheels would be a bad choice right, since they couldn't skid as easily as a round-bottom wheel? Or am I just thinking about it a bit wrongly?

David

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It works Quite well i should have put a metal ring onthe bottom of the dome as i used ply and the weight of the dome has caused the btu's to wear a grove in the marine ply..

Ok, that's a good tip. Although I was considering using some kind of track with V-Groove casters to eliminate this problem a different way. I saw one post where a guy made a track from 0-gauge wire and then ran the V-grooves on that. I really like that because the wire would be easy to work with.

David

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About the tengential force being absorbed, I'm not sure about this. Let's say I push exactly where there is a wheel causing that wheel to want to start moving in a straight line. The next castor ahead of the one I pushed at will have a little bit of side force applied, the next one again will have even more side force, and the one at 90 degrees around the perimeter of the dome from where I pushed will have mostly all sideward force. Are you saying that the side force gets absorbed by the bearing in the caster and the remaining vector component of the force in the direction the wheel is facing will cause movement, just with a lesser force than what I applied at "source"?
Ummm, sort-of. The preceding wheel only transfers onwards the force-component in it's own direction of travel: the rest being partially absorbed or translated into forward-force within the bearings. That force is then transferred into the body of the dome along it's axis of rotation, to be applied to the next caster on the rail. In theory, if you could balance the dome, you could get by with just 2 wheels on opposite sides and it would still rotate. The reason it would still rotate, rather that being pushed off the rail is that you're not applying the force to the dome's centre of gravity but to it's edge, which naturally makes it rotate, due to inertia. Edited by pete_l

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I agree with what is being said. There is a good article by Chris Lord which delves into these issues.

My concern with the roller ball transfers is that dirt or rainwater may manage to enter inside the bearing over time and increase friction.. oh well just throw it away and install a new one ;) Then as rightly pointed out since all the force is being transferred to a point then the pressure increases and it gouges its way into the plywood...

I shall be using nylon castors for mine (8 in no.). The profile is not flat but curved.

http://www.bearingbo...at-1400-cat.jpg

I also considered a geodesic to avoid cutting curves, but there is so much more cutting to be done! And too many lengths of joints to be sealed! Better one out of marine plywood ribs. I am going to avoid fibreglass too, and clad mine with aluminium sheeting overlapped onto the ribs and screwed into by means of stainless steel screws. Then seal the edges (inside and out with Sikaflex or similar). Credit goes to Nytecam (google for his website).

Edited by teleskopjo

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I shall be using nylon castors for mine (8 in no.). The profile is not flat but curved.
The other thing to look out for with casters is that with the dome closed, they will spend long periods under load, but with the same part of the wheel bearing the load. On some materials that can cause flat spots on the wheel.

More casters around the circumference means less load on each wheel and a smaller tendency for flats to develop.

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The other thing to look out for with casters is that with the dome closed, they will spend long periods under load, but with the same part of the wheel bearing the load. On some materials that can cause flat spots on the wheel.

More casters around the circumference means less load on each wheel and a smaller tendency for flats to develop.

that is definitely true with rubber castors but nylon ones are completely rigid - which brings a disadvantage - rolling noise when rotating the dome at 3am!

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I like the idea of minimal noise while rotating, which maybe means I should use rubber casters. I also agree that spherical casters may have a tendency to become clogged.

To avoid flat spots on the bottom of rubber casters, maybe there is a way to kind of jack up the dome slightly in a few places when "parking" it so that the rubber is not under load. Maybe this could be part of the dome lockdown mechanics to prevent it flying away.

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I was thinking of a geodesic because fitting triangles together sounded easier than fitting curved (in 2 axes) cladding.

Am I misjudging the difficulty of either or both of these things?

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I was thinking of a geodesic because fitting triangles together sounded easier than fitting curved (in 2 axes) cladding.

Am I misjudging the difficulty of either or both of these things?

If you search on the www, no amateur dome consists of double-curvature cladding unless using fibreglass and resin onto pre-formed moulds. 90% of amateur domes consist of single curvature plywood/hardboard/aluminium sheets bent over singly onto timber ribs or even tied onto themselves (nytecam's stitch and glue dome).

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Here's my 12 sided Hybrid dome "slide off roof" for Widefield use...

Obs_Sideview.jpg

I drew up the roof timbers in cad and had them laser cut from 19mm marine ply same for the dome ring which is made in 4 segments the top ring has two sets of these set 45 degrees apart and glue together using PU glue... The doem ring had the mortices for the ribs laser cut as well... Originally it was a dome with zenith hole and reovable sections but that needed tweaking to often so in the second summer I changed it to this hybrid design...

Peter..

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Here's my 12 sided Hybrid dome "slide off roof" for Widefield use...

I drew up the roof timbers in cad and had them laser cut from 19mm marine ply same for the dome ring which is made in 4 segments the top ring has two sets of these set 45 degrees apart and glue together using PU glue... The doem ring had the mortices for the ribs laser cut as well... Originally it was a dome with zenith hole and reovable sections but that needed tweaking to often so in the second summer I changed it to this hybrid design...

Peter..

CNC/laser cutting has to be the ultimate in accuracy when cutting curved shapes!!

Any pointers to the cost ? :)

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I had a dome (fibreglass) and ring (25mm angle iron) made local, ring has eight rollers attached and the ring is attached to the dome. The roof circle has a six inch wall which the wheels orbited on the roof, this caused a problem with some catching of the wall so I put a hosepipe outside the bottom of the wall and this keep the wheels at an even spacing away from the wall, rotates at the slightist touch. I have put a 12v winch on the roof and this will rotate the dome from the outside but controlled from the inside, almost finished but first try worked perfect, just a bit of finishing off.

Jim

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CNC/laser cutting has to be the ultimate in accuracy when cutting curved shapes!!

Any pointers to the cost ? :)

For me it was Free as we do a lot of buisness with the company that did it and I had done the CAD/CAM myself so it was just the machine time and ply cuts pretty quickly.

The only downside was the carbon from the burnt edges so they took a bit of cleaning up before gluing..

Peter...

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Here's my 12 sided Hybrid dome "slide off roof" for Widefield use...

Obs_Sideview.jpg

I drew up the roof timbers in cad and had them laser cut from 19mm marine ply same for the dome ring which is made in 4 segments the top ring has two sets of these set 45 degrees apart and glue together using PU glue... The doem ring had the mortices for the ribs laser cut as well... Originally it was a dome with zenith hole and reovable sections but that needed tweaking to often so in the second summer I changed it to this hybrid design...

Peter..

I think this is a really inspirational design :) Great if you've got access to a laser cutter :) I bet all that marine ply wasn't cheap :D I know that plastic shiplap cladding isn't cheap - I used it on our back porch. But this is a design that is buildable by many people I should think, without costing the enormous price of a pre-fabricated dome.

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See what happens when you get bored one weekend... the ply came to about £90..

National plastics now do dual wall simulated shiplap - each panel looks its several pieces of shiplap joined together this can have a section removed from the inner surface and then bent so you don't have so many joints to seal... i used it when i added the slide off roof...

Peter...

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