Bent Dome UPDATE 16/5/05

[Astroman]

Hi all,

I am having a little trouble with the rollers on my dome. I think I may have found the problem, but would like some advice on how to go about fixing it, or thow some ideas in.

The problem I am having is on my Dome which is home built is the dome is not quite round it actually is an egg shape. It varies by 15mm. I measured this by using a Pole with another pole inside it. Then I marked one way then 90 degrees from it and came up with the measurement. I measured the top of the base and this varies by only about 4mm. I have 6 rollers on it like the picture below which are fastened onto the dome and roll on the round tubing on the base.

Anyway I think the main problem is the dome being egg shaped. The problem I have is correcting it. I have a couple of ideas which could work let me know what you think.

1. Remove the dome, stand it on it's edge and hang from the other end to try and squash it back to round.

2. Attach some sort of cable to it with some sort of way to reduce the length, squashing the dome in.

Mind you the dome is made of 20mm square tubing, is sheeted with colourbond steel. Weighs quite a bit.

Anyway let me know what you think.

http://www.theastroweb.com/astronomy/temp/obsroll1.jpg


http://www.theastroweb.com/astronomy/temp/obsroll2.jpg

More pictures can be found on my observatory page at http://www.theastroweb.com/astronomy/

[acropolite]

A steel cable and a couple of turnbuckles should do the trick. i.e. yacht fittings. Maybe you could modify your roller assembly so that the rollers are spring loaded. This technique is used on aluminium sliding doors and screens. :work:

[[1ponders]]

What's the actual problem with the rollers Andrew. What is it they are doing that they are not supposed to?

[Astroman]

It binds in some areas as I turn the dome around. The rollers seem ok but it's like pulling a car up a hill. Basically I am after ideas on bending the dome top back to round.

[[1ponders]]

The steel cable and turnbuckle sound like a good idea. They would need to be strong though to deform the steel pipe the rollers are resting on, far enough so that its not going to spring back out after tensioning and removing the cable. I'm imagining that you don't want to leave anything permanent accross your dome.

Is it binding on the short axis or the long axis? IF on the short axis you may be able to rig up some sort of strut with a hydraulic car jack in the middle and push the short side out. I'm no engineer but that might help prevent spring back as you could put more pressure on the axis than using a turn buckle and cable which would have a certain amount of stretch in it.

Good luck with it. Hope you don't have to take it off. That doesn't sound like a pleasand task.

[Astroman]

I think I will have to seperate the dome from the base to do it no matter which way I go. I don't want to risk bending the base out of round.

[ballaratdragons]

Andrew,

because of the nature of steel you will have to pull in with cables (or push apart with jacks) further than you think as metal will always spring back a certain amount!

If you need to pull in with cables and turnbuckles about 15mm you will have to actually pull in to about 25mm. When you release the cable it will spring back to about 15mm. If not, you will have to pull it in even further. Keep tightening and releasing until it relaxes at the required shape.

Same if you use Jacks. Push out and back repeatedly until the shape is right.

Rather than lifting the dome all the way off try lifting it enough to put long timbers under it.

[ballaratdragons]

Example

[Astroman]

That sounds like a quite doable idea there Ken. Now if this bloody rain goes away (I know we need it) I will have a look to see if I can do it.

If not I will have to hire some help.

[ballaratdragons]

Let us know how it goes Andrew.

Send some rain this way too. We are still on Water restrictions.
3 years now!

[Astroman]

I didn't realise how much of a problem I have until I made this little movie up about 30 minutes ago. It clearly shows the amount of movement I am experiencing. Also shows how the dome is affecting the base.

Have a look and let me know what you think.

http://www.theastroweb.com/astronomy/observatory/domemove.gif

Looks like a bad case of human engineering gone terribly wrong.

Edit- No I am not moving it that fast.

[slice of heaven]

The movie shows it all.
I agree, pull the top off to rework it.
The top is a long way out of round, but a bit of effort should get it back in shape as long as the 'ring' for the dome is the right size.

[Astroman]

Yeah thats one of the other things I am afraid of, the actual ring not being the right diameter. I don't think I am doing too badly considering I have no idea what I am doing and I have no instructions for the things I am doing. The whole thing has been put together with no knowledge of the way the designer intended.

[[1ponders]]

Now that's a tricky little problem. It looks more pear shape than egg shape in that the change in the radius of curvature is not even throughout. Without seeing what's going on inside I was wondering if packing some of the rollers away from the dome might help, so that at least the radius to the roller mountings would be equal and not worry about the pear shape dome. Unless of course if the dome is actually rubbing on the outside walls creating drag.

[slice of heaven]

How much trouble is it to remove the top ring ?
I'd be removing the dome and then the domes ring and altering and checking the ring for fit and movement. Then fit and reattach the dome.

[Orion]

Andrew I have been looking at your problem and I suggest that If you have a welder tack a piece of RHS (rectangular hollow section) to the major axis of your dome and in the center of the RHS cut a section long enough to accommodate a turn buckle and weld the buckle to the RHS. This system is more stable than a cable and will allow you to put some considerable pressure to the turn buckle so you can bring in the difference.

Next time you want to build a round dome weld some sections of pipe into a star formation with equal lengths called a "spider" this will allow you to make a true round dome using the spider as a guide.

[Astroman]

Paul R, Yeah the dome actually rubs on the aluminium uprights I have to hold the outer metal skin to the frame. This thing needs a major overhaul, something I don't rally want to do because of the hours I have spent on it already.

SoH No trouble at all, Nothing an angle grinder cannot do in an hour or so. It is welded onto the main frame. I knew I should have checked all this before getting carried away.

This is one problem that is going to haunt me until I fix it.

[Astroman]

Hi all,

Got around to taking the dome off today. I videoed the event with #1 son watching on. Anyway video is about 2mb, it's in slow motion for some reason.

http://www.theastroweb.com/astronomy/observatory/dome.wmv

Now for the hard part...........

[toetoe]

I liked how #1 son showed how pleased he was when the dome came off.

[Astroman]

Yes he has all his cars inside it now :/

Going to have to fight him to get it back I think.

[Astroman]

Okay quick update.

I put the dome back onto the base and only used three rollers, rolls pretty good now. Still a little off but I think I will have to live with it. When I took it off the first time I may have bent it a little when it came off, seems to roll better. It is still loud as I roll so I am thinking of using polystyrene as an insulator on the walls, may dampen the sound.