Mirror ball.

[sunray]

Discussing arrangements for a lighting effect we have experimented with a pair of mirror balls. The effect is for conventional ball & pin spots and the second with different coloured pin spots to hang on a string so it has to 'wind up' before it starts moving then once it is it starts unwinding and speeds up as it unwinds then slows, stops and starts the cycle again. It's counterintuitive but it does do as described with the cylcle getting shorter until the speed matches the motor.

My question is what string to use? This is a small (maybe 6") ball so lightweight, Initially I tried 3 strands of monofilament fishing line but after a dozen or so cycles in an hour one fatigued and broke, 4mm paracord warmed and went stiff (Although no signs of failure I don't like something changing state) and waxed or plain twine not so long as they 'deconstructed', a rubber luggage bungee was too stiff.

Does anyone have any thoughts for something more reliable?

This is only for one song for 2 or possibly 3 performances (it can be re-rigged between shows if we really do have to) and there will be a safety device below the string fixing point.

[sunray]

A little bit more to the physical  layout; the motor will be tight to the ceiling with the conventional arrangement and the second ball will be fairly tight beneath it with the string attatched inside the top of the first ball. Somewhat like this with the safety wires in red :

The other thing we have tried is a conventional arrangement but using a VSD, in the form of an ampilifier, sig gen and step-up transformer and standard mains mirror ball motors seem to happily run at 400Hz but to achieve the same sort of acceleration as the string winding the motors stall and stutter to overcome the inertia of the ball. We have also tried running the second ball at a faster speed (and a 3rd at an intermediate speed) and varying the lights but the jumps doesn't look right.

[Don Allen]

I use welded chain  for mirrorballs, it does what you want. It is slow to start, then once at speed runs at  constant speed, then when you stop it keeps rotating until the links are wound up, then it rotates in the opposite direction. The longer the chain, the longer the reverse rotation. I put the motor on a separate dimmer circuit with a 60w lamp as a dummy load to stop dimmer bleed.

[sunray]

3 hours ago, Don Allen said:

I use welded chain  for mirrorballs, it does what you want. It is slow to start, then once at speed runs at  constant speed, then when you stop it keeps rotating until the links are wound up, then it rotates in the opposite direction. The longer the chain, the longer the reverse rotation. I put the motor on a separate dimmer circuit with a 60w lamp as a dummy load to stop dimmer bleed.

That I really would not have ever thought to use and I have loads of it from the days of safety chains rule. I usually put it on the same channel as the spots.

[Jivemaster]

It's close to impossible to safety chain a mirror ball, even then you are relying on a cheap Chinese geared motor. It's hard on a rotator to use a short link

the longer support chains transmit the torque better. If you really must use a single link then look at drilling through the ball and fitting a piece of plastic pipe through the ball and secure the chain to the bottom of the ball.

[Robin D]

I stored a mirror ball in a hot room unused for a couple of years over the pandemic. Took it to an event on a cold day. Took it out of the original box it had always been stored in and hung it. Ten minutes later it just fell apart into two vertical hemispheres and fell from the central  bolt that was left hanging. Most of the mirror facets fell away giving quite a clear up job as they were on carpet and caught in it. I think it was perhaps about ten years old. I would be careful drilling through one as clearly the construction  of that one is two hemispheres c=glued together. In engineering terms it would be stronger if constructed with the hemispheres horizontal with the central bolt holding them together. I bought a replacement, but it's impossible to tell how it's constructed. 

 

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[sunray]

7 hours ago, Robin D said:

I stored a mirror ball in a hot room unused for a couple of years over the pandemic. Took it to an event on a cold day. Took it out of the original box it had always been stored in and hung it. Ten minutes later it just fell apart into two vertical hemispheres and fell from the central  bolt that was left hanging. Most of the mirror facets fell away giving quite a clear up job as they were on carpet and caught in it. I think it was perhaps about ten years old. I would be careful drilling through one as clearly the construction  of that one is two hemispheres c=glued together. In engineering terms it would be stronger if constructed with the hemispheres horizontal with the central bolt holding them together. I bought a replacement, but it's impossible to tell how it's constructed. 

 

. 

I made one a lot of years ago using a plastic lampshade, double sided tape and an old mirror. the shade is virtually spherical and appears to be two 3mm plastic tubes wound round and round to form the ball, reinforced with a pair of hoops inside using wire coathangers. I used it for ~35 years in fits and starts, throughout 80 to 95 ish maybe 20 times a year then a few times 2005-2015 ish.  Approximate top and bottom view:

Apart from losing half a dozen mirrors all was good, Christmas 2023 I took it to a job and found the plastic tubes had separated in places, effectively meaning the bottom half of the ball was hanging off and a few more mirrors lost. I was able to botch it for the event by attatching safety wires top to bottom to the wire hoops

I have access to borrowing several mirror balls from ~8" to ~24", the middle sized' ~15", has a hole in the bottom big enough for me to get my hand in to the inside of the bolt.

All of them have a safety arrangement which attatches above the motor in some way:

they have a bolt in the top of some description, I happen to know the middlesize has an eyebolt

there is a nylon washer about 50mm diameter sits on the nut,

The dark grey isa galvanised steel wire about 2mm diameter with a simple knot to form a loop about twice the diameter of the shaft. I've shown a pair of loops on the ends but he has a selection for different situations.

The red is a steel washer which usually sits on the safety wire. If the motor failed the clip shown light grey will stop the ball falling.

 

 

Recently I have been looking around to replaceme my failed mirror ball but I'm finding many these days are a polystyrene ball covered with sticky backed mirror tiles at the sort of price I think would be value for money

[Jivemaster]

A mirror ball can likely be made from foam or moulded plastic with glass mirror facets. Often the glues give out over time.