Spinning Disc

The RSAMD pantomime in 2007 included a vinyl disc,4.5 meters in diameter, as a show piece. This was to spin when in and then fly out. The structure and mechanism were designed and constructed by Peter Strain

Requirements

The disk was required to spin as if an old 45rpm record and fly out. The mechanism to revolve the piece needed to be on the back as there was no detail, such as a stylus arm, to hide any mechanical parts on the front. Furthermore, any equipment used had to me slim enough not to get caught on the gauze hanging immediately upstage behind it.

The Solution

The construction of the piece was looked at in two parts. A wooden and canvas frame, forming the disk, and a metal back frame to house the mechanism. The two were joined by steel plates that each had interlocking spindles. These plates were separated and bolted onto the two frames.

Due to budgetary restrictions and space issues the mechanism was powered by a simple car windscreen wiper motor. This drove a bicycle gear which stepped down the motor's output and spun the disk. The photos below show the component parts and how they fitted together.

Technical Data

• Motor powered by mains supply to a lap top transformer
• Transformer dropped the voltage to 15V
• small gear driven my motor
• mechanical revolution step down
• due to thread of bolts the mechanism had to run anti clockwise so that the nut would not loosen

Even though the mechanism's speed was reduced significantly the disc still spun too fast. The result being that the gauze behind the disc was sucked into it. This introduced the risk of the gauze being caught on a bit of wood and ripped. We tried to page the gauze up stage but the vacuum created was too large and the disc followed. Upon attempting to drop the power input we discovered that there was not enough current to keep the disc spinning. The solution was that we put two power resisters in parallel (as one became very hot) and put them on the output of the transformer. This was enough to slow the disc enough to allow it to run. The resisters, however, only allowed us to run the disc for a short period of time otherwise they may have burnt out. The power came from the fly floor tech. earth so it was simple to unplug and stop that from happening once this disc was flown out.

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Photos

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