University of Oklahoma

Max Weitzenhoffer Theatre

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For the School of Drama production of Everybody, one of the desired scenic effects was a 'sand in an hourglass' effect towards the end of the show.  There was a pit of sand in the forward end of the set, and the desired idea was that a small pour of sand would come out as if the pit was an hourglass, and the performer playing Everybody would interact with the falling sand, and then later be shielded by it.  

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The technical director's initial idea was a dispenser using funnels to give the sand drop a very narrow shape.  The sand would require filtering to ensure no large particles would clog the funnel.  This was the process initally started.  However, it became apparent that the system would need manual operation, someone to dump the sand into the hopper so it would pour.  This presented a noise challenge of the sand landing in the hopper, and the operation of it over the stage would be conspicuous.  I tasked myself with finding a way to allow the drop to occur before audience entered the house, and for the drop to be triggered.

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I decided that a small pneumatic system would provide the means to do this in the most efficient way possible.  A pneumatic cylinder was used with a hook ring, which was attached to a 'knife', a flat portion of steel that would cover the hopper aperture.  When the cylinder was triggered, it would pull the knife away just enough to allow a slow fall of the sand into the narrow funnel, creating the desired effect.  A spring would pull the knife back over the aperture to stop the pour.

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I used a large piece of 2" box steel to build a hinging mechanism, and welded a small square of plate steel to a short length of 1" box steel.  The smaller piece also had holes drilled to allow two eye bolts to be attached for the pulling mechanisms.  With a bolt to act as the swinging hinge, it was aligned in position to cover the aperture and adjusted to allow for just enough opening when triggered.  

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I decided to use a solenoid pneumatic valve to provide the opening mechanism.  By creating a small powered switch box, I was able to give the stage manager control of the drop.  By flipping the switch on the box, an indicator light showed when the mechanism was engaged.  The switch would engage the solenoid valve, pulling the cylinder closed and moving the knife.  When turned off, the solenoid extended the cylinder, and the tension spring pulled the knife back to the closed position.  By timing the amount of sand that came through the aperture and the closing of the knife, we were able to trigger the start and stop of the effect to best reflect the needs of the show.  The exhaust lines from the solenoid valve were ran into a far room, allowing the mechanism to be virtually silent.