Motion System Design

Pneumatics on the upswing

Whoosh. Whoosh.

The sound, accompanied by a high-pitched shriek as cables move rapidly upon a cable loop system, is somewhat reminiscent of that made by a windmill's blades as they slice through the air. No power is generated, however — this sound is the definition of power consumption, as two giant arms swing thrill-seekers high into the air through the use of huge pneumatic cylinders.

Cedar Fair LLP, Sandusky, Ohio, introduces a new ride each year at its Cedar Point amusement park. The Skyhawk, 2006's edition, is a massive swing, designed by S&S Power Inc., Logan, Utah, which specializes in pneumatically (and sometimes hydraulically) driven rides. Its other Cedar Point contributions include the pneumatic Power Tower and the free-falling hydraulic Frog Hopper, a children's ride. More than 100 of its rides entertain riders worldwide.

Seating 40 people — 10 on each side of two large arms — the Skyhawk swings riders up to 125 ft into the air at speeds of 60 mph or more. The one-minute ride peaks at seven swings, giving riders a chance to look straight up into the sky, out over Lake Erie, or straight down to the concrete below.

This is accomplished, says Cedar Point's Vice-President of Maintenance/New Construction and Assistant General Manager Monty Jasper, with the use of a “ready power plant.” Air.

The ride is run by four 300-hp compressors from Ingersoll Rand, which pump 1275 cfm of air at 120 psi to two double-acting air cylinders that are truly massive — each has a 28-in. bore and 21-ft stroke. The two cylinders, situated on either side of the ride's legs, each contain a 300-lb piston, which generates approximately 73,000 lb. of thrust, said S&S founder Stan Checkitts. The cylinder stroke about 22 ft in normal operation within the 31-ft long cylinder.

Quick-acting, 4-in. valves, operated by 3/8-in. pilot valves, shoot the compressed air into the cylinder. S&S' Tim Jacobi, project engineer for the Skyhawk, adds, “To swing the arm one way, we open the two-position push valve on one side of the cylinder and the exhaust valve on the opposite side. Then we do the opposite to swing the arm in the other direction.” The valves shift from fully closed to fully open in about 1 sec. A computer system monitors the ride and its velocity and commands the valves to open or close.

A single loop of high-strength steel cable transmits force from the cylinder to a sheave that applies torque to drive the swing arm. Essentially, then, the assembly acts as a giant rotary actuator. A sheave at the bottom of the tower acts as an idler, and additional sheaves help keep the cable stable. Jacobi says the cable cannot become slack, which is achieved by pre-tensioning the wire rope. Because it's a loop, the tensions oppose each other. Air flowing into one end of the cylinder increases cable tension at that end but decreases tension at the other end, and the pre-tensioning prevents any part of the loop from going slack.

For more information, contact Tim Jacobi at S&S Power [email protected].

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