The world’s largest rodless cylinders?
They just may be. But officials at Cedar Point prefer to point out the fun, thrills, and excitement guests enjoy when riding the Power Tower. Power Tower consists of four 240-ft tubes that accelerate a carriage containing up to 12 riders to 50 mph in.3 sec. Each tube is part of a cable cylinder. A cable attached to both sides of a piston inside the tube runs through the center of the tube, exits through the center of each end, then wraps half-way around a pulley at each end. From the pulley, the cable runs parallel to the tube OD, and its ends attach to the top and bottom of the load carriage.
Because of the cable arrangement, routing compressed air to the top of the tube pushes the piston down, thereby pulling the carriage up. Likewise, routing air into the bottom of the tube raises the piston, moving the carriage down. Power Tower actually consists of two separate towers — one uses compressed air to propel the carriage upward from near-ground level. The other uses compressed air to transport the carriage to the top of the tower, then a charge of air propels the carriage downward.
Air for each of the four tubes is stored in a huge receiver mounted centrally among the four tubes. Once riders are secured in their seats, a charge of compressed air first propels the carriage upward. It then is allowed to freefall, which compresses a measured quantity of air in the top portion of the four tubes. Pressure transducers in the tubes communicate to the ride computer how much pressure the loaded carriage exerts on the charge of air. The computer uses this pressure value to calculate the weight of the carriage with its occupants. Next, air is released from the tops of the tubes, and the carriage returns to earth. From the weight calculation, the computer then meters out the precise amount of compressed air to propel the carriage up or down and to cushion the carriage when it approaches the top or bottom of its travel.
When the four tubes approach the end of their stroke, their pistons begin compressing the air that has been metered into the tubes. This causes the carriage to bounce several times as the air is released from the tubes to allow the carriage to gently return to earth. Power Tower contains a total of eight tubes approximately 240-ft long and with diameters of about 8 in. Plus, two 240-ft central receivers each have a diameter of approximately 16 in., so you might expect this attraction to consume huge quantities of compressed air. It does. Power Tower gets its pneumatic power from three 200-hp rotary screw compressors. (A fourth is used for standby operation.) Each is set at an operating pressure of 125 psi and can deliver 990 cfm of air. Each compressor has its own refrigerated dryer.
Hydraulics helps entertain across the pond
Most Americans who are in regular contact with residents of the U.K. will attest to the British as having an offbeat sense of humor. Most British, however, probably feel that the reverse is true. But playing on the British sense of humor is the essence of a new section at Chessington World of Adventures, a theme park on the outskirts of London. As with other major parks of this size, specific areas cater to a certain theme. The new section that opened earlier this year is Beanoland, modeled after the popular, long-running British comic strip featuring Dennis the Menace, Gnasher, and other colorful characters.
One of the attractions at Beanoland is a stunt show where Dennis gets into trouble while trying to pull a prank by altering the ornamentation of a decorative fountain. Renaissance Entertainment, Orlando, produced the show, and Integrity Attractions, also in Orlando, designed and built a fountain lift system and controls for the attraction as a subcontractor to Renaissance.
In the show, Dennis and a friend decide to perform some harmless vandalism on a nearby fountain. Dennis uses a ladder to climb into the fountain, then attempts to remove a top piece from it. In doing so, he leans over the center of the fountain, and his friend pulls a prank of his own by opening the fountain’s main valve. Intending to drench Dennis, the friend gets more than he bargained for when a powerful geyser of water shoots upward from the fountain, taking Dennis with it. This cascade of water keeps Dennis elevated high above the fountain, much to the surprise of Dennis’ friend. Or so it seems.
The geyser of water shooting upward appears to push Dennis high above the fountain. Matt Lenz, president of Integrity Attractions, reveals that in reality, a hydraulic cylinder located in the center of the cascading water pushes the character upward. “Because it is completely surrounded by water, and is roughly the same color as the water, the piston rod cannot be seen by the audience.”
Lenz continues, “The Dennis character wears a harness under his costume. The harness supports a lock that mates with another lock on the end of the piston rod. Of course, none of this hardware can be seen by the audience. When the Dennis character leans over the fountain, he very subtly locks himself to the end of the retracted piston rod. Once Dennis is locked into place, an off-stage operator actuates a control that starts the flow of water from the fountain while the piston rod begins extending.
“Pure water is used as the hydraulic fluid to actuate the cylinder — the same water used in the fountain. The top of the fountain has eight nozzles that shoot water into the air. Diffusers on all the nozzles aerate the water so it forms a curtain of spray that hides the piston rod.
“The cylinder itself has a 10-in. bore, 6-in. diameter rod, 78-in. stroke, and is constructed entirely of corrosion-resistant materials. For example, the piston rod is made of 6-in. stainless steel tube, the piston is made of UHMW composite, and the cylinder barrel is made of aluminum tubing. To keep the piston rod properly aligned, the piston is approximately 2-ft long. It’s actually two discs of UHMW composite with a section of 10-in pipe in between joining them. This keeps the rod aligned with the cylinder barrel and prevents binding that might otherwise result from side load caused by movement of the Dennis character.
“Because it is not as strong as steel, aluminum tubing generally is not used for hydraulic cylinders. But we operate the cylinder at a low pressure, so aluminum tubing has more than enough strength. The cylinder only has to lift one person, all the rigging, and the weight of the piston rod. So with a piston area of almost 80 sq in., the pump doesn’t need to develop high pressure. However, because the cylinder has to go from fully retracted to fully extended in only a few seconds, the pump does have to deliver high volume. With a 10-in. bore and 78-in. stroke, the pump has to move more than 5600 in.3 of water in under 5 sec. That’s over 300 gpm!”
The cylinder is double-acting, with meter-out flow control for rod extension and retraction. Cylinder direction is controlled by 6-in. butterfly valves, actuated pneumatically. As with many conventional water hydraulic systems, using a group of digital type valves proves more practical than using a single, very large spool-type valve.
Lenz explained that the current setup didn’t necessarily require a double-acting cylinder. However, as constructed, the system is versatile enough to accommodate adjustments or changes that may be necessary later. “We could’ve saved some money by building a single-acting cylinder, letting gravity retract the rod, and metering the water flowing out of the bottom (cap end) of the cylinder. The problem is, gravity limits retraction dynamics, which restricts what you can do.
“But I learned years ago that in this business, you’ll eventually need to make modifications to just about any system down the road. It’s almost inevitable. The system we designed and built meets the design requirements established early on. But if the show producers want to add a little pizzazz later on, they’ll be able to, simply by reprogramming the PLC.”
Lenz refers to a programmable logic controller (PLC) that operates everything in proper sequence and continually monitors key operating and safety functions. Lenz continues, “Everything is designed with double redundancy for safety, including the controls. That way, an operator can”t accidentally cause something to occur out of sequence; he or she has to use two hands to communicate a command to the PLC. The one exception to this are the emergency stop buttons.”
Considering all this behind-the-scenes engineering, Dennis is really pulling a prank on the audience: probably none of them realize that Dennis is held airborne not by a jet of water, but by a hydraulic cylinder.