Assembly in Action / Motion Control / Columns

Linear Actuators Move Observation Platform

June 3, 2014

Forty years ago this month, the Skydeck at the Sears Tower (now known as the Willis Tower) was opened to the public. The observation floor remains one of Chicago’s most famous attractions, drawing nearly 1.3 million visitors annually.

Located on the tower’s 103rd floor, the Skydeck is 1,353 feet high. Elevators reach it from ground level in about 60 seconds. Once there, tourists can see across Lake Michigan to Indiana, Michigan and Wisconsin on a clear day—and feel the building sway on a windy day.

In January 2009, the Skydeck underwent a major renovation. It included the installation of four all-clear-glass viewing boxes that enable people to look straight down to the street. Known as The Ledge, each box extends 4.3 feet over Wacker Drive and retracts back into the tower on a trolley system to ease and quicken panel maintenance and cleaning.

Original Sears Tower architecture firm Skidmore, Owings and Merrill designed The Ledge concept, while the structural engineering firm Halcrow Yolles detailed all the glass and steel components. Engineers there also developed a near-invisible support system that bears 5 tons of weight (about 4.5 metric tons) without any perimeter structural steel.

The Ledge consists of six 250-pound glass panels. Each panel is comprised of three layers of 0.5-inch-thick glass that are fully tempered for durability and laminated so they stay together if shattered. MTH Industries, a Chicago-based glass and architectural metal contractor, installed the ledges.

Locking pins secure each ledge in the extended and retracted positions. A T1T4 electric actuator, made by RACO International L.P., ensures smooth opening and closing of the pins.

The variable-frequency-drive actuator
has a 7.79-inch stroke and a speed of 0.66 inch per second. It uses a digital noncontact encoder (EPS 06) with 4- to 20-milliamp output signal to ensure repeatable, uniform and precise positioning of the pins.

The actuator is also equipped with a hand wheel for manual operation in case of a power loss. It provides power control, integrates easily into a PLC and only consumes energy during operation, making it environmentally friendly.

The four ledges opened to the public on July 2, 2009. Two weeks later, the building was officially renamed Willis Tower after London-based insurance broker Willis Group Holdings Ltd. The company had obtained renaming rights as part of its 2009 agreement to lease more than 140,000 square feet of space on three floors.

 For more information on electric actuators, call 412-835-5744 or visit

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