A movable sunshade on the roof of the Milwaukee Art Museum consists of a pair of wing-like structures, each made up of 36 steel ribs ranging in length from 24 to 105 ft. The ribs are mounted on two rotating steel shafts, in turn connected to the pavilion's spine. To open and close the sunshade, 22 hydraulic cylinders (11 on each side) stroke simultaneously. The cranks swing a 90-degree. arc to open the wings in 3.5 minutes. Engineers equipped the building with two separate ultrasonic wind sensors to monitor velocity and direction. If wind speed exceeds 23 mph for three seconds or longer, the control system automatically closes the wings. Similarly, a lightning sensor is installed to predict imminent lightning strikes. The power unit consists of two identical pump sets. Each 30-hp motor drives a tandem axial-piston pump. One pump of each tandem set powers the north wing, the other powers the south wing. This circuit can move both wings even if one motor/pump group is out of service.