12 A.R. Klembczyk Fig. 2.1 The Millennium Bridge 2.2 Specialized Damper Design [1] Taylor Devices’ Fluid Dampers with metal bellows seals had been previously used exclusively by NASA and other U.S. Government agencies for space based optical systems. These previous applications had similar requirements for long life and high resolution at low amplitudes, but required relatively low damper forces from small, lightweight design envelopes. Figure 2.2 is a photograph of a pair of typical dampers of this design, used in space on more than 70 satellites to protect delicate solar array panels. This figure also shows the metal bellows seals; one in the compressed position and one in the extended position. This type of seal does not slide, but rather flexes without hysteresis as the damper moves. This patented design is known as a Frictionless Hermetic Damper. A cutaway of a typical damper of this type is shown in Fig. 2.3. Two metal bellows seals are used to seal fluid in each damper, one at each end of the damping chamber. As the damper moves, the two metal bellows alternately extend and retract, by flexure of the individual bellows segments. Since the seal element elastically flexes rather than slides, seal hysteresis is nearly zero. The volume displaced by the compressing bellows passes through the crossover ports to the extending bellows at the opposite end of the damper. While this is occurring, damping forces are being produced by orifices in the damping head, and the pressures generated are kept isolated from the metal bellows by high restriction hydrodynamic labyrinth bushings. Because hydrodynamic bushings are used, no sliding contact with the piston rod occurs, assuring near-frictionless performance. Adapting this basic design for use on the Millennium Bridge largely involved simply scaling the small satellite Dampers to the required size range. All parts, including the metal bellows seals, were designed with low stress levels to provide an endurance life in excess of 2 109 cycles. The metal bellows and other moving parts were constructed from stainless steel for corrosion resistance. To assure a high resolution output, it was required that all damper attachment clevises be fabricated with fitted spherical bearings and fitted mounting pins, such that zero net end play existed in the attachment brackets. A total of 37 dampers of this design were manufactured, component-level tested, and installed on the bridge in late 2001. There are three basic types of dampers. These are referred to as the Pier Dampers, the Deck Dampers, and the Vertical Dampers and are described below: Damper Nomenclature: Pier Damper Quantity on the Bridge: 16 Description: Two Pier Dampers are located on each side of each of two piers on both the east and west side of the bridge, for a total of eight dampers per pier. Damping coefficient values for the eight dampers connected directly to the center span of the bridge are significantly higher than the other Pier Dampers. Dampers have varying over-all lengths due to the
RkJQdWJsaXNoZXIy MTMzNzEzMQ==