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Isolation Systems

Technical Brief

76. SIMULATION, DEVELOPMENT, AND FIELD MEASUREMENT VALIDATION OF AN ISOLATION SYSTEM FOR A NEW ELECTRONICS CABINET IN THE SPACE SHUTTLE LAUNCH ENVIRONMENT WITHIN THE MOBILE LAUNCH PLATFORM

This paper describes the dynamic analysis of an isolator system for the cabinet-mounted low voltage power switchgear in the Space Shuttle Mobile Launch Platform (MLP). The addition of electronic sensing and control components to this cabinet combined with the harsh vibration environment experienced during a Shuttle launch necessitated a six degree of freedom isolation system to prevent the spurious tripping of breakers. An added benefit of the isolation system is that it provides vibration isolation during the Shuttle’s approximately three mile journey between the Vehicle Assembly Building (VAB) and either of its two launch pads. The isolation system was designed, built, and integrated within the MLP. Broadband dynamic measurements were made during an actual Shuttle launch to verify the effectiveness of the isolation system and to validate the predictions of the analysis. Measurements made during the launch of STS-115 on September 9, 2006, affirmed the effectiveness of the isolators and validated the predicted performance of the isolation system.

Case Study

75. STRUCTURAL CONTROL USING HYBRID SPRING-DAMPER ISOLATOR WITH INTEGRAL GAPPING FUNCTION

The spring-damper isolators described in this paper were used on the world’s largest cable stayed bridge – the Sutong Bridge over China’s Yangtze River, completed in 2008. The Sutong Bridge is located north of Shanghai in China’s Jiangsu Province at a site where catastrophic earthquakes, typhoons, and ship impact are key design issues. The total length of the bridge is 4.7 miles, with a .67 mile long center span. The tall support towers of this bridge and the long support cables create long period motions along the primary axis of the bridge. The need to accommodate thermal expansion and contraction of the deck axially means that extensive motion can occur in this direction. The configuration of the bridge permits large axial motion of the suspended deck during earthquakes, typhoons, and synchronized truck/car braking loads such as would occur during a mass vehicular accident on the bridge. During dynamic earthquake loading, the long period of the suspended deck provides inherent isolation, albeit essentially undamped. Analysis indicated that added viscous damping would reduce deck motions substantially. During other events like typhoons and vehicle loading, analysis determined that the most cost-effective solution was to incorporate a snubbing type spring element that would only engage (become active) when the damper was approaching its end of travel in either extension or compression. The spring-dampers on this bridge have only damping forces for roughly 85% of the available displacement from the neutral (center of travel) position. Beyond this travel the spring element engage and a combined response of spring plus damper forces results. Essentially, the spring elements are “gapped” through all but approximately the last 15% of the damper stroke in either direction.

White Paper

74. INTRODUCTION TO SHOCK AND VIBRATION ISOLATION AND DAMPING SYSTEMS

This paper presents an introduction to shock and vibration isolation of complex structures and mechanisms. It provides an outline of various ways to provide isolation, shock absorbing and damping within a wide array of dynamic systems and structures. This paper presents key definitions that are widely used within the shock and vibration community. Additionally, useful formulae are presented that provide the user with an approach to typical problems. Finally, a comparison of different types of shock isolators, shock absorbers and dampers compares their advantages and disadvantages for use in the commercial, military, and aerospace sectors.

White Paper

69. ANALYSIS, OPTIMIZATION, AND DEVELOPMENT OF A SPECIALIZED PASSIVE SHOCK ISOLATION SYSTEM FOR HIGH SPEED PLANING BOAT SEATS

The Mk V Special Operations Craft (SOC) is used to carry Special Operations Forces (SOF) into and out of combat operations. During operation, particularly during extended training missions, the passengers and crew have reported numerous cases of musculoskeletal injuries from operation in high sea states. This paper describes the analysis, development and operational testing of a highly specialized, non-linear, passive shock isolated seat for this craft. Initial sea trial testing of the isolation system resulted in positive operator feedback that correlated well with earlier field measurements and also validated the analytical predictions.

White Paper

63. VIRTUAL BASE ISOLATION BY BUILDING SOFTENING WITH DRIFT CONTROL PROVIDED BY FLUID VISCOUS DAMPERS

The paper describes “virtual isolation” for buildings with one or more soft stories. Using the 1999 SEAOC Blue Book (SEAOC, 1999) recommendations for passive energy dissipation, the building’s Lateral Force Resisting System (LFRS) is designed for strength requirements only, resulting in a relatively flexible LFRS, while Fluid Viscous Dampers (FVD) are incorporated to limit story drifts to acceptable levels. There are many benefits to this “virtual isolation” system. With the elimination of the maximum drift requirements, the moment frames are substantially lighter than a traditionally framed building, thus lowering the structural steel cost of the LFRS. The long period structure also produces significantly reduced forces in the foundation elements. Velocity and displacement are reduced significantly through the use of the FVDs, which protects the sensitive contents of the building. These benefits lead to a reduced response resulting in an enhanced performance level during a major seismic event.

White Paper

61. A NUMERICAL INVESTIGATION OF COMBINED SHOCK AND VIBRATION ISOLATION THROUGH THE SEMI-ACTIVE CONTROL OF MAGNETORHEOLOGICAL FLUID DAMPER IN PARALLEL WITH AN AIR SPRING

Combining shock and vibration isolation into a single isolation mount is investigated numerically through the use of the Bouc-Wen model of a magnetorheological fluid damper in parallel with an air spring. The stability and dissipative capabilities of the Bouc-Wen model are proven mathematically. The response characteristic of this hybrid isolator to shock and vibration inputs is explored. The advantages of combining shock and vibration isolation into a single package is discussed. It is possible, using this technique, for a single device to perform equally well as a shock and a vibration isolator.

White Paper

55. EXPERIMENTAL STUDY OF BRIDGE ELASTOMER AND OTHER ISOLATION AND ENERGY DISSIPATION SYSTEMS WITH EMPHASIS ON UPLIFT PREVENTION AND HIGH VELOCITY NEAR-SOURCE SEISMIC EXCITATION

A series of shake table tests on an isolated bridge model included low and high damping elastomeric isolation systems, and low damping elastomeric systems with added linear and nonlinear viscous dampers. Each of these configurations could withstand much stronger seismic excitations than the non-isolated configurations. A set of low intensity tests was conducted to form a basis for comparison with the non-isolated configurations and also to test the effectiveness of these systems under low intensity excitation. The results of these tests are presented, followed by a discussion of the effects of scragging, the benefits of seismic isolation, and the significance of damping, the importance of added damping in near source seismic excitation and on the benefits and drawbacks of using nonlinear viscous damping.

White Paper

51. APPLICATIONS OF HERMETICALLY SEALED FLUID DAMPERS FOR LOW LEVEL, WIDE BANDWIDTH VIBRATION ISOLATION

Vibration isolation of sensitive components like high resolution cameras requires extremely low friction in the isolator system. Hydraulic dampers for these systems must be leak-free, which equates to relatively high friction seals. There is always a trade-off between allowable leakage and allowable friction in this type of application. This paper describes the isolation performance of a new hermetically sealed damper with essentially zero friction. It contains both an analytical representation of damper performance and dynamic test results.

Case Study

48. Arrowhead Regional Medical Center

This article describes every aspect of the design and construction of the Arrowhead Regional Medical Center, including the use of base isolators and viscous dampers to insure continuous operation even after a major seismic event. The article even includes many of the financial aspects of this huge project.

White Paper

47. DEVELOPMENT AND TESTING OF AN ELECTRONICALLY CONTROLLED SHOCK AND VIBRATION DAMPER HAVING AN ELECTRORHEOLOGICAL FLUID MEDIUM

An electrorheological (ER) fluid has been developed comprised of zeolite particles suspended in silicone oil. Testing of this fluid in a damper of 1,000 lbs. nominal output force rating has demonstrated the ability to control damper output with internal pressures above 500 psi and control power requirements of less than one watt. The damper control valve is a simple ER duct, with no moving parts, requiring only that a voltage potential exists across the duct’s cross section to activate the ER material, and thus cause the material to exhibit plastic behavior. All tests were successful, with no degradation of the damper or ER material occurring over large numbers of activation cycles or with time.

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