The NRAO Green Bank Telescope (GBT), located in Green Bank, West Virginia, is supported by 16 steel wheels which rest upon a composite steel and concrete Azimuth Track, 210 feet (64 meters) in diameter. From the start of observing in February 2001, the Azimuth Track design presented an operational problem for NRAO. By the spring of 2001, slippage of the top plate on the base plate was causing hold-down bolt failures. In July 2002, wear between the top and base plates (fretting) had become evident around the entire track circumference. NRAO engineers took immediate action to reduce both the track slippage and wear problems. But in January 2003, cracks were discovered in two adjacent top plates; by 2006 the top plates were cracking at a rate of almost one a month -an alarming rate given the design service life of 20 years. This paper will summarize the engineering analysis efforts that were subsequently conducted to assess the root cause of the GBT track degradation problem. We will also discuss a trial modification section that was installed in June 2004. Finally, we will discuss the design solution that was developed to remedy the track performance problem.
In millimeter wavelength telescope design and construction, there have been a number of mysterious failures of simple CFRF-metal joints. Telescope designers have not had satisfactory interpretations of these failures. In this paper, factors which may influence the failure of joints are discussed. These include stress concentration, material creep, joint fatigue, reasons related to chemical process and manufacture process. Extrapolation formulas for material creep, joint fatigue, and differential thermal stresses are derived in this paper. Detailed chemical and manufacturing factors are also discussed. All these issues are the causes of a number of early failures under a loading which is significantly lower than the strength of adhesives used. For ensuring reliability of a precision instrument structure joint, the designer should have a thorough understanding of all these factors.
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