You never know where you’re going to run into thermal processing, as a trip to the National Radio Astronomy Observatory (NRAO) in Green Bank, W.V., illustrated. The Green Bank Telescope (GBT) – the world’s largest fully steerable radio telescope – underwent a track rebuild, which involved thermal processing and materials technology at every turn.
A radio telescope consists of a radio receiver and an antenna system, and it is used to detect radio-frequency radiation created by extraterrestrial sources. Radio telescopes are, by necessity, very large because radio wavelengths are much longer than those of visible light, and the large size attains the required resolution.
The first parabolic radio telescope was built by Grote Reber in his backyard in Wheaton, Ill., in 1937. This original telescope is on display at Green Bank. The science has expanded in over 70 years, and there are a number of observatories around the world.
Along with the science, the size of these telescopes has also expanded from the 30 foot diameter of Reber’s parabola to the 328- x 360-foot GBT, which is a wheel-and-track design. The track is 210 feet in diameter, level within a few thousandths of an inch and bears 16 million pounds of weight. It was this weight that created the need for a track rebuild. In fact, the dead-weight load on each wheel is 1.1 million pounds, which results in a contact pressure of over 100,000 pounds per square inch. This is the highest loading of any wheel-on-rail application known!
Since beginning operation in 2000, the track on the GBT deteriorated. The track is comprised of wear plates and base plates and was fastened down with bolts. Each of these components failed, with the wear plates cracking due to fatigue. After a thorough failure investigation, including finite element modeling, a stiffer design was developed. Higher-strength base plates were used. These plates were joined by welding to a depth of 3.5 inches, and each weld was subsequently ultrasonically inspected.
Redesigned wear plates are not only 50% thicker, they are hardened to approximately 38 RC. Plates were flame cut oversize and were machined and ground to size. Originally, this material was 2.25-inch-thick AISI 4140. It is now 3.5-inch-thick 4340, which has a greater hardenability to meet the upgraded requirements.
The plate fastening system was also redesigned from hardened cap screws to 2-inch-diameter threaded 4340 hardened to a minimum yield strength of 125,000 psi. The studs are also located farther away from the wheel path to reduce fatigue stress. The stud connectors are tensioned hydraulically, and their elongation is measured with an ultrasonic bolt-elongation measurement system. The wheels are also made from 4340. Although not replaced in the rebuild, these wheels are hardened to approximately 45 RC.
The need for a new and improved design is understandable given the sheer size of this piece of equipment (see figure). At 485 feet tall, the GBT stands higher than the Statue of Liberty and weighs 30 times as much. The main dish has a surface area of 2 acres and is comprised of 2,004 aluminum panels, which are individually mounted at their corners by actuators – little motor-driven pistons. A laser system continually monitors the shape of the dish and utilizes the actuators to optimize its signal-receiving ability.
The GBT can receive radio waves from space that are a billion times weaker than signals from a typical AM radio station 60 miles away. For this reason, if you visit the GBT at the NRAO in Green Bank, W.V., you can leave your cellphone at home. That’s because the facility is located in a quiet zone free from outside interference. In fact, we were told that if you live in the quiet zone and your microwave malfunctions, someone from the observatory will show up to assist you with repair or replacement.
The applications for critically designed hardened material is almost limitless. Now you know about an application of thermal-processing technology that brings our universe a little closer.