Flying at zero altitude

Laser weld seams up to 12 meters long are utilized in the design of the Transrapid train

David A. Belforte

Shanghai, the self-styled “Paris of the East,” is a city on the move, with more than 16 million people continuously traversing its streets. Nowhere is movement more obvious than on the trip into the city from Pudong International Airport, where Transrapid-the world’s first commercial magnetic levitation (maglev) system-whisks people 30 kilometers to the Long Yard Road subway station, located in the heart of the city’s financial district. Traveling at speeds up to 450 km/hour (275 mph) passengers make the trip in about 8 minutes.

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The airport authority chose maglev because it has the lowest energy consumption of any transport system, it makes limited use of valuable right-of-way real estate, it has low operational and maintenance costs, and it is environmentally compatible. Transrapid International, a joint venture between Germany’s Siemens and Thyssen Krupp, did the system engineering, system integration, and maintenance support for the Transrapid and completed the job in less than two years.

The magnetic levitation track is the most important innovation in train technology since the construction of the first railroads. The Transrapid does not travel along the track; it hovers without touching its tracks-and at a speed of up to 500 km/hour. An electromagnetic levitation and drive system takes over the function of wheel and rail. The Transrapid is, in fact, the only rail system in the world that functions without wheels.

The Alcan group of companies designed the railcar body for the Transrapid and also provided material optimization. The railcar bodies contain a series of innovative production technologies that guarantee comprehensive stability and thus safety, as well as giving the Transrapid its characteristic lightness. These technologies include laser welding.

A 28-meter-long railcar body consists of extruded aluminum profiles with composite boards, also made of aluminum, welded onto them. This procedure is taken care of by a TRUMPF CW solid-state laser in conjunction with a robot installation. The beam guidance from the laser to the robot-mounted welding head is via a flexible laser fiber cable. The application challenge was the enormous length of a single weld seam-up to 12 meters. With regulated laser power of 3 kW, beam quality of 25 mm/mrad, and a seam detector, optimal weld seam quality was guaranteed.

A fillet weld joins 1mm material to 2mm material, and 2mm material to 3mm material, respectively. The low heat transfer during laser welding is a benefit where welding of the Transrapid is concerned. With conventional welding methods, such as MIG or TIG welding, this kind of joint would not be possible because the aluminum sandwich structure would not withstand the very high heat transfer. Here the laser proves its superiority as a flexible and economically efficient means of production.


Electrodynamic suspension

Electrodynamic suspension is a form of stable magnetic levitation based on the induction of repulsive forces between a mobile magnetic field provided by superconducting coils on the vehicle and a static electrical conductor built into the track in the form of plates and coils. Because a repulsive force, which is sufficiently large for levitation, can be obtained only at or above a critical speed-the transition velocity-complimentary auxiliary wheels are also necessary. In general, EDS is combined with steel-less long-stator liner motor propulsion. Air gap vales vary from about 100 mm to 150 mm.

Courtesy of Transrapid International

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