Revolution in Balancing Technology: Laser Balancing™ Makes the Operation of Turbopumps Even More Efficient
Since its invention in 1958, the turbomolecular pump (turbopump for short) has been regarded as the driving force behind high-vacuum technology. Thanks to its reliable vacuum generation, it has become indispensable in the semiconductor production industry and among others.
Its inventor, Willi Becker, who at that time had been the head of the technical laboratory at Arthur Pfeiffer Vakuumtechnik GmbH (today Pfeiffer Vacuum GmbH) for 13 years, could hardly have dreamed back then that 63 years later, a laser beam would be responsible for further revolutioning the turbopump. This is because laser balancing is the latest and most efficient method of balancing technology that increases the service life and performance of turbopumps. Laser Balancing was developed and patented at Pfeiffer Vacuum - the company where Willi Becker invented the turbopump back in the day.
Structure of the turbopump
To this day, the turbopump remains essential for generating oil-free high and ultra-high vacuum. Immediately after its invention, it gradually replaced existing pumping principles for vacuum generation. In the 1960s, the demand for high vacuum began to increase more and more, quickly establishing the turbopump as the standard for high and ultra-high vacuum generation in a wide variety of applications. Without its use, many process steps in semiconductor manufacturing or coating would not be possible.
The design of the turbopump is similar to that of a turbine. Inside the pump, several rotor disks are mounted on a shaft. Between them are stator disks whose blade orientation is mirror-inverted to that of the rotor blades. As a result, the gas molecules to be pumped are conveyed from the high-vacuum flange along the individual turbo stages to the fore-vacuum flange. The rotor of the turbopump is driven by a brushless three-phase synchronous motor. This enables very high rotational frequencies of up to 1500 Hz to be achieved. The rotor shaft bearing, in turn, consists of a permanent magnet bearing on the high vacuum side and a high performance ball bearing on the fore vacuum side. Although the ball bearing is minimally lubricated, the pump generates an oil-free vacuum.
This hybrid bearing, which is rarely found in mechanical engineering, represents a special feature to the bearing and thus balancing technology and differs from usual bearing technologies. In combination with the very high speeds, balancing the turbopump rotors in particular is a technological challenge. This is because the balancing quality in particular has a major influence on the service life and performance of the turbopump.
Sectional model of a hybrid-bearing turbopump showing the rotor shaft bearing arrangement with a permanent magnet bearing and a ball bearing.
Background to rotor balancing
In practice, every rotating component exhibits a certain amount of unbalance, which cannot be completely avoided. In order to enable operation with as little vibration as possible later on, it is essential to reduce the unbalance of the ever faster rotating rotors by taking appropriate measures. The best-known process is probably that of balancing car tires. If the wheels exhibit an imbalance, this becomes noticeable through vibrations on the steering wheel. This physical phenomenon is also known as centrifugal force: The DIN ISO definition describes the unbalance of a rotor as a condition in which oscillating forces and movements are transmitted to the bearings due to unbalanced centrifugal forces.
Even bodies that appear visually symmetrical in reality exhibit slight inequalities in the distribution of mass. This can result, for example, from the manufacturing process of the component or an inhomogeneity in the density of the raw material. The term unbalance describes this uneven mass distribution. Other causes can stem from the design or assembly. In addition, unbalance can also occur during operation due to wear or deposits.
This is only an excerpt.
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