How liquid metal bearings deliver cool operation, quiet and tube longevity
Use of liquid metal bearings sidestepped the pitfalls that shortened the life of tubes with traditional ball bearings. The liquid metal bearing of a rotating anode X-ray tube typically has four individual load-carrying components to take radial forces, gyroscopic moments, and thrust loads. The liquid metal fills a thin gap between stationary shaft and rotating sleeve. As soon as the rotor drive overcomes sticking friction, the rotation of the sleeve forces the liquid to separate the members. Capillary forces prevent leakage of fluid into vacuum. The bearing members touch each other for only seconds during start and landing.
The design of liquid metal bearing tubes results in several clinical and workflow benefits:
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- Large surfaces of the bearings and a gap filled with metallic liquid guarantee excellent heat conduction as well as conduction of tube current. Because the tube doesn't heat up as easily as metal bearing tubes, there is no need to wait between exams for the tube to cool down, even in demanding applications that require many scans or ultra-high image quality.
- Liquid metal bearing tubes are ready instantaneously, after the first ramp up in the day. Because the anode is spinning continuously, no preparation time is necessary for the anode to speed up, liquid metal bearing tubes also are well suited for emergency situations.
- The absence of wear during rotation allows for constant high-speed rotation through the entire working day, allowing smoother workflow and promoting long life.
- Audible bearing noise is absent, which promotes a pleasant scanning experience for patients.
- There is abundant primary photon flux available with the enhanced tube power. Hard filtering becomes feasible, e. g. with an extra slab of copper, that is eliminating excessive patient dose from the soft portion of the X-ray spectrum.
The current liquid metal bearing was introduced in 2007 with the advent of the straddle-type bearing for premium tier CT. This state-of-the-art bearing is suspended on both ends of the axis to enable extreme rotor speed while operating below the intrinsic resonance of the rotor and suspension system. That guarantees stability and low vibration. However, special measures are required to encapsulate the liquid metal. The cooling rate could be further enhanced, which allows to omit a graphite back of the anode disk.
