Ultra-high-speed set-up

Record speed: 1 200 000 rpm on aerodynamic bearings

1. Set-up

A special set-up (see figure 1) is used to test new bearing geometries. This set-up uses a simple 6 mm diameter caliper pen as rotor. This rotor is supported on aerodynamic radial bearings and aerostatic thrust bearings. Two symmetrically placed nozzles drive the pen by means of a small Pelton turbine in the center of the rotor. Shaft whirl is measured via optical probes mounted radially at both ends of the rotor.

Fig. 1. Ultra-high-speed set-up.

2. Measurements

Driven by compressed air

The rotor was first tested with the turbine driven by compressed air, reaching a speed of 685 000 rpm at full throttle. The maximal speed was clearly determined by the nozzle performance, NOT by bearing instability. The air speed at the exit of the nozzle is limited to sonic speed (about 343 m/s). When comparing this to the rotor's circumpherential speed (215 m/s), we see that the Pelton turbine is operating well above its point of maximum efficiency (this point lies at half the sonic speed). Figure 2 shows a waterfall diagram for a controlled run-up experiment from 0 to 683 280 rpm.

Figure 3 shows the spectra of the rotor whirl measured at both ends of this rotor, at the maximal speed.


Fig. 2. Waterfall diagram for controlled run-up experiment. Turbine driven by compressed air.		Fig. 3. Spectra of the rotor whirl at the maximal speed obtained with the air-driven turbine.

Driven by helium

To obtain higher speeds, the turbine nozzles are fed with compressed helium, because helium has a much higher sonic speed than air (1007 m/s versus 343 m/s at room temperature). Regarding turbine performance, the increased velocity more than compensates for the lower density of helium. A record speed of 1.2 million rpm was obtained. Helium may flow into the aerodynamic bearing gap, but this has no influence on the bearing performance as both air and helium have very similar viscosities.

Fig. 4. Waterfall diagram for controlled run-up experiment. Turbine driven by helium.

3. Comparison

The table below compares gas bearing speed records obtained by institutes and companies around the world. To make a fair comparison between small and large rotors, the DN-number should be considered: diameter x speed. The highest DN-number was obtained by J. W. Beams in 1937, a time when considerable effort was put in the development of ultra-centrifuges for uranium enrichment. An important 'trick' was the use of hydrogen as bearing gas, because its low viscosity postpones rotor instability with respect to air, and its high speed of sound increases the speed at the nozzle outlets.

To our knowledge, we realised the fastest air bearings in the world. Furthermore, our bearings are of the aerodynamic (self acting) type, requiring no external supply of compressed gas or energy. This is quite important for application in microgasturbines.

Author	Institute	Year	Diameter [mm]	Speed [rpm]	DN-number [mm.rpm]	Bearing type	Subtype
G. Belforte	Politecnico di Torino	2008	37	75 000	2 775 000	Aerostatic	O-ring support
H. Signer	NASA Lewis Research Center	1973	120	25 000	3 000 000	Ball bearings
C. Zwyssig	ETH Zürich	2008	3.175	1 000 000	3 175 000	Ball bearings
S. Tanaka	Tohoku University	2003	4	1 250 000	5 000 000	Aerostatic	Hydroinertia
S. Tanaka	Tohoku University	2009	8	642 000	5 136 000	Aerodynamic	Foil bearings
-	Bruker Biospin	2005	1.3	4 200 000	5 460 000	Aerostatic	O-ring support
F. D. Doty	Doty Scientific	2006	4	1 380 000	5 520 000	Aerostatic	O-ring support
A. Epstein	MIT	2006	4.2	1 700 000	7 140 000	Aerostatic	Small length/diameter ratio
T. Waumans	K.U.Leuven - PMA	2010	6	1 200 000	7 200 000	Aerodynamic	Rigid surface, externally damped
J. W. Beams	University of Virginia	1937	9	1 300 000	11 700 000	Aerostatic	Hydrogen fed
J. W. Beams	University of Virginia	1946	0.521	37 980 000	20 130 000	Magnetic	In vacuum

4. Future work

These new aerodynamic bearings will now be implemented on the turbo-shaft set-up and later on in the complete gasturbine.