Gas Bearings

A gas cushion for highest purity and lowest maintenance: Gas bearings not only increase the lifetime but also enable complete oil-free operation. With Celeroton gas bearings this is also achievable at the highest possible speeds for various gases.

Functionality

Gas bearings have appeared throughout the history of drive systems in several places. They had a first commercial success in the application of foil bearings in aviation and aerospace in the 1950s and 60s. Common synonyms for gas bearings are the terms “fluid bearings” or often simplified “air bearings”. This means that the bearing of, say, a rotor-stator-system is formed from a thin gas cushion between the rotor and the stator case. To establish the required lifting power, an overpressure is built in the gas cushion, which increases with deflection. The gas exerts force against the deflection of the rotor and keeps it in the centre of the stator case, and therefore ensures a contactless bearing of the rotor from the stator. The main required technical characteristic of the gas cushion is having the correct stiffness and damping in order for the gas cushion to absorb disruptions such as unbalance and vibration. The following figures show profile examples of a gas bearing and a ball bearing rotor.

Basically, there is a difference in how the pressure can be built-up by the gas cushion. On the one hand there is the so called externally pressurized gas bearing and on the other hand there is the self-acting gas bearing.

In externally pressurized gas bearings the pressure within the air gap is ensured by an external pressure feed. The rotor can be kept in the centre of the stator case with a suitable implementation of the pressure feed, e.g. with porous material or with the arrangement of injectors. Rotation can be realized from standstill without any stiction.
In self-acting gas bearings the pressure within the gas cushion is built-up by the rotation of the rotor relative to the stator. No external pressure feed is necessary and the overall system is therefore considerably easier and more compact. However, the rotor has to be operated at a minimum rotational speed, the so called lift-off rotational speed. From this speed on, the force build-up of the gas cushion is large enough to overcome gravity. Besides the lifting force, the stiffness and cushioning are different to externally pressurized gas bearings. These gas bearings are strongly dependent on the rotational speed. This type of gas bearing is applied in Celeroton products as they enable a more compact design without external pressure feed.

Figure 3: Externally pressurized gas bearing

Advantages and disadvantages

The pros and cons of gas bearing technology compared to ball bearings can be summarised as follows:

Advantages

Long lifetime
100% oil-free

Disadvantages

More complex rotor construction
Tighter manufacturing tolerances required

An extended comparison of different bearings technologies can be found here.

Challenges in designing a gas bearing drive system

The behaviour of a gas bearing is strongly dependent on the viscosity of the gas in the bearing clearance. The viscosity again is dependent on temperature of the gas and the gas itself. The gas bearing is designed to run stably over the expected range of these parameters and throughout the entire speed range and with a range of manufacturing tolerances (e.g. the lowest gas bearing clearance can be critical at high temperature and maximum speed or the largest bearing clearance at minimum temperature at medium speeds). Furthermore, the gas bearing is designed to withstand the required maximum shock and vibration levels throughout the entire speed range (where low speeds are usually more critical than high speeds).

The specific challenges in designing a high-speed turbo compressor with gas bearings:

The gas bearing must work robustly for all expected operating conditions, which include:
- Varying viscosity due to changing gas mixtures within the same compressor. For example in noble gas cooling applications where the system is not completely sealed then there is a variable percentage of air in the gas mixture.
- Fluctuating pressure levels resulting from operation at different altitudes. For example in fuel cells in mobile applications, operating point specific inlet pressure levels in noble gas recirculation applications or heat exchanger temperature dependent pressure levels in heat pumps.
- Widely varying gas temperatures in the bearing clearance at different operating points. For example, between -30°C ambient temperature at cold start and 200°C (329 °F) when heated up by internal losses.
- Reliable operation during high levels of vibration and shock, e.g. up to 25 G shock in automotive applications.
The gas bearing, rotor dynamics and thermodynamic behaviour of the system are strongly coupled. Therefore an interdisciplinary design is required to ensure that the total system is fully functional.
The overall efficiency of the thermodynamic, motor and the gas bearing must be maximised. This often leads to a different solution than just using the combined individual maximum efficiencies.
The manufacturing tolerances must be considered already during the design phase, in order to produce a robust and stable product series.
Last but not least the total system has to be economic to produce.

The design challenges increase significantly with increasing the rotational speed. The following considerations are needed:

Higher rotational speed leads to higher performance but also higher losses. These higher losses subsequently result in higher temperatures, especially in the rotor.
The damping required for a stable gas bearing increases with higher rotational speed.
Higher rotational speed leads to miniaturization, but the manufacturing tolerances must reduce accordingly.

An air bearing as in the Celeroton turbo compressor CT-2X-series for the supply of air to fuel cells, is designed according to the expected viscosity, temperature, and pressure range during operation, but only for air. For a gas other than air, e.g. noble or inert gases, which has a different viscosity, the gas bearing behaviour is different, and therefore the compressor that designed for air is usually not directly applicable. However, the gas bearing can be adapted to the new gas properties and operating specifications. For the Celeroton turbo compressor CT-NG-2000, which is operated with different inert and rare gases, there are gas bearing designs already available for several gas types.