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.
Gas bearings have appeared in 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 fifties and sixties. 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 on 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 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 realised 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 within Celeroton gas bearings as they enable a more compact design without external pressure feed.
Advantages and disadvantages
The pros and cons of gas bearing technology compared to ball bearings is summarised as:
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
There are the following challenges in designing a gas bearing drive system:
The gas bearing must robustly work over an operating condition range:
Fluctuating or changing pressure levels resulting from operation at different altitudes (e.g. automotive industry) or in heat pumps.
Wide operating temperature range, e.g. between -30°C and 200°C gas temperature in the air gap.
Reliable operation during high levels of vibration and shock, e.g. of 25 G
The gas bearing, rotor dynamics and thermodynamic behaviour have to combine to ensure a stable total system and therefore have to be designed in close coordination.
The overall efficiency of the motor and the gas bearing has to be maximised. This leads often to a different solution than taking the combined individual maximum efficiency.
When considering the manufacturing tolerances, the system of rotor, stator case and gas bearing has to result in robust and stable operation.
Last but not least the total system has to be economic to produce.
These challenges increase with the rotational speed:
Higher rotational speed leads to higher performance and losses, and results in higher temperatures, especially in the rotor.
The required damping for a stable gas bearing increases with higher rotational speed.
Higher rotational speed leads to miniaturisation and therefore to smaller manufacturing tolerances.
Therefore, the complete design and calculation of high-speed gas bearings together with the application is conducted by the team at Celeroton. The gas bearings have been successfully designed and tested for air as well as for other gases (e.g. refrigerants such as butane and methane). Contact us at infocelerotoncom or call us on +41 44 250 52 20 if you want to learn more about our innovative products.