Operating Principle

MAGER srl - Air Bearings _ Pneumostatic Principle

The lifting and guide pneumostatic systems uses a gas as lubricant, usually the air.
For this reason they are commonly called Air Bearings.
Gas is introduced under pressure inside of the component by suitable more or less complex diffusers (nozzles and microscores), iand it is distributed among the interacting surfaces, creating a very thin (a few thousands of millimetre) air gap, acting as lubricant.

General application and advantages

PNEUMOSTATIC
AIR BEARINGS
--- WHY YES ---

ROLLING AND SLIDING
BEARING
--- WHY NOT ---

In case of: high dynamics on free-maintanance guide systems

A correctly dimensioned air bearing ensures air gap stability and consequently the lubrication even at high speed, accelerations and positioning frequency.
→No wear, high reliability and durability
→No strokes limitation.
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As motion inversions frequences and accelerations increase, the behaviour and consequently the durability of rolling bearings comes close to the behaviour of NON lubricated sliding bearings.
→Premature wear, reduced reliability
→Significant run limitations
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In case of: High precision of positioning and/or measurement

The air pads enable higher measurement precision, higher positioning and speed control.
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Using rolling bearings, extremely high precision positioning and accurate speed control is quite difficult.
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In case of: No environmental pollution

The air bearings DO NOT pollute. They are the priority choice for whiteroom applications.
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Systems based on rolling technology may significantly pollute particular working environments. .
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In case of: Low noise and No vibrations

Air bearings are very quiet and their vibrations are limited and indipendent by the system dynamic.
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Systems based on rolling technology may feature unacceptable noise and vibrations depending by the system dynamic.
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In case of: High temperature and/or radioactivity

Air bearings are reliable even with high temperatures and radioactive sources.
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The durability of systems based on rolling technology with high temperatures and radioactive sources may be limited.
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MAGER Air Bearings - Pneumostatic Characteristic Testing Machine ....

Pneumostatic Characteristic Testing Machine and related Test Report (click to see zoomed image)

SOME COUNTERINDICATION TO THE USE OF THE PNEUMOSTATIC TECHNOLOGY

--- HIGHLY DINAMIC MOVEMENT ON FREE-MAINTENANCE GUIDE SYSTEMS ---

PNEUMOSTATIC ADVANTAGE
Since the air bearings feature creep and stick-slip: there is no mechanic contact and consequently no wear among the sliding elements even with high speed, accelerations and positioning frequency.
The guide systems are highly reliable and durable, with limited or no maintenance.
The friction is significantly lower than the friction of sliding and rolling bearings, and it is also much lower than the typical friction of hydraulic couplings, enabling to reduce the power required.
In contrast to the caged rollers systems, the air bearings feature no intrinsic limit in operating runs.

DISADVANTAGE OF ROLLING / SLIDING BEARINGS
the sliding and rolling bearings suffer from creep and stick-slip.
What is creep? The creep is a parasitic effect of the traditional rolling bearings (a sfere o a rulli).SIt occurs at the beginning of the movement, causing a temporary drag (-> sliding friction) between the rolling element (roller or ball) and the track on which the element slides, before the element starts to roll (-> rolling friction).
Creep becomes more significant as the frequency of motion inversion and acceleration increase, since the loads are higher and the distance travelled by the rolling element in sliding friction condition before rolling start increases.
For accelerations up to 10g, in rolling systems the reduction of damages due to creep depends on lubrication efficiency (with grease or oil), which must be periodically restored. For very high dynamics, the layer of grease or oil breaks down, causing metal on metal contact and significantly reducing the durability due to premature wear of the parts.

MAGER srl - Air Bearings _ Rolling Bearings Defects

SGuide systems with preloaded caged rollers and pinion-rack drive system solved the issue of roller and cage misalignment due to creep, but the wear issue is only partially solved. Often, to achieve an acceptable durability, it is necessary to oversize the rolling guide system.

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---- HIGH PRECISION OF POSITIONING AND/OR MEASUREMENT ----

PNEUMOSTATIC ADVANTAGE
The air bearings, thanks to the low viscosity of air, feature very low, constant friction. Movement is very fluid, and actuation CN can achieve accurate positioning without difficulty.

MAGER srl - Air Bearings _ Friction Diagram

The air gap can also “correct” small shape errors, due to waviness and roughness, of the guides on which it slides, either linear or of revolution, avoiding microjams and partly compensating the shape errors of the guide systems, increasing its accuracy.

DISADVANTAGE OF ROLLING / SLIDING BEARINGS
Rolling bearings suffer for the presence of creep and stick-slip phemonena.
Due to creep, at motion start (and in opposite direction at positioning end) a rolling part meets 3 (not only 2) decreasing friction coefficient values: static sliding friction (first detachment friction, called stick-slip), kinetic sliding friction and rolling friction.
This means that at very low speeds, near zero, the friction force of the rolling guide system changes, making precision positioning quite difficult: it is as if you'd try to stop your car in a specific point with an engine brake which, when the machine is almost still, suddenly increases its power.
Positioning control relies on motors power and on the readiness of the actuators used. Finally, in radial bearings the roller or ball rotation, due to imperceptible uneveness, can lead to microjams, which affect measurements, positioning control and speed.

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---- NO ENVIRONMENTAL POLLUTION ----

PNEUMOSTATIC ADVANTAGE
The lubricant in pneumostatics is suitably filtered, deoiled, dehumidified air. Such air has no polluting power. Besides, with no wear between parts, no dust is generated.
A further benefit is the pressurization of the environments due to the air supplied to the pads which escapes from the air gaps. This enables air tight environments, protected from dust.

DISADVANTAGE OF ROLLING / SLIDING BEARINGS
The presence of oils or greases is a pollutant: in case of high dynamics there may be significant overheat which in turn may cause the evaporation of the lubricants. Besides, if lubrication is poor, metal wear on parts may produce dusts.
Such pollution may not be acceptable in different industries, for several reasons.

MAGER srl - Air Bearings _ Application Table

Generally, the guide systems may require optical transducers (photodiodes, lasers, etc.): the deposit of the lubricant vapours on the optic components over time may alter their performance, up to complete malfunction.
In case of hydraulic systems or hydrostatic systems, leaks of water or oil cannot be avoided, and it is necessary to set up a system to recover and recycle the fluids.

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---- LOW NOISE AND NO VIBRATION ----

PNEUMOSTATIC ADVANTAGE
The air gap avoids the contact between rigid parts: the noise is limited to the hiss of air escaping from air gaps.The air has as well a damping effect, which absorbs the vibrations in a specific range of frequencies.

DISADVANTAGE OF ROLLING / SLIDING BEARINGS
The creep and the stick-slip effects generate noise and vibrations, due to the high rigidity of the contact combined with the difficulty to control the movement: at high frequencies, this may become a quite important design parameter.

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---- HIGH TEMPERATURE AND RADIOACTIVITY ----

PNEUMOSTATIC ADVANTAGE
Air viscosity virtually does not change as the temperature changes, and is not sensible to radioactivity: the pneumostatic systems ensure maximum reliability even in difficult environmental conditions.

DISADVANTAGE OF ROLLING / SLIDING BEARINGS
The durability of the parts in the rolling guide system decreases significantly when the operating temperature increases, because there is a decay of the mechanic characteristics of the metals (surface strength, stress resistance) and of the lubrication effectiveness of lubrication oils and greases. The latter are quite sensible also to radioactivity.

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COUNTERINDICATIONS

The pneumostatic technology has a few counterindications:

• LOWER SPECIFIC LOADS AND STIFFNESS

The air bearings, compared with rolling bearings, have lower specific loads and stiffness. It means that, with the same load capacity, the air bearing has bigger dimensions (which cannot be quantified a priori), and does not withstand significant changes of the applied load.
Its application is not recommended in case of significant variability of the loads (>200% compared with the rated load), for instance in the industry of heavy machine tools.

• COSTS RELATED TO THE PRODUCTION OF COMPRESSED AIR
Thanks to pneumostatics, the reliability and the maintenance costs of the guide and translation systems are optimized, yet it is necessary to take into account the costs for air compression and cleaning.
Typical supply pressures are approx. 4÷5 bar. Standard compression equipment available in most industries can be used.
Filters for particles with 1 µm power, oil extraction power, and dryers which provide air with dewpoint (at operating pressure) of 3°C are required.