What are the properties of Fluid Film Bearings?
Due to the fact that there is no rubbing or rolling contact between bearing surfaces the following benefits arise:
• Exceptional Accuracy - The fluid film averages out errors of manufacture in the bearing surfaces and provides an accurate spin axis in a shaft, or linear motion of a slider. A recent estimate of the rotational accuracy of a pressurised air bearing is that the spin axis can be defined to ± 1.5 nanometres ( ± 6 x 10-8 inches). This result was obtained with a spindle designed to operate within an Ultra High Vacuum environment, to produce Data Tracks for a Mastering Machine aimed at the DVD Blue Ray Format. Deviations in Track concentricity of the Data Tracks were verified by Atomic Force Microscopy. The spindle design was optimised for dynamic stiffness.The spindle is shown in the accompanying photograph.
• Low Friction and the elimination of "Stick Slip" - Under conditions of low relative velocity between sliding surfaces, frictional drag is reduced to negligible proportions. There is an almost total elimination of the "stick slip" action associated with plain bearings. Recent investigations have shown that a spindle with a dedicated design can have a freedom from "stick slip", with a drag torque below 10 -7 Nm (8.8x10-7 Lb.inch). This latter effect is still maintained when bearings are heavily loaded. An excellent example of these properties has been demonstrated by the development of the Torque Machine at the National Physical Laboratory. The Aerostatic bearings that form all the principal bearings on the Torque Machine were designed and built at Fluid Film Devices. The machine is capable of applying torque levels from 2 Nm to 2 kNm with a discrimination of ± 0.002%
• Linear Stiffness of the Fluid Film - If the design is properly optimised, the stiffness of a fluid film bearing can be considered to be almost linear when deflected through 50% of the film thickness. This latter property allows the bearing to be used as a transducer under conditions of dynamic loading. This technique has been applied to flowmeters and instruments for measuring tensile stress in metal foil as it is rolled.
• Virtually Wear Free - Because of the absence of any rubbing contact fluid film bearings will remain operational for very long periods. Some of the metrology instruments using Aerostatic bearings are as accurate today as when they were first made 40 years ago. In the case of Aerodynamic bearings material selection is all important, as rubbing contact is inevitable when starting and stopping. By use of selected materials many thousands of stop-starts have been successfully accomplished.
A good example of the potential long life properties of an Aerodynamic bearing is the spindle developed by FFD in conjunction with Air Liquide of France. The bearing system which runs on process gas, in this case Nitrogen, is designed to run in the Manned Space Laboratory and assist in cooling instruments. The shaft is 22 mm (0.866 inch) dia. and can run at speeds of 90,000 rpm. The materials used for both bearings and shaft are Tungsten Carbide. The device has to survive the vibration spectrum at launching and have an operational life of 10 years. The accompanying illustration does not show the assembled spindle, but does show the shaft, journal bearings and the thrust runner.
• High Speed Operation - Absence of any rubbing can also promote running at high speeds. A typical example is a prototype for a heat pump using Freon Vapour as the working fluid. The shaft was ½ inch( 12.7mm) in diameter. The operating speed was 160,000 rpm. The design was based on a herringbone grooved pattern and is an example of an Aerodynamic bearing. Power dissipated per bearing did not exceed 17 Watts.
• Using Process Fluid as a Bearing Fluid - Both Aerodynamic and Hydrodynamic Bearings can often use the process fluid in a system as the working fluid in the bearing. This technique enables a rotor to run in what is effectively a closed circuit environment without the necessity of seals.
An example of using a process fluid to form a non-contacting bearing, was a pump designed and developed at FFD, to deliver a precisely metered flow of Diesel to a burner. The grooved bearing not only supported the rotor, but, due to the characteristics of the grooves, also delivered a regulated flow of Diesel. |
