Spiral groove bearing

Spiral groove thrust bearings are self-acting bearings used to reduce friction and wear without the use of pressurized lubricants. They have this ability due to special patterns of grooves. Spiral groove bearings are self-acting because their own rotation builds up the pressure needed to separate the bearing surfaces. For this reason, they are also 'contact-less' bearings.

Operation

Spiral groove bearings produce the required pressure to keep the bearing surfaces lubricated and separated by the wedging and pumping effect. When the parts of the bearings are rotated with respect to each other the grooves push the lubricants towards the small apertures between the surfaces causing an overall rise in the pressure.

The motion of the surfaces will then cause the fluid to flow over the grooves and a pressure ripple, perpendicular to the direction of the motion, is formed. Between the flat surface of the bearings and the fluid, a pressure rise occurs because this flow is limited. At sufficient speeds and thicknesses of the film of fluid, the bearing surfaces are completely separated. It is the pressure acting perpendicular to the direction of motion that supports the bearing load.

This explanation neglects the effects of inertia, compressibility of the lubricant and other factors.[1]

Fabrication

The dimensions of the grooves are tailored to the intended operating conditions of the bearing. If the indentations on the grooved surface are too deep, then there will be significant leakage of the lubricant. If the depth is reduced, the pump effect will stop. The speed of the rotation of the bearing surfaces and the accuracy of the dimensions must also be taken into account. Designers and manufactures calculate the optimal dimensions for greatest efficiency.[2] The grooves are made by the following methods;

Etching

Etching is easiest way to make spiral groove bearings. The surface of the metal is coated with an etchant - resistant lacquer, then the intended locations of the grooves are removed by hand. The factors that affect the properties of the grooves in this method are;

Despite the simplicity of this method, there is a significant disadvantage: the groove depth is non-uniform and is therefore fairly inaccurate.

Selective etching

This method differs from regular etching as two layers are placed on the surface to be grooved, but only the upper layer is exposed to the etchant, leaving the lower surfaces protected.

Mechanical grooving

This method is used when more accurate and more uniform grooves are required. The grooves are cut by an electrical diameter cutter, The disc surface is rotated, and the cutter it is steered by a guider ring, so that the spirals have the required logarithmic shape. One disadvantage of this method is that more specialized equipment is required to accurately cut smaller grooves. (approximately 6 cm and less).

Soldering

Soldering is used when other fabrication methods are unavailable or inapplicable to the given situation. e.g. the bearing is too large for an etching bath. A foil on which the grooves have been etched is obtained, and is soldered onto the flat bearing surface.
The factors that are considered in this method are;

Types

The main types of spiral groove bearings are:[3]

Flat thrust bearings

Flat thrust bearings, the most common spiral groove bearings, are so named because one consists of a flat surface that opposes the grooved surface.

Variations in this type of bearing come from the nature of the spiral surface and the type of fluid flow. The following is a list of the different types of flat thrust bearings.

Spherical thrust bearings

A spherical thrust bearing consists of a sphere that rotates concentrically in a spherical cup with groove patterns

Conical thrust bearings

In these bearings, a cone is cut out of the end of a cylindrical shaft. On the surface of the cone next to the cylindrical part, the grooves are made.

History and applications

Spiral groove bearings were used most successfully in inertial gyroscopes for planes and ships.[4]

In this application, the spiral groove bearings, made of metal alloys, were operating at low speeds, high precision and high efficiency.[5] Due to the multiple technical advantages, thrust bearings continue to be used in gyroscopes such as in the Hubble Telescope.[6]

Another notable use of spiral groove bearings is in cryogenic expanders. They are used here to support the high speed rotation of turbines, and to minimize power losses due to inefficiency. Cryogenic expanders extract energy from the streams of gases that enter it, causing a rapid decrease in temperature, and the energy extracted is used to rotate the turbines.[7][8]

Advantages

The following lists the advantages of using spiral groove bearings as opposed to other self-acting bearings.

References

  1. ↑ "Ceramic Spiral Groove Bearings in Oil-Free Compressors". Docstoc.com. 6 February 2012.
  2. ↑ Muijderman, E. A.. Spiral groove bearings. New York: Springer-Verlag, 1966. Print. <http://www.emeraldinsight.com/journals.htm?articleid=1688586&show=abstract
  3. ↑ Muyderman, E.A. (1966). "Constructions with spiral-groove bearings". Wear 9 (2): 118–141. doi:10.1016/0043-1648(66)90129-3.
  4. ↑ Ceramic Spiral Groove Bearings in Oil-Free Compressors. Docstoc – Documents, Templates, Forms, Ebooks, Papers & Presentations. N.p., n.d. Web. 6 Feb. 2012. <http://www.docstoc.com/docs/68697394/Ceramic-Spiral-Groove-Bearings-in-Oil-Free-Compressors>.
  5. ↑ Breadman, G. Development of the series 700 gas bearing gyroscope. 5th International Gas Bearing Symposium, 1971. University of Southampton, UK.
  6. ↑ http://www.spacetelescope.org/about/general/gyroscopes/
  7. ↑ Molyneaux, A K (1989). The Use of Spiral Groove Gas Bearings in a 350 000 rpm Cryogenic Expander. Tribology Transactions 32. pp. 197–. doi:10.1080/10402008908981879.
  8. ↑ "FUNDAMENTALS OF THE TURBOEXPANDER - MIT File." Document Finder, Converter & Download - MIT File. N.p., n.d. Web. 13 Feb. 2012. <http://www.mitfile.com/pdf/fundamentals-turboexpander.html>.

External links

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