Bearings for ore mills Increasing ore grinding mill size, and the greater feed openings required, have resulted in mill trunnion dia- meters exceeding the size limit for roller bearings. Hydrodynamic plain bearings and rollers operating against girth rings have inherent problems due to the high surface pressures involved and edge loading problems.

For diameters over the limit where rolling bearings can be used, above about 2m, and where alternative bearing types do not perform satis- factorily, SKF have developed a self- aligning hydrostatic bearing comprising a number of 'shoes'. Out of roundness of the girth ring or trunnions is not critical.

Each shoe consists of an upper sliding section, designed as a piston, a steel ball, and a lower section. The oil duct- ing arrangement relieves the load on the ball joint, and allows the shoe to tilt, compensating for angular misalign- ments. The centre of rotation of the trunnion is determined by two 'master. shoes. Further 'slave' shoes are incor- porated into the system, depending on the total load. An equal supply of oil to each shoe is delivered from a central oil system.

SKF started developing the system in 1969, and the first hydrostatic shoe bearing was installed in 1972. Operat- ing results to date are described as "very good" and it has recently been suggested that, with a well maintained system, lives of up to 50 O00h can be expected.

SKF (UK) Ltd, Luton LU3 1JF, Bedfordshire, UK or SKF Group Headquarters, S 415 50 Goteborg, Sweden.

Hydrostatic shoe bearing

Ore mill with a 4.8m internal diameter, fitted with a hydrostatic shoe bearing arrangement on the feed side

Coal pitch carbon fibres Carbon fibres are normally produced by pyrolysis of rayon or polyacrylon- itrite fibres at high temperatures. The Centre d'Etudes et Recherches des Charbonnages de France have devel- oped a process, starting from coal pitch, which gives lower cost fibres with slightly inferior mechanical properties.

Research workers at the Institut National de Recherche Chemique App- lique have recently completed an evaluation of these fibres, in an orga- nic matrix, sliding against a metallic counterface. The results obtained indi- cate that coal pitch carbon fibres give lower wear rates than PTFE and MoS2. In thermosetting polymers, graphite gives a lower friction coefficient and wear rate, but coal pitch carbon fibres

performed better in thermoplastic polymers. High modulus fibres from polyacrylonitrite generally gave better results than the low modulus fibres obtained from coal pitch, but at a higher cost.

The workers acknowledge the need for further work on these fibres, indicat- ing that their behaviour may change when rubbing against a different coun- terface. However, on the basis of the results obtained so far, they suggest that composites containing coal pitch carbon fibres could well be used in dry bearings, bearing cages, seals, thrust washers, etc.

IRCHA, Centre de Recherche, Boite Postale No 1,91710 Vert le Petit, France.

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TRIBOLOGY international April 1977 81