Four limiting factors af iecting lubricating grease production are discussed: scientific knowledge of grease structure, raw materials, the manufacturing process, methods of test.
The precise physico-chemical nature of greases is still in doubt. The now generally accepted view that they are micro-dispersions of solid soap in oil, the soap crystals being peptized by suitable materials and accompanied by varying proportions of soaplmineral oil gel, accords widely with practical experience.
Nevertheless, the older theory that they are emulsions, though inadequate as a general explanation, is wholly apposite in certain cases and must continue to be given consideration. The authors believe that a combination of the two views may be acceptable.
Only fatty acids in the comparatively narrow range of C16 or C18 are really suitable for grease manufacture, preferably as glycerides, which account for some 70 % of the fatty stocks used. Calcium, sodium, and aluminium are the commonest soaps employed and greases made from them are able to meet almost all requirements. The properties of the mineral oil component must be considered in connection with the solubility of the soap, the phase transition temperature, and the nature of the colloidal structure. The use of solvent-refined oils causes more syneresis than conventionally-refined oils but the more paraf finic oils while reducing the tendency of the grease to stiffen after heating may give rise to poor resistance to shear. Conventional manufacturing processes are reviewed and the simplicity of the soap body method for lime base grease is stressed. Reference is made to systems for continuous grease production and thP importance of cooling rate is emphasized with reference to aluminium soaps.
Control tests are vital at all stages in manufacture from raw material to finished product. Adjustment to consistency is usually the final stage though devices may be installed for controlling it during manufacture. The flow properties of greases are imperfectly understood but in bearing applications * Vacuum Oil Co., Ltd., London, U.K. the thixotropic effect and the rate and extent of recovery may greatly influence the extent to which the material thins out. The plunger type of rheometer is useful for controlling manufacture of semi-fluid greases. For assessing mechanical stability laboratory tests, though useful as a guide, are less satisfactory than performance tests on equipment similar to that employed in service. Oil separation and oxidation are principal factors determining storage behaviour.
On discute quatre facteurs limitant la production de graisse lubrifiante: la connaissance scientifique d: la structure des graisses, les matières premières, les procédés de fabrication, les méthodes d'épreuve.
On est encore dans le doute quant à l'exacte nature physico-chimique des
