This invention relates to polyorganosiloxane traction fluids. The polyorganosiloxane traction fluids of this invention are particularly well suited for use in traction drive systems and transmissions subject to wide operating temperature conditions.
A traction drive is a device by which torque can be transmitted from one smooth rolling element to another wherein the rolling elements are in nominal point or line contact. One such simple traction drive might consist of two parallel cylindrical elements in nominal line contact where one element is the input member and the other is the output member. As is well known in the art, both fixed speed and variable speed traction drives can be made by proper selection of the number, size, shape, and geometrical configuration of the roller elements. The continuously variable speed traction drive is attracting current interest for automotive applications because it has been estimated that use of such a traction drive could result in increased fuel efficiencies of 30-50% without sacrificing vehicle performance. Another advantage of traction drives over conventional transmissions is the smooth and quiet operation of the traction drive.
The limited lifetime and load carrying capabilities of traction drives have substantially prevented their wide-spread use except for light-duty applications. Recently, however, the development of better lubricants, called traction fluids, have allowed the development of traction drive transmissions which are suitable for heavy-duty applications. Indeed, the properties of the traction fluid, which also acts as a lubricant and coolant in the traction drive, determines to a large degree the performance, capacity, and lifetime of the traction drive. Of critical importance are the properties of the traction fluid under the high pressure and high shear conditions found in the area of contact between the roller elements. Although the roller elements are usually spoken of as being in contact, it is generally accepted that the roller elements are separated by a thin film of the tractive fluid. It is through the traction fluid's resistance to shear that the torque transmitting ability of a given fluid arises. The torque transmitting ability of a fluid, and thus its suitability as a traction fluid, is measured by and is directly related to its traction coefficient.
Hammann, et al. disclosed in U.S. Pat. No. 3,440,894 (Apr. 29, 1969) that certain classes of fluids characterized by high traction coefficient and molecular structure were superior traction fluids. Wygant in U.S. Pat. No. 3,994,816 (Nov. 30, 1976) discloses that certain hydrogenated dimers of alpha-methylstyrene (e.g. 2,4-dicyclohexyl-2-methyl pentane) are suitable as traction fluids. U.S. Pat. No. 3,994,816 is further discussed in an article entitled "Base Fluids" in Functional Fluids for Industry, Transportation and Aerospace, M. W. Ranney (Ed), Noyes Data Corporation, Park Ridge, N.J. (1980). Among the disclosed traction fluids were the cycloaliphatic hydrocarbon species. Preferred cycloaliphatic hydrocarbons are now offered as traction fluids by Monsanto Company under the tradename Santotrac. Although the Santotrac fluids offer high traction coefficients they have one major disadvantage which has prevented the more wide-spread utilization of traction drives. At sub-zero temperatures the viscosity of the Santotrac fluid increases dramatically. For example, one Santotrac fluid has a viscosity of 31,600 centistokes at -20.degree. F. and an estimated viscosity of 200,000 centistokes at -40.degree. F. Clearly such fluids could not be successfully used in applications subjected to low temperature extremes. One such application, for example, in which a Santotrac fluid traction drive might not be successfully employed would be vehicles exposed to sub-zero temperatures.
Several attempts have been made to develop fluids with reasonable low temperature viscosity and high traction coefficients. Wygant in U.S. Pat. No. 3,652,418 (Mar. 28, 1972) discloses that a low temperature traction fluid can be prepared by blending 30-60% by weight hydrogenated dicumyl, 30-60% by weight tercyclohexyl, and at least 5% by weight dicyclohexyl or certain alkyl dicyclohexyl. The disclosed blends of Wygant gave acceptable traction coefficients and improved low temperature viscosities as compared to U.S. Pat. No. 3,440,894 traction fluids. However, Wygant admits that his blended traction fluids gave only "operable viscosity ranges over temperatures of 0.degree. to 210.degree. F."
Kulik and Smith in U.S. Pat. No. 4,190,546 (Feb. 26, 1980) disclosed that a traction fluid with acceptable low temperature properties and traction coefficients could be obtained by blending a Santotrac fluid with a silicone fluid containing from 15 to 25 methyl groups per phenyl group if, and only if, about 2 to 10% by weight of an aromatic hydrocarbon or aromatic ether co-solvent is added. The co-solvent is required to ensure complete miscibility of the siloxane and Santotrac fluids.
Siloxanes have been evaluated for use as traction fluids. F. G. Rounds ("Effect of Lubricant Composition on Friction as Measured With Thrust Ball Bearings," J. Chem. Engn. Data, 5, 499-507 (1960)) found that several different siloxanes had traction coefficients approximately equal to that found for mineral oils. The mineral oils have traction coefficients much lower than that of cycloaliphatic hydrocarbons such as Santotrac. One siloxane, a chlorophenyl silicone, was reported by Green and Langenfeld ("Lubricants for Traction Drives," Mach. Design, 46, 108,113 (1974)), to have a traction coefficient approaching that found for the cycloaliphatic hydrocarbons. However, this chlorophenyl silicone was not suitable as a traction fluid because of its poor resistance to oxidation and moisture and the resulting tendency to gel.
More recently, certain siloxanes suitable for use as traction fluids were described in Groenhof, U.S. patent application Ser. No. 304,726, filed Sept. 23, 1981 which is hereby incorporated by reference. The siloxanes of Ser. No. 304,726 could be used as traction fluids either alone or in combination with cycloaliphatic hydrocarbons. The disclosed siloxane fluids contained [(CH.sub.3).sub.3 SiO.sub.1/2 ] units, [(CH.sub.3)R'SiO] units, and, optionally, [(CH.sub.3).sub.2 SiO] units where R' is a phenyl radical or a cyclohexyl radical and where there are about 1.6 to 14 methyl radicals for each R' radical. The disclosed siloxane fluids may also contain a limited amount of [RSiO.sub.3/2 ] units where R is methyl, phenyl, or cyclohexyl radicals. The [RSiO.sub.3/2 ] content of the siloxanes of Ser. No. 304,726 should be kept below 5 percent by weight and preferably below 1 percent by weight. The most preferred siloxanes of Ser. No. 304,726 do not contain any [RSiO.sub.3/2 ] units.
It has now been found that siloxane fluids containing significant amount of [RSiO.sub.3/2 ] units make excellent traction fluids. It is, therefore, one object of this present invention to provide polyorganosiloxanes which contain significant amounts of [RSiO.sub.3/2 ] units and which are useful as traction fluids. Another object is to provide polyorganosiloxane traction fluids suitable for use at low temperatures. Another object is to provide polyorganosiloxane traction fluids which contain only methyl radicals as the organic groups attached to silicon. Still another object of this invention is to provide improved traction drive systems particularly well suited for operating under widely varying temperature environments.