This invention relates to a grease for constant velocity joints, in particular, a grease for constant velocity joints which has a good extreme pressure property, good durability and vibration inhibiting effect by adding organic molybdenum compound, antimonydialkyl dithiocarbamate (hereinunder referred as Sb-DTC), a zinc dithio phosphate and organic sulfur compound.
The conventionally used greases include greases containing sulfur-phosphorus extreme pressure agent and an extreme pressure grease containing molybdenum disulfide and these greases are in general used in lubricating parts where wears and fretting corrosions are easily caused by extreme pressure, such as constant velocity joints used in motorcars (C.V.J), universal joint, steer linkage, spline shaft gear, coupling in industrial machine, gear motor and transmission gear.
Greases for wear-inhibiting and extreme pressure composed of sulfur-phosphorus compound were disclosed in U.S. Pat. Nos. 4,466,895 and 3,322,802 and Japanese Patent Publication Soh 66-47099. In these greases, by using sulfur-phosphorus compound independently or in complex, the friction coefficient and extreme pressure were improved. But in order to increase the extreme pressure and decrease the friction coefficient high temperature, a comparatively large amount of additives are required to be used. Some problems remained unsolved such as thermal decomposition of grease by active sulfide derived from the decomposition of sulfur-phosphorus compound in causing high temperature, corrosion and aging by acidic compound.
Greases using organic molybdenum, were disclosed in U.S. Pat. Nos. 3,840,463, 4,466,901, 4,428,861, 3,400,140 and 4,208,292 which describes greases using organic molybdenum compound (Mo-DTP) independently of other extreme pressure additives. Further U.S. Pat. No. 3,509,051 disclosed a grease which is characterized in using polyurea thickener, organic molybdenum compound, especially molybdenum dialkyl dithiocarbamate (Mo-DTC) and organic zinc compound in mixed condition to the basic oil. However, with respect to the use of organic molybdenum independently, wear-resistance is increased owing to a decrease in the friction coefficient, and there is no synergistic effect between the organic molybdenum and other extreme pressure additives. And as there are limits in extreme pressure of molybdenum disulfide (MoS.sub.2) compound produced by the decomposition of organic molybdenum, in friction condition where extreme pressure property is greatly required, great heat radiation due to lubrication in friction area and great deal of wears like scoring caused.
And in case that a mixture of an organic molybdenum compound and an organic zinc compound (Zn-DTP) is used as with a lithium grease there is an increase in both, friction coefficient and wear-resistance. Though the critical temperature of lithium grease is 120.degree. C., particularly in flanging type constant velocity joints wherein the rolling friction and sliding friction simultaneously occur, the temperature the of surrounding area increases to over the maximum 120.degree. C. because the of impulse load and frictional heat caused by sliding friction. Furthermore, the thermal decomposition temperature of Mo-DTP and Zn-DTP is low therefore are readily decomposed at 120.degree. C. into molybdenum disulfide compound and some cause some detrimental side-effects such as corrosion, sludge and slight-corrosions remain unsolved.
Further Japanese Patent Publication Pyung 5-62639 disclosed a grease composition comprised of molybdenum a compound and sulfur compound, which improved oxidation stability, wear resistance and corrosion-inhibiting effects but failed to reduce the beating noise and vibrations.
Conventionally used greases do not infiltrate into the lubricating area well in bad lubrication conditions which can result in wear and wear vibrations. And in the parts where slight vibrations do occur, the oxide produced by initial corrosion accelerates the wear, and abnormal beating noise, and vibrations occur.
Therefore, the inventors have made efforts to solve the aforementioned problems and at last have succeeded invent a grease which is characterized in that the extreme pressure and the wear-resistance properties are greatly improved, using organic molybdenum, antimony dialkyl dithiocarbamate, zinc dithiophosphate and organic sulfide compound in mixed condition; sludge occurrence possibility is reduced by improving thermal stability of additives; infiltration into the lubricating area is made easy by low viscosity; and good durability is aquired when it applied to constant velocity joints.