Grease composition for constant velocity joints

Abstract: A grease composition for constant velocity joints which consists essentially of: (a) a base oil; (b) a diurea compound represented by the following general formula: R 1 NH-CO-NH-p-C 6 H 4 -CH 2 -p-C 6 H 4 -NH-CO-NHR 2 (wherein R 1 and R 2 may be the same or different and each represents an aryl group or a cyclohexyl group); (c) melamine cyanurate; (d) molybdenum disulfide; and (e) a phosphorus-free sulfur extreme pressure agent, and optionally (f) molybdenum dithiocarbamate. The grease composition exhibits excellent pitting-inhibitory effect and heat resistance.

Description: BACKGROUND OF THE INVENTION The present invention relates to a grease composition for use in constant velocity joints (CVJs), especially for ball type fixed and plunging constant velocity joints. A very high contact pressure is developed between the parts of the constant velocity joints to be lubricated and the joint parts undergo complicated rolling and sliding motions. This often results in abnormal wear and metal fatigue and, in turn, leads to a spalling phenomenon, i.e., pitting of the joint parts. More specifically, the present invention relates to a grease composition for constant velocity joints which can effectively lubricate such constant velocity joints to effectively reduce the wear of joints and to effectively reduce the occurrence of any pitting of the parts. Examples of lubricating greases conventionally used in such constant velocity joints include a lithium soap thickened extreme pressure grease containing molybdenum disulfide and a lithium soap thickened extreme pressure grease containing molybdenum disulfide and extreme pressure agents, e.g., sulfur-phosphorus or a lead naphthenate. However, these greases for constant velocity joints have not always been satisfactory in the severe working conditions which occur in the present high-performance motorcars. The double offset type constant velocity joints and cross groove type constant velocity joints used as the plunging joints as well as Birfield joints used as the fixed joints have a structure in which torques are transmitted through 6 balls. These joints cause complicated reciprocating motions such as complicated rolling and sliding motions during rotation under high contact pressure, and stresses are repeatedly applied to the balls and the metal surfaces which come in contact with the balls,and accordingly, the pitting phenomenon is apt to occur at such portions due to metal fatigue. The recent improvement in the power of engines is accompanied by an increase in the contact pressure as compared with conventional engines. Motorcars are being made lighter to improve fuel consumption and the size of joints has correspondingly been down-sized. This leads to a relative increase in the contact pressure and thus the conventional greases are ineffective in that they cannot sufficiently reduce the pitting phenomenon. In addition, the greases must also be improved in their heat resistance. JP-A-62207397 discloses a grease composition containing base oil, diurea thickener, Mo dithiocarbamate and sulfurised olefin or sulfurised oil and fat as extreme pressure agent. JP-A-61012791 discloses a grease compositons containing base oil, melamine cyanurate, and one of polytetrafluoroethylene, MoS 2 and Mo dithiocarbamate. SUMMARY OF THE INVENTION The present invention can provide a novel grease composition for constant velocity joints which has an excellent pitting-inhibitory effect and heat resistance. We have conducted studies to develop a grease composition capable of optimizing frictional wear of CVJs and of eliminating pitting of joints due to abnormal wear and metal fatigue and having improved heat resistance. We have evaluated greases under lubricating conditions accompanied by complicated reciprocating motions such as complicated rolling and sliding motions under high contact pressure as discussed above using a lubricity tester known as a high speed four ball tester, to determine extreme pressure properties of various kinds of extreme pressure agents, solid lubricants or combinations of additives. As a result, we have found that a grease comprising a combination of a base oil, a diurea compound, melamine cyanurate, molybdenum disulfide, phosphorus-free sulfur extreme pressure agent and molybdenum dithiocarbamate exhibits high extreme pressure properties and have confirmed, by a durability test performed using a practical constant velocity joint, that the grease can prevent the occurrence of pitting phenomena, unlike the conventional greases for constant velocity joints. The present invention provides a grease composition for constant velocity joints consisting essentially of ( a ) a base oil; (b) a diurea compound of formula: R 1 NH-CO-NH-p-C 6 H 4 -CH 2 -p-C 6 H 4 -NH-CO-NHR 2 wherein R 1 and R 2 are the same or different and selected from aryl and cyclohexyl groups; (c) melamine cyanurate; (d) molybdenum disulfide; (e) a phosphorus-free sulfur extreme pressure agent; and (f) molybdenum dithiocarbamate. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will hereunder be explained in more detail. The base oil as Component (a) is not restricted to specific ones and may be, for instance, lubricating oils currently used such as mineral oils, ester type synthetic oils, ether type synthetic oils, hydrocarbon type synthetic oils or mixture thereof. The diurea compound as Component (b) can be prepared through reaction of an aromatic amine such as aniline or p-toluidine, cyclohexyl amine or a mixture thereof with a diisocyanate compound. In the diurea compound, any aryl group preferably has 6 or 7 carbon atoms and the mol.% of aryl group in the diurea compound amine moieties ranges from 100 to 0. The melamine cyanurate as Component (c) is an adduct of melamine and cyanuric acid. Cyanuric acid is a tautomer of isocyanuric acid. Melamine isocyanurate commercially available is an adduct of one mole of melamine and one mole of cyanuric acid and is in the form of melamine isocyanurate. In the present specification, the term "melamine cyanurate" is used to mean an adduct of melamine and cyanuric acid or isocyanuric acid. Melamine cyanurate may easily be obtained as white precipitate when an aqueous melamine solution is mixed with an aqueous cyanuric acid or isocyanuric acid solution. Melamine cyanurate is commercially available as finely divided white powder having an average particle size of about 1 to 2 µm, wherein a melamine molecule of 6-membered ring structure is combined with cyanuric acid molecule of 6-membered ring structure through hydrogen bonds to form a planar structure. It is believed that the planar structures are piled up and show cleavability and lubricity like molybdenum disulfide. The molybdenum disulfide as Component (d) has widely been used as an extreme pressure agent. With regard to the lubricating mechanism thereof, the molybdenum disulfide is easily sheared under sliding motions through the formation of a thin layer since it has a layer lattice structure and it shows effects of reducing frictional force and of preventing seizure of joints. The phosphorus-free sulfur extreme pressure agent as Component (e) has preferably a sulfur content ranging from 35 to 50% by weight. The molybdenum dithiocarbamate used in the present invention as Component (f) is preferably represented by the following formula: (R 4 R 3 N-CS-S) 2 -Mo 2 OmSn wherein R 3 and R 4 are selected independently from alkyl groups having 1 to 24 carbon atoms, preferably 3 to 18 carbon atoms, m is 0 to 3, n is 4 to 1 and m + n = 4. The composition according to the present invention may include antioxidants, corrosion inhibitors and/or rust inhibitors in addition to the foregoing essential components. The grease composition according to the present invention preferably contains, on the basis of the total weight of the composition, 55.0 to 98.2% by weight of the base oil (a); 1 to 25% by weight of the diurea compound (b); 0.1 to 5.0% by weight of melamine cyanurate (c); 0.5 to 5.0% by weight of the molybdenum disulfide (d); 0.1 to 5.0% by weight of the phosphorus-free sulfur extreme pressure agent (e); and 0.1 to 5.0% by weight of the molybdenum dithiocarbamate (f). If the amount of the diurea compound (b) is less than 1% by weight, the thickening effect thereof tends to become too low to convert the composition into a grease, while if it exceeds 25% by weight, the resulting composition tends to become too hard to ensure the desired effects of the present invention. Moreover, it becomes difficult to obtain the desired effects of the present invention if the amount of the melamine cyanurate (c) is less than 0.1% by weight, the amount of the molybdenum disulfide (d) is less than 0.5% by weight, or the amount of the phosphorus-free sulfur extreme pressure agent (e) is less than 0.1% by weight. On the other hand, if the amount of the melamine cyanurate (c) is more than 5% by weight, the amount of the molybdenum disulfide (d) is more than 5% by weight, or the amount of the phosphorus-free sulfur extreme pressure agent (e) is more than 5% by weight, any further improvement in the effects cannot be expected and these components rather inversely affect the pitting-inhibitory effect of the present invention. If the amount of the molybdenum dithiocarbamate (f) is less than 0.1% by weight or more than 5% by weight, the effects obtained by the addition of the component are not remarkable. The present invention will hereunder be described in more detail with reference to the following non-limitative working Examples and Comparative Examples. Examples 1 to 2 and Comparative Examples 1 to 10 There were added, to a container, 4100 g of a base oil and 1012 g of diphenylmethane-4,4'-diisocyanate and the mixture was heated to a temperature between 70 and 80°C. To another container, there were added 4100 g of a base oil, 563 g of cyclohexylamine and 225 g of aniline followed by heating at a temperature between 70 and 80 °C and addition thereof to the foregoing container. The mixture was then reacted for 30 minutes with sufficient stirring, the temperature of the reaction system was raised up to 160 °C with stirring and the reaction system was allowed to cool to give a base urea grease. To the base grease, there were added the following additives listed in Table 1 in amounts likewise listed in Table 1 and an optional and additional amount of the base oil and the penetration of the resulting mixture was adjusted to the No. 1 grade by a three-stage roll mill. In all of the abovementioned Examples and Comparative Examples, a mineral oil having the following properties was used as the base oil. Viscosity at 40°C 130 mm 2 /s at 100°C 14 mm 2 /s Viscosity Index 106 Moreover, a commercially available lithium grease containing molybdenum disulfide , a sulfur-phosphorus extreme pressure agent and a lead naphthenate was used as the grease of Comparative Example 10. Physical properties of these greases were evaluated according to the methods detailed below. The results thus obtained are also summarized in Table 1. [Penetration] According to ISO 2137 [Dropping point] According to ISO 2176 [High Speed Four Ball Extreme Pressure Test] According to ASTM D 2596 Evaluated Item: Weld Point [Durability Test on Bench Using Real Joints] The greases were inspected, under the following conditions, for the occurrence of pitting by a durability test on a bench using real joints. Test Conditions: Number of Revolutions 1500 rpm Torque 196 N· m Angle of Joint 10° Operation Time 200 hours Type of Joint Used Birfield Joint Evaluated Item: Occurrence of pitting at each part of the joints after operation. After the durability test was conducted for 200 hours, these greases were evaluated according to the following criteria: ○ : No pitting was observed; X : Pitting was observed.

Claims: A grease composition for constant velocity joints consisting essentially of (a) a base oil; (b) a diurea compound of formula: R 1 NH-CO-NH-p-C 6 H 4 -CH 2 -p-C 6 H 4 -NH-CO-NHR 2 wherein R 1 and R 2 are the same or different and selected from aryl and cyclohexyl groups; (c) melamine cyanurate; (d) molybdenum disulfide; (e) a phosphorus-free sulfur extreme pressure agent; and (f) molybdenum dithiocarbamate. A composition according to claim 1 containing, based on the total composition weight, 55.0 to 98.2% (a); 1 to 25% of (b); 0.1 to 5.0% of (c); 0.5 to 5.0% of (d); 0.1 to 5.0% of (e); and 0.1 to 5.0% of (f). A composition according to claim 1 or 2 wherein the phosphorus-free sulfur extreme pressure agent contains sulfur in the amount of from 35 to 50% by weight. A composition according to any preceding claim wherein R 1 and/or R 2 represents an aryl group of 6 or 7 carbon atoms. A composition according to any preceding claim wherein the diurea contains a diurea compound in which cyclohexyl amine and aniline provide monoamine moieties. A composition according to any preceding claim wherein the molybdenum dithiocarbamate is of formula (R 4 R 3 N-CS-S) 2 -Mo 2 O m S n wherein R 3 and R 4 are selected independently from alkyl groups having 1 to 24 carbon atoms, m is O to 3, n is 4 to 1 and m + n = 4. Use of a composition according to any preceding claim for lubricating constant velocity plunging ball joints. Use of a composition according to any one of claims 1 to 6 for lubricating constant velocity fixed ball joints.

Inventors: HANAWA TOMIO, OGURA TAKAHIRO, SASAKI TSUYOSHI, TAKEUCHI KIYOSHI, USUI YOSHIMI, HANAWA, TOMIO, OGURA, TAKAHIRO, SASAKI, TSUYOSHI, TAKEUCHI, KIYOSHI, USUI, YOSHIMI, Hanawa, Tomio, c/o Tochigi Factory, Ogura, Takahiro, c/o Tochigi Factory, Sasaki, Tsuyoshi, c/o Tsujido Plant, Takeuchi, Kiyoshi, c/o Tsujido Plant, Usui, Yoshimi, c/o Tochigi Factory
Applicants: HONDA MOTOR CO LTD, KYODO YUSHI, HONDA GIKEN KOGYO KABUSHIKI KAISHA, KYODO YUSHI CO., LTD.
Application year: 1996
Main classification: C10M169
IPC codes: C10N30, C10M169, C10N10, C10N50, C10N40