Grease composition for constant velocity joints

Abstract: A grease composition for constant velocity joints which consists essentially of: (a) a base oil; (b) an urea thickener; (c) molybdenum disulfide; and (d) a metal salt or an overbasic metal salt selected from the group consisting of metal salts or overbasic metal salts of oxidized waxes, petroleum sulfonates, alkyl aryl sulfonates, salicylate, and phenates. In addition to (a) to (d), it may further contain, (e) an extreme pressure agent selected from the group consisting of a metal-free sulfur-phosphorus extreme pressure agent and molybdenum dithiophosphate, or (f) molybdenum dithiocarbamate. The grease composition exhibits excellent wear-resistance and pitting-inhibitory effect.

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. US-A-5207936 discloses a grease composition containing base oil, urea thickener and MoS 2 . US-A-5059336 discloses a grease composition containing base oil, urea thickener and, as rust inhibitor, Ba sulfonate and lanolate. SUMMARY OF THE INVENTION We have conducted studies to develop a grease composition capable of optimizing the 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 notions under high contact pressure as discussed above using an SRV (Schwingung Reibung und Verschleiss) tester known as an oscillating friction and wear tester, to determine lubricating characteristics (such as friction coefficient and wear) of various kinds of extreme pressure agents, solid lubricants or combinations of additives. As a result, we have found that a grease consisting essentially of a combination of a base oil, urea thickener, molybdenum disulfide and calcium salt or overbasic calcium salt of specific compounds (or of the combination mentioned above and an extreme pressure agent selected from metal-free sulfur-phosphorus extreme pressure agents and molybdenum dithiophosphate, or of the combination mentioned above and molybdenum dithiocarbamate) exhibits desired lubricating characteristics such as a good friction coefficient and low wear 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 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 urea thickener; (c) molybdenum disulfide; and (d) a salt selected from calcium and overbasic calcium salts of oxidized waxes, phenates, and petroleum sulfonic, alkyl aryl sulfonic and salicylic acids. A preferred composition of the present invention further includes (e) an extreme pressure agent selected from metal-free sulfur-phosphorus extreme pressure agents and molybdenum dithiophosphate in addition to the components (a) to (d). Another preferred composition of the present invention further includes (f) molybdenum dithiocarbamate in addition to the components (a) to (d). 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 and mixtures thereof. The urea thickener as Component (b) is not restricted to specific ones and may be, for instance, diurea compounds and polyurea compounds Examples of the diurea compounds include those obtained through reaction of a monoamine with a diisocyanate compound. Examples of the diisocyanates include phenylene diisocyanate, diphenyl diisocyanate, phenyl diisocyanate, diphenylmethane diisocyanate, octadecane diisocyanate, decane diisocyanate, and hexane diisocyanate. Examples of the monoamines include octylamine, dodecylamine, hexadecylamine, octadecylamine, oleylamine, aniline, p-toluidine, and cyclohexylamine. Examples of the polyurea compounds include those obtained through reaction of a diamine with a diisocyanate compound. Examples of the diisocyanates include those used for the formation of the diurea compounds as mentioned above.. Examples of the diamines include ethylenediamine, propanediamine, butanediamine, hexanediamine, octanediamine, phenylenediamine, tolylenediamine, and xylenediamine. Preferred examples of urea thickeners include those obtained through reaction of aryl amine such as aniline or p-toluidine, cyclohexyl amine or a mixture thereof with a diisocyanate. The aryl group in the diurea compounds preferably has 6 or 7 carbon atoms and the amount of aryl group in the diurea compound ranges from 100 to 0 mole% based on the total moles of the aryl and the cyclohexyl groups in the diurea compounds. The molybdenum disulfide as Component (c) 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. There have been known molybdenum disulfide products having various particle sizes, but it is preferable, in the present invention, to use those having a particle size ranging from 0.25 to 10 µm expressed in terms of an average particle size as determined by the Fisher method (by the use of a Fisher Sub-Sieve sizer), in particular, those having an average particle size of 0.55 to 0.85 µm. The calcium or overbasic calcium salts as Component (d) are selected from those known as metal cleaning dispersants or rust-inhibitors which are used in lubricants such as engine oils, being such salts of oxidized waxes, of petroleum sulfonates which are obtained by the sulfonation of aromatic hydrocarbons in lubricating oil fractions, of synthetic sulfonates such as dinonylnaphthalene sulfonic acid and alkylbenzene sulfonic acid, of salicylate, and of phenates. Preferred metal-free sulfur-phosphorus extreme pressure agents as Component (e) have a sulfur content ranging from 15 to 35% by weight and a phosphorus content ranging from 0.5 to 3% by weight and exhibit excellent effects of inhibiting wear and of preventing seizure of the joints through the well-established balance between the sulfur and phosphorus contents. More specifically, if the sulfur content exceeds the upper limit defined above, joints are easily corroded, while if the phosphorus content exceeds the upper limit defined above, any wear-inhibitory effect cannot be expected. On the other hand, if the sulfur and phosphorus contents are both less than the corresponding lower limits, any desired effect of the present invention cannot likewise be expected. As an extreme pressure agent (e), molybdenum dithiophosphates can also be used. Preferred molybdenum dithiophosphates is represented by the following formula (1): wherein R 1 , R 2 , R 3 and R 4 independently represent primary or secondary alkyl groupshaving 1 to 24, preferably 3 to 20,carbon atoms or aryl groups having 6 to 30, preferably 8 to 18, carbon atoms. The molybdenum dithiocarbamate as Component (f) is preferably represented by the following formula: (R 5 R 6 N-CS-S) 2 -MO 2 O m S n wherein R 5 and R 6 independently represent 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 grease composition of the invention may further include antioxidants, corrosion inhibitors, and rust inhibitors in addition to the foregoing essential components. A first preferred composition of the invention consists essentially, based on the total composition weight, of 55.0 to 98.0% of base oil (a); 1 to 25% of urea thickener (b); 0.5 to 5.0% of molybdenum disulfide (c); and 0.5 to 15% of salt (d). A second preferred composition of the invention consists essentially, based on the total composition weight, of 52.0 to 97.9% of base oil (a); 1 to 25% of urea thickener (b); 0.5 to 5.0% of molybdenum disulfide (c); 0.5 to 15% of salt (d); and 0.1 to 3% by weight of the extreme pressure agent (e). A third preferred composition of the invention consists essentially, based on the total composition weight, of 50.0 to 97.9% of base oil (a); 1 to 25% of urea thickener (b); 0.5 to 5.0% of molybdenum disulfide (c); 0.5 to 15% of salt (d); and 0.1 to 5% of molybdenum dithiocarbamate (f). A fourth preferred composition of the invention consists essentially, based on the total composition weight, of 63.0 to 91.5% of base oil (a); 5 to 20% of urea thickener (b); 2 to 4% of molybdenum disulfide (c); 1 to 10% of salt (d); and 0.5 to 3% of molybdenum dithiocarcamate (f). If the amount of urea thickener (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 molybdenum disulfide (c) is less than 0.5% by weight or the amount of salt (d) is less than 0.5% by weight, or if the amount of extreme pressure agent (e) or molybdenum dithiocarbamate (f) (when present) is less than 0.1% by weight. On the other hand, if the amount of molybdenum disulfide (c) is more than 5% by weight, the amount of salt (d) is more than 15% by weight, the amount of extreme pressure agent (e) is more than 3% by weight, or the amount of molybdenum dithiocarbamate (f) is more than 5% by weight, any further improvement in the effects cannot be expected and these components rather inversely effect the pitting-inhibitory effect of the present invention. 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 11 and Comparative Examples 1 and 2 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 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 No. 1 grade by a three-stage roll mill. Examples 12 and 13 There were added, to a container, 440 g of a base oil and 58.9 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 440 g of a base oil and 61.1 g of octylamine followed by heating at a temperature between 70 and 80°C and addition thereof to the forementioned container. The mixture was then reacted for 30 minutes with sufficient stirring, the temperature of the reaction system was raised to 160 °C with stirring and the reaction system was allowed to cool to give a base aliphatic amine 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 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 3. 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. [SRV Test] Test Piece: ball: diameter 10 mm (SUJ-2) cylindrical plate: diameter 24 mm × 7.85 mm (SUJ-2) Conditions for Evaluation: Load: 300 N Frequency: 15 Hz Amplitude: 1000 µm Time: 10 min Test Temperature: 150 °C Items evaluated: Maximum coefficient of friction Average diameter of wear scar observed on balls (mm) Maximum depth of wear observed on plates (µm) [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 200 rpm Torque 785 N · m Angle of Joint 7 ° Operation Time 100 hours Type of Joint Used Birfield Joint Cross Groove Joint Item evaluated Occurrence of pitting at each part after operation.

Claims: A grease composition for constant velocity joints consisting essentially of: (a) a base oil; (b) a urea thickener; (c) molybdenum disulfide; and (d) a salt selected from calcium and overbasic calcium salts of oxidized waxes, phenates, and petroleum sulfonic, alkyl aryl sulfonic and salicylic acids. A composition according to claim 1 including (e) an extreme pressure agent selected from metal-free sulfur-phosphorus extreme pressure agents and molybdenum dithiophosphate. A composition according to claim 1 including (f) molybdenum dithiocarbamate. A composition according to claim 1 which, based on the total composition weight, consists essentially of 55.0 to 98.0% of (a); 1 to 25% of (b); 0.5 to 5.0% of (c); and 0.5 to 15% of (d). A composition according to claim 2 which, based on the total composition weight, consists essentially of 52.0% to 97.9% of (a); 1 to 25% of (b); 0.5 to 5.0% of (c); 0.5 to 15% of (d); and 0.1 to 3% of (e). A composition according to claim 2 or 5 wherein the extreme pressure agent (e) is a metal-free sulfur-phosphorus extreme pressure agent which has a sulfur content ranging from 15 to 35% by weight and a phosphorus content ranging from 0.5 to 3% by weight. A composition according to claim 3 which, based on the total composition weight, consists essentially of 50.0 to 97.9% of (a); 1 to 25% of (b); 0.5 to 5.0% of (c); 0.5 to 15% of (d); and 0.1 to 5% of (f).

Inventors: OKANIWA TAKASHI, OSAWA HISAYUKI, OKANIWA, TAKASHI, OSAWA, HISAYUKI, Okaniwa, Takashi, c/o Tsujido Plant, Osawa, Hisayuki, c/o Tsujido Plant
Applicants: KYODO YUSHI, KYODO YUSHI CO., LTD.
Application year: 1996
Main classification: C10M169
IPC codes: C10M169