DE69708974T2 - Grease composition for constant velocity joints - Google Patents

Description

BACKGROUND OF THE INVENTION

The present invention relates to a grease composition for use in constant velocity joints, particularly fixed ball and immersed ball constant velocity joints. High contact pressure develops between the parts of the constant velocity joint that require lubrication, and the joint parts undergo complicated rolling and sliding movements. This often results in abnormal wear and fatigue of the metal and leads to a chipping phenomenon, i.e. pitting of the joint parts. More specifically, the present invention relates to a lubricant composition for constant velocity joints that effectively lubricates such constant velocity joints to effectively reduce joint wear and the occurrence of any pitting of the parts.

Examples of conventional constant velocity joint lubricants include a lithium soap thickened extreme pressure lubricant containing molybdenum disulfide and a lithium soap thickened extreme pressure lubricant containing molybdenum disulfide and extreme pressure agents such as sulfur-phosphorus or a lead naphthenate. However, these lubricants have not always been satisfactory under the severe operating conditions encountered in current high performance automobiles.

Recently, the number of four-wheel drive (FF) type automobiles has increased dramatically. For application in these vehicles, constant velocity joints must be made as light and small as possible. Both double offset type and cross groove type constant velocity joints used as plunge joints and Birfield joints used as fixed joints have a structure in which torque is transmitted through 6 balls. These joints cause complicated alternating motions such as complicated rolling and sliding motions during rotation under a high contact pressure, the balls and the metal surfaces in contact with the balls are repeatedly subjected to stress, and accordingly, pitting phenomena due to metal fatigue tend to occur in such parts. Recent improvements in engine power are associated with an increase in contact pressure compared with conventional engines. Vehicles are being lightened to improve fuel economy and the size of the joints has been reduced accordingly. This leads to a relative increase in contact pressure and thus the conventional lubricants ineffective in that they cannot sufficiently reduce the pitting phenomenon. In addition, the lubricants must also be improved in terms of their heat resistance.

EXPERT BACKGROUND

EP-A-0 773 280 has priority dates earlier than those of the present invention, but was published after the present specification was filed. It describes and claims a lubricating grease comprising a base oil, a urea thickener, molybdenum disulfide, a metal salt or an overbased metal salt of a selected group of metals (Ca, Mg, Ba, Na, K, Pb, Zn, Al) and either an extreme pressure agent selected from a metal-free sulfur-phosphorus extreme pressure agent and molybdenum dithiophosphate or molybdenum dithiocarbamate. The patent also discloses compositions consisting of the above ingredients, but the metal salt is selected from magnesium, barium, sodium, potassium, lead, zinc and aluminum salts.

US-A-5 207 936 discloses a grease with a urea thickener which includes molybdenum disulfide and molybdenum dithiocarbamate. In addition, it includes zinc dithiophosphate and a vegetable oil.

US-A-5 516 439 discloses a grease containing a lithium thickener and a metal salt of various types including a calcium salt and molybdenum dithiocarbamate or molybdenum dithiophosphate. Additionally, the lubricant includes zinc dithiophosphate.

US-A-5 160 645 discloses a urea-thickened grease including a metal-free sulfur-phosphorus extreme pressure agent and molybdenum dithiocarbamate. Additionally, the lubricant contains zinc dithiophosphate and lead dithiocarbamate.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a novel grease composition for constant velocity joints which has excellent pitting prevention effect and heat resistance.

The inventors of the present invention have conducted various studies to develop a grease composition which can optimize the frictional wear of the constant velocity joints and eliminate the problem of pitting of joints due to abnormal wear and fatigue of the metal and has improved heat resistance.

The inventors have carried out a quality evaluation of lubricating greases used under lubrication conditions accompanied by complicated alternating movements such as complicated rolling and sliding movements under a high contact pressure as discussed above, whereby a vibration friction and Wear and tear (SRV) tester, known as an oscillating friction and wear tester, was used to determine lubricating characteristics (such as friction coefficient and wear) of various kinds of extreme pressure agents, solid greases, or combinations of additives. As a result, the inventors have found that a grease comprising a specific combination of a base oil, a urea thickener, molybdenum disulfide, a calcium salt or an overbased calcium salt of a specific compound, a metal-free sulfur-phosphorus extreme pressure agent, and molybdenum dithiocarbamate has desired lubricating characteristics such as a good friction coefficient and low wear, and have confirmed through a durability test conducted using a constant velocity joint in practice that the grease can prevent the occurrence of any pitting phenomena, unlike conventional constant velocity joint greases, thus completing the present invention.

The above object of the present invention can be effectively achieved by providing a constant velocity joint grease composition, consisting essentially of:

(a) a base oil;

(b) a urea thickener;

(c) molybdenum disulfide;

(d) a calcium salt or an overbased calcium salt selected from calcium salts of oxidized waxes, calcium salts of petroleum sulfonates, calcium salts of alkylaryl sulfonates, calcium salts of salicylate, calcium salts of phenates, overbased calcium salts of oxidized waxes, overbased calcium salts of petroleum sulfonates, overbased calcium salts of alkylaryl sulfonates, overbased calcium salts of salicylate and overbased calcium salts of phenates;

(e) a metal-free sulphur-phosphorus extreme pressure agent; and

(f) Molybdenum dithiocarbamate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in more detail below.

The base oil as component (a) is not limited to specific oils. It may be, for example, lubricating oils currently in use such as mineral oils, ester-type synthetic oils, ether-type synthetic oils and hydrocarbon-type synthetic oils, and mixtures thereof.

The urea thickener as component (b) is not limited to specific ones. It may be, for example, diurea or polyurea compounds. Examples of the diurea compounds include those obtained by reacting 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 by reacting a diamine with a diisocyanate compound. Examples of the diisocyanates include those used for forming 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 by reacting arylamine such as aniline or p-toluidine, cyclohexylamine or a mixture thereof with a diisocyanate. The aryl group in the diurea compounds is preferably that having 6 or 7 carbon atoms and the ratio of the aryl group in the diurea compound is in the range of 100 to 0 mol% based on the total number of moles of aryl groups and cyclohexyl groups in the diurea compounds.

The molybdenum disulfide as component (c) has been widely used as an extreme pressure agent. In view of its lubricating mechanism, molybdenum disulfide is easily sheared under sliding motions by forming a thin layer since it has a layered lattice structure, and it exhibits effects of reducing frictional force and preventing joint seizure. Molybdenum disulfide products having various particle sizes are known, but in the present invention, it is preferable to use those having a particle size in the range of 0.25 to 10 µm in terms of average particle size as determined by the method called Fisher method (by using a Fisher sub-sieve sizer), particularly those having an average particle size of 0.55 to 0.85 µm.

The calcium salts or overbased calcium salts as component (d) are selected from those known as metal cleaning dispersants or rust inhibitors used in lubricating greases such as machine oils, such as the calcium salts of oxidized waxes, calcium salts of petroleum sulfonates obtained by sulfonylation of aromatic hydrocarbons in lubricating oil fractions, calcium salts of synthetic sulfonates such as dinonylnaphthalenesulfonic acid and alkylbenzenesulfonic acid, calcium salts of salicylate, calcium salts of phenates, overbased calcium salts of oxidized waxes, overbased calcium salts of petroleum sulfonates, overbased calcium salts of alkylarylsulfonates, overbased calcium salts of salicylate and overbased calcium salts of phenates.

Preferred metal-free sulfur-phosphorus extreme pressure agents as component (e) have a sulfur content of 15 to 35 wt.% and a phosphorus content of 0.5 to 3 wt.% and, due to the well-balanced ratio of the sulfur and Phosphorus content has excellent effects on wear inhibition and the prevention of seizure of the joints.

The molybdenum dithiocarbamate as component (f) is preferably represented by the following formula:

(R¹R²N-CS-S)₂-Mo₂OmSn

where R¹ and R² each independently represent an alkyl radical having 1 to 24, preferably 3 to 18 carbon atoms, m ranges from 0 to 3, n ranges from 4 to 1 and m + n = 4.

The constant velocity joint grease composition of the present invention may further contain, in addition to the above essential components, antioxidants, corrosion inhibitors and rust inhibitors.

The constant velocity joint grease composition of the present invention preferably comprises, based on the total weight of the grease composition, 52.0 to 97.8% by weight of the base oil (a); 1 to 25% by weight of the urea thickener (b); 0.5 to 5% by weight of the molybdenum disulfide (c); 0.5 to 15% by weight of the calcium salt or overbased calcium salt (d); 0.1 to 3% by weight of the metal-free sulfur-phosphorus extreme pressure agent (e) and 0.1 to 5% by weight of the molybdenum dithiocarbamate (f).

The present invention will be described in more detail below with reference to the following non-limiting working examples and comparative examples.

Examples 1 to 9 and comparative examples 1 to 3:

In a container, 4100 g of a base oil and 1012 g of diphenylmethane-4,4'-diisocyanate were charged, and the mixture was heated to a temperature between 70 and 80°C. In another container, 4100 g of a base oil, 563 g of cyclohexylamine and 225 g of aniline were charged, followed by heating to a temperature between 70 and 80°C and adding the contents to the above container. The mixture was then reacted with sufficient stirring for 30 minutes, the temperature of the reaction system was raised to 160°C with stirring, and the reaction system was allowed to cool to obtain the base urea grease. To this base grease, the following additives listed in Table 1 were added in amounts also listed in Table 1, as well as an optional and additional amount of the base oil and the consistency of the resulting mixture was adjusted to grade No. 1 by a trio-roll mill.

Example 10

In one container, 440 g of a base oil and 58.9 g of diphenylmethane-4,4'-diisocyanate were added and the mixture was heated to a temperature between 70 and 80ºC. In another container, 440 g of a base oil and 61.1 g of octylamine were added, followed by heating to a temperature between 70 and 80ºC and adding the contents to the above container. The mixture was then reacted with sufficient stirring for 30 minutes, the temperature of the reaction system was raised to 160°C with stirring, and the reaction system was allowed to cool to obtain the alkylamine urea-based base grease. To this base grease were added the following additives shown in Table 1 in amounts also shown in Table 1 and an optional and additional amount of the base oil, and the consistency of the resulting mixture was adjusted to No. 1 grade by a triple roll mill.

In all of the above examples and comparative examples, a mineral oil having the following properties was used as the base oil:

Viscosity: at 40ºC 157 mm²/s

at 100ºC 14 mm²/s

Viscosity index: 88

Further, a commercially available lithium grease containing molybdenum disulfide, a sulfur-phosphorus extreme pressure agent, and lead naphthenate was used as the lubricant of Comparative Example 4.

The physical properties of these greases were evaluated using the procedures described in detail below. The results 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 500 N

Frequency: 15 Hz

Amplitude: 3000 um

Time: 10 minutes

Test temperature: 25ºC

evaluated values: maximum friction coefficient average diameter of wear scars observed on the balls (mm) maximum wear depth on the plates (um)

[Durability test on a test bench using real joints]

The greases were tested for pitting corrosion by conducting a durability test on a test bench using actual joints under the following conditions.

Test conditions:

Number of revolutions 1000 rpm

Torque 392 N·m

Angle of the joint 8º

Operating time 100 hours

Type of joint used: Birfield joint Cross groove joint

Rated size: Occurrence of pitting corrosion on each part after service. Table 1 Table 1 (continued) Table 1 (continued)

*Measurement impossible

1) Diurea grease using a diurea compound in which cyclohexylamine and aniline are used as a monoamine.

2) Diurea grease using a diurea compound in which octylamine is used as a monoamine.

3) Molybdenum disulfide available from Climax Molybdenum Company and the trade name Molysulfide; average particle size: 0.7 µm.

4) Calcium salt of oxidized wax, available from Alox Corporation under the trade name Alox 165

5) Calcium salt of petroleum sulfonate, available from Matsumura Petroleum Laboratory Co. Ltd. under the trade name Sulfol Ca-45

6) Calcium salt of dinonyl naphthalene sulfonate, available from King Industries Co., Ltd. under the trade name NA-SUL 729

7) Calcium salicylate, available from Osca Chemical Co., Ltd. under the trade name OSCA423

8) Calcium phenate, available from Oronite Chemical Ca., Ltd. under the trade name OLOA 218 A

9) overbased calcium sulfonate 1, available from Lubrizol Japan under the trade name Lubrizol 5283

10) overbased calcium sulfonate 2, available from Witco Corporation under the trade name Bryton C-400

11) Sulphur-phosphorus extreme pressure agent available from Lubrizol Japan under the trade name Anglamol 99

12) Molybdenum dithiocarbamate 1, available from R.T. Vanderbilt under the trade name Molyvan A

13) Molybdenum dithiocarbamate 2, available from R.T. Vanderbilt under the trade name Molyvan 822

14) Penetration according to ISO 2137

15) Dropping point according to ISO 2176 (ºC)

16) Maximum friction coefficient

17) Average diameter of wear scars observed on the balls (mm)

18) Maximum wear depth observed on plates (um)

19) Durability test on a test bench using real joints Birfield joint

20) Durability test on a test bench using real joints Cross groove joint

In the durability test, these greases were evaluated according to the following criteria:

O: no pitting was observed;

X: pitting corrosion was observed.

As explained in detail above, the grease composition for constant velocity joints of the present invention consists essentially of (a) a base oil; (b) a urea thickener; (c) molybdenum disulfide; (d) a specific calcium salt or a specific overbased calcium salt; (e) a metal-free sulfur-phosphorus extreme pressure agent; and (f) molybdenum dithiocarbamate, and thus exhibits an excellent wear-resistant effect and an excellent pitting-preventive effect, as is also clear from the comparison of the results of the examples with those of the comparative examples.

Claims (5)

1. Grease compositions for constant velocity joints, consisting essentially of: (a) a base oil; (b) a urea thickener; (c) molybdenum disulfide; (d) a calcium salt or an overbased calcium salt selected from calcium salts of oxidized waxes, calcium salts of petroleum sulfonates, calcium salts of alkylaryl sulfonates, calcium salts of salicylate, calcium salts of phenates, overbased calcium salts of oxidized waxes, overbased calcium salts of petroleum sulfonates, overbased calcium salts of alkylaryl sulfonates, overbased calcium salts of salicylate and overbased calcium salts of phenates; (e) a metal-free sulphur-phosphorus extreme pressure agent; and (f) Molybdenum dithiocarbamate. 2. A constant velocity joint grease composition according to claim 1, wherein the grease composition consists essentially of, based on the total weight of the composition, 52.0 to 97.8% by weight of the base oil (a); 1 to 25% by weight of the urea thickener (b); 0.5 to 5% by weight of the molybdenum disulfide (c); 0.5 to 15% by weight of the calcium salt or overbased calcium salt (d); 0.1 to 3% by weight of the metal-free sulfur-phosphorus extreme pressure agent (e); and the molybdenum dithiocarbamate (f). 3. A constant velocity joint grease composition according to claim 1 or 2, wherein the metal-free sulfur-phosphorus extreme pressure agent (e) has a sulfur content in the range of 15 to 35% by weight and a phosphorus content in the range of 0.5 to 3% by weight. 4. A constant velocity joint grease composition according to claim 1 or 2, wherein the constant velocity joints are immersion ball type constant velocity joints. 5. A constant velocity joint grease composition according to claim 1 or 2, wherein the constant velocity joints are fixed ball constant velocity joints.