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1. WO1999002629 - GRAISSE POUR JOINTS HOMOCINETIQUES

Note: Texte fondé sur des processus automatiques de reconnaissance optique de caractères. Seule la version PDF a une valeur juridique

[ EN ]

Title: GREASE COMPOSITION FOR CONSTANT VELOCITY JOINTS

Description of Invention
This invention relates to a lubricating grease which is intended primarily for use in constant velocity joints which are used in the drivelines of motor vehicles.
The motions of components within constant velocity joints are complex with a combination of rolling, sliding and spinning. When the joints are under torque, the components are loaded together which can not only cause wear on the contact surfaces of the components, but also significant frictional forces between the surfaces. The wear can result in failure of the joints and the frictional forces can give rise to noise, vibration and harshness (NVH) in the driveline. Ideally the greases used in constant velocity joints need not only to reduce wear, but also have to have a low co-efficient of friction to reduce the frictional forces and to reduce or prevent NVH.
Various additives are known which help reduce wear and/or friction. Greases which have both low friction and low wear characteristics are available, but the grease is generally formulated with a high quantity of high cost additives: this results in a high-cost grease.
The object of the present invention is to provide a grease composition primarily for constant velocity joints which has both low wear and low friction characteristics and which is economical to produce.
In broad terms a lubricating grease according to the invention is obtained by mixing together the following constituents:- a) a base oil comprising one or more mineral or synthetic oils or mixtures thereof, b) a thickener,

c) 0.5 to 5% by weight of the total weight of the constituents of a molybdenum dithiophosphate of the following general formula,


wherein X or Y represents S or O and each of R1 to R4 inclusive may be the same or different and each represents a primary (straight chain) or secondaiy (branched chain) alkyl group having between 3 and 20 carbon atoms,

d) between 0.5 to 5% by weight of the total weight of the constituents of powdered copper (I) oxide (cuprous oxide) having a particle size of less than 200μm and, if desired,

e) some or all of the normal grease additives such as anti-wear additives, anti-oxidants, friction-modifying additives, corrosion inhibitors, tackiness agents and viscosity index improvers.

The powdered oxide preferably has a particle size of less than 50μm. ticle sizes of less than 200μm, 100% of the particles must pass through esh sieve as defined in ASTM El 1 or ISO 595. For particle sizes of less than 50μm (45μm), 100% of the particles must pass through a 325 mesh sieve as defined in ASTM El 1 or ISO 595.
It has previously been proposed to use copper-containing additives in grease. In a paper by Y Ischuk et al published in Lubrication Science Volume 6, Part 2, January, 1994 there are described experiments involving adding copper-containing additives to grease. The conclusions arrived at were that such additions decrease a grease's tribological properties under AW (anti-wear) conditions but somewhat improve these properties under EP (extreme pressure) conditions, such conditions being described in relation to Figure 1 of the paper. The paper states that greases with copper-containing additions are undesirable in friction couples operating under AW conditions. The conditions in a CV joint in use vaiy continuously between AW and EP conditions because of the wide spectrum of loads, angles and speeds encountered in service. Thus a skilled reader of the Ischuk paper would not consider using a copper-containing additive in a grease for use in a constant velocity joint.
The prior art does not show the synergistic addition of copper (I) oxide and a molybdenum dithiophosphate to a grease suitable for use in constant velocity joints to give excellent anti-wear and anti-friction properties as will be described below.
The invention will now be described in detail.
The base oil preferably consists of a mixture of mineral oils and comprises 25% of naphthenic oils and 75% of paraffinic oils by weight based on the total base oil. The invention is not limited to the use of mineral oils, synthetic hydrocarbon oils or ester oils may be used either singly or mixed together or mixed with mineral oils.
The thickener may be a simple lithium soap formed from stearic acid, 12-hydroxy stearic acid or from other similar fatty acids or mixtures thereof or methyl esters of such acids. Alternatively a lithium complex soap may be used formed e.g. from a mixture of long chain fatty acids together with a complexing agent, e.g. a borate or one or more dicarboxylic acids. The use of complex lithium soaps allows the grease to operate up to a temperature of about 180°C whereas with simple lithium soaps the grease will only operate up to a temperature of about 120°C. Other thickeners may be used such as simple or complex calcium, sodium, barium or aluminium soaps; clays; polymers such as polyethylenes and polytetrafluorethylenes; and urea derivatives.
The grease constituents may include between 0.5 to 5% by weight of a zinc dithiophosphate as an anti-wear additive which is preferably selected from the group of those represented by the following general formula:

(R50)(R60)SP-S-Zn-S-PS(OR7)(OR8)

wherein each of R5 to R8 inclusive may be the same or different and each represents an alkyl group having 1 to 24, preferably 3 to 20 carbon atoms or an aiyl group having 6 to 30, preferably 8 to 18 carbon atoms. The alkyl group may be a primary or secondary. Preferably the substituents R to R inclusive each represents a primary or secondary alkyl group having 3 to 8 carbon atoms. Alternatively potassium borate may be used as an anti-wear additive.
The grease constituents may also include between 0.5 and 5% by weight of a molybdenum dithiocarbamate complex which acts as a minor friction modifier.
The molybdenum dithiocarbamate may be in accordance with the formula: -


wherein X or Y represents S or O and each of R9 to R12 inclusive may be the same or different and each represents a primary (straight chain) or secondary (branched chain) alkyl group having between 3 and 20 carbon atoms.
The grease constituents may also include between 0.10 to 5% by weight of a metal-free, sulphur-containing extreme pressure additive which may be a sulphur/phosphorus additive or one containing sulphur but no phosphorous. The additive preferably contains 20-30% of an amine phosphate and 55-65% of sulphurised oil, the percentages being by weight.
The grease constituents may include 0.1 to 2% by weight of an anti-oxidant in the form of an amine, preferably an aromatic amine, which may be phenyl α-naphthylamine or di-phenylamine or derivatives thereof.
The grease may also contain a corrosion inhibitor such as a natural or synthetic petroleum sulphonate salt of an alkali, alkaline earth or transition metal, e.g. calcium petroleum sulphonate, a tackiness agent, e.g. polyisobutene, and a viscosity index improver, e.g. polymethylmethacrylate.

First Examples
Two base greases were made up according to the following compositions and are referred to herein as Greases A and B; the figures in the table are percentages by weight of the total grease.


Note:

The components of the base greases were as follows:

1. Lithium 12-hydroxystearate.
2. Base oil 25% by weight naphthenic oils and 75% by weight paraffinic oils based on the total of the base oil.
3. Molybdenum dithiophosphate (Molyvan L, available from R.T.
Vanderbilt Company).
4. Molybdenum dithiocarbamate (Molyvan 822, available from R.T.
Vanderbilt Company).
5. Zinc dithiophosphate (Lubrizol 1360, available from Lubrizol Great Britain Ltd.).
6. E P additive (Mobilad G-305, available from Mobil Chemical Company).
7. Corrosion inhibitor (Alox 165, available from Alox Corporation).

8. Anti-oxidant additive (Additin RC7130 available from Rhein Chemie).

Test samples were made up comprising 98% by weight of Grease A or Grease B and 2% by weight of a copper additive. This was either copper powder, cuprous oxide (copper (I) oxide), cupric oxide (copper (II) oxide) or copper (II) naphthenate. Each of the copper powder and the copper oxides had a particle size of less than 45μm as defined above. The additive was blended with the grease in a mixer until thoroughly blended and the grease was then milled to achieve homogenisation.
Laboratory sliding friction and wear tests are widely used in oil and grease development. These types of tests are used to mimic contact conditions between mating surfaces. One apparatus for canying out such tests is the Optimol Instruments SRV (Schwingungen Reibung Verschliess) tester. The test consists of an upper ball specimen reciprocating under load on a flat disc lower specimen, with the grease lubricating the contact. It is an industry standard test and is especially relevant for the testing of greases for CV Joints.

A series of test methods using the SRV tester have been identified which are appropriate for the testing of greases for use in constant velocity joints. In this instance, the following conditions were used:


The batches of greases with and without the copper additive were tested in duplicate for wear and friction. The results were averaged and are shown in the attached Figures 1 to 3 and 8 to 10 which will now be described.
In the following description A is Grease A with no copper additive, Ap0 is Grease A with 2% of powdered copper, Apι is Grease A with 2% of copper (I) oxide, Ap2 is Grease A with 2% of copper (II) oxide and An2 is Grease A with 2% of copper naphthenate, all the percentages being by weight of the total grease including the copper or the copper-containing additive.
Figure 1 shows the static and dynamic friction co-efficients measured in the SRV tests referred to above for Grease A without any additives and for the Greases Ap0, Apι, Ap2 and A1)2. I will be seen that the greatest reduction in coefficient of friction, both static and dynamic, is shown by Ap2, i.e. Grease A with the addition of copper (II) oxide and is closely followed by Grease Apι containing the copper (I) oxide. Smaller reductions in co-efficient of static friction are shown by Greases Ap0 and A„2. With regard to the co-efficient of dynamic friction, the situation is the same as with static friction with the copper oxide powders; powdered copper gives a greater reduction in co-efficient of dynamic friction than does copper naphthenate.
Figure 2 shows the wear depth measured in the SRV tests for Grease A and Greases Ap0, Api, Ap2 and An2. It will be seen that the wear depth using Grease Apι is considerably less than that obtained with any of the other greases and the reduction in wear depth using Grease Apι as compared with Grease A is 74%; in five out of eight tests there was no measurable wear. The reduction in wear depth using copper or copper naphthenate is about the same at 27% and the reduction in wear depth using Grease Ap2 is rather less at 18% (see Figure

3).
It will be seen from Figures 1, 2 and 3 that the best performing grease overall giving low co-efficient of friction and low wear is the Grease Apl which contains 2% of copper (I) oxide.
The effect of adding copper (I) oxide to Grease A to produce Grease Ap] is also shown in Figures 4 and 5 which show the results of tests in a constant velocity joint. In Figure 4, A shows the two dimensional profile of the virgin surface of the tracks in the outer race of a plunging tripod joint before testing. B shows the wear scar depth in the tracks parallel to the direction of movement of the rollers using Grease A and C shows the wear scar depth obtained using Grease Apι after the test. This is shown graphically in Figure 5 where it is seen that the average wear depth of a test sample lubricated by Grease Apι is approximately a quarter of the wear depth of a test sample lubricated with Grease A. The test conditions were 106 hours at a torque of 1,000 Newton metres at 160 revolutions per minute (rpm) at an installed angle of 6°. This is a high torque/low speed test.
Figure 6 shows the damage grades after running a plunging tripode joint in a test under the conditions referred to in relation to Figures 4 and 5. The average damage grades are assessed according to the following table. A damage grade of 10 is as new and at a damage grade of 1 the component is severely damaged. It will be seen that there is considerable increase in damage grades (i.e. less damage) in the parts of the joint when using Grease Apι, as compared with test using Grease A.


Figure 7 shows the reduction in third order axial force generated in a plunging tripode joint at different degrees of articulation when using Grease Apl as compared with Grease A. The test conditions were 300 Newton metres torque at 200 rpm after running in for 20 minutes at 100 Newton metres torque at 200 rpm.
Figure 8 shows the effect of the addition of different percentages of copper (I) oxide to Grease A with respect to the wear depth and static and dynamic co-efficients of friction. It will be seen that with the addition of 1% by weight of the total grease of copper (I) oxide the wear depth is reduced virtually to zero. It will also be seen that an increase in the content of copper (I) oxide over about 1.5% does not make a great deal of difference to the static and dynamic co-efficients of friction.

The main difference between the composition of Grease A and Grease B is that the content of molybdenum dithiophosphate in Grease B is only 2% whereas it is 3% in Grease A. Figures 9 and 10 show comparative results between Greases A and B, the composition of Grease B being as set out in the above table and the composition of Grease Bpι being 98% Grease B plus 2% by weight of the total grease of powdered copper (I) oxide having a particle size of less than 45 μm as defined above.
It will be seen from Figure 9 that both the static friction of Grease B and Grease Bpι are slightly higher than Grease A and Grease Apι respectively. However if one looks at the wear depth as a result of the SRV test it will be seen that there is a considerable reduction in the wear depth with Grease Apι as compared with Grease A (as described above) and that there was no measurable wear depth in the SRV tests using Grease Bpι. As mentioned above, in five of the eight tests using Grease Apι there was no measurable wear depth in the SRV tests.
It will be seen from the foregoing that Grease Bp] has a comparable performance to Grease Apι and Figure 1 1 shows how a cost saving can be obtained by replacing some of the molybdenum dithiophosphate with copper (I) oxide. Thus the saving on the cost of raw materials by substituting 1% of copper (I) oxide for 1% of molybdenum dithiophosphate is approximately 9% rising to about 17% if 2% of the molybdenum dithiophosphate is replaced by copper (I) oxide.
It is clear from the foregoing that the grease compositions which give the best overall wear and friction properties are the base Greases A and B to which copper (I) oxide has been added.
Further sample greases were made up and tested to establish what combination of the constituents of Greases A and B when combined with copper (I) oxide were primarily responsible for the improved wear and friction properties. These samples are described below.

Second Examples

Eight greases were made by mixing together the following constituents in percentages by weight.


Constituents 1 to 8 of the samples were the same as set out above for the First Examples referred to above.
Sample No. 1 contained no copper, is similar in composition to Grease A described above and is included as a control sample of an acceptable non-copper-containing grease.
The samples of greases according to the invention are Samples 3, 4 and 6, the remaining samples are comparative examples all of which, except Sample 1, include copper or a copper-containing compound.

Each of the samples of grease were tested for wear and friction in an SRV tester under conditions identical to those applied to Greases A and B as described above. The results of the two tests were averaged and the results are shown in Figures 12 and 13, Figure 12 shows the measured static friction for each sample and Figure 13 shows the measured wear depth for each sample.
Referring to Figure 12, with reference to the control grease, Sample 1, the friction co-efficient is lower for each of the copper containing samples except for Samples 7 and 8 which do not contain any MoDTP. The lowest friction co-efficient is that of Sample 3 which contains copper (I) oxide and MoDTP closely followed by Samples 4 and 6, each of which contains MoDTP and copper oxide, Sample 6 containing copper (I) oxide and Sample 4 containing copper (II) oxide.
Referring to Figure 13, the lowest wear depth was achieved by Sample 6, the wear being unmeasurable, and the next lowest wear being achieved by Sample 3. Each of Samples 3 and 6 contains MoDTP and copper (I) oxide.
Consideration of the test results in conjunction with the sample composition table shows the following:

1. The inclusion of MoDTP and copper (I) oxide in the grease (Samples 3 and 6) gives the best test results for both friction and wear.

2. The inclusion of MoDTP and copper (II) oxide or copper naphthenate gives test results which are an improvement on those given by the grease of Sample 1, which contains no copper, but which are considerably less good than those given by Samples 3 and 6.

3. The omission of MoDTP from the grease - even when copper (I) oxide is included (Samples 7 and 8) - gives substantially worse friction co-efficients than any of the other samples although Samples 7 and 8 included the EP additive. The wear depth of Sample 8 is better than that of Sample 1 but unsurprisingly the wear depth of Sample 7 is worse since the ZDTP is omitted.

The conclusion that the applicants draw from the test results is that there is an unexpected synergy between MoDTP and copper (I) oxide when combined in a grease comprising a base oil and a thickener, whether or not the other constituents described herein are included in the grease.
It will be seen that the invention provides a grease, particularly but not exclusively for use in constant velocity joints, which has low wear and low friction characteristics and is economical in the cost of raw materials.