Grease composition

Abstract

A grease composition includes at least one mineral-based or synthetic-based oil, individually or in a mixture; at least one fatty acid simple or complex metal soap as thickener; at least one molybdenum dithiocarbamate; and graphite. The grease can be used for constant velocity joints of a motor vehicle transmission.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application is a National Phase Entry of International Application No. PCT / IB2011 / 055621, filed on Dec. 12, 2011, which claims priority to French Patent Application Serial No. 1060441, filed on Dec. 13, 2010, both of which are incorporated by reference herein.BACKGROUND AND SUMMARY[0002]The present invention relates to grease compositions which can be used in constant velocity joints of drive trains of motor vehicles, or of other gear mechanisms for which greases with a low friction coefficient are sought.[0003]A transmission or mechanical coupling joint is a mechanical system consisting of several parts which are moveable relatively to each other, or deformable, which allows mutual driving of two rotating parts, the axes of rotation of which occupy variable relative positions during operation. In other words, it is a link which allows transmission of the rotation of one axis to another axis which is moveable relatively to the first....

AI Technical Summary

Benefits of technology

[0004]The movements inside constant velocity joints are complex, with a combination of rolling, sliding and rotations. Wear on the contact surfaces of the components occurs therein, but also significant frictional forces between the surfaces. The wear may result in a failure of the joints and the frictional forces may cause noise, vibrations and jolts in the drive train. Thus, the greases used in constant velocity joints should have an anti-wear effect and preferentially a low friction coefficient in order to reduce or prevent noises, vibrations and jolts.

[0006]Application FR 2 723 747 discloses high-temperature greases for constant velocity joints comprising mineral and / or synthetic base oils, polyurea thickeners and MoS2 as a solid lubricant, as well as graphite and at least one organic molybdenum compound, preferentially molybdenum dithiocarbamate. The solid lubricants graphite, MoDTC, PTFE, make it possible to reduce the content of expensive solid lubricant MoS2, but without however replacing it completely.

[0007]Solid lubricants of the MoS2 type have a high cost and introduce a high metal content in the formulations, which is not desirable for environmental reasons. Nothing in this application suggests the lowering of the metal (Mo) contents of the grease by totally substituting other solid lubricants for the MoS2. The molybdenum content of the disclosed compositions is of at least of the order of 5,000 ppm. No mention is made of specific molybdenum and graphite contents allowing optimization of the performances of the greases as regards wear and / or friction. Nothing in this application either suggests substitution of lithium soaps for polyureas as thickeners.

[0008]The greases thickened with polyureas are technically more complicated to manufacture, in particular because the components used in their manufacture, such as isocyanates and amines, are highly toxic and not very stable upon storage. The precautions which have to be taken, make the manufacturing methods more complex and more expensive. The availability of the raw materials is also lower as compared with those of greases thickened with metal soaps, in particular with lithium and lithium complex. The polyureas are superior in thermal resistance but their thixotropic nature also leads to problems of destructuration under mechanical stress and hardening upon storage. Therefore, technically easier and more economical greases for constant velocity joints will be preferred for formulating greases from non-toxic and non-hazardous materials using fatty acid metal soaps as thickeners.

[0013]Therefore, there exists a need for greases for constant velocity joints, containing non-toxic, available and inexpensive raw materials, easy to make, having reduced molybdenum (Mo) and phosphorus (P) content and having adequate anti-wear properties and friction properties (low friction coefficient) for this application. The applicant has shown that greases for constant velocity joints thickened with fatty acid metal soaps and comprising graphite combined with molybdenum dithiocarbamate give the possibility of meeting the need discussed above.

[0014]These greases according to the invention have improved anti-wear properties as compared with known greases for constant velocity joints based on lithium soap. The friction properties are equivalent to those of known greases. These greases may moreover attain these performances with lower molybdenum and optionally lower phosphorus content than the greases of the prior art.

Problems solved by technology

The movements inside constant velocity joints are complex, with a combination of rolling, sliding and rotations.

Wear on the contact surfaces of the components occurs therein, but also significant frictional forces between the surfaces.

The wear may result in a failure of the joints and the frictional forces may cause noise, vibrations and jolts in the drive train.

Solid lubricants of the MoS2 type have a high cost and introduce a high metal content in the formulations, which is not desirable for environmental reasons.

Nothing in this application either suggests substitution of lithium soaps for polyureas as thickeners.

The greases thickened with polyureas are technically more complicated to manufacture, in particular because the components used in their manufacture, such as isocyanates and amines, are highly toxic and not very stable upon storage.

The precautions which have to be taken, make the manufacturing methods more complex and more expensive.

The polyureas are superior in thermal resistance but their thixotropic nature also leads to problems of destructuration under mechanical stress and hardening upon storage.

Below this MoS2 content, the wear and friction performances are insufficient.

The disclosed composition examples either have a high molybdenum content (about 8,500 ppm) and / or a high phosphorus content (about 2,000 ppm of phosphorus), which leads to environmental problems.

This study is limited to a single natural lubricant oil and does not deal with the field of greases.

It is therefore absolutely impossible to conclude from this publication whether the booster effect of graphite on the friction and wear performances of MoDTC will be reproduced in a grease comprising a particular thickener.

However, the use of graphite (or MoS2) in combination with MoDTC is not recommended, since this combination, according to this application, has a detrimental effect on the anti-wear properties and on the friction coefficient of the greases.

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