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Title:
Grease composition
United States Patent 3928214
Abstract:
A grease composition having a consistency of 200 to 300 which is composed of a mineral oil and, dispersed therein, a high melting wax and a non-volatile solid lubricant powder. The grease composition is long in life because the mineral oil used as the base oil is not only maintained stably in the composition but also is thermally stable. When incorporated with a non-volatile solid lubricant powder, the composition shows excellent lubricating characteristics as a lubricant for contacts used in electric equipment.


Inventors:
Naka, Reishi (Hitachi, JA)
Komatsuzaki, Shigeki (Hitachi, JA)
Tomobe, Tomotsuna (Hitachi, JA)
Moniwa Deceased., Yoshihiro (LATE OF Hitachi, JA)
Application Number:
05/445892
Publication Date:
12/23/1975
Filing Date:
02/26/1974
Assignee:
Hitachi, Ltd.
Primary Class:
Other Classes:
252/510, 252/512, 252/519.2, 252/519.33, 508/113, 508/128, 508/129, 508/150, 508/165, 508/166, 508/168, 508/171, 508/589, 585/2, 585/3, 585/9
International Classes:
C10M169/00; (IPC1-7): C10M1/10; C10M1/32; C10M3/10; C10M3/24
Field of Search:
252/11,25,26,29,30,51.5A,58,59,52A,510,512,518
View Patent Images:
US Patent References:
Primary Examiner:
Gantz, Delbert E.
Assistant Examiner:
Vaughn I.
Attorney, Agent or Firm:
Craig & Antonelli
Parent Case Data:


CROSS-REFERENCES TO RELATED APPLICATION

This is a continuation-in-part application of Ser. No. 247,384 filed on Apr. 25, 1972 and now abandoned.
Claims:
1. A grease composition consisting essentially of a mineral oil having a dynamic viscosity of 50 to 600 centistokes at 37.8°C. (100°F.) and a pour point higher than -30°C. and a wax poor in solubility in the mineral oil and dispersed in the mineral oil having a molecular weight of 300 to 1,000 and a melting point higher than 100°C. in an amount larger than the solubility limit thereof in the mineral oil and sufficient to provide the consistency of 200 to 330 at room temperature with the grease composition.

2. A grease composition according to claim 1, wherein the mineral oil is a member selected from the group consisting of a turbine oil having a dynamic viscosity of 60 to 130 centistokes at 37.8°C. and a pour point higher than -30°C., a cylinder oil having a dynamic viscosity of 400 to 500 centistokes at 37.8°C. and a pour point of about 10°C., a gear oil having a dynamic viscosity of 200 to 300 centistokes at 37.8°C. and a pour point of about 5°C. and mixtures thereof.

3. A grease composition according to claim 1, wherein the wax is a member selected from the group consisting of microcrystalline wax having a molecular weight of 500 to 700, chlorowax having a molecular weight of 400 to 500, amidowax having a molecular weight of 500 to 700 and mixtures thereof.

4. A grease composition according to claim 1, wherein an amount of the wax is 5 to 20 parts by weight per 100 parts by weight of the mineral oil.

5. A grease composition consisting essentially of a turbine oil having a dynamic viscosity of 60 to 130 centistokes at 37.8°C. (100°F.) and a pour point higher than -30°C. and 13 to 17% by weight of amidowax having a molecular weight of 500 to 700, the particles of the wax being dispersed in the mineral oil whereby the grease composition has consistency of 270 to 285 at room temperature.

6. A grease composition comprising a mineral oil having a dynamic viscosity of 50 to 600 centistokes at 37.8°C. (100°F.) and a pour point higher than -30°C., a wax poor in solubility in the mineral oil and dispersed in the mineral oil having a molecular weight of 300 to 1,000 and a melting point higher than 100°C. in an amount larger than the solubility limit thereof in the mineral oil and sufficient to provide the consistency of 200 to 330 at room temperature with the grease composition, and a non-volatile solid lubricant powder having a particle size smaller than 5 microns and a specific resistance less than 0.1 ohm-cm in an amount sufficient to provide conductivity with the grease composition upon application of a contact pressure larger than 0.1 kg/mm2.

7. A grease composition according to claim 6, wherein the mineral oil is a member selected from the group consisting of a turbine oil having a dynamic viscosity of 60 to 130 centistokes at 37.8°C. and a pour point higher than -30°C., a cylinder oil having a dynamic viscosity of 400 to 500 centistokes at 37.8°C. and a pour point of about 10°C., a gear oil having a dynamic viscosity of 200 to 300 centistokes at 37.8°C. and a pour point of about 5°C., and mixtures thereof.

8. A grease composition according to claim 6, wherein the wax is a member selected from the group consisting of microcrystalline wax having a molecular weight of 500 to 700, chlorowax having a molecular weight of 400 to 500, amidowax having a molecular weight of 500 to 700, and mixtures thereof.

9. A grease composition according to claim 6, wherein the solid lubricant is a member selected from the group consisting of graphite, intermetallic compounds of elements belonging to Groups 4, 5 and 6, and mixtures thereof.

10. A method for lubricating slidable contacting surfaces of an electric apparatus, the contacting surfaces being operated under a contact pressure higher than 0.1 kg/mm2, at least one of said contacting surfaces coming into contact with an electric arc during operation of said electric apparatus, said method comprising coating at least one contacting surface coming into contact with an electric arc with a grease composition comprising (a) a mineral oil having a dynamic viscosity of 50 to 600 centistokes at 37.8°C (100°F) and a pour point higher than -30°C, (b) a wax having limited solubility in said mineral oil and a melting point higher than 100°C, said wax being dispersed in said mineral oil in an amount larger than the solubility limit thereof in the mineral oil and sufficient so that said grease composition has a consistency of 200 to 330 at room temperature, and (c) a non-volatile solid lubricant powder having a particle size smaller than 5 microns and a specific resistance less than 0.1 ohm-cm in an amount sufficient so that said grease composition is electrically conductive.

11. A grease composition according to claim 10 consisting essentially of a mineral oil having a dynamic viscosity of 60 to 130 centistokes at 37.8°C. (100°F.) and a pour point higher than -30°C., 8 to 12% by weight of amidowax having a molecular weight of 500 to 700, the particles of the wax being dispersed in the mineral oil whereby the grease composition has consistency of 270 to 285 at room temperature, and 4 to 6% by weight of graphite having a particle size less than 5 microns.

12. A grease composition according to claim 10, further including an oiliness improver powder selected from the group consisting of copper, zinc, iron, lead, chromium, titanium, magnesium and mixtures thereof in an amount of 0.5 to 6% by weight based on the grease composition, the powder having a particle size less than 5 microns.

13. A grease composition according to claim 10, further including an oiliness improver powder selected from the group consisting of lead oxide, copper oxide, magnesium oxide and mixtures thereof in an amount of 0.5 to 6% by weight based on the grease composition, the powder having a particle size less than 5 microns.

14. A grease composition according to claim 10, further including an oiliness improver selected from the group consisting of halogenated polymers and hydrogenated diphenyl compounds.

15. A grease composition according to claim 10 consisting essentially of 100 parts by weight of a mineral oil having a dynamic viscosity of 50 to 600 centistokes at 37.8°C. (100°F.) and a pour point higher than -30°C.; 5 to 20 parts by weight of a wax poor in solubility in the mineral oil having a melting point higher than 100°C. and a molecular weight of 300 to 1,000, particles of the wax which are not dissolved in the mineral oil being dispersed in the mineral oil whereby the grease composition has the consistency of 200 to 330; 0.5 to 30 parts by weight of a non-volatile solid lubricant powder having a particle size less than 5 microns and a specific resistance smaller than 0.1 ohm-cm; and an oiliness improver powder selected from the group consisting of copper, zinc, iron, lead, chromium, titanium, magnesium, lead oxide, copper oxide, magnesium oxide, polyvinyl chloride, polyvinylidene chloride, polyvinylether chloride, trifluoromonochloroethylene, in an amount of 0.5 to 6% by weight based on the total amount of the mineral oil, wax and solid lubricant powder, the oiliness improver powder having a particle size less than 5 microns.

16. A grease composition according to claim 15, wherein the grease composition is 100 parts by weight of grade No. 200 turbine oil having a dynamic viscosity of 103 centistokes at 37.8°C. and a pour point of -27.5°C., 5 parts by weight of amidowax having a molecular weight of about 600, 10 parts by weight of graphite powder and 5% by weight of copper powder.

17. A grease composition according to claim 15, wherein the grease composition is 100 parts by weight of grade No. 200 turbine oil having a dynamic viscosity of 103 centistokes at 37.8°C. and a pour point of -27.5°C.; 5 parts by weight of amidowax having a molecular weight of about 600, 10 parts by weight of NbS2 powder and 5% by weight of zinc powder.

18. A grease composition according to claim 15, wherein the grease composition is 100 parts by weight of grade No. 200 turbine oil having a dynamic viscosity of 103 centistokes at 37.8°C., 5 parts by weight of amidowax having a molecular weight of about 600, 10 parts by weight of graphite powder and 5% by weight of polyvinyl chloride.

19. A grease composition according to claim 15, wherein the grease composition is 100 parts by weight of grade No. 200 turbine oil having a dynamic viscosity of 103 centistokes at 37.8°C. and a pour point of -27.5°C., 5 parts by weight of amidowax having a molecular weight of about 600, 10 parts by weight of NbS2 and 5% by weight of trifluoroethylene chloride.

20. A grease composition according to claim 15, wherein the grease composition is 100 parts by weight of grade No. 200 turbine oil having a dynamic viscosity of 103 centistokes at 37.8°C. and a pour point of -27.5°C., 5 parts by weight of a 1 : 1 mixture of microcrystalline and wax amidowax, each having a molecular weight of about 600, 10 parts by weight of graphite powder and 5% by weight of copper powder.

21. A method of lubricating slidable contacting surfaces of an electric apparatus, the contacting surfaces being operated under a contact pressure higher than 0.1 kg/mm2, at least one of said contacting surfaces being so formed that said one contacting surface contacts no electric arc during operation of said electric apparatus, said method comprising coating said one contacting surface with the grease composition of claim 1.

22. A method of lubricating slidable contacting surfaces of an electric apparatus, the contacting surfaces being operated under a contact pressure higher than 0.1 kg/mm2, one of said contacting surfaces coming into contact with an electric arc during operation of said electric apparatus, another of said contacting surfaces contacting no electric arc during operation of said electric apparatus, said method comprising coating said one contacting surface with the grease composition of claim 1 and coating said another contacting surface with a second grease composition including the ingredients of the grease composition of claim 1 and further including a non-volatile solid lubricant powder having a particle size smaller than 5 microns and a specific resistance less than 0.1 ohm-cm in an amount sufficient to provide conductivity with said second grease composition upon application of a contact pressure larger than 0.1 kg/mm2.

23. The method of claim 10, wherein the mineral oil is a member selected from the group consisting of a turbine oil having a dynamic viscosity of 60 to 130 centistokes at 37.8°C. and a pour point higher than -30°C., a cylinder oil having a dynamic viscosity of 400 to 500 centistokes at 37.8°C. and a pour point of about 10°C., a gear oil having a dynamic viscosity of 200 to 300 centistokes at 37.8°C. and a pour point of about 5°C., and mixtures therof.

24. The method of claim 23, wherein the wax is a member selected from the group consisting of microcrystalline wax having a molecular weight of 500 to 700, chlorowax having a molecular weight of 400 to 500, amidowax having a molecular weight of 500 to 700, and mixtures thereof.

25. The method of claim 24, wherein the solid lubricant is a member selected from the group consisting of graphite, intermetallic compounds of elements belonging to Groups 4, 5 and 6, and mixtures thereof.

26. The method of claim 23, wherein the solid lubricant is a member selected from the group consisting of graphite, intermetallic compounds of elements belonging to Groups 4, 5 and 6, and mixtures thereof.

27. The method of claim 10, wherein the wax is a member selected from the group consisting of microcrystalline wax having a molecular weight of 500 to 700, chlorowax having a molecular weight of 400 to 500, amidowax having a molecular weight of 500 to 700, and mixtures thereof.

28. The method of claim 27, wherein the solid lubricant is a member selected from the group consisting of graphite, intermetallic compounds of elements belonging to Groups 4, 5 and 6, and mixtures thereof.

29. The method of claim 10, wherein the solid lubricant is a member selected from the group consisting of graphite, intermetallic compounds of elements belonging to Groups 4, 5 and 6, and mixtures thereof.

30. The method of claim 10, wherein at least one contacting surface covered with said grease composition is formed from a noble metal.

31. The method of claim 30, wherein said noble metal is silver.

32. The method of claim 10, wherein the solubility of said wax in said mineral oil is about 2 to 3% at normal or ambient temperature.

33. The method of claim 10, wherein said wax is homogeneously dispersed in said mineral oil.

34. The method of claim 10, wherein said grease composition further comprises 0.5 to 6% by weight based on the total amount of said mineral oil, wax and solid lubricant of a powdered oiliness improver having a particle size of less than 5 microns.

35. The method of claim 34, wherein said powdered oiliness improver is selected from the group consisting of copper, iron, zinc, lead, titanium, magnesium, copper oxide, lead oxide, magnesium oxide, polyvinyl chloride, polyvinylidine chloride and polyvinyl ether chloride.

36. The method of claim 34, wherein said powdered oiliness improver is selected from the group consisting of a hydrogenated diphenyl compound and halogenated polymer.

37. The method of claim 34, wherein said grease composition further comprises 0.5 to 6% by weight based on the total amount of said mineral oil, wax and solid lubricant of a liquid oiliness improver comprising polyfluoroethylne chloride having a molecular weight of 349 to 1,398.

38. The method of claim 10, wherein said grease composition has a consistency of about 280 to 330 at 25°C.

39. The method of claim 10, wherein said grease composition has a consistency of about 270 to 285 at 25°C.

40. A grease composition comprising a mineral oil having a dynamic viscosity of 50 to 600 centistokes at 37.8°C. (100°F.) and a pour point higher than -30°C. and a wax poor in solubility in the mineral oil and dispersed in the mineral oil having a molecular weight of 300 to 1,000 and a melting point higher than 100°C. in an amount larger than the solubility limit thereof in the mineral oil and sufficient to provide the consistency of 200 to 330 at room temperature with the grease composition.

41. A conductive grease lubricant mixture comprising:

42. parts be weight of a liquid mineral oil selected from the group consisting of turbine oil, cylinder oil and gear oil;

43. to 20 parts by weight of a high melting point wax selected from the group consisting of microcrystalline wax, chlorowax and amidowax, said wax being dispersed in the mineral oil; and

44. 5 to 30 parts be weight of powder having a particle size smaller than 5 microns and a specific resistance smaller than 0.1Ω-cm, said powder being a member selected from the group consisting of powders of graphite and intermetallic compounds of element of groups 4, 5 and 6 of the Periodic Table, said mixture being formed as a grease.

45. A lubricant mixture according to claim 41, further including powder selected from the group consisting of copper, zinc, iron, lead, chromium, titanium, magnesium, lead oxide, copper oxide, silicon oxide, magnesium oxide and titanium oxide, in an amount of 0.5 to 6% by weight of said mixture.

46. A lubricant mixture according to claim 42, wherein said mixture is 100 parts by weight of grade No. 200 turbine oil, 5 parts by weight of amidowax, 10 parts by weight of graphite powder and 5% by weight of copper powder.

47. A lubricant mixture according to claim 42, wherein said mixture is 100 parts by weight of grade No. 200 turbine oil, 5 parts by weight of amidowax, 10 parts by weight of NbS2 powder and 5% by weight of zinc powder.

48. A lubricant mixture according to claim 42, wherein said mixture is 100 parts by weight of grade No. 200 turbine oil, 5 parts by weight of amidowax, 10 parts by weight of TiSe2 powder and 5% by weight of lead oxide powder.

49. A lubricant mixture according to claim 42, wherein said mixture is 100 parts by weight of grade No. 200 turbine oil, 5 parts by weight of a 1:1 mixture of microcrystalline wax and amidowax, 10 parts by weight of graphite powder and 5% by weight of copper powder.

50. A lubricant mixture according to claim 41, wherein said mixture is 100 parts by weight of grade No. 200 turbine oil, 5 parts by weight of amidowax, 10 parts be weight of graphite powder and 5% by weight of polyvinyl chloride.

51. A lubricant mixture according to claim 41, wherein said mixture is 100 parts by weight of grade No. 200 turbine oil, 5 parts by weight of amidowax, 10 parts by weight of NbS2 powder, and 5% by weight of polytrifluoroethylene chloride.

52. A conductive grease lubricant mixture comprising:

53. parts by weight of a liquid mineral oil selected from the group consisting of turbine oil, cylinder oil, and gear oil;

54. to 20 parts by weight of a high melting point wax selected from the group consisting of microcrystalline wax, chlorowax, and amidowax, said wax being dispersed in the mineral oil; and

55. 5-30 parts of an electroconductive powder having a particle size smaller than 5 microns and a specific resistance smaller than 0.1Ω-cm, said mixture being formed as a grease.

56. A lubricant mixture according to claim 49, wherein said electroconductive powder is at least one member selected from the group consisting of graphite and intermetallic compounds of elements of groups 4, 5 and 6 of the Periodic Table, copper, zinc, iron, chromium, titanium, magnesium, lead oxide, and copper oxide.

57. A lubricant mixture according to claim 50, further including powder selected from the group consisting of copper, zinc, iron, lead, chromium, titanium, magnesium, lead oxide, copper oxide, silicon oxide, magnesium oxide and titanium oxide, in an amount of 0.5 to 6% by weight of said mixture.

58. A lubricant mixture according to claim 49, further including powder selected from the group consisting of copper, zinc, iron, lead, chromium, titanium, magnesium, lead oxide, copper oxide, silicon oxide, magnesium oxide and titanium oxide, in an amount of 0.5 to 6% by weight of said mixture.

Description:
BACKGROUND OF THE INVENTION

This invention relates to a grease composition which can maintain a stable grease state over a long period of time. More particularly, the invention is concerned with a grease composition having such characteristics that it is thermally and chemically stable and is easier in application. The grease composition of the present invention is particularly suitable for lubrication of metal contact surfaces which are subjected to a contact pressure of more than 0.1 kg/mm2.

The contact pressure applied to contacts used in relays and the like small-sized electric equipments is approximately in the range from 10 g/mm2 to 30 g/mm2, so that the lubrication of contact portions of such electric equipments is not serious. However, the contact pressure applied to contact portions of electric equipments larger in size is ordinarily high. For example, in the case of disconnecting switches of air-blast or gas-blast circuit breakers, the contact pressure applied to the contacts used therein is more than 1 kg/mm2. Moreover, portions which operate said contacts are also subjected to a high pressure. In such electric equipments as mentioned above, therefore, it is necessary to use a lubricant suitable for lubrication of said contact portions.

Quite many kinds of lubricants have been known hitherto. These may be roughly classified into liquid lubricants, solid lubricants, emulsion type lubricants and greases. The liquid lubricants, which are composed of mineral or synthetic oils, are liquids and hence easily flow out of the portions to which they have been applied, and thus tend to lose their lubricating ability in a short period of time. The solid lubricants, which are graphite or molybdenum disulfide powders, have such drawback that they are difficulty applicable to and retainable at portions to be lubricated and are simply dissipated. As the emulsion type lubricants, there have been known lubricants composed of mixtures of mineral oils and graphite powders. These lubricants, however, are essentially liquid and hence easily flow out of the portions to which they have been applied. Further, the said lubricants tend to cause phase separation due to great difference in specific gravity between the mineral oils and the graphite, and thus are easily deprived of their lubricating ability during storage and application thereof. As the greases, there have been known those which are composed of a mineral oil and a silica powder as a tackifier; a mineral oil and a metallic soap; and a synthetic oil, e.g. polyalkylene glycol, and a metallic soap. However, the grease compound of a mineral oil and silica tends to cause phase separation due to difference in specific gravity between the two components. Further, the metallic soap is decomposed into a liquid fatty acid and a metal in the presence of O3 and NOx which are produced by arc in electric apparatus, and the fatty acid is separated and flowed out, with the result that the grease is deprived of its lubricating ability in a short period of time. Further, in the case of the grease composed of a synthetic oil and a metallic soap, not only the metallic soap brings about such undesirable decomposition as mentioned above, but also the synthetic oil is easily oxidized and ring-opened in the presence of decomposed gases of sulfur hexafluoride, an insulating gas to cause block polymerization and thus is cured, with the result that the grease is easily deprived of its lubricating ability. Particularly in the case of a grease composed of a synthetic oil and a benton powder (composed mainly of SiO2 and MgO), there is brought about such a problem that SiO2 acts as a catalyst for said reaction.

Since the contact surfaces of metals used as contacts in electric equipments are not smooth when observed microscopically, it is necessary, both for successful lubrication of the contact surfaces and for minimization of contact resistance between the contact surfaces, that lubricants used for lubrication should be those which can be easily retained in micropores formed between the contact surfaces due to fine unevenness of the surfaces. For the above-mentioned purposes, the liquid lubricants and emulsion type lubricants are not preferable since they are easily squeezed off, but the greases may be said to be the most preferable lubricants since the base oils thereof as lubricants can be characteristically retained in the aforesaid micropores.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a grease composition which is chemically stable and can maintain a grease state over a long period of time.

Another object of the invention is to provide a grease composition suitable for lubrication of contact portions operated under a contact pressure of more than 0.1 kg/cm2.

A further object of the invention is to provide a grease composition suitable for lubrication of electric contact positions of electric equipments which are operated under a contact pressure of more than 0.1 kg/mm2.

A still further object of the invention is to provide a grease composition suitable for lubrication of contact portions of electric equipments which are operated under a contact pressure of more than 0.1 kg/mm2 and have surfaces coated with a noble metal.

The present invention provides a grease composition maintained in a grease state which comprises a mineral oil and, dispersed therein to more than the solubility limit, a high melting wax. The present invention also provides a grease composition containing the above two ingredients and a non-volatile solid lubricant powder. The grease compositions of the present invention are thermally and chemically stable, can maintain a grease state over a long period of time, and have an excellent lubricating ability.

The above-mentioned objects, other objects and advantages of the present invention are clarified from the detailed explanation made below with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relation between temperatures and consistency or penetration values of various greases.

FIG. 2 is a graph showing the relation between average surface pressures and static frictional coefficients produced by use of grease compositions according to the present invention.

FIG. 3 is a rough sketch of a means for measuring voltage drops due to contact resistance between electric contact surfaces.

FIG. 4 is a rough sketch of a means for measuring frictional coefficients between contact surfaces.

FIG. 5 is a graph showing voltage drops of electric contacts due to application of various lubricants.

FIG. 6 is a graph showing the relation between contact pressures and temperatures at which the frictional coefficients of contact surfaces reach 0.4. FIG. 7 is a graph showing oil separation characteristics, i.e. life characteristics, of various lubricants.

DETAILED DESCRIPTION OF THE INVENTION

In the grease composition of the present invention, a mineral oil is used as the base oil in view of its chemical stability. The mineral oil is preferably a highly purified oil having a viscosity of 50 to 600 centistokes at 37.8°C. (100°F.) and a pour point higher than -30°C. Examples of such mineral oil include a turbine oil having a viscosity of 60 to 130 centistokes and a pour point higher than -30°C., a cylinder oil having a viscosity of 400 to 500 centistokes and a pour point of about 10°C., and a gear oil having a viscosity of 200 to 300 centistokes and a pour point of about 5°C. These may be used either singly, or in the form of a mixture in order to provide a proper hardness or consistency as a grease. The said mineral oils have been purified and hence form substantially no sludges during use. Further, the use of such mineral oils is advantageous in that they are so inexpensive as about one-hundredth the costs of synthetic oils such as polyalkylene glycols, diester oils or polyphenyl ethers.

The above-mentioned mineral oil is, of course, liquid at normal temperatures, so that no stable lubrication can be accomplished when the mineral oil is used alone. In the present invention, therefore, a wax having a melting point of more than 100°C. and a molecular weight of 300 to 1,000 is dispersed in the mineral oil to prepare a mineral oil-based grease composition. The wax used in the present invention should not be well-soluble in the mineral oil. If the wax is well-soluble in the mineral oil, it is difficult to bring the mineral oil to a grease state. Examples of the wax include a microcrystalline wax having a molecular weight of 500 to 700, an amidowax having a molecular weight of 500 to 700, and a chlorowax having a molecular weight of 400 to 500. These waxes are soluble merely by 2 to 3 % in the mineral oil at normal temperature. The mineral oil is mixed with the wax in an amount larger than the solubility limit thereof in the mineral oil, and the resulting mixture is homogenized by heat-melting or the like and then cooled, whereby the wax is uniformly dispersed in the mineral oil to bring the mixture to a grease state.

The mixing of the mineral oil with the wax is carries out in such a manner that the two are thoroughly mixed with each other under stirring at a temperature above the melting point of the wax. The resulting mixture is cooled to deposit wax particles which have not been dissolved in the mineral oil, and the deposited wax particles are dispersed in the mineral oil to prepare a grease composition. In this state, the grease composition is somewhat low in consistency (i.e. is somewhat hard). For example, a grease composition comprising 85 parts by weight of a turbine oil of grade No. 200, and 15 parts by weight of an amidowax has a consistency of about 260. When kneaded with rolls or a mixer, the grease composition is increased in consistency. While the consistency is variable considerably freely by said kneading treatment, a proper consistency may be selected in consideration of easier application of the grease composition as a lubricant. The most preferable consistency of the composition as a lubricant is ordinarily in the range from 270 to 285.

In case the above-mentioned grease composition is desired to be incorporated with other additive, a mixture of the composition and the additive is sufficiently kneaded by means of rolls or a mixer. Even when the grease composition is incorporated with the additive, the consistency of the composition is dominated by the blending proportions of the mineral oil and the wax and the extent of kneading thereof, and does not vary due to the presence of the additive.

The melting point of a wax varies depending chiefly on the molecular weight thereof. Since the grease composition of the present invention is used for lubrication of contact portions of electric equipments, there is used a wax having a melting point higher than the operational temperature of the electric equipments. If the melting point of the wax used is lower than the operational temperature of the electric equipments, the wax is melted to make the composition impossible to maintain its grease state. Ordinary electric equipments, e.g. circuit breakers, disconnecting switches thereof, pantographs of electric cars, contacts of transformers, and ordinary switches, are operated at temperatures below 100°C., so that the use of a wax having a melting point of more than 100°C. is preferable.

The amount of the wax used is 5 to 20 parts by weight per 100 parts by weight of the mineral oil. If the amount of the wax is less than 5 parts by weight, it is impossible to obtain a grease composition having a consistency of 200 to 330 at normal temperature which is usable as a lubricant. That is, the resulting composition is too high in consistency and too soft to be used as a grease. On the other hand, if the amount of the wax is more than 20 parts by weight, the resulting grease composition becomes too hard (less than 200 in consistency) to be used for lubrication and increases frictional resistance between the sliding contact surfaces. The consistency is a numeral representing the hardness of a grease composition, and is a value measured according to ASTM D-217. According to the studies of the present inventors, it has been found that among the above-mentioned waxes, the amidowax is most preferable since it is particularly excellent in lubricating ability and heat resistance. It appears that the amidowax, which has amide groups, well-adsorbs on metal contact surfaces to decrease the contact resistance between the metal contact surfaces. Each of the aforesaid waxes gives a grease composition having such characteristic that a film of the composition is easily cut when a contact pressure of more than 0.1 kg/mm2 is applied to the film, whereby the contact resistance, i.e. the frictional resistance, of the contact surfaces is descreased. It is needless to say that the waxes may be used either singly or in the form of a mixture.

The consistency of a grease composition, which is most easily usable as a lubricant and is high in retainability, is in the range from 275 to 285. A grease composition having such desirable consistency as mentioned above is prepared by dispersing 16 to 18 parts by weight of an amidowax having a molecular weight of about 600 in 100 parts of a turbine oil having a dynamic viscosity at 37.8°C. of 60 to 130 centistokes and a pour point of more than -30°C. A wax having a molecular weight of less than 300 is low in melting point, and hence gives a grease composition which is decreased in consistency with increasing temperature and thus is low in retainability, while a wax having a molecular weight of more than 1,000 is too high in melting point to be easily dispersed in the mineral oil.

The above-mentioned mineral oil-wax system grease composition is usable for lubrication of contact portions used to connect or disconnect electric circuits, i.e. for lubrication of contacts used in electric equipments because of its excellent resistance to O3 and NOx, but is preferably incorporated with a proper amount of a nonvolatile solid lubricant powder in order that the composition can minimize the electric contact resistance of contacts, i.e. the voltage drop between electric contact members. An emulsion type lubricant comprising a mineral oil and a solid lubricant powder such as graphite has already been used for lubrication of electric contact surfaces. In the present invention, the mineral oil-wax system grease composition is incorporated with a proper amount of a solid lubricant, like in the case of the said emulsion type lubricant. The solid lubricant is used in the form of a powder of less than 5 microns in particle size. In order to minimize the contact resistance between contact surfaces, the solid lubricant should have a specific resistivity as low as possible. Each of such solid lubricants as graphite, WTe, NbS2, NbSe2, NbTe2, TaSe, TaSe2, TaTe2, TiS2, TiSe2, TiTe2, and ZrTe2 has a resistivity of less than 0.01 ohm-cm. Any solid lubricant may be used so far as it has a specific resistivity of less than 0.1 ohm-cm. In this respect, molybdenum disulfide, which has a specific resistivity of 8.5 × 102 ohm-cm, cannot be said to be a preferable solid lubricant, though it is excellent in lubricating ability. Specific resistivity values of the above-mentioned solid lubricants are as shown in Table 1.

Table 1 ______________________________________ Solid lubricant Specific resistivity (ohm-cm) ______________________________________ WTe2 3.10 × 10-3 NbS2 " " NbSe2 5.35 × 10-4 NbTe2 5.74 × " TaS2 3.33 × 10-3 TaSe2 2.23 × " TaTe2 1.37 × " TiS2 8.00 × " TiSe2 2.00 × " TiTe2 1.00 × 10-4 ZrTe2 " × 10-3 Graphite 2.64 × 10-3 ______________________________________

The above-mentioned graphite and intermetallic compounds of elements of Groups 4, 5 and 6 of the Periodic Table may be used either singly or in the form of a mixture. According to the studies of the present inventors, it has been found that graphite is most preferable for lubrication of contacts of air-blast and SF6 gas-blast circuit breakers and contacts of disconnecting switches thereof. A grease composition suitable for the abovementioned purpose is prepared by adding 11 to 13 parts by weight of the wax and 5 to 7 parts by weight of the solid lubricant to 100 parts by weight of the mineral oil. However, the amount of the solid lubricant varies depending on the application purpose of the resulting grease composition, and is in the range from 0.5 to 30 parts by weight. If the amount of the solid lubricant is less than 0.5 part by weight, the resulting compostion cannot display the effect derived from addition of the solid lubricant, and cannot sufficiently minimize the voltage drop due to contact resistance of contact portions. On the other hand, if the amount of the solid lubricant is more than 30 parts by weight, the resulting composition comes to contain excessively small amounts of mineral oil and wax, and hence cannot display its characteristics as a lubricant.

As is well known, the lubrication of contact surfaces composed of noble metals high in surface energy such as silver-silver, gold-gold or platinum-platinum contact surfaces is not easy since the lubricant applied thereto tends to cause the cutting of oil film, whereas the lubrication of base metal contact surfaces such as copper-copper or iron-iron contact surfaces is not so difficult since the lubricant is less in cutting of oil film. It is therefore easily understandable that a lubricant, which can satisfy such most severe condition as the lubrication of noble metal contact surfaces, is successfully applicable also to the lubrication of base metal contact surfaces. The mineral oil-wax-solid lubricant system grease composition of the present invention was tested in characteristics of lubricating noble metal contact surfaces to show such markedly excellent lubricating characteristics that no cutting of oil film was caused at all over a long period of time even when the composition was subjected to contact pressures of more than 0.1 kg/mm2. Accordingly, the grease composition of the present invention is useful for the lubrication of noble metal contact surfaces, e.g. contacts of circuit breakers, contacts of disconnecting switches, and the like contacts operated under high contact pressures, which have heretofore been considered to be most difficultly lubricated. Of course, the present composition, when used for the lubrication of copper-copper contact surfaces, is longer in life and greater in lubricating characteristics than the conventional lubricants, and hence can successfully be applied to the lubrication of, for example, pantographs of electric cars.

The above-mentioned ternary system grease composition of the present invention is useful for the lubrication of contact portions to which a contact pressure in the range from 0.5 to 3 kg/mm2 is chiefly applied. However, in case electric equipments have been made greater in capacity and the contact pressure has been made higher in order to further decrease the electric resistance to minimize the voltage drop, it might be necessary to improve the composition in lubricity. In such a case, the composition may be incorporated with a oiliness improver, which is a powder of such metal as, for example, copper, iron, zinc, lead, titanium or magnesium. When incorporated with the oiliness improver, the composition is enhanced in ability of lubricating noble metal contact surfaces, which tend to fuse to each other, and prevents the surfaces from mutual fusion. Alternatively, a powder of such metal oxide as copper oxide, lead oxide or magnesium oxide also shows the same action as that of the metal powder. According to the experiments of the present inventors, copper oxide and lead oxide were great in action of minimizing the surface energy of noble metal contact surfaces, and showed excellent effects as oiliness improvers. Particularly, lead oxide has a lubricating ability by itself and hence is a preferable additive. The oiliness improver is used in the form of a powder of less than 5 microns in particle size. If the particle size is excessively large, the oiliness improver is deteriorated in dispersibility in the composition, and the resulting composition rather increases the contact resistance.

As the oiliness improver, there may also be used a powder of a hydrogenated diphenyl compound or a halogentad polymer such as ordinary polyvinyl chloride, polyvinylidene chloride or polyvinyl ether chloride, or a liquid of polyfluoroethylene chloride having a molecular weight of 349 to 1,398. The oiliness improver of this kind is great in effect of minimizing the surface energy of a noble metal such as silver, and can effectively enhance the lubrication of silver contact surfaces.

The above-mentioned oiliness improver may be used in an amount of 0.5 to 6% by weight based on the total amount of the aforesaid mineral oil, wax and solid lubricant. If the amount of the oiliness improver is less than 0.5%, the effect of the oiliness improver cannot sufficiently be displayed, while if the amount thereof is more than 6%, the resulting grease composition is deteriorated in lubricating ability.

The present invention is illustrated in detail below with reference to examples.

Example 1

Grease compositions of the compositions shown in Table 2 were measured in consistency at various temperatures according to the method regulated in ASTM D-217 to obtain such results as shown in FIG. 1.

Table 2 ______________________________________ Grease composition Composition No. (parts by weight) ______________________________________ 1 Turbine oil of grade No. 200 80 Lithium soap 20 2 Polytrifluoroethylene chloride 90 Benton powder (particle size less than 5 μ) 10 3 Turbine oil of grade No. 200 85 Amidowax 15 4 Turbine oil of grade No. 200 85 Amidowax 10 Graphite 5 ______________________________________

From FIG. 1, it is clear that at below 0°C., the grease composition No. 2 (represented by the curve d) becomes so hard as to have such a low consistency as less than 150 and hence is not usable as a lubricant, and the grease composition No. 1 (represented by the curve c does not become so low in consistency as the grease composition NO. 2 but is still unusable at below 0°C. In contrast thereto, each of the grease composition Nos. 3 and 4 (represented by the curves b and a, respectively) according to the present invention has such a high consistency as 150 to 400 at temperatures ranging from about -30°C. to 100°C. and thus satisfies the function as a lubricant. At normal temperature (25°C.), the grease compositions of the present invention have consistency values of about 280 to 330.

Example 2

The grease composition No. 3 of Example 1 was measured at various temperatures in frictional characteristics to the surfaces of Ni-Cr-Mo steel (SNCM 8) and soft steel (SS 41) to obtain such results as shown in FIG. 2. As is clear from FIG. 2, the grease composition of the present invention shows low static frictional co-efficients (μs) over wide average surface pressures (kg/mm2) at temperatures from 25° to 100°C. Since the conventional grease (e.g. the grease composition No. 1 OF Example 1) ordinarily shows a static frictional coefficient of 0.3 to 0.4, it is understood that the grease composition of the present invention is extremely high in lubricating ability. According to the experiments conducted by the present inventors, it has been found that when a frictional coefficient of the contact surfaces exceeds 0.4, it is difficult to effect slidable contact operation because biting phenomenon of the contact surfaces occurs. In FIG. 2, the curves, e, f, g, h and i shows frictional characteristics at 25°, 40°, 60°, 80° and 100°C., respectively.

Example 3

Three kinds of grease compositions shown in Table 3 were prepared. These compositions are compared with a conventional emulsion type lubricant comprising No. 200 turbine oil and 10% of graphite powder in conductivity and frictional coefficient between silver and silver. The consistency values of grease compositions I, II and III are 270, 290 and 300, respectively.

Table 3 __________________________________________________________________________ I II III __________________________________________________________________________ Turbine oil of grade No. 200 (103 c.s. at 100°F; pour point -27.5°C; 100 parts 100 parts 100 parts specific gravity 0.884; molecular weight about 600) Amidowax (melting point 5 " 5 " 5 " 141°C 80 - 100 mesh) Artificial graphite 10 " -- -- (4 μ>) NbS2 (4 μ>) -- 10 " -- TiSe2 (4 μ>) -- -- 10 " Copper powder (5 μ>) 5 % -- -- Zinc powder (5 μ>) -- 5 % -- Lead oxide powder (5 μ>) -- -- 5 % __________________________________________________________________________

The conductivity of each lubricant was measured in the following manner.

As shown in FIG. 3, the grease compositions were applied between a fixed electrode 1 and a movable electrode 2 which has individually been prepared by plating a copper plate with silver to form a silver layer of 80 μm in thickness. Subsequently, the switch 3 was closed to flow 100 A of a direct current of 100 V, a definite load was applied to the fixed electrode 1, and the movable electrode 2 was reciprocally moved in the directions of the arrows. After the movable electrode has been moved a predetermined number of times, the switch 4 was closed to measure the voltage drop caused by the contact resistance between the electrodes 1 and 2. In the above manner, the lubricants were compared with each other in conductivity.

The frictional coefficient was measured in the following manner.

Each lubricant was applied between fixed elements 5 and 6 and a movable element 7 inserted between said fixed elements all of which had individually been prepared by plating square a copper rod with silver to form a silver layer 0f 80 μm in thickness. After applying a load P to the fixed element 5, the movable element 7 was pushed in the direction of the arrow, and a load F, which allowed the movable element 7 to move, was measured. The frictional coefficient in this case was calculated according to the following equation: ##EQU1##

Voltage drops and frictional coefficients observed by use of the aforesaid three kinds of the grease compositions of the present invention and the conventional lubricant were as set forth in Table 4. As is clear from Table 4, the grease compositions according to the present invention are better in lubricating characteristics than the conventional lubricant. Though the conventional lubricant displays satisfactory lubricating characteristics under a lower contact pressure or load as can be seen in Table 4, it cannot perform its lubricating ability under such a higher contact pressure as 5 kg/mm2. To the contrary, the grease compositions according to the present invention show the excellent lubricating characteristics under the higher contact pressure without developing welding of contactors, as shown in Table 4.

Table 4 __________________________________________________________________________ Present grease composition Conventional I II III lubricant __________________________________________________________________________ Load (kg/mm2) 1 5 1 5 1 5 1 5 Reciprocation (times) 100 100 100 100 100 100 100 welding Temperature (°C.) 80 80 80 80 80 80 80 80 Voltage drop (mV) 2 2 2 2 2 2 2 -- Load P (kg/mm2) 1 5 1 6 1 7 1 5 Load F (") 0.2 1.2 0.2 1.2 0.2 2.1 1.0 welding Temperature (°C.) 80 80 80 80 80 80 80 80 Frictional coefficient 0.1 0.12 0.1 0.10 0.1 0.15 0.5 -- __________________________________________________________________________

Example 4

Using three kinds of grease compositions shown in Table 5, the same conductivity and frictional coefficient measurements as in Example 3 were effected. The results obtained were as set forth in Table 6. The consistency values of the grease compositions V, VI and VII are 270, 290 and 300, respectively.

Table 5 __________________________________________________________________________ V VI VII __________________________________________________________________________ Base Oil (Turbine oil of grade 100 parts 100 parts 100 parts No. 200) Amidowax (same as in Table 3) 5 " 5 " 5 " Graphite (same as in Table 3) 10 " -- -- NbS2 (same as in Table 3) -- 10 " -- TiSe2 (same as in Table 3) -- -- 10 " Polyvinyl chloride (5 μ>) 5 % -- -- Polytrifluoroethylene chloride (liquid -- 5 % -- molecular weight 933.4) Diphenyl pentachloride (liquid) -- -- 5 % __________________________________________________________________________

Table 6 ______________________________________ Present grease composition V VI VII ______________________________________ Load (kg/mm2) 5 5 5 Reciprocation (times) 100 100 100 Temperature (°C) 80 80 80 Voltage drop (mV) 2 2 2 Load P (kg/mm2) 5 5 5 Load F (") 1.0 1.2 1.2 Temperature (°C) 80 80 80 Frictional coefficient 0.10 0.12 0.12 ______________________________________

As is clear from Table 6, the grease compositions according to the present invention have improved lubricating characteristics.

Example 5

A mixture comprising 100 parts of a turbine oil of the grade No. 200, 5 parts of amidowax (molecular weight about 600) and 10 parts of artificial graphite powder was incorporated with 5% of each of zinc powder having a particle size less than 4μ and polytrifluoroethylene chloride and kneaded by use of a ball mill to prepare two grease compositions of the present invention each having consistency of 280. The thus prepared grease compositions were compared with the conventional lubricant as in Example 3 and with an electron grease consisting of polyphenyl ether 90 parts and benton 10 parts which is used for lubrication of contacts of ordinary electronic parts by measuring at room temperature (25°C.) and 100°C. the relation between load and voltage drop in the case where each lubricant was used. The results obtained were as shown in FIG. 5. In FIG. 5, the curve 8 shows the voltage drops observed with respect to the invention lubricants and conventional lubricants after reciprocally moving 100 times at room temperature. The movable electrode 2 is shown in the means of FIG. 3. All of the present grease compositions, the conventional lubricant and the electron grease brought about voltage drops shown on the same curve and thus were identical in efficiency with each other. However, when the movable electrode was reciprocally moved at 100°C., the present grease composition containing zinc powder, the present grease composition containing polytrifluoroethylene chloride having a molecular weight of 933.4, the conventional lubricant and the electron grease caused voltage drops represented by the dotted lines 9, 10, 11 and 12, respectively. As is clear from the dotted lines, the grease compositions of the present invention produce lower voltage drops and are excellent in conductivity.

In the next place, frictional coefficients between silver and silver, to which has been applied the above-mentioned lubricants, were measured by use of the means of FIG. 4, and the lubricants were compared in the range of frictional coefficient ≤0.4 in the case where the temperature and the load were varied. The results obtained were as shown in FIG. 6.

The curves in FIG. 6 were formed in such a manner that contact surfaces lubricated with various lubricants were measured in static frictional coefficient at various temperatures under various loads, and the loads and temperatures at which the static frictional coefficient reached 0.4 were plotted. Thus, FIG. 6 shows the relation between loads (or contact pressures) and temperatures when the frictional coefficient is 0.4 (i.e. lubrication limit). According to experiments, the inventors have found that under a definite load, the frictional coefficient becomes greater with increasing temperature, i.e. the load reaching the frictional coefficient of 0.4 becomes lower with increasing temperature.

In order to obtain the curves of FIG. 4, it was deemed that each lubricant was usable when the frictional coefficient attained by use of the lubricant was 0.4 or less. The curve 13 shows the limit of the frictional coefficient when the electron grease was used. For example, the point X on the curve 13 shows that the frictional coefficient becomes more than 0.4 if the load becomes more than 1.5 kg/mm2, and that even when the load is maintained at 1.5 kg/mm2, the frictional coefficient becomes more than 0.4 as well if the temperature becomes higher than 23°C. The same is the case with the curves 14, 15 and 16. The curve 14 shows the case where the conventional lubricant was used, and the curves 15 and 16 show the cases where the grease compositions of the present invention were used. From FIG. 6, it is clear that the grease compositions according to the present invention have excellent lubricating characteristics under higher loads and at higher temperatures than in the cases of the conventional lubricant and the electron grease.

Example 6

A mixture comprising 100 parts of a turbine oil of the grade NO. 200, 5 parts of a 1 : 1 mixture of microcrystalline wax and amidowax, and 10 parts of graphite powder was incorporated with 5% of each of copper powder and diphenyl pentachloride and kneaded by use of a ball mill tp prepare two grease compositions of the present invention.

If, in the case of a greasy lubricant, the base oil separates during the use of the lubricant from the thickener or other additives used, the lubricant is greatly degraded in lubricating ability and is undesirably shortened in life.

Accordingly, the grease compositions of the present invention were compared in base oil separation ratio with the conventional lubricant and the electron grease used in Example 5. The base oil separation ratio was measured in the following manner:

A copper plate of 1 mm. in thickness, 60 mm. in length and 20 mm. in width was subjected to silver plating to form thereon a silver layer of 80 μm in thickness. On the surface of said silver layer was uniformly coated 1 g. of each of the above-mentioned lubricants, and the copper plate was suspended in an electric thermostat tank and heated at 100°C. for a definite period of time. Subsequently, the oil drops which had separated from the lubricant were collected, and the ratio of the separated oil was represented by weight %. The relation between the oil separation ratio and the time was as shown in FIG. 7. The grease composition containing 5% of copper powder had the properties of the curve 17. Curve 18 shows the case of the present grease composition containing 5% of diphenyl pentachloride, the curve 19 shows the case of the electron grease, and the curve 20 shows the case of the conventional lubricant. From FIG. 7, it is understood that the lubricants according to the present invention are markedly low in oil separation ratio even when heated at 100°C., and hence keep predetermined lubricating characteristics for a long period of time.

Example 7

Each of the grease compositions of the composition I in Table 3 and the composition I in Table 5 was subjected to load friction tests 60 times during 6 months, using a model disconnecting switch of a 50 KVA circuit breaker under such conditions as a contact surface pressure of 5 kg/mm2, an alternating current of 50 KV and 1,000 A, and a temperature of 80°C. As a result, the conventional lubricant brought about welding on the rubbed surface after one test, whereas the grease compositions according to the present invention caused no marked degradation even after 20 tests.

The fact that the grease compositions of the present invention are markedly excellent in lubricating characteristics as compared with the conventional lubricant has been substantiated with reference to the aforesaid examples.

Example 8

A grease composition (I) comprising 85 parts by weight of No. 200 turbine oil and 15 parts by weight of amidowax (molecular weight about 600) and having a consistency of 280; a grease composition (II) comprising 85 parts by weight of No. 200 turbine oil, 10 parts by weight of amidowax and 5 parts by weight of natural graphite and having a consistency of 280; a grease composition (III) comprising 85 parts by weight of No. 200 turbine oil, 10 parts by weight of amidowax and 5 parts by weight of synthetic graphite; a grease composition (IV) comprising 90 parts by weight of polytrifluoroethylene chloride (molecular weight 933.4) and 10 parts by weight of benton powder and an emulsion type lubricant comprising 90 parts by weight of No. 200 turbine oil and 10 parts by weight of synthetic graphite were individually tested in chemical influence on silver-silver contact surfaces by applying the said compositions onto the silver surfaces in air at 100°C. for 100 hours. When the grease compositions (I), (II) and (III) were applied, the silver surfaces were not colored at all except the portions which had not been brought into contact with the compositions. At portions contacted with the grease compositions (I) and (III), the silver surfaces were not colored at all, whereas at portions contacted with the grease composition (II), the silver surfaces were colored to pale purple since the composition contained natural graphite. When the grease composition (IV) was applied, the silver surfaces were colored to purple even at portions which had not been contacted with the composition, and colored to deep purple at portions contacted therewith. In the case of the emulsion type lubricant, the silver surfaces were not colored at portions contacted with the lubricant but were colored to purple at portions contacted therewith. From the above, it is understood that the grease compositons of the present invention are not only excellent in lubricating ability but also effective for prevention of silver contact surfaces from oxidation.

Each of the above-mentioned grease compositions was heated at 98.9°C. for 500 hours, and the oxidation stability thereof was measured from the amount of oxygen consumed due to oxidation. The results obtained were such that in the case of the grease compositions of the present invention, the partial pressure of oxygen was lower only by 5 p.s.i. than the original value 110 p.s.i., whereas in the case of the grease composition (IV), the partial pressure of oxygen became 0 in such a short period as 150 hours to show that the composition was easily oxidizable, and in the case of the emulsion type lubricant, the partial pressure of oxygen decreased to 90 p.s.i. in 300 hours and to 78 p.s.i. in 500 hours to show that the lubricant was low in oxidation stability.

At atmospheric pressure or under a contact pressure of less than 0.1 kg/mm2, the grease composition according to the present invention is a homogeneous greasy mixture having a specific resistivity of less than about 1012 ohm-cm and thus is a substantially complete insulator. When a contact pressure of more than 0.1 kg/mm2 is applied to the grease composition, however, the mineral oil component in the composition oozes out little by little to display its lubricating ability. At the same time, the wax component, which is low in shearing force, does not disturb the lubrication of metal contact portions nor substantially increase the contact resistance between electric contact portions. Particularly when the grease composition is incorporated with the aforesaid low resistivity solid lubricant, the wax does not increase the contact resistance even at the time of lubrication of noble metal contact surfaces. In some experiments conducted by the inventors using the composition of the present invention, the voltage drop between contacts was such a low value as 0.2 to 0.3 mV. In the case of the conventional lubricant comprising a synthetic oil and a metallic soap, the metallic soap is low in specific resistivity, so that the lubricant has a specific resistivity of 107 to 108 ohm-cm and hence is markedly low in insulating ability. Accordingly, the conventional lubricant has had such drawback that if it adheres to improper portions of electric equipments, short circuit of the electric equipments is necessarily brought about. In the case of the grease composition of the present invention, however, no such problem is brought about since the composition is an insulator.

Example 9

Two kinds of grease compositions of the following compositions shown below were prepared, and the characteristics thereof were measured in the same manner as in Example 3. The results are shown in Table 7.

______________________________________ Composition VIII Gear oil (325 centistokes at 37.8°C) 100 parts Amidowax (Molecular weight 579) 11 parts Consistency 275 Composition IX Gear oil (325 centistokes at 37.8°C) 50 parts Turbine oil (105 centistokes at 37.8°C) 50 parts Amidowax (molecular weight 579) 17.6 parts Consistency 285 ______________________________________

Table 7 ______________________________________ VIII IX ______________________________________ Load (kg/mm2) 5 5 Reciprocation (times) 100 100 Temperature (°C) 80 80 Voltage drop (mV) 2 2 Load P (kg/mm2) 5 5 Load F (kg/mm2) 2.5 2 Temperature (°C) 80 80 Frictional coefficient 0.25 0.2 ______________________________________

The above experiments were conducted with respect to lubrication of silver-silver contact surfaces. Since the above grease compositions contain no solid lubricant such as graphite, the lubricating properties thereof are some what worse than those of the grease compositions of Examples 3 and 4, as can be seen from Tables 4 and 6. However, the grease composition consisting of a mineral oil and lithium soap produced welding between contact surfaces at a load of 5 kg/mm2 to render the contacting operation impossible. When the grease compositions were applied to lubrication of the contact surfaces of SNCM-8 steel and SS-41 steel under the same condition as above, frictional coefficients with respect to composition VIII and composition IX were 0.15 and 0.12, respectively. That is, these coefficients are sufficiently smaller than the critical coefficient 0.4. Therefore, the above grease compositions can display the sufficient lubricating properties on the base metals, i.e. non-noble metals.