Gear assembly and gear oil composition

1. A gear assembly comprising at least one pair of meshing gears situated inside a gear case having a sump portion and a wall portion constructed and arranged to provide a head space above the sump portion, and a gear oil mixture disposed in said sump portion; at least one of said gears being immersed at least partially in said mixture and said mixture comprising a base oil, and a coolant having an endothermic liquid/vapor transition between the lower and upper operating temperatures of the gear assembly, the coolant having a viscosity lower than the base oil, and a heat exchanger in an interior portion of the gear case constructed and arranged to fit in the head space, and the heat exchanger being constructed and arranged to condense coolant vapor into a liquid form.

2. A gear assembly as set forth in claim 1 , wherein at least one of said meshing gears is of a hypoid or a helical gear type.

3. A gear assembly comprising at least one pair of meshing gears situated inside a gear case having a sump portion and a wall portion, and a gear oil mixture disposed in said sump portion; at least one of said gears being immersed at least partially in said mixture and said mixture comprising a base oil, and a coolant having an endothermic liquid/vapor transition between the lower and upper operating temperatures of the gear assembly, the coolant having a viscosity lower than the base oil, and further comprises a heat exchanger disposed in said wall portion and configured to condense the vapor of said coolant into a liquid and to return said liquid to the sump portion.

4. A gear assembly as set forth in claim 1 , wherein said endothermic liquid/vapor transition occurs at a temperature between about 80° C. and about 200° C.

5. A gear assembly as set forth in claim 1 , wherein the at least heat exchanger includes a section of said wall portion made of a heat conductive metal and having at least one of channels, dents, spikes or voids configured to condense the vapor of said coolant, conduct heat away from said gear assembly, and return the condensed liquid coolant to said sump portion.

6. A gear assembly as set forth in claim 1 further comprises a pressure regulator configured to adjust the pressure inside said gear case.

7. A gear assembly as set forth in claim 1 is an automotive differential gear box, rear axle gear box, or transmission gear box.

8. A method comprising: providing a gear assembly comprising at least one pair of meshing gears and a gear case having a sump portion and a wall portion constructed and arranged to provide a headspace above the sump portion; disposing in said sump portion a gear oil comprising a base oil, and a coolant having an endothermic liquid/vapor transition between the upper and lower operating temperatures of said gear assembly, and immersing at least one of said gears in said gear oil, said coolant having a viscosity lower than the base oil, and at least one of a heat exchanger in an interior portion of the gear case constructed and arranged to fit in the head space, or a plurality of fins extending from the exterior of the gear case.

9. A method as set forth in claim 8 further comprises: operating said gear assembly to allow at least a portion of said gears and/or sump portion to reach a temperature near said endothermic liquid/vapor transition temperature, causing said coolant to vaporize into the headspace of said gear case; condensing the coolant vapor in the headspace; returning the condensed coolant liquid to the sump portion; and mixing said condensed coolant liquid with said gear oil mixture in said sump portion through the rotating action of said gears.

10. A method comprising: providing a gear assembly comprising at least one pair of meshing gears and a gear case having a sump portion and a wall portion with a headspace and a heat exchanger disposed in said wall portion and configured to condense the vapor of said coolant into a liquid and to return said liquid to the sump portion; disposing in said sump portion a gear oil comprising a base oil, and a coolant having an endothermic liquid/vapor transition between the upper and lower operating temperatures of said gear assembly, and immersing at least one of said gears in said gear oil, said coolant having a viscosity lower than the base oil, wherein said wall portion comprising a heat conductive metal configured to condense the vapor of said coolant, conduct heat away from said assembly and return the condensed coolant liquid to said sump portion.

11. A method as set forth in claim 8 , wherein said coolant is an organic alcohol having a heat of vaporization of at least 0.3 kJ/g.

12. A method as set forth in claim 8 , wherein said gear assembly is an automotive differential gear box, rear axle gear box, or a transmission and at least one of said meshing gears is of a hypoid or helical gear type.

13. A gear assembly as set forth in claim 1 wherein the gear oil mixture is such that the gear oil mixture has a first viscosity at the contact point of the at least one pair of meshing gears when the coolant at the contact point is at a temperature below 80° C., and so that the gear oil mixture has a second viscosity at the contact point of the at least one pair of meshing gears when the coolant at the contact point is at a temperature between 80° C. and about 200° C. and is in the vapor phase, wherein the second viscosity is greater than the first viscosity.

14. A gear assembly as set forth in claim 1 wherein the gear oil mixture is such that the gear oil mixture has a first viscosity when the coolant is at a temperature below 80° C., and so that the gear oil mixture has a second viscosity when the coolant is at a temperature between 80° C. and about 200° C. and is in the vapor phase, wherein the second viscosity is greater than the first viscosity.

15. A gear assembly as set forth in claim 1 wherein the wall portion defines a headspace above the a gear oil mixture disposed in said sump portion so that the coolant may vaporize into the headspace and condense vapor of the coolant into a liquid and to return said liquid to the sump portion.

16. A gear assembly as set forth in claim 1 wherein the oil coolant mixture is such that coolant is vaporizable from the mixture at a temperature ranging from 80° C. and about 200° C.

17. A gear assembly as set forth in claim 1 , wherein said coolant has a melting point of −40° C. or less, an endothermic boiling point of about 190° C. or less, and a heat of vaporization at boiling point of at least 0.25 kJ/g.

18. A gear assembly as set forth in claim 1 , wherein said coolant is present in said mixture at about 0.1% to about 20% by weight.

19. A gear assembly as set forth in claim 1 further comprises at least one anti-wear and/or an extreme pressure additive.

20. A gear assembly as set forth in claim 1 , wherein said coolant comprises a molecule having a polar group and a lipophilic group.

21. A gear assembly as set forth in claim 1 , wherein said coolant comprises an organic alcohol.

22. A gear assembly as set forth in claim 1 , wherein said coolant comprises at least one of diethylene glycol monobutyl ether, ethylene glycol monobutyl ether, diethylene glycol monoethyl ether, 1,2-butanediol, nonyl alcohol, dipropylene glycol n-butyl ether, dipropylene glycol methyl ether, dipropylene glycol monopropyl ether, propylene glycol methyl ether, propylene glycol n-butyl ether, propylene glycol propyl ether, isobutyl alcohol, 1-butanol, 2-chloroethanol, 1-hexanol, 1,2-propanediol, propylene glycol phenyl ether, ethylene glycol mono-propylether, diethylene glycol methyl ether, 4-methyl-2-pentanol, 1-methyl cyclohexanol, 2-methyl-1-pentanol, or n-pentanol.

23. A gear assembly as set forth in claim 1 , wherein said coolant comprises at least one of propylene glycol methyl ether or diethylene glycol methyl ether.

24. A gear assembly as set forth in claim 1 , wherein said coolant has a heat of vaporization of at least 0.3 kJ/g and a boiling point between about 80° C. to about 150° C.

25. A gear assembly as set forth in claim 1 , wherein said coolant is present in said mixture at about 1.0% to about 20% by weight.

26. A gear assembly as set forth in claim 8 , wherein said coolant is present in said mixture at about 1.0% to about 20% by weight.

27. A method comprising: providing a gear assembly comprising at least one pair of meshing gears and a gear case having a sump portion and a wall portion with a headspace; disposing in said sump portion a gear oil comprising a base oil, and a coolant having an endothermic liquid/vapor transition between the upper and lower operating temperatures of said gear assembly, and immersing at least one of said gears in said gear oil, said coolant having a viscosity lower than the base oil, and a coil filled with heat conductive fluid fit in the headspace of the gear case.

28. A gear assembly comprising at least one pair of meshing gears situated inside a gear case having a sump portion and a wall portion, and a gear oil mixture disposed in said sump portion; at least one of said gears being immersed at least partially in said mixture and said mixture comprising a base oil, and a coolant having an endothermic liquid/vapor transition between the lower and upper operating temperatures of the gear assembly, the coolant having a viscosity lower than the base oil, and a heat exchanger disposed in said wall portion and configured to condense the vapor of said coolant into a liquid and to return said liquid to the SUMP portion, and a plurality of fins extending from the exterior of the gear case.

29. A gear assembly as set forth in claim 1 , wherein said coolant is present in said mixture at about 1.0% to about 10% by weight.

30. A gear assembly as set forth in claim 8 , wherein said coolant is present in said mixture at about 1.0% to about 10% by weight.

31. A gear assembly comprising at least one pair of meshing gears situated inside a gear case having a sump portion and a wall portion, a heat exchanger disposed in said wall portion and configured to condense the vapor of said coolant into a liquid and to return said liquid to the sump portion, and a gear oil mixture disposed in said sump portion; at least one of said gears being immersed at least partially in said mixture and said mixture comprising a base oil, and a coolant having an endothermic liquid/vapor transition between the lower and upper operating temperatures of the gear assembly, the coolant having a viscosity lower than the base oil, the coolant comprising an alcohol having an ether group.

32. A gear assembly comprising at least one pair of meshing gears situated inside a gear case having a sump portion and a wall portion, a heat exchanger disposed in said wall portion and configured to condense the vapor of said coolant into a liquid and to return said liquid to the sump portion, and a gear oil mixture disposed in said sump portion; at least one of said gears being immersed at least partially in said mixture and said mixture comprising a base oil, and a coolant having an endothermic liquid/vapor transition between the lower and upper operating temperatures of the gear assembly, the coolant having a viscosity lower than the base oil, the coolant comprising at least one of diethylene glycol monobutyl ether, ethylene glycol monobutyl ether, diethylene glycol monoethyl ether, dipropylene glycol n-butyl ether, dipropylene glycol methyl ether, dipropylene glycol monopropyl ether, propylene glycol methyl ether, propylene glycol n-butyl ether, propylene glycol propyl ether, 1,2-propanediol, propylene glycol phenyl ether, ethylene glycol mono-propylether, or diethylene glycol methyl ether.