Encapsulated balance shaft and method

1. A shaft assembly comprising:

a shaft including a body, wherein the body includes a sealing portion, a balancing portion and a hollow portion;

a spacer in proximate contact with the body and configured to substantially enclose the hollow portion; and

a cover encapsulating the shaft body and the spacer and defining a substantially continuous cylindrical surface in sealing engagement with the sealing portion.

2. The shaft assembly of claim 1 , wherein:

the shaft assembly is configured for installation in an engine such that the shaft assembly is rotatable about an axis of rotation defined by the shaft, and the balancing portion is partially submerged in an oil bath in the engine when the shaft assembly is in the installed position; and

such that the substantially continuous cylindrical surface of the cover minimizes at least one of turbulence and aeration of the oil bath when the shaft assembly is rotated.

3. The shaft assembly of claim 1 , wherein at least one of the spacer and the shaft body are configured to be operatively attached to the other of the spacer and the shaft body.

4. The shaft assembly of claim 1 , wherein at least one of the spacer and the shaft body includes a retention feature configured to retain the spacer in proximate contact with the shaft body.

5. The shaft assembly of claim 1 , wherein the cover includes a polymer-based material with a melting point above 250° C.

6. The shaft assembly of claim 1 , wherein the cover is configured to exert a compressive force on the spacer such that the spacer is held in proximate contact with the shaft body.

7. The shaft assembly of claim 1 , wherein the sealing portion defines a recessed portion and the cover is configured in sealing contact with the recessed portion.

8. The shaft assembly of claim 1 , wherein the cover includes one of a polyphthalamide-based material, a high performance polyamide-based (PPA) material, a polyphenylene ether-based (PPE) material, and a filler-reinforced polyamide material.

9. The shaft assembly of claim 1 , wherein the shaft is configured as a balance shaft for rotation in synchrony with a crankshaft.

10. The shaft assembly of claim 1 , wherein the spacer is configured to provide structural support to the cover.

11. A method for encapsulating a shaft, the method comprising:

positioning a spacer in proximate contact with a shaft including a body, wherein:

the body includes a sealing portion, a balancing portion and a hollow portion, and

the spacer is configured to substantially enclose the hollow portion of the body; and

forming a cover encapsulating the body and the spacer wherein the cover as formed defines a substantially continuous cylindrical surface in sealing engagement with the sealing portion of the body.

12. The method of claim 11 , wherein the shaft is configured for installation in an engine such that when the shaft assembly is installed in the engine the shaft is rotatable about an axis of rotation defined by the shaft and the balancing portion is partially submerged in an oil bath in the engine; and

such that the substantially continuous cylindrical surface of the cover minimizes at least one of turbulence and aeration of the oil bath while the shaft assembly in the installed position is rotated.

13. The method of claim 11 , further comprising:

retaining the spacer in proximate contact with the body.

14. The method of claim 11 , wherein at least one of the spacer and the body includes a retention feature configured to retain the spacer in proximate contact with the shaft body.

15. The method of claim 11 , wherein forming the cover includes:

shrinking the cover during forming such that the cover exerts a compressive force on the spacer and against the shaft body.

16. The method of claim 11 , wherein the sealing portion defines a recessed portion and wherein forming the cover includes configuring the cover in sealing contact with the recessed portion.

17. The method of claim 11 , wherein the cover is formed from one of a polyphthalamide-based material, a high performance polyamide-based (PPA) material, a polyphenylene ether-based (PPE) material, and a fiber-reinforced polyamide material.

18. The method of claim 11 , further comprising:

forming the spacer by one of blow molding, insert molding, and injection molding.

19. An engine including a balance shaft assembly, the engine comprising:

a balance shaft assembly including:

a shaft including a body, wherein the body includes a sealing portion, a balancing portion and a hollow portion;

a spacer retained in proximate contact with the body and configured to substantially enclose the the hollow portion; and

a cover encapsulating the body and the spacer and defining a substantially continuous cylindrical surface in sealing engagement with the sealing portion; and

an oil bath;

wherein the balance shaft assembly is rotatably positioned in the engine such that the balancing portion is rotatable through the oil bath; and

such that the substantially continuous cylindrical surface of the cover seals the balance shaft assembly from ingression of oil into the hollow portion and minimizes at least one of turbulence and aeration of the oil bath when the shaft assembly is rotated.

20. The engine of claim 19 , wherein the spacer is configured to reinforce the cover to substantially prevent distortion of the cover in the engine.