Pre-stretched tie-bolt for use in a gas turbine engine and method

1. A tie-bolt assembly for securing a rotor in a gas turbine engine, the tie-bolt assembly comprising:

an engine main shaft;

a rotor mounted about the main shaft, the rotor having a central bore therethrough;

a tie-bolt mounted to the main shaft, the tie-bolt having an elongated tubular portion having a first and a second end, the first end mounted to the main shaft, the elongated portion extending through the central bore, the tie-bolt having a retaining portion having a first and a second end, the first end of the retaining portion being unitary with the second end of the tubular portion, the retaining portion having an outer diameter larger than that of the tubular portion;

an annular spacer removably provided between the retaining portion and the rotor; and

wherein the tie-bolt is in an axial-pretension condition when mounted to the rotor and wherein said pretension at least partially maintains the annular spacer between the retaining portion and the rotor.

2. The tie-bolt assembly as defined in claim 1 , further comprising a locking ring to be mounted around the spacer to prevent the spacer from moving outwards.

3. The tie-bolt assembly as defined in claim 2 , further comprising a removable and outwardly-biased C-shaped ring provided in complementary slots made between the retaining portion and the locking ring for preventing the locking ring from moving.

4. The tie-bolt assembly as defined in claim 1 , further comprising a thermally insulated member interposed between the spacer and the rotor.

5. The tie-bolt assembly as defined in claim 1 , wherein the tubular portion and the retaining portion are made integral with each other.

6. A tie-bolt for use in a gas turbine engine having a main shaft, the tie-bolt comprising:

a rotor having a central bore;

an elongated tubular portion having a first and a second end and configured to pass through the central bore;

an apparatus for connecting the first end of the tubular portion to the main shaft;

a retaining portion having a first and a second end, the first end of the retaining portion being unitary with the second end of the tubular portion, the retaining portion having a diameter larger than that of the tubular portion;

an apparatus on the retaining portion for connection to a device pulling the tie-bolt; and

an annular spacer removably provided between the retaining portion and the rotor, the tie-bolt having a pretension to retain the spacer against the rotor, the annular spacer having a thickness corresponding to a required elongation of the tie-bolt to achieve a desired value for said tie-bolt pre-tension.

7. The tie-bolt as defined in claim 6 , further comprising an apparatus for restricting radial movement of the spacer.

8. The tie-bolt as defined in claim 7 , wherein the apparatus for restricting movement of the spacer comprises a locking ring mounted on the retaining portion.

9. The tie-bolt as defined in claim 6 , further comprising a thermally insulating member between the spacer and the rotor.

10. A method of mounting a tie-bolt to a main shaft in a gas turbine engine, the tie-bolt having opposite first and second ends, the method comprising:

threading a first end of the tie-bolt on the main shaft until the tie-bolt abuts a component to be retained;

exerting an axial pulling force at the second end of the tie-bolt for stretching the tie-bolt with reference to the first end thereof, thereby creating an axial spacing between the tie-bolt and the component to be retained;

inserting a spacer in the axial spacing; and then

releasing the axial pulling force.

11. The method as defined in claim 10 , further comprising:

mounting a locking ring around the spacer to prevent the spacer from moving outwards.

12. The method as defined in claim 11 , further comprising:

mounting a removable and outwardly-biased C-shaped ring to prevent the locking ring from moving.

13. The method as defined in claim 10 , further comprising:

inserting a thermally insulated component between the spacer and the a rotor retained by the tie-bolt.

14. The method as defined in claim 10 , further comprising the step of selecting said spacer with a width configured to provide an elongation of the tie-bolt associated with a desired level of pre-stretching of the tie-bolt.

15. A rotor assembly comprising:

an engine main shaft having an axis, the axis defining an axial direction;

a turbine rotor mounted about the main shaft, the main shaft passing through a central bore of the rotor, the rotor having an aft face;

a tie-bolt having a forward end coaxially mounted to an aft end of the main shaft, the tie-bolt extending co-axially through the rotor central bore to an aft end of the tie-bolt, the tie-bolt having a face adjacent the tie-bolt aft end which face opposes the rotor aft face, the tie-bolt having a portion configured to be engaged by an axial pretensioning apparatus, the tie-bolt being in an axial pre-stretched state when initially mounted to the rotor assembly; and

a spacer disposed between the rotor aft face portion and the tie-bolt face and there maintained at least partially by compression between the rotor aft face and the tie-bolt face, wherein the spacer has a thickness configured to maintain a desired axial pre-stretching on the tie-bolt when a load of the axial pretensioning apparatus is released.

16. The rotor assembly as defined in claim 15 , wherein the tie-bolt face extends radially outwardly of the rotor central bore.

17. The rotor assembly as defined in claim 16 , wherein the tie-bolt first end and tie-bolt face are provided on a unitary body.

18. The rotor assembly as defined in claim 16 , further comprising a component disposed between the spacer and the rotor aft face, the component composed of a low thermal conductivity material and thereby configured to insulate the spacer from the rotor.

19. The rotor assembly as defined in claim 16 , further comprising a ring disposed around the spacer and the ring is thereby configured to impede radial expansion of the spacer.

20. The rotor assembly as defined in claim 19 , wherein the ring is further disposed around a portion of the rotor adjacent the aft face and the ring is thereby configured to impede radial movement of a portion of the rotor having the aft face.

21. The rotor assembly as defined in claim 16 , wherein the tie-bolt having a portion configured to be engaged by an axial pretensioning apparatus comprises threads disposed at a second end of the tie bolt.

22. The rotor assembly as defined in claim 21 , wherein the threads are disposed on radially inward face of the tie-bolt.