Method and apparatus for varying the critical speed of a shaft

1. A method for changing the critical speed of a shaft, comprising:

providing a shaft having forward and aft ends supported for relative rotational movement with respect to a stationary structure;

providing a second shaft having forward and aft ends mounted coaxially with said first shaft, said second shaft having a second diameter greater than said first diameter and a second length shorter than said first length;

providing a forward bearing disposed at said forward end of said shaft, providing an aft bearing disposed at said aft end of said shaft;

providing a first active bearing disposed between said forward bearing and said aft bearing of said second shaft; and changing the support stiffness of said first active bearing.

2. The system for changing the critical speed of shaft of claim 1 wherein said means for changing the support stiffness of said first active bearing comprise a first fluid film damper disposed between said first active bearing and said stationary structure and means for selectively supplying fluid to said first fluid film damper.

3. The method for changing the critical speed of a shaft of claim 1 further comprising:

providing a second active bearing disposed between said first active bearing and said aft bearing; and

providing means for changing the support stiffness of said second active bearing.

4. The system for changing the critical speed of shaft of claim 3 wherein said means for changing the support stiffness of said second active bearing comprise a second fluid film damper disposed between said second active bearing and said stationary structure and means for selectively supplying fluid to said second fluid film damper.

5. A system for changing the critical speed of a shaft, comprising:

a first shaft supported for relative rotational movement with respect to a stationary structure, said shaft having forward and aft ends, a first diameter, and a first length;

a forward bearing disposed at said forward end of said shaft,

an aft bearing disposed at said aft end of said shaft;

a first active bearing disposed between said forward bearing and said aft bearing;

a second shaft having forward and aft ends mounted coaxially with said first shaft, said second shaft having a second diameter greater than said first diameter and a second length shorter than said first length, wherein said first active bearing is disposed between said forward bearing and said forward end of said second shaft; and

means for changing the support stiffness of said first active bearing.

6. The system for changing the critical speed of a shaft of claim 5 wherein said means for changing the support stiffness of said first active bearing comprise a first fluid film damper disposed between said first active bearing and said stationary structure and means for selectively supplying fluid to said first fluid film damper.

7. The system for changing the critical speed of a shaft of claim 5 further comprising:

a second active bearing disposed between said aft end of said second shaft and said aft bearing; and

means for changing the support stiffness of said second active bearing.

8. The system for changing the critical speed of shaft of claim 7 wherein said means for changing the support stiffness of said second active bearing comprise a second fluid film damper disposed between said second active bearing and said stationary structure and means for selectively supplying fluid to said second fluid film damper.

9. The system for changing the critical speed of a shaft of claim 5 wherein said means for changing the support stiffness of said first active bearing comprise a first fluid film damper disposed between said first active bearing and said stationary structure and means for selectively supplying fluid to said first fluid film damper.

10. The system for changing the critical speed of shaft of claim 5 further comprising:

a second active bearing disposed between said first active bearing and said aft bearing; and

means for changing the support stiffness of said second active bearing.

11. The system for changing the critical speed of a shaft of claim 10 wherein said means for changing the support stiffness of said second active bearing comprise a second fluid film damper disposed between said second active bearing and said stationary structure and means for selectively supplying fluid to said second fluid film damper.

12. A system for changing the critical speed of a gas turbine engine shaft, comprising:

a turbine shaft supported for relative rotational movement with respect to a stationary structure, said turbine shaft having forward and aft ends, a first diameter, and a first length;

a core shaft having forward and aft ends mounted coaxially with said turbine shaft, said core shaft having a second diameter greater than said first diameter and a second length shorter than said first length;

a forward bearing disposed at said forward end of said turbine shaft,

an aft bearing disposed at said aft end of said turbine shaft;

a first active bearing disposed between said forward bearing and said aft bearing of said core shaft; and

means for changing the support stiffness of said first active bearing.

13. The system for changing the critical speed of a gas turbine engine shaft of claim 12 wherein said means for changing the support stiffness of said first active bearing comprise a first fluid film damper disposed between said first active bearing and said stationary structure and means for selectively supplying fluid to said first fluid film damper.

14. The system for changing the critical speed of a gas turbine engine shaft of claim 12 further comprising:

a second active bearing disposed between said first active bearing and said aft bearing; and

means for changing the support stiffness of said second active bearing.

15. The system for changing the critical speed of a gas turbine engine shaft of claim 14 wherein said means for changing the support stiffness of said second active bearing comprise a second fluid film damper disposed between said second active bearing and said stationary structure and means for selectively supplying fluid to said second fluid film damper.

16. The system for changing the critical speed of a gas turbine engine shaft of claim 14 further comprising:

a second active bearing disposed between said aft end of said core shaft and said aft bearing; and

means for changing the support stiffness of said second active bearing.

17. The system for changing the critical speed of a gas turbine engine shaft of claim 16 wherein said means for changing the support stiffness of said second active bearing comprise a second fluid film damper disposed between said second active bearing and said stationary structure and means for selectively supplying fluid to said second fluid film damper.