Variable turbine vane actuation mechanism having a bumper ring

1. A variable vane actuation assembly for gas turbine engine having a turbine section with a plurality of rotatable stator vanes, the variable vane actuation assembly comprising:

an engine casing configured to encase the plurality of rotatable stator vanes;

a unison ring disposed concentrically with the engine casing;

a bumper ring disposed concentrically between the engine casing and the unison ring;

a radial spline connection extending from the engine casing and joining with the bumper ring, wherein the radial spline connection permits the bumper ring to float radially with respect to the engine casing, but prevents the bumper ring from rotating circumferentially with respect to the engine casing; and

a plurality of bumper shims positioned between the unison ring and the bumper ring to limit deformation of the unison ring.

2. The variable vane actuation assembly of claim 1 wherein the radial spline connection comprises:

a flange extending radially from the engine casing;

radial slots extending into the flange; and

lugs extending axially from the unison ring and configured to slide within the radial slots.

3. The variable vane actuation assembly of claim 2 wherein the radial spline connection further includes a washer plate connected to the lugs to prevent the bumper ring from axially disengaging the flange.

4. The variable vane actuation assembly of claim 3 wherein the flange extends radially inward from the engine casing.

5. The variable vane actuation assembly of claim 2 wherein the bumper ring comprises a C-shaped cross section having an inner bumper and an outer bumper and wherein the unison ring is positioned between the inner and outer bumpers.

6. The variable vane actuation assembly of claim 5 wherein the bumper shims are positioned on inner and outer surfaces of the unison ring to mate with the inner and outer bumpers of the bumper ring.

7. The variable vane actuation assembly of claim 6 and further comprising hardfacing applied to inner and outer surfaces of the plurality of bumper shims and the inner and outer bumpers of the bumper ring.

8. The variable vane actuation assembly of claim 1 wherein the radial spline connection comprises:

holes extending radially through the bumper ring; and

pins extending radially from the engine casing and through the holes.

9. The variable vane actuation assembly of claim 8 wherein the pins extend radially outward from the engine casing.

10. The variable vane actuation assembly of claim 1 and further comprising a plurality of actuation arms extending from the unison ring to connect to outer diameter ends of the plurality of rotatable stator vanes.

11. The variable vane actuation assembly of claim 1 wherein there is a clearance between the plurality of bumper shims and the bumper ring of approximately 0.010 inches (approximately 0.0254 cm) at temperatures generated within the engine at idle operation.

12. A bumper assembly for a variable vane actuation mechanism, the bumper assembly comprising:

an annular engine casing configured to enshroud outer diameter ends of variable vanes;

projections extending radially from the engine casing to form an annular array;

a bumper ring comprising:

an annular body concentrically positioned with the annular array of projections; and

receptacles for receiving the projections;

an annular unison ring comprising:

a first circumferential surface for engaging the bumper ring; and

bores for connecting with actuation arms of the variable vanes; and

bumper shims positioned on the first circumferential surface between the bores, and between the first circumferential surface and the bumper ring such that the bumper shims inhibit deformation of the unison ring.

13. The bumper assembly of claim 12 wherein:

the projections comprise a plurality of tabs arranged to form a plurality of slots between the tabs, wherein the tabs are formed from an annular flange extending radially from the engine case; and

the bumper ring comprises:

a C-shaped annular bracket having an interior channel into which the unison ring is receivable; and

a plurality of axial lugs positioned within the plurality of slots in the annular flange.

14. The bumper assembly of claim 13 wherein the annular flange extends radially inward from the engine case.

15. The bumper assembly of claim 12 wherein:

the projections comprise a plurality of pins extending from the engine case; and

the receptacles comprise a plurality of holes in the annular body configured to receive the plurality of pins.

16. The bumper assembly of claim 15 wherein the plurality of pins extend radially outward from the engine case.

17. The bumper assembly of claim 12 and further comprising hardfacing applied to mating surfaces of the bumper shims and the bumper ring.

18. The bumper assembly of claim 12 wherein the projections are spaced approximately 1.0 inch (approximately 2.54 cm) apart along the circumference of the engine casing.

19. The bumper assembly of claim 12 wherein the unison ring, the bumper ring and the engine casing are all comprised of a nickel-based alloy.

20. A method for maintaining circularity of a unison ring in a variable vane assembly of a gas turbine engine, the method comprising the steps of:

forming a plurality of projections on an engine casing that extend in a radial direction;

positioning a bumper ring having a plurality of radial openings between the engine casing and the unison ring such that the plurality of projections engage the plurality of radial openings;

positioning a bumper shim between the unison ring and the bumper ring;

thermally deforming the engine casing, the unison ring and the bumper ring during operation of the gas turbine; and

floating the bumper ring on the plurality of projections such that the bumper shim engages the unison ring to maintain circularity of the unison ring, and to prevent binding of the unison ring with the engine casing.

21. The method of claim 20 wherein the step of floating the bumper ring further comprises the steps of:

permitting radial expansion of the bumper ring along the plurality of projections; and

preventing rotation of the bumper ring with respect to the engine casing with the plurality of projections.

22. The method of claim 20 wherein the thermally deforming comprises radial expansion and contraction.