Platform motion restraints for freestanding turbine blades

A platform motion restraint for freestanding blades having Christmas tree shaped roots comprising a plurality of L shaped members which fit in circumferential notches machined in the outer portion of rotor steeples, one leg of the L shaped member being forced into the groove under platform portions of the blade to move the blades radially outward and the other leg filling the gap between adjacent blades.

CROSS REFERENCE TO RELATED APPLICATIONS 
This application is related to a co-pending application entitled Turbine 
Blade Assembly, filed Apr. 29, 1991 and given Ser. No. 07/693,256. 
BACKGROUND OF THE INVENTION 
The invention relates to a steam turbine and more particularly to platform 
motion restraints for freestanding turbine blades when the turbine is on 
turning gear. 
When the turbine is operating, the blade roots are held tight in the rotor 
blade grooves by centrifugal force, but when the turbine is on turning 
gear rotating at about 3 revolutions per minute there is negligible 
centrifugal force. Gravity pulls the distal ends of the blades downwardly 
when in the 3 and 9 o'clock positions producing a rocking motion at the 
root due to the required clearances between the blade root and the rotor 
grooves. This repetitive small motion between the blade root and rotor 
groove causes fretting, a combination of wear and corrosion, that leads to 
cracking in the highly stressed root area of the blade. 
SUMMARY OF THE INVENTION 
Among the objects of the invention may be noted the provision of means for 
preventing motion between the blade root and the rotor groove when on 
turning gear to substantially reduce fretting between the blade root and 
the rotor groove while not affecting the natural frequency of the 
freestanding blades. 
In general, a platform motion restraint for freestanding turbine blades 
having a Christmas tree shaped root with platform portion disposed 
adjacent thereto and a turbine rotor having blade grooves which form 
Christmas tree shaped steeples on the outer periphery of the rotor, when 
made in accordance with this invention, comprises a circumferential notch 
extending across the distal end of each steeple and a plurality of L 
shaped members having one leg which fits within said notch and another leg 
which fills a gap disposed between the platform portions of adjacent 
blades.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings in detail and in particular to FIG. 1 there 
is shown a portion of a row of rotating freestanding blades 1 disposed in 
grooves 3 machined in a turbine rotor 5. The freestanding blades 1 have 
Christmas tree shaped roots or root portions 7 and platform or platform 
portions 9 which are curved to provide support for curved freestanding 
blade air foil portions 11 which extends from the platform portions 9. The 
grooves 3 machined in periphery of the rotor 5 are curved to accept the 
curved blade roots 7 forming curved Christmas tree shaped steeples 13. The 
steeples 13 have three circumferential notches extending across the distal 
end of each steeple 13. 
As shown in FIG. 2, a semicircular notch 15 is disposed in the central 
portion of the steeple 13 and registers with a semicircular notch (not 
shown) in the platform 9 for receiving a locking pin 19, which when 
installed prevents axial movement of the blades 1 locking the blade roots 
7 in the grooves 3. The other two notches 21 are rectangular in cross 
section and are disposed adjacent the inlet and exit ends of the steeples 
13. These outboard notches 21 are shaped to receive one leg 23 of an L 
shaped member 25. The one leg 23 is thicker than the notch 21 is deep, so 
that when installed, as shown in FIG. 3, there is an interference fit 
between the bottom of the notch 21, the platform 9 and the one leg 23 
forcing the blade 1 radially outward to generally provide contact between 
the blade root 7 and the steeple 13. 
As shown in FIGS. 3 and 4, another leg 27 of the L shaped member fills a 
gap 29 between the platforms 9, the gap 29 is exaggerated to illustrate 
the other legs 27. The other legs 27 provide tangential contact with the 
platforms 9 and cooperate with the one leg portion 23 of the L shaped 
members 25 to prevent the rocking motion of the blades 1 in the turbine 
rotor 5. A portion of a blade row 31 is shown in FIG. 4 along with the 
direction of rotation of the turbine rotor 5 which is indicated by an 
arrow R. Steam flow to the blade row 31 is from right to left. A group of 
arrows I indicate the inlet steam flowing to the blade row 31 and a group 
of arrows E indicate steam exiting from the blade row 31. 
When assembling the freestanding blades 1 in the turbine rotor 5 a first 
blade 41 is inserted into the groove 3 in the rotor 5 and the one leg 23 
of the L shaped members 25 are forced into the notches 21 on the convex 
side of the platform 9 producing an interference fit. A root portion 7 of 
a second blade 42 is inserted into the adjacent groove 3 on the convex 
side of the first blade 41. The second blade 42 is normally inserted from 
the inlet side of the blade row 31, however if interference of the blade 
air foils 11 is encountered and installation from inlet side is not 
possible then the blades 1 are inserted from the exit side of the blade 
row 31. With the inserted blade in place in the groove 3, two L shaped 
members are forced into the notches 21 from the convex side of the second 
blade 42. Successive blades 1 are installed in the direction of rotation R 
in the same manner. In installing a last blade or closing blade N the L 
shaped member 25 is installed with the one leg 23 forced into the notch 21 
under the platform 9 on the concave side of the first installed blade 41 
on the exit side of the blade row 31. A flat strip 45 is inserted into the 
open notch 21 on the inlet side of the blade row 31 and forced into the 
notch 21 under the platform 9 of the first blade 41 from the concave side 
and extends into the notch 21 over which the last blade N is to be 
inserted. The portion of the flat strip 45 inserted under the platform of 
the first blade 41 is slightly thicker than the portion of another strip 
45 which will reside under the platform of the last blade N. The blade 
root 7 of the last or closing blade N is inserted into the remaining open 
groove 3 from the inlet side of the blade row 31 and forced over the flat 
strip 45. The first blade 41 may have a wedge shaped portion of its 
platform 9 cut away as generally indicated at 47 to allow the closing 
blade N to be inserted into the last open groove 3. If necessary the 
closing blade N may be inserted by scissoring the last blade N with an 
adjacent blade N minus 1. 
With all of the blades 1 in place in the rotor 5, the other legs 27 of the 
L shaped member 25 are upset causing them to expand to fill the gap 
between adjacent blades. Thus restraining the blades in the grooves 3 
substantially in the position they take when the turbine is running at low 
speed even when operating on turning gear and restraining the rocking 
motion which results in fretting. These platform restraints can be used 
alone on smaller blade roots or in conjunction with biasing springs 
disposed between the bottom of the blade root and the bottom of the groove 
when the blade roots are large. When the rotor is operating at normal 
rotating speed the high centrifugal force will loosen the L shaped members 
slightly. Advantageously the natural frequencies of the rotating blades 1 
will not be affected by the L shaped members. The L shaped members 25 also 
advantageously form a barrier to reduce the leakage of steam through the 
gaps between and under the platforms 9. 
While the preferred embodiments described herein set forth the best mode to 
practice this invention presently contemplated by the inventors, numerous 
modifications and adaptations of this invention will be apparent to others 
skilled in the art. Therefore, the embodiments are to be considered as 
illustrative and exemplary and it is understood that the claims are 
intended to cover such modifications and adaptations as they are 
considered to be within the spirit and scope of this invention.