|Title of Invention||
METHOD OF REMOVING DOVETAIL PINS.
|Abstract||A method of removing a pin 16 from a hole 26 comprising a) drilling a first hole 28 of a first diameter in thepin 16 a slected fraction of a length dimension of the pin and attempting removal of the pin; b) if the pin 16 cannot be removed drilling a second hole 32 of a second diameter smaller that the first diameter an extended fraction of the length dimension of the pin; and c) removing the pin. Fig.1.|
|Full Text||BACKGROUND OF THE INVENTION
This invention relates to a method of drilling dovetail pins, as well as auxiliary and
cross keys installed in turbine rotors, to facilitate removal thereof.
Certain steam turbine buckets utilize a finger dovetail configuration to provide
attachment of the buckets to the turbine rotor. Specifically, generally radially oriented
fingers on a bucket mate with opposite mating fingers machined in the rotor. Each
bucket dovetail has three axially extending dovetail pin holes that may be aligned
with corresponding holes pre-drilled in the rotor. After the buckets are fit to the rotor,
the dovetail pin holes in the buckets are aligned with the corresponding holes in the
rotor. After all of the buckets have been installed, the pin holes are final-reamed to
allow the installation of the dovetail pins that hold the buckets in place on the rotor.
During operation in a severe environment, the dovetail pins may become stuck in the
bucket and/or rotor pin holes due to pressure and temperature effects, oxidation, as
well as stepping of the pins due to the buckets pulling radially on the pins as a result
of centrifugal forces. Removal of these dovetail pins for inspection or replacement
can be difficult, time consuming and costly.
Conventional methods of removing dovetail pins involve using a jack, a peening gun,
and/or a powder actuated gun to push the pins out of the holes. If the pins do not
move, and in the event the buckets were to be scrapped, one option has been to cut off
the main section of the buckets and gang mill out the dovetail portion of the buckets
from the rotor fingers. Residual material was removed by hand. If the buckets were
to be saved, however, the dovetail pins were drilled through, using conventional
manual drilling methods, and the leftover shells were removed again by hand.
Another method used was to drill the pins part way through, insert a driver rod, and
then push the pins out with a peening gun or powder actuated gun. Additional
problems with the prior drilling methods involve: (1) a tendency for conventional
drill bits to "walk" off center when drilling through a hard pin, and subsequently
extend through the side of the pin and into the rotor; (2) the extensive time involved in
hand picking out shells left when pins are drilled; (3) if the drilling is completed too deep into the pin, the
driving operation can separate the end of the pin, leaving a partial shell in a hole which needs to be removed
by hand; (4) when the pins are drilled, any initial misalignment of the drilling machine can also result in the
drill bit or EDM electrode extending through the side of the pin and into the rotor. The deeper the drilling, the
more profound the effect of misalignment and the greater the chance of rotor damage.
The disclosure of US Patent 2,165,485 to Jackman is incorporated herein by reference. It describes a drill
guide for quickly positioning a drill to drill a hole along the axis of a broken stud. The drill guide includes a
pair of collars 12 telescopically disposed on a cylindrical shank 4. A drill is disposed in an axial bore 18 of
the shank 4. The drill is operated to drill a guide hole down along the axis of, and into, the broken stud. The
guide role having been made with the drill guide, a larger drill can then be used to drill a hole in the broken
stud to the desired size, after which an extractor can be used to remove the broken stud.
The present invention discloses a method which is superior than the above method, as it involves "drilling a
first hole in the pin of a first diameter", the length of the hole being restricted to about half the axial length
dimension. Jackman also does not disclose drilling a second hole of a smaller diameter as well as drilling
the specified third hole.
BRIEF SUMMARY OF THE INVENTION
The present invention seeks to eliminate the above described problems associated with prior drilling
methods. In the exemplary embodiment, a step drilling procedure is utilized which involves drilling a
relatively large diameter hole (preferably using rapid EDM drilling) in the pin to be removed, for
approximately 1/3 of the pin length. The electrode is then removed and pin removal is attempted via
conventional means. If the pin does not move, the electrode is replaced with a smaller electrode and a
smaller diameter hole is continued to a point beyond the midpoint of the pin. After the electrode is removed,
pin removal is again attempted. If the pin still does not release, a final drilling step utilizes the same smaller
electrode of the previous step to continue to drill the hole to about 3A of the length of the pin. Testing has
demonstrated that at this point, the pressure on the pin will be sufficiently relieved to enable removal of the
pin. It is significant that the step down in electrode size while drilling deeper, accommodates any slight
misalignment of the electrode, reducing the potential for drilling through the side of the pin and into the
Accordingly, in its broader aspects, the present invention relates to a method of removing a pin from a hole
comprising a) drilling a first diameter in the pin a selected fraction of a length dimension of the pin and
attempting removal of the pin; b) if the pin cannot be removed, drilling a second hole of a second diameter
smaller than said first diameter an extended fraction of the length dimension of the pin; and c) removing the
In another aspect, the invention relates to a method for removing a dovetail pin from a
turbine bucket-to-rotor finger dovetail arrangement wherein generally radially
oriented fingers on the bucket and on a rotor wheel are interleaved and where a
plurality of axially oriented dovetail pins secure the buckets to the rotor, the method
comprising a) drilling a first hole in the pin of a first diameter and extending about 1/3
of an axial length dimension of the pin; b) drilling a second hole, continuing from the
first hole, of a second diameter less than the first diameter, and extending about 1/2
the axial length dimension of the pin; c) drilling a third hole, continuing from the
second hole, of a diameter substantially equal to the second hole, and extending about
3/4 the axial length of the pin; and d) removing the pin.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a cut-away perspective view illustrating in simplified fashion a finger
dovetail configuration in a row of buckets on a turbine rotor wheel, with dovetail pins
securing the buckets to the rotor;
FIGURE 2 is a schematic section through a dovetail pin, illustrating a first drilling
step in accordance with the invention;
FIGURE 3 is a view similar to Figure 2, but illustrating continuation through a second
drilling step in accordance with the invention;
FIGURE 4 is a view similar to Figures 2 and 3, but illustrating continuation of the
drilling through a third drilling step in accordance with the invention; and
FIGURE 5 is a view similar to Figure 4 but illustrating how the present drilling
operation accommodates axial misalignment of the drilling electrode.
detailed description of the invention
In Figure 1, a section 10 of a turbine wheel is shown including a plurality of buckets
12 secured to a turbine rotor wheel 14 by a finger dovetail arrangement coupled with
a plurality of axially extending dovetail pins 16. More specifically, each bucket 12
has a plurality (four in the exemplary embodiment) of bucket dovetail projections or
fingers 18 that are received within a plurality of rotor dovetail slots 20 therebetween.
Similarly, the turbine wheel has a plurality of generally radially extending dovetail
projections 22 received in a plurality of bucket dovetail slots 24 therebetween.
During installation, after all of the buckets have been loaded onto the rotor wheel 14,
the buckets 12 are adjusted so that the dovetail pin holes 26 in the rotor and in the
buckets are in axial alignment. Figure 1 illustrates some dovetail pins 16 in place as
well as some empty holes 26 for ease of understanding. The holes 26 are formed in
the bucket dovetail fingers as well as the rotor dovetail fingers, and are axially
alignable. After the buckets have been loaded onto the wheel, the dovetail and rotor
holes are aligned and then final-reamed to allow installation of the dovetail pins 16
which hold the buckets in place on the wheel. This invention relates to a drilling
technique for subsequent removal of the pins 16 for inspection and/or replacement
With reference to Figures 2-4, the first step in the procedure is to drill a relatively
large hole 28, with a diameter of, for example, .44 inch in a pin between about 5 and
about 9 inches in length and having a diameter of about 1/2 - 5/8 inch, preferably
using high speed EDM drilling procedures. This hole or bore is drilled to an axial
length approximately equal to 1/3 of the pin length, terminating at 30. The electrode
is then removed and pin removal is attempted, for example, by driving the rod through
the hole with a driver, peening gun or powder actuated gun. If the pin cannot be
removed, the electrode is replaced with a smaller electrode and drilling of a smaller
hole 32, with a diameter of about .260 inch, is continued to a point about halfway or
just beyond halfway along the length of the pin as shown in Figure 3, terminating at
34. The electrode is removed and pin removal is again attempted, utilizing similar
conventional procedures described above. If the pin still cannot be removed, a third
and final drilling step employs the same smaller electrode and the hole 32 is continued
with the same diameter of about .260 inch from the prior termination point 34 forward
along an extended hole or bore 36 to about 3/4 the length of the pin, terminating at 38.
After the electrode is removed, the pin is again pushed, using conventional methods.
All testing to date indicates that after this third drilling stage, pressure on the pin will
be sufficiently relieved to enable it to be removed.
With reference to Figure 5, it will be appreciated that the stepping down procedure in
hole diameter accommodates slight axial misalignment, minimizing the chance that
the drill bit will pass through the pin and damage the rotor. Thus, the Figure
illustrates a pin 40 with a first hole 42 drilled generally axially of the pin, but with a
slight axial misalignment. The misalignment is exaggerated through one or more
smaller diameter drillings 44 but, because of the step-down feature, the drilled hole
does not break through the side of the pin.
It will be understood that the invention is also applicable for the removal of auxiliary
and cross keys used to secure the last-installed notch buckets and/or blocks used with
pine tree or other bucket dovetail configurations.
While the invention has been described in connection with what is presently
considered to be the most practical and preferred embodiment, it is to be understood
that the invention is not to be limited to the disclosed embodiment, but on the
contrary, is intended to cover various modifications and equivalent arrangements
included within the spirit and scope of the appended claims.
1. A method for removing a dovetail pin(16)from a turbine bucket-to-rotor finger
dovetail airrangement wnerein generally radially oriented fingers(18, 22) on the bucket
(12)and on a rotor wheel (14)are interleaved and where a plurality of axially oriented
dovetail pins(16)secure the buckets to the rotor, the method comprising.
a) drilling a first hole (28) in the pin of a first diameter and extending about 1/3 of an
axial length dimension of the pin;
b) drilling a second hole(32) continuing from said first hole, of a second diameter less
than said first diameter, and extending about 1/2 the axial length dimension of the pin;
c) drilling a third hole (36) continuing from said second hole, of a diameter
substantially equal to said second hole, and extending about 3/4 the axial length of the
d) removing the pin(16)
2. The method of claim 1, wherein steps a), b) and c) are carried out utilizing EDM
3. The method of claim 2 wherein the pin (16)has a diameter of between about 1/2
and 5/8 inch, and the first hole (28) has a diameter of about .4 inch.
4. The method of claim 3 wherein the second hole(32)has a diameter of about .26
5. The method of claim 4 wherein the third hole (36) has a diameter substantially
equal to said second hole.
A method of removing a pin 16 from a hole 26 comprising a) drilling a first hole 28 of a
first diameter in the pin 16 a selected fraction of a length dimension of the pin and
attempting removal of the pin; b) if the pin 16 cannot be removed, drilling a second
hole 32 of a second diameter smaller than the first diameter an extended fraction of the
length dimension of the pin; and c) removing the pin
|Indian Patent Application Number||423/CAL/2002|
|PG Journal Number||06/2008|
|Date of Filing||16-Jul-2002|
|Name of Patentee||GENERAL ELECTRIC COMPANY|
|Applicant Address||One River Road, Schenectady,New York 12345, united States of America. A corporation of the State of New York.|
|PCT International Classification Number||B23h 9/14|
|PCT International Application Number||N/A|
|PCT International Filing date|