Note: Descriptions are shown in the official language in which they were submitted.
1 31 77~C) 47812-1
NOTCH GAUGE TO L
This invention relates to a tool and method for
measuring turbine blade outer shroud notch wear in a jet
engine. It is particularly useful for measuring shroud notch
S wear in the Pratt and Whitney JT8D *engine u~ed in the 80eing
72~, 737 and Douglas DC9~ or MD 80*series of aircraft.
Turbine rotors in modern jet engine~ and other turbine
equipment generally comprise a circular array of blades
extending outwardly from a rotatable shaft, the outer ends of
the each blade being formed into outer shroud portion~ that
interfit with the shroud portions of adjacent blades to
create an annulu~ about the blades. The shroud portions are
dimensioned to interfit together along joint lines called
notches such that the shroud poxtions provides a tight
external ring about the blade~.
The tool and method of the pre~ent invention wa~
developed particularly to address the problem of measuring
shroud notch wear in Pratt ~ Whitney JT8D engines. The~e
engines experience wear of the 92 third xtage turbine blade
outer shroud notches. If undetected the wear will result in
failure of one or more blades and consequent damage to the
downstream turbine component~. Repair costs for this type of
failure can exceed ~300,000.
*Trade-mark
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The engine manufacturer's recommendation to avoid this
type of failure is to dismantle the -turbine section aE-ter a
set period of service to physically measure wear on the
blades. Dismantling and examining of several engines should
provide guidance for the degree of wear in other engines, but
experience has shown -that the wear rate of the notches is
random. Some engines may be in danger of failure before a
shop visit is due while other engines are able to con-tinue in
service long after the scheduled service periocl before notch
wear reaches a critical state.
At present, due to the random nature of -the shroud notch
wear a lot of engines are dismantled unnecessarily. Since an
engine shop dismantling and examination of -the third stage
turbine blades can cost over $100,000, there is obviously a
lS need for a tool or method that will allow for -testing o
shroud notch wear without dismantling an engine at great
expense.
Prior art method~ for measuring wear in turbine blades
are known. For example, ~nitecl States Patent 4,741,205 to
Keller teaches a method of measuxing shroud wear on the
integrally shrouded rotating blades of a steam turbine. This
method involves pre-forming special wedge notches in the
shroud portions adapted to accept a wedge member that is used
to apply a wedging force to move two adjacent shroud portions
apart. The distance the shroud portions are moved apart is
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measured ancl provides an indication of wear on the shroud
por-tions. This process requires that special wedgin~ notches
be formed in -the rotor blades. These wedge notches are used
only for measuring shroud portion wear and are a completely
different item from the standard no-tches found in the shroud
por-tions of the Pra-tt and Whitney JT8D jet engine. Keller's
method therefore involves modifying an existing shroud
design. In addition, it is necessary that -the turbine be
completely dismantled so that -the distance between shroud
portions can be measured as they are wedged apart.
There is a need for a -tool and a method of measuring
shroud notch wear that can be used without dismantling an
engine and -that does not involve modifying the existing
shroud portion structure.
Accordingly, the present invention is a tool for
measuring turbine blade outer shroud notch wear in situ in a
turbine having multiple stages of turbine blades, each stage
being arranged in a circular array comprising:
a longitudinal member having first and second ends;
means for applying a separating force between two
adjacent turbine blades at said first end;
positioning means mounted about said longitudinal member
intermediate said first and second ends;
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driving means at said seconcl end and adapted to engage a
device for applying a known torque force;
whereby inserting said first end of said longitudinal
member through a first outer s-tage of blades and into -the
interior of the turbine such -that said positioning means
abuts said first outer stage of blades to place said means
for applying a separa-ting force between two acljacent blades
of an interior stage of blades, and applying a rotary force
to said tool acts to separate said adjacent blades, -the
measure of the magnitude of the force required to separate
the blades serving as an indication of outer shroud notch
wear.
The present invention also inclucdes the method of
measuring outer shroud notch wear comprising the steps of:
providing means for applying a separating force between
two adjacent turbine blades;
positioning said means for applying a separating force
between two adjacent blades in a fully assembled turbine
adjacent said outer shroud;
applying a -torque force to said means for providing a
separating force to force apart said adjacent blades;
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measuring said torque force to determine an indication
of outer shroud notch wear; and
repeating the above steps at several different positions
around the circular arra~ of blades.
The apparatus of the present invention is shown by way
of example in the accompanying drawings in which:
Figure 1 is a section view -through a JT8D -turbine
showing the tool in use;
Figure 2 is a pictorial view showing the turbine blade
outer shroud arrangement.
Figure 3 is an elevation view of the tool; and
Figure 4 is a plan view of the tool.
Referring to Figure 1, there is shown a cross-section
through a typical jet engine turbine, in this case, a Pratt &
Whitney JT8D engine. There are a plurali-ty of turbine stages
5, each stage including a circular array of blades 6 arranged
about a rotatable main shaft 7. Associated with each set of
blades 6 are sets of stationary vanes B that the blades
rotate between. In Figure 1, there are 3 stages shown:
fourth stage blades and vanes 4, third stage blades and vanes
3 and second stage ~lades and vanes 2.
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As best shown in Figure 2, each turbine blade 6 is
formed at its outer end 9 into a shroud por-tion 10. The
shroud portions of the each blade interfit to form a tight
annulus that surrounds each stage of turbine blades. The
shroud portions are shaped to interfi-t along a shroud notch
13. With use, it is these notches that become worn leading
to flexing of the turbine blades 6 which can result in
failure of one or more blades. Par-ticularly in Pra-tt &
Whitney JT8D jet engines, it is the third stage turbine blade
outer shroud notches that are susceptible to wear.
The tool of the present application is designed to allow
for measurement of outer shroud notch wear in a turbine
without requiring dismantling of the turbine as is presently
the case.
Referring to Figures 3 and 4, the tool comprises a
longitudinal member 12 having first and second ends 14 and 16
respectively. Preferably, longitudinal member 12 is a
quarter inch diameter steel rod. For use in the JT8D engine,
member 12 is 20 inches long.
Adjacent first end 14 there is attached means for
applying a separating force between two adjacent turbine
blades comprising a gauge pin 18 welded across -the end of the
longitudinal member to form a "T". For use in the JT8D
engine, pin 18 is preferably a cylindrical member one quarter
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inch in diame-ter and .9 inches in length. The pin is rounded
at both ends and has a cen-tral hole 19 drilled across the
width of the cylinder. First end 14 is formed with a
pro~ection 20 and gauge pin 18 is pinned onto this
projec-tion. Pin 18 is also welded to the longitudinal member
to ensure it is firmly attached.
Positioning means are mounted on longitudinal member 12
intermediate ends 14 and 16. In the illustrated embodiment,
the positioning means comprises a disc 25 having a central
hole through which the longitudinal member is fitted. For
use in a JT8D engine, the disc is three quarter inches in
diameter is located 3.880 inches from gauge pin 18.
Preferably, the face of disc 25 facing toward second end 16
is formed with a groove 27 across a diameter of the disc
intersecting the central hole through the disc. This groove
is aligned with gauge pin 18 and is filled with white paint
or a similar easily visible material and provides an
indication of the rotational angle of the tool.
At second end 16, there is mounted driving means
comprising a hexagonal nut 30. Nut 30 is engageable by a
standard drive socket of a torque wrench or other torque
measuring or torque limiting tool. Alternatively,
longitudinal member 12 can be formed from a piece of
hexagonal bar. If appropriate, the section of the tool
between gauge pin 18 and disc 25 is be machined down such
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that -the section is sufficien-tly narrow to be inserted
between the blades and vanes of a turbine.
Referring to Figure 1, the tool is used in the following
manner on a fully assembled Pratt & Whitney JT8D engine:
First end 14 is inserted through the fourth stage blades
and vanes 4 adjacent the outer shroud location until disc 25
of the positioning means abuts the rear or trailing edges 32
of the fourth stage blades. Disc 25 is positioned on
longitudinal member 12 such that when the disc abuts the
fourth stage blades, gawge pin 18 is positioned between a
pair of adjacen-t third stage blades. Gauge pin 18 is
initially introduced between the blades in a radially
aligned positioned so that it does not contact the blades.
Groove 27 on disc 25 provides an indication of the angle of
the gauge pin once the gauge pin disappears behind the fourth
stage blades and vanes. Obviously, the distance between
disc 25 and gauge pin 18 can be adjusted in order to measure
shroud wear in other stages of blades or in different engines
that space apart the stages differently. As long as the rear
or trailing edge of the foremost stage of blades is used as a
reference point, positioning the gauge pin between the
appropriate stage of blades is easily accomplished.
In the present example, once the gauge pin is in place
between a pair of adjacent third stage blades a torque
measuring or tor~ue limiting device such as a torque wrench
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or a torque limiting screw driver :is app:Lied to hexagonal nut
30 which pro-trudes from the rear of the engine. The tool is
rotated through 1~0 degrees thereby causing the rounded ends
of gauge pin 18 to engage and force apart the adjacent third
stage turbine blades. The shroud notches 13 adjacent -the
turbine blades being forced apart are likewise forced apart.
A measure of the ~orce required to rotate the tool through
180 degrees is taken. If the blade shroud notches are
unworn, considerable torque will be required -to rota-te the
tool. If notch wear has occurred, the -torque required to
rotate -the tool will be less. A relationship between the
torque required to rotate the tool between two adjacent
blades and the degree of wear on the blade outer shroud
notches must be previously established. Once known, this
relationship can be used to determine outer shroud blade
notch wear simply by measuring the torque required to rotate
the tool. The above procedure is repeated at several
positions around the turbine to gauge the accumulated wear on
all 92 of the third stage turbine blades.
The tool and method of the present invention allow the
jet engines being -tested to be examined while in place on the
aircraft without disassembly. Engines can remain safely in
service under periodic monitoring using the present apparatus
and method until a normally scheduled shop visit or until the
tool and method indicate that blade notch wear is excessive
and corrective shop action is required. This procedure
thereby avoids the expensive, time consuming and unnecessary
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operation of disassembly and examination of -the turbine
blades when outer shroud notch wear is within acceptable
limits.
Although the present invention has been described in
some detail by way of example for purposes of clarity and
understanding, it will be apparent that certain changes and
modifications may b~ practiced within the scope of the
appended claims.
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