FIELD OF INVENTION
The present invention relates to a fixture for checking of aerofoil sections of the banana
type hollow guide blade of low pressure steam turbine and a method for the same.
More particularly, the invention relates to the checking of the aerofoil cross section of
the blade to achieve the blade sectional dimensions within the specified tolerances of
the blades.
BACKGROUND OF THE INVENTION AND PRIOR ART
As this banana type of hollow guide blades are being designed and developed in the
recent past only, the methods established for the above inspection are not reported
readily. The patent search has been carried out on this subject, but the information
related to this invention is not directly available. The following patents
(US2002197156al, US006272861B1, 815236, U1332679 etc..) related to the present
invention were checked and found there is no mention of this kind of inspection fixture.
Normally the Steam or Gas Blades are inspected with the help of template type
inspection fixtures, by holding the blade roots, such as T-root, Fir-Tree root, Fork root
etc. which are of prismatic nature and the tolerances are closer on these root surfaces.
But in case of Banana type hollow guide blades there is no prismatic root available to

hold the blade while inspecting the aerofoil section of the blade and hence the methods
available are not suitable to inspect the banana type hollow guide blades.
OBJECTIVES OF THE INVENTION
Therefore it is an object of the invention to propose a fixture for checking of aerof,
sections of the banana type hollow guide blade of low pressure steam turbine and a
method for the same which is capable of checking the aerofoil sections of the blades
within the specified tolerances.
Another object of the invention is to propose a fixture for checking of aerofoil sections
of the banana type hollow guide blade of low pressure steam turbine and a method for
the same which is able to inspect the guide blade without any prismatic root and shroud
ends.
A further object of the invention is to propose a fixture for checking of aerofoil sections
of the banana type hollow guide blade of low pressure steam turbine and a method for
the same which is capable of inspecting the profile with tapered pins when there is no
machined/reference surface available on the blade.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1 - shows the fixture and the blade from suction side of the blade.

Fig.2 - shows the fixture and the blade from the pressure side of the blade.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
The present invention is to inspect the Banana type hollow guide blades of the Steam
Turbine LP Module, where the aerofoil cross sections of the blade are to be checked for
the deviations so as to achieve the blade sectional dimensions within the specified
tolerances of the blades.
Banana type hollow guide blade is a stationery blade of Low pressure (LP) module of
the Steam Turbines of power generating equipment. This guide blade has typically
deigned geometrical aerofoil profiles with bent stacking axis. Moreover these blades do
not have any prismatic root such as T-Root, Fir Tree, Fork type etc. which will be
normally found in all the other types ST/GT blades. The prismatic roots will have
stringent tolerances though the profile will have wider tolerances. Hence the blades
with root can be held in a fixed position and repeatable profile dimensions can be taken
within the root tolerances, which are very closer. But in case of Banana blade, without
any prismatic root, which has got profiled 3-D surface throughout its body, will have
wider profile tolerances throughout its length. Banana blade ends cannot be held in a
fixed position, because of its wider tolerance bands. Hence, the middle sections can be
inspected in the inspection fixture, with free floating ends.
For the above purpose an Inspection fixture has been designed and developed to
facilitate checking of the blade aerofoil sections. In this fixture, holding of the blade is

done, on the profile itself, at the two aerofoil sections with the help of tapered pins.
The gap of 4mm is given between the blade nominal profile and the aerofoil sectional
template profiles to accommodate the tapered pins and cylindrical pins. The cylindrical
pins are used to check the deviations between actual to nominal profiles of the blade.
The profile inspection Fixture (F) and the operation of the fixture for checking the
nominal blade sectional aerofoil profile requirements, which is main claim of this
invention, is described with the help of line diagrams (Fig.l ad Fig.2). The banana type
hollow guide blade (1), which is to be inspected for its sectional profiles, has got suction
(2) and pressure (3) aerofoil surfaces constructed by the aerofoil sections, suction side
(4) and pressure side (5), at different levels of the blade height. These sectional
dimensions are important design parameters, which are to be inspected and maintained
for the validation of the turbine blade. For which the inspection methodology is deviseu
to suit to the configurations of the blade. Since there is no flat surface available in the
blade construction, the proposed fixture should have necessarily holding arrangements
to mount the blade, on the blade aerofoil surface itself.
The inspection fixture is designed with a base plate (6) to mount the support plate (7),
the end butting plate (8) and the suction side (9) sectional templates and pressure side
(10) sectional templates. The end butting plate is acting like a stopper, during operation
this will stop the blade movement in lateral way and it will also match to the end face of
the blade by which the blade will be in position for checking requirements. The support
plate (7), which is in T shape, will hold the templates in position by arresting the motion
of the templates in vertical directions. The templates are also butted against the face of

base plate by which the template movement is arrested in horizontal direction. The
templates are located on the base plate with the help of three cylindrical pins (14), to
arrest the lateral movements of the templates.
The mounting arrangement of the banana blade in the fixture is done by holding the
aerofoil surface of the blade at two extreme positions with the help of sectional
templates and tapered pins (11). The mounting is done with help of tapered pins to
accommodate the variations of the actual sectional profile of the blade. The tapered
pins are positioned at four locating points on suction and pressure surfaces of the
blade. With this arrangement the blade will be held in position firmly for carrying out
further inspection checks of the blade. The templates are designed in such a way that
they form a constant 4 MM gap (13) between the template profile and the blade
sectional nominal profile. By this construction the deviations on trie sectional profiles at
various sections can be measured with the help of cylindrical pins (14) of different sizes
by checking in go-no go procedure. When there is excess material on the blade, the
gap becomes less and when there is less material on the blades it becomes more. By
the above procedure the deviations in the blade aerofoil sections can be measured.

WE CLAIM
1. A fixture (F) for checking the aerofoil sections of the banana type hollow guide
blade of low pressure steam turbine comprising;
a base plate (6);
a support plate (7) mounted on the base plate (6) for supporting a plurality of
templates (9, 10) and for arresting the movement of templates in vertical
direction;
an end butting plate (8) butting one end face of the blade (1) for keeping the
blade in position required for checking;
a plurality of sectional templates (9, 10) butted against the face of base plate (6)
for arresting the movement of templates in horizontal direction;
characterised in that,
the said sectional template and tapered pins (11) are disposed in the fixture for
holding the aerofoil surface of the blade at two extreme positions for the
mounting arrangement of the banana blade (1) in the fixture wherein the said
tapered pins (11) are disposed at four locating points of suction (2) and pressure
(3) surface of the blade (1) for accommodating the variations of the actual
sectional profile of the blade (1).

2. A fixture as claimed in claim 1, wherein the end butting plate (8) is disposed for
stopping the lateral movement of the blade (1).
3. A fixture as claimed in claim 1, wherein the templates (9, 10) are located on the
base plate by a plurality of cylindrical pins (14) for arresting the lateral
movement of the templates (9, 10).
4. A fixture as claimed in claim 1, wherein the templates (9, 10) are designed for
maintaining a constant gap (13) of 4 mm between the template profile and the
blade sectional nominal profile.
5. A method for checking the aerofoil sections of the banana type hollow guide
blade of low pressure steam turbine, the said method comprising the steps of:
holding the templates (9, 10) in position by the support plate (7) to arrest the
motion of the templates (9, 10) in vertical direction;
butting a plurality of templates (9, 10) against the face of the base plate (6);
locating the said templates on the said base plate by three cylindrical pins (14) U
arrest the lateral movement of the said templates wherein the banana blade is
mounted in the fixture (F) by holding the aerofoil surface of the blade at two
extreme positions with sectional templates (9,10) and tapered pins (11) and the

tapered pins (11) are positioned at four locating points on suction and pressure
surfaces of the blade (1) to position the blade (1) firmly to perform checking.

A fixture (F) for checking of aerofoil sections of the banana type hollow guide blade of
low pressure steam turbine consists of a base plate (6), a support plate (7) mounted on
the base plate (6) for supporting a plurality of templates (9, 10) and for arresting the
movement of templates in vertical direction. An end butting plate (8) butts one end face
of the blade (1) for keeping the blade (1) in position for checking. The templates (9,
10) are located on the base plate by a plurality of cylindrical pins (14). The said
templates are butted against the face of the base plate for arresting the movement of
the said templates in horizontal direction. The fixture (F) is disposed with the sectional
templates (9, 10) and tapered pins (11) for holding the aerofoil surface of the blade at
two extreme position for the mounting arrangement of the banana blade (1) in the
fixture (F) when the said taper pins (11) are disposed at four locating points of suction
(2) and pressure (3) surfaces of the blade (1) for accommodating the variations of the
actual sectional profile of the blade (1).