FIELD OF INVENTION
The invention generally relates to a pitch traversing mechanism for an aerodynamic
probe. In particular, it relates to an angle positioning device for aerodynamic
probes in the measurement of pressures in turbo machines like compressors and
turbines. The angle positioning is towards the pitch plane of the angle
measurement.
BACKGROUND OF INVENTION
A pitch traversing mechanism is developed for pitch angle setting of aerodynamic
probes. These probes are used for aerodynamic testing of turbomachines like
compressor, turbine and fans. The probes are traversed at inlet and outlet of the
stage of the turbomachine to generate the performance parameters like efficiency,
pressure ratio, temperature ratio and mass flow. The specific probe parameters
investigated are total pressure, static pressure, velocity and flow angle. Probe with
three sensing hole are used for measuring velocity at two planes and the yaw angle.
Probes with five are higher sensing holes are used for velocity measurements and
flow angles in pitch and yaw planes. The variation in velocity in pitch direction
effects the performance of the turbomachine where the inlet and the out let flow
pattern are skewed. The present invention can set the aerodynamic probe in pitch
direction with a high degree of accuracy close 0.05° during the probe calibration.
Aerodynamic probe calibrated with a device like pitch traversing mechanism gives
the higher accuracy in the measurement of efficiency and other aerodynamic
parameters.
The pitch angle setting of aerodynamic probes during calibration should be carried
out with high accuracy of pitch angle to avoid shifting of probe from the
measurement location and give erroneous measurement of velocity and pitch angle.
The difficulty of setting the pitch angle and measurement of flow angles are
encountered in the traversing mechanism based on lead screw concept. This is due
to the inherent backlash in the system. To avoid this backlash and inaccuracy
inherent in the lead screw mechanism alternative mechanisms were studied. The
invention came out with a combination of two such alternative mechanisms viz.
An indexing head and worm-worm wheel. In this mechanism the accuracy is
improved with a least count of 0.05°. Backlash is reduced with the spring loaded
indexing device coupled with a worm- worm gear mechanism. Probe calibration
charts generated with the probe mounted in the above pitch traversing mechanism
gave a more accurate picture of the flow parameters in and around the
turbomachine and they are more reliable in the estimation of the efficiency.
US Patent 6, 058, 769 discloses a probe positioning system. This system includes a
frame having forward portion for attachment to fluid flow tunnel mechanism. An aft
frame portion aligned with the forward frame portion. The probe supporting strut
mounted on the base can move the probe in the radial direction.
US 5, 929, 331 describes the development of a pressure probe with 18 holes for the
measurement of three velocity components and pressure measurements.
Determination of pitch and yaw angle does not require a traversing mechanism.
However, a complicated a traversing mechanism is used for the yaw and pitch angle
mechanism.
Document US 3, 977, 249 (A) provides a probe traversing mechanism for
measurement of three dimensional investigation of fluid flow in a turbomachine. The
probe with a five hole sensing head is mounted on a movable probe finger. A
movable probe shaft supports the probe finger. The probe finger is movable in a
radial, circumferential and the polar direction. This is referring to a standard
orthogonal co ordinate system the axis can be defined corresponding to X,Y and Z
axis. Probe support member is rotatable and slidable within the opening in the
second rotatable member. The probe finger is pivotally mounted on the probe
support member. The probe finger is displaced from first position to second position
on the Z axis by movement of the probe support along the Z axis. The rectilinear
displacement of the probe finger on the Z axis is defined as being in the redial
direction. The probe support member is also rotatable about its axis. Thus the probe
support member cab be angularly displaced from a first angular position to a second
angular position. The angular displacement thus generated is commonly referred as
yaw or circumferential angle. In addition to the radial movement and the
circumferential movement of the probe support member the probe finger member is
angularly displaceable from first angular position to a second position. The
displacement of the probe finger is defined as polar or pitch movement.
Document US 3, 731, 547 provides a traversing mechanism capable of traversing a
number of differently sized lead screws which carry the probe. Accuracy of the
traverse is independent of the size of the traverse .The patent also mentioned how
the mechanism can be adopted to traverse a remotely situated probe.
Document US 3, 699, 811 discloses an instrument for providing for measurements of
both the magnitude and direction of flow velocities in a flow field. A probe is
oriented in the flow field by two servo systems which impart two angular motions to
the probe to align it with the direction of the flow. The two angular motions are
indicative of the direction of flow and the magnitude of the flow velocity is then
measured by conventional means.
In the prior art documents described above, the pitch movement and measurement
has been carried out in the predominantly with a lead screw mechanism or its
derivative. The measurement accuracy in the case of probe angular measurement
with a lead screw is not very high because of the back lash present in the screw.
There is therefore, a need for a pitch traversing mechanism for an aerodynamic
probe capable of attaining angular movement and measurement with high accuracy.
SUMMARY OF THE INVENTION
The main object of the present invention is to provide a pitch traversing mechanism
for an aerodynamic probe capable of carrying out probe angular measurement much
more accurately than what is possible with prior art systems.
This object is achieved in the present invention by carrying out the angular
movement with a combination of mechanisms like indexing head and worm-worm
wheel resulting in a least count of 0.05° such combination of mechanisms is not
reported in the prior art. The least count of the pitch traversing i.e. the smallest
angle the probe can be rotated is substantially smaller than that possible in a lead
screw based pitch angle displacement.
Thus the present invention provides a pitch traversing mechanism for an
aerodynamic probe for pitch angle rotation and measurement thereof, said pitch
traversing mechanism comprising: a combination of an indexing head and worm-
worm wheel, thereby providing a high measurement accuracy.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention can now be explained with the reference to the accompanying
drawings where
Figure 1 shows an impeller passage geometry.
Figure 2 shows the pitch traversing mechanism of the present invention.
Figure 3 shows a photograph of the pitch traversing mechanism
with the probe in position.
Figure 4 shows a photograph of the pitch traversing mechanism with
Vernier scale.
DETAILED DESCRIPTION
The relationship between the performance parameter like loss coefficient or
efficiency and traversing pitch angle is described below.
The performance parameter of the centrifugal compressor is based on the total
pressure loss incurred in the impeller and the diffuser. Ten components of total
pressure loss had been modeled by Aungier (Centrifugal Compressor By R.H Aungier
ASME Press page 81).
Out of these ten loss parameter components the sixth one viz. mainly hub to shroud
loading loss is dependent on the pitch angle being measure by the aerodynamic
probe at the inlet and the outlet of the impeller. Thus, the pitch angle measured by
the aerodynamic probe and pitch traversing mechanism influences the total pressure
loss and efficiency of the compressor stage.
Meridional impeller passage geometry is shown in Fig.l along with pitch angle at
exit.
Pertaining to a meridian or southern zone
The hub to shroud loading loss mentioned above is estimated from the following
equation
(PHs=(Km*b*W/W1)2/6
Where, Average velocity W = (Wi+W2)/2
Average passage width b = (bl+b2)/2
Stream line curvature Km = (a a - a Ci) / L
L is the mean stream line meridional length.
a a and a ci are the exit and the inlet pitch angles.
The above equation indicates that the pitch angle, a a and a Ci measured by the
probe and the traversing mechanism determines the hub to shroud loading loss, q>
hs ¦ This loss component cp Hs in turn influences the efficiency of compressor stage,
which is the major performance parameter. In addition the pressure ratio and the
temperature ratio of a compressor stage is dependent on the blade loading and the
loss coefficient.
As shown in Figure 2, a knob 1 is fitted onto a pointer 2. The pointer 2 is provided
with a spring 3 so that the spring loaded pointer 2 can sit tightly in one of the slots
in the indexing wheel 9. The knob can be lifted up from the slot with a lifting
pressure and then it is free to rotate with worm shaft 5 to which it is firmly
connected through a pointer support bracket 4. The worm shaft 5 can rotate with
respect to its own vertical axis, because of the planetary motion of the knob and the
pointer 2 around the shaft 5. The indexing head 9 below the pointer has got 60
slots giving a least turning of the pointer by l/60th of 360° or 6° for an angular
movement of the pointer 2 from one slot to the adjacent one. The worm shaft 5 is
mounted on two ball bearings 7, 8 one located in the pointer side 7 and the other
on the bracket site 8.
A worm 6 fitted to the worm shaft 5 rotates with the shaft and is engaged to a
worm gear 12 which rotates with respect to an axis perpendicular to the worm
shaft 5. The worm gear is mounted on a shaft 13 and is supported by two ball
bearings 14, 15 one inner 14 and the other outer 15 with reference to the worm
gear. One rotation of the worm shaft gives an angular movement of the worm gear
by l/120th of a revolution. Since the number of teeth in the worm gear is 120 and
the worm has got only one start.
The individual effect of one slots movement of the pointer on the indexing head is
as follows.
> Rotation of the worm shaft by 0 = 360°/ 60°= 6°
> Turing of the worm gear by 8 /120°
Thus the combined effect of pointer in the indexing head and the worm - worm gear
is of
360°/ 60° X 1/120° = 0.05°
The rotation of the worm gear effectively turns the probe 19.
The circular scale and vernier 18 gives the reading for the pitch angle movement of
the probe. The bearings 16,17 are in turn mounted on the main bracket bottom and
top.
The prior art documents described above refer to the probe angular measurement
predominantly with a lead screw mechanism. This measurement accuracy is not so
high because of the backlash present in the screw. The present invention refers to
a pitch angular movement with a combination of indexing head and worm- worm
wheel. The least count of the angular system works out to 0.05° which is a much
accurate value compared to the lead screw type of mechanism. None of the prior
art material obtained through a patent search refers to a combination of
mechanisms to get a best least count.
Figures 3 and 4 show the photographs of the pitch traversing mechanism with probe
in position and with Vernier scale respectively.
WE CLAIM
1. A pitch traversing mechanism for an aerodynamic probe for pitch angle
rotation and measurement thereof, said pitch traversing mechanism
comprising: a combination of an indexing head and worm-worm wheel,
thereby providing a high measurement accuracy.
2. The pitch traversing mechanism as claimed in 1, wherein said indexing head
is provided with 60 slots giving a least turning of 6° for an angular movement
of the pointer of the indexing wheel.
3. The pitch traversing mechanism as claimed in preceding claims, wherein the
worm gear is provided with 120 teeth for giving an angular movement of the
worm gear by 1/120* of a revolution with one rotation of the worm shaft.
4. The pitch traversing mechanism as claimed in the preceding claims, wherein
the combined effect of the pointer in the indexing head and the worm-worm
gear is 0.05°.
5. A pitch traversing mechanism for an aerodynamic probe, substantially as
herein described and illustrated in the figures of the accompanying drawings.

The main object of the present invention is to provide a pitch traversing mechanism
for an aerodynamic probe capable of carrying out probe angular measurement much
more accurately than what is possible with prior art systems.

This object is achieved in the present invention by carrying out the angular
movement with a combination of mechanisms like indexing head and worm-worm
wheel resulting in a least count of 0.05° such combination of mechanisms is not
reported in the prior art. The least count of the pitch traversing i.e. the smallest
angle the probe can be rotated is substantially smaller than that possible in a lead
screw based pitch angle displacement.
Thus the present invention provides a pitch traversing mechanism for an
aerodynamic probe for pitch angle rotation and measurement thereof, said pitch
traversing mechanism comprising: a combination of an indexing head and worm-
worm wheel, thereby providing a high measurement accuracy.