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1.
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| What is Dynamic Balancing? |
Excessive vibrations
in any rotating machinery, can cause unacceptable levels of noise and
more importantly, substantially reduce the life of the shaft bearings.
One of the important causes of these vibrations can be attributed to
shafts of the rotating machinery, couplings etc. being out of balance.
The unbalance is caused by an effective displacement of the mass
centerline from the true axis due to some mass eccentricity.
The procedure of reducing the out-of balance forces that cause
vibrations in rotating machinery is called Balancing. In the process of
which, weights are either added or removed such that, the effective
mass centerline approaches the true axis. Balancing thus understood can
be either Static or Dynamic.
Static balancing is the process adopted for low speed machines, wherein
the unit is placed in low friction bearings and rotated momentarily,
allowing it, to come to a stand still. In this process it will be
observed, that the unit will settle with the heaviest portion stopping
at the bottom. Material is then removed from this point and the process
is repeated till no obvious heavy point exists.
Dynamic balancing is the process adopted for high speed machines,
wherein the unit is placed on a purpose built balancing machine, which
has its bearings connected to sensors, which detect the heavy point in
relation to the datum of the unit. This increases the sensitivity and
hence accuracy of the balancing process.
Dynamic balancing is described above, can be either single plane or
multiple planes. Single plane balancing is used, if the unit is very
short in length vis a vis its diameter. However for longer units the
unbalance at two planes is independently identified and corrected.
EUROFLEX PRACTICE: All Euroflex couplings are dynamically balanced in
two planes as per appropriate grade of API 671, by adhering to the
following procedure.
1.Hubs,Spacers and Adaptors: They are first dynamically balanced in two
planes and the residual unbalance is corrected by removal of weights.
For the purpose of mounting on the balancing machine, custom built
balancing mandrels are used.
2.Other components of the coupling: are weight-matched and grouped into
sets with the weight differential kept to less than 0.1gm between
components,so that they can be assembled randomly without effecting
unit balance.
3.Assembly balance: The coupling assembly is then balanced as a unit
for removal of all residual balance. The unit is then match marked
along the length, so that field engineers can get a repeatability of
balance. For meeting needs of field balancing, trim balance holes are
provided on all couplings. Compliance is also offered to API 671 latest
edition.
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2.
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Is
it necessary to balance couplings at its operating speed? |
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The Euroflex Disc
Couplings operate well below their lateral critical or whirl speeds and
hence are considered as rigid bodies which are stable with regard to
the speed of operation, hence there is no necessity for the dynamic
balancing operation to be carried out, at the unit's operating speed.
Further the any balance grade, for example, to be it Q0.6 or 4W/N,
both of which, consider operational speed in the computation of actual
permitted unbalance, will balance out at a speed much below the
operational speed, and will result in the same final level of balance.
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3.
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While
a Turbine rotor needs balancing at operating speed,why does the
coupling, not need balancing at operating speed? |
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A turbine rotor is
classified as a flexible body, since it is designed to generally
operate at speeds higher than its critical speed. Consequently any
machine which is to operate at higher than its crirical speed, may
demonstrate characteristics different from those displayed by it below
its critical speed.
Hence to avoid out of balance situations, at the operating speed it is
necessary to balance such units like turbine rotors at their operating
speed.
In the case of couplings as was explained in the earlier question, the
design ensures that the critical speed of the coupling would always be
far higher than the operating speed of the unit to which it is coupled.
Hence it would never pass through its critical speed, thus eliminating
the need for balancing it at its operating speed.
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| 4. |
What
are the advantages of the Euroflex flexible disc coupling vis a vis
Gear Coupling? |
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The Eurolfex
Coupling has the following advantages over the gear coupling:
1.The Euroflex Disc coupling is of the dry type, not requiring any
lubrication and / or maintenance. hence the entire lubrication system
is eliminated, contributing to lower system costs.
2.The Euroflex Disc coupling do not have any relative moving parts,
hence there is no wear and tear, consequently the usefull life is in
excess of 1,50,000 hours.
3.The Euroflex Disc coupling offer both a higher power to weight ratio
and misalignment capacity.
4.Unlike in a gear coupling, in the Euroflex Disc coupling, the
misalignment capacity does not reduce with increase in speeds.
5.The Euroflex Disc coupling generate lower axial thrusts hence smaller
sized thrust bearings are adequate, results in savings.
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5.
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Is
it necessary to limit the end float of Euroflex Disc Coupling? |
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If a rotating
equipment has its shafts located in thrust bearings, there is no reason
for a coupling to have any form of limited end float. If however, one
of the machines has no axial location for the rotor and the float of
the rotor is purely limited by the end stops in the machine, then it
may be necessary, to consider some form of end float limitation in the
coupling, to prevent the rotor hitting or running against its own end
stops.
It is in this situation, that the Euroflex Disc coupling offers a very
good advantages. The flexing elements in the Euroflex coupling behave
as springs having a non linear characteristic, consequently any axial
movement of the rotor is restrained by a force of increasing magnitude.
Under normal running conditions, the rotor will tend to run at its
magnetic center and any attempt to move it from its natural position
will induce an axial restoring force, increasing non-linearly, in
proportion to the axial movement.
In view of the above inherent design advantage of the Euroflex Disc
coupling, there is no need, under normal circumstances, to provide for
end float limits in the coupling, however should the need arise, many
filed proven design options of end float are possible.
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6.
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Can
the torsionall stiffness of the Euroflex disc coupling be varied to
suit the results of torsional analysis of the turbo machinery train? |
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Torsional analysis
of a rotating equipment train, is performed to rule out the possibility
of resonance within the equipment's running range. Generally the
results of such an analysis, require some modification, in the design
of the coupling to accommodate the corrective factors, since
alternation to the main equipment are diffucult.
Even though Euroflex couplings are by nature torsionally rigid, they do
exhibit some small level of torsional flexibility, by virtue of the
physical modulus of the material of construction. Consequently it is
possible to vary the torsional stiffness of the coupling by modifying
the spacer tube to adjust the overall stiffness and remove the
resonance from problem zones.
Torsional stiffness is generally increased by using larger diameter
spacer, with increased wall thickness, while lowering of torsional
stiffness calls for using close to solid spacer.
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| 7. |
Can
an Euroflex coupling be used in vertical operation? |
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Euroflex disc
couplings can be used in vertical operations and generally do not
require a support. Even though a disc coupling when suspended in
vertical mode will experience sag of the spacer section directly
proportional to the stiffness of the two flexing elements taken in
parallel. The degree of this sag will vary with the coupling size,
design, length and weight of the spacer section. However since the
axial stiffness of the disc coupling increases with speed, the level of
sag will consequently reduce as the unit begins to operate. The
Euroflex coupling design takes into consideration the resulting
stresses caused by the imposed sag.
However if the spacer section is too large or heavy and there is a need
for a support to be introduced, then the Euroflex design practice, is
to provide spherical buttons as in the fig. shown. These buttons are
placed at the central point of the flexing element, to ensure that any
angular misalignment causes rolling of the surface and not rubbing. |
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| 8. |
What
is windage and how to reduce it? |
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Windage is resistance that atmospheric
air provides to any rotating body. Since all couplings for safety
reasons need to necessarily have coupling guards,it is very important
that the coupling guards are properly designed to reduce windage and
its related problems like temperature raise, high frequency sound, oil
sucking from bearings etc.
Apart from the benefits which are derived from proper design of the
guards, the Euroflex coupling recognises the importance of windage
related problems and offers certain design solutions on the coupling
itself for windage reduction. The figures shown below show the design
changes adopted in the form of introducing shrouding and windage plates
on high speed couplings.
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Standards Flanges
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Windage Shroud
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Windage Plate
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The design modifications given above
generally meet most requirements. Should the need arise, Euroflex also
offers streamlined designs, which not only offers a smooth surface to
the unit, but also eliminates the rapid changes in the coupling profile
and hence introduces the best possible laminar air flow around the
coupling.
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9.
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What
is the typical Quality Plan used in the manufacture of Euroflex
Couplings? |
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A typical
Quality Plan followed by Euroflex is as follows and the same is offered
to the customer at the stage of ordering, for his review and comments.
Based on the final Quality plan agreed between the Customer and
Euroflex, the manufacturing operations are conducted.
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| Component |
Operation |
Type of
Check |
% Check |
Standard |
Acceptance
Norm |
Agency |
Adaptors
Hubs
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Chemical
Mechanical
NDT
Machining |
Chemical
Mechanical
U.T
Dim.Insp. |
100%
100%
100%
100% |
BS 970
BS 970
ASTM A388
Approved drawing |
BS 970
BS 970
PARA 10
Drawing |
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Spacer
|
Chemical
Mechanical
NDT
Machining |
Chemical
Mechanical
U.T
Dim.Insp. |
100%
100%
100%
100% |
BS 970
BS 970
ASTM A388
Approved drawing |
BS 970
BS 970
PARA 10
Drawing |
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Coupling bolts & Lock Nuts
|
Chemical
Mechanical
NDT
Machining |
Chemical
Mechanical
U.T
Dim.Insp. |
100%
100%
100%
100% |
BS 970
BS 970
ASTM A388
Approved drawing |
BS 970
BS 970
PARA 10
Drawing |
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Element bush & Washers
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Chemical
NDT
Mechanical |
Chemical
U.T
Dim.Insp. |
100%
100%
100% |
BS 970
ASTM A388
Approved drawing |
BS 970
PARA 10
Drawing |
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Coupling Assembly
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Inspection
Dynamic bal. |
Dimensions
Unbalance |
100%
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Approved drawing
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10.
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Can
torque limiting feature be provided in the Euroflex coupling? |
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Torque limiting
features can be provided in Euroflex couplings by employing shear pins
designed for shearing at particular over load levels.
The couplings employing shear pins are designed with a bearing
arrangement in the central spacer to prevent spacer oscillation after
shear pin breakage. The shear pin arrangements ensures that pin
replacements are fast and easy.
The figure below gives a typical shear pin arrangement employed.
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