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FAQ

1.
What is Dynamic Balancing?


2.
Is it necessary to balance couplings at its operating speed?


3.
While a Turbine rotor needs balancing at operating speed,why does the  coupling, not need  balancing at operating speed?


4.
What are the advantages of the Euroflex flexible disc coupling vis a vis Gear Coupling?


5.
Is it necessary to limit the end float of Euroflex Disc Coupling?


6.
Can the torsionall stiffness of the Euroflex disc coupling be varied to suit the results of torsional analysis of the turbo machinery train?


7.
Can an Euroflex coupling be used in vertical operation?


8.
What is windage and how to reduce it?


9.
What is the typical Quality Plan used in the manufacture of Euroflex Couplings?


10.
Can torque limiting feature be provided in the Euroflex coupling?


1.
 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.

2.
Is it necessary to balance couplings at its operating speed?

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.

3.
While a Turbine rotor needs balancing at operating speed,why does the coupling, not need balancing at operating speed?

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.

4. What are the advantages of the Euroflex flexible disc coupling vis a vis Gear Coupling?

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.

5.
Is it necessary to limit the end float of Euroflex Disc Coupling?

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.

6.
Can the torsionall stiffness of the Euroflex disc coupling be varied to suit the results of torsional analysis of the turbo machinery train?

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.

7. Can an Euroflex coupling be used in vertical operation?

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?

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.

Standard

Windage shroud discs

Windage Plate
Standards Flanges

Windage Shroud

Windage Plate





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.


9.
What is the typical Quality Plan used in the manufacture of Euroflex Couplings?

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.

Component Operation Type of Check     % Check Standard Acceptance Norm Agency
Adaptors
Hubs

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
 

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
 
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
 
Element bush & Washers
Chemical
NDT
Mechanical
Chemical
U.T
Dim.Insp.
100%
100%
100%
BS 970
ASTM A388
Approved drawing
BS 970
PARA 10
Drawing
 
Coupling Assembly

Inspection
Dynamic bal.
Dimensions
Unbalance
100%

Approved drawing
 
   


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10.
Can torque limiting feature be provided in the Euroflex coupling?

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.


Shear Pin

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