Page 280 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer
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Be st Practice 4 .8           Gear and Coupling Best Practices
                                                              Coupling
       B.P. 4.7. Supporting Material
                                                              Driven shaft/coupling fit
                                                              The driven shaft
       The coupling system                                    The coupling spacer system
                                                              Lubrication (if required)
       It has been the writers’ experience that if couplings are properly    Cooling system (if required)
       selected, the root cause of failure, if it occurs, is in the coupling
       system.                                                A schematic of a coupling system is shown in Figure 4.7.1.
          The coupling system must continuously transmit torque  The reliability of the coupling is a function of the coupling
       safely between the driver and driven equipment and must allow  system design and assembly. If any of the items noted above are
       for changes in shaft misalignment and axial movement. The  not properly designed or assembled, then a coupling failure can
       components that make up the shaft system are:        occur. Coupling assembly/disassembly errors and enclosed
                                                            coupling guard design are two important areas that are critical to
         The driver shaft                                   coupling system reliability.
         Driver shaft/coupling fit






























       Fig 4.7.1   The coupling system (Courtesy of M.E. Crane, Consultant)







         Best Practice 4.8

         Require a review of dry, flexible coupling, shaft end, ther-  Coupling breakage under load conditions can cause steam turbine
         mal, axial growth calculations, and never exceed coupling  overspeed and blades to exit through the exhaust case, even though
         axial installation limits since they can cause coupling fail-  the overspeed trip system is set correctly and functions properly. The
         ure and possible catastrophic safety issues.       trapped steam between the trip valve and throttle valves has sufficient
                                                            energy to accelerate the turbine rotor beyond its design limits.
           Flexible couplings (disc and diaphragm) have limited axial movement.
           The driver and driven equipment suppliers calculate the maximum
         shaft axial growth of the shafts.                  Lessons Learned
           These values, along with the coupling maximum axial movement  High shaft vibration and/or coupling breakage has occurred
         limits, determine the dimensions of the complete coupling assembly as  as a result of improper shaft end thermal axial growth and/
         compared to the between shaft extensions (BSE) dimension.  or improper flexible coupling assembly installation.
           In many cases, the shaft end axial growth will exceed the dry,
         flexible coupling, maximum axial movement dimension. In these cases,  Benchmarks
         the coupling is not assembled in a neutral position, but is either pre-  This best practice has been used since flexible ‘dry’ couplings were
         compressed or pre-stretched, so that the installed coupling assembly  first used in the late 1960s, and has resulted in trouble free coupling
         will ensure that the maximum safe amount of movement will not be  assemblies without any coupling failures.
         exceeded at any time during operation.

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