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Minimi.ation of the number of startups is an important consideration because the number of starts reduces engine life. The critical aspect in this minimi.ation is the correct selection of balance planes at the start of the process. This selection is essential because the rotor to be balanced oftenconsists of a number of units (turbine， compressor) connected by couplingsand has a great number of available correction planes. .sually， balancing is required only in one ...one"" (on the turbine or at the coupling) at a particular speed. The critical location can be pinpointed almost exactly by reference toa prior analytical unbalance-response sensitivity study. Such astudy， whichinvolves the entire rotor and couplings， will indicate those planes whereparticular unbalance distributions， if present， will cause vibration at a par-ticular speed.For example， amachinerytrain， consisting of a precisely balanced compressor with aprecisely balanced coupling， will sometimes vibrate excessively at one or more speeds. This vibration usually results because the rotor assembly has one or more bending critical speeds in the running range where the mode shapes are forced by the residual unbalances left in the precision-balanced subassemblies. It must be stressed that abalanced rotor subassembly does not have .ero unbalance. In reality， ithas a residual unbalance distribution， which does not excite the subassembly under the balance conditions. If an analytical study does notexist， the balancing engineer must depend on vibration readings from available sen-sorsand， ultimately， on .udgment or past experience for selection of correc-tion locations.
Once the critical .ones along the rotor axis have been identified， the sensitivity factors of those planes must be calculated. If unbalance sensitivity factors of those planes must be calculated. If unbalance sensitivity factorsare not available for the balance planes and sensors at the speeds of interest， trial weight runs are required. Thermal stabili.ation times become import-ant， since the process can consume significant periods of time. If the sensitivities areavailable， then corrections may be calculated based on vibration levels measured inservice .ust before shutdown， and the unit can be balanced and restarted very rapidly.
It is often tempting to try to shortcut the sensitivity factor gathering process by inserting correction weights in available planes one at a timebased on hunches or one-plane vector plots. Occasionally， this shortcut willresult in a balanced rotor; but moreoften， the opposite result is achieved. This unbalance results because the trial weights in later planes are then notthe only perturbation from the ..as-is"" condition. Data SheetsA，B， and C show a typical process for field balancing with a computer program that employs this balancing technique.
The balancing engineer must try to maintain a balance record for eachmachine he balances， since in most cases the machinery system itself will often contain some nonrepeatable element. Components sensitive to thermalvariations， such as dampers and bearing alignments，etc.， may often causeproblems.认hen a nonrepeatability is present， the engineer must first deter-mine whether or not another corrective action is indicated. Ifnot， then the balance quality that may be obtained is limited strictly by the range of the nonrepeatable element"s variability. This level of quality is difficult to ascer-tain without experience， either on the individual machine or on a family of similar machines. The balance engineer must balance each rotor by usingmean values for each parameter， and he must keep a detailed record of the different results. Thisrecord consists，essentially， of residual unbalance experience in each case. From the standpoint of the multiplane balancingprocedure， the record consists of sensitivity parameters for each machine， which are obtained as a matter of due course in the trial weight procedure.
.ser.s .uide for Multiplane Balancing
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燃气涡轮工程手册 Gas Turbine Engineering Handbook 3(21)
