1. Analyze the transmission torque
　　Under normal circumstances, the transmission torque should be determined according to the actual transmission power and actual working speed of the unit. The diaphragm coupling selected in this way can not only meet the torque transmission requirements, but also has the lightest structure, which is an addition to the unit. Forces and bending moments are also minimal.In order to pursue safety, some users increase the size of the coupling layer by layer when choosing the coupling. The couplings selected in this way are often too bulky and the use effect is not good.The coupling is subjected to the combined action of steady-state load CAL torque, axial misalignment medium ring mouth alternating load CAL alternating torque, angular and radial misalignment during operation.Coupling manufacturers usually leave a certain fatigue safety factor when designing their products. The composite stress point of key force components generally falls on the lower left of the revised Goodman curve (like a constant life curve). Make the fatigue safety factor greater than 1.When selecting the coupling, the manufacturer will multiply a reasonable working condition coefficient according to the severity of torque fluctuations under different working conditions. The actual working point will be farther away from the Goodman curve.The actual compound stress point position when choosing different working condition coefficients.According to the working condition coefficient of the diaphragm coupling, the working condition coefficient should not be less than 1.5. When it is necessary to match the shaft system, the supply and demand parties can negotiate to reduce the working condition coefficient, but the minimum shall not be lower than 1.2.
　　In short, the selection of the diaphragm coupling should be based on the machine type provided by the complete set or the host manufacturer and the shaft power under normal conditions, and should not be overweighted.Of course, other conditions should also be considered in some occasions.
　　1) When the coupling is required to work normally when the driven machine is overloaded, the type should be selected according to the power of the driving machine;
　　2) When the driving machine is a motor, the instantaneous short-circuit torque of the motor should be considered if necessary.
　　2. Estimating the thermal growth value
　　Due to the complexity of the temperature field, it is very difficult to accurately calculate the thermal rise value of a machine from a cold state to a hot state.Of course, the diaphragm coupling has a relatively large axial compensation capacity, so there is no need to calculate it so accurately, just estimate it.In the estimation, the position of the thrust bearing of the rotor and the position of the dead center of the casing should be considered, the expansion direction of the rotor and the casing and the influence of the casing expansion on the position of the rotor should be determined, and then the expansion of the rotor and the casing should be calculated separately according to the linear expansion coefficient of the material. Finally, it is superimposed to obtain the change in the position of the shaft head caused by the thermal expansion of the machine.
　　2. The selected coupling should be able to adapt to the unit's maximum continuous speed of 1100. The working speed of the unit is different, and the coupling material selection, structural design, machining accuracy, dynamic balance accuracy and other requirements are different.Under normal circumstances, coupling manufacturers have different series to meet the needs of different speed ranges.Therefore, when selecting a diaphragm coupling model, it should be checked whether the maximum allowable speed of the model can meet the requirements of the unit.