Forces and moments

A solid coupling is only designed and constructed to be sub­jected to torsional power transmission torques and axial forces. Flexible couplings can be subjected to bending moments as well as axial and radial forces. The solid coupling does not allow the shafts to move independently of each other. Torque and ax­ial movement are transmitted directly from one shaft to the other. Diaphragm and gear couplings transmit torque directly but react differently to axial and radial movement.

• A diaphragm coupling allows the shafts to move axially and radially, the diaphragms are deformed, and both an axial force and a radial moment are generated.

• The double gear coupling also allows axial and radial move­ment. No axial force is produced, but a radial load is pro­duced rather than a moment.

• A torsionally-flexible coupling produces radial loads rather than moments.

• The rubber ring coupling will produce an axial force when axial movement takes place, whereas the other types of coupling will slide to accommodate axial movement.

  Forces and moments



Equ 12.1




= coupling torque at start (Nm)


= internal motor torque (air-gap torque) at start



= moment of inertia of motor (kgm2)


= moment of inertia of driven machine (kgm2)


= motor starting time without load (s)


= motor starting time with load (s)

By inserting appropriate figures in equation 12.1 and assuming that Mi may be 6 to 10 times the rated torque, values for cou­pling torque at starting may be up to 4 times the rated torque for 4-pole motors and 8 times the rated torque for 6-pole motors. Care must therefore be taken when sizing couplings for fans which are started direct-on-line, especially when the fan has a large inertia.

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