Choice of coupling


In general the cost per kW of a coupling is only a fraction of that of a fan or motor, a fan usually costing at least 30 times that of a coupling and a 4-pole electric motor at least 20 times. The cost varies according to the size and the type. The market for cou­plings is very competitive; the cost difference between manu­facturers is usually small.

Gear couplings are the most costly. If a spray oil lubrication sys­tem is required this obviously increases the total cost consider­ably. Diaphragm and flexible spring couplings, together with the rubber buffer couplings, are about the same cost. Some of the rubber ring couplings are surprisingly expensive.

A good way to compare the cost of couplings is to set the price in relation to the torque and range of shaft end sizes to which the coupling can be fitted. The same fan shaft can, for example, be used for a torque range of 1:20 which occasionally means that the shaft end dimension and not the torque is used when select­ing the size of a coupling.

Furthermore, the motor shaft may be larger than the corre­sponding fan shaft. The motor shaft may be dimensioned for bending stress to a greater degree than the fan shaft; for exam­ple a motor is often used for belt drive. This can also mean us­ing a larger size coupling.

Factors influencing choice

It is important, not least of all from an initial cost point of view but also cost and space required for spare parts, to establish a via­ble internal standard by which a small number of type or style variations can cover the majority of coupling requirements within a company or plant. The factors reviewed in the check-list, Table 12.1 should be considered.


Influencing parameters

Type of coupling

Non-disengaging Disengaging Torque limitations Torsionally rigid Torsionally flexible

Type of movement

Radial and axial deviation Angular deviation

Forces and moments

Torsional moment Bending moment Axial and radial forces

Operational factors

Frequency of starting Connection frequency Operating time Ambient temperature Moment of inertia Method of calculation




Throw protection (safety flange)

Size, weight

Shaft bore

Space requirements

Spacer for disassembly





Explosive (spark-free, flameproof)

Installation and disassembly

Horizontal and vertical shafts




Attachment facilities etc. for alignment measuring device.

Replaceable wear elements Service life Routine maintenance Internal standard Costs

Coupling safeguards

Table 12.1 Check-list for shaft coupling selection

For many centrifugal fans, the diaphragm spacer coupling has become the standard. These couplings are very reliable and can easily cope with the loads and speeds encountered in most situations. For higher speed applications, e. g. fans driven by steam turbines, the gear coupling is preferred by some users. Smaller fans operate better with a torsionally flexible coupling; flexible spring and couplings with rubber cushioning are favour­ites.

Users who have a large number of fans usually choose a single coupling manufacturer whenever possible. This philosophy in­creases the purchasing power of the user while reducing inven­tory requirements for spares.

Posted in Fans Ventilation A Practical Guide