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.
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.
|
Factor |
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 |
|
Speed |
Balancing Strength Throw protection (safety flange) |
|
Size, weight |
Shaft bore Space requirements Spacer for disassembly |
|
Environment |
Corrosive Abrasive Temperature Explosive (spark-free, flameproof) |
|
Installation and disassembly |
Horizontal and vertical shafts Alignment Fit |
|
Others |
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