This Chapter sets out the factors which influence the relationship between shaft couplings and the fan unit. It includes a short review of the different types of coupling and continues with an explanation of the various types of misalignment and the forces and moments which are transmitted. Advice is given on “service factors” with special emphasis on the torque produced when starting electric motors. Several other factors are dealt with, and as shaft alignment is considered to be of importance, several different methods are explained. A check-list of important factors related to couplings is also included.
Chapter 9 showed that there are a considerable number of mechanical arrangements for fans, both centrifugal and axial flow. When looking at how the drive is transmitted from the prime mover to the fan impeller, it can immediately be seen that these can be resolved into three basic classifications:
• where the fan impeller is directly mounted on the motor shaft extension and thus runs at the motor speed.
• where the fan impeller is mounted on a separate shaft running in its own bearings and there is an indirect connection
Through belts, chains or gears to the prime mover.
• where the fan impeller is mounted on a separate shaft running in its own bearing(s) and there is a direct connection
Through a shaft coupling to the prime mover.
In this Chapter we are particularly interested in the last category. The coupling may be “rigid” or “flexible”, transferring torque between two in-line, or nearly in-line, rotating shafts. Torque in the two shafts will of course, be equal in magnitude. If slipping or disengagement is possible however, there may be variations in speed. In its basic form the coupling is used as a simple way of joining shafts. Another requirement is to join two shafts which are not necessarily in perfect alignment with each other — indeed the author’s experience is that they rarely are.
Perfection is not possible in this world and so the coupling must be capable of accommodating such misalignment. Modern couplings, between fans and their drivers, must be capable of rapid disassembly, especially in capital intensive plant where down-time can affect profitability.
It should be noted that coupling drives are invariably used on larger fans where the impeller is too heavy for the motor shaft or where vee belt drive would require lay-shafts and/or too many belts.
Shaft couplings can perform many different functions and have varying characteristics. They are usually divided into three main groups with sub-divisions, namely:
Clutch with manual over-ride mechanism
Limited torque couplings
Controlled and variable
Some of the requirements for flexible couplings, including definitions, performance and operating conditions, dimensions of bores, reference to components as well as an appendix on alignment are to be found in BS 3170. Friction clutches and power-take-off assemblies for engines, and their requirements are included in BS 3092. Process fans to API 610 Standard may have spacer couplings in accordance with API 671.
For fan applications it is common to use a coupling from the first group above, although special installations make use of disengaging clutches and limited torque couplings. Thus it is possible to incorporate centrifugal clutches to reduce starting loads when using a direct-on-line starting induction motor. Hydrodynamic clutches can be used for reducing starting loads and speed regulation. Combinations of brakes and reverse locks can be used to prevent reverse fan rotation.
Power recovery hydraulic turbines have been used in public utility and process fans when they have been coupled to the non-drive end of the fan motor so that the turbine can “unload” the motor.
The coupling used is a free-wheel type with manual over-ride so that the fan/motor can start-up before the turbine. Once the turbine runs up, as it tries to rotate faster than the motor, the clutch locks automatically and power is transmitted.
Posted in Fans Ventilation A Practical Guide