When determining the size of flexible and solid couplings, it is usual to evaluate a so-called “service factor". The cynics amongst us would suggest that this is a euphemism and should more correctly be designated a “safety factor”. It will cover our lack of knowledge of all the operating conditions.
Most coupling manufacturers publish nominal ratings for each of their products, together with lists of service factors for various applications. User groups also give advice and it is perhaps significant that those published by the American Petroleum Institute in its Standard No 613 are higher than those given by designers.
Drives with squirrel-cage motors and fans are usually stated by manufacturers to have a service factor of 1.0. However, it is wise to remember that where the absorbed power can vary, then this should be taking into account. Increased power can result from measurement uncertainties in the original base design manufacturing variations between nominally identical units, temperature variations in the gas/air handled, and whether the system resistance varies or has been incorrectly calculated (especially important with fans having a rising power characteristic e. g. forward curved bladed centrifugal fans).
To compare different couplings objectively a method has been developed which takes into consideration the frequency of starting, temperature, the moments of inertia of the driving and driven machine, normal torque and maximum torque.
This method has been presented in the German coupling Standard DIN 740, which, apart from the method of calculation, also contains dimensional standards. There are, however, two additional service factors which should be considered.
The first is the effect that shaft misalignment can have on the coupling. A factor based on the extent of allowable misalignment expressed as a percentage of the maximum permissible deviation, should also be given.
The second factor should take into consideration the level of vibration of both the fan and its driver.
Note that for fans, vibrational velocities above 5 mm/sec may well be permissible. In this respect the reader is referred to ISO 14694 (BS 848 Part 7) for the appropriate grades AN1 to AN4 and their corresponding balance quality grades.
The size of the various factors and their influence on coupling speed varies with different types, which is why the calculations and values given in DIN 740 must be used with a certain amount of caution and always with due regard to the suppliers’ instructions, which must apply.
Avery important point in this context, to which too little consideration is given, is the magnitude of the starting torque in the case of direct-on-line starting of a squirrel-cage induction motor. Measurements have shown that almost immediately after connection, approximately 0.04 s, a maximum torque is reached which is between 6-10 times the rated torque and even higher in some cases. This is a result of the electrical sequence in the actual motor and the fact that connection of the three phases does not occur absolutely simultaneously. The actual maximum torque is therefore much greater than the starting torque quoted in motor catalogues.
An important factor for coupling calculations is the relationship between the moments of inertia of the driving and driven machine. This quotient determines the percentage of torsional moment which is to be used for the acceleration of the motor and fan rotors. When starting, the torque passing through the shaft coupling is:
Type of deviation. It is important to know the maximum permissible values of combined misalignment, see Figure 12.6, and how the maximum permitted deviations are influenced by speed and the torque transmitted.
The service life of both couplings and machines, normally machine bearings, are influenced by misalignment. Just how much the life of the machine is affected can only be judged when information regarding the precise magnitude of the torque and forces transmitted due to misalignment is known. It is usual to refer only to the amount of misalignment permitted for a specific coupling type. But it is the amount of misalignment tolerated by the machine, Figure 12.7, which should really be investigated.
— = moments transferred by angular misalignment
— ■ force transferred by axial deviation
Figure 12.7 Relationship between misalignment and transmitted forces/ moments
Posted in Fans Ventilation A Practical Guide