SEED Guides Unit Selection - Shaft Coupling < >

The most important information is usually related to its purpose, constituent elements, life, performance, environment and the economic situation for which it is required. This information is needed in order to understand the total system so that the coupling selected consistent with the rest of that system. The temptation to the mechanical engineer is to consider only purpose, performance and perhaps environment should be strongly resisted. Although this stage is sometimes difficult and time consuming it must be completed thoroughly if an appropriate unit is to be identified.

Factors which influence the choice of coupling must now be identified. A listing of most of the important and common factors appears below. Not all of these factors are important on every occasion so careful study of the system is required to ensure that those considered are actually relevant. Reference must be made to the Product Design Specification for the system.

Each factor should then be defined in terms which are as objective as possible. Thus, where appropriate, numerical information should be given, terms must be explained and vagueness avoided. Then the boundaries of satisfaction must be defined for each of the chosen factors. This helps the designer to decide which couplings meet the requirements in each respect. The boundaries must be numerical whenever possible. When subjective judgements are necessary a means of comparison must be established.

Manufacturers' data should now be collected and arranged into a suitable format. There is a finite number of couplings available from manufacturers and the selection process is heavily constrained by the form and content of the information presented by them and by the range of catalogues available to the designer at the time.

There is a good case for maintaining a 'rolling' catalogue library or data on microfilm/computer since this stage can be very time consuming if a unique set of data is collected separately on each occasion. Data on, say, size, performance, cost can be noted in numerical form, giving a range where appropriate. In the case of less objective data a rating may be shown based on advice or opinion gathered.

Optimising the choice is now a process of finding the best compromise (in the opinion of the designer) between the priorities of the system and the availability of the hardware. As far as the factors involving numerical data are concerned, some yield a 'go/no-go' situation which will eliminate those couplings which are e.g.: too costly, too heavy, too big etc.

Other requirements involving more subjective data should be compared on the basis of their ability to meet the criteria as laid down in the Product Design Specification. This may be an iterative process which converges on the best compromise.

The evaluation technique used here will be similar to that used elsewhere in the design activity.

Further advice on the detail of installation or specifying and ordering will be required from the manufacturer's information. Normally this would be available from the catalogue but sometimes it is necessary to communicate directly with a representative of the company.