SEED Guides Unit Selection - Shaft Coupling <

For satisfactory performance and long life the selection of a shaft coupling depends not only on the power, speed and geometrical requirements but also on the environmental conditions, the flexibility requirements and the nature of the loads applied.

Figure 4 Factors Affecting Coupling Choice

The mechanical performance required of the coupling is mainly a function of the power to be transmitted and thus of the torque and speed of the drive. Couplings are often rated in terms of the torque they can transmit at a standard speed (e.g.: 100 rev/min) and will be constrained by a limiting speed. Flexibility required in torsional and other modes should also be established.

The application factors relate to the coupling's use in original fitting or in service. Thus size, in relation to the space available, accessibility to features requiring adjustment or maintenance, the need for lubrication and the ease of assembling/dismantling will need to be considered.

Geometry factors refer to the overall space available, the tolerances required in alignment and the probability of significant misalignment should be specified. Particularly important is the need to ensure compatibility of shaft bores with those of the driving and driven machines.

For many couplings the ambient conditions are critical. Maximum tolerance of vibration and pollutants, and the acceptable ranges of humidity and/or temperature must be compared with the actual conditions of the system. Extremes of condition will exclude some coupling types.

The failure of a correctly selected and applied coupling is an unlikely event. However it could also be very dangerous. The reliability of the coupling selected may be difficult to establish except by reputation but manufacturers will usually quote the maximum loads and conditions. For flexible couplings the duty and total service life of the coupling are important because fatigue loading may be present. The frequency acceptable and likely manner of possible failure should be established and related to the consequences of failure in the context of the total system.

Although price may be the most obvious commercial factor, other considerations relate to any company policy there may be in dealing with standardisation of suppliers or products, the ease and speed of obtaining the required unit and the terms of trading and reputation of the supplier.

SELECTION PROCEDURE

Figure 5 Coupling Selection Procedure

After obtaining as much information as possible about the system, especially the driving and driven machines to be coupled, and the conditions in which it is operating, the performance required of the coupling can be established. In order to take into account the nature of the loads and the duty required a service factor, S, is applied to the theoretical power. Suitable values for the service factor are suggested in Appendix 1.

The design power, P; is the designer's estimate of the maximum equivalent power likely to be transmitted by the coupling, taking into account the conditions of its use. It is obtained by applying the service factor to the nominal power.

Both maximum and minimum speed requirements for the drive should be established. The speed limitations on couplings will be a function of centrifugal force, at high speed, and the torque rating of the coupling, at low speed. For a constant speed drive system it is simply necessary to ensure that the required speed lies within the limits. If the drive requires variable speed then the whole range must lie within the limits. Some small allowance may also be applied to permit the possibility of over-speed.

The shaft diameters of the driving machine (output) and the driven machine (input) must be established since the coupling must be compatible with both. It is not necessary that both shafts should have the same diameter, though they will be expected to be similar. Most couplings have hubs which incorporate some adjustment or can be fitted with collets to match a range of diameters. The coupling must also be suitable for the driving and driven machine shaft-ends to permit satisfactory fitting. Standard keyed or splined fittings are normal but friction fittings may be used for light drives.

Fig. 6 shows the options in this procedure.

Requirements for misalignment and torsional flexibility should now be checked. Fig 6. shows the options in this procedure. The limits of flexibility (under the four types defined above) for flexible couplings is usually quoted in the manufacturer's catalogue. Care should be exercised in ensuring that all modes of misalignment are taken into account to the extent that they are likely to be present in the system. It should also be remembered that the flexibility of the coupling will contribute to the overall flexibility of the system, with consequences for its deflection and vibration characteristics.

At this point several couplings may present themselves as suitable and it remains to choose the best compromise between the 'other factors'. Since only performance factors have been considered so far, commercial and human factors should now be investigated. Such requirements as cost, availability, standardisation etc. with respect to the former, and ease of use, assembly, maintenance, space available etc. under the latter heading. For a given type of coupling the competition is generally on commercial grounds since manufacturers have taken into account the operational and application factors in their design.

Manufacturers' catalogues are a valuable source of information and should be consulted for data on operation and application as well as performance. The initial choice of coupling will depend on catalogue information although this may sometimes be incomplete or incompatible between manufacturers. It may be that further information will be required direct from the manufacturer, regarding the commercial aspects or to clarify catalogue data, in order to confirm the choice.

APPENDIX 1: Service Factors

APPENDIX 3: RELEVANT BRITISH STANDARDS

BS 3170 Flexible Couplings for Power Transmission
BS 4506 Metric Shaft Ends
BS 3979 Metric Series Electric Motors

APPENDIX 4: COUPLING MANUFACTURERS

This is a selection of coupling manufacturers and is not complete. Technical Indexes or a buyers' guide should be consulted for a more complete list. Manufacturers should be contacted to check the availability of products.

NOTE: Whilst every effort has been made to ensure that the information given is correct and up-to-date, SEED the publishers and authors cannot be held responsible for any errors or omissions that might occur in this Guide. Use of the methods or data on projects for application outside the educational environment should be justified and validated during the course of the designer's normal professional duties.