SEED Guides Component Selection - Chain Drives < >

For any application it is necessary to initially establish certain essential information. This typically includes the power to be transmitted, the drive shaft and driven shaft speeds (and hence the reduction ratio), the duty (specified in terms of the power source, the severity of load and the design life), the shaft centre distance, the operating environment (i.e. dampness, temperature, etc.) and the likely level of maintenance (thus indicating lubrication types that may be permissible).

Drive selection is at two levels, at the transmission type level (i.e. deciding whether to use chain drives, gears or pulleys) and at the component level (i.e. choosing the chain pitch size, number of strands etc.).

Transmission Type Selection

Many factors might influence why a chain drive will be selected for a power transmission in preference to gears or a belt drive. Typically, a chain drive will be favoured when

1. the centre distance between shafts is relatively large (making gears impractical);
2. a synchronous drive is necessary (making wedge pulley belts unsuitable); 3. a high operating efficiency and long life is required;
3. a high torque (or power) at slow speed is needed;
4. operating conditions are severe (pulleys are unsuitable in a wet, hot or abrasive environment);
5. costs have to be minimal;
6. a number of shafts need to be driven in synchronism;
7. shaft alignment and centre distance may be of low precision.

Component Selection Level

The sequence taken in identifying suitable chain drives and then choosing a preferred alternative is described in the flow diagram. A summary of a worked example which follows the procedure taken in this figure is given on a later page of this chapter. The following comments will help to clarify details of the flow diagram and the example.

1. When establishing requirements, essential information will include the drive and driven shaft speeds, the power to be transmitted, the shaft centre distance, the operating duty and permissible lubrication types.
   
   
2. To determine the number of teeth required for the drive and driven sprockets it is first necessary to obtain the reduction ratio and this is found from knowledge of the shaft speeds. Frequently, though, it is not possible to achieve exactly the reduction required (since sprockets are available only in a limited number of teeth sizes) and so some tolerance on the driven shaft speed may have to be accepted. Also, the greater the number of teeth in the pinion, the quieter and smoother the transmission, and if low noise and very long life are desirable, a larger and thus more expensive chain assembly will be necessary.
   
   
3. Operating with a severe drive source and/or a driven load will require a more substantial size of chain - and possibly larger sprockets also. To provide a reserve of capacity to compensate for more demanding operational conditions, an appropriate Service Factor is used. This is obtained from a table and applied to the actual power to be transmitted thus producing an imaginary power which is then used for determining suitable chain pitch sizes from a Power Rating Chart. To avoid the necessity for having unique charts for each driving sprocket size and multi-strand alternative, a term called the Selection Power is defined. This quantity is calculated by simply applying factors to the actual power to be transmitted so that a single Rating Chart may then be used.
   
   
4. Suitable chain pitch sizes, based on power considerations, will be found from referring to the appropriate chain manufacturer's 'Transmission Rating Chart' (found in a catalogue) and using the calculated Selection Power and the drive shaft speed. Generally, the larger the chain pitch and the greater the number of strands, the greater the power that can be transmitted.
   
   
5. The chain pitch sizes must be checked for practical suitability, Sprocket diameters and minimum centre distances must be established so as to ensure that alternatives are possible (see Figure 4).
   
   
6. Frequently the designer is faced with a number of viable alternatives comprising different pitch sizes and multi-strand options and each will have inherent advantages and disadvantages. For example, increasing the pitch size results in larger chain links and sprocket diameters but with consequent increased cost and size and reduced chain tension and speed capability. In contrast, using multi-strand chain results in an increase of face width rather than diameter but it too is more expensive because it necessitates having tandem chain and sprocket sets.
   
   
7. Full catalogue details of the chosen chain drive should be summarised, together with connecting link type and tensioner (if required) so that mating items can be designed. The required length may be calculated by applying the appropriate equation. Usually it is more practical to know the length in terms of the number of links - rounding up to the next integer number if part of a link is calculated, and using a chain tensioner to take up any slack. The chain tension is important when determining shaft forces and hence establishing shaft and bearing suitability.

Figure 4 A Roller Chain Sprocket

Typical Selection Strategy for a Roller Chain Transmission