Main Construction Principles of Chain Drives
A wide range of factors should be taken into account when selecting a chain drive in order to secure its long service life and reliable operation. The service life of roller chains is supposed to be around 10 000 running hours providing two chain wheels are used and all the principles of correct assembly and maintenance are respected. Non-compliance with or undervaluation of the constructional and/or maintenance principles will lead to considerably shorter service life of the chain.
Main factors influencing chain selection and service life:
- transmitted power
- load type
- chain running speed
- number of teeth of the small chain wheel
- transmission ratio
- axial distance and sag
- specific pressure at chain joint
- lubrication method
- work conditions
- correct assembly of the complete drive
- chain drive maintenance.
For reference purposes, the maximum loading force can be determined as follows: for static load on a correctly installed and properly maintained roller and bush chains the loading force should not exceed 1/3 of the breaking load. With Gall chains the loading force should not surpass the value of 1/5 of the breaking load. Continuous operation, impact strain or less favourable work conditions require for the permissible load to be multiplied by the safety coefficient. When selecting and calculating roller and bush chains the stipulations of DIN 8195 or ČSN 01 4809-69 should be respected. Also, the chain wheel should be shaped correctly. For their construction the provisions of DIN 8192, DIN 8196 and ČSN 01 4811-82 apply.
Type of load
When in operation, chains are rarely exposed to steady pull and that is why the loading force increased by operational impacts should be taken into consideration. For continuous operation, a larger force should also be considered. Shafts and bearings should be dimensioned in such a way that vibrations are avoided. Overhung installation of chain wheels is not suitable for heavy-duty service.
Chain running speed
The permissible peripheral velocity gets lower with the increasing pitch and with decreasing number of the chain wheel teeth, the marker being 19. At chain pitch of 25.4 mm and 19 teeth the maximum permissible velocity is 9.7 m/sec, the maximum permissible velocity for a pitch of 50.8mm being 6.2 m/sec. Where the peripheral velocity is higher than 4 m/sec the effect of the centrifugal force should be included in the calculation.
Number of Chain Wheel Teeth
Chain wheel girdled with a chain forms a polygon with leg size equal to that of the chain pitch. When the chain is approaching the wheel periphery a certain degree of acceleration comes into play and increases together with the increasing difference between the circumscribed circle and the circle inscribed by the chain. The acceleration grows proportionally with the pitch and quadruple to the number of revolutions. The acceleration results in a jerking motion which can be observed in case of low speeds of the chain and less number of pinion teeth. Chain wheels with number of teeth below 17 show a dramatic growth of work consumed for the acceleration while there are minute changes apparent where the pinion teeth are more than 25 in number. Pinions featuring small number of teeth show faster wear not only due to the more frequent engagement of the teeth but also due to the fact that they together with the chain consume the work exerted upon uneven chain movement. Uneven chain movement is caused by the two chain wheels. To avoid the harmonic oscillation of the whole system an odd number of teeth should be selected, adopting prime numbers as far as possible, while the transmission ratio should not be an integer.
The use of chain drives is, as a rule, connected with reduction transmission ratios. The number of the pinion teeth should preferably be 17 as a minimum. The teeth of the large (driven) wheel should not be more than 70 in number. Chains showing larger pitch should not feature transmission ratio higher than 1:7 while the transmission ratio of smaller-pitched chains should not be above 1:9. A step-up transmission ratio is unfavourable and larger ratios of this type should be avoided as much as possible. Where applicable, the smaller (driving) wheel should have at least 25 teeth.