N2 Fitting and Machining Theory|Hands-On! 149


• Chain drives, if properly lubricated, do not generate heat. • Chains do not undergo the same degrading effects of what time has on belts.


Advantages of chain drive over gears


• Tere is no initial tension. • Chain drives are versatile and are equally suited for short or long centre distances, low or high speeds and light and heavy loads. Since they are symmetrical, they can be driven from either side.


• Tey are easily maintained, manually or automatically, without interference to other parts of the drive.


• Tere may be, as in the case of belt drives, multiple drives on the same chain. • Chain drives are more economical than gears and have low maintenance costs.


Disadvantages of chain drives


• Chain drives are noisy. • Chain drives fail without prior warning. • If a chain is stretched, it may become too slack and can vibrate or jump the sprockets. • When replacing the chain, inspect the sprockets and replace if required. • Tey are only flexible in one plane and cannot transmit motion to perpendicular shaſts.


13.5 Chain drive terms


Sprockets for chain drives are precisely manufactured and the teeth are machined to standard specifications to obtain the correct contact between sprocket and chain. For low speeds, sprockets (chain wheels) with less than 16 teeth are generally used, whilst for high speeds sprockets with 18 to 24 teeth are more appropriate. Sprockets with fewer than 25 teeth which are used at speeds of more than 500 revolutions per minute, must be made of medium or high carbon steel.


(a)


driver sprocket


jockey or idler (b)


driven sprocket


Figure 13.5(a) Simple/singular and (b) compound chain drives


Tey must also be heat-treated to make them wear resistant to wear and tough enough for a long and productive life. Where a big velocity ratio (8 : 1 or bigger) or a large sprocket of more than 128 teeth is needed, it is recommended that a compound drive be used.


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