142 Module 12 • Gear drives


Te Velocity Ratio (VR) of a gear train is the ratio of the speed of the first driver to the last driven gear.


Principle of mechanical advantage


Te mechanical advantage of using gear drives is that by using different gear ratios more torque, stronger drive, more speed and more productive drive can be achieved.


The purpose of intermediate gears Intermediate gears are commonly referred to as idler or pinion gears. Tese gears have no influence on the velocity ratio of the mating (meshing) gears. If the driven (output) is to turn in the same direction as the driver (input), than an idler (intermediate) gear is used. Intermediate gears are also used where the centre distance between driver and driven gears has to be varied.


A


B


C


driver


idler Figure 12.4 Gear train with idler gear


Factors which determine the correct meshing of gear teeth Gears can only mesh and work together if they have the same module and circular pitch as this fixes the size and pitch of the gear teeth. The gear teeth guarantee that no slippage takes place between the two gears (unless the teeth are stripped).


12.7 Simple and compound gear trains


Gear trains (simple and compound systems) form an indispensable part of many power transmission systems.


Simple gears If a gear system consists of two gears, it is called a simple or singular gear train. A driver and a driven gear are shown in Figure 12.5 on the next page. If one gear turns clockwise, the other will automatically turn anti-clockwise. Tese gears work in a straight line, and are fitted independently on their individual shaſts.


When gears rotate, the same number of teeth of each gear will pass the point of mesh (D) in the same time. Thatmeans that for any period of time:


driven


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