Gear systems New words


mesh: fit together as they move


parallel: lines which are side by side, drawn in the same direction and an equal distance apart, as in your exercise book, are parallel lines


clockwise direction: rotating in the same direction as the hands on a clock


In Term 1 you were introduced to gears as wheels with teeth sticking out along the rim. The teeth of one gear fit into the notches on another gear. They mesh with the teeth of the other gear so that the driving gear turns the other one. Gears are rotating machine parts.


Gears come in a variety of shapes and sizes, and they are used to either increase the speed of rotation or decrease it. They can also be used to change the direction of rotation or to move something backwards and forwards or up and down.


Spur gear


The spur gear is perhaps the most common type of gear. Its teeth are straight and cut so that they are parallel to the axle. They are used when the axle to which each gear is connected must rotate in the same plane. When one gear turns in a clockwise direction the other gear will turn anti-clockwise. This is known as counter rotation. By using gears we can control speed and direction, and increase the mechanical advantage. If we want the gears to turn in the same direction, we introduce an idler


idler follower driver


Figure 15 An idler gear connecting two spur gears


gear. This gear sits between the driver gear and the gear being driven and synchronises the rotation of the two gears. The speed of the rotation of the output gear will depend on the speed of rotation of the input gear. The speed is measured in revolutions per minute (RPM).


When designing a machine using gears, we need to calculate the speed at which the final gear will turn the axle. The input speed will be determined by the speed of the motor used. The speed of a motor is measured in RPM. When calculating gear ratio, the gear wheel being turned is called the input gear and the gear being driven is called the output gear. The first figure in the ratio refers to how many turns the input makes in relation to the output gear. First we calculate the gear ratio.


Outut gear 30 Input gear 90


=


1 3


1 : 3


A ratio of 1 : 3 means that the output gear turns through three revolutions for every single revolutions of the input gear. The output gear is three times as fast as the input gear. Remember, the first number in the ratio is the input gear.


If the RPM of the larger input gear is 120 RPM, we multiply this by 3, which means the output gear is turning at 360 RPM.


Example: Input gear has 25 teeth with RPM speed of 120. Output gear has 100 teeth.


Outut gear 100 Input gear 25


=


4 1


4 : 1


A ratio of 4 : 1 means that the input gear turns through a four revolutions for every one revolution of the output gear. This means that the speed of the output gear is slower than the input gear. To find the final RPM of the driven gear, we divide the 120 by 4 = 30 RPM.


106 Topic 5 Mechanical systems and control


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