Home Chapter 15 Cranks drives and gearing
Cranks drives and gearing


This bell crank converts a circular motion to a side-to-side linear motion. The bell crank can also be used to change an up-and-down linear motion to a side-to-side linear motion if there is no circular wheel driving the crank. The origin of the name “bell crank” came about because the mechanism was originally used to ring a servant’s bell.


Bell Crank


By increasing or decreasing the size of the arms and linkage, you can pronounce the leverage available at each point. Notice the circular motion will push and pull on the linkage and notice the pivot and resting or constraining points for parts of this mechanism. 


Bell crank in motion


This crank above allows the cam to push and pull the lever, however, you would need to use a rolling surface like a piece of step tubing. Step tubing is metal tubes which are nested inside other metal tubes. They can be used in creating bearing like linkages.




 Crank in Motion


The crank converts a rotary motion to a back and forth motion. The size of the turning wheel and the distance of the wheel from the center of the shaft will alter the reciprocating action.




Crank in Motion


Drives and Gearing

Gears are important simple machines that function in many ways in mechanical devices, but they are most commonly used to give small motors more torque. Gear reduction makes it possible for a small fast motor attached to a small gear to turn a larger gear, therefore slowing down the speed of the spinning motion and increasing the torque. Gears can also adjust the direction of rotation and change the type of motion.

In a rack and pinion gear, the toothed straight gear meshes with the toothed round gear. This makes it possible to change a linear motion into a circular motion, and vice versa.


Rack and Pinion Gear


Rack and Pinion Gear in Motion


Gears have teeth in order to prevent slippage and to ensure that the axes are always synchronized. The teeth also make it easy to find the exact gear ratios, which are determined by the circumference of the gears. Different gear ratios make it possible for gears to spin at different rates. For instance, if there are two gears spinning together and one gear is twice as big as the other, the gear ratio would be 2:1, and the larger gear would spin at half the speed of the smaller gear. You can easily find a gear ratio by counting the number of teeth in each gear and dividing.

The gears below have a 1:1 ratio, but one gear will spin in one direction and the other in the opposite direction.


1:1 Ratio Gear


These gears have a 2:1 ratio, and the smaller gear spins twice as quickly as the larger gear.


2:1 Ratio Gear