Tanks and Bells
A short overview of the Bell Crank mechanism which are used in Bells, Material presence sensor and all the way to suspension system of tanks
Introduction
If you’ve seen the 1996 Disney Movie The Hunchback of Notre Dame, there was an instance of when the Hunchback would ring the bell by pulling down on a rope which then makes the bell move from side to side. In between the rope and the bell is the Bell Crank Mechanism which is also how it got its name.
A simplistic explanation of the Bell Crank mechanism is it can change the direction of motion by 90 degrees, what that means is that a vertical push or a pull can be transformed into a horizontal push or pull.
Applications
Tanks : As artillery became heavier tanks needed thicker Armour for defense but adding a few millimeters of metal skin would add hundreds of kilograms in weight. Which wasn't much of a problem for the trade off. But a limit had become to develop due to the fact of adding thicker and longer springs to suspend for the heavier tank.
The suspensions were vertical and posed a problem as the height of the tank had to be increased due to the longer springs which in result raises the center of gravity which could topple a million dollar tank which no one wanted The solution was developed by American engineer J. Walter Christie using a Bell Crank .
Simply put, the motion of the vertical bumps (the upward jerk when going over a uneven surface) was transformed into horizontal motion so that springs can be mounted horizontally and not vertically.
- Material Presence: This was the first area i came across this elegant solution. The working was simple. When no material was on the conveyor the output end would be in a position to block a light from an emitter and when the lever rises due to the material on the conveyor the light from the emitter would hit the receiver and get sensed. The length of the arms can be changed for different deflection lengths
There is a lot more application of the mechanism in planes for rudder control, in cars. Working in the same way
The Construction
The mechanism consists of an “L” shaped lever which is pivoted (fixed) at the intersection as shown and can freely revolve about it
The mechanical advantage (force multiplier) can be obtained by changing the arm lengths which then changes the deflection distance as well
As you can see the motion is radial rather than straight and so some consideration has to be looked into when it is being used.
The Bell Crank is a simple mechanism but the ways in which it has been used is rather innovative.
A note: This is my first post, kindly take 2 mins to review and let me know if you would like to hear more about other mechanisms
