Buzz
At some point, we've all stepped onto a bathroom scale. In this article, we'll delve into the mechanics of how a bathroom scale functions. Here’s the model we’ll be examining today:
Upon removing the cover, you’ll discover a large dial, a rack-and-pinion system that moves the dial, a spring, and an intriguing setup of four crisscrossing levers inside:
Beneath the top cover, you'll find four brackets that fit into the levers:
These brackets ensure that the weight placed on the scale (you) is evenly distributed across the levers.
The four levers inside the scale are designed to function as a unified system. The reason for having four is that it simplifies the distribution of weight. These levers connect to a plate at the end of a spring. This is how the system is arranged:
In this diagram, you can observe that a long lever, anchored at the left end, has weight applied near the fixed point. At the right end, the weight is reduced significantly, according to the ratio between the distance from the left end to the weight and the distance from the left end to the right end. This ratio is 1:12. Therefore, when a 120-pound person steps on the scale, only 10 pounds is 'felt' at the scale's right end. The next three diagrams depict how the levers are arranged inside the scale:
The two main levers end at a plate which contains the scale’s primary spring. This setup explains why a 1:12 ratio is utilized – it dramatically decreases the size of the spring. The spring only needs to handle around 20 pounds. Without the levers, however, the spring would need to support 240 pounds and would be much larger and more precise to do so.
The Spring and Spring-plate
The spring and spring-plate are illustrated in the following two diagrams:
Notice that the spring plate moves vertically. This vertical motion is converted into horizontal movement by a pivot, which then directs the motion to a rack and pinion system, as shown here:
The rack and pinion mechanism converts the horizontal motion into rotational movement, which in turn drives the dial.
