Basic Principle of the Scale
Looking at a bathroom scale from the outside gives no clue about the mechanism inside that allows the scale to accurately display weight. The workings of a bathroom spring scale are, in reality, quite simple. They take advantage of some simple levers, springs and gears to convert the weight being measured to an amount that can be displayed and understood, all in a relatively small space.
The Platform, Base and Dial
Bathroom spring scales consist of a platform to stand on. The platform is set on top of a steel base. A dial display and needle indicator will show the amount of weight being exerted on the platform. The platform is usually held to the base using small connecting springs.
The Levers
Four brackets on the underside of the platform rest on two short and two long levers that are connected within the scale. When weight is placed on the platform, it bears down on the levers evenly, no matter how the weight is positioned on the platform.
The Spring Mechanism
The two long levers are attached to a plate that is connected to a spring called the main spring. When the levers are activated by weight, they pull down on the main spring at a ratio of 12 to 1. This means that if 120 lbs. is placed on the scale, only 10 lbs. is actually transmitted to the spring, allowing it to be significantly smaller than in a 1 to 1 ratio. The more weight, the more the spring will stretch downward.
The Spring Rope, Rack and Pinion
A smaller spring called the spring rope is also attached to the plate operated by the levers. The spring rope then attaches to a gear assembly called the rack. When the plate moves, the spring rope moves the rack. The gear in the rack then turns a geared pinion that holds the needle indicator. As more or less weight is applied to the platform, the pinion turns the needle indicator to display the weight.
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