The intent of a doctor's scale is to convert the large weight of a person into a smaller adjustable weight that can be precisely read on the upper calibrated scale. This is accomplished with a set of three connected levers with high mechanical advantage.
Scale Components
The most obvious part of a doctor's scale is the beam across the top. This holds two sets of weights that slide along separate parts of the beam. The lower part of the beam is marked in 50-lb. or 20-kg increments and holds a large sliding weight. The upper beam is marked in single pound or kilogram increments and holds a small sliding weight. The beam also contains an adjustable counterweight, used to calibrate the scale, and a fulcrum about which the whole beam pivots.
The platform at the bottom of the scale rests on a set of springs connected to an output lever. The output lever connects to a steel rod which runs up the shaft of the scale to its head.
A System of Levers
Inside the head of the scale below the beam is a second class lever. A second class lever is one where the fulcrum is at one end, the effort is applied at the other end and the resistance is somewhere between the two. In a second class lever, the effort always has a higher mechanical advantage than the resistance, since it is always farther away from the fulcrum.
The fulcrum is mounted to the inside of the scale head. The resistance on this lever comes from the steel rod that connects to the platform output lever. If there were no effort put on the lever it would just be pulled down by the weight on the platform. The effort to balance the resistance is applied with a cable which connects to the left side of the top beam.
The top beam is a first class lever, like a see-saw. The resistance comes in the form of the cable from the second class lever, the fulcrum is next and the effort is applied by the the sliding weights on the beam. As the weights are moved to the right, their mechanical advantage increases. This means that the same weight will push down on the right side of the beam harder as it is moved away from the fulcrum.
How It Works
When weight is applied to the platform, the springs stretch and the output lever moves down. The steel rod transmits this resistance to the second class lever inside the scale head. This unbalances the second class lever and causes the top beam to be pulled down on its left side. The user moves the weights on the top beam to the right, until the top beam is perfectly balanced about its fulcrum. This means that the weights on the top beam have perfectly balanced the resistance on the second class lever, and correctly indicate the weight placed on the platform.
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