How to Measure Mechanical Advantages of Screws

A screw is actually a circular version of the inclined plane.

A screw is one of the "simple machines," a category that includes such useful devices as the lever and the pulley. Each simple machine has some way of making work easier, usually by increasing the distance or magnitude of a force. The factor by which the machine increases the force is called the mechanical advantage, which is simply the ratio of output force to input force. In the case of a screw, the mechanical advantage depends on the pitch of the threads and the circumference of the head.

Instructions

1. Measure the diameter of the screw's head. The diameter is the distance from one point on the circle to another point on the circle passing through the center of the circle. As an example, the diameter of the screw is 0.2 inches.

2. Multiply the diameter by 3.14 to find the circumference. In the example, circumference = 0.2 inches x 3.14 = 0.628 inches.

3. Hold a ruler next to the threads of the screw and count how many threads are in one inch. If the screw is less than one inch long, measure the entire length of the screw and count the total number of threads.

4. Divide one by the number of threads you counted, use the length of the screw instead of one if the screw is shorter than one inch. This tells you the distance between threads, called the "pitch." In the example screw, there are 24 threads in one inch, so pitch = 1 / 24 = .042.

5. Divide the circumference of the head by the pitch. This is the formula for the mechanical advantage (MA) of a screw: MA = circumference / pitch. For the example screw, MA = 0.628 / .042 = 14.95.

Tips & Warnings

• The mechanical advantage of the screw itself is not very important, because you typically use a screwdriver to drive the screw. This increases the mechanical advantage. To determine the actual mechanical advantage, use the circumference of the screwdriver handle in place of the circumference of the screw's head.

References

• University of Arkansas: Simple Machines

Resources

• Hyperphysics: Simple Machines