A screw is a simple machine that works as a modified incline plane. As the screw is turned, the screw enters deeper into the substrate. Once inside the substrate, the frictional force of the thread is intended to prevent the screw from rotating back out of the substrate. The thread of the screw may be viewed as an inclined plane wrapped around the shaft of the screw. The slope of the screw is the distance for one complete rotation around the screw while the height of the inclined plane is the distance between the threads, known as pitch. The relationship between the pitch and circumference of the screw gives the mechanical advantage.
Measure the pitch of the screw. The pitch of the screw is the distance between the threads and is determined by measuring the number of threads per inch (or centimeter) on the screw then dividing one by the number of threads (pitch = 1/number of threads per inch or cm). For example, if a screw has eight threads per inch, the pitch is 1/8.
Measure the circumference of the screw. Circumference is calculated by measuring the diameter of the screw and multiplying by pi (circumference = diameter of the screw x pi). For example, if a screw has a diameter of 0.25 inches, then the circumference of the screw is 0.79 inches (0.25 inches x 3.14 = 0.79 inches).
Calculate the mechanical advantage of the screw by dividing the circumference of the screw by the pitch of the screw. Using the previous examples, a screw with a pitch of 1/8 and a circumference of 0.79 inches would produce a mechanical advantage of 6.3 (0.79 inches/ 0.125 = 6.3).