In engines, machines, and systems, there is the need to convert the motion of one kind into motion of another. A lead screw or a power screw is used to transform or translate the turning motion into a linear motion. A simple example of this is a vice, where turning the handle (rotational movement) causes the jaws to open or close (linear motion).
This same principle is used in machine tools, lathes, presses and in jacks, as well as in complex systems such as DVD drives on computers and even to raise and lower the doors on aircraft and other types of equipment. In general, due to the shape and the strength of these types of thread forms, they are used when heavy loads or continuous movement is a factor.
The Trapezoidal Shape
A trapezoidal lead screw has a distinctive profile that forms the shape of a trapezoid. This means there is an angle to the thread that is not straight up and down, but rather on a 29-degree angle, at least with the Acme trapezoidal thread form. This means the base of the thread is thicker than the top, adding strength to the screw and increasing load bearing capacity. The metric trapezoidal lead screw has a slightly increased angle to the thread. This includes a 30-degree angle.
At the same time, the trapezoidal lead screw always has the height of the thread as one half of the pitch, further adding to strength while also decreasing friction. The result is a very strong, durable type of lead screw that is found in heavy duty types of applications.
There are specific designations and standards set by ISO (International Organization for Standardization), and these provide a consistent international thread form, allowing for specialized companies to re-engineer old lead screws or design lead screws that meet all standards and requirements for specific applications.
