Over time, engineers find ways to make systems more efficient. While the thread form of a particular screw may not seem like something that could be improved up, there have been changes over history which have created stronger, easier to produce options.
A Brief History
In the 1800s, the traditional type of square head thread form used for linear motion on items such as lathes and other types of equipment was difficult to produce. Additionally, the sharp, 90-degree angle at the base of the thread was a source of weakness for the entire thread and was subject to wear and tear.
In the latter part of the 1800s, a shape change to the current Acme types of thread forms was developed. These offer a 29% angle at the top of the thread, allowing for a taper to the base, eliminating the weakness of the square thread. Throughout Europe a similar shape appeared, but with metric measurements. With a 30-degree angle, these trapezoids threads served a similar purpose.
The Stub Acme Thread
To further refine the standard Acme thread, engineers developed the stub Acme thread. This has the same shape as the standard Acme thread but with a depth that is less than half the pitch, while the Acme thread has a depth of exactly half the pitch.
This further decrease in the pitch strengthens the already strong thread form, making it ideal for applications where there is concern about vibration, shock or heavy loads on the screw. While standard Acme threads could be used, they would have much shorter life cycles, and the risk of failure or loss of efficiency in the system would be significant.
Another common use for the stub Acme thread is for very long lead screws. With the shape and the structure of the thread combined with the increased strength, these long lengths are free from the common problems associated with lead screws that may not stay perfectly straight over these longer distances and under heavier loads.