The screw thread has been around for over two thousand years and used in a variety of applications ranging from fastening, mechanical advantage devices and power transmission. Believed to be initially developed by Archimedes around 250BC, the screw thread principle was originally designed to raise water by winding it up out of a well with a crude helix around a shaft. It is considered a simple machine in its own right, requiring only two components. Early screw threads were crudely carved helixes into timber shafts.
The basic principle of any screw thread is that rotary motion is transferred into axial motion, achieved with a helix around a shaft that runs through matching helix in a hole. The size, shape and angle of this helix vary for application. Due to the infinite range of combinations, the screw thread can be used in nearly as many applications. Two parts make up a screw thread as a simple machine and are referred to as the male and female. The male refers to a shaft with a helix running around its outer, and a female refers to a hole or bore with a helix running around its inside.
Very fine pitch screw threads can be used in measuring and calibration equipment, where one rotation can equal as little as 0.5mm of axial movement. When paired with a device like an encoder that can read rotational movement of 0.5 of a degree or less, you can accurately move a part 0.00069mm axially. Much coarser threads can be used in the same application for greater movement. A thread with a 25mm lead will move a part 25mm for every 1 rotation. Applied in a commercial valve for instance, the valve seal can be moved 100mm in just 4 rotations, offering very fast opening and closing for very little rotational movement. These are just two examples of the mechanical advantage that can be gained with screw threads.
Screw threads are also commonly used for power transmission, most notably in worm drives. In this case the screw thread is rotated with a worm gear engaged at the side of the screw thread. As the screw thread rotates, so does the worm gear, transferring the power through up to 90 degrees.
The most common application of the screw thread is in fastening. The rotational force is transferred into axial force, giving a clamping effect between two parts. Fastening screw threads are used in just about everything we use today. There is a variety of fastening thread forms from Metric, BSW, BSP, Whitworth, UNC, UNF, Acme, Trapezoidal & Buttress to name a few. These thread forms are also available in a variety of screw types such as socket head cap screws, bolts, grub screws and more. This range of combinations results in a screw thread solution for any application.
Machining of threads can be quite an art form, even with the modern advantages of CNC machines and precision ground carbide cutting tools. Acme and Trapezoidal threads in particular require a very skilled tradesperson to ensure the cutting angles and clearances are precisely correct to allow the thread to work at its best. Too much clearance and the thread will be more likely to wear, reducing the lifetime of the component. Not enough clearance and the component may not move freely, or start to gall, causing wear which in turn may also reduce the lifetime of the component. When cutting large Acme and Trapezoidal threads, the tool can push away altering the angles of the thread. This too can affect wear and movement, and needs to be accounted for when setting up a thread cutting operation. These and other factors are why we use our highly skilled and experienced tradespeople to cut complicated thread forms.
Screw threads are a versatile simple machine used in wide range of items we take for granted everyday. From a timber screw holding a shelf on the wall, to a worm drive in the tuning keys of a guitar, or even a precision ball screw in the power steering in a car, screw threads are a part of everyday life. For more information on how we can help you with screw threads or screw cutting please email enquiries@dbengineering.com.au.
