A Brief History of Screw Threads
The first practical application of the humble screw thread was developed by Archimedes in the 3rd century B.C. He used a pipe wrapped around a shaft in a helical pattern to make a crude bilge pump for ships. Later Archimedes wrote a mathematical treatise on spirals. The family of screw threads including Acme, Unified, Trapezoidal and ISO are known as Archimedean screws because they exhibit straight thread profiles in their axial sections. Archimedes took the basic inclined plane and wrapped it into a spiral shape. Rotating the spiral in one direction raised the load and rotating it in the other direction lowered the load. In the 16th century, Leonardo Da Vinci conceived the first flying machines which used the screw thread principle.
Today’s propeller driven ships, airplanes and helicopters can be thought of as utilizing screws against air or water, which act as the mating nuts. Early screws were made by wrapping wire around plain bar. Nuts were made of softer material (copper for example) by forging them around the wire wrapped rod. In fact, modern day manufacture of earth drills and material augers still uses this technique. Later screws were cut from solid bar using single point cutting tools or chasers.
Modern screw thread rolling whereby threads are formed completely by chipless cold forging first began in the late 1800s. Heavy thread rolling of Acme, Ballscrew and worm thread forms began in the late 1950s. Because of its many advantages, thread rolling is the preferred method of manufacture today. Early screw manufacturing suffered from the absence of accurate and powerful machinery capable of holding minimally accurate tolerances. This was compounded by the lack of accurate inspection methods. For many years screws and nuts were manufactured and used in matched sets, and as a result were not interchangeable.
Screw Thread Standards
In the mid 1800s Sir Joseph Whitworth developed the first screw thread standard for a fastening thread which now bears his name. This standard detailed the diameter and pitch combinations along with limits of size for each combination. Later, Britain, Canada and the United States developed the Unified Screw Thread standard which is in wide use today, and which was copied conceptually by the International Standards Organization when that body standardized the ISO fastener threads and the metric Trapezoidal power screw threads. Most screw thread standards are structured as functions of major diameter and pitch. All thread features and limits of size are deterdetermined by the thread major diameter and thread pitch. Acme screw threads were formulated in the 1890s to replace square threads and a varied group of other power transmission thread forms. Today, Acme and metric Trapezoidal screw threads are highly standardized by government and industry associations. Users are guaranteed good fitups when using nut and screw products from different manufacturers only if both sources adhere to the applicable standards.
Basics of Actuating Screws
Actuating screw threads include Acmes, Hileads(r), Torqsplines(r), Ballscrews, Freewheeling Ballscrews and many other special screws such as Stub Acme, 60 degree Stub, Buttress and Square threads. Unified and ISO fastener “Vee” threads should not be used for actuation as their 30° flank angles and finer pitches are designed for fastening and locking down. (See Identifying Screw Threads section for more detailed information on types and forms of screw threads.) Actuating screws provide a compact means for transmitting motion and power. They are ideal for replacing hydraulic and pneumatic drive systems as they require no compressors, pumps, piping, filters, tanks, valves or any other support items required by these systems. Also, screws don’t leak so there are no problems with seals which are so common to hydraulic and pneumatic systems. And, screw systems are quiet running – no noisy compressors, pumps or exhaust valves. Screw systems are simple, reliable and easy to utilize.
There are four distinct motion converting actions that can be produced by actuating screws and nuts. The two most common involve torque conversion to thrust. In Figure 17, the screw is rotated (torqued) and the nut moves linearly producing thrust or the nut is rotated (torqued) and the screw moves linearly. The two less common motions involve thrust conversion to torque. In Figure 18, the nut undergoes a linear force (thrust) and the screw rotates or the screw undergoes a linear force (thrust) and the nut rotates. These two motions are commonly referred to as “backdriving”, “overhauling”, or, improperly, “reversing”.