The screw has been an integral part of our lives from as far back as memory stretches. This simple and ingenious designed tool has been a most reliable part of basically everything that we see around us today. Imagine trying to build an airplane or vehicle without using screws.
The thread that makes a screw what it is, is believed to have been invented as far back as 4 hundred years BC. A screw-like design was the main feature of the press used to squeeze juices and oil from fruits such as grapes and olives. Initially, these screw devices were manufactured from wood, such as the device that the Egyptians used to irrigate their crops.
Screws were used in that basic form and design until man started to work and manipulate iron. With the ability to design tools came the design of the iron screw. The machine screw is a recent variant of the screw and it is used widely since its introduction in the industrial age. This type of screw can be fastened into tapped holes or used with nuts.The thread on this screw is at an angle of 60 degrees in a continual pattern. Some of the parameters that they are defined by are the pitch, the flank angle, the minor diameter, the major diameter and the pitch diameter. The machine screw has the same diameter from beginning to end while a wood screw is pointed and fastens into the wood grain.
The modern day machine screw is manufactured through thread-rolling. A variety of materials are used in the manufacture of screws – from basic carbon steel wire to better quality nickel alloys, stainless steel, and different aluminum alloys. Different finished are hen applied to the screw such as zinc, chrome or nickel. The modern day screw is classified into 3 thread tolerances aptly called classes 1 to 3. It indicates how tight the fit of the screw is. For instance class, 1 indicates a loose fit, which is not widely used today. Class 3 is a very strong and tight fit that is used for string heavy mechanic connections.
As far as the design of the machine screw heads are concerned, it has escalated onto numerous shapes and variants for basically any use. Just a few of the designs are: pan head, fillister head, flat head, oval head, undercut oval head, trimmed oval head, truss head, indented hex washer head, cheesehead and many more. Screws can be used with several different designed lock systems to hold or grip the nut nearly permanently. Such fastening devices are the patch lock, the epoxy lock, and the strip lock. The socket screw is a variant of the machine screw and is made of either steel or stainless steel. Steel socket screws are made from a steel alloy that is suitable for use where high tensile strength is needed.
Screws are manufactured for tightening nuts or bolts to it, mostly in a clockwise direction. There are cases where a left-handed threat, i.e. anti-clockwise tightening, is needed. On some machines, a counter-clockwise force or torque are placed on the screw and nut. Under normal circumstances, the nut will be undone. Hence the left- handed threat screw. Such screws are used in bicycle pedals.
Quality standards are maintained through various organizations globally, Such as the National Screw Thread Commission which follows a Declaration of Accord from 1948. It standardizes diameter sizes, the pitch and the shape of the thread and the number of threads per centimeter. The introduction of threat rolled- compared to machine-cut screws has also improved standards with no metal lost due the cutting. It also prevents weaknesses in the metal. Combined to the various finishes, modern screws are tough, durable and as strong as we need them to be. Choices between single pitch (where the lead and the pitch are identical), double pitch, and triple pitch screws provide variety in applications. It is also known as threads per centimeter. Only in the design of very small screws are machining used instead of the threading method.
The machine screw as we know it today is one of the most ingenious and practical products that ensure strong and reliable joints on almost any device – from small cellphones to massive steel structures such as bridges. It has remained basically unchanged for more than 2000 years and will be around for many millennia.