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A screw is an externally fastener capable of being tightened or released by a twisting force (torque) to the screw head. The most common uses of screws are to hold objects together and there are many forms for a variety of materials. Screws might be inserted into holes in assembled parts or a screw may form its own thread. The difference between a screw and a bolt is that the latter is designed to be tightened or released by torquing a nut.
The screw head on one end has a slot or other feature that commonly requires a tool to transfer the twisting force. Common tools for driving screws include , , coins and hex keys. The head is usually larger than the body, which provides a bearing surface and keeps the screw from being driven deeper than its length; an exception being the set screw (aka grub screw). The cylindrical portion of the screw from the underside of the head to the tip is called the shank; it may be fully or partially threaded with the distance between each thread called the pitch..
Most screws are tightened by clockwise rotation, which is called a right-hand thread. Screws with a left-hand thread are used in exceptional cases, such as where the screw will be subject to counterclockwise torque, which would tend to loosen a right-hand screw. For this reason, the left-side pedal of a bicycle has a left-hand thread.
The screw mechanism is one of the six classical simple machines defined by Renaissance scientists.
Metal screws used as fasteners were rare in Europe before the 15th century, if known at all. The metal screw did not become a common fastener until for mass production developed toward the end of the 18th century. This development blossomed in the 1760s and 1770s.. along two separate paths that soon converged:.
The first path was pioneered by brothers Job and William Wyatt of Staffordshire, UK,. who patented in 1760 a machine that one might today best call a screw machine of an early and prescient sort. It made use of a leadscrew to guide the cutter to produce the desired pitch, and the slot was cut with a rotary file while the main spindle held still (presaging live tools on lathes 250 years later). Not until 1776 did the Wyatt brothers have a wood-screw factory up and running. Their enterprise failed, but new owners soon made it prosper, and in the 1780s they were producing 16,000 screws a day with only 30 employees.—the kind of industrial productivity and output volume that would later become characteristic of modern industry but which was revolutionary at the time.
Meanwhile, English instrument-maker Jesse Ramsden (1735–1800) was working on the toolmaking and instrument-making end of the screw-cutting problem, and in 1777 he invented the first satisfactory screw-cutting lathe.. The British engineer Henry Maudslay (1771–1831) gained fame by popularizing such lathes with his screw-cutting lathes of 1797 and 1800, containing the trifecta of leadscrew, slide rest, and change-gear gear train, all in the right proportions for industrial machining. In a sense he unified the paths of the Wyatts and Ramsden and did for machine screws what had already been done for wood screws, i.e., significant easing of production spurring commodification. His firm remained a leader in machine tools for decades afterward. A misquoting of James Nasmyth popularized the notion that Maudslay had invented the slide rest, but this was incorrect; however, his lathes helped to popularize it.
These developments of the 1760–1800 era, with the Wyatts and Maudslay as arguably the most important drivers, caused great increase in the use of threaded fasteners. Standardization of threadforms began almost immediately, but it was not quickly completed; it has been an evolving process ever since. Further improvements to the mass production of screws continued to push lower and lower for decades to come, throughout the 19th century.. The mass production thus began: that of wood screws (metal screws for fixing wood) in a specialized, single-purpose, high-volume-production machine tool; and that of the low-count, toolroom-style production of machine screws or bolts (V-thread) with easy selection among various pitches (whatever the machinist happened to need on any given day).
In 1821 Hardman Philips built the first screw factory in the United States – on Moshannon Creek, near Philipsburg – for the manufacture of blunt metal screws. An expert in screw manufacture, Thomas Lever, was brought over from England to run the factory. The mill used steam and water power, with hardwood charcoal as fuel. The screws were made from wire prepared by "rolling and wire drawing apparatus" from iron manufactured at a nearby forge. The screw mill was not a commercial success; it eventually failed due to competition from the lower-cost, gimlet-pointed screw, and ceased operations in 1836.
The American development of the turret lathe (1840s) and of automatic screw machines derived from it (1870s) drastically reduced the unit cost of threaded fasteners by increasingly automating the machine-tool control. This demand curve of screws.
Throughout the 19th century, the most commonly used forms of screw head (that is, drive types) were simple internal-wrenching straight slots and external-wrenching squares and hexagons. These were easy to Machining and served most applications adequately. Rybczynski describes a flurry of patents for alternative drive types in the 1860s through 1890s,. but explains that these were patented but not manufactured due to the difficulties and expense of doing so at the time. In 1908, Canadian P. L. Robertson was the first to make the internal-wrenching square socket drive a practical reality by developing just the right design (slight taper angles and overall proportions) to allow the head to be stamped easily but successfully, with the metal cold forming as desired rather than being sheared or displaced in unwanted ways. Practical manufacture of the internal-wrenching hexagon drive (hex key) shortly followed in 1911...
In the early 1930s American Henry F. Phillips popularized the Phillips screw, with a cross-shaped internal drive.See:
Later improved -head screws were developed, more compatible with screwdrivers not of the exactly right head size: Pozidriv and Supadriv. Phillips screws and screwdrivers are to some extent compatible with those for the newer types, but with the risk of damaging the heads of tightly fastened screws.
Threadform standardization further improved in the late 1940s, when the ISO metric screw thread and the Unified Thread Standard were defined.
Precision screws, for controlling motion rather than fastening, developed around the turn of the 19th century, and represented one of the central technical advances, along with flat surfaces, that enabled the industrial revolution.. They are key components of micrometers and lathes.
The blanks are then polished again prior to threading. The threads are usually produced via thread rolling; however, some are thread cutting. The workpiece is then tumble finishing with wood and leather media to do final cleaning and polishing. For most screws, a coating, such as electroplating with zinc (galvanizing) or applying black oxide, is applied to prevent corrosion.
Sheet-metal screws do not have the chip-clearing flute of self-tapping screws. However, some wholesale vendors do not distinguish between the two kinds.
Once screw turning machines were in common use, most commercially available wood screws were produced with this method. These cut wood screws are almost invariably tapered, and even when the tapered shank is not obvious, they can be discerned because the threads do not extend past the diameter of the shank. Such screws are best installed after drilling a pilot hole with a tapered drill bit. The majority of modern wood screws, except for those made of brass, are formed on thread rolling machines. These screws have a constant diameter and threads with a larger diameter than the shank and are stronger because the rolling process does not cut the grain of the metal.
A machine screw is a small fastener less than in diameter similar to a bolt, but which usually has a recessed drive type (slotted, Phillips, etc.) on its head to allow turning it with a screwdriver. Machine screws are threaded the entire length of the shank, and are intended to screw into a pre-formed thread in either a nut or a threaded (tapped) hole. Machine screws are also made with socket heads (see above), often referred to as socket-head machine screws.
Lug bolt and head bolts are other terms that refer to fasteners that are designed to be threaded into a tapped hole that is in part of the assembly and so based on the Machinery's Handbook distinction they would be screws. Here common terms are at variance with Machinery's Handbook distinction.
The head is typically an external hex. Metric hex-headed lag screws are covered by DIN 571. Inch square-headed and hex-headed lag screws are covered by ASME B18.2.1. A typical lag screw can range in diameter from 4 to 20 mm or #10 to 1.25 in (4.83 to 31.75 mm), and lengths from 16 to 200 mm or or longer, with the coarse threads of a wood-screw or sheet-metal-screw threadform (but larger). The materials are usually carbon steel substrate with a coating of zinc galvanization (for corrosion resistance). The zinc coating may be bright yellow (electroplated), or dull gray (hot-dip galvanized).
Screw Head Shapes - Combo Pan and Countersunk.jpg | Combo pan and countersunk Screw Head Shapes - Combo Flat and Truss.jpg | Combo pan and truss |
| SAE Grade 0 | Strength and hardness is not specified | ||||||||
| SAE grade 1 ASTM A307 Low carbon steel | – | B70–100 | |||||||
| ASTM A307 - Grade B Low or medium carbon steel | –4 | 60 minimum 100 maximum | 410 minimum 690 maximum | B69–95 | |||||
| SAE grade 2 Low or medium carbon steel | – | B80–100 | |||||||
| Greater than | B70–100 | ||||||||
| SAE grade 4 Medium carbon steel; cold worked | – | ||||||||
| –1 | B70–100 | ||||||||
| SAE grade 5 Medium carbon steel; quench and tempered | –1 (inc.) | C25–34 | |||||||
| 1– | C19–30 | ||||||||
| 1– (inc.) | C19–30 | ||||||||
| –3 | Brinell scale 183–235 | ||||||||
| No. 6– | C25–40 | ||||||||
| –1 | 85 | 590 | 120 | 830 | C26–36 | ||||
| C25–34 | |||||||||
| –1 (inc.) | C24–35 | ||||||||
| 1– | C19–31 | ||||||||
| –1 | C24–35 | ||||||||
| 1– | C19–31 | ||||||||
| – (inc.) | C26–36 | ||||||||
| –4 | C22–33 | ||||||||
| – | |||||||||
| – | C32–38 | ||||||||
| – (inc.) | C33–39 | ||||||||
| –4 | C31–39 | ||||||||
| –1 | C33–39 | ||||||||
| ASTM A490 - Type 1 Medium carbon alloy steel; quench and tempered | – | 120 | 830 | 130 | 900 | 150 minimum 170 maximum | 1,000 minimum 1,200 maximum | C33–38 | |
| ASTM A490 - Type 3 Atmospheric corrosion resistant steel; quench and tempered | |||||||||
| 18/8 Stainless Stainless steel with | – (inc.) | 40 minimum 80–90 typical | 280 minimum 550–620 typical | 100–125 typical | 690–860 typical | ||||
| –1 (inc.) | 40 minimum 45–70 typical | 280 minimum 310–480 typical | 100 typical | 690 typical | |||||
| over 1 | 80–90 typical | 550–620 typical | |||||||
Modern screws employ a wide variety of screw drive designs, each requiring a different kind of tool to drive in or extract them. The most common screw drives are the slotted and Phillips in the US; hex, Robertson, and Torx are also common in some applications. Some types of drive are intended for automatic assembly in mass-production of such items as automobiles. More exotic screw drive types may be used in situations where tampering is undesirable, such as in electronic appliances that should not be serviced by the user.
The nominal diameter of a metric screw is the outer diameter of the thread. The tapped hole (or nut) into which the screw fits, has an internal diameter which is the size of the screw minus the pitch of the thread. Thus, an M6 screw, which has a pitch of 1 mm, is made by threading a 6 mm shank, and the nut or threaded hole is made by tapping threads into a hole of 5 mm diameter (6 mm − 1 mm).
Metric hexagon bolts, screws and nuts are specified, for example, in International Standards ISO 4014, ISO 4017, and ISO 4032. The following table lists the relationship given in these standards between the thread size and the maximum width across the hexagonal flats (wrench size):
In addition, the following non-preferred intermediate sizes are specified:
Bear in mind that these are just examples and the width across flats is different for structural bolts, flanged bolts, and also varies by standards organization.
The most common use of a Whitworth pitch nowadays is in all UK scaffolding. Additionally, the standard photographic tripod thread, which for small cameras is 1/4" Whitworth (20 tpi) and for medium/large format cameras is 3/8" Whitworth (16 tpi). It is also used for microphone stands and their appropriate clips, again in both sizes, along with "thread adapters" to allow the smaller size to attach to items requiring the larger thread. Note that while 1/4" UNC bolts fit 1/4" BSW camera tripod bushes, yield strength is reduced by the different thread angles of 60° and 55° respectively.
While not related to ISO metric screws, the sizes were actually defined in metric terms, a 0BA thread having a 6 mm diameter and 1 mm pitch. Other threads in the BA series are related to 0BA in a geometric series with the common factors 0.9 and 1.2. For example, a 4BA thread has pitch mm (0.65 mm) and diameter mm (3.62 mm). Although 0BA has the same diameter and pitch as ISO M6, the threads have different forms and are not compatible.
BA threads are still common in some niche applications. Certain types of fine machinery, such as moving-coil meters and clocks, tend to have BA threads wherever they are manufactured. BA sizes were also used extensively in aircraft, especially those manufactured in the United Kingdom. BA sizing is still used in railway signalling, mainly for the termination of electrical equipment and cabling.
BA threads are extensively used in Model Engineering where the smaller hex head sizes make scale fastenings easier to represent. As a result, many UK Model Engineering suppliers still carry stocks of BA fasteners up to typically 8BA and 10BA. 5BA is also commonly used as it can be threaded onto 1/8 rod.
Ultimate tensile strength is the tensile stress at which the bolt fails. Tensile yield strength is the stress at which the bolt will yield in tension across the entire section of the bolt and receive a permanent set (an elongation from which it will not recover when the force is removed) of 0.2% offset strain. Proof strength is the usable strength of the fastener. Tension testing of a bolt up to the proof load should not cause permanent set of the bolt and should be conducted on actual fasteners rather than calculated. If a bolt is tensioned beyond the proof load, it may behave in plastic manner due to yielding in the threads and the tension preload may be lost due to the permanent plastic deformations. When elongating a fastener prior to reaching the yield point, the fastener is said to be operating in the elastic region; whereas elongation beyond the yield point is referred to as operating in the plastic region of the bolt material. If a bolt is loaded in tension beyond its proof strength, the yielding at the net root section of the bolt will continue until the entire section begins to yield and it has exceeded its yield strength. If tension increases, the bolt fractures at its ultimate strength.
Mild steel bolts have property class 4.6, which is 400 MPa ultimate strength and 0.6*400=240 MPa yield strength. High-strength steel bolts have property class 8.8, which is 800 MPa ultimate strength and 0.8*800=640 MPa yield strength or above.
The same type of screw or bolt can be made in many different grades of material. For critical high-tensile-strength applications, low-grade bolts may fail, resulting in damage or injury. On SAE-standard bolts, a distinctive pattern of marking is impressed on the heads to allow inspection and validation of the strength of the bolt." How to Recognize Metric and SAE Bolts ", Chilton DIY, Retrieved April 26, 2016. However, low-cost counterfeit fasteners may be found with actual strength far less than indicated by the markings. Such inferior fasteners are a danger to life and property when used in aircraft, automobiles, heavy trucks, and similar critical applications.
The Machinery's Handbook describes the distinction between bolts and screws as follows:
This distinction is consistent with ASME B18.2.1 and some dictionary definitions for screw and bolt.
Old USS and SAE standards defined cap screws as fasteners with shanks that were threaded to the head and bolts as fasteners with shanks that were partially unthreaded. The federal government of the United States made an effort to formalize the difference between a bolt and a screw, because different apply to each.
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