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In common usage, the distinction not rigorous is often that screws are smaller than bolts, and that screws are generally tapered while bolts are not. For example, cylinder head bolts are called "bolts" at least in North American usage despite the fact that by some definitions they ought to be called "screws".
Their size and their similarity to a bolt that would take a nut seem linguistically to overrule any other factors in this natural word choice proclivity. Screws are then defined as headed, externally threaded fasteners that do not meet the above definition of bolts.
And it is for that reason, perhaps, that some people favor them. However, they are neither compliant with common usage of the two words nor are they compliant with formal specifications. A possible distinction is that a screw is designed to cut its own thread; it has no need for access from or exposure to the opposite side of the component being fastened to. This definition of screw is further reinforced by the consideration of the developments of fasteners such as Tek Screws, with either round or hex heads, for roof cladding, self-drilling and self-tapping screws for various metal fastening applications, roof batten screws to reinforce the connection between the roof batten and the rafter, decking screws etc.
On the other hand, a bolt is the male part of a fastener system designed to be accepted by a pre-equipped socket or nut of exactly the same thread design. Threaded fasteners either have a tapered shank or a non-tapered shank. Fasteners with tapered shanks are designed to either be driven into a substrate directly or into a pilot hole in a substrate. Mating threads are formed in the substrate as these fasteners are driven in.
Fasteners with a non-tapered shank are designed to mate with a nut or to be driven into a tapped hole. Lag bolts are usually used with an expanding insert called a lag in masonry or concrete walls, the lag manufactured with a hard metal jacket that bites into the sides of the drilled hole, and the inner metal in the lag being a softer alloy of lead, or zinc alloyed with soft iron.
The coarse thread of a lag bolt and lag mesh and deform slightly making a secure near water tight anti-corroding mechanically strong fastening. A socket cap screw, also known as a socket head capscrew , socket screw , or Allen bolt , is a type of cap screw with a cylindrical head and hexagonal drive hole. The term socket head capscrew typically refers to a type of threaded fastener whose head diameter is nominally 1.
Forged heat-treated alloy examples are high strength fasteners intended for the most demanding mechanical applications, with special alloy formulations available that are capable of maintaining strength at temperatures in excess of degrees F degrees C. In addition to the series design, other head designs include low head, button head and flat head, the latter designed to be seated into countersunk holes. A hex key sometimes referred to as an Allen wrench or Allen key or hex driver is required to tighten or loosen a socket screw.
Socket head capscrews are commonly used in assemblies that do not provide sufficient clearance for a conventional wrench or socket. A superbolt, or multi-jackbolt tensioner is an alternative type of fastener that retrofits or replaces existing nuts, bolts, or studs.
Tension in the bolt is developed by torquing individual jackbolts, which are threaded through the body of the nut and push against a hardened washer. Because of this, the amount of torque required to achieve a given preload is reduced. Installation and removal of any size tensioner is achieved with hand tools, which can be advantageous when dealing with large diameter bolting applications.
The field of screws and other hardware for internal fixation within the body is huge and diverse. Like aerospace and nuclear power, this field involves some of the highest technology for fasteners, as well as some of the highest prices, for the simple reason that performance, longevity, and quality have to be excellent in such applications.
Bone screws tend to be made of stainless steel or titanium, and they often have high-end features such as conical threads, multistart threads, cannulation hollow core , and proprietary screw drive types some not seen outside of these applications.
These abbreviations have jargon currency among fastener specialists who, working with many screw types all day long, have need to abbreviate repetitive mentions. The smaller basic ones can be built up into the longer ones; for example, if you know that "FH" means "flat head", then you may be able to parse the rest of a longer abbreviation containing "FH".
These abbreviations are not universally standardized across corporations; each corporation can coin their own. The more obscure ones may not be listed here. The extra spacing between linked terms below helps the reader to see the correct parsing at a glance. Screws and bolts are usually made of steel. Where great resistance to weather or corrosion is required, like in very small screws or medical implants, materials such as stainless steel , brass , titanium , bronze , silicon bronze or monel may be used.
Galvanic corrosion of dissimilar metals can be prevented using aluminum screws for double-glazing tracks for example by a careful choice of material. Some types of plastic, such as nylon or polytetrafluoroethylene PTFE , can be threaded and used for fastenings requiring moderate strength and great resistance to corrosion or for the purpose of electrical insulation. Often a surface coating is used to protect the fastener from corrosion e. Selection criteria of the screw materials include: size, required strength, resistance to corrosion, joint material, cost and temperature.
Structural bolts replaced rivets due to decreasing cost and increasing strength of structural bolts in the 20th century. Connections are formed with two types of joints: slip-critical connections and bearing connections.
In slip-critical connections, movement of the connected parts is a serviceability condition and bolts are tightened to a minimum required pretension. Slip is prevented through friction of the "faying" surface, that is the plane of shear for the bolt and where two members make contact. Because friction is proportional to the normal force, connections must be sized with bolts numerous and large enough to provide the required load capacity.
However, this greatly decreases the shear capacity of each bolt in the connection. The second type and more common connection is a bearing connection. In this type of connection the bolts carry the load through shear and are only tightened to a "snug-fit". These connections require fewer bolts than slip-critical connections and therefore are a less expensive alternative.
Slip-critical connections are more common on flange plates for beam and column splices and moment critical connections.
Bearing type connections are used in light weight structures and in member connections where slip is not important and prevention of structural failure is the design constraint. Common bearing type connections include: shear tabs, beam supports, gusset plates in trusses. The numbers stamped on the head of the bolt are referred to the grade of the bolt used in certain application with the strength of a bolt. High-strength steel bolts usually have a hexagonal head with an ISO strength rating called property class stamped on the head.
The property classes most often used are 5. The number before the point is the ultimate tensile strength in MPa divided by The number after the point is the multiplier ratio of yield strength to ultimate tensile strength. For example, a property class 5. 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.
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.
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 is 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. High-strength steel bolts have property class 8.
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.
Such inferior fasteners are a danger to life and property when used in aircraft, automobiles, heavy trucks, and similar critical applications. SAE J defines the bolt grades for inch-system sized bolts and screws. It defines them by grade , which ranges from 0 to 8, with 8 being the strongest.
Higher grades do not exist within the specification. Some varieties of screw are manufactured with a break-away head, which snaps off when adequate torque is applied. This prevents tampering and also provides an easily inspectable joint to guarantee proper assembly. An example of this is the shear bolts used on vehicle steering columns , to secure the ignition switch.
Modern screws employ a wide variety of 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, and Pozidriv has almost completely replaced Phillips in Europe. 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 home repair person.
The hand tool used to drive in most screws is called a screwdriver. A power tool that does the same job is a power screwdriver ; power drills may also be used with screw-driving attachments.
Where the holding power of the screwed joint is critical, torque-measuring and torque-limiting screwdrivers are used to ensure sufficient but not excessive force is developed by the screw.
The hand tool for driving hex head threaded fasteners is a spanner UK usage or wrench US usage , while a nut setter is used with a power screw driver. There are many systems for specifying the dimensions of screws, but in much of the world the ISO metric screw thread preferred series has displaced the many older systems.
The basic principles of the ISO metric screw thread are defined in international standard ISO and preferred combinations of diameter and pitch are listed in ISO The smaller subset of diameter and pitch combinations commonly used in screws, nuts and bolts is given in ISO The most commonly used pitch value for each diameter is the coarse pitch.
For some diameters, one or two additional fine pitch variants are also specified, for special applications such as threads in thin-walled pipes. ISO metric screw threads are designated by the letter M followed by the major diameter of the thread in millimeters e. If the thread does not use the normal coarse pitch e. 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 British Standard BS general purpose screws and BS precision screws. The following table lists the relationship given in these standards between the thread size and the maximal width across the hexagonal flats wrench size :. The first person to create a standard in about was the English engineer Sir Joseph Whitworth.
Whitworth screw sizes are still used, both for repairing old machinery and where a coarser thread than the metric fastener thread is required. Spanners for Whitworth bolts are marked with the size of the bolt, not the distance across the flats of the screw head. The most common use of a Whitworth pitch nowadays is in all UK scaffolding. 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.
British Association BA screw threads, named after the British Association for Advancement of Science, were devised in and standardised in Screws were described as "2BA", "4BA" etc.
This equipment made extensive use of odd-numbered BA screws, in order—it may be suspected—to reduce theft. 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. 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. The size of a UTS screw is described using the following format: X-Y , where X is the nominal size the hole or slot size in standard manufacturing practice through which the shank of the screw can easily be pushed and Y is the threads per inch TPI.
The integer sizes can be converted to the actual diameter by using the formula 0. For example, a 4 screw is 0. There are also screw sizes smaller than "0" zero or ought. The sizes are 00, , which are usually referred to as two ought, three ought, and four ought.
Most eyeglasses have the bows screwed to the frame with pronounced double ought — seventy two size screws. To calculate the major diameter of "ought" size screws count the number of 0's and multiply this number by. For example, the major diameter of a screw thread is. There are three steps in manufacturing a screw: heading , thread rolling , and coating.
Screws are normally made from wire , which is supplied in large coils, or round bar stock for larger screws. The wire or rod is then cut to the proper length for the type of screw being made; this workpiece is known as a blank. It is then cold headed , which is a cold working process. Heading produces the head of the screw. The shape of the die in the machine dictates what features are pressed into the screw head; for example a flat head screw uses a flat die.
For more complicated shapes two heading processes are required to get all of the features into the screw head. This production method is used because heading has a very high production rate, and produces virtually no waste material. Slotted head screws require an extra step to cut the slot in the head; this is done on a slotting machine.
These machines are essentially stripped down milling machines designed to process as many blanks as possible. The blanks are then polished [ citation needed ] again prior to threading. The threads are usually produced via thread rolling , however some are cut.
The workpiece is then tumble finished with wood and leather media to do final cleaning and polishing. While a recent hypothesis attributes the Archimedes' screw to Sennacherib , King of Assyria , archaeological finds and pictorial evidence only appear in the Hellenistic period and the standard view holds the device to be a Greek invention, most probably by the 3rd century BC polymath Archimedes.
By the 1st century BC, wooden screws were commonly used throughout the Mediterranean world in screw presses for pressing olive oil from olives and pressing juice from grapes in winemaking. Metal screws used as fasteners were rare in Europe before the 15th century, if known at all.
Rybczynski has shown [59] that handheld screwdrivers formerly called "turnscrews" in English, in more direct parallel to their original French name, tournevis [60] have existed since medieval times the s at the latest , although they probably did not become truly widespread until after , once threaded fasteners had become commodified, as detailed below.
There were many forms of fastening in use before threaded fasteners became widespread. They tended to involve carpentry and smithing rather than machining, and they involved concepts such as dowels and pins, wedging, mortises and tenons , dovetails , nailing with or without clenching the nail ends , forge welding , and many kinds of binding with cord made of leather or fiber, using many kinds of knots. Prior to the midth century, cotter pins or pin bolts , and "clinch bolts" now called rivets , were used in shipbuilding.
Glues also existed, although not in the profusion seen today. The metal screw did not become a common fastener until machine tools for their mass production were developed toward the end of the 18th century. This development blossomed in the s and s [61] along two separate paths that soon converged : [62] the mass production of wood screws [meaning screws made of metal to be used in wood] in a specialized, single-purpose, high-volume-production machine tool; and the low-count, toolroom -style production of machine screws V-thread with easy selection among various pitches whatever the machinist happened to need on any given day.
The first path was pioneered by brothers Job and William Wyatt of Staffordshire , UK, [63] who patented in a machine that we 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, [63] and the slot was cut with a rotary file while the main spindle held still presaging live tools on lathes years later.
Not until did the Wyatt brothers have a wood-screw factory up and running. Meanwhile, English instrument maker Jesse Ramsden — was working on the toolmaking and instrument-making end of the screw-cutting problem, and in he invented the first satisfactory screw-cutting lathe. 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. His firm would remain 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 — era, with the Wyatts and Maudslay being 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 unit prices lower and lower for decades to come, throughout the 19th century.
The American development of the turret lathe s and of automatic screw machines derived from it s drastically reduced the unit cost of threaded fasteners by increasingly automating the machine tool control. This cost reduction spurred ever greater use 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 machine and served most applications adequately. Rybczynski describes a flurry of patents for alternative drive types in the s through s, [67] but explains that these were patented but not manufactured due to the difficulties and expense of doing so at the time.
In , Canadian P. 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.
In the early s, the popular Phillips-head screw was invented by American Henry F. Threadform standardization further improved in the late s, 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, were one of the central technical advances, along with flat surfaces, that enabled the industrial revolution.
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