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A screw is a shaft with a helical groove or thread formed on its surface and provision at one end to turn the screw. Its main uses are as a threaded fastener used to hold objects together, and as a simple machine used to translate torque into linear force. It can also be defined as an inclined plane wrapped around a shaft.
rew used as a threaded fastener consists of a cylindrical shaft, which in many cases tapers to a point at one end, and with a helical ridge or thread formed on it, and a head at the other end which can be rotated by some means.
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A screw is a shaft with a helical groove or thread formed on its surface and provision at one end to turn the screw. Its main uses are as a threaded fastener used to hold objects together, and as a simple machine used to translate torque into linear force. It can also be defined as an inclined plane wrapped around a shaft.
Screws and bolts
A screw used as a threaded fastener consists of a cylindrical shaft, which in many cases tapers to a point at one end, and with a helical ridge or thread formed on it, and a head at the other end which can be rotated by some means. The thread is essentially an inclined plane wrapped around the shaft. The thread mates with a complementary helix in the material. The material may be manufactured with the mating helix using a tap, or the screw may create it when first driven in (a self-tapping screw). The head is specially shaped to allow a screwdriver or wrench (British English: spanner) to rotate the screw, driving it in or releasing it. The head is of larger diameter than the body of the screw and has no thread so that the screw can not be driven deeper than the length of the shaft, and to provide compression.
Screws can normally be removed and reinserted without reducing their effectiveness. They have greater holding power than nails and permit disassembly and reuse of objects.
The vast majority of screws are tightened by clockwise rotation; we speak of a right-hand thread. Screws with left-hand threads are used in exceptional cases, when the screw is subject to anticlockwise forces that might undo a right-hand thread. Left-hand screws are used on rotating items such as the left-hand grinding wheel on a bench grinder or the left hand pedal on a bicycle (both looking towards the equipment) or hub nuts on the left side of some automobiles.
Threaded fasteners were made by a cutting action such as dies provide, but recent advances in tooling allow them to be made by rolling an unthreaded rod (the blank) between two specially machined dies which squeeze the blank into the shape of the required fastener, including the thread. This method has the advantages of work hardening the thread and saving material. A rolled thread can be distinguished from a thread formed by a die as the outside diameter of the thread is greater than the diameter of the unthreaded portion of the shaft. Bicycle spokes, which are just bolts with long thin unthreaded portions, always use rolled threads for strength.
Differentiation between bolt and screw
A universally accepted distinction between a screw and a bolt (in the context of threaded fasteners) does not exist. The Machinery's Handbook, Edition 26 describes the distinction as follows:
Differentiation between Bolt and Screw -
A bolt is an externally threaded fastener designed for insertion through holes in assembled parts, and is normally intended to be tightened or released by torquing a nut.
A screw is an externally threaded fastener capable of being inserted into holes in assembled parts, of mating with a preformed internal thread or forming its own thread of being tightened or released by torquing the head.
An externally threaded fastener which is prevented from being turned during assembly and which can be tightened or released only by torquing a nut is a bolt. (Example: round head bolts, track bolts, plow bolts.)
An externally threaded fastener that has thread form which prohibits assembly with a nut having a straight thread of multiple pitch length is a screw. (Example: wood screws, tapping screws.)
This distinction is consistent with ASME B18.2.1 and some dictionary definitions for screw
and bolt
.
The issue of what is a screw and what is a bolt is not completely resolved with Machinery's Handbook distinction, however, because of confounding terms, the ambiguous nature of some parts of the distinction and usage variations. Some of these issues are discussed below:
Machine screws – ASME standards specify a variety of “Machine Screws” in diameters ranging up to ¾ of an inch. These fasteners are often used with nuts and they are often driven into tapped holes. They might be considered a screw or a bolt based on the Machinery's Handbook distinction. In practice, they tend to be mostly available in smaller sizes and the smaller sizes are referred to as screws or less ambiguously as machine screws, although some kinds of machine screws can be referred to as stove bolts.
Hex cap screws – ASME standard B18.2.1 -1981 specifies Hex Cap Screws that range in size from ¼ inch to 3 inches in diameter. These fasteners are very similar to hex bolts. They differ mostly in that they are manufactured to tighter tolerances than the corresponding bolts. The Machinery's Handbook refers parenthetically to these fasteners as “Finished Hex Bolts”. Reasonably, these fasteners might be referred to as bolts but based on the US government document, Distinguishing Bolts from Screws, the US government might classify them as screws because of the tighter tolerance.
Lug bolts, Head bolts – These terms 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. This variance, perhaps, originated from common usage ideas that screws are small and bolts are big.
Lag bolt – These are clearly screws based on the Machinery's Handbook distinction. The term has been replaced by "Lag Screw" in the Machinery's Handbook and probably only continues in common usage because of common language notion that bolts are big.
Government standards – The US government made an effort to formalize the difference between a bolt and a screw, because different tariffs apply to each. The document seems to have no significant effect on common usage and does not eliminate the ambiguous nature of the distinction between screws and bolts for some threaded fasteners. The definition is available online.
Historical issue – Old USS and SAE standards defined cap screws as fasteners with shafts that were threaded to the head and bolts as fasteners with shafts that were partially unthreaded. This is now an obsolete distinction.
Controlled vocabulary versus natural language – The distinctions delineated above are enforced in the controlled vocabulary of standards organizations. Nevertheless, there are sometimes differences between the controlled vocabulary and the natural-language usage of the words among machinists, auto mechanics, and other workers. These differences reflect linguistic evolution shaped by the changing of technology over centuries. The words bolt and screw have both existed since before today's modern mix of fastener types existed, and the natural usage of those words has evolved retronymously in response to the technological change. (That is, the use of words as names for objects changes as the objects themselves change.) Nonthreaded fasteners predominated in fastening technology until the advent of practical, inexpensive screw-cutting in the early 19th century. The basic meaning of the word screw has long involved the idea of a helical screw thread. The word bolt is also a very old word, and it was used for centuries to refer to metal rods that passed through the substrate to be fastened on the other side, often via nonthreaded means (clinching, forge welding, pinning, wedging, etc). The connection of this sense to the sense of a door bolt is apparent. In the 19th century, bolts fastened via screw threads were often called screw bolts in contradistinction to clench bolts or clinch bolts.
Other distinctions – At times the words screw and bolt may be defined in ways similar to these definitions:
Bolts are defined as headed fasteners having external threads that meet an exacting, uniform bolt thread specification (such as M, MJ, UN, UNR, and UNJ) such that they can accept a nontapered nut.
Screws are defined as headed, externally-threaded fasteners that do not meet the above definition of bolts.
These definitions of screw and bolt eliminate the ambiguity of the Machinery's handbook distinction. 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.
Other fastening methods
Alternative fasteners to screws and bolts are nails, rivets, roll pins, pinned shafts, welding, soldering, brazing, and gluing (including taping), and clinch fastening.
Materials and strength
Screws and bolts are made from a wide range of materials, with steel being perhaps the most common, in many varieties. Where great resistance to weather or corrosion is required, stainless steel, titanium, brass, bronze, monel or silicon bronze may be used, or a coating such as brass, zinc or chromium applied. Electrolytic action from dissimilar metals can be prevented with aluminium screws for double-glazing tracks, for example. Some types of plastic, such as nylon or Teflon, can be threaded and used for fastening requiring moderate strength and great resistance to corrosion or for the purpose of electrical insulation. Even porcelain and glass can have molded screw threads that are used successfully in applications such as electrical line insulators and canning jars.
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. 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. Gradings are indicated as markings, while grade 0 is the lowest, grade 10 is the highest. Here is the sequence of bolt strength and markings, from least to most. Grade 0, 1 and 2 bolts have no markings, grade 3 has 2 radial lines, grade 5 has 3, grade 6 has 4, grade 7 has 5, grade 8 has 6, grade 9 has 7, grade 10 has 8.
In some applications joints are designed so that the screw or bolt will intentionally fail before more expensive components. In this case replacing an existing fastener with a higher strength fastener can result in equipment damage. Thus it is generally good practice to replace fasteners with the same grade originally installed.
Mechanical analysis
A screw or bolt is a specialized application of the inclined plane. The inclined plane, called its thread, is helically disposed around a cylinder or shaft. That thread usually either fits into a corresponding (negative or female) helical thread in a nut, or forms a corresponding helical cut in surrounding softer material as it is inserted. A simple screw, such as for fastening, is typically pointed, and thereby is commonly distinguished (in informal terminology) from a bolt or machine screw. Common screws, and usually bolts, have a head which may be mechanically driven or rotated, which usually serves as a stop, and may have an unthreaded shoulder portion beneath the head.
The technical analysis (see also statics, dynamics) to determine the pitch, thread profile, coefficient of friction (static and dynamic), and holding power of a screw or bolt is very similar to that performed to predict wedge behavior. Wedges are discussed in the article on simple machines.
Critical applications of screws and bolts will specify a torque that must be applied when driving it. The main concept is to tension the bolt, and compress parts being held together, creating a spring-like assembly. The stress thus introduced to the bolt is called a preload. When external forces try to separate the parts, the bolt experiences no strain unless the preload force is exceeded.
As long as the preload is never exceeded, the bolt or nut will never come loose (assuming the full strength of the bolt is used). If the full strength of the bolt is not used (for example, a steel bolt threaded into aluminium, then a thread-locking adhesive or insert may be used.
If the preload is exceeded during normal use, the joint will eventually fail. The preload is calculated as a percentage of the bolt's yield tensile strength, or the strength of the threads it goes into, or the compressive strength of the clamped layers (plates, washers, gaskets), whichever is least.
Tensile strength
Screws and bolts are usually in tension when properly fitted. In most applications they are not designed to bear large shear forces. For example, when two overlapping metal bars joined by a bolt are likely to be pulled apart longitudinally, the bolt must be tight enough so that the friction between the two bars can overcome the longitudinal force. If the bars slip, then the bolt may be sheared in half, or friction between the bolt and slipping bars may erode and weaken the bolt (called fretting). For this type of application, high-strength steel bolts are used and should be tightened to a specified torque.
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 5.8, 8.8, and 10.9. The number before the point is the tensile ultimate strength in MPa divided by 100. The number after the point is 10 times the ratio of tensile yield strength to tensile ultimate strength. For example, a property class 5.8 bolt has a nominal (minimum) tensile ultimate strength of 500 MPa, and a tensile yield strength of 0.8 times tensile ultimate strength or 0.8(500) = 400 MPa.
Tensile ultimate strength is the stress at which the bolt fails (breaks in half). Tensile yield strength is the stress at which the bolt will receive a permanent set (an elongation from which it will not recover when the force is removed) of 0.2 % offset strain. 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, since the fastener has suffered permanent plastic deformation.
Mild steel bolts have property class 4.6. High-strength steel bolts have property class 8.8 or above. An M10, property class 8.8 bolt can very safely hold a static tensile load of about 15 kN.
There is no simple method to measure the tension of a bolt already in place other than to tighten it and identify at which point the bolt starts moving. This is known as 're-torqueing'. An electronic torque wrench is used on the bolt under test, and the torque applied is constantly measured. When the bolt starts moving (tightening) the torque briefly drops sharply - this drop-off point is considered the measure of tension.
Recent developments enable bolt tensions to be estimated by using ultrasonic testing. Another way to ensure correct bolt tension (mainly in steel erecting) involves the use of crush-washers. These are washers that have been drilled and filled with orange RTV. When the orange rubber strands appear, the tension is correct.
Large volume users such as auto makers frequently use computer controlled nut drivers. With such machines the computer in effect plots a graph of the torque exerted. Once the torque ceases to rise (the point where the bolt begins to deform) the machine stops. Such machines are often used to fit wheelnuts and will normally tighten all the wheel nuts simultaneously.
Types of screws and bolts
Threaded fasteners either have a tapered shaft or a non-tapered shaft. Fasteners with tapered shafts 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 shaft are designed to mate with a nut or to be driven into a tapped hole.
Fasteners with a tapered shaft (self-threading screws)
Screw : There is not a universally accepted definition of the word, screw. It generally refers to a smaller threaded fastener with a tapered shaft. See the section Differentiation between bolt and screw above for a more detailed discussion.
Wood screw : Generally has an unthreaded portion of the shaft below the head. It is designed to attach two pieces of wood together.
Lag screw (lag bolt) : Similar to a wood screw except that it is generally much larger running to lengths up to with diameters from ¼" to ½" (6.4–12.25 mm) in commonly available (hardware store) sizes (not counting larger mining and civil engineering lags and lag bolts) and it generally has a hexagonal head drive head. Lag bolts are designed for securely fastening heavy timbers (post and beams, timber railway trestles and bridges) to one another, or to fasten wood to masonry or concrete.
- 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 amalgamated 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.
Coach bolts : rather like lag bolts, but normally have a square (4 sided) head, rather than a hexagon. Formally used in (horse-drawn) coaches, and have a large thread with good holding force, and an un-threaded part, rather like a giant wood screw.
Sheet metal screw (self-tapping screw, thread cutting screws) : Has sharp threads that cut into a material such as sheet metal, plastic or wood. They are sometimes notched at the tip to aid in chip removal during thread cutting. The shaft is usually threaded up to the head. Sheet metal screws make excellent fasteners for attaching metal hardware to wood because the fully threaded shaft provides good retention in wood.
Self-drilling screw (Teks screw) : Similar to a sheet metal screw, but it has a drill-shaped point to cut through the substrate to eliminate the need for drilling a pilot hole. Designed for use in soft steel or other metals.
Drywall screw : Specialized screw with a bugle head that is designed to attach drywall to wood or metal studs, however it is a versatile construction fastener with many uses. The diameter of drywall screw threads is larger than the shaft diameter.
Particle board screw (chipboard screw) : Similar to a drywall screw except that it has a thinner shaft and provides better resistance to pull-out in particle board, while offset against a lower shear strength.
Deck screw : Similar to drywall screw except that it is has improved corrosion resistance and is generally supplied in a larger gauge.
Double ended screw (dowel screw) : Similar to a wood screw but with two pointed ends and no head, used for making hidden joints between two pieces of wood.
Screw eye (eye screw) : Screw with a looped head. Larger ones are sometimes call lag eye screws. Designed to be used as attachment point, particularly for something that is hung from it.
Fasteners with a non-tapered shaft
Bolt : There is no universally accepted definition of the word bolt. It generally refers to a larger threaded fastener with a non-tapered shaft. See the section Differentiation between bolt and screw above for a more detailed discussion.
Cap screw : In places the term is used interchangeably with bolt. In the past the term cap screw was restricted to threaded fasteners with a shaft that is threaded all the way to the head, but this is now a non-standard usage.
Hex cap screw : Cap screw with a hexagonal head, designed to be driven by a wrench (spanner). An ASME B18.2.1 compliant cap screw has somewhat tighter tolerances than a hex bolt for the head height and the shaft length. The nature of the tolerance difference allows an ASME B18.2.1 hex cap screw to always fit where a hex bolt is installed but a hex bolt could be slightly too large to be used where a hex cap screw is designed in.
Hex bolt : At times the term is used interchangeably with hex cap screw. An ASME B18.2.1 compliant hex bolt is built to different tolerances than a hex cap screw.
Socket cap screw : Also known as a socket head cap screw, socket screw or Allen bolt, this is a type of cap screw with a hexagonal recessed drive. The most common types in use have a cylindrical head whose diameter is nominally 1.5 times (1960 series design) that of the screw shank (major) diameter. Counterbored holes in parts allow the screw head to be flush with the surface or recessed. Other head designs include 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 screws are commonly used in assemblies that do not provide sufficient clearance for a conventional wrench or socket.
Machine screw : Generally a smaller fastener (less than ¼ inch in diameter) threaded the entire length of its shaft that usually has a recessed drive type (slotted, Phillips, etc.). Machine screws are also made with socket heads (see above), in which case they may be referred to as socket head machine screws.
Self-tapping machine screw : Similar to a machine screw except the lower part of the shaft is designed to cut threads as the screw is driven into an untapped hole. The advantage of this screw type over a self-tapping screw is that, if the screw is reinstalled, new threads are not cut as the screw is driven.
Set screw (grub screw) : Generally a headless screw but can be any screw used to fix a rotating part to a shaft. The set screw is driven through a threaded hole in the rotating part until it is tight against the shaft. The most often used type is the socket set screw, which is tightened or loosened with a hex key or hex driver.
Tap bolt : A bolt that is threaded all the way to the head. An ASME B18.2.1 compliant tap bolt has the same tolerances as an ASME B18.2.1 compliant hex cap screw.
Stud : similar to a bolt but without the head. Studs are threaded on both ends. In some cases the entire length of the stud is threaded, while in other cases there will be an un-threaded section in the middle. (See also: screw anchor, wedge anchor.)
Eye bolt : A bolt with a looped head.
Toggle bolt : A bolt with a special nut known as a wing. It is designed to be used where there is no access to side of the material where the nut is located. Usually the wing is spring loaded and expands after being inserted into the hole.
Carriage bolt (coach bolt) : Has a domed or countersunk head, and the shaft is topped by a short square section under the head. The square section grips into the part being fixed (typically wood), preventing the bolt from turning when the nut is tightened. A rib neck carriage bolt has several longitudinal ribs instead of the square section, to grip into a metal part being fixed.
Stove bolt : A type of machine screw that has a round or flat head and is threaded to the head. They are usually made of low grade steel, have a slot or Philips drive, and are used to join sheet metal parts using a hex or square nut.
Shoulder screw : Screw used for revolving joints in mechanisms and linkages. A shoulder screw consists of the shaft, which is ground to a precise diameter, and a threaded end, which is smaller in diameter than the shaft. Unlike other threaded fasteners, the size of a shoulder screw is defined by the shaft diameter, not the thread diameter. Shoulder screws are also called stripper bolts, as they are often used as guides for the stripper plate(s) in a die set.
Thumb screw : A threaded fastener designed to be twisted into a tapped hole by hand without the use of tools.
Tension control bolt (TC bolt) : Heavy duty bolt used in steel frame construction. The head is usually domed and is not designed to be driven. The end of the shaft has a spline on it which is engaged by a special power wrench which prevents the bolt from turning while the nut is tightened. When the appropriate torque is reached the spline shears off.
Plow bolt : A bolt similar to a carriage bolt, except the head is flat or concave. There are many variations, with some not using a square base, but rather a key, a locking slot, or other means. The recess in the mating part must be designed to accept the particular plow bolt.
Other threaded fasteners
Thread rolling screws : These have a lobed (usually triangular) cross section. They form threads by pushing outward during installation. They may have tapping threads or machine threads.
Superbolt, or multi-jackbolt tensioner : 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.
Hanger screw : A headless fastener that has machine screw threads on one end and self-tapping threads on the other designed to be driven into wood or another soft substrate. Often used for mounting legs on tables.
Shapes of screw head
Pan head: A low disc with chamfered outer edge
Button or dome head: Cylindrical with a rounded top
Round head: Dome-shaped, commonly used for machine screws
Truss head: Lower-profile dome designed to prevent tampering
Flat head or countersunk: Conical, with flat outer face and tapering inner face allowing it to sink into the material
Oval or raised head: Countersunk with a rounded top
Bugle head: Similar to countersunk, but there is a smooth progression from the shaft to the angle of the head, similar to the bell of a bugle
Cheese head: Disc with cylindrical outer edge, height approximately half the head diameter
Fillister head: Cylindrical, but with a slightly convex top surface. Height to diameter ratio is larger than cheese head.
Socket head: Cylindrical, relatively high, with different types of sockets (hex, square, Torx, etc.)
Mirror screw head: Countersunk head with a tapped hole to receive a separate screw-in chrome-plated cover, used for attaching mirrors
Headless (set or grub screw): Has either a socket or slot in one end for driving
Square head: A 4 sided head used for high torque driving with a wrench.
Some varieties of screw are manufactured with a break-away head, which snaps off when adequate torque is applied. This prevents tampering and disassembly and also provides an easily-inspectable joint to guarantee proper assembly. An example of this is the shear bolts used on car steering columns, to secure the ignition switch.
Types of screw drive
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; 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 home repair person.
Slot head : Has a single slot, and is driven by a flat-bladed screwdriver. The slotted screw is common in woodworking applications, but is not often seen in applications where a power driver would be used, due to the tendency of a power driver to slip out of the head and potentially damage the surrounding material.
Cross-head, cross-point, or cruciform : has a "+"-shaped slot and is driven by a cross-head screwdriver, designed originally for use with mechanical screwing machines. There are five types:
- ; Phillips : Has slightly rounded corners in the tool recess, and was designed so the driver will slip out, or cam out, under high torque to prevent over-tightening. The Phillips Screw Company was founded in Oregon in 1933 by Henry F. Phillips, who bought the design from J. P. Thompson. Phillips was unable to manufacture the design, so he passed the patent to the American Screw Company, who was the first to manufacture it.
; Reed & Prince or Frearson : Similar to a Phillips but has a more pointed 75° V shape. Its advantage over the Phillips drive is that one driver or bit fits all screw sizes. It is found mainly in marine hardware and requires a special screw driver or bit to work properly. The tool recess is a perfect cross, unlike the Phillips head, which is designed to cam out. It was developed by an English inventor named Frearson in the 19th century and produced from the late 1930s to the mid-1970s by the former Reed & Prince Manufacturing Company of Worcester, Massachusetts, a company which traces its origins to Kingston, Massachusetts, in 1882, and was liquidated in 1990 with the sale of company assets. The company is now in business.
- ; JIS : Commonly found in Japanese equipment. Looks like a Phillips screw, but is designed not to cam out and will, therefore, be damaged by a Phillips screwdriver if it is too tight. Heads are usually identifiable by a single dot to one side of the cross slot. The standard number is
- ; French recess : also called BNAE NFL22-070 after its Bureau de Normalisation de l'Aéronautique et de l'Espace standard number.
- ; Pozidriv : similar to cross-head but designed not to slip, or cam out. It has four additional points of contact, and does not have the rounded corners that the Phillips screw drive has. Phillips screwdrivers will usually work in Pozidriv screws, but Pozidriv screwdrivers are likely to slip or tear out the screw head when used in Phillips screws. Heads are marked with crossed, single lines at 45 degrees to the cross recess, for identification. (Note that doubled lines at 45 are a different recess: a very specialised Phillips screw.)
- ; Supadriv : similar to Pozidriv.
TORX : a star-shaped "hexalobular" drive with six rounded points. It was designed to permit increased torque transfer from the driver to the bit compared to other drive systems. TORX is very popular in the automotive and electronics industries due to resistance to cam out and extended bit life, as well as reduced operator fatigue by minimizing the need to bear down on the drive tool to prevent cam out. TORX PLUS is an improved version of TORX which extends tool life even further and permits greater torque transfer compared to TORX. A tamper-resistant TORX head has a small pin inside the recess. The tamper-resistant TORX is also made in a 5 lobed variant. These "5-star" TORX configurations are commonly used in correctional facilities, public facilities and government schools, but can also be found in some electronic devices.
TTAP : an improved "hexalobular" drive for without wobbling and stable stick-fit. TTAP is backward convertible with generic hexalobular (Torx) drive.
- Hexagonal (hex) socket : Has a hexagonal hole and is driven by a hex wrench, sometimes called an Allen key or Hex key, or by a power tool with a hexagonal bit. Tamper-resistant versions with a pin in the recess are available. Hex sockets are increasingly used for modern bicycle parts because hex wrenches are very light and easily carried tools. They are also frequently used for self-assembled furniture.
Robertson : Invented in 1908 by P.L. Robertson, has a square hole and is driven by a special power-tool bit or screwdriver. In the United States it is referred to as square drive. The screw is designed to maximize torque transferred from the driver, and will not slip, or cam out. It is possible to hold a Robertson screw on a driver bit horizontally or even pendant, due to a slight wedge fit. Commonly found in Canada in carpentry and woodworking applications and in Canadian-manufactured electrical wiring items such as receptacles and switch boxes. It is increasingly used in the United States for woodworking applications.
Tri-Wing : Has a triangular slotted configuration. They have been used by Nintendo on several consoles and accessories, including the Game Boy, Wii, and Wii Remote, and by Nokia on some phones and chargers to discourage home repair.
Torq-Set or offset cruciform : May be confused with Phillips; however, the four legs of the contact area are offset in this drive type. This type is commonly used in the aerospace industry.
Spanner : Uses two round holes opposite each other, and is designed to prevent tampering. Commonly seen in elevators in the United States. Note that in the UK, "spanner" is the usual word for "wrench".
Clutch Type A or standard clutch : Resembles a bow tie. These were common in GM automobiles, trucks and buses of the 1940s and 1950s, particularly for body panels.
Clutch Type G : Resembles a butterfly. This type of screw head is commonly used in the manufacture of mobile homes and recreational vehicles.
Combination drives
Some screws have heads designed to accommodate more than one kind of driver, sometimes referred to as combo-head or combi-head. The most common of these is a combination of a slotted and Phillips head, often used in attaching knobs to furniture drawer fronts. Because of its prevalence, there are now drivers made specifically for this kind of screw head. Other combinations are a Phillips and Robertson, a Robertson and a slotted, a Torx and a slotted, and a triple-drive screw which can take a slotted, Phillips or a Robertson. The Recex drive system claims it offers the combined non-slip convenience of a Robertson drive during production assembly and Phillips for after market serviceability. Quadrex is another Phillips/Robertson drive. Phillips Screw Company offers both Phillips and Pozidriv combo heads with Robertson.
Tamper-resistant screws
The general theory of tamper-resistant fasteners is to make a fastener whose loosening requires a tool that a tamperer is unlikely to have on hand at the time of opportunity for tampering. There is no expectation that it will be impossible for a tamperer to obtain the driver. Rather, the main idea is simply that most tamperers will not bother to seek out and obtain a driver. In the case of end-users, this reduces the incidence of do-it-yourself repair or modifications (and any resulting injury or product damage). In the cases of vandalism prevention and theft prevention, since most vandalism and theft incidents are simply crimes of easy opportunity, the idea is to "raise the bar" and make the opportunity less convenient.
Many screw drives, including Phillips, Torx, and hex socket, have tamper-resistant variants. These typically have a pin protruding in the center of the screw head, necessitating a special tool for extraction. In some variants the pin is placed slightly off-center, requiring a correspondingly shaped bit. However, the bits for many tamper-resistant screw heads are now readily available from hardware stores, tool suppliers and through the Internet. There are also many commonly used techniques to extract tamper resistant screws without the correct driver — for example, the use of an alternative driver that can achieve enough grip to turn the screw, modifying the head to accept an alternative driver, forming one's own driver by melting an object into the head to mould a driver, or simply turning the screw using a pair of locking pliers. Thus, these special screws offer only modest security. However, it is often enough to discourage the more mindless varieties of vandalism.
The slotted screw drive also comes in a tamper-resistant one-way design with sloped edges; the screw can be driven in, but the bit slips out in the reverse direction.
There are specialty fastener companies that make unusual, proprietary head designs, featuring matching drivers available only from them, and only supplied to registered owners. These tend to be confined to industrial uses that the average layperson does not have contact with. But one example familiar to laypersons is the attachment for the wheels and/or spare tires of some types of car; one of the nuts may require a specialized socket (provided with the car) to prevent theft.
The breakaway bolt is a high-security fastener that is extremely difficult to remove. It consists of a counter-sunk flat head screw, with a thin shaft and hex head protruding from the flat head. The hex head is used to drive the bolt into the countersunk hole, then the wrench or hammer is used to knock the shaft and hex head off of the flat head, leaving only a smooth screw head exposed. Removal is facilitated by drilling a small hole part way into the outer part of the head and using a broken-screw extractor ("easy-out") or a punch and hammer at a sharp angle in a counter-clockwise direction. This type of screw is used primarily in prison door locks.
Tools used
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).
Mechanics of use
When driving in a screw, especially when the screw has been removed and is being placed again, the threads can become misaligned and damage, or strip, the threading of the hole. To avoid this, slight pressure is applied and the screw is driven in reverse, until the leading edges of the helices pass each other, at which point a slight click will be felt (and sometimes heard.) When this happens, the screw will often assume a more aligned position with respect to the hole.
Immediately after the 'click', the screw may be driven in without damage to the threading. This technique is useful for re-seating screws in wood and plastic, and for assuring the proper fit when screwing down plates and covers where alignment is difficult.
Thread standards
- See also: Screw thread
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. Other relatively common systems include the British Standard Whitworth, BA system (British Association), and the SAE Unified Thread Standard.
ISO metric screw thread
The basic principles of the ISO metric screw thread are defined in international standard ISO 68-1 and preferred combinations of diameter and pitch are listed in ISO 261. The smaller subset of diameter and pitch combinations commonly used in screws, nuts and bolts is given in ISO 262. The most commonly used pitch value for each diameter is known as 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.g. "M8". If the thread does not use the normal "coarse pitch" (e.g., 1.25 mm in the case of M8), then the pitch in millimeters is also appended with a multiplication sign, e.g. "M8×1" if the screw thread has an outer diameter of 8 mm and advances by 1 mm per 360° rotation.
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 shaft, and the nut or threaded hole is made by tapping threads in a 5 mm hole.
Metric hexagon bolts, screws and nuts are specified, for example, in British Standard BS 4190 (general purpose screws) and BS 3692 (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):
| ISO metric thread | M1.6 | M2 | M2.5 | M3 | M4 | M5 | M6 | M8 | M10 | M12 | M16 | M20 | M24 | M30 | M36 | M42 | M48 | M56 | M64 |
|---|
| wrench size (mm) | 3.2 | 4 | 5 | 5.5 | 7 | 8 | 10 | 13 | 17 | 19 | 24 | 30 | 36 | 46 | 55 | 65 | 75 | 85 | 95 |
|---|
In addition, the following non-preferred intermediate sizes are specified:
| ISO metric thread | M14 | M18 | M22 | M27 | M33 | M39 | M45 | M52 | M60 | M68 |
|---|
| wrench size (mm) | 22 | 27 | 32 | 41 | 50 | 60 | 70 | 80 | 90 | 100 |
|---|
Whitworth
The first person to create a standard (in about 1841) 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. Whitworth became British Standard Whitworth, abbreviated to BSW (BS 84:1956) and the British Standard Fine (BSF) thread was introduced in 1908 because the Whitworth thread was a bit coarse for some applications. The thread angle was 55° and a depth and pitch of thread that varied with the diameter of the thread (i.e., the bigger the bolt, the coarser the thread). The spanner size is determined by the size of the bolt, not the distance between the flats.
The most common use of a Whitworth pitch nowadays is in all (UK) scaffolding where a 7/16" spanner size is required. A 21mm spanner is frequently used, and works for this application. 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.
British Association screw threads (BA)
A later standard established in the United Kingdom was the BA system, named after the British Association for Advancement of Science. Screws were described as "2BA", "4BA" etc., the odd numbers being rarely used, except in equipment made prior to the 1970s for telephone exchanges in the UK. 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 1 mm pitch. These are still the most common threads in some niche applications. Certain types of fine machinery, such as moving-coil meters, tend to have BA threads wherever they are manufactured.
Unified Thread Standard
The United States of America has its own system, usually called the Unified Thread Standard, which is also extensively used in Canada and in most other countries around the world. At least 85% of the world's fasteners are dimensioned to Unified thread dimensions, and the biggest selection of fastener sizes and materials are found supplied in this standard. A version of this standard, called SAE for the Society of Automotive Engineers, was used in the American automobile industry. The SAE is still associated with inch-based fasteners by the public, even though the U.S. auto industry (and other heavy industries relying on SAE) have gradually converted to ISO preferred series fasteners for some assemblies from the 1970s onward, because global parts sourcing and product marketing favor international standardization. However, all automobiles sold throughout the world contain both metric (engine assemblies) and Imperial fasteners (for example, lug nuts, oxygen sensors, internal electrical assemblies, body fasteners, lamps, steering, brake and suspension parts).
Machine screws are described as 0-80, 2-56, 3-48, 4-40, 5-40, 6-32, 8-32, 10-32, 10-24, etc. up to size 16. The first number can be translated to a diameter using a formula, the second is the number of threads per inch. There is a coarse thread and a fine thread for each size, the fine thread being preferred in thin materials or when slightly greater strength is desired.
The numbering system follows a roughly logarithmic series where an increase in each screw number size approximately doubles the tensile strength of the screw and the screw number is found by , where "d" is the nominal diameter. Using this formula a #5 screw has a major diameter of .125" (1/8"), a #10 screw has a diameter of .190" (or 3/16" in practical terms), etc. The formula applies for screw thread numbers #0 and higher, but does NOT apply to smaller Unified miniature screw thread series. Typically screws smaller than size #0 are supplied in the Unified Miniature Series. The formula for number sizes smaller than size #0 is given by , with the zero size being the number of zeros after the first. So a #00 screw is .047" dia, #000 is .034" dia, etc.
The number series of machine screws once included odd numbers (7, 9, etc.) and extended up to #16 or more. Standardization efforts in the late 19th and the early part of the 20th century reduced the range of sizes considerably. Now, it is less common to see machine screws larger than #14, or odd number sizes other than #1, #3 and #5. Even though #14 and #16 screws are still available, they are not as common as sizes #0 through #12.
Sizes 1/4" diameter and larger are designated as 1/4"-20, 1/4"-28, etc. the first number giving the diameter in inches and the second number being threads per inch. Most thread sizes are available in UNC or UC (Unified Coarse Thread, example 1/4"-20) or UNF or UF (Unified Fine Thread, example 1/4"-28).
A Unified Miniature screw thread series is defined in ANSI standard B1.10, for fasteners of 0.3 to 1.4 millimetres (0.0118 to 0.0551 inch) diameter. These sizes are intended for watches, instruments, and miniature mechanisms and are interchangeable with threads made to ISO Standard 68.
Others
Other historical, specialized or obsolescent thread systems include:
- Acme thread form
- BSP (British standard pipe thread which exists in a taper and non taper variant; used for other purposes as well)
- BSC (British Standard Cycle) a 26tpi thread form
- British Standard Buttress Threads (BS 1657:1950),
- British Standard for Spark Plugs BS 45:1972
- CEI (Cycle Engineers Institute, used on bicycles in Britain and possibly elsewhere)
- British Standard Brass a fixed pitch 26tpi thread
- Edison base lamp holder screw thread
- Fire hose connection (NFPA standard 194)
- Hose Coupling Screw Threads (ANSI B2.4-1966) for garden hoses and accessories
- Lowenhertz thread, a German metric thread used for measuring instruments
- NPT (National Pipe Thread) and NPTF (National Pipe Thread Fuel)
- PG (German: "Panzer-Gewinde"), used in thin plate metal, such as for switches and nipples in electrical equipment housings
-
Society Thread, a 36 threads/inch Whitworth form standarded by the Royal Microscopical Society of London for microscope objective lenses.
History
In antiquity, the screw was first used as part of the screw pump of Sennacherib, King of Assyria, for the water systems at the Hanging Gardens of Babylon and Nineveh in the 7th century BC.
The screw was later described by the Greek mathematician Archytas of Tarentum (428 – 350 BC). By the 1st century BC, wooden screws were commonly used throughout the Mediterranean world in devices such as oil and wine presses. Metal screws used as fasteners did not appear in Europe until the 1400s.
In 1744, the flat-bladed bit for the carpenter's brace was invented, the precursor to the first simple screwdriver. Handheld screwdrivers first appeared after 1800.
Prior to the mid-19th century, cotter pins or pin bolts, and "clinch bolts" (now called rivets), were used in shipbuilding.
The metal screw did not become a common fastener until machine tools for mass production were developed at the end of the 18th century. In the 1770s, English instrument maker Jesse Ramsden (1735-1800) invented the first satisfactory screw-cutting lathe. The British engineer Henry Maudslay (1771-1831) patented a screw-cutting lathe in 1797; a similar device was patented by David Wilkinson in the United States in 1798. These developments 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.
The development of the turret lathe (1840s) and of the screw machine (1870s) 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 (drive) were simple internal-wrenching slots and external-wrenching squares and hexagons. These were easy to machine and served most applications adequately. The 20th century saw the development of many other types of drive. In 1908, Canadian P. L. Robertson invented the internal-wrenching square drive. The internal-wrenching hexagon drive (hex socket) shortly followed in 1911. In the early 1930s, the Phillips-head screw was invented by Henry F. Phillips.
Threadform standardization further improved in the late 1940s, when the ISO metric screw thread and the Unified Thread Standard were defined.
Legal issues
In the United States, a screw and a bolt have different import duties. The difference between them is therefore of keen interest to importers and customs authorities. This was the subject of a court case . The position is outlined in a current US government document .
See also
Bibliography
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External links
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