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Screw mould clamping force

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From Wikipedia, the free encyclopedia

This article is about screws and bolts.
Screws come in a variety of shapes and sizes for different purposes. U.S. quarter coin
(diameter 24 mm) shown for scale.

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.


        1 Screws and bolts
            o 1.1 Differentiation between bolt and screw
        2 Other fastening methods
        3 Materials and strength
        4 Mechanical analysis
        5 Tensile strength
        6 Types of screws and bolts
            o 6.1 Fasteners with a tapered shaft (self-threading screws)
            o 6.2 Fasteners with a non-tapered shaft
            o 6.3 Other threaded fasteners
        7 Shapes of screw head
        8 Types of screw drive
            o 8.1 Combination drives
        9 Tamper-resistant screws
            o 9.1 Protruding obstacle head
            o 9.2 One-way only head
           o  9.3 Proprietary head
           o  9.4 Breakaway head
      10 Tools used
      11 Mechanics of use
      12 Thread standards
          o 12.1 ISO metric screw thread
          o 12.2 Whitworth
          o 12.3 British Association screw threads (BA)
          o 12.4 Unified Thread Standard
          o 12.5 Others
      13 History
      14 Legal issues
      15 See also
      16 References
          o 16.1 Notes
          o 16.2 Bibliography
      17 External links

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. [Circular saw blades are usually fastened on left-hand shafts.]

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

Carriage bolt with square nut.

Structural bolt DIN 6914 with DIN 6916 washer and UNI 5587 nut.

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[1]:

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,
and 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 [2] [3] and bolt [4] [5] [6].

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‖[7] 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‖[8]. 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[9].
Lug bolts[10], Head bolts [11] – 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[12] 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.[1]
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[13]. 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[14]:

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 [and
Ace fittings!].

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

Rotating screw and fixed trough

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

Rusty hexagonal bolt heads

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

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.

A Phillips wood screw being driven into a board with a drill

Fasteners with a tapered shaft (self-threading screws)

        There is not a universally accepted definition of the word, screw. It generally
        refers to a smaller[15] 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 15 inches (381 mm) 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 [―Grabber‖]
        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) [―Grabber‖]
        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 [―Grabber‖]
        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.

Combination flanged-hex/Phillips-head screw used in computers

Fasteners with a non-tapered shaft

       There is no universally accepted definition of the word bolt. It generally refers to
       a larger[15] threaded fastener with a non-tapered shaft. See the section
       Differentiation between bolt and screw above for a more detailed discussion.
Breakaway Bolt
       A Bolt with a hollow threaded shaft. Designed to break away when impacted.
       Typically used to fasten fire hydrants, so they will break away when hit by a car.
       Also used in aircraft, not to break away, but to make them lighter in weight.
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.
        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.[16]
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.[17][18]

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
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
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

(a) pan, (b) button, (c) round, (d) truss, (e) flat (countersunk), (f) oval
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
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,
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
         Part of the series on
       Screw drive types
Slotted (flat or straight)

Phillips ("crosshead")
Pozidriv (SupaDriv)

Robertson (square)

Hex socket (Allen)

    T, TS, TX

Spanner head

Triple square

One-way - Clutch
Spline drive

Double hex

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Phillips vs. Frearson

BNAE driver bit

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:
       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.[citation needed] 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.
       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 JIS B 1012:1985
       French recess
       also called BNAE NFL22-070 after its Bureau de Normalisation de
       l'Aéronautique et de l'Espace standard number.
       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.)
       similar to Pozidriv.
       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.
       an improved "hexalobular" drive for without wobbling and stable stick-fit. TTAP
       is backward convertible with generic hexalobular (Torx) drive.

       Hex socket screws
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.
       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.
       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.
       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

Protruding obstacle head

Tamper-resistant external-TORX driver

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.

One-way only head

One-way slotted screw

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.

Proprietary head

There are specialty fastener companies that make unusual, proprietary head designs,
featuring matching drivers available only from them, and only supplied to registered
owners[19]. 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.

Breakaway head

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

An electric driver screws a self-tapping phillips head screw into wood

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):

metric M1.6 M2 M2.5 M3 M4 M5 M6 M8 M10 M12 M16 M20 M24 M30 M36 M42 M48 M56 M64
  size 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


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.[20] 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

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.[21]


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
      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
      Lowenhertz thread, a German metric thread used for measuring instruments[22]
      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.

A lathe of 1871, equipped with leadscrew and change gears for single-point screw-

A Brown & Sharpe single-spindle screw machine.

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.[23]

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.[citation needed]

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 Rocknel Fastener, inc v. United States: 34 page PDF. The
position is outlined in a current US government document Distinguishing Bolts From
Screws: 21 page PDF.

See also
       Archimedes' screw                  Screw-cutting lathe        Tap and die
       Gender of connectors and           Screw propelled            Thread pitch
        fasteners                           vehicle                     gauge
       Nut (hardware)                     Screw thread               Thread repair
       Screw (simple machine)                                          insert


   1. ^ Jones (2000). Machinery's Handbook 26th edition. p. 1492.
   2. ^ "Cambridge Dictionary of American English". Cambridge University Press.
      http://dictionary.cambridge.org/define.asp?key=screw*1+0&dict=A. Retrieved on
  3. ^ "allwords".
      Find+it%21&Language=ENG. Retrieved on 2008-12-03.
  4. ^ "Merriam Webster Dictionary bolt". http://www.merriam-
      webster.com/dictionary/bolt. Retrieved on 2008-12-03.
  5. ^ "Compact Oxford English Dictionary bolt". Oxford.
      http://www.askoxford.com/concise_oed/bolt_1?view=uk. Retrieved on 2008-12-
  6. ^ "Cambridge Advanced Learner's Dictionary bolt". Cambridge University Press.
      http://dictionary.cambridge.org/define.asp?key=8680&dict=CALD. Retrieved on
  7. ^ Jones (2000). Machinery's Handbook 26th edition. pp. 1568-1598.
  8. ^ Jones (2000). Machinery's Handbook 26th edition. p. 1496.
  9. ^ "Distinguishing Bolts from Screws page 7".
      13.ctt/icp013.pdf. Retrieved on 2009-01-13.
  10. ^ "autorepair.com Glossary - lug bolt".
      http://autorepair.about.com/library/glossary/bldef-900.htm. Retrieved on 2009-01-
  11. ^ "autozone.com Glossary - head bolt".
      8/repairInfoPages.htm. Retrieved on 2009-01-13.
  12. ^ Jones (2000). Machinery's Handbook 26th edition. p. 1497.
  13. ^ "Dyke's Automobile and Gasoline Engine Encyclopedia page 701".
      =result#PRA1-PA701,M1. Retrieved on 2009-01-13.
  14. ^ "Bolt versus Screw Definition". http://euler9.tripod.com/bolt-
      database/boltdef.html. Retrieved on 2009-01-13.
  15. ^ a b Szykitka, p. 465.
  16. ^ Huth, pp. 166–167.
  17. ^ Colvin, p. 569.
  18. ^ Plow bolts, http://www.masterbolt.com/plow_bolts.html, retrieved on 2008-12-
  19. ^ "Key-Rex Security Screws". http://www.brycefastener.com/keyrex.htm.
      Retrieved on April 2 2008. "The keyway is licensed and private for each user"
  20. ^ World Fastener Review, Industrial Press, 2006
  21. ^ Ryffel 1988, p. 1547.
  22. ^ Ryffel 1988, p. 1603.
  23. ^ Stephanie Dalley and John Peter Oleson (January 2003). "Sennacherib,
      Archimedes, and the Water Screw: The Context of Invention in the Ancient
      World", Technology and Culture 44 (1).

   Colvin, Fred Herbert; Stanley, Frank Arthur (1914), American Machinists'
    Handbook and Dictionary of Shop Terms (2nd ed.), McGraw-Hill,
   Green, Robert E., et al (eds) (1996), Machinery's Handbook (25th ed.), New
    York: Industrial Press, ISBN 978-0-8311-2575-2.
   Huth, Mark W. (2003), Basic Principles for Construction, Cengage Learning,
    ISBN 1401838375, http://books.google.com/books?id=BPavrOZxCXAC.
   Rybczynsky, Witold (2000). One Good Turn: A Natural History of the
    Screwdriver and the Screw. Toronto, ON, Canada: Harper Collins. ISBN 978-0-
   Ryffel, Henry H., et al (eds) (1988), Machinery's Handbook (23rd ed.), New
    York: Industrial Press, ISBN 978-0-8311-1200-4.
   Szykitka, Walter (2004), The Big Book of Self-Reliant Living: Advice and
    Information on Just about Everything You Need to Know to Live on Planet Earth,
    Globe Pequot, ISBN 1592280439,

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