Screw

SCREW

Neha Khandelwal F & LA Sem 5INTRODUCTIONThis assignment was given for the purpose of studying screw s not only from the aspect of understanding them technically but understanding how a screw is used in normal day objects. From understanding How to depict a screw with a figure, to understanding the different types of screws that are there, the different screw drives, screw tops and their uses specific to their design. It’s not only about understanding that a screw is an inclined plane wrapped helically around an axis but that how the strap of a watch is put together with the help of screws.

CONTENTo Definition of a screwo Historyo Designo Materialso Classification of thread manufacturing methodso Difference between a bolt and screwo Nomenclatureo Types of Screwo Screw Head shapeso Types of screw drives and their uses

SCREW

A screw is one of the six classical simple machines, essentially an inclined plane wrapped helically around an axis for a number of turns (which may be less than one). A screw can convert a rotational movement to

a linear movement, and a torque (rotational force) to a linear force. When the shaft of the screw is rotated relative to the stationary threads, the screw moves along its axis relative to the medium surrounding it; for example rotating a woodscrew forces it into wood; rotating a fixed Archimedean screw used to pump water moves the water.

The most common uses of screws are to hold objects together and to position objects. The head is usually larger than the body of the screw, which keeps the screw from being driven deeper than the length of the screw and to provide a bearing surface.

 There are exceptions; for instance, carriage bolts have a domed head that is not designed to be driven; set screws have a head smaller than the outer diameter of the screw; and J-bolts do not have a head and are not designed to be driven. 

The majority of screws are tightened by clockwise rotation, which is termed a right-hand thread. Screws with left-hand threads are used in exceptional cases. For example, when the screw will be subject to anticlockwise forces (which would work to undo a right-hand thread), a left-hand-threaded screw would be an appropriate choice.

HISTORY

Even though the concept of the screw dates back to around 200 B.C., the actual metal screw that is known today was not developed until the Renaissance. Early screws had to be handmade, so no two screws were ever alike. The time consuming process of hand filing the threads into the screw form made mass production and use virtually impossible. In 1586, the introduction of the first screw-cutting machine by Jacques Besson, court engineer for Charles IX of France, paved the way for more innovations

DESIGNOn a single thread screw, the lead and pitch are identical, lead is twice the pitch on a double thread model, and three times as much on a triple thread. The pitch of a screw is the distance between two threads (or grooves) from the same point on each thread. It is also more commonly known as the number of threads per inch or centimeter. The lead of the screw measures how far it is driven in for each revolution.

MATERIALSScrews 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  (steel screws can discolor oak and other woods), bronze, monel or silicon bronze may be used, or a coating such as brass, zinc 

orchromium applied.

Classification of Thread Manufacturing Methods –

In manufacturing screw threads, the basic problem of course, is how to produce the desired ridge on the workpiece. Various methods are used: cutting, rolling, grinding and casting. Both external and internal threads can be cast, but this process is used primarily in connection with die casting or the molding of plastics, and relatively few threads are made in this manner.

Rolling also can be employed for making both external and internal threads, provided the material is reasonably ductile. Today the majority of threads are formed by rolling. External threads can be made by cutting in the following ways:

I. On an engine lathe.2. With a die and stock (manual).3. With an automatic die (turret lathe or screw machine).4. By milling.5. By grinding.6. By rolling.

Internal threads are made by cutting by the following methods:

I. On an engine lathe.2. With a tap and holder (manual, semiautomatic, or automatic).3. With an automatic (collapsible) tap (turret lathe, screw machine, orspecial threading machine).4. By milling.

Cutting a thread by using a single point tool on the lathe while indispensable for single and small quantity jobs turned on the lathe, would be too slow for production in large quantities and, moreover, would necessitate long and expensive resetting of work turned on other machines.

DIFFERENCE BETWEEN A BOLT & A SCREW“A bolt is a 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 and released by torquing the head."

There are separate standards available for both bolts and threads, as well, as the grips, heads, etc.

NOMENCLATUREThe standard screw thread nomenclature is illustrated. The crests of external threads may be either rounded or flat. The root usually is made rounded to minimize stress concentration at this critical area. The internal thread has a flat crest in order to mate with either a rounded or V root of the external thread. A small round is used at the root to provide clearance for the flat crest of the

external thread. While all elements of the thread form are based on the pitch diameter, screw thread sizes are expressed in terms of the outside, or major, diameter and the number of threads per inch of length.

Common type of fasteners:

TYPES OF SCREWS:o Wood screwGenerally has an unthreaded shank below the head. It is designed to attach two pieces of wood together.

o Coach screw (UK) or lag screw/bolt (US)

Similar to a wood screw except that it is generally much largerLag 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.

o 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 shank is usually threaded up to the head. 

o Concrete screwA stainless or carbon steel screw for fastening wood, metal, or other materials into concrete or masonry. Concrete screws are commonly blue in color, with or without corrosion coating

o 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. 

o Drywall screwSpecialized 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.

o 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. The threads on particle board screws are asymmetrical.oo

o Deck screw Similar to drywall screw except that it is has improved corrosion resistance and is generally supplied in a larger gauge. Most deck screws have a type-17 (auger type) thread cutting tip for installation into decking materials.

o 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.

o 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.

o Thread rolling screwsThese have a lobed (usually triangular) cross-section. They form threads in a pre-drilled hole in the mating workpiece by pushing the material outward during installation.

o Mirror screwsThese are flat head wood screws with a tapped hole in the head, which is designed to receive a separate screw-in chrome-plated cover. They are usually used to mount mirrors.

Fasteners with a non-tapered shank

o Cap screwIn places the term is used interchangeably with bolt. In the past the term cap screw was restricted to threaded fasteners with a shank that is threaded all the way to the head, but this is now a non-standard usage.

o Hex cap screwCap screw with a hexagonal head, designed to be driven by a wrench (spanner).

o 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

ooo

o Machine screwGenerally a smaller fastener (less

than 1⁄4 inch in diameter) threaded the entire length of its shank that usually has a recessed drive type (slotted, Phillips, etc.)

o Self-tapping machine screw

Similar to a machine screw except the lower part of the shank 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.

o 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..

o Stud/threaded rod Studs are head-less screws. They may be threaded at both ends and unthreaded in the middle or completely threaded; the latter is usually referred to as a threaded rod,

especially when it has a large aspect ratio (that is, quite long compared to diameter). Completely threaded round stock is available in bar stock form and is then usually referred to as "all-thread".

Thumb screw A threaded fastener designed to be twisted into a tapped hole by hand without the use of tools.

Security screwIt’s similar to a standard screw except that once inserted it cannot be easily removed.

SCREW HEAD SHAPESPan head

A low disc with chamfered outer edgeButton or dome head

Cylindrical with a rounded topRound head

A dome-shaped head used for decoration.

Truss headLower-profile dome designed to prevent tampering

Countersunk or flat headConical, with flat outer face and tapering inner face allowing it to sink into the material. The angle of the screw is measured as the full angle of the cone.

Oval or raised headA decorative screw head with a countersunk bottom and rounded top.

Bugle headSimilar to countersunk, but there is a smooth progression from the shank to the angle of the head, similar to the bell of a bugle

Cheese headDisc with cylindrical outer edge, height approximately half the head diameter

Fillister headCylindrical, but with a slightly convex top surface. Height to diameter ratio is larger than cheese head.

Flanged headA flanged head can be any of the above head styles with the addition of an integrated flange at the base of the head. This eliminates the need for a flat washer.

TYPES OF SCREW DRIVES AND THEIR USES

Every threaded fastener needs a way of turning it. It may have a head with a shape that a driver can engage, as a wrench fits a hex-head bolt or a nut, or it may have a shaped hole into which a driver can be inserted (fastener engineers call the hole the “recess”).

Term used frequently:Camming outTo cam out (or cam-out) is a process by which a screwdriver slips out of the head of a screw being driven once the torque required to turn the screw exceeds a certain amount.

External typesAll of these screw drives are characterized by a female tool and a male fastener.

Square

A square screw drive uses square shaped fastener heads. They can be turned with a crescent-type wrench, open-end wrench, or 12-pointsockets.

Hex

A hex screw drive uses six-sided fastener heads. The fastener is known as a hex head cap screw. They can be turned with a crescent-type wrench,combination wrench, or sockets.

Pentagon

A pentagon screw drive uses five-sided fastener heads. The fastener is known as a penta screw. They require a special five-point socket in order to be turned. Water meter covers, natural gas valves, and

electrical cabinets are commonly secured with penta fasteners.

Slotted typesSlot

The slot screw drive has a single slot in the fastener head 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 tool would be used, due to the tendency of a power driver to slip out of the head and potentially damage the surrounding material. The tool used to drive a slot is called a slot-head or flat-tip.

Cross

A cross screw drive has two slots, oriented perpendicular to each other, in the fastener head; a slotted screwdriver is still used to drive just one of the slots. This type is usually found in cheaply made roofing bolts.

The sole advantage is that they provide some measure of redundancy: should one slot be chewed up in service, the second may still be used.

Cruciform typesThe following are screw drives based on a cruciform shape, i.e. a cross shape. Other names for these types of drives are cross-head and cross-point.

Phillips screw heads

The Phillips system was invented for use in assembling aluminum aircraft, with the object of preventing assemblers from tightening screws so tightly that the aluminum threads strip. The driver will cam out before that happens. The driver has a 123° point with a blunt tip, tapered wings. 

Frearson screw heads

A cross drive system referred to in ANSI standards as type II recess.

In the United States, Frearson screws are mainly found as the marine bronze wood screws used in boat building 

Note the difference in points: Frearson has sharper V 

Pozidrive® screw heads

Identified in ANSI standards as type IA. As it doesn't cam out, great torque can be applied. Pozidrive screws can be turned by Phillips screwdrivers, but Pozidrive drivers won't turn Phillips screws.

Supadrive screw heads

Supadrive drivers will turn Pozidrive heads.

Square   recesses and heads Square nuts and four-sided heads are now mainly found in farm

equipment and on lag screws.

Robertson head screws

A square recess design was invented by P. Lymburner Robertson in 1908. Its advantages are great resistance to camout and 4 possible positions for the driver.

Five-sided   screw heads Five-sided heads are used for caps and valves of fire hydrants,

and in other situations in which a fastener that cannot be removed by commonly available wrenches (most of which have parallel jaws) is

needed.

Hexagonal   heads and recesses Probably the most common of all fastener heads, hex heads are also very old. Fasteners with hexagonal heads were used to hold armor together in the 15th century.

To find the size of wrench needed to turn a hex head (or hex recess), measure from flat to flat, not from point

to point.

Tamper resistant types: Clutch head

There are two types of clutch screw drives: type A and type G. Type A, also known as a standard clutch resembles a bow tie. These were common in GM automobiles, trucks and buses of the 1940s and 1950s. Type G resembles a butterfly. This type of screw head is

commonly used in the manufacture of mobile homes and recreational vehicles.

A worn tip on a driver can easily be restored by grinding off the end.

Bristol Spline

The Bristol screw drive is a spline shaped with four or six splines. The main advantage to this drive system is that almost all of the turning force is applied at right angles to the fastener axis, which reduces the possibility of stripping the fastener. For this reason Bristol screw drives are often used in softer, non-ferrous metals. This type of drive is commonly used in avionics, higher-end communications equipment,

cameras, air brakes, construction and farm equipment, astronomy, and military equipment.

Double hex

Double hex is a screw drive with a socket shaped as two hexes. It is shaped similar to triple square and spline screw drives, but they are incompatible. Standard hex keys can be used with these sockets.

This version is even closer to a circle, further decreasing torque and increasing stripping.

One-way

One-way screws are special screws that can only be turned in one direction. They can be installed with a standard slotted screwdriver. One-way screws are commonly used in commercial restroom fixtures, to

prevent vandals from tampering with them. One-way screws are only practical when the need for removal is unlikely. They cannot be removed with conventional tools; instead a screw extractor is used.

Polydrive

The polydrive screw drive is spline shaped with rounded ends in the fastener head. The tool has six flat-tip teeth at equal spacing; the sizes are determined by the diameter of the star points. Its primary

advantage over older screw drives is that it resists camming out. It is used primarily in the automotive industry in high-torque applications, such as brakes and driveshafts.

Protruding obstacle

Tamper-resistant external-torx driver

A protruding obstacle screw drive is a common modification to socket and cruciform style drives to make the fastener more tamper resistant by inserting a pin in the fastener screw drive. The tool then requires a corresponding hole to drive the fastener.

Usually the hole is in the center, but some are slightly off-center.

Polydrive

The polydrive screw drive is spline shaped with rounded ends in the fastener head. The tool has six flat-tip teeth at equal spacing; the sizes are determined by the diameter of the star points. Its primary

advantage over older screw drives is that it resists camming out. It is used primarily in the automotive industry in high-torque applications, such as brakes and driveshafts.

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. These tend to be confined to industrial uses with which the average layperson does not have contact. One example familiar to laypersons is the attachment for the wheels and/or spare tires of some types of car; one of the nuts on each wheel may require a specialized socket, provided with the car, to prevent theft. Security fasteners are also available for bicycle wheels and seats.

Spanner

The spanner screw drive uses two round holes opposite each other and is designed to prevent tampering. This type is seen in elevators in the United States. The driving tool is called a spanner wrench in the U.S.

and a pin spanner in the UK.

Spline

The spline screw drive has twelve splines in the fastener and tool. Spline drives are sized via numbers. Its primary advantage is its ability to resist camming out, therefore it is used in high-torque applications, such

a stamper-proof lug nuts.

Torq-set

A set of torq-set bits

Torq-set is a cruciform screw drive used in torque-sensitive applications. The Torq-set head is similar in appearance to a Phillips drive in that it has a cross with 4 arms. In Torq-set however, the arms of the cross are offset from each other, so they do not align to form intersecting slots across the top of the head.. It is used in aerospace applications. 

TP3

TP3 is a type of screw drive that uses a triangular recess in the screw head.[28] It is used on Nintendo,Gameboy, fast food promotional toys and video games, die-cast toys and Roomba battery packs.

Tri-wing

A tri-wing tool and screw

The tri-wing, also known as triangular slotted, screw drive has three radial slots. It is usually used on electronics equipment. Tri-wing, as the name suggests, is a screw with three "wings" and a small triangular hole in the center. A variation is a kind in which the three "wings" are joined in the center (with no triangular hole). A somewhat similar-looking design in which three short radial slots are not joined in the center is called a tri-groove screw drive.

Triple square

Triple square, also known as XZN, is a type of screw drive with 12 equally spaced tips, each with a 90 degree angle. Its name derives from overlaying 3 equal squares to form such a pattern with 12 right-angled

tips.

Hex socket

The hex socket screw drive has a hexagonal recess and is driven by a hex wrench, also known as an Allen key, hex key or inbus.

This style of head is now very common in trucks and automobiles. The walls of the recess are not tapered. Drivers greatly outlast similar hex head drivers.

Internal Torx

Driver sizes for Torx recesses begin with a T.