Screw: Definition, Types, and Types of Screw Thread

What is screw?

A screw is a broad category of mechanical fastener with a threaded shaft, designed to screw into a part. This includes wood screws and self-topping screws, which have a tapered shaft with sharp threads designed to cut a mating thread in the part to which they are fastened.

These grooves can be either throughout the shank or only in the beginning. These grooves provide necessary friction and traction to avoid pull-out.

Also known as a male (or external) thread. Typically made of metal, screws fasten materials by digging in and wedging into them by turning, upon torque applied. A screw is sometimes also known as a cap screw or tap bolt.

Note: Those who don’t know ‘What is a fastener?’. A fastener is a hardware device that mechanically affixes or joins two or more parts together.

On the basis of threading mechanism there are basically two types of screws.

Self-threading screws

When we move self-threading screws clockwise (with some pressure) against the joining material, it makes its way into the material (mainly because of its sharp tip). They are generally suitable when the joining parts make by either wood or plastic.

In many cases, self-threading screws can also insert into joining parts with the help of a hammer, for example in the case of wood and plastic.

Non-self-threading screws

In the case of non-self-threading screws, we have to first make suitable helical grooves in the joining material and then insert the screw carefully. They commonly use when the joining parts are build of metal.

Parts of Screw

part of Screw
parts of screws

Parts of screw can mainly be divide into five parts.

  1. Head: It is the topmost part of a screw. A screw can be tightening or loosen from the head of the screw. It has provisions for accommodating a screwdriver or wrench.
  2. Not threaded shank: This is the part of the shank which do not have threads. This part could be absent in some screws.
  3. Threaded shank: This is the part of a screw having threads on it. It is the place from where the screw gets its grip on joining bodies. It has helical grooves in it.
  4. Screw Drive Head: A special design is cut into the head of the screw that the screwdriver will fit into when the screw is driven into a surface. This is called a drive. There are many different designs of drives, but the three most common are Schlitz, Philips, and Pozidriv.
  5. Tip: It is the bottom-most part of the screw. Tip helps screw-in penetrating through the joining bodies. It could be missing in some screws where threads are already present in joining bodies. It is an essential part of self-threading screws, as mentioned earlier.

What are screws made of?

Following material can use in the manufacturing of screws:

  • Titanium: Screws made of Titanium are hard, strong, light, and corrosion-resistant. When alloyed with other materials it can increase strength and durability.
  • Stainless steel: Screws made from stainless steel are chemical and corrosion-resistant. They have an appealing surface finish. They cannot harden like carbon steel.
  • Hardened steel: Screws made from hardened steel are hard but brittle in nature.
  • Steel: Screws make from steel are strong but they are vulnerable to corrosion.
  • Plastic: Screws made from plastic are inexpensive and corrosion-resistant. They use for light loads. They commonly use near water, such as water pools.
  • Molybdenum: Screws made from Molybdenum have a very high melting point and are exceptionally strong.
  • Copper alloy: Screws make from copper alloy have good load capacity and are wear-resistant. They are suitable for use near magnets.
  • Brass: Screws made of Brass are strong, conductive, and corrosion-resistant. They have low magnetic permeability.
  • Aluminum: Screws make of Aluminum are light and easy to manufacture. They are thermally and electrically conductive. They are resistant to corrosion.
  • Superalloys: Screws made of superalloys show good mechanical strength, surface stability, corrosion resistance, and resistance to creep at higher temperatures.
  • Apart from the above material, sometimes some finishing material used in screws. It can provide durability and corrosion resistance to the screw. Here are some finishing materials used in screws.
  • Zinc: Its coating acts as a sacrificial anode, protecting the underlying metal. It is applied as fine white dust.
  • Chrome: Its coating gives a bright, reflective finish. It is decorative and very durable. It is applied by electroplating.
  • Black oxide: Its coating is mostly used for aesthetic purposes. It does not enlarge the dimensions of the screw. It processes black rust.

Types of Screw

These are some main types of screw which are generally use in our day to day life.

Wood screw

Perhaps the single most common type of screw is a wood screw. Wood screws aren’t made of actual wood. Rather, they are used to connect two or more solid wooden objects. Wood screws have a sharp point that’s able to dig into wood, making them highly useful for woodworking applications.

Machine screw

A machine screw, as the name suggests, is a type of screw that’s used in machining applications. There are many different types of machine screws, one type is a stove bolt. Machine screws are used to hold heavy-duty metal objects together. According to Wikipedia, machine screws can have a diameter of up to 0.75 inches, making them bigger than most other screws.

Lag Screw

While not as common as wood or machine screws, lag screws are another type of screw worth mentioning. Also known as lag bolts or coach bolts, they are essentially jumbo-sized wood screws coated with zinc. A zinc layer is applied to lag screws to protect them against rust and corrosion. To apply the zinc, companies typically perform electroplating or hot-dip galvanization.

Sheet metal screws

Sheet metal screws, of course, Design to force into sheet metal. They can still use other materials, such as wood, but they fundamentally use it for sheet metal. Sheet metal screws have threading all the way up to the shank at the top, allowing them to easily dig into sheet metal.

Twin fast Screw

Twin fast screws feature a unique design consisting of two threads rather than just one. With twice the threading, they can force into objects twice as quickly when camper to traditional screws. Most drywall screws are twin fast screws. With that said, twin fast screws can use for other applications besides the installation and hanging of drywall.

Security Screw

Security screws design to protect against removal or tampering. Unlike other screws, security screws don’t have an operable head that can reverse to remove the screw. Rather, they generally require a specialized tool like a spanner or square driver to remove. This design makes them an excellent choice for security applications.

Concrete screw or masonry screws

Concrete screws come in all kinds of head designs, from flat and pan to slotted hex. The pan and flat head screw styles can be flush to the surface of the material to make it look nicer, while a hex-head slotted screw is easier to drive in. Because of the high-low threading design of these screws, they provide durable and solid results in attaching to concrete or stone. Stainless or carbon steel and used for fastening materials to concrete.

Drywall screw

Drywall screws are standard fasteners for securing partial or full sheets of drywall to wall or ceiling joists. There are two types of drywall screws available. These are:

  • Coarse: this type of drywall screw is mostly used for wood studs. The widely arranged threads are effective in gripping into the wood. The drywall screw also features an extra sharp tip and a phosphate finish.
  • Fine: this type of drywall screw is self-threading, thus making it perfect for metal studs. The fine drywall screws have double threads for easier and more practical self-starting.

Eye bolt Screw

Eyebolts are perhaps the most commonly used screws by both homeowners and material handling specialists. In fact, you are bound to have come across these bolts in your local hardware store. An eye bolt consists of a threaded shank and a ring at one end. These bolts are designed to lift objects by passing a rope or wire through their eye.

Decking screw

Decking screws are made of stainless steel or carbon steel and specially designed to attach wooden or composite boards to metal or wood. These screws come in various widths and lengths and are the perfect solution for fastening decking boards to frames.

Types of Screw Thread

The following are Different Types of screw thread:

  • British Standard Whitworth (B.S.W.)
  • British Association (B.A.) thread
  • American national standard thread
  • Unified standard thread
  • Square thread
  • Acme thread
  • Knuckle thread
  • Buttress thread
  • Metric thread
Types of Screw Thread

British Standard Whitworth (B.S.W.)

British Standard Whitworth (B.S.W.) thread. This is a British standard thread profile and
has coarse pitches. It’s a symmetrical V-thread in which the angle between the flanks, measured in an axial plane, is 55°. These threads are found on bolts and screwed fastenings for special purposes. The various proportions of B.S.W.

The British standard threads with fine pitches (B.S.F.) used where great strength at the root
required. These threads also used for line adjustments and where the connected parts are
subjected to increased vibrations as in aero and automobile work.

The British standard pipe (B.S.P.) threads with fine pitches used for steel and iron pipes and
tubes carrying fluids. In external pipe threading, the threads are specified by the bore of the pipe.

British association (B.A.) thread

This is a B.S.W. thread with fine pitches. The proportions of the B.A. thread. These threads used for instruments and other precision works.

American national standard thread

The American national standard or U.S. or Seller’s thread has flat crests and roots. The flat crest can withstand more rough usage than sharp V-threads. These threads used for general purposes e.g. on bolts, nuts, screws, and tapped holes.

Unified standard thread

The three countries i.e., Great Britain, Canada, and United States came to an agreement for a common screw thread system with the included angle of 60°, in order to facilitate the exchange of machinery. The thread has rounded crests and roots.

Square thread

The square threads, because of their high efficiency, widely used for transmission of power in either direction. Such types of threads usually found on the feed mechanisms of machine tools, valves, spindles, screw jacks, etc.

The square threads are not so strong as V-threads but they offer less frictional resistance to motion than Whitworth threads. The pitch of the square thread is often taken twice that of a B.S.W. thread of the same diameter.

Acme thread

It is a modification of square thread. It is much stronger than square thread and can be easily produced. These threads are frequently used on screw-cutting lathes, brass valves, cocks, and bench vices. When used in conjunction with a split nut, as on the lead screw of a lathe, the tapered sides of the thread facilitate ready engagement and disengagement of the halves of the nut when required.

Knuckle thread

It is also a modification of square thread. And it has a rounded top and bottom. It can cast or rolled easily and can not economically make on a machine. These threads used for rough
and ready work. They are usually found on railway carriage couplings, hydrants, necks of glass bottles, and large molded insulators used in the electrical trade.

Buttress thread

It used for the transmission of power in one direction only. The force is transmitted almost parallel to the axis. This thread unit the advantage of both square and V-threads. It has a low frictional resistance characteristic of the square thread and has the same strength as that of V-thread. The spindles of bench vices are usually provided with buttress threads.

Metric thread

It is an Indian standard thread and is similar to B.S.W. threads. It has an included angle of 60° instead of 55°.

Types of Screw Heads

There are a variety of types of screw head for different materials, and for different applications; screws are often use to fasten sheet metal, wood, and plastic.

Types of Screw Heads
Types of Screw Heads
  1. Binding/Binding Undercut: The ‘binding’ type is a commonly use screw head type. It is an ideal choice for electrical applications; beneath the head is a prominent undercut area providing space for wire connections. Binding screws are also used for other kinds of projects; they are often using to hold together large manuals, for bookbinding, and in leather products.
  2. Trim: The ‘trim’ screw head is a standard countersunk screw that provides a flat, smooth surface after installation. It is an alternative to flat head screws, with a narrower head a more suitable choice for certain applications. The Flat Trim is often used as a finishing screw for carpentry and woodworking.
  3. Oval Undercut: The ‘oval undercut’ is ideal for shorter screw lengths which require longer thread grip and more shallow countersinking.
  4. Fillister: A smaller diameter, in addition to a higher profile, than round or pan heads, makes the ‘fillister’ screw head effective for a deeper slot.
  5. Flat Fillister: The ‘flat fillister’ is identical to a regular fillister screw, except for a flat top replacing the latter’s oval top.
  6. Flat 82+: The Flat 82+ head is use for flush surfaces. They are standard countersunk flat head screws.
  7. Flat 82+ Undercut: The ‘flat undercut’ is very similar to the standard 82-degree head, having the same angle, but a shorter head. This allows it to provide a longer thread on the same screw length.
  8. Flat 100+: The Flat 100+ uses a 100-degree angle instead of the standard 82-degree kind. It is an ideal choice for thinner materials that require a flatter head.
  9. Flat 100+ Undercut: The Flat Undercut is use instead of the standard flat head for some short sizes. The 100-degree kind enables more shallow countersinking than standard 82-degree flat heads.
  10. Hex: Hex heads allow greater torque to be applied, and are driven with the force working against the outside of the head, unlike other varieties, which are internally driven, They are commonly used for bolts.
  11. Pan: The ‘pan’ head is the most commonly used rounded-top screw head. It works well in a range of applications that require a flat-bottomed screw.
  12. Hex Washer: The ‘hex washer’ head is an updated, more commonly used version of the hex head. It is characterized by an attached washer style flange under the head, creating a large surface connection area. It is sometimes used in combination drives, with Philips or slotted drives.
  13. Oval: It is identical to a flat head, except for its rounded surface. Countersunk screw head that includes a decorative rounded finish at the top. Often used for switch coverings.
  14. Oval trim: The ‘oval trim’ head is the same as the regular ‘oval’ head, but with a smaller diameter and countersink.
  15. Round: The ‘round’ head is less commonly used these days, but it offers a round-surfaced alternative to other screw heads.
  16. Round Countersunk: It is similar to ‘flat’ heads, but with no driving recess. The ‘round countersunk’ head use for shorter screw lengths to provide a longer thread grip and more shallow countersinking.
  17. Round Washer: The round washer is a screw head which provides a larger bearing surface in applications where a round head is preferred. It is quite similar to the Modified Truss head style.
  18. Square (Set Screw): The ‘square’ head is a suitable choice for applications that require more torque. It is most often used for switch coverings
  19. Truss: Truss’s heads characterize by a curved surface and an extra-wide top. They regularly use to prevent tampering., having a lower profile than most round heads. Truss heads are very useful for sheet metal work, insulation, and cabinetry– an ideal choice for applications that require large holes, as the wide head prevents the screw from going through the hole.

Types of Screw Head Drivers

Types of Screws Driver

Slotted head Drive

The first of its kind to develop, the slotted type is still the most widely available screw drive because it is simple, inexpensive, and effective. The design characterizes by a single line running through the middle of the head, into which a flat-bladed screwdriver fits.

The slotted screw drive works best when hand-driven; it regularly not use with power tools because of the danger of the screwdriver slipping out of the slot and damaging the surrounding material. It is most suitable for simple carpentry, where the torque applied is less.

Phillips Head Drive

The Philips screwdriver was introduced to improve the quality of the performance when the head came into contact with the screwdriver. It got its name from the Phillips Screw Company after they bought and modified the design. A Phillips drive characterizes by slightly tapered flanks, a pointed tip, and rounded corners.

In contrast to the slotted drive’s two-point contact, the Phillips head type has four. With a four-point contact, either one of the head types can be used. Like the former, it also burrs when more torque is applied.

Pozidriv

The Pozidriv screw is an eight-point contact — a second cross is added to the first, improving the contact of the screwdriver to the head. The Pozidriv is distinguished by a set of radical indentations at 45 degrees, making it visually distinct from the Phillips drive. The two are broadly interchangeable; but when used incorrectly, either can cause significant damage.

While a Pozidriv may fit a Phillips head, it is not advisable to use the former drive on the latter head, as it may slip out or tear it out. In addition to its superior grip, it is more versatile than the Slotted and Phillips drive.

Square Drive

Also known as the Robertson drive, its distinct design characterizes by a square-shaped socket in the screw head and a square protrusion on the tool. The tool and the socket are both slightly tapered. Originally introduced to make screws more practical through the cold forming of the head, the taper also makes inserting the tool easier, and the screw tends to stay steady on the tip of the tool without needing the user’s support.

The Robertson drive is an extremely popular choice for woodworking and construction, while a combination drive of Robertson and Phillips type are frequently use in device and breaker terminals, as well as in clamp connectors.

Six Lobe Drive

Also, refer to as the ‘Torx’ drive style, or a star drive, its design characterize by a six-pointed star shape. The Torx drive is a more recent competitor to the Phillips drive; as is the case with the latter, screw heads of different sizes require different sized drivers.

Manufacturing process of screws

There are three major steps in the manufacturing a screw.

  • Heading
  • Thread rolling
  • Coating

The screw is usually made from the wire. The wire is then cut to the proper length for the type of screw being made. Heading produces the head of the screw. The shape of the die in the machine dictates the features to press into the screw head, for example, a round head screw uses a round die.

The threads usually produce via thread rolling. However, some are a machine. Finally, a coating, such as electroplating with zinc or black oxide, applied to prevent corrosion.

Uses of Screw

Uses of screw in following conditions:

  • When the parts that join, are thick enough to accommodate the threaded hole.
  • Bolts are good for frequent dismantling and reassembling, unlike screws.
  • Relative advantages and disadvantages of screws and bolts.
  • When the parts that are attached, that have sufficient strength to accommodate durable threads.
  • These use when the parts sometimes dismantle.
  • There is no place to accommodate the nut.
  • Screws are cheaper compare to bolts.
  • Bolts carry the load on a larger shank area when compared to the screw

Advantages and Disadvantages of screw and bolt

  • Screws are cheaper compare to bolts.
  • Bolts are good for frequent dismantling and reassembling, unlike screws.
  • Bolts carry the load on a larger shank area when compared to the screw.

Frequently Ask Question

1. What is Screw?

A screw is a broad category of mechanical fastener with a threaded shaft, designed to screw into a part. This includes wood screws and self-topping screws, which have a tapered shaft with sharp threads designed to cut a mating thread in the part to which they are fastened.

2. What are the types of screw?

Different Types of Screw:

  • Wood screw
  • Machine screw
  • Lag Screw
  • Sheet metal screws
  • Twin fast Screw
  • Security Screw
  • Concrete screw or masonry screws
  • Drywall screw

3. What are the types of screw threads?

The following are Different Types of screw thread:

  • British Standard Whitworth (B.S.W.)
  • British Association (B.A.) thread
  • American national standard thread
  • Unified standard thread
  • Square thread
  • Acme thread
  • Knuckle thread
  • Buttress thread
  • Metric thread

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