Fasteners are mechanical devices to rigidly affix or join two or more mechanical items. Fasteners are widely used for mechanical joints requiring dismantling as they usually create non-permanent joints. Mechanical fasteners or fastening hardware are extensively used in various industries including Automobiles, Construction, Aircraft, Agriculture, Machinery and Appliances, Infrastructure, etc. In this article, we will learn the definition of a fastener, Its types, materials, applications, and relevant codes and standards.

What is a Fastener?

A fastener is a broad range of mechanical tools/elements used to hold two or more objects together as a rigid attachment. Fasteners allow for the separation or dismantling of the pieces without suffering any damage. However, they can be used as permanent joints as well. Screws, nuts, bolts, nails, washers, etc are different types of fasteners.

Types of Fasteners

There are different types of fasteners that are used in industrial applications. The most common types of mechanical fasteners are:

• Nuts and Bolts
• Washers
• Screws
• Nails
• Anchors
• Rivets
• Pins
• Retaining Rings
• Inserts

Fasteners Type- Nuts and Bolts

Nuts and bolts are one of the most common types of fasteners available for industrial use. They work together in tandem and hold two or more components together. The bolt is inserted through the bolt holes between the components and then the nut is fastened on the other end. There are various types of nuts and bolts as mentioned below.

Types of Nuts:

Nuts have internal threads and are always used with a mating bolt. The most popular types of nuts are:

Hex Nuts: The most common variety of nuts, Hex Nuts consist of a hex shape (six-sided) with internal threads. They can be easily tightened or loosen with a wrench accessing from any angle.

Coupling Nuts: They are also hex-shaped nuts. This hollow-threaded fastener joins two male threads together and is also known as an extension nut. They are widely used for installing plumbing pipes.

Lock Nuts: Locknuts are specially designed fasteners to prevent loosening due to vibrations. Also known as prevailing torque nuts, lock nuts find uses in automotive and washing machines where vibration problems have the tendency to loosen parts.

Square Nuts: Feature a square shape, square nuts are the oldest type of nuts with four sides. They are the best for the greater surface area making the fastener stronger and reducing damage from rough edges. Usually, square nuts are used in furniture and metal channel applications.

Flange Nuts: Having a wide, serrated flange on one end, flange nuts serve a similar function as a washer but it does not provide any added movement. They are also known as Tee nuts.

Wing Nuts: Having two projected pieces, wing nuts can be easily loosened or tightened using hands without tools. This type of fastener is good for applications requiring frequent tightening and loosening. Click here to learn more about wing nuts.

Slotted Nuts: In slotted nuts, sections are cut out to create a locking mechanism with the help of a cotter ping.
U-Nuts: Reliable and strong, U-nuts are made from one piece of rolled thread. They are used to hold metal sheet panels together.

Speed Nuts: Speed nuts have two metal pieces that work as one. Also known as sheet metal nuts, speed nuts do the job of both a nut and a locking washer.

Push Nuts: Push nuts can distribute loads easily that reduce surface stresses. They are installed with a special nut driver and used to secure unthreaded bolts and other fasteners.

Jam Nuts: Jam nut is small size nut that is half as tall as hex nuts. They are widely used where space has limitations to use hex nuts. They can easily be fastened onto a bolt without applying torque or force.

Axle Nuts: Also known as cap nuts or dome nuts, axle nuts hide the bolt edges and provide a seamless appearance. It provides a nice surface finish and finds use in electrical panels, stereo cabinets, etc.

Castle Nuts: Castle nuts have notches at one end through which a pin can be inserted to fix the nut’s position. They are used when the torque requirements are low.

Some other types of nuts are

• Rivet Nuts
• Weld Nuts
• Barrel Nuts
• Shear Nuts
• Tri-Groove Nuts
• Keps-K Lock Nuts
• Knurled Thumb Nuts
• Wheel Nuts, etc

Types of Bolts

Many different types of bolts are available in the market. The most common types of bolts that are used for industrial applications are:

Carriage Bolts: Having domed or countersunk heads, Carriage bolts use a square component under the head to keep the bolt from moving (pulling through) while tightening the nut. Carriage bolts are self-locking bolts and are usually used to attach metals to wood.

Hex Bolts: They have six-sided heads with machine threads extended halfway or up to the bolt head. Hex bolts find wide construction and machinery applications as they can be easily tightened using a wrench. Also popular as hex cap screws, hex bolts work with a tapped hole or a nut.

U-Bolts: U-bolts are shaped like the letter “U” and have screw heads on both ends. The bent section of the U-bolt is unthreaded. They are extensively used in piping, plumbing, and HVAC works to secure the position of pipes and tubes without making any holes in them.

Eye-Bolts: Eye bolts are threaded on one end and a loop at the other.

Lag Bolts: These bolts are used independently without a nut. Extremely sturdy and tough Lag bolts can handle a lot of weight. They are used for heavy-duty jobs like installing frames, framing lumber, etc.

Flange Bolts: For even distribution of the loads, Flange Bolts include a circular flange beneath the head.

Allen Bolts: Also known as Socket head bolts, Allen bolts have a hexagonal socket for use with Allen wrenches. These types of fasteners need less space.

There are a few other variations of bolts like:

• Plow bolts: for heavy-duty applications, such as heavy equipment, with non-protruding heads.
• Square-head bolts: having square heads that offer an easier grip for wrenches.
• Peta bolts with 5-sided heads.
• Double-end bolts resembling a threaded rod.
• Shoulder bolts or Stripper bolts.

Fasteners-Washers

Sometimes washers are added in between nuts and bolts to distribute the fastener’s load evenly over the material surface. A washer is a flat circular disc with an opening in the center. Washers can be metallic or can be made from non-metals. Other main purposes of washers are:

• Isolation of Components
• Reduction of leakage
• Alleviation of friction, and
• Prevention of loosening during vibration.

Some common types of washers are:

• Plain Washers: Plain washers are used for load distribution and isolation purposes. Plain washers can be of various types like:
  • Round and thin Flat Washers for general use.
  • Torque Washers for use in woodworking projects.
  • Fender Washer used in car fenders.
  • Finishing Washers used with countersank screws, and
  • C-washers
• Spring Washers: These types of fasteners act like a spring as they develop axial flexibility to make the joint more elastic. This can avoid unintended loosening during vibration. The main types of Spring Washers are:
  • Belleville Washer
  • Crescent Washer
  • Dome Spring Washers, and
  • Wave Spring Washers
• Lock Washers: This type of washer uses various mechanisms to prevent nuts, screws, and bolts from loosening. Lock washers are much better than spring washers and can be of the following types:
  • External tooth lock washer
  • Internal tooth lock washer
  • Split lock washer, and
  • Tab washer
• Beveled Washers: These washers add stability when attaching unparallel surfaces.
• Structural Washers: Usually thicker, Structural washers are used in heavy-duty applications.

Fasteners-Screws

Screw fasteners are the most versatile type of fasteners. It’s very simple to use. One needs to drill a pilot hole in a material and then using a screwdriver the screw can be easily installed in place. They generally have male threads that start from the tip. Screws are usually self-threading and create the thread during installation.

They come in various types and sizes like:

Self-Drilling Screws: Also popular as self-tapping screws, self-drilling screws create the required internal thread while installing it. It contains a fully threaded shaft.

Machine Screws: These types of fasteners are most widely used for machinery applications. It comes in a variety of tip shapes and heads with a slotted socket on the head to tighten it. They are of uniform thickness and don’t taper off at the bottom.

Sheet Metal Screws: With an extremely sharp tip, sheet metal screws are used to fasten two metals together. They have a flat or rounded head.

Other types of Screws are:

• Deck screws having a self-tapping design.
• Wood screw having coarse thread and tapered head.
• Grub Screws without a head used to prevent rotation or movement between two parts.
• Masonry screws having flat tips and hex-head designs.
• Countersunk screw
• Hex lag screw
• MDF screws
• Drywall screws

Nails as Mechanical Fasteners

Nails are the oldest type of fasteners used since ancient times. It is still used as an everyday household item. Nails do not have threads and usually have less power than screws. The most widely used types of nails are:

• Common nails having a thicker shank.
• Box nails with a diamond point tip.
• Roofing nails with a wide head.
• Framing nails are specially designed for flush installation and easy concealment.
• Brad nails for easily blending into wood trims.
• Flooring nails for use with flooring materials.
• Drywall nails for reduced slippage.
• Finishing nails with small flatheads.

Anchors as Industrial Fasteners

Anchors are a particular type of fastener that is used to connect something to a material like concrete or drywall. They embed themselves in the material and hold the object in place. Various types of anchors are used to serve different types of functions. Some common types of well-known anchors are:

• Internally threaded anchors
• Externally threaded anchors
• Acoustical wedge anchors
• Masonry screw and pin anchors
• Bonded anchors
• Screw anchors
• Double expansion shield anchors
• Hollow wall anchors
• Drop-in anchors
• Sleeve anchors
• Plastic anchors
• Drive anchors

Mechanical Fasteners-Rivets

Rivets create a permanent joint between objects and are hence known as permanent fasteners. Consisting of a cylindrical shaft with a head on one end and a tail on the other, Rivets offer great support against shearing forces. This type of fastener is lightweight and remarkably durable. A unique tool popular as a rivet gun is required to install rivets. Rivets can not be reused after removal. Some common types of rivets include:

• Blind rivets
• Pop rivets (Closed-end, Open end, Countersunk)
• Semi-tubular rivets
• Large flange rivet
• Solid rivets
• Tri-fold rivet
• Split rivets
• Drive rivets
• Structural rivets
• Colored rivets
• Multi-grip rivets.

Fastener types-Pins

Pins are unthreaded mechanical fasteners usually inserted through preformed holes. The most common types of pins for industrial usage are:

• Dowel pin
• Slotted pin
• Coiled pin
• Roll pin
• Grooved pin
• Split pin
• Wedge pin or tapered pin, known as cotter.

Retaining Rings

Retaining rings are a non-permanent type of metallic fasteners used to hold multiple parts together. It usually consists of spiral, semi-circular metal pieces. Retaining rings are employed in many applications in machinery and housing components.

The various types of retaining rings that are found are:

• Constant section retaining ring
• Snap retaining ring
• Tapered section retaining ring
• Axially assembled retaining rings
• Spiral retaining ring
• Radially assembled retaining ring
• Circular push-on
• Bowed-E retaining rings
• External shaft retaining rings
• Self-locking retaining rings.

Inserts

Inserts perform a range of tasks as mechanical fasteners. They are usually used for reinforcing joints, fixing eroded internal threads, used as keys in rotating machinery, or fastening anchor bolts to hanging pipelines.
Inserts are available in different forms like:

• Keys in shaft key-ways
• Threaded rod
• Unthreaded rod
• Helical threaded inserts
• Keystock

Other Types of Fasteners

There are many other types of fasteners that are used. Some of those fasteners are

• Clamps
• Staples
• Straps
• Hooks and Eyes
• Twist ties, etc

Fastener Materials

Fasteners are made of a variety of materials. Proper selection of a fastener material should be based on the working environment, weight, expected life, reusability, magnetic properties, and reusability. Common materials that are used for manufacturing fasteners are:

• Steel
• Brass and Bronze
• Copper
• Nickel
• Aluminum
• Stainless Steel
• Inconel
• Monel
• Titanium
• Nylon and Plastics

Fastener Codes and Standards

Widely used standards for industrial fasteners are:

• ASME B18.6.3 for Machine Screws, Tapping Screws, and Metallic Drive Screws (Inch Series)
• ANSI B18.6.1 for Wood Screws (Inch Series)
• ASME B18.6.2 for Slotted Head Cap Screws, Square Head Set Screws, And Slotted Headless Set Screws (Inch Series)
• ASME B18.24 for Part Identifying Number (PIN) Code System Standard for B18 Fastener Products
• ASTM-A31 for Steel Rivets and Bars for Rivets, Pressure Vessels
• ASTM-A320 for Alloy Steel Bolting Materials for Low-Temperature Service
• ASTM-A193 for Alloy-Steel and Stainless Steel Bolting Materials for High-Temperature Service
• ASTM-A194 for Carbon and Alloy Steel Nut for Bolts for High-Pressure and High-Temperature Service
• ASTM-A307 for Steel Bolts and Studs, 60,000 PSI Tensile Strength
• SAE-J78 for Steel Self-Drilling Screws
• SAE-J81 for Thread Rolling Screws
• SAE-J82 for Mechanical and Quality Requirements for Machine Screws
• SAE-J238 for Nut and Conical Spring Washer Assemblies
• SAE-J482 for Hexagon High Nuts
• SAE-J492 for Rivets and Riveting
• SAE-J493 for Rod Ends and Clevis Pins
• SAE-J773 for Conical Spring Washers
• SAE-J891 for Spring Nuts
• SAE-J892 for Push-On Spring Nuts, Inch Series- General Specifications
• ANSI-B1.1 for Unified Inch Screw Threads (UN and UNR) Thread Form
• ANSI-B18.1.1for Small Solid Rivet
• ASME B18.18 for Quality Assurance For Fasteners
• ANSI-B18.1.2 for Large Rivets
• ANSI-B18.22.1 for Plain Washers
• ASME B18.3 for Socket Cap, Shoulder, Set Screws, and Hex Keys (Inch Series)
• ISO 2339, ISO 7089, ISO 7090, ISO 2340
• DIN 125, DIN 126, DIN 1444
• BS4464B

Fasteners Selection Criteria

A variety of factors need to be considered during fastener selection for industrial applications. Some of them are

• Material of construction (stainless steel, carbon steel, or alloy steel).
• Materials to be joined.
• Environment, including temperature, water exposure, and potentially corrosive elements.
• Installation process.
• Re-usability.
• Weight restrictions.
• The applied load on the fastener.
• The stiffness of the fastener.
• Special process conditions (special coatings or plating).

What is Nitriding?

Nitriding is a type of heat treatment process to create a case-hardened surface by diffusing Nitrogen. The most common applications of the nitriding process are valve parts, gears, forging dies, crankshafts, extrusion dies, camshafts, firearm components, bearings, textile machinery, aircraft components, turbine generation systems, plastic mold tools, etc. The material widely used for the nitriding treatment process are low-alloy steels, aluminum, molybdenum, and titanium. Depending on the case depth, the nitriding process can take 4 to 60 hours.

Developed early in 1900, the nitriding process is widely used in many industrial applications. During nitriding, no phase change occurs. It is one of the simplest case-hardening processes.

What are the Purposes of Nitriding?

The main purposes of nitriding treatment are:

• To get high hardness on the surface. Hardness achieved in nitriding is usually higher than carburizing method.
• To increase the wear resistance.
• To improve fatigue life and other fatigue-related properties.
• To improve the corrosion resistance of the material.
• To obtain a high-temperature property of the surface (Resistance to tempering or softening up to the nitriding temperature).
• To get anti-galling properties.

Also, the nitriding process helps in reducing notch sensitivity. It is a diffusion-related surface treatment process that causes very small volumetric changes. Nitriding treatment can significantly improve properties like fatigue strength, resistance to wear, corrosion resistance, friction, and hardness.

What is the Principle of Nitriding?

Nitriding is a thermochemical treatment process to enrich the surface with nitrogen for the purpose of increasing the surface’s hardness. The process is based on the low solubility of nitrogen in the ferritic crystal structure to promote the precipitation of iron nitrides or alloy nitrides. The connecting nitriding is connected to a diffusion zone where precipitated nitrides are evenly diffused in the steel matrix. The usual nitriding temperature range is 350°C to 590°C. With a decrease in temperature, the nitriding time to reach a given depth increases. The depth of nitriding hardness may reach 500 μm with maximum hardness levels of > 1000 HV.

The nitriding layer formation occurs in the following steps:

• adsorption of nitrogen atoms on the surface of the component,
• absorption of nitrogen atoms by the component surface, and
• diffusion of the nitrogen atoms along the grain boundaries and within the grains.

What are the Types of Nitriding?

There are three types of nitriding processes that are commonly used in industries. They are:

• Gas Nitriding,
• Plasma Nitriding, and
• Salt-bath Nitriding.

What is Gas Nitriding?

In the gas nitriding process, the metal is heated to a suitable temperature (500⁰C to 575⁰C for Steel) and held in contact with a nitrogenous gas, usually ammonia. This is why the process is sometimes known as ammonia nitriding. When ammonia comes into contact with the heated steel, it breaks down into hydrogen and nitrogen. This nitrogen then diffuses onto the metal surface and creates a nitride layer. Depending on the temperature the solubility of nitrogen into iron varies as can be seen from the attached iron-nitrogen equilibrium diagram in Fig. 1.

The effectiveness of the gas nitriding process depends on various factors like

• Gas Flow
• Temperature
• Time
• Gas activity control
• Process control
• Process chamber maintenance, etc

What are the Advantages and Disadvantages of Gas Nitriding?

The main advantages of gas nitriding over other types of nitriding treatment are:

• Possibility of larger batch sizes (limited by the furnace size and gas flow)
• An all-around nitriding effect is achieved.
• Relatively low equipment cost as compared with plasma nitriding.

The disadvantages of gas nitriding are:

• Ammonia as a nitriding medium can be harmful if inhaled in large quantities. Also, it has the risk of explosion when heating in the presence of oxygen; so must be controlled carefully.
• Reaction kinetics are heavily influenced by surface conditions. Poor contaminated surface delivers poor results.

What is Plasma Nitriding?

Also popular as ion nitriding or glow-discharge nitriding, plasma nitriding uses a plasma discharge of reaction gases to heat the metal surface and supply nitrogen for the nitriding process. The main advantage of plasma nitriding is that the process is not dependent on the decomposition of ammonia gas to release nascent nitrogen. Invented by Dr. Bernhardt Berghaus of Germany, Plasma nitriding has a shorter cycle time. Fig. 2 below shows a typical schematic diagram of the plasma nitriding furnace layout.

In plasma nitriding, intense high-voltage electric fields in a vacuum are used to generate ionized nitrogen gas molecules. This highly active gas with ionized molecules, known as plasma accelerates to impinge on the work surface. This ion bombardment cleans the surface, heats the workpiece, and provides active nitrogen ions. Plasma nitriding helps in close control of the nitriding microstructure and its efficiency is not dependent on the temperature.

Metallurgically versatile, the ion nitriding process provides excellent retention of surface finish and ensures repetitive metallurgical properties. Ion nitriding can be conducted at temperatures lower than those conventionally employed. The plasma nitriding process does not cause any pollution and insignificant gas consumption which are important economic and public policy factors.

What are the Advantages and Disadvantages of the Ion Nitriding Process?

The main advantages of plasma nitriding are:

• Close control of Nitrided microstructure.
• The working lifespan of the component also increases.
• No polishing or machining is required post-nitriding process.
• Causes very little or no distortion.
• Energy-efficient and the fastest process.

The disadvantages of the plasma nitriding process are:

• Limited only on the compound zone thickness.
• Poor temperature control.

What is Salt Bath Nitriding?

Salt bath nitriding used molten salt as the source of nitrogen. The nitriding process uses the nitrogen liberated from the decomposition of cyanide to cyanate. The process also releases carbon and thus the process is also known as a nitrocarburizing treatment. The metal is normally submerged in a bath of preheated molten cyanide salt. The high concentration of nitrogen in the liquid chemically combines with iron to produce a hard and ductile iron nitride (Fe3N) thin outer layer.

What are the Advantages and Disadvantages of Salt-bath Nitriding?

The advantages of salt nitriding are:

• Quick processing time and higher diffusion in the same period.
• A simple operation involving the heating of the salt and workpieces to a temperature of 550 to 5700C and submerging for the specified duration.

The disadvantages of the salt bath nitriding process are:

• Used cyanide salts are highly toxic.
• Only one process is possible with a particular salt type.

What are the Prerequisites for Nitriding?

To get the best results in nitriding the following prerequisites should be followed:

• The steel should be hardened, quenched, and tempered to get a uniform structure.
• The surface must be cleaned to remove oils, grits, etc using vapor degreasing or abrasive cleaning.

Advantages of the Nitriding Process

The nitriding process provides various advantages like:

• Quick processing time.
• Low-temperature process.
• Clean pollution-free operation.
• Economic and energy-efficient operation.
• Lower friction coefficient.
• Improved fatigue properties.
• Uniform surface.
• High surface hardness.
• Distortion is very less.
• Improved corrosion resistance.
• Quenching is not required for getting high hardness.

Disadvantages of the Nitriding Process

The drawbacks of the nitriding process are

• High initial cost.
• Closer process control is required.
• Skilled personnel requirement.

Nitriding vs Carburizing: What is the difference between nitriding and carburizing?

The main differences between Nitriding and Carburizing are:

Nitriding	Carburizing
In nitriding, the nitrogen is diffused to increase the surface hardness	On the other hand, Carbon is diffused in Carburizing process.
The temperature range for nitriding treatment is lower.	Carburizing is done at very high-temperature ranges.
Energy-efficient and quick process.	Higher energy requirement.