What are fasteners?

Fasteners are described as mechanical components used to join or assemble two or more pieces of materials. The fasteners are used in various machines or structures based on their suitability to produce non-permanent, semi-permanent, and permanent joints.

Types of fasteners

Following are the various types of fasteners:

• Screws
• Nails
• Nut and bolts
• Washers
• Rivets
• Anchors

Screw

The screw is described as a threaded shaft having a durable holding power of joint surfaces. Screws need a small hole in the material; then, a screwdriver tightens it in the material. Commonly, screws are designed as self-threading that create the threads into the material during installation; therefore, no previous treading is necessary for screws.

Following are the common types of screws:

• Wood screw
• Machine screw
• Self drilling screw
• Sheet metal screw
• Hex lag screw
• Deck screw

Wood screw

Wood screws have a smooth shank and sharp point for use in wooden material. The diagrammatical representation of wood screw is shown as:

Machine screw

Machine screws are the screws that are generally installed into the hole of the base material and used to assemble various components in many machines. The diagrammatical representation of machine screw is shown as:

Self drilling screw

Self-drilling screws are the screws that develop internal threads in the material during installation. The material of the self-drilling screw is harder than the base material. It is also termed as a self-tapping screw. The diagrammatical representation of self-drilling screw is shown as:

Sheet metal screw

Sheet metal screws have a shank with the thread over the entire length with a head on one end. The head of the screw may be flat or rounded. The diagrammatical representation of sheet metal screw is shown as:

Hex lag screw

Hex lag screws are the screws that have a six-sided head on one end, and their lags are installed in the wooden materials by rotating head. The diagrammatical representation of the hex lag screw is shown as:

Deck screw

Deck screws are the standard wood screws used to hold timber or ply boards together. The diagrammatical representation of deck screw is shown as:

Nails

Nails are described as primary fasteners similar to screws but do not have threads. It has low holding power than a screw; therefore, it has limited applications.

Following are the various types of nails:

• Box nail
• Finishing nail
• Flooring nail
• Masonry nail
• Screw nail
• Double-headed nail
• Ring-threaded nail
• Casing nail

The diagrammatical representation of various nails is shown as:

Nut and bolts

A nut and bolt system is a two-piece fastening arrangement that holds components between them. A nut and bolt arrangement needs a hole in base materials, and then a bolt passes through a hole and gets locked with the help of a nut.

Following are the various types of bolts commonly used in various joints:

• Carriage bolt
• Eye bolt
• Hex-headed bolt
• Lag bolt
• Machine bolt

The diagrammatical representation of various bolts is shown as:

Following are the various types of nuts used in multiple joints:

• Heavy hex nut
• Nylon insert lock nut
• Jam nut
• Wing nut
• Cap nut
• Square nut
• Castle nut

The diagrammatical representation of various nuts is shown as:

Washers

A washer is a flat disc with a circular opening at the center. It is placed between nut and bolt arrangement to distribute the fastener's load over the base material properly.

Following are the types of washers:

• Beveled washers
• Flat washers
• Lock washers
• Structural washers

Rivets

Rivets are described as cylindrical shafts with a head on one end and a tail on another end. It is used to create a permanent joint between the objects. Rivets are more durable as compared to screws and a nut-bolt system. Rivets joints are used to produce permanent joints where vibrations occur in the system. The vibration in the system can easily damage welding joints. It is also unthreaded fasteners.

Following are the various types of rivets:

• Blind rivets
• Semi-tubular rivets
• Solid head rivets
• Flush rivets
• Drive rivets

Anchors

Anchors are described as a fastening element embedded in a material and hold the object fixing to the material. It is commonly used to attach something to the wall or concrete.

Following are the various types of anchors:

• Internally threaded anchors
• Externally threaded anchors
• Masonry screw anchors
• Hollow wall anchors
• Sleeve anchors
• Drive anchors

Design of non-permanent joints

The non-permanent joints are made up with the help of nut-bolts, keys, anchors, screws, and others. To make the joint durable, the failures and deformations of these components should be eliminated. Generally, the non-permanent joints are failed due to shear loading.

Needs for non-permanent fasteners

Following are the basic needs for non-permanent fasteners:

• Field assembly
• Disassembly
• Adjustment
• Maintenance

Bolted and riveted joints loaded in shear

The shear loaded joints are the same for all fastening elements like bolts, rivets, pins, etc. Following are the several failure modes possible in shear loaded joints:

• Bending of fastener
• Shear of bolt
• Tensile failure of a member
• Bearing stress on bolt or members
• Shear tear-out
• Tensile tear-out

Failure due to bending of fastener

The diagrammatical representation for failure due to bending of a fastener is shown as:

The bending moment acting on the fastening system is given as,

$M=\frac{Ft}{2}$

Here, t is the total thickness of connected parts and F is the force applied.

The bending stress acting on the system is given as,

$\sigma =\frac{M}{I_c}$

Here, 

$I_c$

is the moment of inertia of fastener.

Failure due to shear of bolt

The diagrammatical representation of failure due to shear of the bolt is shown as:

The direct shear acting on the system is given as,

$T=\frac{F}{A}$

Here, A is the total cross-sectional area of the bolt and F is the force applied.

For bolts, it is necessary to determine that the shear is acting across the nominal area or the threaded area. The area based on minimum diameter is considered for determining shear.

Failure due to tensile rupture of member

The diagrammatical representation of failure due to tensile rupture of the member is shown as,

The tensile stress in failure is given as,

$\sigma =\frac{F}{A}$

Here, A is the smallest net area of the member with holes removed.

Failure by bearing stress

The failure of the system due to crushing is known as bearing stress. The member with the lowest strength will fail first. Generally, the bolt crushes first. The load distribution on a cylindrical surface is non-trivial or uniformly distributed over the projected contact area.

The projected contact area is given as,

$A=td$

Here, t is the thickness of the thin plate and d is the bolt diameter.

The diagrammatical representation of failure by bearing stress is shown as,

The bearing stress acting on the system is given as,

$\sigma =-\frac{F}{A}$

Failure by shear-out or tear out

The diagrammatical representation of system failure due to shear-out or tear out is shown as,

The edge shear or tear out is avoided by providing at least 1.5 times hole diameter spacing between the bolts from the edge.

Common Mistakes

Following are the common mistakes performed by students:

• Sometimes, students assume that anchors are only fasteners.
• Sometimes, students get confused that fasteners are used only for non-permanent joints.
• Sometimes, students forget that washers are also fasteners.
• Sometimes, students get confused between different failure modes possible in shear-loaded joints.

Context and Applications

The topic of fasteners and the design of non-permanent joints is significant in various courses and professional exams of undergraduate, graduate, postgraduate, doctorate levels. For example:

• Bachelor of Technology in Mechanical Engineering
• Bachelor of Technology in Automobile Engineering
• Master of Technology in Machine Design
• Doctor of Philosophy in Machine Design

Related Concepts

• Design of machine elements
• Permanent and temporary joints
• Failure of components due to shear
• Shear stress and strain

Practice Problems

Q1. Which of the following fasteners is unthreaded in nature?

1. Screw
2. Nails
3. Bolt
4. Nut

Correct option: (b)

Explanation: The Nails are the type of fastener that is non-permanent in nature, and generally, it does not consist of thread on its surface, so it is considered a threaded fastener.

Q2. ______ is placed between nut and bolt arrangement to properly distribute the fastener's load over the base material.

1. Washer
2. Rivet
3. Plate
4. Damper

Correct option: (a)

Explanation: A washer is generally used for the purpose of distributing the total load that is acting on the fastener equally over the base material. It may be of the shape of circular, triangular, rectangular, etc.

Q3. Which of the following is a correct projected contact area in failure due to bearing stress?

1. $td$
2. $\frac{t^2}{d}$
3. $\frac{t^3}{d}$
4. None of these

Correct option: (a)

Explanation: The term projected contact area in the context of failure due to bearing stress represents the projected area used directly due to bearing stress. In this case, the product of thickness and diameter represents the projected contact area.

Q4. If the shear force acting on the nut-bolt system is 10 N and the cross-sectional area of the bolt is 

$1 {\mathrm{cm}}^2$

. Which of the following value represents the shear stress induced in the nut-bolt system?

1. $10 \mathrm{N}/{\mathrm{m}}^2$
2. $100 \mathrm{N}/{\mathrm{m}}^2$
3. $1000 \mathrm{N}/{\mathrm{m}}^2$
4. $0.1 \mathrm{N}/{\mathrm{m}}^2$

Correct option: (c)

Explanation: The value of shear stress can obtain by taking the ratio of the shear force and the cross-sectional area of the bolt. The relation between shear stress and shear force is a linear one.

Q5. _____ is the minimum distance between the bolt and edge to avoid end tear.

1. 0.5D
2. 1.0D
3. 1.5D
4. 2.0D

Correct option: (c)

Explanation: The minimum distance to avoid end tear between the bolt and edge depends on the diameter of the bolt only. If the diameter increases, then the minimum distance would also increase.