What is a Bolt? Types, Parts, Specifications & Uses

A bolt is a type of mechanical fastener designed to join two or more components together. Unlike welding or riveting, bolts form a non-permanent joint, which means the connected parts can be dismantled and reassembled whenever needed using appropriate tools.

Bolts are almost always used with nuts and sometimes with washers to ensure even load distribution, prevent loosening, and provide a strong, secure fastening. They are widely used in construction, automotive, machinery, aerospace, and household applications.

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Parts of a Bolt

A typical bolt consists of two primary parts:

1. Head

The top portion of the bolt, which allows a tool (like a wrench or spanner) to grip and rotate it.
Heads come in different shapes and designs depending on usage:
- Hexagonal head – Most common, used in machinery and construction.
- Square head – Provides extra grip in heavy-duty applications.
- Round head – Used in furniture or when a smooth finish is required.
- Countersunk head – Fits flush with the surface, commonly in carpentry.

Example: A hex head bolt is the standard choice in automotive engines because it offers excellent grip and torque transmission.

2. Shank

The cylindrical body of the bolt extending below the head.
Divided into two sections:
- Threaded section – Contains spiral grooves (threads) that allow the bolt to engage with a nut or threaded hole.
- Unthreaded section (grip length) – Provides shear strength and helps align parts.
The end of the shank is chamfered (slightly rounded) to make it easier to insert into a hole.

Example: Long shank bolts with partial threading are used in structural steel connections to handle shear loads effectively.

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Specifications of a Bolt

When selecting or describing a bolt, engineers consider several specifications to ensure the correct type is used. These include:

Thread form – The profile of the threads (e.g., Unified National Thread, ISO metric thread).
Pitch – The distance between adjacent threads; fine pitch offers more precision, while coarse pitch provides strength.
Head shape – Hex, square, round, countersunk, etc.
Body shape – Straight shank, tapered, or special profiles.
Size (diameter) – Outer diameter of the threaded portion.
Thread direction – Right-hand (tightens clockwise) is most common; left-hand threads are used where rotation may loosen the fastener.
Length – Total length from head to tip, excluding the head unless countersunk.
Material – Determines strength, corrosion resistance, and weight.

Example: A M12 × 1.75 × 80 hex bolt specifies metric thread (12 mm diameter), 1.75 mm pitch, and 80 mm length.

Dimensions of a Bolt

The dimensions of a bolt are standardized to ensure compatibility with nuts and washers. Key measurements include:

Nominal diameter – The outside diameter of the bolt threads.
Thread length – How much of the shank is threaded.
Grip length – Unthreaded portion of the shank that bears shear loads.
Head size – Distance across flats (spanner size).
Overall length – From under the head to the bolt tip.

Example: In structural engineering, the American Institute of Steel Construction (AISC) specifies standard bolt dimensions for different grades to ensure interchangeability.

Materials Used for Bolts

The choice of material determines the bolt’s strength, durability, corrosion resistance, and application.

Steel / Carbon Steel – Most common; used in construction and machinery.
Hardened Steel (Alloy Steel) – High strength; used in automotive and aerospace.
Stainless Steel – Excellent corrosion resistance; ideal for marine and outdoor use.
Brass & Copper Alloys – Corrosion-resistant and decorative; used in plumbing and electronics.
Aluminum – Lightweight, corrosion-resistant; used in aerospace.
Titanium – Very strong and lightweight; used in aircraft and high-performance vehicles.
Plastic / Nylon – Non-conductive and lightweight; used in electronics and light-duty applications.

Example: Stainless steel bolts are essential in shipbuilding to resist seawater corrosion.

Types of Bolts and Their Applications

Here are the most commonly used types of bolts with their specific functions:

Anchor Bolt – Fixes structures to concrete foundations.
Carriage Bolt – Rounded head with square neck; used in wood connections.
Elevator Bolt – Flat head; commonly used in conveyor belts and elevators.
Flange Bolt – Has a flange under the head for better load distribution.
Hanger Bolt – Headless; one end has wood threads, other machine threads.
Hex Bolt (Tap Bolt) – Most common type; used in all general applications.
Lag Bolt – Heavy-duty wood fastener with hex head and coarse threads.
Machine Bolt – Used in metal-to-metal fastening with a nut.
Plow Bolt – Countersunk head; used in plows, heavy machinery.
Sex Bolt (Chicago Bolt) – Used in furniture, leather goods, and bookbinding.
Shoulder Bolt – Has a smooth shoulder; used in moving parts like pulleys.
Square Head Bolt – Older style, provides high torque grip.
Stud Bolt – Threaded rod without a head; used in flanged connections.
Timber Bolt – Used in heavy timber construction.
T-Head Bolt – Designed to fit into T-slots.
Toggle Bolt – Spring-loaded for fastening in hollow walls.
U-Bolt – U-shaped, used to secure pipes or round objects.
J-Bolt – J-shaped, often embedded in concrete for structural supports.
Eye Bolt – Has a loop for attaching ropes, chains, or cables.
Spline Bolt / Special Bolts – Designed for specific industrial applications.

Example: U-bolts are widely used in automotive exhaust systems to clamp pipes securely.

Bolt Types and Their Applications in Tabular Form

Bolt Type	Description	Common Applications
Anchor Bolt	Used to attach structures to concrete foundations	Building columns, bridges, heavy equipment anchoring
Carriage Bolt	Round head with square neck to prevent turning	Wooden joints, fencing, furniture
Elevator Bolt	Flat, large head with short shank	Conveyor belts, elevators, agricultural machinery
Flange Bolt	Has a flange under the head for load distribution	Automotive, machinery, structural connections
Hanger Bolt	Wood screw threads on one end, machine threads on the other	Wooden furniture, framing, electrical installations
Hex Bolt (Tap Bolt)	Hexagonal head, threaded body	Machinery, construction, automotive
Lag Bolt	Heavy-duty wood fastener with coarse threads	Decks, wooden beams, heavy timber
Machine Bolt	Square/hex head, fully/partially threaded	Metal-to-metal fastening, industrial equipment
Plow Bolt	Countersunk head for flush fitting	Bulldozers, plows, road construction machinery
Sex Bolt (Chicago Bolt)	Two-part fastener (barrel + screw)	Furniture, books, leather goods
Shoulder Bolt	Smooth shoulder between head and threads	Pulleys, linkages, moving parts
Square Head Bolt	Square head for higher torque grip	Heavy machinery, vintage equipment
Stud Bolt	Fully threaded rod, no head	Pipe flanges, pressure vessels
Timber Bolt	Large bolt with oversized head	Heavy timber construction, docks, bridges
T-Head Bolt	“T”-shaped head fits into slots	Machine tool tables, structural frames
Toggle Bolt	Spring-loaded, for hollow surfaces	Drywalls, ceilings, wall-mounted fixtures
U-Bolt	“U”-shaped with threaded ends	Pipe fittings, automotive exhausts, marine applications
J-Bolt	“J”-shaped, embedded in concrete	Foundation supports, structural columns
Eye Bolt	Circular eye-shaped head	Lifting, rigging, securing cables and ropes

Conclusion

Bolts are one of the most essential fasteners in engineering and construction. By understanding their parts (head and shank), specifications, materials, and types, you can select the right bolt for any job.

From anchor bolts securing buildings to eye bolts handling lifting loads, each type has a unique purpose. Choosing the correct bolt ensures safety, efficiency, and durability in mechanical, structural, and industrial applications.

Frequently Asked Questions (FAQs)

1. What is a bolt used for?

A bolt is used to join two or more parts together in a non-permanent way. They are widely used in construction, automotive, machinery, and household applications.

2. What are the main parts of a bolt?

A bolt has two main parts:

(i) Head – the top portion used for gripping with tools.
(ii) Shank – the cylindrical body, part of which is threaded for nut engagement.

3. How is a bolt different from a screw?

Bolts usually require a nut to fasten objects, whereas screws are driven directly into a material without needing a nut. Bolts are also generally stronger and used for heavy-duty applications.

4. What materials are bolts made of?

Bolts are commonly made of steel, stainless steel, brass, aluminum, titanium, and copper alloys. The choice of material depends on strength requirements and environmental conditions.

5. Which type of bolt is most commonly used?

The hex bolt (hexagonal head bolt) is the most widely used type due to its strength and versatility in construction, automotive, and machinery.

6. What is the difference between a stud bolt and a machine bolt?

(i) Stud Bolt: A fully threaded rod with no head, usually used with two nuts.
(ii) Machine Bolt: Has a head and may be partially or fully threaded, commonly used for metal-to-metal fastening.

7. How do I choose the right bolt size?

Choosing the right bolt depends on application, load requirements, bolt material, thread type, and environmental conditions (e.g., corrosion resistance in marine use).