Key Characteristics of Machine Bolts
Head Shape:
Machine bolts typically feature a hexagonal head (hex head), which allows for easy tightening
with wrenches or power tools.
The hex head provides a good grip and distributes the load across a larger surface, making it
ideal for heavy-duty applications.
Thread Type:
Unified National Coarse (UNC) and Unified National Fine (UNF) thread types are common.
UNC threads are generally preferred for applications that require frequent installation and
removal, as they are less likely to strip. UNF threads are used when higher clamping force or
precision is needed.
Shank:
The unthreaded portion of the bolt, referred to as the shank, provides additional shear
strength. This makes machine bolts ideal for applications where both tensile and shear forces
are present.
Material:
Machine bolts are manufactured from a variety of materials, depending on the application:
Carbon Steel: Offers high strength and is commonly used in construction and machinery.
Stainless Steel: Provides corrosion resistance and is used in marine and outdoor environments.
Brass: Used in applications requiring electrical conductivity and corrosion resistance.
Alloy Steel: For applications requiring high tensile strength and resistance to wear or extreme
temperatures.
Finish:
Machine bolts are available in a variety of finishes to enhance durability and resistance to
corrosion, such as:
Zinc-Plated: Provides moderate corrosion resistance for general applications.
Galvanized: Hot-dip galvanizing offers superior corrosion protection, ideal for outdoor or
industrial applications.
Black Oxide: Improves appearance and offers mild corrosion resistance, typically used indoors.
Types of Machine Bolts
Hex Head Machine Bolts:
Description: The most common type of machine bolt, featuring a hexagonal head for easy
tightening with a wrench.
Use Case: Used in construction, manufacturing, and automotive industries.
Applications: Structural steel connections, heavy machinery assembly, and bridge construction.
Square Head Machine Bolts:
Description: These bolts have a square-shaped head, offering more grip for installation and
removal, particularly in older or rustic settings.
Use Case: Commonly used in historical restoration projects or where a traditional aesthetic is
desired.
Applications: Timber framing, railroad construction, and industrial machinery.
Carriage Bolts:
Description: These bolts have a round, smooth head and a square neck beneath the head,
preventing rotation during tightening.
Use Case: Frequently used in woodworking or where a flush surface is needed on one side.
Applications: Wood structures, furniture assembly, and decking.
Anchor Bolts:
Description: These are machine bolts designed to secure objects to concrete or other heavy
materials. They are often threaded on one end, with the other end bent or embedded.
Use Case: Used to anchor structural components to concrete.
Applications: Foundation work, securing machinery, and anchoring steel columns.
Heavy Hex Bolts:
Description: These bolts have a larger hexagonal head than standard hex bolts and are used for
heavy-duty applications.
Use Case: Ideal for environments requiring greater load-bearing capacity and clamping force.
Applications: Large steel structures, oil and gas industry, and power generation equipment.
Elevator Bolts:
Description: These bolts have a flat, countersunk head with a square neck that prevents
rotation.
Use Case: Designed for use in conveyor systems and material-handling equipment.
Applications: Elevators, conveyors, and agricultural equipment.
Machine Bolt Specifications
Thread Size:
Machine bolts are available in standard thread sizes, typically ranging from 1/4 inch to 2
inches in diameter for imperial sizes, and M6 to M30 in metric sizes. The thread size determines
the bolt’s load capacity.
Length:
Lengths of machine bolts vary from 1 inch to over 20 inches, depending on the application. The
length is measured from the tip of the threaded end to the underside of the head.
Tensile Strength:
Tensile strength is a key specification for machine bolts, as it determines how much load the
bolt can handle before failure. Machine bolts made from carbon steel or alloy steel typically
have tensile strengths exceeding 60,000 psi, while stainless steel bolts provide a balance of
strength and corrosion resistance.
Grade and Standard:
Machine bolts must meet specific industry standards to ensure their performance in different
applications. Common standards include:
ASTM A307: Covers carbon steel machine bolts for general-purpose use in construction.
ASTM A325: High-strength structural bolts used in heavy steel construction.
ASTM A490: Heat-treated steel bolts designed for high-stress applications.
ISO 898-1: Specifies mechanical properties of carbon steel and alloy steel bolts.
Finish:
Machine bolts are often coated or treated to improve their performance in harsh environments:
Hot-dip galvanized: Provides superior corrosion protection for outdoor applications.
Zinc-plated: Suitable for general indoor use where moderate corrosion resistance is required.
PTFE-coated: Used in chemical industries for added corrosion resistance and low friction.
Applications of Machine Bolts
Construction:
Machine bolts are widely used in construction for joining beams, columns, and other structural
components.
Examples: Steel-framed buildings, bridges, and heavy equipment assembly.
Machinery and Equipment:
In industrial machinery, machine bolts provide secure fastening for parts that require high
clamping forces.
Examples: Securing motors, conveyor systems, and large mechanical parts in factories.
Automotive Industry:
Machine bolts are used to assemble various automotive parts, such as engines, transmissions, and
frames.
Examples: Bolting engine blocks, securing suspension systems, and attaching body panels.
Electrical and Power Generation:
Machine bolts are commonly used in power plants and electrical installations to fasten
components like turbines, generators, and transformers.
Examples: Wind turbines, hydroelectric plants, and power transmission towers.
Marine and Offshore Applications:
Stainless steel or galvanized machine bolts are frequently used in marine environments due to
their resistance to corrosion.
Examples: Securing ship decks, offshore platforms, and dock installations.
Timber Construction:
Square-head machine bolts are often used in timber framing and rustic-style construction due to
their traditional look.
Examples: Log cabins, barns, and timber bridges.