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Ever wondered how structures stay intact? Bolts, like door bolts, are crucial in construction and engineering. They hold everything together, ensuring stability. In this post, you'll learn about the types of bolts, their importance, and how they impact various projects.
When working with bolts, it’s important to know their main parts. Each part plays a role in making sure the bolt holds things together securely.
The bolt shaft is the long, round part that goes through the materials being fastened. It has two sections:
Shank: This is the smooth, unthreaded part just below the head. It provides a snug fit inside the hole, helping to keep the parts aligned. The shank also spreads the stress evenly, reducing wear on the threads. Having a shank prevents bolts from loosening in places where vibrations happen.
Thread: This is the spiral ridge that wraps around the shaft. Threads allow the nut to grip the bolt tightly. They can cover the entire shaft or just part of it. The thread’s design lets the bolt pull parts together and keep them from moving apart.
The bolt head is what you turn to tighten or loosen the bolt. Different heads fit different tools and uses:
Hex Head: Six flat sides, easy to grip with wrenches or sockets. Common in many applications.
Countersunk Head: Cone-shaped so it sits flush with the surface, good for a clean look.
Dome Head: Rounded and smooth, harder to grip but looks neat.
Other heads include square, socket, and button heads, each designed for specific needs or tools.
Bolts often work together with nuts and washers to form a strong joint.
Nuts: These have internal threads that match the bolt’s threads. When tightened, nuts clamp the parts together. They stop the bolt from sliding out. Sometimes, extra features like lock washers or thread-locking fluids help keep nuts from loosening due to vibrations.
Washers: Thin, flat discs placed under the bolt head or nut. Washers spread the load over a bigger area, protecting the parts from damage. Soft washers made of rubber or plastic can absorb vibrations and prevent bolts from loosening. Some washers even seal joints to keep out water or dirt.
Together, these components make bolted connections strong and reliable.
Understanding fastener terminologies helps you select and specify bolts correctly. These terms describe the key dimensions and features of bolts, ensuring compatibility and strength in your projects.
Shank Diameter: This is the diameter of the smooth, unthreaded part of the bolt shaft. It’s also called the nominal diameter or body diameter. The shank diameter is important for determining the bolt’s fit in holes and its shear strength.
Major Diameter: This is the largest diameter of the threaded part, measured from crest to crest of the threads. It usually matches the nominal diameter. The major diameter ensures the bolt fits properly into nuts or tapped holes.
Minor Diameter: Also known as root diameter, this is the smallest diameter of the thread, measured between the roots (bottom) of the threads. It affects the bolt’s shear strength and thread engagement.
Pitch Diameter: This lies between the major and minor diameters, roughly at the midpoint of the thread thickness. It’s critical for ensuring the bolt and nut threads fit together tightly without gaps.
Thread Pitch: The distance between adjacent thread crests, measured along the length of the bolt. It’s expressed in millimeters for metric bolts. A smaller pitch means more threads per length, offering finer adjustment and higher tensile strength.
Threads Per Inch (TPI): Used in imperial bolts, TPI counts the number of threads in one inch. Higher TPI means finer threads. Selecting the right TPI ensures compatibility with nuts and prevents cross-threading.
Bolts used in steel structures fall into three main types:
Black Bolts
Turned and Fitted Bolts
High Strength Friction Grip (HSFG) Bolts
Each type has unique characteristics and uses, especially in construction and engineering.
Black bolts are made from mild steel and usually have a Grade 4.6 rating. They are unfinished and offer adequate strength and ductility for many applications. When tightened snugly, black bolts hold parts firmly. However, overtightening can cause them to twist off easily. Because of their lower cost and decent performance under static loads, black bolts remain popular in simple structures.
Turned and fitted bolts have uniform shanks and fit into drilled holes with close tolerances. The holes are slightly larger—about 1.5 to 2 mm bigger than the bolt diameter—to allow easy fitting. These bolts are tightened snugly using box spanners. They come in grades from 4.6 up to 8.8. Higher-grade bolts may not show a clear yield point, so proof stress at 0.2% strain is used instead.In these bearing-type connections, plates are pressed firmly against the bolt, which carries shear loads. However, under dynamic loading, nuts may loosen, so turned and fitted bolts are not recommended for such conditions. They are also less suitable where small slips cause significant issues, like beam splices. Still, their lower cost and ease of installation make them common in static load applications.
HSFG bolts provide highly efficient connections, especially under fluctuating or fatigue loads. Made from quenched and tempered alloy steels, these bolts range from grade 8.8 to 10.9. The most common grade is 8.8, which balances strength and ductility. Grade 10.9 bolts are stronger but less ductile.HSFG bolts are tightened to their proof loads, ensuring no slip occurs during use. This means load transfers through friction between plates rather than bearing on the bolts. Hardened washers distribute the load under bolt heads, often tapered to fit rolled steel sections.
There are several tightening methods for HSFG bolts:
Turn-of-nut method: After snug tightening, the nut is turned a specified amount to reach proof load.
Calibrated wrench method: A wrench calibrated to apply the correct tension tightens the bolt.
Direct tension indicator method: Special washers with protrusions compress as the bolt tightens, indicating tension by measuring the gap.
HSFG bolts require larger holes than usual to accommodate erection tolerances and lack-of-fit. Hole types include standard, extra large, and slotted (short or long). The hole type affects connection strength, so pitch and edge distances must meet standards.
Bolts come in many types, each designed for specific uses and materials. Knowing the right bolt type helps ensure your project stays strong and secure. Here are some common types of bolts you’ll encounter:
Anchor bolts secure structures to concrete. They’re widely used in construction for fastening steel poles, columns, or signs to concrete foundations. Two main types exist:
Cast-in-place anchor bolts: Installed by setting the bolt into wet concrete, allowing it to harden around the bolt for a strong hold.
Drilled-in-place anchor bolts: Inserted into pre-drilled holes in hardened concrete, often stabilized using adhesives.
Anchor bolts come with different ends:
Headed anchor bolts: Have a hex or square head embedded into concrete to prevent pull-out.
Bent anchor bolts: Feature an L- or J-shaped bend in the shaft, which locks the bolt inside concrete.
Carriage bolts have a smooth, rounded head and a square neck just below the head. The square neck fits tightly into wood or metal, preventing the bolt from turning when tightened. These bolts are common in wood projects like furniture and fences, offering a clean look and secure fastening.
Elevator bolts have a large, flat, round head and a square neck. Designed originally for grain elevators, they now fasten conveyor belts and machinery parts. The flat head sits flush, preventing interference with moving parts. Threads typically cover part of the shaft.
Eye bolts feature a looped head shaped like a ring or eye. They serve as anchor points for ropes, cables, or lifting hooks. Variations include:
Shoulder eye bolts: Have a shoulder for extra strength and load support.
Anchor eye bolts: Used in masonry.
Screw eyes: Threaded like screws for wood applications.
Roller eye bolts: Include rollers for easier cable movement.
Hex bolts have a six-sided head, making them easy to grip with wrenches or sockets. They come fully or partially threaded and suit many applications, from machinery to construction. Their shape allows tightening from multiple angles, speeding up assembly.
J-bolts are shaped like the letter "J," with threads on the straight end. The curved end acts as an anchor point embedded in concrete or wood. They’re ideal for securing walls, beams, or machinery to foundations.
Lag bolts, or lag screws, are heavy-duty fasteners with coarse threads and a hex or square head. They’re designed to be driven into wood, creating strong, friction-based connections without nuts. Common in decks, retaining walls, and heavy framing, lag bolts provide excellent grip but can be difficult to remove once installed.
Selecting the right bolt for your project is crucial for ensuring safety, durability, and performance. Several key factors influence this decision, including load requirements, environmental conditions, material compatibility, and installation constraints.
First, understand the load your bolt must support. Bolts experience tensile forces (pulling apart) and shear forces (sliding forces). Choose a bolt grade and size that can handle these stresses without failure. For example, high-strength bolts are necessary for heavy machinery or structural steel, while standard bolts may suffice for lighter applications.Next, consider the environment. Moisture, temperature, chemicals, and exposure to the elements affect bolt performance. Stainless steel bolts resist corrosion well and are ideal for outdoor or marine environments. Coated bolts, like zinc-plated or hot-dip galvanized, offer protection against rust in less demanding settings. For harsh chemical environments, specialty alloys such as titanium may be necessary.Material compatibility matters too. Coarse thread bolts work better with softer materials like wood or plastic, providing better grip and reducing stripping risk. Fine thread bolts suit harder materials like metal, offering higher tensile strength and better resistance to vibration-induced loosening.
Bolt grades indicate strength and material properties. Common grading systems include:
ASTM (American Society for Testing and Materials): Defines material specs and strength levels for bolts used in structural and mechanical applications.
ISO (International Organization for Standardization): Uses property classes like 8.8, 10.9, and 12.9 to classify bolt strength.
SAE (Society of Automotive Engineers): Uses numeric grades such as Grade 2, 5, and 8 to denote strength levels.
Each grade corresponds to specific tensile strength, yield strength, and proof load values. For example, a Grade 8 bolt offers higher tensile strength than a Grade 5, making it suitable for heavy-duty tasks. Always select bolts adhering to recognized standards to ensure reliability and safety.
Installation conditions influence bolt choice. If space is tight, socket head bolts with Allen drives can be easier to install. For flush finishes, countersunk or flat head bolts work best. Also, consider the tools available and ease of access during assembly.Maintenance is vital for long-term performance. Periodically inspect bolts for signs of corrosion, loosening, or wear. Replace damaged bolts promptly. Using lock washers, thread-locking compounds, or specialized nuts can help prevent loosening in vibrating environments.
Bolts are essential in construction and engineering, with three primary types: black bolts, turned and fitted bolts, and high-strength friction grip bolts. Selecting the right bolt ensures safety and performance, considering load, environment, and material. Choosing the appropriate bolt type is crucial for project success. Ningbo Tongyi Metalwork Co.,Ltd offers high-quality bolts that enhance durability and reliability in various applications, providing exceptional value to customers.
A: A Door Bolt is a type of fastener used to secure doors in place, preventing them from being opened without authorization.
A: Door Bolts operate by sliding a metal rod into a catch or socket, effectively locking the door in a closed position.
A: Door Bolts are crucial for enhancing security by providing an additional locking mechanism to deter unauthorized access.
A: The cost of Door Bolts varies based on material, design, and brand, typically ranging from a few dollars to higher-end models costing more.
