A bolt is a mechanical fastener with external threads that needs to be used in conjunction with a nut (or threaded hole) to connect and secure two or more components through axial preload force.
Bolts and Bolt Connection Components
Bolt connections are the most common fastening method in mechanical assembly. The core components include the bolt itself and matching connecting parts. Below is a systematic classification and explanation:
Source: https://www.mechdaily.com/what-is-a-bolt/
I. Bolt Body Composition
Head
- Function: Transfers tightening torque and bears axial preload force.
- Common Types:
- Hexagon Head—Most versatile, operated with a wrench.
- Socket Head—For space-constrained scenarios.
- Flange Head—Integrated anti-loosening washer.
- Round/Countersunk Head—Requires countersunk holes.
Shank
- Unthreaded Portion:
- Bears shear force and reduces stress concentration on threads.
- Length designed based on connection thickness.
- Threaded Portion:
- Standards: ISO metric threads (e.g., M6×1.0), UN imperial threads.
- Fine Pitch Thread—Better anti-loosening, used in vibrating environments.
End
- Chamfer—Facilitates assembly.
- Lead-in Thread—Reduces insertion resistance.
II. Bolt Connection Accessories
Joined Parts
- Clearance Hole:
- Diameter > bolt diameter (e.g., 6.6mm hole for M6 bolt).
- Avoids assembly interference.
- Tapped Hole:
- Requires threading (e.g., M6 hole depth ≥ 1.5× bolt diameter).
Washer
- Flat Washer: Distributes pressure and protects soft material surfaces.
- Spring Washer: Prevents loosening (limited effectiveness under high-frequency vibration).
- Serrated Washer: Resists vibration loosening, used for soft substrates like aluminum.
Nut
- Hex Nut—Standard configuration.
- Nylon Insert Lock Nut—Preferred for anti-loosening.
- Flange Nut—Integrated anti-loosening teeth.
Auxiliary Anti-Loosening Components
- Threadlocker—e.g., Loctite® 243.
- Jam Nut—Low-cost anti-loosening solution.
- Cotter Pin—Prevents detachment in critical areas.
III. Types of Bolt Connections
| Type | Characteristics | Application Scenarios |
| Standard Bolt Connection | Relies on friction, requires precise preload control. | General mechanical structures. |
| Fitted Bolt Connection | Shank precisely fits the hole to bear shear force. | Positioning transmission parts (e.g., couplings). |
| High-Strength Bolt | Grade 8.8 or higher, preload reaches 70% of yield strength. | Bridges, steel structures. |
| Double-Ended Stud | Threads on both ends with an unthreaded middle section. | Flange sealing connections. |
Bolt Terminology
Thread Diameter
Thread Diameter is one of the most critical dimensions in bolts and nuts. It mainly consists of the following three types:
Source: https://accu-components.com/us/p/454-imperial-thread-chart-for-imperial-bolt-sizes
1. Major Diameter (External Diameter)
Definition: The largest diameter measured from crest to crest of the thread.
Purpose: Commonly referred to as the “bolt diameter” or “nominal diameter” (e.g., the “12” in M12).
Example: The major diameter of an M12 bolt is approximately 12.00 mm.
2. Pitch Diameter
Definition: The diameter measured at the midpoint of the thread height, where the thread flanks engage most effectively.
Purpose: The pitch diameter is the key control dimension for the fit between a bolt and a nut.
3. Minor Diameter (Internal Diameter)
Definition: The smallest diameter measured from root to root of the thread.
Purpose: Related to thread strength and affects the shear resistance of the bolt.
Example Table (M12 × 1.75 Thread):
| Item | Value (mm) |
| Major Diameter | 12 |
| Pitch Diameter | ≈10.863 |
| Minor Diameter | ≈10.188 |
| Thread Pitch | 1.75 |
Bolt length
Bolt length refers to the straight-line distance from a specified starting point to the threaded end of the bolt. This is the size used when selecting, designing, or specifying bolts and is equivalent to the nominal length.
How Bolt Length Is Measured (Depends on Head Type):
| Bolt Head Type | Measurement Method |
| Hex Head Bolt | Measured from under the head to the end of the thread (head not included) |
| Socket Head Bolt | Measured from under the head to the end |
| Countersunk Head Bolt | Measured from the top of the head to the end (head included) |
Example:
- M10 × 60 Hex Head Bolt:
- M10: Thread diameter is 10 mm
- 60: Length is 60 mm measured from under the bolt head to the threaded end (head height not included)
- M6 × 25 Countersunk Bolt:
- Total bolt length is 25 mm measured including the head, since the head is flush-mounted into the surface
Thread Pitch
Thread pitch is the axial distance between two adjacent thread crests on a bolt, typically measured in millimeters (mm) for metric threads, and is a key parameter that indicates the “coarseness” or “fineness” of the thread.
Source: https://www.omnicalculator.com/construction/thread-pitch
Thread Pitch = Distance between two adjacent thread crests measured parallel to the bolt axis
Examples
| Bolt Size | Pitch (mm) | Type |
| M10 × 1.5 | 1.5 mm | Coarse pitch (standard) |
| M10 × 1.25 | 1.25 mm | Fine pitch |
| M10 × 1.0 | 1.0 mm | Extra fine pitch |
Coarse Pitch vs Fine Pitch
| Aspect | Coarse Thread | Fine Thread |
| Pitch Size | Larger | Smaller |
| Strength | Higher tensile strength | Higher shear strength |
| Resistance to Loosening | Average | Better |
| Typical Applications | General purpose | Vibration-prone or precision applications |
Source: https://www.gdandtbasics.com/common-thread-information-diameters-pitch-and-threads-per-inch/
Thread Pitch in Imperial System (TPI)
In the imperial system, pitch is expressed as TPI (Threads Per Inch), the number of threads per inch.
Example:
1/2″-13 UNC means there are 13 threads per inch (pitch ≈ 1.954 mm).
Right-Hand Thread & Left-Hand Thread
Right-Hand Thread: Tightens when turned clockwise (the most common thread direction).
Left-Hand Thread: Tightens when turned counterclockwise; used in applications where reverse rotation may cause loosening (e.g., fan blades, gas cylinders).
Chamfer & Chamfer Angle
Chamfer: A beveled edge cut at the end of a bolt or nut to ease installation and prevent thread damage.
Chamfer Angle: The angle of the chamfer, typically between 30° and 45° (commonly 45° in ISO standards).
Thread Class / Fit
Indicates the precision and tightness of the thread engagement between bolt and nut.
- Imperial (Unified): Classes like 2A (external), 2B (internal) for standard fits; 3A/3B for tighter fits
- Metric (ISO): Classes like 6g (external), 6H (internal)
Higher classes = tighter fit, but increased manufacturing cost
Indicates the mechanical strength of the fastener.
- Metric: Property classes like 4.6, 8.8, 10.9, 12.9 (higher numbers = higher strength)
- Imperial (SAE): Grades like 2, 5, 8
Example: Class 10.9 = 1000 MPa minimum tensile strength, 900 MPa yield strength
Coating / Plating
Surface treatments for corrosion protection, aesthetics, or lubrication:
| Coating Type | Purpose / Features |
| Zinc Plating | Basic corrosion resistance; economical |
| Hot-Dip Galvanizing | Thick protective layer for outdoor/heavy-duty use |
| Dacromet | Chromium-free, high corrosion resistance; used in automotive |
| Nickel Plating | Decorative, corrosion- and wear-resistant |
| Black Oxide | Light corrosion resistance, dark finish; indoor use |
Washer Compatibility
Washers must be selected based on bolt type and material:
| Washer Type | Description |
| Flat Washer | Distributes load; protects workpiece surface |
| Spring Washer | Adds anti-loosening function |
| Hardened Washer | Used with high-strength bolts; resists deformation |
| Material Matching | Stainless steel bolts should use stainless washers to prevent galvanic corrosion |
Common Bolt Types
Purpose: Used to secure structures to concrete foundations (e.g., steel columns, equipment bases).
Features: One end is threaded, while the other has a hook or L-shape for enhanced anchoring.
Common types: Expansion anchors, Chemical anchors.
2. Arbor Bolts
Purpose: Fixing rotating tools like saw blades and grinding wheels.
Features: Short threads + long shank, often with a flange head to prevent loosening.
3. Blind Bolts
Purpose: Used in enclosed spaces where only one-sided access is possible (e.g., pipes, enclosures).
Features: The tail expands after installation (e.g., Pop blind rivets).
Purpose: Wooden structures or metal-to-wood connections (e.g., furniture, agricultural machinery).
Features: Round head + square neck to prevent rotation.
Purpose: Securing elevator rails, conveyor belts, and similar equipment.
Features: Large flat head + short threads, resistant to vibration.
Purpose: Lifting and hanging (e.g., equipment hoisting).
Features: Head is ring-shaped, available in shouldered (dynamic load) and non-shouldered (static load) variants.
Purpose: Sealed connections like pipe flanges and automotive exhaust systems.
Features: Integrated flange washer for anti-loosening and pressure distribution.
Purpose: General mechanical structure connections.
Features: Hex head + full/partial threads, available in various strength grades (e.g., 8.8, 10.9).
9. J-Bolts
Purpose: Embedded in concrete (e.g., guardrail foundations).
Features: One end is bent into a J-shape for enhanced pull-out resistance.
Purpose: Heavy-duty fastening in wood or soft materials (e.g., timber framing).
Features: Coarse threads + hex/square head, does not require a nut.
Purpose: Machine bolts are versatile fasteners featuring hexagonal/square heads with full/partial thread options, made of carbon or stainless steel, designed for high-strength connections in mechanical equipment and steel structures. They offer multiple surface treatments, standardized certifications, and flexible installation with washers or nuts for anti-loosening functionality.
Features: As fundamental industrial fasteners, machine bolts combine functional features (like shear-resistant shanks) and material adaptability (from standard carbon steel to corrosion-resistant stainless steel), meeting both routine assembly requirements and specialized environmental demands (e.g., high-temperature/humidity conditions) while maintaining stable performance.
12. Hanger Bolts
Purpose: Transitional connections between wood and metal (e.g., ceiling installations).
Features: Threads on both ends (one for wood, one for metal), no head.
13. Plow Bolts
Purpose: Agricultural machinery (e.g., plows, buckets).
Features: Countersunk head + square neck, resistant to loosening and impact.
14. Rock Bolts
Purpose: Support in mines and tunnels.
Features: Full-length threads, used with resin or mechanical anchors.
Purpose: Vibration-prone environments (e.g., engines).
Features: Flange face has serrations to grip the material and prevent loosening.
16. Sex Bolts / Chicago Bolts
Purpose: Joining leather or panels (e.g., handbags, signs).
Features: Internally and externally threaded sleeve + countersunk bolt for a flush finish.
Purpose: Axial positioning of bearings and gears.
Features: Precision cylindrical shank (tolerance h6).
Purpose: High-torque applications in compact spaces (e.g., molds).
Features: Cylindrical head + internal hex drive, high tensile strength.
Purpose: Traditional wood or cast iron structures.
Features: Square head + square neck to prevent rotation.
Purpose: Sealed flange connections (e.g., petrochemical pipelines).
Types: Equal-length (threads on both ends), single-end threaded (one threaded end, one plain shank).
21. T-Head Bolts
Purpose: Securing to T-slot tracks (e.g., machine tool tables).
Features: T-shaped head slides into the slot.
22. Tap Bolts
Purpose: Applications requiring a completely hidden head.
Features: Fully threaded, head can be countersunk into the workpiece.
23. Toggle Bolts
Purpose: Hanging heavy objects on hollow walls (e.g., drywall).
Features: Wings expand behind the wall after installation for increased load-bearing surface.
24. Track Bolts
Purpose: Fixing railway tracks.
Features: High strength, anti-corrosion coating (e.g., zinc-nickel alloy).
Purpose: Securing pipes or cables (e.g., automotive chassis).
Features: U-shaped bend + threaded ends, used with clamping plates.
Classification of Bolt Materials
| Material Category | Key Alloys/Standards | Properties |
| Monel Alloys | Monel 400 (UNS N04400) | Excellent seawater corrosion resistance, high strength. |
| Monel K500 (UNS N05500) | ||
| Nickel Alloys | Nickel 200 (UNS N02200) | High-temperature stability, alkali resistance. |
| Nickel 625 (UNS N06625) | ||
| Hastelloy Alloys | Hastelloy C276 (UNS N10276) | Superior acid resistance (HCl, H₂SO₄), stress-corrosion cracking resistance. |
| Hastelloy B2 (UNS N10665) | ||
| Inconel Alloys | Inconel 718 (UNS N07718) | Oxidation-resistant at high temps (up to 700°C), high strength. |
| Inconel 625 (UNS N06625) | ||
| Incoloy Alloys | Incoloy 800 (UNS N08800) | Heat/corrosion resistance, good mechanical properties. |
| Incoloy 925 (UNS N09925) | ||
| Urea Steel | 316L Mod (UNS S31603) | Resistant to urea/ammonia corrosion. |
| 25-22-2 (UNS S31050) | ||
| PH Stainless (Precipitation Hardening) | 17-4PH (UNS S17400) | High strength after heat treatment, corrosion-resistant. |
| 15-5PH (UNS S15500) | ||
| Corrosion-Resistant | 304 (UNS S30400) | General-purpose corrosion resistance (food-grade, marine). |
| 316 (UNS S31600) | ||
| Duplex Stainless | 2205 (UNS S31803) | Mixed austenitic-ferritic structure, high strength + chloride resistance. |
| 2507 (UNS S32750) | ||
| High-Temp Alloys | Waspaloy (UNS N07001) | Retains strength at extreme temps (>1000°C). |
| Haynes 230 (UNS N06230) | ||
| Stainless Steel | A2-70 (304) | Cost-effective, versatile corrosion resistance. |
| A4-80 (316) |
Conclusion
- Bolts are used to fasten and assemble parts in aligned unthreaded holes, typically with a matching nut.
- A bolt primarily consists of the bolt shank and the bolt head. The unthreaded portion of the shank is referred to as the bolt shank, while the threaded portion is called the bolt thread.
- Bolts are generally used in conjunction with nuts and sometimes with washers.
- Key fastener terminology related to bolts includes: shank diameter, major diameter, minor diameter, pitch, nominal length, grip length, thread length, thread pitch, threads per inch (TPI), right-hand thread, left-hand thread, chamfer, and chamfer angle.
- Bolts are specified using either the metric or imperial system.
