Industrial Fastener Types Explained: Bolts, Screws, Nuts and Washers
Fasteners are the most frequently specified yet most commonly under-engineered components in industrial assembly. A misspecified bolt grade or an incorrect thread standard can cause joint failure, production delays, and costly rework. This guide covers every major fastener type used in industrial applications — bolts, screws, nuts, and washers — with grade references, material options, and selection criteria.
Industrial fasteners are standardised mechanical components used to join two or more parts together, either permanently or allowing disassembly. The main categories are bolts, screws, nuts, washers, and studs — each available in multiple head types, thread standards, grades, and materials. Correct fastener selection requires matching thread standard (metric DIN/ISO or imperial UNC/UNF), mechanical grade (strength class), material (carbon steel, stainless steel, or alloy steel), and surface treatment (zinc, hot-dip galvanized, or plain) to the application’s load, environment, and assembly requirements.
Fastener Thread Standards: Metric vs Imperial
The two dominant thread systems in industrial fasteners are metric (ISO/DIN) and imperial (UNC/UNF). Metric threads are defined by nominal diameter and pitch in millimetres — for example, M10×1.5 means 10 mm diameter with 1.5 mm pitch. Imperial threads use fractional or numbered diameters with threads-per-inch (TPI). The two systems are not interchangeable. European and most Asian manufacturing uses metric; North American and some British legacy equipment uses imperial. Always confirm the thread standard before ordering.
| System | Designation example | Standard | Primary markets |
|---|---|---|---|
| Metric coarse | M10 × 1.5 | ISO 68-1 / DIN 13 | Europe, Asia, global OEM |
| Metric fine | M10 × 1.25 | ISO 68-1 | Automotive, precision machinery |
| UNC (Unified Coarse) | 3/8–16 | ASME B1.1 | USA, Canada |
| UNF (Unified Fine) | 3/8–24 | ASME B1.1 | USA, aerospace |
| BSW / BSF (Whitworth) | 3/8 BSW | BS 84 | Legacy UK equipment |
| Trapezoidal (Tr) | Tr 20 × 4 | ISO 2904 / DIN 103 | Lead screws, actuators |
Bolt Types for Industrial Applications
A bolt is an externally threaded fastener designed to be used with a nut, or inserted into a tapped hole, to clamp two or more components together. Bolts are classified by head style, which determines the tool required for installation, the clamping footprint, and the torque capacity. The most common industrial bolt types are hex head (HEX), socket head cap (SHCS), flange head, carriage, and eye bolts, each suited to different assembly configurations and load directions.
Hex Head Bolt (HEX)
Six-sided head driven by spanner or socket. Standard for structural connections, flanged joints, and general machinery assembly. Available M4–M100+. DIN 931 (partial thread) / DIN 933 (full thread) / ISO 4014 / ISO 4017.
Socket Head Cap Screw (SHCS)
Cylindrical head with internal hex (Allen) drive. Used where space is limited or a flush / recessed head is needed. Higher torque capacity than hex head of equivalent size. DIN 912 / ISO 4762.
Flange Bolt
Hex head with integrated flanged washer face. Distributes clamping load over a wider area without a separate washer. Common in automotive, HVAC, and pipe flanges. DIN 6921.
Carriage Bolt
Domed head with a square neck that locks into the mating material when tightened. Used in timber connections, conveyor structures, and panel assemblies where the bolt head must not rotate. DIN 603.
Eye Bolt
Threaded shank with a looped head for attaching lifting slings, ropes, or chains. Load rating is direction-dependent — axial loading only unless shoulder type is specified. DIN 580.
High-Strength Structural Bolt
Heavy-hex head bolt for structural steel connections. Grade 8.8, 10.9, or ASTM A325/A490. Used with hardened washers and heavy hex nuts in steel frame construction and bridges. EN 14399 / ASTM F3125.
Screw Types for Industrial Use
Unlike bolts, screws are designed to thread directly into a material — either a tapped hole in metal or directly into softer substrates such as wood, plastic, or sheet metal. Industrial screws are classified by their drive type (Phillips, Torx, hex socket, slotted), point type (machine thread, self-tapping, self-drilling), and head style (pan, countersunk, button, cheese). Selecting the wrong point type for the substrate — for example, a machine screw into sheet metal without a tapped hole — is one of the most common specification errors in assembly design.
| Screw type | Key feature | Application | Standard |
|---|---|---|---|
| Machine screw | Fully threaded, requires tapped hole or nut | Precision assemblies, electrical panels | DIN 84 / ISO 1207 |
| Self-tapping screw | Cuts its own thread in thin sheet metal or plastic | Sheet metal, enclosures, HVAC ductwork | DIN 7971–7976 |
| Self-drilling screw (Tek) | Drills and taps in one operation | Steel purlins, roofing, cladding | DIN 7504 |
| Countersunk screw | Flush head sits below surface when installed | Panels, covers, any flush-finish requirement | DIN 965 / ISO 7046 |
| Set screw (grub screw) | Headless — sits fully within tapped hole | Shaft collars, gear hubs, locking mechanisms | DIN 913–916 / ISO 4026–4029 |
| Wood screw | Coarse thread, sharp point for wood | Timber structures, crating, packaging | DIN 97 |
Nut Types and When to Use Them
Nuts are internally threaded fasteners used with bolts or studs to clamp a joint. Standard hex nuts (DIN 934) are the default for most industrial applications. Locking nuts — nylon insert (DIN 985), all-metal prevailing torque (DIN 980), or castle nuts with cotter pins — are specified where vibration could cause loosening over time. Flange nuts, coupling nuts, and weld nuts serve specific assembly functions that standard hex nuts cannot fulfil.
Hex Nut
Standard nut for general industrial use. Available in regular (DIN 934 / ISO 4032) and thin (DIN 439 / ISO 4035) versions. The default specification for structural and machinery assemblies.
Nyloc Nut (Nylon Insert)
Nylon ring in the top of the nut creates prevailing torque, preventing loosening under vibration. Single-use in high-vibration applications. DIN 985 / ISO 7042.
Flange Nut
Integrated serrated or smooth flange distributes load and resists vibration-induced loosening without a separate washer. Common in automotive and HVAC. DIN 6923.
Castle Nut (Slotted)
Slots align with a hole in the bolt to accept a cotter pin — providing a positive mechanical lock. Used in steering, axle, and critical safety joints. DIN 935 / ISO 7035.
Coupling Nut (Long Nut)
Extended hex nut for joining two threaded rods end-to-end. Used in threaded rod assemblies, suspended ceilings, and pipe support systems. DIN 6334.
Weld Nut
Designed to be welded to a structure before assembly. Provides a captive threaded hole for blind-side fastening — common in automotive body panels and sheet metal enclosures. DIN 929.
Washer Types and Their Functions
Washers serve four distinct functions in bolted joints: distributing clamping load over a larger surface area, preventing damage to the mating surface, providing a hardened bearing surface for the bolt head or nut, and acting as a locking device to resist loosening. Flat washers, spring washers, and hardened structural washers are specified for different combinations of these functions. Using the wrong washer type — or omitting washers entirely — is a common cause of joint relaxation and surface damage in service.
| Washer type | Function | Standard | Typical use |
|---|---|---|---|
| Flat washer (Form A) | Load distribution, surface protection | DIN 125 / ISO 7089 | General purpose — under bolt head and nut |
| Flat washer (Form B, large OD) | Load distribution on soft or oversized holes | DIN 9021 / ISO 7093 | Timber, plastics, sheet metal with punched holes |
| Spring washer (split) | Vibration resistance — maintains preload | DIN 127 | Machinery, panels, any vibrating assembly |
| Nord-Lock washer | Wedge-locking — mechanically prevents loosening | Proprietary | Heavy machinery, rail, wind turbines |
| Hardened structural washer | Bearing surface for high-strength bolts | EN 14399-6 / ASTM F436 | Structural steel connections |
| Sealing washer (bonded) | Seals against fluid or gas passage at bolt hole | DIN 7603 | Pipe flanges, hydraulic ports, roofing fixings |
| Tab washer | Mechanical lock — tab bent against bolt flat | DIN 93 / DIN 432 | Bearing retainers, gearboxes, positive locking applications |
Fastener Grades and Strength Classes
Metric fastener strength is defined by a two-number property class marked on the bolt head or nut — for example, 8.8, 10.9, or 12.9. The first number indicates one-tenth of the minimum tensile strength in MPa divided by 10; the second indicates the ratio of minimum yield strength to tensile strength multiplied by 10. An 8.8 bolt therefore has a minimum tensile strength of 800 MPa and a yield-to-tensile ratio of 0.8. Higher grades provide greater clamping force in smaller diameters but require controlled tightening and hardened washers.
| Property class | Min. tensile strength | Min. yield strength | Typical use |
|---|---|---|---|
| 4.6 | 400 MPa | 240 MPa | Light-duty, non-critical assemblies |
| 5.8 | 500 MPa | 400 MPa | General machinery, moderate loads |
| 8.8 | 800 MPa | 640 MPa | Standard structural and machinery default |
| 10.9 | 1,000 MPa | 900 MPa | High-load assemblies, automotive, flanges |
| 12.9 | 1,200 MPa | 1,080 MPa | Maximum strength, precision critical joints |
| A2-70 (stainless) | 700 MPa | 450 MPa | Corrosion-resistant, moderate load |
| A4-80 (stainless) | 800 MPa | 600 MPa | Marine, chemical, high-load stainless |
Fastener Materials and Surface Treatments
Industrial fasteners are produced in carbon steel, alloy steel, stainless steel, and non-ferrous materials such as brass and aluminium. Carbon steel fasteners are the lowest cost and highest strength option but require surface treatment for corrosion resistance. Stainless steel fasteners (A2 = 304, A4 = 316) provide inherent corrosion resistance at higher cost and lower maximum strength. Surface treatments on carbon steel fasteners range from zinc electroplating for light indoor use to hot-dip galvanizing for outdoor and structural applications.
| Material / treatment | Corrosion resistance | Max strength class | Typical application |
|---|---|---|---|
| Plain carbon steel (black) | None | 12.9 | Dry indoor assemblies, short-term use |
| Zinc electroplated | Moderate (indoor, light exposure) | 12.9 | General machinery, electrical enclosures |
| Yellow zinc (dichromate) | Moderate+ | 10.9 | Automotive, appliances |
| Hot-dip galvanized (HDG) | High (outdoor, structural) | 8.8 (HDG) | Construction, HVAC, infrastructure |
| Stainless A2 (AISI 304) | Good — general corrosive environments | A2-70 / A2-80 | Food industry, architecture, outdoor |
| Stainless A4 (AISI 316) | Excellent — marine, chlorides, acids | A4-70 / A4-80 | Marine, chemical processing, pharma |
| Geomet / Dacromet | Very high — no hydrogen embrittlement | 10.9 / 12.9 | Automotive, wind energy, rail |
Fastener Selection Checklist
Before specifying or ordering industrial fasteners, confirm the following:
- Thread standard confirmed: metric (DIN/ISO) or imperial (UNC/UNF/BSW)
- Nominal diameter and pitch or TPI specified
- Property class / strength grade selected based on joint load calculation
- Material chosen for operating environment (carbon steel, stainless, alloy)
- Surface treatment matched to exposure (zinc, HDG, stainless, Geomet)
- Head type selected for assembly tool access and clamping footprint
- Locking method specified if vibration is present (nyloc, flange, Nord-Lock)
- Washer type specified: flat (load distribution), spring (preload), hardened (structural)
- Certifications required: DIN, ISO, or EN 14399 structural bolts
- Mill test certificates or batch traceability required?
Industrial Fasteners from Turkish Manufacturers
Turkey is a major producer and exporter of DIN and ISO standard industrial fasteners, supplying hex bolts, socket head screws, nuts, and washers in property classes from 4.6 to 12.9 and stainless grades A2 and A4. Turkish fastener manufacturers hold ISO 9001 certification, produce to DIN 931, DIN 933, DIN 912, DIN 934, DIN 985, and related standards, and export to automotive, construction, machinery, and HVAC manufacturers across Europe and the Middle East at competitive prices with full material traceability.
Steeling.net provides direct access to certified Turkish fastener manufacturers. Whether you need standard hex bolts in bulk, non-standard thread specifications, or stainless fastener sets for a specific application, submit your requirements to steeling.net/get-a-quote/ and we will return competitive quotes with lead times. No fees for buyers.
Frequently Asked Questions
What is the difference between a bolt and a screw?
A bolt is an externally threaded fastener designed to be used with a nut or inserted into a pre-tapped hole — it requires a tool on both ends for tightening. A screw is designed to thread directly into a material, either into a tapped hole (machine screw) or by cutting its own thread (self-tapping screw). In common usage the terms overlap, but in engineering specifications the distinction determines the assembly method and mating component.
What does the grade marking 8.8 on a bolt mean?
The property class 8.8 indicates the bolt has a minimum tensile strength of 800 MPa and a minimum yield-to-tensile ratio of 0.8, giving a minimum yield strength of 640 MPa. The first digit (8) × 100 = minimum tensile strength in MPa; the product of both digits (8 × 8 × 10) = minimum yield strength in MPa. Class 8.8 is the standard grade for most structural and machinery applications.
What is the difference between DIN 931 and DIN 933?
DIN 931 is a hex bolt with a partially threaded shank — a plain (unthreaded) grip section under the head with thread only at the end. DIN 933 is a hex bolt with a fully threaded shank from head to tip. DIN 931 is used when the shank must pass through a clearance hole in structural applications; DIN 933 is the standard for most general assembly where full thread engagement is needed throughout.
When should I use a nyloc nut instead of a standard hex nut?
Use a nyloc nut (DIN 985) wherever vibration could cause a standard nut to loosen over time — in machinery, vehicle assemblies, HVAC equipment, or any application subject to dynamic loading or cyclic stress. The nylon insert creates a prevailing torque that maintains clamping force without additional locking devices. Note that nyloc nuts are typically rated as single-use; replace them if removed and reinstalled in a critical joint.
What is the difference between A2 and A4 stainless steel fasteners?
A2 stainless fasteners are made from AISI 304 stainless steel — suitable for most general corrosion-resistant applications including food industry, architecture, and outdoor exposure. A4 fasteners are made from AISI 316 stainless steel, which contains 2–3% molybdenum for enhanced resistance to chlorides, acids, and marine environments. A4 is specified for seawater, salt spray, pharmaceutical, and chemical processing environments where A2 would eventually pit or corrode.
What is hot-dip galvanized (HDG) coating on fasteners?
Hot-dip galvanizing involves immersing steel fasteners in molten zinc at approximately 450°C, creating a thick zinc-iron alloy layer (typically 45–85 μm) that provides long-term cathodic protection against corrosion. HDG fasteners are specified for outdoor structural applications, infrastructure, and HVAC systems exposed to weather. Note that HDG coating increases thread dimensions, so nuts must be ordered oversized (“after galvanizing” size) for proper assembly.
Can metric and imperial fasteners be mixed?
No. Metric and imperial threads are not interchangeable and must never be mixed. A metric nut driven onto an imperial bolt (or vice versa) will appear to engage for a few turns but the thread flanks do not mate correctly, resulting in a joint with no reliable clamping force and a high risk of seizure or stripping. Always confirm the thread system of both the fastener and the mating component before assembly.
Hex bolts, socket screws, nuts, washers — DIN and ISO standard, grades 4.6 to 12.9, carbon steel or stainless. Send your specification and Steeling.net will return competitive quotes with full traceability. No fees for buyers.