Mechanical Joint and Fastener Selection: Metric Fastener Designation

Metric Fastener Designation

Metric screws and bolts are designated as follows:

Product name, nominal diameter × thread pitch × nominal length, property class, protective coating

Examples:

• Hex cap screw, M10 × 1.5 × 50, class 9.8, zinc plated
• Heavy hex structural bolt, M24 × 3 × 80, ASTM A490M
• Hex lag screw, 6 × 35, silicon bronze
• B18.3.1M—6 × 1 × 20 Hexagon Socket Head Cap Screw, Alloy Steel

Rivets and Riveted Joints — The Original Permanent Fastener

Before the age of high-strength bolts and advanced welding, rivets were the backbone of structural engineering. The Eiffel Tower. The Titanic. The Empire State Building. All riveted.

Understanding riveted joints remains essential for:

• Maintaining and inspecting legacy structures
• Aerospace applications (blind rivets, specialty alloys)
• Applications where welding is impractical or prohibited

Classes and Types of Riveted Joints

Riveted joints are classified by application:

1. Pressure vessel (boiler code — refer to ASME Boiler Code)
2. Structural (buildings, bridges)
3. Machine member (equipment frames, guards)

Two fundamental joint types exist:

• Lap joint: Plates overlap and are held by one or more rows of rivets
• Butt joint: Plates are in the same plane, joined by a cover plate (butt strap) riveted to both

Terminology:

• Single riveting = one row of rivets in a lap joint or one row on each side of a butt joint
• Double riveting = two rows
• Pitch = spacing between rivet centers in a row
• Back pitch (transverse pitch) = spacing between row centerlines
• Diagonal pitch = center-to-center distance between nearest rivets in adjacent rows
• Margin = distance from plate edge to nearest rivet row centerline

Failure Modes of Riveted Joints

Rivet failures:

1. Shearing through one cross-section (single shear)
2. Shearing through two cross-sections (double shear)
3. Crushing

Plate failures: 4. Shearing along two parallel lines from rivet hole to plate edge 5. Tearing from middle of rivet hole to plate edge 6. Crushing 7. Tearing between adjacent rivets (tensile failure)

Failures 4 and 5 are prevented by placing the rivet center at a minimum of 1.5 × rivet diameter from the plate edge. Diagonal tearing between adjacent rows is prevented by making the transverse pitch at least 1.75 × rivet diameter.

Design Assumptions for Simplified Analysis

1. Load is carried equally by all rivets
2. No combined stresses cause failure
3. Shearing stress is uniform across the cross-section
4. Double shear failure load = 2 × single shear failure load
5. Bearing stress is distributed equally over the projected rivet area
6. Tensile stress is uniform between rivet holes

Allowable Stresses

Joint Strength Formulas

For Single-Riveted Lap Joint:

For Double-Riveted Lap Joint:

Where:

• $d$ = diameter of rivet holes
• $t$ = plate thickness
• $p$ = rivet pitch
• $S_s$ = allowable shear stress
• $S_t$ = allowable tensile stress
• $S_c$ = allowable compressive/bearing stress

Joint efficiency = joint strength ÷ strength of unperforated plate × 100%

Worked Example: Single-Riveted Lap Joint

Given: 12-inch section, 1/4-inch thick plates, six 5/8-inch rivets, rivet holes 1/16 inch larger than rivets. Design stresses: shear = 8,500 psi, bearing = 20,000 psi, tension = 10,000 psi.

A) Safe load based on single shear of rivets:

$$L = 6 \times \frac{\pi}{4}(0.625)^2 \times 8{,}500 = 15{,}647 \text{ lbs}$$

B) Safe load based on bearing:

$$L = 6 \times (0.625 \times 0.25) \times 20{,}000 = 18{,}750 \text{ lbs}$$

C) Safe load based on tension:

$$L = 0.25 \times [12 - 6(0.625 + 0.0625)] \times 10{,}000 = 19{,}688 \text{ lbs}$$

Safe tensile load = 15,647 lbs (governed by rivet shear)

Efficiency:

$$\eta = \frac{15{,}647}{12 \times 0.25 \times 10{,}000} \times 100 = 52.2\%$$

Rivet Sizing Rule

The rivet diameter $d$ commonly falls between:

$$d = 1.2\sqrt{t} \quad \text{and} \quad d = 1.4\sqrt{t}$$

Where $t$ is the plate thickness.

Nails, Spikes, and Wood Screws

the practitioner's colleague the practitioner the practitioner specialized in timber construction — a field where nail selection could mean the difference between a structure that stands for a century and one that collapses under its first heavy snow load.

Standard Wire Nails and Spikes

Nails are sized by the penny system (abbreviated "d"), an ancient measurement that originally referred to the cost of 100 nails. Today, it simply designates length and wire gauge.

Complete Nail Reference Table:

Spikes are heavy-duty nails sized from 10d (3 inches) to 12 inches:

Specialty Nail Types

Beyond common wire nails, the standard covers:

• Flooring brads — finer gauge for hardwood flooring
• Fence nails — heavier gauge for outdoor structural use
• Clinch nails — designed to be bent over (clinched) on the exit side
• Barbed car nails — barbed shank for superior holding in wood
• Boat nails — corrosion-resistant, used in marine construction
• Slating nails — large flat heads for roofing slate
• Hinge nails — heavy and light varieties for hinge attachment

Wood Screws (ANSI B18.6.1-1981, R1997)

Wood screws are available in flat head, pan head, and oval head configurations. They're designated by number size (0 through 24), which corresponds to a specific basic screw diameter.

Key Dimensions:

Thread Length Rule: On wood screws with cut threads, the thread length equals approximately two-thirds of the nominal screw length. For rolled threads, the thread length is at least 4 × basic screw diameter or two-thirds of the nominal length, whichever is greater.

Pilot Hole Sizes:

Machine Screws — Precision in Small Packages

Machine screws are the precision fasteners of the fastener world — smaller, finer-threaded, and designed for tapped holes or mating nuts in metal, plastic, and composite assemblies.

Head Types

Machine screws come in an extraordinary variety of head styles:

• Flat countersunk — sits flush with the work surface
• Oval countersunk — decorative, partially flush
• Pan head — wide, low-profile bearing surface
• Fillister head — tall, cylindrical head with deep slot
• Truss head — extra-wide, low-profile
• Binding head — with undercut for wire binding
• Round head — traditional domed shape
• Hex head — for wrench-driven applications
• Washer head — integral washer for load distribution
• Cheese head (British) — similar to fillister, traditional British style

Cross Recesses

Modern machine screws use standardized cross recesses (Phillips, Pozidriv) in addition to slotted drives. These recesses provide:

• Self-centering during driving
• Higher torque transmission
• Compatibility with power drivers

ANSI Metric Machine Screws

Metric machine screws follow ANSI standards for thread lengths, head dimensions, and header points. Available head types include flat countersunk, oval countersunk, pan head, hex head, and hex flange head.

Cap Screws and Set Screws — Holding, Positioning, and Locking

Cap Screws

Cap screws are high-quality fasteners designed for precision applications. They include:

Slotted head cap screws:

• Flat countersunk head
• Round head
• Fillister head

Socket head cap screws:

• The workhorse of precision mechanical assemblies
• Hexagon and spline socket drives
• Available in ANSI inch and metric series
• Alloy steel (property class 12.9) and corrosion-resistant steel

Thread Length: The length of complete thread on cap screws equals twice the basic screw diameter plus 0.250 in (with a positive tolerance of 0.188 in, or 2.5 × pitch, whichever is greater).

Socket Head Shoulder Screws

Shoulder screws have an enlarged, unthreaded "shoulder" portion that serves as a precision bearing or pivot surface. The shoulder diameter is ground to tight tolerances and the thread is smaller than the shoulder.

Application: Shoulder screws are commonly used as pivot pins, guide pins for die sets, and bearing surfaces for moving linkages.

Set Screws — Holding Power

Set screws hold pulleys, gears, collars, and other components on shafts. They transmit torque through friction or mechanical interference (cup point digs into the shaft).

Available point types:

• Cup point — most common, digs into shaft for maximum holding
• Flat point — for repeated adjustment, doesn't damage shaft
• Cone point — locates in a drilled dimple for precise positioning
• Dog point — cylindrical end fits into a hole or slot
• Half dog point — shorter cylindrical end

Safe Holding Force in the supplied reference:

Power Transmission Formula:

$$P = \frac{D \times N \times d^{2.3}}{50}$$

$$T = 1{,}250 \times D \times d^{2.3}$$

Where:

• $P$ = horsepower transmitted
• $T$ = torque in inch-pounds
• $D$ = shaft diameter (inches)
• $N$ = shaft speed (RPM)
• $d$ = set screw diameter (inches)

Example: How many 1/2-inch set screws are needed to transmit 3 hp at 1,000 rpm on a 1-inch shaft?

$$P = \frac{1 \times 1{,}000 \times (0.5)^{2.3}}{50} = 4.1 \text{ hp}$$

A single 1/2-inch set screw is sufficient (4.1 hp > 3 hp required).

Self-Threading Screws — When the Fastener Creates Its Own Thread

Self-threading screws are divided into two fundamental categories: thread-forming (displacing material) and thread-cutting (removing material).

Thread-Forming Types

Thread-Cutting Types

Metallic Drive Screw (Type U)

Type U is a multiple-threaded drive screw with a large helix angle and pilot point. It's forced into the work by pressure alone (no turning) and is intended for permanent fastenings in metal and plastics.

Screw Thread Inserts

For applications in soft materials (aluminum, magnesium, zinc die castings, plastics, wood), screw thread inserts provide a way to achieve strong, durable threads:

• Self-tapping thread inserts: Hard bushings with internal and external threads. The external thread has cutting edges for the self-tapping feature. Available in case-hardened carbon steel, stainless steel, and brass.
• Helical coil inserts (Heli-Coil type): Helically formed coils of diamond-shaped stainless steel or phosphor bronze wire. Available in sizes from 4-40 to 1-1/2-6 (coarse) and 6-40 to 1-1/2-12 (fine). Thread classes 2, 2B, 3, and 3B.

T-Slots, T-Bolts, and T-Nuts — The Fixturing System

T-slots are the backbone of machine tool tables, jigs, and fixtures. They allow workpieces and fixtures to be clamped anywhere along the table using T-bolts and T-nuts.

Standard T-Slot dimensions are specified to ensure interchangeability across machine tool manufacturers:

Pins and Studs — Alignment, Retention, and Shear Resistance

Dowel Pins

Dowel pins are used to retain parts in a fixed position or preserve alignment. Under normal conditions, a dowel pin is subjected solely to shearing strain at the junction of the surfaces of the two parts being held.

Sizing rules:

• For locating nests, gage plates, etc.: 1/8 to 3/16 inch diameter
• For locating dies: never less than 1/4 inch — the general rule is to use dowel pins the same size as the screws used in fastening the work
• Length: 1.5 to 2 × diameter in each part being doweled

Fitting recommendations:

• Hardened dowel pins in soft parts: ream the hole 0.001 inch under pin diameter
• Both parts hardened: grind or lap the hole 0.0002 to 0.0003 inch under — straight, without taper or bell-mouth

Taper Pins

Taper pins are the preferred choice for parts that must be frequently disassembled while maintaining absolute alignment. Standard taper is 1:48 (1/4 inch per foot).

Available in sizes 7/0 through 14, with basic pin diameters from 0.0625 to 1.5210 inches.

Grooved Pins

Grooved pins have three equally spaced longitudinal grooves that create an expanded diameter over the crests of the raised ridges. This provides excellent retention without requiring a reamed hole — the pin compresses during installation and the ridges grip the hole walls.

Seven types (A through G) cover various head styles, chamfer configurations, and retention characteristics.

Recommended hole size: As close to nominal pin diameter as possible. The maximum limits are suitable for length-to-diameter ratios of 4:1 to 10:1.

Spring Pins

Two types of spring pins are standard:

• Slotted type: A cylindrical pin with a longitudinal slot. The pin compresses during insertion, creating a spring force that holds it in place.
• Coiled type: Shaped into a coil, providing more uniform stress distribution than slotted pins.

Materials: SAE 1070–1095 carbon steel, SAE 6150H alloy steel, SAE types 51410–51420, 30302 and 30304 corrosion-resistant steels, and beryllium copper alloy.

Cotter Pins

The classic safety device for preventing nut removal. After the nut is tightened to the correct torque, a cotter pin is inserted through a hole in the bolt and the slotted nut, then the prongs are spread to prevent the pin from falling out.

Retaining Rings — The Artificial Shoulder

Retaining rings act as removable shoulders that retain components on shafts (external rings) or in housings (internal rings), eliminating the need for machined shoulders, threads, or other integral retention features.

Two Fundamental Types

Stamped (snap) rings:

• Stamped from tempered sheet metal
• Non-uniform cross-section
• Most can only be installed at or near the end of a shaft or housing

Spiral-wound rings:

• Uniform cross-section
• Made of two or more turns of coiled, spring-tempered steel
• Provide a continuous, gapless shoulder
• One-turn variants are common

Retaining Ring Failure Modes

Ring failure (shear): Occurs when the ring is installed in a groove and loaded by a retained part where both groove and retained part have compressive yield strength > 45,000 psi, or when the ring is too thin relative to its diameter.

Allowable thrust based on ring shear:

$$P_s = \frac{\pi D t S_s}{K}$$

Where:

• $P_s$ = allowable thrust (lbf)
• $D$ = shaft or housing diameter (inches)
• $t$ = ring thickness (inches)
• $S_s$ = shear strength of ring material (psi)
• $K$ = factor of safety

Groove failure (yielding): The most common failure mode. The thrust load, applied through the retaining ring against the groove corner, exceeds the compressive yield strength of the groove material, causing the groove to deform and the ring to tilt out.

Thrust load that initiates groove deformation (spiral-wound rings):

$$P_G = \frac{\pi D d S_y}{K}$$

Where:

• $d$ = groove depth
• $S_y$ = yield strength of groove material

Critical Rule for Rotating Applications: External rings should be wound in the direction of rotation of the retained part. Internal rings should be wound against the direction of rotation. Failure to observe this causes the ring to wind out of the groove. Stamped retaining rings do not have this limitation.

Ring Materials

Wing Nuts, Wing Screws, and Thumb Screws — Hand-Operated Fastening

For applications requiring frequent hand adjustment without tools:

Wing Nuts (ANSI B18.17-1968, R1983)

Four types based on manufacturing method:

Designation Example: 10-32 Type A Wing Nut, Regular Series, Steel, Zinc Plated

Wing Screws

Wing screws have wing-shaped heads designed for manual turning without a driver or wrench. Four types (A through D) cover cold-formed, hot-forged, die-cast, and welded construction methods.

Materials:

• Type A: Carbon steel (shank case hardened), also available in corrosion-resistant steel or brass
• Type B: Carbon steel, also corrosion-resistant steel or brass
• Type C (Style 1): Die-cast zinc alloy
• Type D: Carbon steel

British Fasteners — The Other System

British Standard Square and Hexagon Bolts, Screws, and Nuts

British fastener standards have evolved through several thread systems:

• BSW (British Standard Whitworth) — the original, declining use
• BSF (British Standard Fine) — finer pitch variant, declining use
• Unified (UNC/UNF) — harmonized with American standards (BS 1768:1963)
• ISO Metric — the modern standard (BS 3692:1967)

Key insight for international procurement: Unified nominal and basic dimensions in British Standards are the same as comparable American Standards, but tolerances applied to these basic dimensions may differ due to rounding-off practices and other factors.

British Standard Screwed Studs (BS 2693: Part 1:1956)

Studs have three distinct zones:

• Metal end: Screwed into the component
• Nut end: Receives the nut for tightening
• Plain portion: Unthreaded length between threaded zones

Fitting practice: Holes tapped to Class 3B limits (per BS 1580) for Unified threads or Close Class limits (per BS 84) for Whitworth threads.

British Spring Washers (BS 4464:1969)

British spring washers come in double-coil rectangular section and square section types, with specific grades for different load and vibration conditions.

The Fastener Selection Decision Matrix

After thirty years, the practitioner distilled his entire fastener philosophy into a single decision framework:

Step 1: Define the Joint

Step 2: Select the Fastener Type

Step 3: Verify the Design

1. Calculate required preload based on joint loads
2. Select fastener grade with appropriate proof strength
3. Verify thread engagement length exceeds stripping requirements
4. Specify torque based on friction conditions
5. Add locking mechanism if vibration is present
6. Specify washer type based on surface hardness and preload requirements
7. Document the complete fastener designation including material and finish

Your Next Step

This guide is your permanent reference. But knowledge without application is just information.

Here's your challenge: Pick one assembly in your current project. Pull the engineering drawings. Verify that every fastener is correctly specified — grade, material, torque, washer, and locking method. Check the preload calculations. Verify the thread engagement length.

If you find a single fastener that's underspecified or unverified, you've just prevented the next the practitioner story.

What's the most critical bolted joint in your current work — and when was the last time you verified every fastener in it?

📌 Bookmark this guide. Share it with your team. Print the decision matrix and tape it to the wall above your drafting station. The fastener you verify today is the failure you prevent tomorrow.

Every Bolt, Screw, Rivet, Pin, and Ring You Need to Master

The Bridge That Wasn't Supposed to Fail

the practitioner stared at the pile of Grade 2 bolts scattered across his workbench. Forty-three of them. Every single one pulled from the same structural joint that had cracked open on a conveyor frame — a frame rated for 10,000 pounds of continuous load.

The bolts were the right diameter. The right length. The right thread pitch.

They were the wrong grade.

Someone — months ago, during a late-night maintenance run — had substituted SAE Grade 2 bolts (rated at 74,000 psi tensile) for the SAE Grade 8 bolts (rated at 150,000 psi tensile) specified in the original design. The heads looked the same to an untrained eye. The threads mated perfectly. But under load, those bolts stretched, relaxed, and eventually let the joint walk apart.

The cost? Three weeks of downtime. A replacement frame. And a safety audit that shut the entire line down for another two weeks.

the practitioner learned something that day that every engineer, fabricator, and maintenance technician eventually learns the hard way:

Fasteners are not interchangeable commodities. They are precision-engineered components with specific mechanical properties, and choosing the wrong one doesn't just cause inconvenience — it causes failure.

This guide exists so you never make that mistake. Whether you're a first-year apprentice learning to tell a bolt from a screw, or a senior engineer calculating preload for a critical pressure vessel joint, everything you need is here.

What Separates a Bolt from a Screw (And Why It Matters)

Before you can select the right fastener, you need to speak the language correctly. The distinction between a bolt and a screw is not casual — it is formally defined by ANSI/ASME standards and it determines how you design, specify, and install the fastener.

A bolt is an externally threaded fastener designed for insertion through holes in assembled parts, and is normally intended to be tightened or released by torquing a nut.

A screw is an externally threaded fastener capable of being inserted into holes in assembled parts, of mating with a preformed internal thread or forming its own thread, and of being tightened or released by torquing the head.

Here is the practical decision framework:

Why this matters: Bolts and screws are designated differently in engineering drawings, procurement documents, and inspection protocols. Using the wrong term can result in receiving the wrong product — and the wrong product in the wrong joint is how failures start.

Bolts, Screws, and Nuts: The Foundation of Mechanical Assembly

The Families You Need to Know

The American National Standards (ANSI/ASME B18.2.1-1996 and B18.2.2-1987) define the primary families of bolts, screws, and nuts used across industries worldwide. Here is the complete taxonomy:

Designation: How to Properly Specify a Fastener

Every fastener must be designated with the following data in this exact sequence:

1. Nominal size (fractional and decimal equivalent)
2. Threads per inch (omit for lag screws)
3. Product length (fractional or two-place decimal)
4. Product name
5. Material, including specification where necessary
6. Protective finish, if required

Examples of correct designation:

• 3/8-16 × 1-1/2 Square Bolt, Steel, Zinc Plated
• 1/2-13 × 3 Hex Cap Screw, SAE Grade 8 Steel
• .75 × 5.00 Hex Lag Screw, Steel
• 1/2-13 Square Nut, Steel, Zinc Plated
• 3/4-16 Heavy Hex Nut, SAE J995 Grade 5 Steel

Thread specification: When rolled, threads shall be Unified Coarse, Fine, or 8-thread series (UNRC, UNRF, or 8 UNR Series), Class 2A. Threads produced by other methods may be UNC, UNF, or 8 UN Series, Class 2A.

Grade Marks: Reading the Code Stamped Into Every Bolt Head

This is where the practitioner's story hits home. Every bolt and screw is stamped with a symbol on the head that identifies its grade — the specification that establishes minimum mechanical properties. Additionally, industrial fasteners must carry a registered head mark identifying the manufacturer.

If you cannot read these marks, you cannot verify what you are installing.