Mechanical Joint and Fastener Selection: Wing Screws

Wing Screws

Wing screws have wing-shaped heads for manual turning. Available in Types A through D with various wing configurations. Type A are two-piece construction (cold-formed), Type B are one-piece (hot-forged).

Materials: Carbon steel (shank case-hardened for Type A), corrosion-resistant steel, brass, or as agreed upon between manufacturer and user.

Lock Wire Procedure: Safety-Wiring Critical Fasteners

For applications where fastener loosening could cause catastrophic failure, safety wire (lock wire) provides a positive mechanical locking method.

Rules for Lock Wire Application

1. No more than three bolts may be tied together
2. Bolt heads may be tied only when the female thread receiver is captive
3. Pre-drilled nuts may be tied with the following conditions:
   • Nuts must be heat-treated
   • Nuts are factory-drilled for lock wire
4. Lock wire must fill a minimum of 75% of the drilled hole
5. Lock wire must be aircraft-quality stainless steel

Wire Diameter Selection

Note: Larger wire may be used in smaller fasteners for convenience, but smaller wire must never be used in larger fasteners.

British Fasteners: Bridging the Standards

British Standards for fasteners have evolved through several generations:

In 1965, British industry formally adopted the policy that ISO metric threads should be the first choice for all future designs, with ISO Unified as second choice. Whitworth and BA threads should be superseded by ISO metric in preference to an intermediate change to ISO inch.

Key British Standards:

• BS 1083:1965 — Precision hexagon bolts, screws, nuts (BSW/BSF)
• BS 1768:1963 — Unified precision hexagon bolts, screws, nuts (UNC/UNF) — obsolescent
• BS 3692:1967 — ISO metric precision hexagon bolts, screws, nuts — obsolescent
• BS 4168:1981 — Hexagon socket screws (metric)

The Master Decision Framework: Choosing the Right Fastener

Meet the practitioner, a mechanical design engineer two years into her career. She's just been handed her first solo project: designing the fastening scheme for a new material handling system. The system will be exposed to vibration, moderate loads, and occasional maintenance disassembly.

Here is the framework Leila used — and the one you should use for every fastener decision:

Step 1: Define the Joint Requirements

Step 2: Select the Fastener Type

Based on Leila's answers:

• Primary fastening: Hex cap screws, Grade 5 or 8
• Locking method: Helical spring lock washers (extra duty series for vibration)
• Alignment: Hardened dowel pins at critical interfaces
• Quick-access panels: Wing screws for hand-operated covers

Step 3: Calculate the Size

Using the preload formulas and service load analysis:

1. Determine required clamping force from joint loads
2. Select bolt size where $0.75 \times A_t \times S_p >$ required clamping force
3. Calculate required torque: $T = K \times F_i \times d$
4. Verify joint-length to bolt-diameter ratio ≥ 4:1

Step 4: Specify Completely

Write the complete designation per ANSI standards, including size, thread, length, product name, material, grade, and finish.

Troubleshooting Fastener Failures: The Diagnostic Checklist

When a bolted joint fails, the root cause usually falls into one of these categories:

Your Next Step

You have just absorbed the essential knowledge of fastener engineering — from the mathematics of preload and torque to the practical realities of grade identification, joint design, and failure analysis.

But knowledge without action is worthless.

Here is your immediate next step:

Go to your shop floor, your garage, or your current project. Pick up five fasteners. For each one:

1. Read the head marking. Can you identify the grade?
2. Check the specification. Does the installed fastener match the design requirement?
3. Estimate the preload. Using $F_i = 0.75 \times A_t \times S_p$, is the bolt adequately loaded for its application?
4. Look for distress signals. Rust, elongation, looseness, or wear patterns that indicate a problem developing.

If you cannot answer all four questions for all five fasteners, you have just discovered your highest-priority learning gap — and now you have this guide to close it.

What fastener failure have you encountered — or narrowly avoided — in your work? The lessons in those stories are the ones that stick forever.

This guide covers fastener types, specifications, and engineering principles per ANSI/ASME, SAE, ASTM, ISO, and British Standards. All formulas use generic notation applicable to any unit system. For specific dimensional data and tolerance tables, consult the referenced standards directly.

The Fastener That Failed at 40,000 Feet — And Everything You Need to Know So Yours Never Does

A bolt snapped. A joint opened. A pressure seal broke. And an aircraft skin panel peeled away mid-flight.

That is not a hypothetical scenario. It is the kind of cascading failure that has killed people — all because someone chose the wrong fastener, applied the wrong torque, or skipped a preload calculation they assumed did not matter.

Fasteners are the most overlooked, most underestimated, and most mission-critical components in every machine, structure, and vehicle ever built. They hold the world together — literally. And yet most engineers treat them as catalogue items to be selected by size and forgotten.

This guide will make sure you never make that mistake.

What follows is the most comprehensive fastener reference you will find anywhere — covering bolts, screws, nuts, washers, rivets, pins, retaining rings, self-threading screws, torque calculations, preload engineering, failure analysis, and every critical standard from ANSI to ISO to British Standards. Whether you are a first-year apprentice or a 30-year veteran, there is something here that will save you from a catastrophic failure you did not see coming.

How Fasteners Hold Everything Together — The Fundamentals

Before you touch a wrench, you need to understand what a fastener actually does. It is not just "holding two things together." A fastener creates clamping force — a controlled compressive load between mating parts that resists separation, prevents slipping, seals gaskets, and distributes stress.

Every fastened joint is a spring system. The bolt stretches. The clamped parts compress. And the balance between those two forces determines whether your joint survives the next million load cycles — or fails on the first.

Bolt vs. Screw — The Distinction That Matters

This is not a trivial vocabulary exercise. The ANSI/ASME standard (B18.2.1-1996) establishes a positive identification procedure that determines whether an externally threaded fastener is a bolt or a screw:

• A bolt is designed for insertion through holes in assembled parts and is normally intended to be tightened or released by torquing a nut
• A screw is 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

The practical difference:

Why this matters: A bolt that is specified as a screw (or vice versa) may be installed incorrectly, loaded in ways it was never designed for, or fail to meet code requirements for the application.

Nails, Spikes, and Wood Screws — Where Fastening Begins

Wire Nails and Spikes

The simplest fasteners remain among the most widely used. Standard wire nails and spikes are classified by the penny system (abbreviated "d"), where larger penny numbers indicate longer nails.

Different nail types serve different purposes: common nails for general construction, finishing nails (smaller heads, finer gauge) for trim work, casing nails for exterior casings, flooring brads for tongue-and-groove installations, and boat nails for marine applications.

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

Wood screws come in three primary head styles:

• Flat head — sits flush with the surface (82° countersink)
• Pan head — low-profile rounded top
• Oval head — decorative, partially countersunk

Critical installation detail: The thread length on wood screws with cut threads is approximately two-thirds of the nominal screw length. This means you need sufficient unthreaded shank to clamp the top piece without thread engagement causing separation.

Pilot hole guidance:

Rivets and Riveted Joints — The Original Permanent Fastener

Meet the practitioner, a structural engineer reviewing a 1940s-era bridge slated for rehabilitation. Every connection on that bridge was riveted — no bolts, no welds. And despite 80 years of freeze-thaw cycles, truck traffic, and corrosion, most of those riveted joints were still sound. Understanding why requires understanding how rivets work.

Classes and Types of Riveted Joints

Riveted joints fall into three application categories:

1. Pressure vessel joints (governed by ASME Boiler Code)
2. Structural joints (governed by AISC specifications)
3. Machine member joints

The two fundamental joint types:

• Lap-joint — plates overlap, rivets pass through both layers
• Butt-joint — plates align end-to-end, joined by one or two cover plates (butt straps)

Riveting terminology:

How Riveted Joints Fail

Daniela's bridge inspection checklist covered every known failure mode. Understanding these failures is essential for both design and assessment:

Rivet failures:

• Shearing through one cross-section (single shear)
• Shearing through two cross-sections (double shear)
• Crushing (bearing failure)

Plate failures:

• Shearing along two parallel lines from rivet hole to plate edge
• Tearing from rivet hole to plate edge (single line)
• Crushing of the plate
• Tensile failure (tearing between adjacent rivets)

Edge distance rule: Place the rivet center at a minimum of 1.5 times the rivet diameter from the plate edge. This prevents shear-out failures.

Diagonal tearing rule: When pitch is four times the rivet diameter or less, the transverse pitch must be at least 1.75 times the rivet diameter to prevent diagonal tearing between rows.

Design of Riveted Joints

The simplified design method makes six key assumptions:

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

Allowable Stresses for Riveted Joints

Worked Example — Single-Riveted Lap-Joint Analysis

Consider a 12-inch section of single-riveted lap-joint: ¼-inch plate, six rivets at ⅝-inch diameter, rivet holes 1/16-inch larger than rivets.

Using design stresses of 8,500 psi (shear), 20,000 psi (bearing), 10,000 psi (tension):

A) Shear capacity:

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

B) Bearing capacity:

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

C) Tensile capacity:

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

Safe load = least of the three = 15,647 lbs (governed by rivet shear)

Joint efficiency:

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

Riveted Joint Formulas — Complete Reference

Single-Riveted Lap-Joint:

Double-Riveted Lap-Joint:

Double-Riveted Butt-Joint:

Where: $d$ = hole diameter, $t$ = plate thickness, $p$ or $P$ = pitch, $S_s$ = shear stress, $S_t$ = tensile stress, $S_c$ = bearing stress.

Standard Rivets

Rivet size rule of thumb: The rivet diameter $d$ commonly falls between $d = 1.2\sqrt{t}$ and $d = 1.4\sqrt{t}$, where $t$ is the plate thickness.

Rivet holes are typically made 1/16 inch larger than the nominal rivet diameter. When holes are punched in heavy plate, reaming is necessary to remove weakened metal — this increases hole diameter by 1/16 to 1/8 inch.

Important physical behavior: Hot-driven rivets contract on cooling. This contraction in length draws plates together and induces a stress approximately equal to the yield point of the rivet steel. The contraction in diameter creates slight clearance between rivet and hole. The resulting clamping friction often carries a significant portion of the joint load before the rivets experience shear.

Torque and Tension in Fasteners — Where Most Engineers Go Wrong

This is the section that separates competent engineers from the ones who cause joint failures. Torque is not tension. Understanding that distinction is worth its weight in gold.

The Torque-Tension Problem

When you tighten a bolt, you apply torque with a wrench. That torque causes the bolt to stretch, producing preload — the clamping force that holds the joint together.

Here is the problem: a torque wrench does not measure bolt tension accurately. The relationship between applied torque and resulting preload depends on:

• Bolt, nut, and washer material
• Surface smoothness and machining accuracy
• Degree of lubrication
• Number of previous installations
• Thread condition and class of fit

As much as 90% of the applied torque goes to overcoming friction — only about 10% actually produces useful bolt tension. Change the lubrication, and you change the preload by as much as 25% or more.

Recommended Preload Formulas

The recommended preload $F_i$ for bolted joints:

$$F_i = 0.75 \times A_t \times S_p \quad \text{(reusable connections)}$$

$$F_i = 0.9 \times A_t \times S_p \quad \text{(permanent connections)}$$

Where:

• $F_i$ = bolt preload
• $A_t$ = tensile stress area of the bolt
• $S_p$ = proof strength of the bolt

If proof strength is unknown: $S_p \approx 0.85 \times S_y$, where $S_y$ is the yield strength.

Warning: Soft materials should never be used for threaded fasteners.

Measuring Preload — The Best Methods

Best method: Direct measurement with a strain gage.

Second best: Measure bolt elongation during tightening with a micrometer or dial indicator.

Elongation formulas:

$$\delta = \frac{F_i(A_d \times l_t + A_t \times l_d)}{A_d \times A_t \times E} \quad \text{(detailed)}$$

$$\delta = \frac{F_i \times l}{A \times E} \quad \text{(simplified)}$$

Where:

• $\delta$ = change in bolt length
• $A_d$ = major-diameter area
• $A_t$ = tensile-stress area
• $E$ = modulus of elasticity
• $l_t$ = threaded length within grip
• $l_d$ = unthreaded length within grip

The General Torque-Preload Relationship

If elongation measurement is not possible:

$$T = K \times F_i \times d$$

Where:

• $T$ = wrench torque
• $K$ = torque coefficient
• $F_i$ = preload
• $d$ = nominal bolt diameter

Values of K for steel bolts (¼ to 1 inch):

Approximate Tightening Torque Formula

For a rough estimate of proper torque using the bolt diameter $d$ (in inches):

$$T = 10^{b + m \log d} \quad \text{(ft-lb)}$$

Where $b$ and $m$ are coefficients from the following table:

Adjustment factors: For cadmium-plated cap screws, multiply torque by 0.9. For cadmium-plated nuts and bolts, multiply by 0.8. For special lubricants, multiply by 0.9.

Why Preload Matters — The Fatigue Life Connection

Picture the practitioner, a maintenance engineer who inherited a fleet of industrial pumps with chronic bolt-loosening problems. Every few weeks, flange bolts would back off, gaskets would leak, and production would halt for emergency repairs.

the practitioner discovered that his predecessors had been using low preload values "to avoid overstressing the bolts." The result was exactly the opposite of what they intended.

Here is the physics: In an axially loaded joint with no preload, the bolt load equals the joint load — the bolt absorbs 100% of every load cycle. But when proper preload is applied:

• The joint members are compressed
• External loads are partially absorbed as a reduction of compression
• The bolt sees a much smaller proportion of the cyclic load variation
• Fatigue life increases dramatically

The relationship is illustrated by this principle: with preload $P_{B1}$, the joint is compressed and bolt load changes more slowly than the joint load because some of the applied force is absorbed as decompression of the clamped material.

Preload for Shear-Loaded Joints

In joints where members slide, preload must be sufficient to hold members in contact. In non-sliding joints, shear loads are transmitted by friction — and friction comes directly from preload. The preload must generate friction forces greater than the applied shear force.

Preload Application Methods and Their Accuracy

Coefficients of Friction for Bolts and Nuts

Note: "None added" means residual machine oil from manufacturing. Threads cleaned to remove all lubrication may have dramatically higher friction coefficients unless a plating or film is acting as a lubricant.

Preload Relaxation — The Hidden Enemy

Even after correct preload is applied, tension decreases over time due to:

• Local yielding under bolt heads and nut faces (high spots, rough surfaces, non-square bearing surfaces)
• Thread deformation redistributing load
• Embedment of mating surfaces
• Vibration causing relative motion
• Temperature cycling changing material dimensions
• Creep at elevated temperatures

General rule: Allow for about 10% preload loss when designing a joint.

Best practice for resilience: Maintain a joint-length to bolt-diameter ratio of 4:1 or more (e.g., ¼-inch bolt with 1-inch or greater joint length). Use through bolts, far-side tapped holes, spacers, and washers to improve this ratio.

Preload Adjustments for Combined Loading

When preload is applied by turning the nut or bolt head, a torsion component adds to the axial bolt load. The combined tensile stress (von Mises stress) is:

$$F_{tc} = \sqrt{F_t^2 + 3F_s^2}$$

Where $F_t$ is axial tensile stress and $F_s$ is shear stress from torsion.

For single-start Unified inch screw threads:

$$F_{tc} = F_t \sqrt{1 + 3\left(\frac{1.96 + 2.31\mu}{1 - 0.325P/d_2} - 1.96\right)^2}$$

The Complete Torque-Tension Relationship

The total torque $T$ required to develop axial bolt load $P_B$:

$$T = P_B\left[\frac{l}{2\pi} + \frac{d_2 \mu_1}{2\cos\alpha} + \frac{(d + b)\mu_2}{4}\right]$$

For 60° threads (α = 30°) with no loose washer ($b ≈ 1.5d$, $d_2 ≈ 0.92d$):

$$T = P_B[0.159l + 0.531\mu_1 d + 0.625\mu_2 d]$$

If thread and bearing friction are equal ($\mu_1 = \mu_2 = \mu$):

$$T = P_B(0.159l + 1.156\mu d)$$

Metric Torque Coefficients

For metric hexagon head bolts, the torque coefficient $K$ varies with both thread friction $\mu_s$ and bearing surface friction $\mu_w$:

Values are averages for coarse-pitch metric threads (JIS B 0205/ISO 724)

Tensile Stress Area

For Unified threads:

$$A_s = \frac{\pi}{4}\left(\frac{d_m + d_p}{2}\right)^2$$

Where $d_m = d - 1.2990P$ and $d_p = d - 0.6495P$

For metric threads (JIS B 1082/ISO 898):

$$A_s = 0.7854(d - 0.9382P)^2$$

Grade Identification — Know What You Are Installing

Every bolt tells you what it is made of — if you know how to read the marks on its head. Grade markings are not decorative. They are the primary defense against installing a Grade 2 bolt where a Grade 8 is required.

SAE and ASTM Grade Marks for Steel Fasteners

Detecting Counterfeit Fasteners

Counterfeit fasteners — those marked with grade identifiers they do not meet — are a serious and documented safety hazard. They may break unexpectedly at loads far below their rated capacity.

The law now requires testing of fasteners used in some critical applications.

Detection is difficult because counterfeits look genuine. The only sure way to verify compliance is laboratory testing:

• Hardness testing
• Elongation testing
• Ultimate load testing
• Chemical analysis

Best practice: Purchase from reputable, certified distributors. For critical applications, independently test samples from each lot.

Mechanical Properties of Nuts

SAE J995 specifies three grades of hex and square nuts (Grades 2, 5, and 8) in the ¼ to 1½-inch diameter range. The nut grade must match or exceed the bolt grade to ensure the bolt — not the nut — is the weakest link.

Bolts, Screws, Nuts, and Washers — The Complete Catalogue

Square and Hex Bolts (ANSI/ASME B18.2.1-1996)

Designation format:

[Nominal Size]-[TPI] × [Length] [Product Name], [Material], [Finish]

Examples:

• 3/8-16 × 1½ Square Bolt, Steel, Zinc Plated
• 1/2-13 × 3 Hex Cap Screw, SAE Grade 8 Steel
• .75 × 5.00 Hex Lag Screw, Steel

Thread specification: When rolled, threads conform to 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.

Bolt Types and Their Applications

Hex Nuts (ANSI/ASME B18.2.2-1987, R1999)

Plain Washers (ANSI B18.22.1-1965, R2003)

Washers serve multiple critical functions: distributing load, protecting surfaces, spanning oversized holes, providing a consistent bearing surface, and preventing galvanic corrosion between dissimilar metals.

Type A (Regular):

• General-purpose washers with broad tolerances
• Suitable for most applications

Type B (Narrow, Regular, Wide):

• Tighter tolerances than Type A
• Available in three width series for different load-distribution requirements

Lock Washers

Helical Spring Lock Washers (ANSI/ASME B18.21.1-1994):

• Available in Regular, Heavy, Extra Duty, and Hi-Collar series
• Work by spring action and edge bite into the bearing surface
• Most effective at preventing loosening under vibration

Tooth Lock Washers:

• Available in Internal, External, and Combination types
• Teeth dig into bearing surface and fastener
• External types provide greater resistance to loosening
• Internal types have a cleaner appearance