Mechanical Joint and Fastener Selection: SAE and ASTM Grade Identification Marks

SAE and ASTM Grade Identification Marks

Nut Grades (SAE J995)

Three grades of hex and square nuts are specified: Grades 2, 5, and 8, covering the 1/4- to 1-1/2-inch diameter range. Always match your nut grade to your bolt grade — a Grade 8 bolt mated with a Grade 2 nut creates a joint limited by the weakest component.

Detecting Counterfeit Fasteners

Fasteners that carry grade markings but do not meet the mechanical standards for that grade are counterfeit. They are typically made from incorrect material or improperly heat-treated. Counterfeit fasteners may break at loads far below what the marking implies.

Detection methods include:

• Hardness testing — quick field verification
• Elongation testing — confirms ductility
• Ultimate load testing — verifies tensile strength
• Chemical analysis — confirms material composition

Critical fact: The law now requires testing of fasteners used in some critical applications. The only certain way to verify a fastener meets its specification is to test it. Reputable distributors will assist in verifying authenticity.

Torque, Tension, and Preload: The Science That Holds Joints Together

This is where fastener engineering separates the amateurs from the professionals. Understanding the relationship between the torque you apply with a wrench and the tension (preload) that actually holds the joint together is the single most important skill in bolted joint design.

Why Preload Matters

When you tighten a bolt, you are stretching it. That stretch creates bolt tension — also called preload — which is the clamping force that holds the joint together.

High preload delivers four critical benefits:

• Keeps bolts tight under service loads
• Increases joint strength by maintaining compression between parts
• Creates friction between parts to resist shear loads
• Improves fatigue resistance by reducing cyclic load variation in the bolt

The Preload Problem: Torque ≠ Tension

Here is the problem every engineer faces: torque is easy to measure (just use a torque wrench), but torque does not accurately predict bolt tension because it does not account for friction.

Friction depends on:

• Bolt, nut, and washer material
• Surface smoothness
• Machining accuracy
• Degree of lubrication
• Number of times a bolt has been installed

Roughly 85-90% of the torque you apply goes to overcoming friction. Only 10-15% actually goes into stretching the bolt.

Calculating Recommended Preload

The recommended preload $F_i$ for standard applications:

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

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

Where:

• $F_i$ = bolt preload (force)
• $A_t$ = tensile stress area of the bolt
• $S_p$ = proof strength of the bolt material

For materials without published proof strength: $S_p \approx 0.85 \times S_y$, where $S_y$ is the yield strength.

Measuring Preload: From Best to Worst

Bolt Elongation Formula

The most reliable indirect method is measuring bolt elongation. The required change in length $\delta$ to achieve the recommended preload:

$$\delta = F_i \times \frac{A_d \times l_t + A_t \times l_d}{A_d \times A_t \times E}$$

Or the simplified version (when bolt area is approximately constant):

$$\delta = \frac{F_i \times l}{A \times E}$$

Where:

• $l_t$ = length of threaded portion within the grip
• $l_d$ = length of unthreaded portion within the grip
• $A_d$ = major-diameter area of the bolt
• $E$ = modulus of elasticity

The Torque-Tension Relationship

When direct measurement is not possible, estimate torque from preload:

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

Where:

• $T$ = wrench torque
• $K$ = torque coefficient (friction-dependent constant)
• $d$ = nominal bolt diameter

Standard K values for steel bolts (1/4 to 1 inch range):

Torque Approximation Formula

For a rough estimate of tightening torque using bolt diameter $d$ (inches) and tabulated coefficients:

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

Note: Values are for standard, unplated fasteners as received from manufacturer. For cadmium-plated cap screws, multiply torque by 0.9. For cadmium-plated nuts/bolts, multiply by 0.8. For lubricated fasteners, multiply by 0.9.

Coefficients of Friction

Friction between threads and bearing surfaces directly controls the torque-tension relationship. These values assume some residual machine oil lubrication ("dry" threads):

Warning: Values are NOT valid for threads cleaned to remove all traces of lubrication — friction may be drastically higher unless plating or film acts as lubricant.

Detailed Torque-Tension Analysis

For precision work, the total torque $T$ is the sum of three components:

$$T = T_1 + T_2 + T_3$$

Where:

• $T_1 = \frac{P_B \times l}{2\pi}$ — torque to develop axial load (lead contribution)
• $T_2 = \frac{d_2 \times \mu_1 \times P_B}{2 \cos\alpha}$ — torque to overcome thread friction
• $T_3 = \frac{(d + b)}{4} \times \mu_2 \times P_B$ — torque to overcome nut/head bearing friction

For 60° thread fasteners ($\alpha = 30°$, $d_2 \approx 0.92d$), with no loose washer ($b \approx 1.5d$):

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

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

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

Worked Example:

Estimate the torque to tighten a UNC 1/2-13 Grade 8 bolt to 55% of minimum tensile strength. Assume unplated, µ = 0.15.

Step 1 — Find stress area:

$$A_s = \frac{\pi}{4}\left(\frac{0.4500 + 0.4001}{2}\right)^2 = 0.1419 \text{ in}^2$$

Step 2 — Calculate preload:

$$P_B = 0.55 \times 150{,}000 \times 0.1419 = 11{,}707 \text{ lbf}$$

Step 3 — Calculate torque:

$$T = 11{,}707 \times \left(\frac{0.159}{13} + 1.156 \times 0.15 \times 0.500\right) = 1{,}158 \text{ lb-in} = 96.5 \text{ lb-ft}$$

Preload Relaxation: Why Joints Loosen Over Time

Even a perfectly tightened bolt will lose preload over time. Causes include:

• Local yielding under nut/bolt head bearing surfaces due to rough finish or high spots
• Thread deformation as load redistributes from uneven initial seating
• Vibration that causes gradual loosening
• Temperature cycling — ambient changes and thermal expansion mismatch
• Creep — especially at elevated temperatures

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

Design recommendation: A joint-length to bolt-diameter ratio of 4:1 or greater improves resilience and reduces preload loss. Use through-bolts, spacers, and washers to achieve this ratio when possible.

Preload for Shear-Loaded Joints

In joints where members slide, preload must be sufficient to hold joint members in contact. In joints that do not slide, shear loads are transmitted by friction resulting from preload. Therefore:

$$\text{Friction force from preload} > \text{Applied shear force}$$

For joints with combined axial and shear loads, the analysis must verify the bolt will not fail in either tension or shear independently.

Metric Fasteners: The Global Standard

American National Standards for metric bolts, screws, nuts, and washers have been coordinated with ISO Standards. The dimensional differences are few, relatively minor, and none affect functional interchangeability.

Metric Fastener Identification

Metric fasteners are identified by property class numbers rather than SAE/ASTM grade marks. The marking system works as follows:

• First number × 100 = minimum tensile strength in MPa
• First number × second number × 10 = minimum yield strength in MPa

Example: A class 8.8 bolt has:

• Minimum tensile strength: 8 × 100 = 800 MPa
• Minimum yield strength: 8 × 8 × 10 = 640 MPa

Metric Fastener Designation

Metric fasteners are designated by: nominal size (M prefix + diameter); thread pitch; nominal length; product name; property class; material; and protective finish.

Example: M10 × 1.5 × 30 Hex Cap Screw, Class 8.8 Steel, Zinc Plated

Key Metric Standards

Metric Torque Coefficient Table (Coarse Thread)

The torque coefficient $K$ for metric hex head bolt-and-nut combinations varies with thread and bearing surface friction:

Worked Example — Metric Bolt to Yield:

Find the torque to tighten an M10 × 1.5 (coarse) Grade 8.8 bolt to yield, with µs = µw = 0.12.

Step 1: $\sigma_y = 640$ MPa (minimum for 8.8)

Step 2: $A_s = 0.7854(10 - 0.9382 \times 1.5)^2 = 57.99$ mm²

Step 3: From the table, $K = 0.164$

Step 4: Calculate yield clamping force $F_{fy}$ considering combined loading, then:

$$T_{fy} = K \times F_{fy} \times d = 0.164 \times 38{,}075 \times 10 = 62.4 \text{ N·m}$$

Washers: The Unsung Heroes of Joint Integrity

Washers are not optional accessories. They serve critical functions in bolted joints: distributing load, providing uniform bearing surfaces, preventing surface marring, and — in the case of lock washers — resisting loosening.

Plain Washers

ANSI/ASME B18.22.1 covers two types of plain washers:

Plain washers are available in narrow, regular, and wide series. The series determines the ratio of outside diameter to bolt size. Inside and outside diameters must be concentric within the inside diameter tolerance, and washers must be flat within 0.005 inch for OD through 0.875 inch and 0.010 inch for larger ODs.

Metric Plain Washers (ANSI B18.22M)

Available in three series: Narrow, Regular, and Wide, in nominal sizes from 1.6 mm through 36 mm.

Helical Spring Lock Washers

ANSI/ASME B18.21.1-1994 covers helical spring lock washers in four series:

Helical spring lock washers provide:

• Good bolt tension per unit of applied torque
• Hardened bearing surfaces for uniform torque control
• Uniform load distribution through controlled radii
• Protection against looseness from vibration and corrosion

Available materials: Carbon steel, boron steel, corrosion-resistant steel (Types 302/305), aluminum-zinc alloy, phosphor-bronze, silicon-bronze, and K-Monel.

Tooth Lock Washers

Tooth lock washers serve to lock fasteners to assembly components or increase friction. Available in three configurations:

Each type is available in Type A (narrow) and Type B (wide) constructions.

Riveted Joints: Permanent Fastening for Structural Integrity

While bolted joints dominate modern construction, riveted joints remain essential in specific applications — particularly where vibration, thermal cycling, or permanent assembly requirements exist.

Classes of Riveted Joints

1. Pressure vessel joints (governed by ASME Boiler Code)
2. Structural joints (buildings, bridges)
3. Machine member joints (equipment assemblies)

Types of Riveted Joints

There are two fundamental types:

Lap Joints: Plates overlap each other, held by one or more rows of rivets.

Butt Joints: Plates are in the same plane, joined by a cover plate (butt strap) riveted to both plates.

Riveting terminology:

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

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 rivet hole to plate edge
6. Crushing
7. Tearing between adjacent rivets (tensile failure)

Design rule: Place rivet centers at minimum 1.5× rivet diameter from the plate edge to prevent Types 4 and 5 failures. Maintain transverse pitch of at least 1.75× rivet diameter to prevent diagonal tearing.

Riveted Joint Analysis: Worked Example

Single-Riveted Lap-Joint: 12-inch section, 1/4-inch plate, six 5/8-inch rivets. Holes 1/16 inch larger than rivets. Design stresses: 8,500 psi shear, 20,000 psi bearing, 10,000 psi tension.

A) Shear of rivets (single shear):

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

B) Bearing stress:

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

C) Tensile stress:

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

Safe load = minimum of A, B, C = 15,647 lb (governed by rivet shear)

Joint efficiency:

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

Riveted Joint Strength Formulas

Single-Riveted Lap-Joint:

Double-Riveted Lap-Joint:

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

Nails, Spikes, and Wood Screws: The Oldest Fasteners Still in Use

Standard Wire Nails and Spikes

The penny system (abbreviated "d") remains the standard sizing convention for nails:

Wood Screws (ANSI B18.6.1)

Wood screws are available with flat, pan, and oval heads, in both slotted and cross-recessed (Phillips) configurations. The thread length is approximately two-thirds of the nominal screw length.

Pilot hole sizes for wood screws:

Cap Screws and Set Screws: Precision Fastening

Cap Screws (ANSI/ASME B18.6.2)

Cap screws are precision fasteners designed for direct installation into tapped holes. They are available in several head styles:

Thread length: Full-form thread length equals 2× basic diameter + 0.250 inch (with +0.188 inch tolerance or 2-1/2 × pitch, whichever is greater).

Socket Head Cap Screws

These are the workhorse of precision machine assembly. Available in metric and inch sizes per ANSI/ASME B18.3-1998 and British Standard BS 4168:1981.

Key metric property classes: Class 12.9 (alloy steel) is the standard for socket head cap screws, providing minimum tensile strength of 1,220 MPa.

Set Screws: Holding Power

Set screws transmit torque between a shaft and a hub by pressing against the shaft surface. The power capacity of a set screw:

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

Or as torque:

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

Where:

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

Example: How many 1/2-inch set screws 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 Makes Its Own Thread

Self-threading screws (commonly called "self-tapping screws") eliminate the need for pre-tapped holes, reducing assembly time and cost. They are covered by ANSI B18.6.4-1981 and ANSI/ASME B18.6.5M-1986 (metric).

Thread Forming vs. Thread Cutting

There are two fundamental categories:

Thread Forming Screws — displace material to create threads:

• Generate internal stresses (desirable for locking)
• No chips produced
• Best in ductile materials

Thread Cutting Screws — remove material to create threads:

• Minimal internal stress
• Produce chips
• Best where high driving torques are a concern

Complete Type Guide

Head Types for Self-Tapping Screws

Self-Tapping Thread Inserts and Screw Thread Inserts

Self-tapping inserts are hard bushings with internal and external threads. The external thread has cutting edges for self-tapping installation. Available in case-hardened carbon steel, stainless steel, and brass. Used in magnesium, aluminum, cast iron, zinc, plastics, and wood.

Screw thread inserts (e.g., Heli-Coil) are helically formed coils of diamond-shaped stainless steel or phosphor bronze wire. They provide a convenient means of:

• Repairing stripped threads
• Providing stronger threads in soft materials than direct tapping
• Available from 4–40 to 1-1/2–6 (coarse) and 6–40 to 1-1/2–12 (fine)
• Support thread classes 2, 2B, 3, and 3B

Dowel Pins: Precision Alignment

Dowel pins serve two critical functions: retaining parts in a fixed position and preserving alignment. Under normal conditions, a properly fitted dowel pin is subjected only to shearing strain at the junction between mating surfaces.

Selection rules:

• One or two dowel pins are normally sufficient
• For locating nests and gage plates: 1/8 to 3/16 inch diameter
• For locating dies: never less than 1/4 inch — use the same size as the fastening screws
• Pin length should be 1.5 to 2× diameter in each part

Three types are available:

Installation fits:

Safety note: Dowel pins should NOT be installed by striking or hammering. Use a press with a shield, and wear safety glasses.

Taper Pins

Taper pins (ANSI/ASME B18.8.2-1995) have a standard taper of 1/4 inch per foot (1:48). They are numbered from 7/0 (smallest, 0.0625 inch basic diameter) through 14 (largest, 1.5210 inch basic diameter).

Available in Commercial Class and Precision Class tolerances.

Grooved Pins

Grooved pins have three equally spaced longitudinal grooves and an expanded diameter over the groove ridges. Seven types exist (Types A through G), each with different combinations of crowned/chamfered ends and groove configurations.

Materials: Normally low carbon steel; also available in alloy steel, corrosion-resistant steel, brass, and Monel.

Hole sizing: For optimum retention, hold holes as close as possible to the basic pin diameter. Maximum hole limits are suitable for length-to-diameter ratios of 4:1 to 10:1.

Spring Pins

Available in two types:

Spring pins are heat-treated or cold-worked to achieve required hardness and performance characteristics.

Cotter Pins and Clevis Pins

Cotter pins are split pins used to retain other fasteners (typically slotted or castle nuts on clevis pins and similar applications).

Clevis pins are headed pins with a transverse hole for a cotter pin, used in clevis-type connections where the pin is loaded primarily in shear.

Retaining Rings: The Artificial Shoulder

Retaining rings act as artificial shoulders to retain objects in housings (internal rings) or on shafts (external rings). They are critical in bearing retention, gear positioning, and component location.

Two Fundamental Types

Key advantage of spiral-wound rings: They provide a continuous, gapless shoulder — no gap like stamped rings.

Retaining Ring Failure Modes

Failure can occur in the ring itself, the groove, or both.

Ring Shear Failure: Occurs when:

• Ring is loaded by a retained part with compressive yield > 45,000 psi
• Sharp corners create line-to-line contact
• Ring is too thin relative to diameter

The allowable thrust based on shear strength:

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

Where $D$ = shaft/housing diameter, $t$ = ring thickness, $S_s$ = shear strength, $K$ = safety factor.

Groove Failure: The most common failure mode — yielding of groove material under thrust load. The ring tilts and exits the groove.

For spiral-wound rings, the thrust load initiating groove deformation:

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

Where $d$ = groove depth and $S_y$ = yield strength of groove material.

Rotation Considerations

Stamped rings: No rotation limitations — can be used regardless of rotation direction.

Spiral-wound rings:

• External rings: Wind in direction of shaft rotation
• Internal rings: Wind against direction of rotating part
• Failure to observe these rules causes the ring to unwind from the groove

T-Slots, T-Bolts, and T-Nuts: Machine Tool Clamping

T-slots are machined channels that accept T-bolts and T-nuts for clamping workpieces to machine tables. Dimensions are standardized per ANSI/ASME specifications.

Key Dimensions

The T-slot width determines the compatible bolt and nut size. Standard T-slot widths range from 1/4 inch (6 mm) to 1-1/2 inch (36 mm).

T-bolts have a head shaped to fit the T-slot channel, with the threaded shank extending upward through the workpiece or clamp.

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

Wing nuts are designed for manual turning without tools. Four types exist: