Washers: Selection, Standards and Joint Function: ANSI Metric Plain Washers

ANSI Metric Plain Washers — The Foundation Standard

Understanding ANSI B18.22M-1981 (R1990)

The American National Standard ANSI B18.22M-1981 (R1990) is the foundational specification for metric plain washers in North American engineering. It covers flat, round-hole washers in both soft (as fabricated) and hardened configurations, intended for general-purpose applications.

Critical Note: Nominal washer sizes are intended for use with comparable screw and bolt sizes. Always verify the actual inside diameter clearance against your specific fastener.

The Three Series: Narrow, Regular, and Wide

Every nominal washer size in the ANSI standard is available in three series, each designed for a different load-distribution requirement:

The choice of series directly affects how much bearing area you provide. A wide series M16 washer has an outside diameter of 56 mm compared to just 32 mm for the narrow series — nearly three times the bearing area. That difference can mean the difference between a joint that holds and one that crushes its way into the base material.

Complete ANSI Metric Plain Washer Dimensions

Table: ANSI B18.22M-1981 (R1990) — American National Standard Metric Plain Washers

All dimensions in millimeters.

† Note: The 18.80/18.37 mm and 25.40/24.48 mm outside diameters are specifically chosen to avoid washers that could be used in coin-operated devices — an often-overlooked detail that has real anti-counterfeiting implications.

Comparison with ISO Standards

Where ANSI and ISO Align — And Where They Don't

Here is where the practitioner's story takes its second twist. Six months after the hydraulic press incident, her company won a contract with a European equipment manufacturer. The drawings called for ISO metric washers per ISO 887. the practitioner assumed her ANSI B18.22M washers would be interchangeable.

They weren't. Not exactly.

The washers covered by ANSI B18.22M are nominally similar to those covered in various ISO documents. Outside diameters were selected, where possible, from the ISO/TC2/WG6/N47 document — the "General Plan for Plain Washers for Metric Bolts, Screws, and Nuts."

However, there are critical differences:

The practical impact: You cannot assume a washer manufactured to ANSI tolerances will meet an ISO specification, or vice versa. The nominal dimensions may match, but the tolerance bands do not overlap in all cases. When working on international projects, always verify which standard governs and procure accordingly.

Key Standard References:

• ANSI B18.22M-1981 (R1990) — American National Standard Metric Plain Washers
• ISO 887 — Plain Washers for Metric Bolts, Screws, and Nuts – General Plan
• ISO/TC2/WG6/N47 — General Plan for Plain Washers (the source document for OD selection)

Types of Metric Plain Washers

Soft (As Fabricated) Washers

Soft washers are the workhorses of general assembly. They are generally available in nominal sizes 1.6 mm through 36 mm in a variety of materials.

Primary functions of soft washers:

• Distribute bearing load in low-strength applications
• Provide a uniform bearing surface for consistent torque-tension behavior
• Prevent marring of the work surface during assembly and tightening

When to use soft washers: Any time the base material is strong enough to resist embedment at the applied bolt preload. This typically means mild steel flanges at moderate bolt tensions, aluminum assemblies with controlled torque, and any application where surface protection is the primary concern.

Hardened Steel Washers

Hardened washers represent the critical difference that the practitioner's procurement team missed. They are normally available in sizes 6 mm through 36 mm in the narrow and regular series.

Primary functions of hardened washers:

• Minimize embedment in high-strength joints — this is their most critical function
• Provide a uniform bearing surface with significantly higher contact stress capacity
• Bridge large clearance holes and slots without deforming

When to use hardened washers: Any time you are using high-strength bolts (property class 8.8 and above), any time the bolt preload is significant relative to the base material hardness, and any time the bolt hole is oversized or slotted.

The Embedment Problem — Why Hardness Matters

Embedment is the single most common failure mode that washers are designed to prevent. Here is the engineering reality:

When you tighten a high-strength bolt, the preload creates enormous bearing stress under the bolt head and nut. The bearing stress is calculated as:

$$\sigma_{\text{bearing}} = \frac{F_{\text{preload}}}{A_{\text{bearing}}}$$

Where:

• $F_{\text{preload}}$ = bolt preload force (N)
• $A_{\text{bearing}}$ = annular bearing area under the washer (mm²)

The bearing area of the washer is:

$$A_{\text{bearing}} = \frac{\pi}{4} \left( D_{\text{OD}}^2 - D_{\text{ID}}^2 \right)$$

If the bearing stress exceeds the yield strength of the washer material, the washer deforms plastically — it gets thinner, it spreads outward, and the bolt loses preload. With a soft washer under a high-strength bolt, this can happen during the initial tightening and continue to creep for hours or days afterward.

Hardened washers solve this by providing a bearing surface with a yield strength (38–45 HRC corresponds to roughly 1,200–1,500 MPa tensile strength) that far exceeds the bearing stress, even under the highest bolt preloads.

Materials and Finish

Soft Washer Materials

Soft (as fabricated) washers are made of nonhardened steel unless otherwise specified by the purchaser. This provides adequate strength for low- to moderate-load applications while keeping costs minimal and manufacturing straightforward.

For specialized applications, the purchaser can specify alternative materials including:

• Stainless steel — for corrosion resistance in marine, chemical, or food-processing environments
• Brass or bronze — for electrical conductivity or non-sparking applications
• Aluminum — for weight-critical aerospace or automotive applications
• Nylon or other polymers — for electrical isolation, vibration damping, or chemical resistance

Hardened Washer Materials

Hardened washers are made of through-hardened steel tempered to a hardness of 38 to 45 Rockwell C (HRC).

This hardness range is critical:

Engineering Note: The "through-hardened" specification means the hardness is uniform throughout the washer cross-section, not just a surface treatment. This prevents the softer core from deforming under sustained loading.

Standard Finish

Unless otherwise specified, washers are furnished with a natural (as fabricated) finish — unplated or uncoated, with a light film of oil or rust inhibitor.

For applications requiring corrosion protection, common finish specifications include:

Warning on hydrogen embrittlement: Electroplated zinc coatings on hardened steel washers (above 32 HRC) can introduce hydrogen into the steel during the plating process, leading to delayed brittle fracture. For hardened washers requiring corrosion protection, specify mechanical zinc plating, zinc flake coatings, or hot-dip galvanizing instead of electroplating, or require hydrogen embrittlement relief baking per ASTM B850 within 4 hours of plating.

Metric Plain Washer Designation

How to Specify a Washer Correctly

When specifying metric plain washers on drawings, purchase orders, or bills of materials, the designation should include the following data in the sequence shown:

1. Description — Plain washer
2. Nominal size — in millimeters
3. Series — Narrow, Regular, or Wide
4. Material type — Soft or Hardened steel
5. Finish — if required

Designation Examples

Plain washer, 6 mm, narrow, soft, steel, zinc plated

Plain washer, 10 mm, regular, hardened steel

Plain washer, 24 mm, wide, soft, stainless steel

Why this sequence matters: The designation order follows a hierarchy from what it is → what size → what shape → what material → what surface. Following this sequence prevents ambiguity in procurement and ensures every parameter is addressed.

Quick-Reference Designation Builder

British Standard Metric Washers — BS 4320:1968 (1998)

the practitioner's European Education

When the practitioner began working with the European client, she discovered an entirely different washer classification system governed in the supplied reference BS 4320:1968 (1998). This standard specifies bright and black metal washers for general engineering purposes — and the form designations (A through G) bear no resemblance to the ANSI narrow/regular/wide system.

Understanding both systems is essential for anyone working in international manufacturing, multinational maintenance operations, or global supply chains.

BS 4320 Bright Metal Washers

Bright metal washers per BS 4320 are made from either:

• CS4 cold-rolled strip steel to BS 1449: Part 3B, in the hard condition
• CZ 108 brass strip to BS 2870:1980, in the hard condition

By mutual agreement between purchaser and supplier, washers may also be made from other materials, in other conditions, or with protective or decorative finishes to appropriate British Standards.

Physical characteristics:

• Washers are reasonably flat and free from burrs
• Normally supplied unchamfered
• May optionally have a 30-degree chamfer on one edge of the external diameter

Bright washer classification:

Key relationship: The thickness of a light-range washer (Form B or D) is from ½ to ⅔ the thickness of a corresponding normal-range washer (Form A or C).

BS 4320 Bright Metal Washers — Normal Diameter Sizes (Forms A & B)

All dimensions in millimeters. Sizes in parentheses are non-preferred.

BS 4320 Bright Metal Washers — Large Diameter Sizes (Forms C & D)

All dimensions in millimeters.

BS 4320 Black Metal Washers

Understanding Black Washers

Black metal washers per BS 4320 are made from mild steel and are intended for general engineering applications where the surface finish quality of bright washers is not required. They are available in three size categories:

A protective finish can be specified by the purchaser in accordance with any appropriate British Standard.

BS 4320 Black Metal Washers — Normal Diameter (Form E)

All dimensions in millimeters. Sizes in parentheses are non-preferred.

BS 4320 Black Metal Washers — Large Diameter (Form F)

All dimensions in millimeters.