Washers: Selection, Standards and Joint Function: Type A Plain Washers

Context and scope

A conveyor system at a grain processing facility rattled itself apart in seventy-two hours.

Not because the bolts were wrong. Not because the steel was underspec. The maintenance crew had replaced every fastener on the bearing housings with Grade 8 hex bolts torqued to factory specifications. They'd even used thread-locking compound.

But they'd skipped the washers.

Within three days, vibration-induced loosening had backed out seventeen bolts. The conveyor chain jumped its sprockets, shearing two drive pins and sending forty meters of product onto the floor. The total cost — replacement parts, emergency labor, lost production, contaminated inventory — exceeded what those missing washers would have cost by a factor of three thousand.

The engineer who investigated the failure found the root cause in under sixty seconds. Bare bolt heads seated directly against soft mounting brackets. No load distribution. No vibration resistance. No bearing surface protection. Every bolt had been slowly grinding its way through the bracket material, losing clamp load with every operating cycle.

His name was the practitioner Elliot. And in his twenty-three years of structural fastener work, he'd seen this exact failure mode more times than he cared to count.

"A washer is the cheapest insurance policy in mechanical engineering," the practitioner told the facility's operations manager. "And it's the first thing people leave out when they think they know what they're doing."

This is the story of how the practitioner rebuilt that facility's entire fastener specification program — and the comprehensive washer reference guide he created in the process. It covers every inch-series washer type governed in the supplied reference, from the humblest Type A plain washer to the most aggressive tooth lock washer configuration.

If you've ever wondered which washer to use, which series to specify, or why one type matters more than another in a given application — this guide was written for you.

Table of Contents

• The Status Quo: Why Most People Get Washers Wrong
• Type A Plain Washers — Preferred Sizes (ANSI/ASME B18.22.1)
• Type A Plain Washers — Additional Selected Sizes
• ANSI Standard Plain Washers — The Evolution from Type A to Type B
• Type B Plain Washers — Narrow, Regular, and Wide Series
• Helical Spring Lock Washers — The Vibration Fighter
• Hi-Collar Helical Spring Lock Washers
• Tooth Lock Washers — Internal, External, and Combination
• Internal-External Tooth Lock Washers
• The Washer Selection Decision Framework
• the practitioner's Transformation: From Failure to Fastener Authority
• Your Next Step

Current-state problem

the practitioner had seen it a hundred times. An assembler reaches into the parts bin, grabs a flat washer that "looks about right," drops it onto the bolt, and torques the nut down. No thought given to whether it's a Type A or Type B. No consideration of narrow versus wide series. No understanding of whether the application demands a plain washer, a spring lock washer, or a tooth lock washer.

The consequences of this casual approach range from annoying to catastrophic:

• Bolt heads and nuts embed into soft materials — reducing clamp force and creating stress concentrations that initiate fatigue cracks
• Oversized washers bridge structural gaps they weren't designed to span — transferring loads to edges rather than distributing them uniformly
• Undersized washers fail to cover clearance holes — allowing the bolt head to pull through under load
• Missing lock washers allow vibration-induced loosening — the single most common cause of non-fatigue fastener failure in dynamic assemblies
• Wrong series selection creates tolerance stack-up problems — particularly in precision equipment where every thousandth of an inch matters

The American National Standards Institute recognized these problems decades ago. The result was a systematic classification of inch-series washers into distinct types, each engineered for specific load conditions, material combinations, and assembly requirements.

Here is the complete technical reference for every inch-series washer type you will encounter in professional practice.

Type A Plain Washers — Preferred Sizes

Standard: ANSI/ASME B18.22.1-1965 (R1998)

All dimensions in inches.

When the practitioner rebuilt the grain facility's fastener program, he started here — with Type A plain washers. These are the workhorses. The most commonly specified flat washers in general-purpose mechanical assembly.

What Makes Type A Washers Distinct

Type A plain washers trace their lineage to two earlier series: "Standard Plate" and "SAE." Where common sizes existed in both series, the ANSI standard preserved both — designating the SAE size as "N" (narrow) and the Standard Plate size as "W" (wide).

Critical ordering note: All Type A plain washers — both preferred and additional sizes — are specified and ordered by their inside diameter (ID), outside diameter (OD), and thickness dimensions. There is no "series" designation for Type A. You call out the exact dimensions you need.

Preferred Sizes Reference Table

These preferred sizes represent the most commonly stocked and specified dimensions. If your application can use one of these sizes, availability and lead time will always be better than non-preferred alternatives.

* Note: The 0.734-inch, 1.156-inch, and 1.469-inch outside diameters were specifically selected to avoid creating washers that could be used in coin-operated devices. This is a deliberate anti-counterfeiting design feature embedded in the standard.

Key Engineering Insights for Type A Preferred Sizes

Tolerance pattern: Notice how the ID tolerance shifts from +0.000/−0.005 on the smallest sizes to +0.008/−0.005 at No. 6, and then to +0.015/−0.005 at 3/16 inch and above. This reflects manufacturing reality — larger washers require proportionally larger clearances for reliable bolt passage.

Thickness control: The thickness tolerance is expressed as a max/min range rather than a bilateral tolerance. For the 0.049 basic thickness common across the No. 6 through 1/4 inch range, the actual part can be anywhere from 0.036 to 0.065 inches — a total spread of 0.029 inches. In precision stack-up calculations, always use the worst-case thickness values.

Flatness requirement: Inside and outside diameters shall be concentric within at least the inside diameter tolerance. Washers shall be flat within 0.005 inch for basic outside diameters up through 0.875 inch, and flat within 0.010 inch for larger outside diameters.

Type A Plain Washers — Additional Selected Sizes

Standard: ANSI/ASME B18.22.1-1965 (R1998)

All dimensions in inches.

the practitioner learned early in his career that the preferred sizes don't cover every application. When the grain facility needed washers for non-standard bracket geometries and imported equipment with odd bolt patterns, he turned to the additional selected sizes table.

These sizes are ordered by ID, OD, and thickness dimensions only — there is no series designation.

* Note: The 0.734-inch and 1.469-inch outside diameters avoid washers usable in coin-operated devices.

The Pattern the practitioner Noticed

"Look at the OD-to-ID ratios," the practitioner told his junior engineers during the facility rebuild. "The additional sizes give you two options at many ID values — a smaller OD for tight spaces and a larger OD for greater load distribution. The 0.438 ID, for example, comes with either a 0.875 or 1.375 OD. That's not random — it's intentional design flexibility."

For the larger bolts (0.938 ID and above), the OD tolerance widens to +0.045/−0.010, reflecting the realities of stamping thicker, larger-diameter washers.

ANSI Standard Plain Washers — The Evolution from Type A to Type B

Before diving into the Type B tables, you need to understand why Type B exists at all.

The original Type A plain washers were developed in four series: light, medium, heavy, and extra heavy. Over time, these series designations were discontinued. The washers previously classified under those series are now simply designated by their nominal dimensions — which is exactly what you see in the Type A tables above.

Type B plain washers represent a fundamentally different design philosophy. Rather than offering multiple thickness options at each size, Type B washers are organized into three width series — Narrow (N), Regular (R), and Wide (W) — with proportions specifically engineered to distribute load over larger areas of lower-strength materials.

This is the critical distinction the practitioner emphasized to every technician at the grain facility:

Type A = general purpose, ordered by exact dimensions Type B = engineered load distribution, ordered by nominal size and series

Materials and Tolerances

Plain washers (both Type A and Type B) are made of ferrous or non-ferrous metal, plastic, or other material as specified. The tolerances indicated in the standard tables are intended for metal washers only. If you're specifying plastic or composite washers, consult the manufacturer for applicable tolerances.

Type B Plain Washers — Narrow, Regular, and Wide Series

Standard: ANSI/ASME B18.22.1-1965 (R1998)

All dimensions in inches.

This was the table the practitioner kept laminated and mounted inside every maintenance tool crib at the facility. "If you can only memorize one washer table," he said, "make it this one."

Series Designation Key

• N = Narrow Series — minimum footprint, for tight spaces and hard materials
• R = Regular Series — standard load distribution for general applications
• W = Wide Series — maximum bearing area for soft materials and oversized holes

Complete Type B Plain Washer Dimensions

Screw Sizes No. 0 through No. 6

Fractional Sizes: 1/4 inch through 1-1/8 inch

Fractional Sizes: 1-1/4 inch through 2 inch

* Note: The 0.734-inch and 1.469-inch outside diameters avoid washers that could be used in coin-operated devices.

Bearing Area Comparison: Why Series Selection Matters

the practitioner created this quick-reference comparison to show his technicians the real-world impact of series selection. Consider a 1/2-inch bolt (Nominal Size 0.500):

The Wide series washer provides more than four times the bearing area of the Narrow series for the same bolt size. When you're fastening into aluminum, softwood, fiberglass, or any material with a compressive yield strength below 20,000 psi, that difference is the margin between a reliable joint and a crushed bearing surface.

Designation and Ordering Convention

Type B plain washers are designated by the following data in sequence:

1. Product name — "Type B Plain Washer"
2. Nominal size — number, fraction, or decimal equivalent
3. Series — Narrow, Regular, or Wide
4. Material — steel, stainless, brass, etc.
5. Protective finish — if required

Example: Type B Plain Washer, 1/2, Regular, Steel, Zinc Plated

Helical Spring Lock Washers — The Vibration Fighter

Standard: ANSI/ASME B18.21.1-1994

All dimensions in inches.

The grain facility's original failure was a textbook case for helical spring lock washers. When the practitioner inspected the failed bearing housings, the evidence was unmistakable: smooth, polished bolt-seat surfaces with no scoring, no tooth marks, no mechanical interference of any kind to resist rotation.

"Vibration doesn't need to be dramatic to kill a bolted joint," the practitioner explained to the plant manager. "A conveyor running at 1,200 RPM creates thousands of micro-movements per minute. Each one relaxes the bolt tension by a microscopic amount. Without something to resist that back-off, you get progressive loosening until the joint fails."