ASME B27.2-1965 — Plain Washers

⚙️ ASME B27.2-1965 (Plain Washers, American Standard ASA B27.2-1965) is the classic specification for general-purpose plain washers, developed by the B27 sectional committee (Standardization of Washers and Machine Rings) jointly sponsored by the Society of Automotive Engineers and ASME. The Standard divides general-purpose plain washers into Type A and Type B: Type A descends from the original light, medium, heavy, and extra heavy series, while Type B is offered in narrow, regular, and wide width series designed to distribute load over larger areas of lower-strength materials. All dimensions are given in inches unless otherwise stated, and washers may be made of ferrous or non-ferrous metal, plastic, or other material as specified.

1. Scope and Washer Types 📌

This Standard covers two types of general-purpose plain washers:

• Type A: originally developed in light, medium, heavy, and extra heavy series with proportions related to the dimensions of bolt heads and nuts, principally to minimize embedding and aid torquing. The Standard intends these series designations to be discontinued, with washers identified by their nominal dimensions; selection should preferably use the preferred sizes of Table 1A, and Table 1B may be referenced when the desired Type A washer is not listed there.
• Type B: available in narrow, regular, and wide series with proportions designed to distribute load over larger areas of lower-strength materials; select by referring in Table 2 to the nominal screw or bolt size and choosing the appropriate width series.

The Standard explicitly notes that listing dimensional data does not imply all products are production items; consumers should consult manufacturers for lists of stock production sizes.

2. Dimensions, Tolerances, and Designation 🔧

The Standard indexes washers by nominal screw or bolt size and gives basic dimensions and tolerances for inside diameter, outside diameter, and thickness; tolerances are set to good commercial stamping practice for metal washers. The inside diameter must clear the bolt shank for easy assembly, while the outside diameter and thickness determine bearing area and load-distribution capability.

Type A “series” designations are to be phased out in favor of nominal-dimension identification, simplifying marking and procurement: for example, instead of “3/8-inch medium washer,” specify the inside diameter, outside diameter, and thickness from Table 1A directly. Type B washers are always selected by “nominal size plus width series,” e.g., a 1/2-inch “wide” series washer.

3. Materials and Hardened Washers 📏

Plain washers may be made of ferrous or non-ferrous metal, plastic, or other material as specified; the table tolerances apply to metal washers only. When hardened washers are required, two product forms are available:

• Stamped hardened washers: one-piece rings without break or gap;
• Flat coiled hardened washers: wound from wire with a very narrow gap, narrow enough to prevent tangling.

Suitable mechanical properties for hardened stamped plain washers are covered by ASTM specification A-325 (high-strength bolting); wire-coiled washers follow the applicable wire material specification. Hardened washers are used in high-strength structural bolting and anywhere embedding under heavy preload must be resisted.

4. Engineering Practice Essentials 🛠️

• 🔍 Function distinction: a plain washer carries bearing load and protects surfaces; it is not a locking device. For loosening resistance use lock washers, lock nuts (e.g., nylon-insert), or thread-locking compounds.
• 🔧 Low-strength materials: when bolting through aluminum, plastic, wood, or thin sheet, use the wide series to spread the load and avoid local crush; verify washer bearing area against the material’s allowable bearing stress.
• 📊 Torque control: the washer reduces friction scatter, stabilizing the torque-to-preload relationship; still account for finish and lubrication, and verify preload by bolt elongation or ultrasonic measurement where critical.
• 📋 Corrosion and galvanic couples: match stainless hardware with stainless washers; isolate aluminum structures from carbon-steel fasteners with a separating washer to prevent galvanic corrosion; specify zinc or other protective finishes outdoors.

5. Frequently Asked Questions ❓