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ASME B18.21.1-2009 — Washers: Helical Spring-Lock, Tooth Lock, and Plain Washers (Inch Series)
🔧 ASME B18.21.1-2009 is the core ASME B18 standard governing inch-series washers. It is a revision and consolidation of ASME B18.21.1 and ASME B18.22.1 into one document covering three major product families: helical spring-lock washers (#0 through 3 in.), tooth-lock washers (#2 through 1¾ in.), and plain washers (#0 through 3 in.). It states dimensional requirements, physical properties, and related test methods. All dimensions are in inches and apply to unplated or uncoated product.
💡 The Standard explicitly notes that the word “lock” in the product names is a generic historical term and does not imply indefinite permanency of fixity. Lock washers provide elastic tension and friction under compression — evaluate loosening resistance together with proper preload control.
1. Scope and Product Families 📌
The Standard covers three families of products:
- Helical spring-lock washers: sections designated regular, heavy, extra duty, and high-collar.
- Tooth-lock washers: internal tooth, external tooth, countersunk external tooth, and internal/external tooth, in two constructions designated Types A and B.
- Plain washers: three constructions designated Type A, Type B, and Fender washers.
The locking products have no comparable ISO standards, which makes B18.21.1 the primary acceptance reference for inch-series washers in U.S. procurement and export trade. Dimensional data apply to the unplated state, so users should account for coating thickness (e.g., zinc plating) when specifying.
⚠️ Washer manufacturers are not responsible for product malfunction caused by plating or other modifications when such plating or modification is not performed under the manufacturer’s control or direction. Specify “before coating” versus “after coating” acceptance in purchase orders.
2. Helical Spring-Lock Washers 🔩
Helical spring-lock washers are split, helically shaped elastic rings. When tightened flat, their spring-back tension applies a frictional torque against the nut/bolt head to resist loosening. The Standard tabulates regular, heavy, extra-duty, and high-collar sections (Tables 1–4), plus minimum bearing width verification and the washer twist test.
| Section | Typical Size Range | Section Features | Typical Use |
|---|---|---|---|
| Regular | #0 – 3 in. | Standard rectangular-section split ring | General machinery, appliances |
| Heavy | #0 – 3 in. | Thicker section, higher load capacity | Automotive, agriculture, vibration |
| Extra Duty | #0 – 3 in. | Thickest section, high stiffness | Heavy equipment, mining |
| High-Collar | #0 – 3 in. | One end with raised collar | Centering with specific bolt heads |
The Standard also states decarburization limits (Table 5): excessive surface decarburization lowers surface hardness and elasticity, degrading locking performance — verify section microstructure and hardness distribution during inspection.
3. Tooth-Lock Washers 🦷
Tooth-lock washers carry inclined teeth stamped on the inner edge, outer edge, or both. In assembly the tooth tips embed into the bearing surfaces and the work piece, producing a strong anti-rotation torque. Tables 6–10 give dimensions for internal tooth, external tooth, countersunk external tooth, and internal/external tooth types, in Type A and Type B constructions. Tooth-lock washers suit thin sheet, non-ferrous, and plastic parts where surfaces are relatively soft.
🚨 Tooth-lock washers are very sensitive to bearing-surface hardness: if the surface is as hard as or harder than the washer teeth, the teeth cannot embed and the locking effect largely disappears. On hardened steel surfaces use elastic preload solutions instead.
4. Plain Washers (Type A, Type B, Fender) 📏
Plain washers enlarge the bearing area, protect part surfaces, and absorb hole-position tolerance. The Standard gives the preferred sizes for Type A (Table 11), Type B dimensions (Table 12), and Fender washer dimensions (Table 13). Type A is the general-purpose series, Type B has a narrower outside diameter, and Fender washers have a greatly increased OD for large holes or soft surfaces.
| Construction | Features | Typical Use |
|---|---|---|
| Type A | General proportions, fullest size range | General machinery, construction, maintenance |
| Type B | Narrow OD series, matches standard bolt heads | Confined spaces, appearance-sensitive joints |
| Fender | Large thin OD, generous clearance hole | Body panels, oversized holes, soft materials |
5. Physical Properties and Test Methods 🧪
- Hardness: helical spring-lock and tooth-lock washers are heat treated (hardened and tempered) to specified hardness ranges to secure elasticity and resistance to permanent set.
- Washer twist test: the washer is loaded in a test fixture simulating service and its resistance to twisting/opening is measured — the key verification of lock-washer elasticity (Fig. 2).
- Minimum bearing width: verified per Fig. 1 to ensure sufficient contact with the nut or bolt head.
- Decarburization limits: controlled per Table 5 for surface hardness and wear resistance.
💡 All dimensional tolerances are stated on the unplated basis. For plated products (Zn, Cd, or Dacromet), choose sizes allowing coating allowance or agree on post-plating dimensional ranges with the supplier.
6. Engineering Practice Essentials 🛠️
- 🔍 Selection: for well-defined vibration directions, prefer tooth-lock washers; for joints dismantled repeatedly, use a plain washer plus thread-locking adhesive.
- 🔧 Installation: orient helical spring-lock washers with the split end toward the bolt head/nut side; do not re-use a flattened washer.
- 📊 Preload control: washers cannot replace torque control — apply specified preload with a calibrated torque wrench on critical joints.
- 📋 Procurement marking: specify size series and type per B18.21.1; optionally use the B18.24 Part Identifying Number (PIN) system to simplify ordering codes.
7. Frequently Asked Questions ❓
❓ Q: Do helical spring-lock washers really “lock”?
A: They add frictional torque through spring-back tension and can delay loosening under normal vibration, but they are not absolute locking devices. For heavy duty or high temperature, combine with nylon-insert lock nuts, thread-locking adhesive, or positive mechanical locking.
A: They add frictional torque through spring-back tension and can delay loosening under normal vibration, but they are not absolute locking devices. For heavy duty or high temperature, combine with nylon-insert lock nuts, thread-locking adhesive, or positive mechanical locking.
❓ Q: How does B18.21.1 relate to metric washer standards?
A: B18.21.1 is the inch series; metric washers follow ISO 7090/7092 (plain) and corresponding metric lock-washer standards. The two systems are not size-interchangeable — verify ID and OD before mixing.
A: B18.21.1 is the inch series; metric washers follow ISO 7090/7092 (plain) and corresponding metric lock-washer standards. The two systems are not size-interchangeable — verify ID and OD before mixing.
❓ Q: Can plating push a washer out of tolerance?
A: Yes. Dimensions apply to the unplated state; plating adds thickness. Use a pre-plating allowance or a looser tolerance grade, and fix the acceptance criterion in the contract.
A: Yes. Dimensions apply to the unplated state; plating adds thickness. Use a pre-plating allowance or a looser tolerance grade, and fix the acceptance criterion in the contract.
🔍 Q: Why are tooth-lock washers unsuitable on hardened surfaces?
A: Their locking depends on tooth tips embedding into the bearing surface. On very hard surfaces the teeth slip or break off. Use thread-locking adhesive or spring washers with preload control instead.
A: Their locking depends on tooth tips embedding into the bearing surface. On very hard surfaces the teeth slip or break off. Use thread-locking adhesive or spring washers with preload control instead.
