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ASME B18.21.3-2008 (2013) — Double Coil Helical Spring Lock Washers for Wood Structures
🌲 ASME B18.21.3-2008 is the ASME B18 washer-family standard dedicated to double coil helical spring lock washers for wood structures. It covers the dimensional and physical properties and the methods of testing for these washers used with bolts and screws in timber connections. The Standard was prepared by Subcommittee 21 (Lock Washers) of the ASME B18 Committee, approved by ANSI on March 28, 2008, and reaffirmed in 2013. Interestingly, the proposal was originally driven by connections on high-voltage wood structures (power poles); the Committee later removed the words “high voltage” so the standard could serve any wooden-structure application.
💡 Wood swells and shrinks with moisture content, and common flat washers often lose preload as timber compresses and dries. The double coil spring washer provides greater elastic travel and a larger bearing area, sustaining clamping force as wood dimensions change — that is its engineering reason to exist.
1. Scope and General Framework 📌
This Standard applies to double coil helical spring lock washers for wood structures. Core content:
- Dimensional requirements: dimension tables for standard-travel and extended-travel double coil washers (Tables 1 and 2).
- Physical properties: material hardness, decarburization limits (Table 3), and travel limits (Tables 4 and 5).
- Test methods: the washer twist test (Fig. 1) and related verification methods.
- Units: all dimensions are in inches and apply to unplated or uncoated product.
- ISO relation: no comparable ISO standard exists for this part.
The Standard references ASME B18.12 (glossary of fastener terms), B18.18.1 (inspection and quality assurance for general-purpose fasteners), B18.21.1 (inch-series lock washers), and B18.24 (PIN code system for B18 fastener products), plus ASTM B117 (salt spray), ASTM E140 (hardness conversion), SAE J403/J411 (carbon steel chemistry), and SAE J419 (decarburization measurement).
⚠️ This Standard covers only the double coil helical spring form. Conventional single-coil helical spring lock washers follow B18.21.1, and plain washers follow B18.22.1 (now consolidated into B18.21.1). Products from different standards are not dimensionally interchangeable.
2. Double Coil Design and Types 🔩
A double coil helical spring lock washer is wound from spring wire into a two-coil helix with split ends. Compared with a conventional single-coil spring washer, it offers:
- Longer elastic travel: the two-coil construction provides a longer compression stroke, sustaining preload as timber compresses and dries.
- More uniform bearing area: the coils form a ring-shaped bearing surface that reduces localized crushing of the wood.
- Two travel options: the standard-travel type suits ordinary timber members; the extended-travel type fits thick, soft, or high-compression connections.
| Series | Design Features | Typical Applications | Dimension Table |
|---|---|---|---|
| Standard Travel | Conventional coil count and height | Ordinary timber, pole cross-arms, roof trusses | Table 1 |
| Extended Travel | Taller coils, longer stroke | Softwood, thick members, high expected compression | Table 2 |
Travel limits are stated separately for the two types (Table 4 standard travel; Table 5 extended travel). A washer with insufficient travel cannot hold clamping force as the wood deforms; excessive travel risks collapse or loss of elasticity during installation.
3. Dimensions and Tolerances 📏
Tables 1 and 2 give the key dimensions of standard- and extended-travel double coil washers: inside diameter (ID), outside diameter (OD), free height, wire diameter, and number of coils. All dimensions are in inches and refer to the unplated state. Select the size from the nominal diameter of the mating bolt/screw, the timber thickness, and expected compression, and use the B18.24 PIN coding system to standardize purchasing identification.
🚨 In timber connections the washer ID must match the bolt/screw shank: an oversized ID lets the washer sit eccentrically and dig into the wood; an undersized ID will not slide over the threads. Check every washer — never force-fit a “near enough” size.
4. Material, Hardness, and Decarburization Control 🧪
- Material: typically carbon spring wire conforming to SAE J403/J411 chemistry.
- Hardness: hardened and tempered to the specified range to secure elasticity and resistance to permanent set.
- Decarburization limits: per Table 3; a decarburized layer sharply reduces surface hardness and fatigue life and is a common hidden cause of field failure.
- Corrosion protection: outdoor or pole hardware is normally zinc or hot-dip galvanized and verified by salt spray testing per ASTM B117.
💡 Outdoor timber hardware lives in hot, humid conditions, and coating quality governs service life. Require an ASTM B117 salt spray report and state the coating thickness grade in the purchase order.
5. Test Methods and Acceptance 🛠️
The key test in the Standard is the washer twist test (Fig. 1): the washer is loaded in a fixture and its resistance to twisting, opening, or permanent set is measured. This verifies elastic travel and crush resistance — the decisive acceptance criterion that distinguishes these from plain washers. In addition, general fastener inspection per B18.18.1 covers dimensions, appearance, hardness, and coating.
6. Engineering Practice Essentials 🛠️
- 🔍 Selection: pick the ID from the bolt nominal diameter; then choose standard or extended travel from the wood species (softwood/hardwood) and member thickness.
- 🔧 Installation: seat the washer flat against the timber and drive the bolt square to the surface; angled loading crushes one edge of the washer.
- 📊 Preload control: use torque or tension control on timber joints to avoid over-tightening that crushes the wood; consider assemblies with a limiting collar where practical.
- 📋 Maintenance: inspect outdoor structures periodically for corrosion and loosening; replace severely rusted washers promptly.
7. Frequently Asked Questions ❓
❓ Q: What is the essential difference between a double coil washer and a common spring washer?
A: The double coil washer has two helical coils and a longer elastic stroke, designed for compressible, shrinking materials like timber; it sustains preload as the wood changes size. A common spring washer has limited travel and suits metal-to-metal joints.
A: The double coil washer has two helical coils and a longer elastic stroke, designed for compressible, shrinking materials like timber; it sustains preload as the wood changes size. A common spring washer has limited travel and suits metal-to-metal joints.
❓ Q: How do I choose standard versus extended travel?
A: Use standard travel for ordinary hardwoods or thin members; use extended travel for softwood, thick members, or connections with high expected compression. Cross-check against the travel limits of Tables 4 and 5.
A: Use standard travel for ordinary hardwoods or thin members; use extended travel for softwood, thick members, or connections with high expected compression. Cross-check against the travel limits of Tables 4 and 5.
❓ Q: Can these washers be used on metal structures?
A: Not recommended. These products are optimized for timber; on metal the elastic travel and bearing characteristics do not match. Use helical spring-lock or plain washers of B18.21.1 instead.
A: Not recommended. These products are optimized for timber; on metal the elastic travel and bearing characteristics do not match. Use helical spring-lock or plain washers of B18.21.1 instead.
🔍 Q: Why did the Standard drop the word “high voltage”?
A: The original proposal targeted high-voltage pole connections, but the Committee recognized that the locking and bearing properties apply to general timber structures as well, so the restriction was removed to widen the standard’s scope.
A: The original proposal targeted high-voltage pole connections, but the Committee recognized that the locking and bearing properties apply to general timber structures as well, so the restriction was removed to widen the standard’s scope.
