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ASME B18.8.200M-2000 — Cotter Pins, Headless Clevis Pins, and Headed Clevis Pins (Metric Series)
📌 ASME B18.8.200M-2000 is the consolidated metric standard in the ASME B18.8 machine pin system, merging three previously separate metric pin standards into one document: B18.8.6M (cotter pins), B18.8.7M (headless clevis pins), and B18.8.8M (headed clevis pins). These three products form the standard “pivot + lock” combination in metric mechanical design: a clevis pin installed in a clevis-type joint carries the pivot load and shear, while a cotter pin passes through the hole in the pin end and is bent over to lock the pin axially.
💡 The metric cotter pin + clevis pin combination is widely used on tractors, construction machinery, hydraulic cylinder trunnions, and implement hitches. Assembly and disassembly need only needle-nose pliers — low cost, high reliability — making it the standard solution for quick-detachable pivots.
1. Scope and General Framework 📌
The Standard specifies three pin products by structure:
- Cotter pins: half-round wire bent into a U shape with spreadable prongs, inserted through a pin-end hole for locking; types include the extended prong type and the hammer lock type.
- Headless clevis pins: straight cylindrical pins with no heads at either end, located by cotter pins or retaining rings at both ends.
- Headed clevis pins: one end with a shoulder/head that limits axial position, the other end drilled for a cotter pin.
All dimensions are in millimeters and standardized to the metric series. The Standard continues the B18.8 lineage and forms the metric counterpart to the inch-series B18.8.1.
The three incorporated documents were developed over successive years: B18.8.6M (cotter pins), B18.8.7M (headless clevis pins), and B18.8.8M (headed clevis pins) were each proposed by Subcommittee 8 and approved as American National Standards before being consolidated. Combining them into B18.8.200M gives designers and purchasers a single reference for the complete “pin-and-lock” pair, with one consistent set of tolerances, terminology, and referencing requirements. The consolidated format also simplifies inspection: acceptance follows the ASME B18.18 quality assurance system unless the purchaser specifies otherwise, and terminology follows the ASME B18.12 glossary of fastener terms.
⚠️ A cotter pin is a locking element, not a load-bearing one: it carries only locking load and does not resist shear or tension. The capacity of a clevis pin is calculated from pin diameter, material, and fit. Never use a cotter pin as a structural pin in design.
2. Product Classification and Size Series 🔧
| Product | Source Standard | Structural Features | Typical Applications |
|---|---|---|---|
| Cotter pins | B18.8.6M | U-shaped wire, two bendable prongs | Locking clevis pins, bolts, shaft ends |
| Headless clevis pins | B18.8.7M | Straight cylinder, no heads, drilled at both ends | Lockable at both ends; bidirectional position adjustment |
| Headed clevis pins | B18.8.8M | One shoulder/head end, drilled other end | Unidirectional retention, more secure against pull-out |
The Standard provides diameter and length series, head/end structures, prong spread, material, and surface treatment requirements with associated tolerances. Cotter pins are selected by diameter-length combination; clevis pins by shank diameter, clamping width, and end hole size.
3. Engineering Practice Essentials 🛠️
- 🔍 Selection workflow: size the clevis pin shank from pivot load and hole diameter → set length from clamping width → choose the cotter pin from the pin-end hole → choose headless (bidirectional) or headed (unidirectional) design based on accessibility.
- 🛡️ Correct bending: after insertion, spread and bend both prongs symmetrically around the pin (about 180 deg) — never bend only one side; keep bends snug against the part face to reduce snagging.
- 📋 Material selection: carbon steel cotter and clevis pins are commonly zinc plated; stainless steel offers good corrosion resistance for outdoor and humid service; verify material temperature limits for hot applications.
- 📊 Periodic inspection: cotter pins may fatigue and fracture under vibration and shock — include them in routine inspection; replace immediately if prongs loosen or corrode; never reuse an old cotter pin.
4. Frequently Asked Questions ❓
❓ Q: How do I choose between headless and headed clevis pins?
A: Headless pins are symmetrical, allow bidirectional locating and disassembly, and suit applications with access at both ends. Headed pins use the head for axial retention and a cotter pin at the other end, giving more secure one-way retention for installations with limited access.
A: Headless pins are symmetrical, allow bidirectional locating and disassembly, and suit applications with access at both ends. Headed pins use the head for axial retention and a cotter pin at the other end, giving more secure one-way retention for installations with limited access.
❓ Q: How does B18.8.200M correspond to the inch B18.8.1?
A: B18.8.200M is the metric series (merging B18.8.6M/7M/8M) covering cotter pins and clevis pins, while B18.8.1 is the inch series covering clevis pins and cotter pins. Content corresponds but units differ — choose by the project’s unit system and never mix.
A: B18.8.200M is the metric series (merging B18.8.6M/7M/8M) covering cotter pins and clevis pins, while B18.8.1 is the inch series covering clevis pins and cotter pins. Content corresponds but units differ — choose by the project’s unit system and never mix.
❓ Q: Why is cotter pin prong spread important?
A: Spread determines locking reliability: too little spread and the pin can work out under vibration; too much spread over-deforms and fatigue-cracks the prongs. The Standard specifies spread and bend radius to ensure reliable locking and one-time removability.
A: Spread determines locking reliability: too little spread and the pin can work out under vibration; too much spread over-deforms and fatigue-cracks the prongs. The Standard specifies spread and bend radius to ensure reliable locking and one-time removability.
🔍 Q: How are metric cotter pin sizes designated?
A: By a “diameter × length” combination, e.g., 5 × 40 (5 mm diameter, 40 mm length). The pin diameter should be slightly smaller than the clevis pin end hole or bolt hole so insertion is smooth and locking remains reliable.
A: By a “diameter × length” combination, e.g., 5 × 40 (5 mm diameter, 40 mm length). The pin diameter should be slightly smaller than the clevis pin end hole or bolt hole so insertion is smooth and locking remains reliable.
