Where Does The Locking Washer Go

Where Does the Locking Washer Go? A Complete Guide to Proper Installation

The silent failure of a single fastener can lead to catastrophic equipment breakdown, safety hazards, and costly downtime. At the heart of preventing this lies a small, often misunderstood component: the locking washer. Knowing exactly where a locking washer goes is not just a minor detail—it is a fundamental principle of mechanical integrity. Incorrect placement renders this critical safety device useless, while proper installation ensures that bolts and nuts remain securely tightened against the relentless forces of vibration, shock, and dynamic loads. This guide will definitively answer the placement question for all common types of locking washers, explain the science behind their function, and provide the step-by-step knowledge to install them correctly every time.

Understanding the Core Purpose: Why Placement is Non-Negotiable

Before addressing where to place a locking washer, it is essential to understand why it must be placed correctly. A standard washer’s primary job is to distribute load and provide a smooth bearing surface. A locking washer, also known as a retaining washer or security washer, has an additional, vital function: to prevent the self-loosening of a nut or bolt. It achieves this by creating a prevailing torque—a constant frictional resistance against the rotating action of the fastener. This friction is generated through the washer’s unique design, which typically involves a split, serrated teeth, or a bent tab. The placement determines which parts of the fastener system this friction acts upon and, consequently, how effectively it counteracts loosening.

The Golden Rule: General Placement Principles

For the vast majority of locking washer designs, one universal rule applies: the locking washer must be placed directly adjacent to the component whose rotation you are trying to prevent. In a typical bolted joint, this means:

• Under a nut: The locking washer is installed on the bolt shank, directly beneath the nut. As the nut is tightened, the locking feature (e.g., the split ends) bites into the underside of the nut’s bearing surface, creating friction that resists the nut’s tendency to rotate counter-clockwise and loosen.
• Under a bolt head: If the bolt head is the component at risk of rotating (common in applications where the bolt is threaded into a tapped hole and the head is accessible), the locking washer is placed directly under the bolt head. Its locking feature engages with the underside of the bolt head.

Crucially, the locking washer should be placed on the same side as the component you are locking. It should never be sandwiched between two standard washers or placed in a position where its locking feature cannot engage the intended metal surface.

Specific Placements for Common Locking Washer Types

1. Split Lock Washer (Helical Spring Washer)

This is the most common type, recognizable by its cut helix. Its ends are sharp and designed to "bite" into the mating surface.

• Placement: Directly under the nut or bolt head.
• How it Works: When the nut or bolt is tightened, the split ends are forced outward, pressing against the underside of the fastener. This creates a spring tension and a sharp point contact that resists rotation. It must be used against a hardened surface (the nut or bolt head) to allow the sharp ends to embed. Using it against a soft material or a second washer will cause it to embed into the wrong surface, losing its locking effect.
• Visual Cue: The split should be oriented so the ends are pointing away from the direction of tightening (clockwise for standard right-hand threads). This is a debated point, but the primary goal is ensuring the ends can flare out and engage.

2. Tooth Lock Washer (Serrated or Internal/External Tooth)

These washers have teeth (either on the inner diameter, outer diameter, or both) that dig into the mating surfaces.

• Placement: Directly under the nut or bolt head.
• How it Works: The teeth are designed to penetrate the surface of the nut or bolt head and the clamped material (if it’s soft enough) or a standard flat washer placed between the locking washer and the workpiece. For external tooth washers, the teeth face outward and must be in contact with the workpiece surface. For internal tooth washers, the teeth face inward and engage with the fastener itself.
• Critical Note: Never install a tooth lock washer on a flanged nut or bolt head. The flange’s own serrations will conflict with the washer’s teeth, preventing proper engagement and potentially damaging both components. The locking washer must have a clean, flat surface to bite into.

3. Tab Washer (or Tab Lock Washer)

This is a more robust locking device, often used in high-vibration applications like automotive or heavy machinery. It consists of a flat washer with one or more protruding tabs.

• Placement: The washer sits between the nut/bolt head and the workpiece, like a standard washer. The placement of the tab is the critical factor.
• How it Works: After the nut is tightened to the correct torque, the tab is bent over (using a hammer or pliers) to engage a slot in the nut or a flat on the bolt head, or to press against the side of the workpiece. This physically prevents the nut from rotating at all.
• Placement Process: The tab must be aligned with a suitable feature (a slot, a flat, or an edge) before final tightening. The nut is tightened, and then the tab is bent into the locked position. The washer itself is placed normally, but the tab’s final locked position defines its functional placement.

4. Beveled or Conical Washer (like a Belleville washer)

While often used for spring loading, certain conical washers with a split or serrated edge can act as locking washers.

4. Beveled or Conical Washer (like a Belleville washer)

While often utilized for spring loading, certain conical washers – particularly those with a split or serrated edge, such as Belleville washers – can function effectively as locking washers. These washers leverage a spring effect to maintain pressure against the fastener and the clamped material.

• Placement: Typically placed between the nut/bolt head and the workpiece, similar to a standard washer.
• How it Works: The conical shape and internal spring force create a constant compressive load. When tightened, the washer’s edges press firmly against the surfaces, resisting rotation. The split or serrations enhance this gripping action. Belleville washers, with their stacked, curved surfaces, offer a particularly strong and consistent locking force.
• Considerations: The effectiveness of a beveled or conical washer depends heavily on the material’s hardness and the applied torque. Softer materials will yield more, potentially reducing the locking effect. Selecting the correct spring rate (for Belleville washers) is crucial for optimal performance.

5. Star Washers (or Multi-Point Washers)

These washers feature a star-shaped pattern of indentations or slots around their perimeter.

• Placement: Positioned between the nut/bolt head and the workpiece.
• How it Works: The star pattern creates multiple points of contact with the fastener and the surrounding material. As the nut is tightened, these points dig into the surfaces, generating significant friction and preventing slippage. They are particularly effective in situations where a high level of vibration or loosening resistance is required.
• Important Note: Star washers are most effective when used with fasteners that have a relatively smooth surface. Rough surfaces can interfere with the star’s ability to engage properly.

Conclusion:

Selecting the appropriate locking washer is paramount to ensuring the long-term reliability and safety of any bolted joint. Each type – split lock, tooth lock, tab washer, conical washer, and star washer – offers a unique mechanism for preventing fastener loosening. Understanding their specific applications, limitations, and proper installation techniques is crucial for engineers and technicians alike. Careful consideration of the materials involved, the anticipated loads, and the operating environment will guide the selection process, ultimately contributing to robust and dependable assemblies. It’s always recommended to consult with fastener manufacturers and engineering guidelines to determine the optimal locking solution for a given application.