Spring Lock Washer dimensions

Metric

(DIN 127B / ISO 12944)

Imperial

ASME B18.21.1 - Helical Spring Lock Washers (Regular)

Design Parameters

Spring lock washers (ASME B18.21.1 / DIN 127) act as a spring element within a bolted joint. The split in the ring creates a "helical spring" effect that provides axial tension, helping to prevent the fastener from backing out under light vibration.

Engineering Note: While widely used, spring lock washers are generally effective only under light loads. In high-vibration or critical structural applications, they are often replaced by wedge-locking washers (like Nord-Lock) or chemical thread-lockers, as a spring washer can actually act as a "bearing" that aids loosening once the initial preload is lost.

Technical Guidance for Spring Lock Washers

Helical spring lock washers (split washers) are among the most common fasteners in mechanical assembly, yet they are frequently misapplied in modern engineering. While their intended purpose is to prevent a nut or bolt from backing out, their effectiveness is highly dependent on the type of load and the vibration profile of the assembly.

The Mechanics of the Helical Split

A spring lock washer is essentially a single-coil helical spring. When the fastener is tightened, the washer is compressed until it is almost flat. This compression stores energy, creating a constant axial force (tension) against the fastener.

In theory, this axial force increases the friction between the threads of the bolt and the nut, making it harder for the fastener to rotate. Additionally, the split ends of the washer form two "tangs" or sharp edges that are intended to bite into the seating surface and the fastener head, providing mechanical resistance to counter-clockwise rotation.

The Junker Test and the "Bearing" Effect

Extensive research and "Junker" vibration testing have shown that standard spring lock washers often fail to prevent loosening in high-vibration structural joints.

When a bolt is tightened to its proper preload, the spring washer is compressed completely flat. At this point, it behaves exactly like a standard flat washer. If the joint experiences enough vibration to cause a slight loss of preload, the spring washer "uncoils." However, the spring rate of a standard lock washer is usually far lower than the stiffness of the bolt. By the time the washer starts providing spring force, the bolt has already lost the vast majority of its clamping tension.

Furthermore, because the compressed washer is a smooth, hardened ring, it can actually act as a thrust bearing, reducing the friction between the bolt head and the part and making it easier for the bolt to rotate loose once the initial friction is overcome.

Surface Marring and Electrical Grounding

One area where spring lock washers excel is in creating a gas-tight electrical connection. Because the sharp edges of the split bite into the parent material, they effectively "plow" through paint, oxidation, or anodized coatings to reach the bare metal beneath.

This makes them the standard choice for:

• Grounding lugs: Ensuring a low-resistance path to the chassis.
• Busbar connections: Maintaining contact despite thermal expansion cycles.

However, this same "biting" action is a disadvantage in precision machinery. The marring of the surface can create stress risers in the parent material, which may lead to fatigue cracks in aluminum or high-stress steel components.

Proper Installation and Orientation

To maximize the limited effectiveness of a split washer:

1. Use a Flat Washer Underneath: If you must use a lock washer on a soft material or a painted surface where you want to protect the finish, place a flat washer between the lock washer and the part. Note that this reduces the "locking" effect of the tangs.
2. Hardness Matching: Ensure the lock washer is harder than the nut or bolt head. If the washer is too soft, the tangs will simply flatten out rather than digging in.
3. Correct Torque: Lock washers require a minimum torque to reach the "flat" state. Under-tightening leaves the split open, which can cause the fastener to sit at an angle, introducing bending stresses into the bolt shank.

When to Use Alternatives

For critical structural joints, or environments with high-frequency vibration, engineers should consider superior locking methods:

• Wedge-Locking Washers (e.g., Nord-Lock): These use a pair of cams to create a "wedge" effect that increases tension if the bolt tries to rotate.
• Prevailing Torque Nuts (Nyloc or All-Metal): These use an insert or deformed threads to provide constant friction regardless of preload.
• Chemical Thread-lockers: Anaerobic adhesives (like Loctite) fill the gaps between threads to prevent any relative motion.

Summary for Designers

Split washers are appropriate for non-structural, low-vibration applications where you want to prevent components from rattling loose, or for electrical grounding. For any joint where a failure would be catastrophic or where high clamping force must be maintained, move beyond the split washer to a more robust mechanical or chemical locking solution.

Standard Reference Comparison

Note: DIN 127 has been officially withdrawn by the German Institute for Standardization due to the effectiveness concerns mentioned above, but remains widely available and used in non-critical commercial applications.