What Is a Lock Washer?

A lock washer is a fastener component placed under the nut or bolt head to resist loosening caused by vibration, thermal cycling, or dynamic loads. Lock washers work by creating either a spring force, a mechanical interference, or a wedge-locking action between the nut bearing surface and the joint surface. They are widely used in structural steel, pipe supports, equipment bolting, and mechanical assemblies.

Types of Lock Washers

Type	Standard	Mechanism	Effectiveness
Split (helical spring)	ASME B18.21.1	Spring tension + edge bite into surfaces	Low to moderate
Toothed (internal)	ASME B18.21.1	Serrated teeth grip bearing surface	Moderate
Toothed (external)	ASME B18.21.1	External serrations grip joint surface	Moderate
Belleville (disc spring)	DIN 6796	Conical spring maintains preload	High
Nord-Lock (wedge-locking)	ASME B18.21.2	Cam-action wedge pairs resist rotation	Very high
Tab washer	BS 7084	Bent tabs lock against nut flat and joint	High (positive lock)

When Lock Washers Are Used

Lock washers are common in the following piping and structural applications:

• Pipe support clamps: U-bolts and clamp bolts on pipe racks subject to vibration
• Equipment mounting: Pump, compressor, and motor anchor bolts exposed to dynamic loads
• Structural connections: Bolted steel connections on platforms, ladders, and handrails
• Instrumentation: Mounting bolts for field instruments and junction boxes

Common Mistake

Split lock washers are not effective on hardened steel surfaces (above 40 HRC) because the edges cannot bite into the bearing surface. For hardened flanges, use wedge-locking washers or Belleville springs instead.

Lock Washers vs. Flanged Piping Joints

Lock washers are generally not used on ASME B16.5 flanged piping joints. Flange bolting relies on controlled preload via torque or hydraulic tensioning to maintain gasket seal, not on anti-rotation devices. Adding a lock washer to a flanged joint changes the effective bolt length and alters the load-deflection behavior, which can compromise gasket compression.

Application	Lock Washer Required?	Reason
ASME B16.5 flanged joints	No	Controlled torque maintains preload
Pipe support U-bolts	Yes (recommended)	Vibration loosening risk
Equipment anchor bolts	Yes (recommended)	Dynamic loads
Structural bolted connections	Per design code	AISC specifies hardened washers
Electrical grounding bolts	Yes	Maintain continuity

Key Specifications

Standard	Coverage
ASME B18.21.1	Lock washers (spring, toothed)
ASME B18.21.2	Wedge-locking washers
DIN 6796	Conical spring (Belleville) washers
DIN 127	Split lock washers (metric)
DIN 25201	Junker vibration test for locking devices

Pro Tip

For critical bolted joints subject to severe vibration (rotating equipment, offshore platforms), Nord-Lock wedge-locking washers outperform all other types in Junker vibration testing (DIN 25201). They maintain over 95% of initial preload under sustained vibration.

Belleville washers also serve a secondary role: compensating for bolt relaxation by maintaining a spring force that keeps the bolt stretched even as embedment and creep reduce the clamped length.

Read the full guide to stud bolts for flanges