Types of Lock Washers: Practical Guide to Selection and Use

What “types of lock washers” means in real assemblies

The phrase types of lock washers covers multiple designs that resist loosening in different ways. Some add spring force (spring washers), some increase friction (serrated/tooth washers), and some create a mechanical wedge effect (wedge-lock pairs). Choosing the wrong type can turn a “locked” joint into a joint that still loosens under vibration, thermal cycling, or embedment.

A practical approach is to match the washer’s locking mechanism to the failure mode you expect:

• Vibration loosening (transverse slip) → consider wedge-lock washers or proven prevailing-torque solutions.
• Low torque, thin sheet metal, electrical bonding → consider tooth (star) washers.
• Need controlled spring behavior (maintain force during thermal changes) → consider Belleville (disc spring) or wave washers.
• Positive mechanical retention (safety critical) → consider tab washers or lock plates.

Split lock washers (helical spring washers)

Split lock washers are the familiar “cut and twisted ring.” They are intended to add a small spring effect and create edge bite. In practice, their locking performance depends heavily on joint stiffness, surface hardness, and whether the joint experiences transverse motion.

Where split lock washers fit best

• General-purpose, low-to-moderate vibration where historical practice/spec requires them.
• Assemblies where the mating surface is hard enough to resist the washer flattening early.

Practical cautions

Under higher clamp loads, many split washers flatten quickly, behaving like a plain washer while also adding variability to torque-to-tension due to changing friction. If your design problem is true vibration loosening, treat split washers as “not your first choice” unless test data or a customer spec supports them.

Common specifications include DIN 127 / similar legacy standards, but many industries prefer alternatives for vibration-critical joints.

Tooth (star) lock washers: internal, external, and combination

Tooth lock washers use serrated “teeth” to increase friction and bite into surfaces. They are widely used in electrical and light mechanical assemblies because the teeth can break through oxides/paint and improve electrical continuity while resisting rotation.

Internal tooth vs external tooth

• External tooth: larger effective radius for better resistance to rotation; may mar visible surfaces.
• Internal tooth: teeth inside the inner diameter; better for tight OD constraints and cleaner outer appearance.
• Combination (internal/external): more aggressive bite, but also more surface damage risk.

Best-use example

A common application is bonding a ground lug to a chassis. The tooth washer is placed so teeth contact the conductive base metal. If the chassis is painted, the teeth can cut through the coating, improving contact. In this scenario, the “locking” benefit comes largely from higher friction and surface bite, not spring action.

Wedge-lock washers (paired cam washers)

Wedge-lock washers are used as a matched pair with cams on the inside and radial serrations on the outside. The cams have a wedge angle designed so that any loosening rotation must climb the cam ramps, which increases clamp length and resists back-off.

When to choose wedge-lock

• High vibration or dynamic shear where transverse slip can occur.
• Safety- or uptime-critical joints where field loosening is costly.
• Applications where you can accept surface serration marks (or use compatible hardened bearing surfaces).

Key practical note

These washers depend on correct pairing and orientation. Install them as a mated set (cams facing each other). A common field error is splitting the pair across multiple joints, which defeats the wedge mechanism.

If your requirement is “resist vibration loosening,” wedge-lock designs are frequently selected because the locking effect is not merely friction-based; it is a geometric resistance to back-off.

Belleville (disc spring) washers

Belleville washers are conical disc springs. They are chosen less for “anti-rotation bite” and more for maintaining clamp force when there is settling, thermal cycling, gasket creep, or differential expansion. They can be stacked in series/parallel to tune deflection and load.

What they solve well

• Maintaining preload during embedment/creep (e.g., polymers, gaskets, soft aluminum interfaces).
• Thermal cycling where joint length changes over temperature.
• Designs where a defined spring rate is required (controlled clamp behavior).

Simple numeric example (preload context)

Suppose an M10 bolt property class 8.8 has a proof stress near 580 MPa. Using a typical engineering target of about 70% of proof for preload and a tensile stress area near 58 mm², an approximate preload is:

Preload ≈ 0.7 × 580 MPa × 58 mm² ≈ 23.5 kN.

A Belleville washer can be selected so that expected joint settling (for example, a small loss of stack height) results in only a modest preload change, improving retention compared with a rigid stack.

Wave washers and curved spring washers

Wave washers (multi-wave) and curved spring washers (single-wave/curved) provide lighter spring forces and more deflection than many helical split washers. They are commonly used to reduce rattling, control axial play, and compensate for tolerance stack-up in light-duty assemblies.

Best-fit applications

• Bearing preload in low-load mechanisms (where specified by the bearing/mechanism design).
• Noise/rattle control for panels and light brackets.
• Assemblies needing compliance without aggressive surface bite.

Limitations

These are not usually the first choice for severe vibration loosening. Their value is primarily controlled spring behavior, not anti-rotation geometry.

Tab washers and lock plates (positive mechanical locking)

Tab washers and lock plates use a bend-up tab that physically blocks nut/bolt rotation by engaging a flat, slot, or feature on the fastener and a stationary feature on the assembly. This is a “positive lock” concept rather than a friction/spring concept.

Where they make sense

• Safety-critical joints with inspection requirements (visual confirmation of tab engagement).
• Rotating equipment where historical standards call for positive locking.
• Applications where surface bite or serrations are undesirable.

Practical cautions

Bending tabs is a form of plastic deformation; many designs are treated as single-use or limited-reuse depending on specification. Ensure the tab washer material and thickness match the torque and flat geometry so the tab does not crack or relax.

Comparison table: selecting among common lock washer types

Use the table below as a fast filter. Then validate against your joint conditions (vibration level, surface hardness/coatings, temperature, and whether you can tolerate surface marking).

How to choose the right lock washer (decision checklist)

Use this checklist to narrow down the correct option quickly, then validate with testing or prior qualification when the joint is mission-critical.

Selection steps

1. Define the loosening driver: vibration, thermal cycling, embedment, or operator re-torque issues.
2. Confirm surface condition: painted, plated, anodized, soft aluminum, hardened steel, or composites.
3. Decide if surface marking is acceptable: serrations/teeth often leave visible damage.
4. Assess serviceability: will it be removed often? If yes, prefer solutions with consistent reuse behavior.
5. For vibration-critical joints, prioritize solutions with proven performance under transverse loading (often wedge-lock or prevailing torque strategies) rather than relying on split washers.

A robust rule of thumb: locking starts with clamp force. If the joint is under-tightened, no washer will reliably prevent loosening.

Installation tips and common mistakes

Many “lock washer failures” are actually assembly process failures. The points below prevent the most common field issues.

Best practices

• Place the lock washer directly under the rotating element (usually the nut), unless your spec requires otherwise.
• Avoid stacking multiple “locking” elements that fight each other (e.g., tooth washer plus wedge-lock pair) unless validated by test.
• If using tooth washers on coated surfaces, ensure the teeth actually contact base metal where electrical or friction bite is required.
• For wedge-lock pairs, keep the pair together and confirm the cams face inward (cams-to-cams).

Mistakes that reduce locking effectiveness

• Using a lock washer to “fix” incorrect torque or an oily/contaminated joint surface.
• Using a spring-type washer where joint stiffness is so high that the washer contributes negligible deflection.
• Choosing a washer OD/ID that does not seat properly, causing eccentric loading and inconsistent clamp force.

When not to use a lock washer

Sometimes the right answer is “no lock washer.” If you need controlled preload and repeatable performance, other strategies can outperform washers:

• Use prevailing-torque nuts, all-metal lock nuts, or thread-locking adhesives where appropriate for the environment.
• Use hardened flat washers to protect surfaces and maintain consistent friction when torque-to-tension repeatability matters.
• If vibration is severe, consider joint redesign (increase clamp length, add dowels/shear keys, improve bearing area) in addition to locking hardware.

The most reliable outcome comes from selecting among the types of lock washers based on joint physics: clamp force, slip risk, surface condition, and service environment.