Technical Foundation
What Is a Lock Washer?
A lock washer sits under a fastener head or nut and resists joint loosening by maintaining spring tension and, in toothed designs, biting into the mating surfaces. Split ring (spring) washers work by staying partially compressed under the bolt head, keeping axial preload on the threads even as minor settling occurs; internal- and external-tooth washers add sharp radial teeth that dig into the bearing surface for additional resistance to rotation under vibration.
Independent testing including the DIN 25201 vibration series has shown split-ring and standard tooth lock washers provide far less anti-loosening benefit than commonly assumed once a joint is properly torqued, which is why many engineering standards now favor prevailing-torque nuts, thread-locking compound, or Belleville washers sized for a specific preload rather than a lock washer alone on critical or cyclic-load joints. Belleville (conical disc) washers are a different category entirely — they're a calibrated spring, selected by load-deflection curve, used to maintain consistent clamp load across thermal cycling or gasket relaxation.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Washer Type | Split ring, internal tooth, external tooth, Belleville, curved | Determines the locking mechanism — spring tension, tooth bite, or calibrated conical spring load |
| Bolt Size | Matched to fastener nominal diameter (e.g., 1/4", M6) | Undersized or oversized washers won't seat correctly under the bolt head or nut |
| Material | Spring steel, stainless steel (18-8, 316), silicon bronze | Determines corrosion resistance and whether the washer retains spring temper in the service environment |
| Finish | Zinc plated, black oxide, passivated | Affects corrosion resistance and galvanic compatibility with the mating fastener and joint material |
| Belleville Load Rating | Specified per washer by load-deflection curve | Belleville washers are selected to a target clamp force, not just a bolt size, for thermal-cycling or gasketed joints |
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Applications
Where Lock Washers Get Used
Lock washers are specified wherever a bolted joint sees vibration, needs an electrical bond point, or requires calibrated spring preload across thermal cycling.
General Machine Assembly
Split ring and tooth lock washers specified on standard bolted joints as a low-cost secondary locking measure alongside proper torque.
Electrical Grounding & Bonding
External tooth lock washers used to bite through paint or anodizing and establish a low-resistance electrical bond at grounding points.
High-Vibration Equipment
Belleville spring washers selected by load-deflection curve to maintain clamp force on pump, motor and compressor mounts through vibration cycles.
Gasketed Flange Joints
Belleville washers maintaining consistent bolt preload as gasket material relaxes and compresses over thermal cycling.
HVAC & Sheet Metal Assembly
External tooth washers on thin sheet metal joints where the teeth also help resist pull-through under light loads.
Marine & Outdoor Equipment
316 stainless lock washers specified where corrosion resistance is required alongside vibration resistance.
FAQ
Lock Washers Questions, Answered
Do split lock washers actually prevent bolts from loosening?
Testing under standards like DIN 25201 has generally found that once a joint is properly torqued, standard split-ring lock washers provide limited anti-loosening benefit compared to alternatives like prevailing-torque nuts or thread-locking compound. They're still widely specified as a low-cost secondary measure, but for critical or high-vibration joints, engineers increasingly favor other locking methods.
What's the difference between internal tooth and external tooth lock washers?
Internal tooth washers have teeth bent inward from an inner diameter, keeping the outer edge smooth — useful where a clean outer appearance or smaller footprint matters. External tooth washers have teeth around the outer diameter, giving more contact points and better bite through coatings like paint or anodizing, which is why they're the common choice for electrical grounding and bonding connections.
What is a Belleville washer and how is it different from a standard lock washer?
A Belleville washer is a conical disc spring, selected by a specific load-deflection curve rather than just a bolt size, used to deliver a calibrated clamping force that stays consistent even as the joint settles or a gasket relaxes under thermal cycling. It's a precision spring component, not a simple anti-rotation washer like a split ring or tooth washer.
Can I stack multiple Belleville washers for more force?
Yes — Belleville washers can be stacked in series (same orientation, nested) to increase deflection at the same load, or in parallel (alternating orientation) to increase the load capacity at the same deflection. The stacking configuration is chosen from the washer's published load-deflection curve to hit a target clamp force and travel.
What material should I choose for a lock washer in a corrosive environment?
Stainless steel (18-8 for general corrosion resistance, 316 for marine or chemical exposure) is standard for outdoor or wash-down environments. Plain spring steel with zinc plating or black oxide is common for indoor, dry-environment general assembly where cost is the priority and the finish provides adequate short-term protection.
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