How To Use A Lock Washer: The Complete Engineering And Installation Guide
To properly use a lock washer, place it directly beneath the nut or bolt head (whichever is the rotating element during tightening) to prevent loosening caused by vibration and thermal expansion. Ensure the washer split-ends or internal/external teeth bite into both the mating surface and the fastener face to establish tension. Always torque the fastener to the specified engineering limit to prevent over-flattening and premature joint failure.
Pre-Assembly Planning and Lock Washer Selection Checklist
Achieving a structurally sound bolted joint requires selecting the correct lock washer for the specific mechanical load, vibration profile, and environmental conditions. Using the wrong washer type or misinterpreting material grades can lead to rapid clamp-load loss, joint slippage, or structural fatigue.
Before beginning assembly, you must identify the mechanical forces at play. Helical split washers rely on spring tension and sharp edge biting, while tooth lock washers utilize frictional resistance over a larger surface area. Conical (Belleville) washers excel in high-preload, thermal-expansion environments. Refer to ASME B18.21.1 standards to match washer specifications to your fastener class.
Essential Gear, Materials, and Tools
- Lock Washers: Helical split, internal/external tooth, conical, or wedge-locking style (matched to bolt size and grade).
- Fasteners: High-tensile bolts and matching nuts (e.g., Grade 5/Class 8.8 or Grade 8/Class 10.9).
- Calibrated Torque Wrench: Dial or click-type, calibrated to handle the target torque range.
- Cleaning Agents: Solvent-based degreaser (isopropyl alcohol or acetone) and lint-free microfibers.
- Safety Equipment: ANSI-approved safety glasses and protective work gloves.
Prerequisite Standards and Knowledge
- Thread Condition: All mating threads must be clean, dry, and free of burrs or deformed leads unless a specific thread lubricant is specified.
- Galvanic Compatibility: Match washer metallurgy to fastener and substrate metallurgy (e.g., use 316 stainless steel washers with stainless steel fasteners to avoid galvanic corrosion).
- ASME B18.21.1 Compliance: Ensure dimensions and hardness match standardized engineering specifications.
Execution Metrics
- Estimated Duration: 2 to 5 minutes per fastener joint.
- Budget Range: Nominal cost ($0.05 to $2.00 per washer, depending on material, size, and proprietary wedge-lock designs).
Step-by-Step Mechanical Installation of Lock Washers
Executing a proper lock washer installation involves precise sequencing, alignment, and controlled torque application. Follow this systematic workflow to ensure optimal joint security.
Step 1: Inspect and Prepare Mating Surfaces
Inspect the bolt threads, nut threads, and the parent substrate material for imperfections, dirt, scale, or lubricant buildup. Any contaminant on the joint face acts as a dampening zone, which will rapidly dissipate clamp load under mechanical stress.
Clean the contact areas thoroughly using a solvent degreaser and a wire brush if necessary. Ensure the substrate surface is flat and perpendicular to the bolt axis. If the parent metal is significantly softer than the lock washer (such as aluminum or plastic), recognize that tooth or split washers will dig deeply into the material, which may cause structural degradation of the substrate.
Step 2: Establish the Proper Stacking Sequence
Determine whether you are tensioning the bolt head or the nut. The lock washer must always be positioned immediately adjacent to the component that is being rotated during the tightening process.
If you are using a flat washer to distribute the clamping load over a larger area, place the flat washer directly against the substrate first. Next, place the lock washer on top of the flat washer. Finally, thread the nut onto the bolt. Stacking in the reverse order (putting the lock washer against the substrate and the flat washer on top) entirely defeats the function of the lock washer, as the flat washer will simply spin freely on top of it.
Warning: Never stack two lock washers on top of one another on a single bolt. Double-stacking reduces joint stability, prevents proper torque transmission, and increases the likelihood of catastrophic thread failure.
Step 3: Align and Orient the Lock Washer
Slide the lock washer onto the bolt shank. Pay close attention to orientation based on the washer type:
- Helical Split Washers: These are symmetrical, so either face can point toward the nut.
- Conical/Belleville Washers: Ensure the crown (the peak of the cone) points directly toward the underside of the bolt head or nut. The wider base of the cone must face down toward the joint substrate.
- Tooth Lock Washers: Ensure the teeth lie flat and concentric with the bolt hole to prevent uneven loading.
- Wedge-Lock Washers (e.g., Nord-Lock): Ensure the cam faces are mated together facing inside, while the outer radial ridges face outward toward the bolt head and mating surface.
Step 4: Engage the Threads Hand-Tight
Thread the nut onto the bolt or insert the bolt into the tapped hole by hand. Ensure the lock washer remains centered and concentric with the bolt shank. Hand-tighten until the lock washer makes light contact with both the fastener head/nut and the mating surface.
This step prevents cross-threading and guarantees that the lock washer does not slip off-center or become pinched diagonally in the clearance hole, which would cause eccentric loading.
Step 5: Apply Torque to Engineering Specifications
Use a calibrated torque wrench to tighten the fastener to its final calculated torque value. Avoid using impact wrenches for final tightening, as they can deliver unregulated torque spikes that over-compress and permanently deform the lock washer.
As torque is applied, the helical split washer will compress until it is completely flat. This transition zone indicates that the washer is exerting its spring rate force against the fastener face.
For tooth washers, the tightening torque forces the twisted teeth to bite into both mating surfaces, creating a mechanical lock.
Pro-Tip: Adjust your torque calculations based on joint friction. If using a dry split lock washer, increase torque by 10% to 15% compared to standard flat washer values to overcome the added rotational friction of the split ends biting into the fastener face during installation.
Metric Split Lock | Split Lock Washers | Wurth Canada
Lock Washer Types, Material Properties, and Torque Metrics
Selecting the correct lock washer requires balancing material hardness, spring tension, and resistance to environmental degradation. The table below outlines the primary configurations of lock washers used across industrial applications.
| Lock Washer Type | Engineering Standard | Ideal Application | Torque Coefficient Adjustment (K-Factor) | Material Limits & Compatibility |
|---|---|---|---|---|
| Helical Split | ASME B18.21.1 | Medium-duty joints; general mechanical assembly with moderate vibration. | Increase dry torque by 10%–15% (K = 0.22) | Use zinc-plated carbon steel for steel bolts; stainless steel for SS hardware to prevent galvanic corrosion. |
| External Tooth | ASME B18.21.1 | Electrical grounding; maximum torsional resistance on oversized clearance holes. | Increase dry torque by 20% (K = 0.24) | Soft to medium metals. Avoid on hardened alloy surfaces where teeth cannot bite. |
| Internal Tooth | ASME B18.21.1 | Low-clearance assemblies; aesthetic applications where teeth must be hidden under the screw head. | Increase dry torque by 15% (K = 0.23) | Best used on small machine screws (under 1/2 inch) with light clamping loads. |
| Conical (Belleville) | DIN 6796 | High-load joints subject to thermal expansion and contraction cycles. | Maintain standard flat washer torque (K = 0.20) | High-tensile alloy steels and spring steels; ideal for heavy machinery and busbars. |
| Wedge-Lock (Nord-Lock) | Proprietary / ISO | Extreme-vibration environments; high-stress structural joints. | Apply lubrication; reduce torque based on manufacturer tables (K = 0.16–0.18) | Hardened steel or stainless steel configurations. Must match or exceed bolt grade hardness. |
Fastener Joint Failures and Structural Remedies
When a lock washer fails, it compromises the clamp load of the entire joint. Recognizing the indicators of mechanical failure allows for quick field intervention.
Scenario 1: Helical Split Washer Has Flattened and Cracked
- Root Cause: The fastener was subjected to extreme over-torque, or a low-grade carbon steel washer was paired with a high-tensile Grade 8 (Class 10.9) bolt. The excessive compressive force exceeded the elastic limit of the washer material, causing it to crack at its weakest cross-section.
- Actionable Fix: Replace the damaged washer with a high-strength, through-hardened alloy steel split washer that matches the grade of the bolt. Tighten the assembly using a calibrated torque wrench to the exact torque limit specified for that fastener grade.
Scenario 2: Joint Experiences Clamp Load Loss in High-Vibration Environments
- Root Cause: Split lock washers and tooth lock washers rely on friction and spring tension. Under severe harmonic or transverse vibration, these forces can be easily overcome, causing the fastener to back off and lose preload.
- Actionable Fix: Replace the tooth or split washer with a pair of wedge-locking washers. Wedge-lock washers utilize tension rather than friction, using cam wedges with a greater slope than the bolt thread pitch to mechanically lock the fastener in place under extreme vibration.
Scenario 3: Significant Surface Galling and Substrate Deformation
- Root Cause: An external tooth lock washer was installed directly against a soft aluminum or copper substrate. The hardened teeth dug too deeply into the parent metal during tightening, tearing the material and causing metal shavings to compromise the joint interface.
- Actionable Fix: Disassemble the joint and clean the damaged substrate. Install a hardened flat washer directly against the soft metal surface to distribute the load, place the lock washer on top of the flat washer, and then secure the nut. Alternatively, switch to a smooth conical (Belleville) washer.
Frequently Asked Questions
Does the lock washer go before or after the flat washer?
The lock washer must always be placed after the flat washer. In a standard stacking sequence, the flat washer sits directly against the parent material of the substrate to distribute the load, and the lock washer is positioned between the flat washer and the nut or bolt head. Placing the flat washer on top of the lock washer isolates the lock washer from the nut, which renders its locking capabilities completely useless.
Can you reuse a helical split lock washer?
No, helical split lock washers are designed for single-use applications. During the initial tightening sequence, the washer undergoes significant plastic deformation as its split ends are forced flush. Reusing a split washer compromises its spring rate tension and dulls the sharp edges required to bite into the metal surfaces, drastically increasing the risk of joint loosening.
Why do lock washers fail in high-vibration applications?
Many traditional lock washers, particularly helical split styles, lose effectiveness when the dynamic transverse forces of vibration exceed the static friction forces holding the joint together. Once the joint experiences minor slip, the spring force of the split washer is often insufficient to prevent the bolt from spinning, necessitating the use of wedge-locking washers or anaerobic threadlocking compounds for high-vibration environments.
Which side of a conical Belleville washer should face the nut?
The crown (the raised, narrow center cone) of a Belleville or conical lock washer must always point toward the underside of the nut or bolt head. The wide, flat circular base of the cone must rest against the flat washer or joint substrate. Mounting a conical washer upside down prevents it from compressing correctly under load, which can destroy its spring-tensioning properties.
Optimize Your Mechanical Fastening Systems
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