Reuse Assessment: Durability of Wedge-Locking Washers

1. Assessment Background and Failure Mechanisms

DIN 25201 wedge-locking washers achieve locking through radial teeth biting into the connected parts and relative sliding on the wedge surfaces. Each tightening and disassembly causes:

1. Plastic Deformation and Wear of Teeth
   The tooth tips embed into the surface of the connected part under high pressure. During disassembly, they are forcibly extracted, leading to blunting and height reduction of the tooth tips. Reduced bite depth decreases the slip resistance and the "mechanical locking" effect.

2. Wedge Surface Wear
   Sliding friction occurs between the inner and outer washers on the wedge surfaces during tightening and loosening, especially during loosening (large-angle climbing), where wear is more severe. Changes in wedge surface roughness, material transfer, or coating peeling can alter the friction coefficient $\mu_{cam}$, thereby affecting the locking torque $M_{lock}$ and unlocking torque $M_{unlock}$.

3. Hardness-Related Fatigue
   Washers experience high contact stresses under repeated loading. Insufficient hardness can lead to micro-cracks at the tooth root or wedge surface, ultimately resulting in fatigue fracture.

Core Conclusion: After reuse, the locking capability of the washer will decrease progressively, manifested as a reduction in locking torque and a decrease in the safety factor $S_{lock}$.

2. Decisive Role of Hardness in Reuse Performance

2.1 Hardness and Wear Resistance

According to Archard's wear law, the wear volume of a material is inversely proportional to its hardness:

where $F$ is the contact force, $s$ is the sliding distance, and $H$ is the material hardness.

Therefore, the higher the surface hardness of the washer, the smaller the wear on the teeth and wedge surfaces, and the longer the locking performance is maintained.

2.2 Typical Hardness Levels and Reuse Cycles

Rule of Thumb: Washers with a hardness of HV 350 or above, under normal operating conditions (no severe overload, no extreme corrosion, correct installation), can be reused 5 times while maintaining locking performance within an acceptable range.

3. Performance Degradation Assessment After Reuse

3.1 Locking Torque Degradation Model

The locking torque $M_{lock,n}$ after the $n$-th use can be approximated as:

where $M_{lock,1}$ is the initial assembly locking torque, and $\eta$ is the torque retention coefficient (related to hardness and friction pair).

Reference values:

• HV ≥ 350: $\eta \approx 0.95 \sim 0.98$ (maintains about 80% ~ 90% after 5 uses)
• HV 300–350: $\eta \approx 0.85 \sim 0.90$ (may drop to 60% after 3 uses)
• HV < 300: $\eta$ is low and unstable; reuse is not recommended

3.2 Safety Factor Re-evaluation

Before each reuse, the locking safety factor should be verified:

decreases with wear. When , the washer must be replaced.

3.3 Visual Inspection Criteria

Before reuse, the following inspections must be performed. Failure to meet any item requires scrapping:

4. Other Factors Affecting Reuse Cycles

1. Hardness of Connected Parts
   Harder connected parts result in shallower tooth embedding and less wear on the washer teeth, allowing for increased reuse cycles. Soft materials like aluminum and magnesium accelerate tooth engagement wear.

2. Coating Type

3. Dacromet and zinc-nickel coatings have good wear resistance, protect the substrate, and improve reusability.

4. Pure lubricating coatings (e.g., MoS₂) may peel off after repeated compression and require more frequent inspection.

5. Tightening Method and Overload

6. The torque method, if over-tightened due to friction variations, increases single-use plastic deformation and shortens reuse life.

7. The angle method or yield point method provides more uniform preload and causes less damage to the washer.

8. Environmental Factors
   High temperatures may cause tempering and softening of the washer (hardness reduction); corrosive environments may create pitting, which acts as a fatigue initiation site.

9. Assembly Process

10. Each installation must ensure correct centering, wedge surfaces facing each other, and teeth perpendicular to the contact surface.

11. Misaligned installation can cause localized overload on the teeth, potentially damaging them in a single use.

5. Assessment Process

┌─────────────────┐
│ Inspect Washer After Disassembly │
└────────┬────────┘
│
┌──────────────▼──────────────┐
│ Teeth intact? Wedge surface undamaged? │
└──────────────┬──────────────┘
│ │
No │ │ Yes
▼ ▼
┌──────────┐ ┌──────────────────┐
│ Scrap & Replace │ │ Confirm Hardness ≥ HV350 │
└──────────┘ └────────┬─────────┘
│
┌──────────▼──────────┐
│ Evaluate Use Count n ≤ 5 │
└──────────┬──────────┘
│
┌──────────▼──────────┐
│ Calculate S_lock,n ≥ 1.5│
└──────────┬──────────┘
│ │
No │ │ Yes
▼ ▼
┌──────────┐ ┌──────────┐
│ Scrap & Replace │ │ Allow Reuse │
└──────────┘ └──────────┘

6. Design Recommendations

• Critical Connections: Strictly specify single-use washers; clearly state replacement requirements in maintenance manuals.
• Frequently Disassembled Structures: Select washers with HV 450 or above, and specify a maximum reuse limit (e.g., 5 times) in the maintenance schedule.
• Record Keeping: Implement batch management for reused washers, record the number of assembly/disassembly cycles, and avoid exceeding the service life.
• Failure Warning: In torque-angle monitoring systems, if a significant change in the tightening curve of the same connection is observed (e.g., a decrease in the slope of the locking segment), investigate for washer wear.

Summary:
The reusability of wedge-locking washers is dominated by hardness. Washers with HV ≥ 350, under strict inspection, can be reused 5 times. Before each use, visual and critical dimensional inspections must be performed, and the locking safety factor must be verified. Higher hardness, harder connected parts, and milder environments increase the potential for reuse.