Preload Decay of Bolted Connections — Causes and Solutions: Diamond Friction Shims to Stabilize Bolt Preload

I. Phenomenon and Hazards of Preload Decay

Bolted connection is the most widely adopted connection method in industrial equipment, and its reliability depends on the stability of bolt preload. Nevertheless, under working conditions such as vibration, shock, alternating load and temperature variation, bolt preload gradually decays over time, resulting in connection loosening, seal failure, accuracy degradation and other problems. In severe cases, equipment breakdown and safety accidents may be triggered.

Preload decay is a common engineering problem. Especially in high-end equipment sectors including wind power, robotics, automotive and aerospace, the preload stability of critical connections directly affects the overall performance and safety of complete machines. Conventional solutions mainly rely on periodic retightening and anti-loosening components, yet they deliver limited effectiveness with high maintenance costs.

II. Root Causes of Preload Decay

1. Interfere Micro-slip This constitutes the primary cause of preload decay. Under vibration and alternating loads, micron-scale relative slip occurs between the two contact surfaces of bolted joints. The slip per cycle is tiny, yet it accumulates continuously under cyclic loads and gradually reduces bolt preload. Ordinary flat washers feature a low friction coefficient (0.1–0.2) and can hardly resist such micro-slip.
2. Embedment Effect After bolt tightening, microscopic asperities on contact surfaces undergo plastic deformation and flattening under pressure, shortening bolt elongation and reducing preload. The embedment effect is particularly prominent in the early stage of connection, normally causing 10–30% loss of preload.
3. Vibration-induced Loosening Under sustained vibration, relative rotation may take place between bolt threads and nuts, leading to nut loosening. Transverse vibration is the main trigger for bolt loosening. When transverse force exceeds the maximum static friction of contact surfaces, slip emerges and changes the stress state of thread pairs, which may cause nut rotation.
4. Impact of Thermal Cycles Temperature variation creates differential thermal expansion and contraction between bolts and clamped members and triggers preload fluctuation. High temperature further induces material creep and stress relaxation and aggravates preload decay.
5. Material Creep and Stress Relaxation Under long-term high temperature and constant load, bolt materials creep while stress relaxation appears on the surface of clamped components, bringing about slow preload reduction.

III. Limitations of Traditional Anti-loosening Methods

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IV. Solution by Diamond Friction Shims

DAOLER® Diamond Friction Shims adopt the Diamond Claw® micro-engagement technology to fundamentally solve preload decay.

1. Suppress Interfacial Micro-slip Diamond micro-particles on the shim surface "bite into" the mating surface under preload and form microscopic mechanical interlock. This interlocking mechanism physically eliminates relative slip between contact surfaces and serves as the most effective approach to tackle preload decay.
2. High Friction Coefficient Static friction coefficient ≥0.6, 3–5 times that of ordinary flat washers. A high friction coefficient enables the connection to withstand larger transverse loads at the same preload and resist slip.
3. Mitigate Embedment Effect Diamond particles have extremely high hardness (Mohs hardness 10) and barely produce plastic deformation under preload, which greatly cuts preload loss caused by embedment effect.
4. Anti-vibration Loosening The combination of high friction coefficient and micro interlocking mechanism maintains connection stability under vibration and effectively prevents nut loosening.
5. Stable Performance in Wide Temperature Range Stable performance from -40℃ to 150℃, minimizing the influence of thermal cycles on preload.
6. Direct Replacement & Upgrade Ultra-thin specification starting from 0.1 mm. It can directly replace original flat washers without modifying connection structure or bolt specifications.

V. Application Effects & Test Data

After applying diamond friction shims in various critical connections, preload decay is significantly improved:

• Preload decay rate reduced by 60–70%
• Retightening cycle extended by 2–3 times (from 6 months to 12–18 months)
• Preload retention raised from 60% to over 90% after vibration test
• Improved connection stiffness and reduced overall machine vibration
• Extended bolt fatigue life, lower risk of bolt fracture
• Less downtime for maintenance and higher equipment operation efficiency

VI. Typical Application Scenarios

1. Wind Turbine Tower Flange Sustained wind load and vibration lead to preload decay. Diamond friction shims extend retightening cycles and cut downtime maintenance.
2. Reducer on Robot Joint Frequent start-stop and alternating shock cause flange surface slip. Diamond friction shims stabilize preload and improve positioning accuracy.
3. Automotive Chassis & Powertrain Engine vibration and road shock induce connection loosening. Diamond friction shims enhance connection reliability.
4. Railway Bogie Continuous vibration and impact loads. Diamond friction shims guarantee long-term stability of critical connections.
5. Aerospace Structural Components Extreme thermal cycles and high-vibration environment. Diamond friction shims deliver highly reliable connections.

VII. Selection & Installation Guidelines

Selection Tips

• Select specifications according to load and vibration grade of connection positions
• Verify temperature resistance for high-temperature environments
• Prioritize ultra-thin specifications for precision connections
• Conduct compatibility test with mating surface materials
• Custom non-standard dimensions available

Installation Guidelines

• Clean contact surfaces to remove oil contamination and rust before installation
• Place friction shim between the two mating surfaces
• Tighten bolts following specified torque and sequence
• No thread-locking adhesive or other anti-loosening parts required
• Perform one retightening 24 hours after initial installation, then inspect periodically as scheduled

VIII. Conclusion

Preload decay of bolted connections is a critical issue affecting equipment reliability, and its root causes lie in interfacial micro-slip and embedment effect. Centered on DAOLER® Diamond Claw® interlock technology, DAOLER® Diamond Friction Shims feature a static friction coefficient ≥0.6 and micro mechanical interlocking mechanism. It fundamentally restrains interfacial slip, greatly slows preload decay, and provides an independently controllable high-reliability solution for critical connections across industries.

DJKJ (Shandong) Co., Ltd. specializes in superhard friction materials. We can provide customized selection plans and technical support according to actual working conditions. Clients from all industries are welcome for communication and cooperation to jointly advance the development and application of connection technology.