HPC/AI Servers: Anti-Loosening Fastener Design under Fan Vibration and Transport Shock
Executive Summary
High-frequency micro-vibration from fans plus whole-machine transport shock are the two exams server fasteners must pass. Based on the YF Zhichengjia HPC/AI-server handbook, this article explains the working principles of double-lock washers and single-face serrated conical washers, pre-applied adhesive anti-loosening, fatigue strength and IP68 sealing reliability design.
Table of Contents
Two Vibration Sources inside a Server
AI servers in data centers run almost year-round without shutdown, and fasteners face two fundamentally different kinds of vibration. One is fan rotational vibration: multi-kilowatt power corresponds to multiple high-speed fan sets, and the chassis sheet metal, hard-drive carriers and accelerator-card pressure plates continuously bear high-frequency micro-vibration. The other is transport and loading vibration shock: the jolting of the whole machine from factory to data center and the bumping of rack insertion form a low-frequency, large-amplitude shock spectrum. Following the YF Zhichengjia HPC/AI-server handbook, anti-loosening design must account for these two working conditions separately.
Mechanism of Vibrational Loosening and Graded Countermeasures
The essence of threaded-joint loosening is transverse micro-slip: when relative displacement perpendicular to the bolt axis occurs between clamped parts, friction at the thread pair and bearing surface is repeatedly overcome, and the bolt gradually backs out as if "unscrewed." Countermeasures increase in strength roughly across four levels: ordinary flat washers (only preventing scratching) → spring washers (limited effect) → serrated/double lock washers and pre-applied adhesive (targeting transverse vibration) → prevailing-torque nuts (GB/T 3098.9, the most reliable). The handbook's standard practice for server screws is that the bolt achieves anti-loosening through combined washers and pre-applied thread-locking adhesive (chemical and physical); the two together cover most fan-vibration conditions.
| Vibration Condition | Characteristics | Recommended Anti-Loosening Solution |
|---|---|---|
| Continuous high-frequency fan micro-vibration | Transverse micro-motion, temperature cycling | Pre-applied thread adhesive + serrated washer |
| Hard-drive / accelerator-card pressure plates | Alternating load, clearance prone | Single-face serrated conical lock washer |
| Large transverse dynamic load positions | Large relative displacement | Double lock washer (removal torque > installation torque) |
| Cabinet sheet-metal splicing | Vibration + sealing requirement | Pre-applied rivet nuts/rivets, IP68 |
| Critical load-bearing joints | No loosening allowed | Prevailing-torque lock nuts (GB/T 3098.9) |
Working Principles of Two Kinds of Lock Washer
The description of lock-washer principles in the low-altitude-economy and HPC handbooks can be applied directly to server vibration design:
- Double lock washer: used where transverse dynamic load is large. It exploits the principle that the lift angle of the two contacting washer faces exceeds the thread lead angle, achieving "removal torque greater than installation torque"—to unscrew it, you must first overcome greater resistance than when tightening, thus locking the loosening tendency.
- Single-face serrated conical lock washer: used at positions with vibration and alternating load. The conical structure provides clearance compensation—after creep and settlement of the clamped parts, the washer can still spring back to supplement pressure; the serrated face adds anti-loosening through increased contact friction.
Fatigue Strength and Preload Decay
Anti-loosening does not equal no fatigue. Under alternating external load, stress amplitude is the key determinant of bolt life: the more fully the joint is clamped and the less the interface slips, the smaller the actual stress amplitude the bolt bears. The handbook's logic of post-heat-treatment thread rolling for aluminum bolts is also worth borrowing for server fasteners—forming compressive residual stress in thread roots reduces notch effect and raises fatigue strength. High-strength carbon-steel screws (10.9 class and above) must watch for plating hydrogen embrittlement and must be hydrogen-baked per specification after surface treatment.
Transport Vibration: Flat Rivets and Cabinet Flatness
Server sheet-metal cabinets must bear stacking and jolting during transport. The handbook specifically mentions flat (countersunk) rivets for sheet-metal cabinets with high flatness requirements: after countersunk riveting the panel is flush, not interfering with rail insertion or door fitting. At positions needing water and dust protection, sealed self-clinching studs, pre-applied rivet nuts and pre-applied rivets achieve IP68 sealing—this line of defense blocks dust in transit and condensation in the data center.
Reliability Verification Checklist
- Calibrate the tightening process per GB/T 16823.2 and record torque-clamp-force curves.
- Fan-position screws are 100% pre-coated with anti-loosening adhesive; double lock washers added at transverse-vibration positions.
- Critical load-bearing joints use prevailing-torque nuts per GB/T 3098.9, retaining locking-torque data.
- Post-transport sampling: re-tighten so torque decay does not exceed the design allowable upper limit.
- IP68 positions undergo leakage sampling per the sealing grade.
Server reliability is designed and verified. Drawing the two load spectra of "fan micro-vibration plus transport shock," then selecting parts against the four-level anti-loosening countermeasure table, keeps the fastener link from becoming the weak point of whole-machine reliability.
Conclusion
Reliability design against vibration in servers is really a battle against preload decay. Fans generate continuous high-frequency micro-vibration that loosens joints by relative rotation between threads, while transport shock applies single high-amplitude events that can cause permanent settling. The countermeasures stack up: under-head serrations or pre-applied adhesive resist rotational loosening, spring or tab washers compensate for axial relaxation, and correct torque control keeps preload in a band that is high enough to stay clamped yet low enough to avoid fatigue. Fatigue strength is often overlooked in this context: a bolt tightened too close to its yield point accumulates fatigue damage over millions of cycles and eventually fractures, which is worse than a loose joint. Transport vibration is handled differently—flat rivets and reinforced cabinet mounts maintain flatness and resonance margin so the chassis does not amplify road input into the boards. Validation should combine random-vibration profiling, shock testing, and preload-relaxation measurement over time, not a one-time static torque check. A mature server fastener program treats loosening, fatigue, and transport as three parallel failure modes and designs countermeasures for each.
The core of HPC/AI-server vibration and reliability design is preload retention: pre-applied adhesive and serrated washers resist transverse micro-vibration, conical washers compensate long-term relaxation, and flat rivets plus IP68 sealing withstand transport and environment. YF Zhichengjia, founded in Shenzhen in 2005, is a fastener manufacturer serving the HPC/AI-server industry with anti-loosening screws, lock washers and sealed joints. For verification methods and selection material see the technical document library; reliability issues can be analyzed by contacting our technical engineers for failure analysis.
