Why is it not recommended to use spring washers for anti-loosening in high-strength bolts

In bolted connections that carry real load — machinery, structural steel and process lines where flange gaskets are squeezed to make the seal — high-strength bolts of grade 8.8 and above hold everything together, and their resistance to self-loosening decides how long the joint stays safe. Yet many engineers still fit spring washers as a locking device. On grade 8.8 and above that habit is not only ineffective; it can become a safety risk.

Why Spring Washers Fail on High-Strength Bolts

A spring washer works by elastic deformation: it pushes back along the bolt axis and adds friction between the thread flanks. In light, low-torque joints this is simple, cheap and reasonably effective. On high-strength bolts the design runs out of capacity:

• Elastic limit exceeded — installation torque on high-strength bolts is usually above 200 N·m, far beyond what an ordinary spring washer can take. The washer flattens, loses its elastic recovery and stops resisting rotation.
• Stress concentration — the sharp edges of the washer bite into the contact surface under high stress, which can leave micro-cracks and cut fatigue strength.
• Standard provisions — industry rules such as JG/T 5057 state clearly that spring washers are not recommended for anti-loosening on high-strength bolts of grade 8.8 and above.

Spring washer under load: above 200 N·m of installation torque the washer flattens and gives up its spring-back.

Self-Locking Preload and the Double-Nut Method

High-strength bolts are designed with a strong anti-loosening foundation, so the first locking element is the preload itself.

• A high preload clamps the joint faces with a force far above ordinary bolts, and the resulting static friction resists vibration and lateral slip in most service conditions.
• High-strength material and accurate thread geometry keep the friction coefficient of the thread pair stable, which lowers the chance of spontaneous loosening.

Where the duty is severe — continuous vibration, impact loads or wide temperature swings — the double-nut arrangement is a proven active method. Tighten the lower nut to the specified torque, then fit the upper nut and tighten it against the lower one. The two nuts load each other and lock the threads axially, blocking rotational loosening. The same logic applies to bolted joints on pipe flanges, where the clamping force has to survive thermal cycling.

• No special parts are required; standard nuts are enough.
• Under dynamic loads it outperforms a single nut with washers.
• Easy to disassemble, reusable and cheap to maintain.

Table 1. Typical behaviour of anti-loosening arrangements on high-strength bolts — confirm the actual torque for every joint size.

Single nut versus double nuts on a high-strength bolt.

Selection, Torque and Inspection in Practice

Three habits decide whether a joint keeps its preload through the service life.

• Select by duty — bolt grade and locking method should follow the load type, vibration level and environment. For static or lightly vibrating loads the standard preload of high-strength bolts is enough.
• Control the tightening process — use a torque wrench or a hydraulic tensioner so the preload reaches the design value. With the double-nut method follow the sequence tighten first, lock second, and never over-tighten: excess torque damages threads.
• Inspect on schedule — even a proven locking method loses preload over time, so check critical bolts during maintenance rounds and re-torque them one at a time.

Frequently Asked Questions

Technical answers to the questions that come up most often when locking high-strength bolted joints.

Why do spring washers stop working on grade 8.8 bolts?

Installation torque is usually above 200 N·m, which is past the elastic limit of an ordinary spring washer. The washer flattens, loses its spring-back and provides no further locking.

What anti-loosening method suits high-strength bolts?

Rely on the high preload first: it creates clamping force and static friction between the joint faces. For severe vibration, impact loads or temperature swings, add the double-nut method.

How is the double-nut method tightened?

Tighten the lower nut to the specified torque with a torque wrench or a hydraulic tensioner, then tighten the upper nut against it so the two nuts load each other and lock the threads.

When should bolt preload be checked?

During maintenance cycles. Critical connections should be inspected and re-torqued even when a proven locking method is used, because preload always drops with time and thermal cycling.