Anti-loosening Types Of Bolts And Nuts & Anti-loosening Principle Of Self-locking Nuts

Loosening of bolt and nut connections is a common failure problem in mechanical engineering, which has always been a hot research topic in the fastener industry. Based on practical assembly experience and actual working conditions, this document sorts out mainstream anti-loosening structures for bolts and nuts, elaborates on the working principle, applicable scenarios and inherent defects of each anti-loosening method. Meanwhile, it explains the core mechanism of long-term anti-loosening performance of self-locking nuts, providing reliable references for on-site fastener selection and assembly process formulation.

1. Common Anti-loosening Types and Principles of Bolts and Nuts

1.1 Double Nut Anti-loosening (Butt Nut Anti-loosening)

Also known as butt nut anti-loosening. After two nuts are tightened against each other reversely, continuous axial compression force will be generated between them and act on the meshed thread contact surfaces. The tighter the two nuts are locked, the greater the normal pressure on thread contact surfaces, as well as the friction torque between thread pairs. Any rotational loosening of the nuts must overcome the large friction force between meshed threads. Even if external load fluctuates during equipment operation, the jacking pressure between two nuts remains constant, realizing sustainable anti-loosening effect.

Applicable Scenarios: Suitable for bolt connections with or without preload requirements, only applicable to working conditions with slight vibration and stable load.

Disadvantages: Occupies large axial installation space, limited anti-loosening capacity, and fails easily under severe vibration conditions.

1.2 Hard Lock Concave-convex Anti-loosening Nut

The Hard Lock anti-loosening nut consists of a lower convex nut and an upper concave nut for matched use. The lower convex nut adopts eccentric machining with slight center offset to play the role of a wedge block; the upper concave nut is processed into a standard concentric circle without center deviation. The joint surfaces of upper and lower nuts are all tapered concave-convex surfaces, forming a mechanical self-locking structure similar to hammer wedging after tightening.

A small axial preload can generate huge radial locking pressure on tapered joint surfaces. The pressure is transmitted to meshed threads synchronously, greatly increasing friction torque on both thread contact surfaces and concave-convex tapered surfaces. It locks the thread rotation tendency mechanically and delivers excellent anti-loosening performance.

Applicable Scenarios: Applicable to bolt connections with or without preload requirements, and can be used for long-term service under harsh working conditions with high-frequency severe vibration and alternating loads.

Disadvantages: Requires extremely high machining precision for eccentric structure, difficult production process and high procurement cost.

1.3 Spiralock Anti-loosening Nut

The core structural feature of Spiralock nuts is a 30° wedge-shaped slope machined at the root of internal threads. When bolts and nuts are tightened and meshed, the tip of bolt threads closely presses against the wedge-shaped slope of internal threads to form high-strength rigid locking force.

For standard ordinary threads, the included angle between normal contact force on thread surfaces and the bolt axis is 30°. Spiralock threads change the thread profile angle, making the included angle reach 60°. The normal pressure of thread pairs is far greater than axial fastening pressure, which greatly improves friction resistance and thoroughly restrains thread rotational loosening.

Applicable Scenarios: Only applicable to connection structures requiring stable preload, and the connected parts shall not be made of excessively soft materials.

Disadvantages: The anti-loosening effect will fail completely once bolt preload decreases; larger tightening torque is required to overcome friction resistance between thread teeth to reach the required bolt preload.

1.4 Split Spring Washer Anti-loosening

After the nut is tightened to flatten the split spring washer, the washer generates continuous elastic rebound force to keep internal and external threads closely fitted. Friction torque formed by constant contact force prevents nut rotation. Meanwhile, sharp cutting edges at the split opening embed into the bottom surface of the nut and the surface of connected parts respectively, further preventing relative rotation of nuts through physical locking.

Application Limitations: Not suitable for connections with ultra-hard joint surfaces; the cutting edges cannot embed into hard substrates and lose anti-loosening function completely. It is also not applicable to connections requiring high preload, as it will accelerate preload attenuation of bolts.

1.5 Conical Spring Washer Anti-loosening

It shares the same anti-loosening principle as ordinary split spring washers, relying on elastic rebound force after compression to compress thread pairs and realize friction anti-loosening. Compared with ordinary split spring washers, conical spring washers have higher structural stiffness, and can provide larger axial pressure under the same compression stroke, delivering better anti-loosening stability and performance.

Application Limitations: Still not suitable for high-precision connection parts with strict preload requirements.

1.6 Nord-Lock Dual-layer Self-locking Washer

This kind of washer is used in pairs, with large-angle inclined teeth on one side and radial anti-slip serrations on the other side. During assembly, two washers are installed reversely with inclined tooth surfaces facing each other. After tightening, the outer radial serrations bite tightly on the nut end face and connected part surface to fix the outer contact surfaces completely, leaving only relative sliding between the inner inclined tooth surfaces of two washers.

When bolts tend to loosen and rotate, the wedge jacking effect between inner inclined tooth surfaces will generate axial lifting force. The lifting distance generated by washers is larger than the axial lifting distance caused by thread loosening, which reversely locks the loosening tendency of threads and achieves permanent anti-loosening effect.

Application Limitations: Cannot be used for ultra-hard or ultra-soft joint surfaces. The washer has specific installation direction, and loses anti-loosening function if installed reversely. It is also invalid for loose connections without preload.

2. Core Principle of Non-loosening Self-locking Nuts

The long-term anti-loosening performance of self-locking nuts originates from specially optimized non-standard thread structure.

A 30° wedge-shaped slope is processed at the root of internal threads of self-locking nuts. After meshing and tightening, bolt thread tips press rigidly against the wedge-shaped slopes to form high-strength mechanical locking force. For standard 60° V-shaped ordinary threads, the included angle between normal contact force and bolt axis is 30°, while the included angle changes to 60° for self-locking threads, leading to fundamental change of stress state.

Mechanical Force Comparison: Under the same axial bolt tension P0, the normal contact force of standard ordinary threads is only 1.15P0, while that of self-locking threads reaches 2P0. The force ratio of the two is about 7:12, which greatly improves the anti-loosening friction force of thread pairs.

In addition, self-locking threads solve the inherent uneven stress defect of ordinary threads. For conventional 60° V-shaped threads, the first and second engaged threads bear 70%~80% of the total load, while the remaining threads bear almost no load. Under long-term vibration and alternating loads, the front threads are prone to overload failure, resulting in rapid attenuation of locking force, thread rotation loosening, even thread seizure and stripping.

Self-locking threads distribute load evenly on all engaged thread turns via wedge-shaped slopes, realizing synchronous stress of all threads. It avoids local thread overload failure, eliminates bolt loosening caused by vibration fundamentally, and achieves maintenance-free long-term anti-loosening effect.