Why Do Bolts Get Tighter The More You Tighten Them?

In mechanical engineering, bolt failure is a common issue, and its causes are typically analyzed from four main aspects: the quality of the bolt, the preload torque, the strength of the bolt, and the fatigue strength of the bolt. However, in most cases, bolt failure is not caused by insufficient strength or fatigue capacity but rather by loosening, which leads to damage.

1. Bolt Failure Is Not Due to Tensile Strength

Taking an M20×80 8.8-grade high-strength bolt as an example, it weighs only 0.2 kilograms, yet its minimum tensile load is 20 tons—100,000 times its own weight. In practice, this bolt is typically used to fasten components that only weigh around 20 kilograms, using only a fraction (about 0.001%) of its tensile capacity. Therefore, the bolt’s tensile strength is more than sufficient, and failure is not caused by the bolt's strength.

2. Bolt Failure Is Not Due to Fatigue Strength

In lateral vibration tests, threaded fasteners can loosen after fewer than 100 cycles. However, in fatigue tests, they need to endure millions of cycles. In other words, the fastener loosens after experiencing only a tiny fraction (1/10,000) of its fatigue strength. Thus, the loosening of the bolt is not a result of insufficient fatigue strength.

3. The True Cause of Bolt Failure: Loosening

When a bolt loosens, it generates significant kinetic energy (mv²), which directly impacts the fastener and its connected components, causing damage. Once the fastener fails, the equipment cannot function properly, leading to further damage. If the fastener is subjected to axial force, the threads may be damaged, causing the bolt to break. When subjected to radial forces, the bolt may shear off, and the bolt hole may deform into an oval shape.

4. The Key to Solving the Problem: Effective Anti-Loosening Measures

For example, in the GT80 hydraulic hammer, the side plate bolts are M42 10.9-grade bolts with a tensile strength of 110 tons each, and the preload is set at 50% of the tensile strength (around 300–400 tons). Despite this, the bolts still fail. The solution is not simply increasing the bolt diameter or strength but rather improving anti-loosening measures. The root cause of bolt failure is not the bolt's strength but the inability to prevent loosening. In summary, bolts are: “If they don’t loosen, they won’t break; but once they loosen, they will break.”

Analysis of Bolt Loosening Causes

The loosening of bolts is generally due to a reduction or loss of the frictional force between the threads. Under static load, the friction between the threaded pair ensures that the bolt stays tightened. However, under dynamic conditions such as shock, vibration, variable loads, or significant temperature changes, the frictional force can decrease or even disappear, causing the bolt to loosen.

Common Anti-Loosening Methods

There are three primary methods for preventing bolt loosening: friction-based, mechanical, and permanent anti-loosening methods.

1. Friction-Based Anti-Loosening Methods

1. Spring Washer Anti-Loosening: A spring washer generates continuous spring force after being compressed, maintaining friction between the nut and the bolt threads, which prevents the nut from loosening.

2. Jam Nut (Double Nut) Anti-Loosening: Two nuts are used to increase friction and prevent loosening.

3. Self-Locking Nut Anti-Loosening: The nut is designed with a non-circular or slotted opening. When tightened, the opening expands and applies additional force to compress the threads.

4. Elastic Ring Nut Anti-Loosening: An elastic ring inside the nut provides additional friction to prevent loosening.

2. Mechanical Anti-Loosening Methods

1. Slotted Nut and Cotter Pin Anti-Loosening: A slotted nut or cotter pin is used to prevent loosening.

2. Locking Washers: After tightening the nut, a single- or double-ear locking washer is bent against the nut and the connected part to prevent loosening.

3. Wire Locking Anti-Loosening: A low-carbon steel wire is inserted through holes in the heads of bolts and linked together to prevent loosening by mechanical interlocking.

3. Permanent Anti-Loosening Methods

1. Spot Welding: The nut or bolt is spot-welded to prevent loosening.

2. Riveting: Rivets are used to permanently fix the bolt and connected parts.

3. Adhesive Anti-Loosening: Anti-loosening adhesive is applied to the bolt threads, which cures and prevents loosening once the nut is tightened.

Summary

Bolt loosening and the resulting failure are common issues in mechanical systems, typically caused by dynamic forces and not due to the bolt’s tensile or fatigue strength. The key to preventing bolt failure lies in using effective anti-loosening methods such as friction-based, mechanical, or permanent solutions. Proper anti-loosening measures can significantly improve equipment stability and avoid failures caused by bolt loosening and subsequent damage.