Why Bolt Failures Are Rarely About Strength—The Hidden Link to Loosening

Veröffentlicht: January 01, 2026

Kategorien: Neuigkeiten

When a critical bolt snaps on a heavy-duty machine, the gut reaction for most maintenance teams is to blame the material. We look at the broken pieces and assume the bolt wasn’t strong enough or that “metal fatigue” finally caught up with it. It’s a logical guess, but in the world of high-performance fastening, it’s often a mistake.

Bei Qewit, we’ve seen countless cases where switching to a thicker or higher-grade bolt didn’t stop the breakage. Why? Because the real culprit isn’t the tensile strength of the steel; it’s the fact that the bolt started to back off. In the fastening industry, we have a saying: “A bolt that doesn’t loosen, doesn’t break.” Once you understand that thread loosening is actually the root cause of almost every “fatigue” failure, your approach to equipment maintenance changes forever.

The Myth of Weak Steel

If you take a standard M20x80 high-strength bolt (Grade 8.8), it weighs next to nothing—maybe 0.2kg. Yet, its minimum tensile load is around 20 tons. That is roughly 100,000 times its own weight. In most real-world applications, that bolt is only holding a component that weighs a fraction of that capacity. Even with the dynamic forces of a running machine, you are rarely using more than 1% of the bolt’s actual breaking strength.

So, why does it still snap? It’s not because the load exceeded the bolt’s capacity. Instead, the failure happens because the connection lost its “tightness” first. When a bolt is properly pre-tensioned, it acts like a stiff spring holding two parts together. As long as that tension stays, the bolt and the parts move as one unit. But once the tension drops, even by just a slight extent, everything changes immediately.

Why Fatigue is Often a Misdiagnosis

When we analyze a broken bolt, the fracture surface often looks exactly like a classic fatigue failure. This leads engineers to spend weeks calculating vibration cycles and stress limits. However, laboratory tests tell a different story. In transverse vibration tests, a standard fastener can shake loose in as little as 100 cycles. Compare that to a fatigue test, which usually requires over a million cycles to cause a break.

This gap is huge. It means a bolt will loosen 10,000 times faster than it will fatigue. If a bolt is loose, it’s no longer doing its job. The parts it’s supposed to hold begin to shift, creating tiny gaps. This is where the real damage starts.

The Role of Kinetic Energy in Bolt Damage

Once a thread becomes loose, the bolt is no longer just a static fastener; it becomes a target for kinetic energy. Think of it this way: if you hold a hammer against a nail and push, nothing happens. But if you back the hammer up and swing it, the impact is massive.

A loose bolt allows for a “gap” (v²). When the machine vibrates or shifts, the bolt is suddenly hit by the moving mass of the equipment.

• For axial loads: The constant “hammering” effect rips the threads apart or stretches the bolt until it snaps under the impact.
• For radial loads: The bolt gets sheared off sideways, and you’ll often see the bolt holes getting beaten into an oval shape.

This isn’t a failure of material quality; it’s a failure of the locking mechanism. This is why Qewit focuses so heavily on precision-engineered fastening solutions. We know that if we can stop that initial turn of the nut, we stop the eventual break.

Why Oversizing Your Bolts Isn’t a Real Fix

When a bolt breaks, many people try to “over-engineer” the problem. They move from an M42 bolt to an M48, or they jump from Grade 8.8 to 12.9. Does this help? Sometimes, but it’s a bit of a brute-force tactic. A bigger bolt allows for more torque and higher friction, which might slow down the loosening process, but it doesn’t solve the vibration issue.

Take a hydraulic hammer as an example. These machines are essentially professional “bolt-breakers.” Some models use M42 bolts with over 100 tons of tensile strength each. Even with hundreds of tons of clamping force, the bolts still snap if the locking method isn’t right. Spending more money on massive bolts is just an expensive way to ignore the real problem: thread security.

Breaking Down How Threads Actually Work

To fix the problem, we have to look at how threads stay tight in the first place. Most bolt designs rely on the “self-locking” principle, where the friction between the male and female threads is supposed to be higher than the force trying to turn it. Under a static load, this works great.

But machines don’t sit still. Impact, heavy vibration, and heat cycles cause the friction in the threads to drop or even disappear for a split second. Once that friction hits zero, the bolt turns. At Qewit, our job is to find ways to limit that relative movement or make it physically impossible for the nut to rotate back.

Common Methods for Stopping the Spin

There are dozens of ways to keep a bolt from turning, and they generally fall into three buckets: friction, mechanical, and permanent.

Friction-Based Locking

This is the most common approach for everyday tasks.

1. Frühling Wascher: These provide a constant elastic force to keep the friction high. The sharp edges of the washer also try to “bite” into the surface to stop rotation.
2. Self-Locking Nuts: These usually have a non-circular end or a “crimped” top. When you screw it on, the nut has to deform slightly, creating a constant squeeze on the bolt threads.
3. Nylon Inserts: You’ve probably seen these—nuts with a blue or white plastic ring inside. The nylon increases the “drag” on the bolt, making it much harder for vibration to spin the nut off.

Mechanical Locking

If you want to be sure the nut isn’t going anywhere, you go mechanical.

• Split Pins and Slotted Nuts: You put a pin through a hole in the bolt and a slot in the nut. The nut physically cannot turn unless the pin shears off.
• Tab Washers: After tightening, you bend a metal tab up against the side of the nut. It’s simple, effective, and very visual—you can tell at a glance if it’s locked.
• Safety Wire: You’ll see this a lot in aerospace. A wire connects multiple bolt heads together so that if one tries to loosen, it actually pulls the other one tighter.
  

Permanent Locking

Sometimes, you don’t ever want that bolt to come off. In these cases, people use spot welding, riveting, or high-strength thread lockers (liquid adhesives). These are great for security, but they usually mean you have to destroy the bolt if you ever need to take the machine apart for repairs.

Conclusion: Prevention is Cheaper than Repair

At the end of the day, a bolt is only as good as its ability to stay tight. If you are dealing with recurring breakages, stop looking at the steel grade for a minute and start looking at your anti-loosening strategy. Whether it’s a simple spring washer or a high-tech locking nut from the Qewit catalog, the goal is the same: keep the tension, stop the movement, and prevent the break.

If you’re struggling with fasteners that won’t stay put, check out our full range of solutions at Qewit Fasteners. We’ve spent years figuring out how to keep things together so you don’t have to keep fixing them.

FAQ (häufig gestellte Fragen)

Q1: If I use a higher-grade bolt (like 12.9), will it stop my bolts from breaking?

A: Not necessarily. While a higher grade is stronger, it is also more brittle. If the root cause of your failure is the bolt coming loose and then getting hit with impact (kinetic energy), a “stronger” bolt might actually snap sooner than a more flexible lower-grade one. You need to solve the loosening first.

Q2: Can I reuse a bolt after it has come loose?

A: It depends on how loose it got. If the bolt was rattling around, the threads are likely damaged or the bolt may have been stretched beyond its “elastic limit.” It’s usually much safer and cheaper to replace the fastener than to risk a second failure.

Q3: Is thread-locking fluid better than a mechanical lock?

A: Each has its place. Adhesives are great for sealing against liquids and handling fine vibrations. However, in high-heat environments or where you need to perform regular maintenance, a mechanical lock (like a tab washer or a slotted nut) is often more reliable and easier to work with.