“The twistlock must have bent due to metal fatigue,” the customer said confidently. Some years ago, I received a damage report from a customer using a Kalmar reachstacker that was less than a year old.
Now having worked in sales and workshops for years, I was used to these kinds of discussions whether it was damage, wear, or a manufacturing issue? To make things more tangible, I decided to take a loose twistlock with me to the site, along with its material certificate. That visit became a perfect opportunity to explain what a twistlock really is, how it’s made, and why it’s such a crucial (and costly) part of a reachstacker spreader.
Turns out that, unfortunately for the customer, the twistlock was bended due to a mispick with the spreader.
How a spreader twistlock is made
A twistlock starts its life as a forged steel component. Forging, unlike casting, compacts the steel’s grain structure, giving it exceptional strength and fatigue resistance. The steel alloy used is heat-treated for a perfect balance of hardness and ductility.
After forging, precision machining shapes the locking head and stem with tolerances often below 0.05 mm. Every contour matters, as it determines how smoothly the twistlock can engage and disengage with a container’s corner casting.
Surface treatment follows: shot blasting, zinc or manganese phosphate coating, and corrosion protection paint prepare the twistlock for harsh maritime environments. Each unit is then laser-marked with a serial number and accompanied by a traceable certificate a document linking it to the steel batch, heat treatment data, and inspection results.
Testing and certification
Before a twistlock enters service, it undergoes both static and dynamic testing to ensure it can handle extreme forces safely:
- Static load test: Verifies the ultimate breaking strength. A single twistlock on a reachstacker spreader can withstand up to 45–50 tonnes in tension before permanent deformation.
- Dynamic fatigue test: Simulates tens of thousands of loading cycles to ensure durability over years of heavy use.
- Non-destructive inspection: Ultrasonic and magnetic tests detect micro-cracks invisible to the naked eye.
Only after passing all these checks is the twistlock approved for use and shipped with its certificate of conformity.
Why a Twistlock Doesn’t Just Bend
A genuine twistlock is not designed to flex like a spring it’s engineered to resist bending entirely within its operating range. The combination of forged steel, heat treatment, and geometric strength ensures it can withstand immense forces without deformation.
If a twistlock appears bent, it’s almost always the result of forces far beyond its design limits, such as:
- A container lifted while still locked to a container chassis
- Misalignment of the spreader and container, or
- Shock loads during improper handling.
So when someone says, “It must have bent from metal fatigue,” it’s worth remembering: twistlocks don’t get tired: they get overloaded.
A costly part for good reason
Though small, a twistlock carries an enormous responsibility. It’s the only point of contact between a 45-tonne container and the spreader lifting it several meters above the ground. A failure here could mean serious safety risks and costly downtime.
That’s why genuine, certified twistlocks are expensive because they are precision-engineered safety components, not just “pieces of steel.”
At Heavy Cargo Lifters, we believe in sharing knowledge as much as supplying parts. Understanding how a component works helps operators and maintenance teams appreciate why quality matters and why investing in certified components always pays off in safety and uptime.
