Defective Clevis Pin Culprit in Runaway Trailer


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    • Cranes & RIgging
    • Failure Analysis
    • Fires & Explosions
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    • Machinery Damage & Assessment

A large agricultural trailer had been connected to a truck using a clevis pin with a spring locking clip.  The trailer became disconnected from the truck and collided with an oncoming vehicle.  The trailer was in poor condition, did not have safety chains, and had substantial recent modifications by the owner.

The truck driver was insistent that she had properly installed the clevis pin and locking pin.  An investigation concerning the trailer and the means that it became disconnected was conducted.  Despite the condition of the trailer, it should have remained attached to the truck unless the clevis pin was not properly installed or was defective.

The clevis pin and spring clip were missing after the incident, however they had been recently purchased and the trailer owner had several spares from the same box. Examination of these other clevis pins revealed they were an appropriate size and strength for use with the trailer.  The spring clips were nickel plated steel and locked properly on the clevis pins the first time they were used.

A clevis pin with the spring clip attached.

A clevis pin with the spring clip attached.

Repeated installations of the spring clips on the clevis pins revealed that most of them either fractured forcibly or became too weak to lock onto the clevis pin after five installations.  Several of the spring clips failed as soon as the second installation.

Laboratory evaluation of the spring clips included optical and scanning electron microscopy.

Scanning electron microscope (SEM) photographs showing the metal structure.

Scanning electron microscope (SEM) photographs showing the metal structure.

This evaluation determined that the spring clips were failing due to very low cycle fatigue cracking that was caused by hydrogen embrittlement of the steel.  Instead of being ductile and deflecting within its elastic limits, the steel was brittle and cracked when the spring clips were opened to install them on the clevis pins.

Hydrogen embrittlement of the base metal is a common result of metal plating processes.  Metal plating can improve the corrosion resistance, appearance, and wear resistance of a component but can lead to decreased fatigue life if proper pretreatment and plating procedures are not used.  Hydrogen from plating can enter the base metal from the exterior and become trapped in the base metal.  Pretreatment and plating procedures can be designed to minimize this condition by limiting the amount of hydrogen available to enter the base metal or removing it following the plating.

A fractured spring clip pictured with an intact clip.

A fractured spring clip pictured with an intact clip.

In this case, it was determined that poor manufacturing technique had produced at least one box of clevis pins with defective spring clips.  The probable cause of the trailer separation was a defective clevis pin spring clip.

John Phillips, senior consulting engineer at Warren, has more than 30 years of crane and heavy equipment experience and more than 20 years of experience in forensic engineering.  A licensed professional engineer in South Carolina, North Carolina, Georgia, Louisiana and Ohio, he’s NCEES registered both as a model engineer and with The United States Council for International Engineering Practice, USCIEP. John has designed crane systems, supervised installation, tested and certified lifting equipment even serving as a project engineer for maintenance and certification of nuclear weapon lifting and handling systems. He is a certified fire and explosion investigator and fire and explosion investigator instructor by the National Association of Fire Investigators. John is a member of the American Society of Materials and American Society of Testing and Materials, as well as a voting member of ASTM Ships & Marine Forensic Sciences, Forensic Engineering, and Performance of Buildings committees.

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