Loctite - How To Prevent Threaded Assembly Failure

Our thanks to Loctite for allowing us to reprint the following article.

Threaded fasteners are the most common detachable hardware, used on thousands of products including automobiles, aircraft, household appliances, and industrial machinery. With more than 300 billion fasteners used annually in the U.S., it is crucial that these fasteners predictably and reliably maintain clamp force on the parts they join.

Why Threaded Assemblies Fail

Threaded fastener loosening due to vibration is the number one cause for catastrophic machinery failure. Such failure occurs when clamp load is not maintained. Gaps that naturally exist between the mating surfaces of threads directly impact the fastener's ability to stay tightly fixed. These tiny gaps allow side-to-side movement when exposed to vibration and thermal expansion or contraction. Side-to-side movement loosens the mated parts, reducing clamp load and ultimately causing the fastener to fail.

Mechanical locking devices were invented to solve the problem of loosening, but all designs have inherent flaws:

  • Split ring or spring washers are designed for increased friction, which reduces clamp loads. These mechanical locking devices are not reliable when exposed to dynamic loads.
  • Tooth or ribbed flange bolts are expensive and require large flange-bearing surfaces. They also can damage the surfaces of the mating parts.
  • Tab washers, split pins, and castle nuts are costly and time consuming to lock as they require that components be lined up appropriately before being set.
  • Nylon nuts increase friction, which results in inaccurate torque during assembly.

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