L9 bolts -- designed for work environments with extreme stresses and strains -- have ratings in four areas, including tensile strength, proof load, yield strength and hardness. These values vary depending on the bolt’s diameter and number of threads per inch.
Tensile strength refers to the pulling force required to break a bolt. An L9 bolt with a quarter-inch diameter and 20 threads per inch, or a ¼-20, has a tensile strength of 5,720 lbs. A 1-inch, eight-thread L9 bolt has a tensile strength of 109,080 lbs.
The proof load means the force required to deform or strip the threads. A ¼-20 L9 bolt has a proof load of 4,610 lbs., while a 1-inch, eight-thread bolt has a proof load of 87,870 lbs.
Yield strength refers to the force required to permanently deform, without breaking, the bolt. The yield strength of an L9 bolt equals approximately 86 percent of its tensile strength.
A Rockwell hardness value indicates the brittleness of the bolt. L9 bolts are heat-treated to a Rockwell hardness of R/C 38-42.
Greg Epperly retired in 2002 from the United States Army after 20 years in calibration electronics. He has been writing professionally since 1985. He has written various calibration and electronics training materials, and numerous operational and quality-control procedures. Epperly has a Bachelor of Science in management studies from the University of Maryland.