Substandard threaded rod resulting in dangerous installations

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Manufacturers of threaded rod are warning contractors and engineers that sub-standard, defective and counterfeit threaded rod is being imported into Canada. Widely used to support piping, ducts and equipment, this non-standard threaded rod can result in unsafe installations. “The problem is much more prevalent in Canada than in the U.S., reported Salim Brahimi, director of engineering & technology for the Industrial Fasteners Institute (IFI), Independence, Ohio.

“These products are sold as “A307” threaded rod. However, in many cases they do not meet the requirements of ASTM A307, raising concerns about product safety and unfair competition.”

“Some of this substandard threaded rod is being supplied with falsified paperwork, claiming A307 compliance,” added Don Lockard, general manager/engineer, for National Socket Screw Mfg. Ltd., a threaded rod manufacturer based in Beamsville, Ont.

Sloppy fit

It’s usually easy to spot counterfeit threaded rod and not just by it’s lower price. Typically the nuts are a loose or sloppy fit because of a reduced thread pitch, said Brahimi.

Threaded rod, made in accordance with ASTM A307, has a minimum tensile strength of 60,000 psi. They are typically made of low carbon steel and not heat-treated.

Dimensional characteristics of ASTM A307 threaded rods are given in ASME B18.31.3. The thread form is Unified Inch with the standard 60 degrees UN thread profile, as specified in ASME B1.1. Threads are typically manufactured to Class 1A, which is the thread class with the largest tolerance and the loosest fit.

Quality inspection of imported threaded rods has revealed in many cases the basic 60° profile of UN screw threads is not being respected, said Brahimi. These threaded rods have thread flank angles in the range of 43 to 48 degrees, but flank angles as low as 37 to 40 degrees are common.

But these parts do pass the GO GAGE inspection, which is the most common dimension verification, he added. They also pass the minimum tensile strength requirement.

Poor grip strength

However, the consequence of this nonconformance is that the thread engagement between the rod-nut assembly is reduced and cannot withstand the same load. When assembled with a nut, an aberrant looseness of fit, i.e., “slop” is evident. The nut literally “jiggles” on the threaded rod. The reduced ability to carry a load – i.e., “grip strength” – can result in the unexpected and catastrophic failure of a loaded assembly in service.

Test results obtained from an independent laboratory show a direct correlation between flank angle and grip strength. Test data shows that grip strength is reduced by as much as 40 percent when the flank angle is decreased from 60 to 45 degrees.

“This significant reduction of grip strength is cause for concern as threaded rods are often integral to any load-bearing assembly,” added Brahimi.

One common application is for pipe hanger assemblies such as those used for water supply and sprinkler systems. The use of sub-standard and nonconforming threaded rod raises concerns over the structural integrity of these assemblies.

This problem can be further exacerbated if the external threads of the rods and internal threads of the nuts happen to be respectively at the minimum and maximum of Class 1A thread tolerances.

Sold by weight

The motivation for supplying threaded rod products with reduced thread angle is simple. It reduces the quantity of steel used to manufacture a part by as much as 10 to 15 percent.

Given that threaded rods are sold by weight, the lower weight of nonconforming parts means lower prices. This practice is tolerated and sometimes even encouraged because a common misconception is that, “it’s just threaded rod” and is not used in critical applications, said Brahimi.

Although it may be true that threaded rods are not always used in critical applications, they are often used in load bearing joints where failure of the assembly can have serious consequences in terms of safety, damage and inevitable litigation. This practice certainly has a negative impact on product integrity, but it also amounts to unfair competition for manufacturers and suppliers who make and sell conforming product, he added.

“It is contingent upon importers to determine if they are in fact importing and distributing nonconforming threaded rod and to understand what safety and legal implications may be involved,” remarked Lockard.

“We are doing our best to let our customers know – but we are reliant on them passing on the information to their customers who don’t always want to listen.”

More information is available from the Industrial Fasteners Institute at www.indfast.org.

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