Ever notice that when a bolt breaks inside a casting or nut, it usually shears two or three threads below the surface? That’s because not all threads carry an equal load. According to engineers who specialize in fastener technology, the first complete thread of a nut or bolt carries 38% of the total load. The second thread bears around 25% of the load and the third thread roughly 18%—adding up to 81% of the total load borne by the first three threads.
That’s why there’s no advantage to using a bolt with more threads or a casting that is threaded more deeply. The higher threads of a bolt carry only 19% of the total load, with the final threads often carrying 1% or less.
This is why double-nutted bolts always shear just inside the first nut. The extra threads of the second nut add no strength to the junction—they merely lock the first nut so it can’t come loose.
Job performance. The tricky thing about threaded fasteners is that even though increased thread length on a bolt doesn’t increase strength when that bolt is installed into a threaded hole, thread length does influence the strength of a bolt if it’s used to fasten together two pieces of metal. It all depends on the type of load the joint experiences.
If the joint experiences shear load—the two surfaces trying to slide past each other—the bolt should be sized so its shank extends through the shear point where the two pieces of metal meet. The shank is thicker than the threaded portion and better able to withstand shear forces.
But if the joint will be subjected to tensile loads (pulled apart), the critical design consideration becomes the number of threads between the bottom of the bolt’s head and the nut, when the nut is torqued and in its final position.
Imagine a 3"-long bolt used to fasten two 1"-thick steel plates. If the bolt’s shank is 17⁄8" long, there will be 1⁄8" of threads inside the bolt hole once the nut is tight.
Now imagine those same 1"-thick plates clamped together with a bolt that has a 1½" shank, so there is ½" of threads inside the bolt hole once the nut is tightened.
If the nuts are tightened to apply 10,000 lb. of tensile force against both bolts, the bolt with two threads between the nut and the bottom of the bolt’s head is exposed to 5,000 lb. of load against each thread, while the bolt with eight threads between nut and bolt head experiences 1,250 lb. of loading against each thread.
Both bolts have the same tensile strength, but the extreme loading of the few threads in the first example makes it more prone to failure than the second bolt, where the load is spread across more threads.
Discussions of bolt failure inevitably lead to questions about bolt strength. Bolts come in grades that reflect their resistance to both shear and tensile failure. Farmers are most familiar with Grade 2, Grade 5 and Grade 8 bolts. SAE Grade 2 bolts, often called "shear grade," and their metric equivalent have no markings on their heads. SAE Grade 5 bolts have three radial marks on the head; Grade 5 metric bolts have the number "8.8" on the head. SAE Grade 8 bolts have six radial marks on the head; their metric equivalent has the number "10.8" on the head.
We’re all guilty of replacing a shear bolt with a Grade 5 or Grade 8 bolt in order to finish a field or a harvest. When asked what harm "upgrading" bolts can do to a machine, an engineer once told me that shear bolts are designed to break before other, more expensive components. Replace a shear-grade bolt with a stronger bolt, and you transfer the excess load to another component. That component could be a lot more expensive to replace than a 15¢ shear bolt.
In many cases, problems with shear bolts are due to "egged-out" holes, where the hole is drilled out and a larger shear bolt is used. This increases the shear value of that joint and transfers increased loading to other components in the machine.
The best fix for frequent shear bolt failure is to replace both sides of the joint and then use the proper-sized shear bolt.
Cool Tool of the Month
Roll pin punches have special knobs on the end of long straight shanks to center them when removing roll pins. Once you’ve worn off, chipped off or broken off these centering knobs, the punches can have useful second lives driving sheared bolts and pins from driveshafts. Roll pin punch sets range from $50 to $150, depending on the quantity and size of punches in the set.