Three Critical Fastener Truths
14 June 2022
“Your bolts are breaking in my engine blocks!”
The purchasing manager was furious!
This was a massive, costly failure. Someone would pay.
The unimpressed fastener engineer on the other end of the phone replied wryly. “What did you want to break?”
WHAT????
It was simple. Tremendous shear and tension had built, and something had to break. “Did you want it to be the bolt? Or the block?”
This made the auto executive stop.
The fastener engineer did not design the engine. He did not design the joint. His company supplied the bolts that held the engine together. And the bolts were up to the specifications they should have been made to. There was no disagreement as to any of these facts.
The problem here was not a bad bolt, it was a bad joint design, resulting in a tremendous strain on the joint. The joint design was creating shear on the bolt, far exceeding what the bolt was designed for.
Something had to give, and the preferred item to give was the bolt going into the block, not the block itself. Now this happened around the 1980’s, so the dollars might have changed, but the principle hasn’t: It’s much less expensive to replace a $15 bolt than it is to replace a $1,500 engine block.
Critical Fastener Truth #1: “Bolts are made to break.”
In the case of our Detroit friend here, the bolts were actually performing up to specifications. If a stronger bolt was used, it could have resulted in damage to the block itself, and that would render the block as scrap iron. The problem was the design of the joint itself, not the bolt.
Often, even experienced engineers tend to forget their fastening technology basics. It’s important that the proper fasteners are chosen, and as tension is brought to bear, It may require a period of relaxation and then a final tightening in order to reach and retain the proper tension.
Only when all the pieces of the puzzle are together can you be assured of holding the proper tension.
Critical Fastener Truth #2: Bolts Stretch.
What about disassembly and reassembly? Bolts are like rubber bands, they stretch and become deformed. They are subject to elasticity. Once put under load they cannot be returned to their pristine new shape. This often-overlooked principle can cause havoc on a high strength application, for example, a 60-ton capacity heavy duty construction crane turntable. And once one bolt fails, the entire turntable deck is compromised. Sadly, when a device of that critical strength fails, lives can easily be lost.
Critical Fastener Truth #3: Hydrogen Embrittlement.
One of the first things I ever learned in this business was learned the hard way. Well, the hard way, but it could have been far worse.
A customer had called desperately searching for stock on a high strength bolt. We happened to have stock, and we were ready to send it, when we saw some discoloration on the bolts.
Knowing the customer needed a cosmetically appealing solution, we quickly sent the parts out to our local plater to be flashed zinc before we sent them.
This worked wonderfully at the time, until the customer used their air guns to install the bolts, and watched the heads pop off.
Being new, I was not aware of the critical potential for hydrogen embrittlement to weaken the strength of the bolt and make it much more susceptible to break far below the required specifications. For some reason no one ever realized the need to heat treat these bolts to dissipate the hydrogen atoms throughout the bolt.
Thankfully, while this was a painful lesson, these bolts never made it through the rejection process, and no one was hurt.
In short, there are many factors that go into a good fastener joint. This includes proper fastener selection, a good understanding of what a joint is designed to do, and potential factors that can degrade a high strength fastener to the extent that they are unable to properly hold the load they were designed to hold.
When looking for fastener supply, look not only for the item, or for the cost-effective price. Look for a fastener supplier who is trained in fastener technology, who understand the fundamentals of fastener joint design, and is aware of the potential issues resulting from plating and secondary processes.
Fast-Rite engineers are available to discuss your fastener challenges. Just reach out to Fast-Rite Engineering at 888.327.8077 or email sales@fast-rite.com.