Nuts And Bolts: Keeping It Tight

It's not much of a stretch to say that without nuts and bolts, the world would fall apart. Bolted connections are everywhere, from the frame of your DIY 3D printer to the lug nuts holding the wheels on your car. Though the penalty for failure is certainly higher in the latter than in the former, self-loosening of nuts and bolts is rarely a good thing. Engineers have come up with dozens of ways to make sure the world doesn't fall apart, and some work better than others. Let's explore a few of these methods and find out what works, what doesn't work, and in the process maybe we’ll learn a little about how these fascinating fasteners work.

There are plenty of ways for a bolted joint to fail, but vibration-induced self-loosening is perhaps the most insidious. Anyone who has ever pounded on a stuck bolt or used an impact wrench to remove a rusty nut knows that vibration really helps. Put that same joint into service and subject it to the right kind of vibration, and there's a good chance the connection will self-loosen and cause the joint to fail.

In the 1960s, German engineer Gerhard Junker studied self-loosening and came to the conclusion that transverse vibrations were responsible for the failure of bolted connections. He devised a simple test apparatus that provided rapid transverse vibrations while monitoring fastener preload tension with a load cell. Graphing preload as a function the number of vibratory cycles yielded clues as to the effectiveness of various locking methods. The test became known as the Junker test and as standard DIN 65151, it remains the gold standard for testing self-loosening.

There are some fascinating videos out there showing Junker tests in action, and some are downright scary. Typically, we’ll throw a simple helical spring lock washer on a stud or bolt, torque down the nut nice and snug, and call it a day, feeling like we’ve made a secure joint. But nothing could be further from the truth. In fact, the video below shows that not only do lock washers add very little security to bolted connections, none of the other common methods — plain washers, nylon insert nuts, and stacked nuts — provide much help either.

Obviously, the video above is aimed at marketing the company's fancy wedge-locking washers, and it's pretty clear that they work well. But why do they work when a simple lock washer fails? To answer that, it pays to look at what else works, something that wasn't tested in the video — a properly installed jam nut.

A jam nut is a low-profile nut, typically about half the height of a standard nut, that's installed below the larger nut. When the jam nut is installed, it's tightened only to about a quarter to a half of the full final torque. The thick nut is installed next and torqued to the final value while the jam nut is held in place with a wrench. This effectively pulls the bolt up through the jam nut. The threads of the bolt are then in contact with the top flanks of the threads within the jam nut, while simultaneously contacting the upper or pressure flanks of the top nut. With the top and bottom nuts providing opposing forces on the bolt, the nuts are far less likely to self-loosen.

A similar mechanism is at work in the wedge-locking washers. The two halves of the washer have interlocking wedges, the angle of which exceeds the pitch of the bolt threads. As the bolt is tightened, the higher pitch of the washers pulls the bolt back upwards, providing an opposing force to jam the threads and prevent the fastener from self-loosening.

If we look at all the locking methods that fail, they all have something in common: they all rely on friction. Jam nuts and wedge-lock washers work by providing tension to oppose the transverse vibrations that lead to self-loosening and are therefore much more effective.

Of course, there are other methods of locking threaded fasteners. Adhesive thread lockers come to mind, as do more complicated methods like wired nuts and tabbed washers, and they can be very effective methods. But for low cost and ease of installation, it's hard to beat a simple jam nut to keep the world from falling apart.

Featured image source: Nord-LockGroup