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Having worked in factories for over 28 years, I have been able to document that fastener failure leads to 30% of factory downtime. A prime culprit in fastener failure is the average mechanic (and engineer) lack of understanding of how fasteners work. Today I am going to address lock washers, sometimes known as split washers.

Lock washers have been experimentally proven to be ineffective locking devices and can even cause self loosening over time.

In theory lock washers (aka split washers or helical spring washers) are supposed to work by squishing flat between the nut and the mounting surface when you tighten them. Once flat, the sharp edges of the washer are supposed to dig into the nut and mounting surface to prevent counter-clockwise rotation.

In practice, a lock washer is unable to gain any purchase against hard surfaces and does not actually prevent rotation. The problem is that lock washers make for poor springs and bottom out after only a small percentage (on order of 10%) of a bolt’s total clamping load. In effect, the spring action of the lock washer causes the nut to turn and loosen as it expands and contracts with load.

The only time a lock washer might prove useful would be for fastening onto soft, easily deformed surfaces such as wood, where the washer's springiness & sharp edges could actually work.

The evidence against lock washers started stacking up in the 1960’s when a gentleman named Gerhard Junker published some of his lab experiments. He invented a machine for testing the effect of vibrations on threaded fasteners. The first thing he discovered was that transverse vibration loads generate a much greater loosening effect than do axial vibrations.

His second discovery was made by plotting the bolt tension vs vibration cycles to create a ‘preload decay chart’. When he compared the preload decay of a bolt and lock washer combo to a bolt by its lonesome, he found that the lock washer caused the connection to loosen sooner.

So what is a mechanic to do? Chemical lockers like Loctite, deformed thread lock nuts, and Nyloc nuts should be your everyday go-to locking devices. Just remember that Nyloc nuts are a one-time use item.

When lives are on the line you may want to employ a ‘positive locking device’ such as a castle nut or a slotted nut. No amount of vibration will break this kind of connection:

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Share 9 5 Comments Hidenori Araki 4y

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An additional technology that I recommend is a wedge-lock washer (Nord-Lock washer, the original manufacturer of this type) which passes the Junker vibration test (DIN 65151/25201). Product info video: https://youtu.be/IKwWu2w1gGk

Like Reply 1 Reaction 2 Reactions Phil Bates 8y

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Nuts, Bolts, Fasteners and Plumbing Handbook by Carroll Smith. This has been known since before 1990 when Carroll Smith's book was published. He has a fairly extensive section on locking devices that do not work as well as what does work. It is unfortunate that dysfunctional locking methods continue to be sold, and are so well accepted.

Like Reply 1 Reaction D. Andrew Ward, P.E. 8y

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Awesome, thanks!

Like Reply 1 Reaction Abe Clark 8y

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Very interesting!

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