Hey there!

Today let's dive right in: what do you really know about washers?
Most people recognize flat washers and spring washers and are familiar with using them. However, many people may use them habitually, without fully understanding their purpose.
Today, we're going to take a closer look at the actual function of washers.

Preventing Bearing Surface Sinkage and Ensuring Secure Fastenings

Flat washers are available in a wide range of sizes for any given screw diameter, from miniature to extra-large.

List of sizes for flat washers regulated by JIS

Now, here's a common question. Which way do washers go? Since flat washers are typically manufactured by stamping, one side will often have slightly sharper edges or burrs, while the other is more rounded.

While there's no universal rule about which side of a washer goes down, considering the bearing surface pressure, it's generally recommended to position the burred side against the fastened material. However, if you're concerned about scratching to the surface, such as plating from peeling, placing the rounded side down is preferable.

Do They Really Prevent Loosening?

Spring washers are made by cutting and twisting a flat washer, giving them a spring-like shape. The idea is that this spring action, or elastic force, along with the biting action of the cut edge, helps prevent screws from loosening.

List of sizes for spring washers regulated by JIS

But, honestly, I have my doubts about their effectiveness. The elastic force of a spring washer is pretty small compared to the force you use when tightening a screw properly. By the time the spring washer force kicks in, the screw is probably already loose.

For example, a high-strength M8 steel bolt can handle around 23,000 N of axial force. But an M8 spring washer? It only takes about 500 N to compress it (that's from my own test). That's barely a fraction of the bolt's strength!

Maximum allowable axial bolt force found here

Sure, the spring action might add a little friction, but it's not much.

And the "biting" effect? That only works if the material you're fastening is softer than the washer. In addition, it scratches the surface, which isn't great for repeated use. You might see them used with flat washers to prevent damage, but once everything's scratched up, it doesn't really stop rotation.

Take a look at this graph from a Junker vibration test. It shows a screw with spring and flat washers loosening over time.

And they don't do much better in an NAS vibration test either.

NAS vibration test video

Long story short, there are many reasons why screws loosen, and spring washers might help in some cases. But proper tightening is way more important.

I'd love to dive into all the different causes of loosening another time, but for now, I'll wrap it up here.
Thank you!

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