You know what? Bolts don’t keep themselves tightened. The combination of vibration, temperature fluctuations and simple settling will cause any threaded joint to lose it’s grip. Sometimes, it is as simple as the little disk beneath the nut or head. That disk makes the difference between a fastener that holds and one that walks out.
This is where a lock washer come in. If you’ve selected a bolt for a job then this is the time when you need to select a lock washer. This is where it gets interesting. There are several types of lock washers. Each one address the loosening issue in its own way: wedge, external tooth, internal tooth, or split ring (see the infographic above).
How to Choose the Right Lock Washer
These is the dimensions that are important when making your selection. The important dimensions are the inside diameter, outside diameter, thickness, and how much load they’ll stand up to (as determined by testing). This chart translates that information back to design decisions so you know what size washer will fit on bolt and won’t require guesswork.
The spring tension is provided by a split ring. The cut ends press against the joint surface, as well as the fastener. When squeezed together, they want to open up again, which creates a spring effect that presses those cut ends back onto the joint surface. That’s why you’ll still find them on general machinery and even structural steel. Spring force will handle shock loads pretty well.
This is a tooth washer. Mechanical interference happen when teeth bite into the underside of the bolt head (or softer material) and resist rotation. For smaller screws used in electronics where space is tight, internal tooth versions are suitable; for larger bearing surfaces, an external tooth version spreads that grip over a larger area.
Washers go further with a wedge shape. Combining two cams creates a steeper angle than the thread’s helix. This means if you try to back the bolt off, you just tighten it more instead of loosening it. That’s why in the comparison graphic above, the wedge design ranks highest in terms of holding up over time to vibration; in higher cycle applications such as rail equipment or pumps, that added margin is vital.
And then there’s material selection. For cost, indoor / protected environments: zinc plated carbon steel. For automotive or other structural applications requiring higher spring force, use hardened alloy. If it is outdoors but not in contact with water or road spray, 18-8 stainless won’t rust. And if it is near water / exposed to chemical environments too, like a road spray joint: go up to 316 which has even more resistance to those things.
The charts list this out without naming any specific solution as the best one; that depends on the environment the joint will encounter. Here’s the deal on size. It has to be sized so that the inner diameter clears the bolt shank while the outer diameter stays within the bearing area of the head or nut. If not it will bind and lose its effectiveness.
If it’s too large. The table gives the actual dimensions which allows you to check your fit before grabbing for hardware. The truth is none of these will perform unless installed correctly. It should fit flush to the bottom and be tight to the wall so the split ends digs into the material being attached. Do not reuse a crushed washer and torque according to the spec for the fastener. If you guess wrong, the washer loses its spring and won’t lock like it used to. It becomes just a piece of metal spacer.
The number tables is not as much about memorizing measurements as they are about selecting the proper lock washer that best matches the forces the joint will face and the type of locking mechanism used. Read the charts with that query and the numbers cease to be abstract and instead become a useful roadmap for holding things together.
