The difference between Spring Lock Washer and Flat Washer and their combined use

1. Introduction
Washers are very common auxiliary fasteners in bolt and nut joints. Although they are small in size, their role is significant. Different types of washers are designed for different working conditions. Improper selection or installation can lead to loose connections, component damage, and even equipment failure. The two most common washers are flat washers and spring lock washers. They differ significantly in appearance, function, and applicable environments, and can even be used together in some cases. Understanding the differences between the two and how to properly combine them is of practical significance for project installation and maintenance.

2. Flat Washers
A flat washer is a thin, uniformly thick metal or non-metallic sheet with a hole in the center. It is typically circular in shape, with a hole diameter slightly larger than the bolt diameter, making it easier to install. Its primary function is to increase the contact area between the bolt or nut and the connected component, thereby distributing the pressure generated during bolt tightening and preventing the nut or bolt head from directly injuring the workpiece surface.
Flat washers can be made from a variety of materials, including carbon steel, stainless steel, copper, aluminum, and even plastic. Metal washers are mostly used in applications requiring high strength or high loads, while plastic or rubber washers are suitable for lighter loads, scratch protection, or insulation.

In addition to surface protection, flat washers can also reduce the impact of uneven contact surfaces on tightening force, ensuring uniform force distribution across the bolt. However, it's important to note that flat washers themselves offer little anti-loosening properties; their primary function is to distribute pressure and protect components. Therefore, if the working conditions are subject to vibration or shock, using flat washers alone will not effectively prevent loosening.

3. Spring Lock Washers

Spring washers differ from flat washers in appearance. They are a metal ring bent into a spiral shape, offset along a cut, and are typically made of carbon steel or stainless steel. Their most notable features are their elasticity and the cut end surface. When the nut or bolt is tightened, the spring washer is flattened, generating a rebound force. Simultaneously, the sharp edges of the cut dig into the underside of the nut or workpiece surface, increasing friction and thus reducing loosening caused by vibration, shock, or thermal expansion and contraction. The anti-loosening effect of spring washers primarily stems from mechanical friction combined with elastic preload. While they can prevent loosening to a certain extent, their effectiveness in high-frequency, high-vibration environments is not as robust as that of modern anti-loosening devices (such as nylon locknuts, anti-loosening adhesives, and double nuts). Therefore, industry standards no longer recommend relying solely on spring washers for loosening prevention for high-strength bolts (e.g., grade 8.8 and above).

They are primarily used in medium- and low-speed machinery, pumps, motor installations, and transportation machinery, all subject to moderate vibration. Spring washers can bite into soft surfaces (such as aluminum and plastic) and cause damage, making them unsuitable for direct use in these areas.

4. Differences Between the Two

Although both flat washers and spring washers are called "washers," their design purposes are completely different. Flat washers primarily provide protection and pressure distribution. They are passive protective devices, lack elasticity, and are not primarily designed to prevent loosening. Spring washers are designed to prevent loosening, using elastic force and friction from the cuts. However, they offer little surface protection and may even scratch the workpiece.
Flat washers are suitable for installation anywhere uniform force distribution is required, especially on soft materials, while spring washers are more suitable for installation in hard areas that may be subject to vibration or impact.

5. Combination Methods and Principles
In some assembly processes, flat washers and spring washers are used together to leverage both the protective properties of the flat washers and the anti-loosening properties of the spring washers.
The common installation sequence is:
Starting from the bolt or nut side, the order is: bolt/nut → spring washer → flat washer → workpiece.
In this order, the flat washer rests against the workpiece, preventing the sharp edges of the spring washer from directly contacting the workpiece surface and causing damage, while the spring washer rests against the nut or bolt head, creating friction and preventing loosening.
Installation principles include:
The flat washer should be placed on the side of the workpiece to protect the surface.
Spring washers should be in direct contact with the nut or bolt head to ensure proper anti-loosening function.

If anti-loosening is required in high-strength bolted connections, consider more reliable anti-loosening devices rather than relying solely on spring washers.

When using a combination of bolts, pay attention to the effective thread length of the bolts to avoid insufficient penetration due to the addition of extra washers, which can reduce joint strength.