Fender Washer vs Flat Washer

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Fender washers and flat washers are both common types of plain washers. As a result, engineers often need to select which of them is the best fit on an application-by-application basis. In this article, we present an overview of the fender washer vs the flat washer, compare them, and answer commonly asked questions.

Overview of Fender Washer

At first glance, the fender washer may seem exactly like a flat washer. However, the fender washer has a larger outer diameter in proportion to its center hole. Also, fender washers are made from very thin gauge metals and often has zinc plating.

The large diameter and thin gauge metal enable a fender washer to evenly distribute load over a larger surface area than a flat washer. Moreover, the large outer diameter is useful in covering oversized holes that could be a result of corrosion or wear. Although the name fender comes from its use in mounting fenders in the automotive industry, it is popular in plumbing, sheet metal, and electrical work. Largely because of the additional reinforcement it gives to bolt joints.

Fender washer showing the significant difference between outer and inner diameter, unlike flat washer.
Courtesy: Boltdepot

Flat Washer Overview

Of all the washer types, this is the most common, with its name coming from its two flat sides. This feature means that it is possible to install them in either direction. In addition, its flatness helps to reduce friction and heat generation during installation.

Generally, flat washers are circular general-purpose washers and come in a variety of sizes and thicknesses, mostly thicker than fender washers. Also, they come in both metric and imperial dimensions. Standard imperial washers are either SAE washers for fine threaded nuts and bolts, or USS flat washers for coarse-threaded nuts and bolts. But for standard metric washers, flat washers are available in several gauges in line with JIS standards.

Flat washer with a smaller outer diameter to inner diameter ratio than fender washer.
Courtesy: Theengineerspost

Comparing Fender Washer vs Flat Washer

Although a fender washer and flat washer share several similarities, there are some differences between them as the table below outlines.

Fender WasherFlat Washer
The difference between inner and outer diameter is significant. Usually, the outer diameter is at least three times larger.The difference between the inner and outer diameter is not significant. Generally, the outer diameter is less than three times the inner diameter.
A fender washer has a much larger bearing surface for spreading load than a flat washer. Thus, creates less pressure during tightening.Comparatively, has a smaller bearing surface. So, creates more pressure on the object’s surface during tightening.
Because of its large surface area, it is suitable for covering oversized holes.Flat washers cannot adequately cover oversized holes or holes enlarged by corrosion or wear.
Its geometry makes it ideal for the fitting of thin-walled components.Flat washers are not the best for fitting thin-walled components.
Fender Washer vs Flat Washer

Frequently Asked Questions

This section provides answers to common questions concerning the fender washer and flat washer.

Where did the term fender washer come from?

The term fender washer comes from the most common use of this type of washer, which is for mounting fenders of automobiles.

Which applications use a fender washer over a flat washer?

Unlike a flat washer, a fender washer is ideal for sheet metal applications. This largely owes to its geometry that allows for better distribution of contact pressure when tightening. Also, it is useful in plumbing and electrical work.

Another use of fender washers is for covering oversized holes, despite using a small bolt. Even previously small holes may expand over time due to wear or corrosion. Rather than changing the size of the bolt and rethreading the hole, a fender washer may provide a quick fix.

On the other hand, using a flat washer is preferable over a fender washer when spacing is a goal. Flat washers come in a variety of thicknesses and can be selected to suit applications where spacing is needed. But fender washers are typically very thin and do not provide spacing.

Can you install a flat washer in either direction?

As the name suggests, flat washers are flat with both sides being symmetrical. As a result, it is safe to install them in either direction without a negative effect on their performance.

Is it ok to stack washers on a bolt?

Whether it is a fender washer or flat washer, it is not good practice to stack washers on a bolt. Why? Because each of the washers will act like a spring in series, which stores energy when compressed/tightened. Over time, this energy could undo the tightening, thus defeating the purpose of using a washer.