Flat Washer dimensions

Metric

(ISO 7089 / DIN 125)

Imperial

ASME B18.21.1 - Type A Plain Washers (Narrow)

Design Parameters

Flat washers (ISO 7089 / DIN 125) are used to distribute the load of a threaded fastener and protect the surface of the parts being joined. They are essential for preventing the bolt head or nut from "marring" the parent material.

Engineering Note: Not all "M6" washers are the same. "Fender" washers have a significantly larger Outside Diameter for use with thin materials, while "Small" or "Narrow" washers are used in tight clearances. Always ensure the washer thickness is sufficient to prevent "dishing" (deforming into a cone shape) under the required clamping force.

Technical Guidance for Flat Washer Selection

While often viewed as the simplest component in a fastener assembly, flat washers (Plain Washers) perform several critical mechanical functions. Beyond simply protecting a painted surface, they are essential for managing the distribution of force and ensuring the long-term stability of a bolted joint.

The Critical Role of Hardness (HV Ratings)

The most common engineering failure involving washers is a mismatch in material hardness. In the metric system (ISO 7089), washers are typically rated by their Vickers Hardness (HV).

• 140 HV / 200 HV: Standard washers intended for use with Property Class 8.8 bolts or lower.
• 300 HV: Hardened washers required for Property Class 10.9 and 12.9 bolts.

If you use a soft (140 HV) washer with a high-strength 12.9 bolt, the washer will "yield" under the intense clamping force. This causes the washer to dish (deform into a funnel shape) or allow the bolt head to embed into the washer material. As the washer deforms, the bolt loses its preload (tension), which leads to the nut backing off or the bolt failing due to fatigue. Rule of thumb: The washer must always be at least as hard as the fastener head.

Standard vs. Large (Fender) Washers

The choice of Outside Diameter (OD) depends entirely on the parent material's compressive strength.

• ISO 7089 (Form A): The standard "Normal" series. The OD is sized to provide a balance between load distribution and space efficiency.
• ISO 7093 (Fender Washers): These feature a significantly larger OD (typically 3x the nominal thread diameter). They are indispensable when fastening into soft materials like wood, plastic, or thin sheet metal. By spreading the load over a larger area, they prevent the fastener from pulling through the material or crushing the fibers of the substrate.

Surface Physics: Friction and Preload

A washer introduces two new friction interfaces into your joint: one between the bolt head and the washer, and another between the washer and the part surface. This is beneficial because it provides a consistent, smooth surface for the bolt head to rotate against during tightening.

Without a washer, the bolt head might "gall" against the part, especially if the part is made of a different alloy. This galling creates erratic friction, making it impossible to accurately correlate torque with actual clamping force. Using a hardened, lubricated washer ensures that the torque you apply is actually stretching the bolt rather than just overcoming surface roughness.

Assembly Orientation: Rounded vs. Sharp Sides

Flat washers are typically stamped out of sheet metal using a die. This process leaves one side with slightly rounded edges (the side the die entered) and the other side with sharp edges or a small burr (the side the die exited).

In high-precision or aerospace applications, orientation matters:

1. Rounded side up: The rounded edge should face the bolt head. This prevents the sharp edge of the washer from scoring the radius (fillet) underneath the bolt head, which could create a stress concentration point.
2. Sharp side down: Placing the sharp/burred side against the part provides a slightly better grip to prevent the washer from spinning during the tightening sequence.

Material Selection and Galvanic Corrosion

When selecting washers, ensure chemical compatibility with the rest of the stack.

• Zinc-Plated Steel: Standard for most indoor applications.
• Stainless Steel (A2/A4): Excellent for corrosion resistance but significantly softer than hardened alloy steel washers. Never use a standard stainless washer with a Grade 12.9 bolt in a structural application without verifying the compressive yield.
• Galvanic Warning: Avoid using stainless steel washers with aluminum parts in salt-spray environments unless a barrier (like a zinc-rich primer or Tef-Gel) is used, as the aluminum will act as an anode and corrode rapidly around the washer.

When to Omit a Flat Washer

Washers are not always required. Many modern industrial designs utilize Flanged Bolts or Flanged Nuts. These components have a "built-in" washer that is precisely engineered to be concentric and of the same hardness as the fastener. Flanged hardware is often preferred in automated assembly because it reduces the part count and eliminates the risk of a technician forgetting to install the washer.

Standard Reference Table

Note: For critical joints, check the HV rating stamped on the packaging or the manufacturer's certs to ensure compatibility with high-tensile bolts.