Washer Types Complete Guide — Flat, Spring, Lock & Square Washers

Locking - Prevents Rotational Loosening

Locking washers are designed to resist rotational loosening caused by vibration, thermal cycling, or dynamic loading. They achieve this through various mechanisms - spring washers generate axial force that maintains thread friction, tooth lock washers bite into the bearing surface to create mechanical interference, and wedge lock washers use ramp surfaces to create a positive locking action. Each type has specific performance characteristics that determine its effectiveness in different vibration environments and load conditions.

Gap Adjustment and Sealing

Washers compensate for manufacturing tolerances, slotted holes, or misalignment in assembled components. Square taper washers provide a flat bearing surface on inclined structural steel flanges. Sealing washers incorporate elastomeric elements such as bonded O-rings or rubber coatings to prevent fluid leakage through the bolt hole, which is essential in hydraulic, pneumatic, and containment applications.

Flat Washer - The Most Common Washer

Function - Pressure Distribution, Surface Protection, Friction Reduction

Flat washers are the most widely used washer type, serving to distribute the clamping force evenly, protect the surface finish of the clamped part from scoring during tightening, and sometimes reduce friction between the rotating fastener and the stationary surface. They are available in three main series: SAE (small outer diameter), USS (large outer diameter), and metric (ISO) series. The choice between SAE and USS depends on the bearing area required and the space available under the fastener head.

Materials and Dimension Standards

Flat washers are manufactured from carbon steel (typically with zinc plating), stainless steel 304/316, copper, nylon, PTFE, and other engineering plastics. Carbon steel washers with zinc plating are the standard for general applications. Stainless steel washers are required for corrosive environments. Nylon and PTFE washers provide electrical insulation and chemical resistance. Dimensional standards include ASME B18.22.1 for inch series and ISO 7089/7090 for metric series, specifying inner diameter, outer diameter, and thickness for each bolt size.

Spring Washer and Elastic Washers

Split Lock Washer, Wave Washer, Belleville Washer

Split lock washers (helical spring washers) have a gap that creates spring force when compressed, providing axial tension that resists loosening. They are effective in low to moderate vibration applications but have limited effectiveness under high-frequency vibration. Wave washers feature a wavy profile for lighter spring applications, providing axial compensation for stack-up tolerances. Belleville washers (conical spring washers) deliver the highest spring force in the smallest space, making them ideal for precision preload control in high-temperature bolted joints where thermal expansion cycling occurs.

Spring Washer Locking Principle and Limitations

The locking action of a spring washer comes from the elastic spring force pushing the nut away from the joint surface, maintaining tension in the bolt threads. However, research including Junker vibration testing has shown that spring washers have limited effectiveness under severe vibration - the washer can lose its spring tension through relaxation or embedment. For high-vibration environments, tooth lock washers, wedge lock washers, or chemical thread locking provide more reliable solutions.

Tooth Lock Washers

External Tooth, Internal Tooth, and Nord-Lock Wedge Lock Washers

External tooth lock washers have teeth on the outer circumference that bite into the nut or bolt head bearing surface when tightened, creating mechanical resistance to rotation. Internal tooth lock washers place teeth on the inner diameter, engaging under the fastener head. Internal/external combination washers provide locking on both surfaces. Nord-Lock wedge lock washers use a pair of washers with mating cam surfaces - when the joint is tightened, the cams lock together and only allow movement across the radial teeth, providing positive locking that withstands the highest vibration levels.

Special Purpose Washers

Square taper washers compensate for the slope on I-beam flanges and channel sections, providing a level bearing surface for the nut or bolt head. Dome washers and spherical washers accommodate angular misalignment between the fastener and the bearing surface. Sealing washers incorporate rubber O-rings or full rubber bonding for leak-proof assemblies in hydraulic and pneumatic systems. Insulating washers made from reinforced plastics provide electrical isolation between dissimilar metals to prevent galvanic corrosion. Tab washers have protruding tabs that can be bent against a flat surface of the nut or mating part for positive anti-rotation locking.

Washer Material and Surface Treatment Selection

Material selection for washers must consider the base material hardness, corrosion environment, and galvanic compatibility with both the fastener and the clamped material. Carbon steel washers with zinc plating or hot-dip galvanizing are standard for carbon steel fasteners. Stainless steel washers in 304 or 316 grades must be used with stainless steel fasteners to prevent galvanic corrosion. Copper washers are used in high-temperature sealing applications. Nylon and PTFE washers provide electrical insulation and chemical resistance. The general rule for hardness matching is that the washer should be softer than the clamped part to prevent surface damage but hard enough to distribute the load effectively.

Flat Washer vs Spring Washer vs Tooth Lock Washer - Selection Guide

For static, non-critical connections, a flat washer alone is sufficient. For low to medium vibration environments, spring washers or tooth lock washers provide adequate locking. For high-frequency or severe vibration applications, Nord-Lock wedge lock washers or chemical thread locking compounds are recommended. For high-temperature cycling service where thermal expansion differences cause preload variation, Belleville washers provide the necessary axial spring compensation. The selection must account for the specific vibration frequency, load magnitude, temperature range, and accessibility for maintenance.

ASME / ISO / DIN / GB Washer Standards Comparison

Washer standards differ between major industrial regions. ASME B18.21.1 covers American standard flat and lock washers. ASME B18.22.1 specifies flat washer dimensions for inch series. ISO 7089/7090 define metric flat washers with two series (regular and large OD). DIN standards include DIN 125 (flat washers), DIN 127 (spring lock washers), and DIN 6798 (tooth lock washers). GB/T standards cover Chinese equivalents. Understanding the applicable standard ensures dimensional compatibility with the bolt and nut assembly.

Washer Installation and Torque Effects

The type and number of washers used in a bolted joint affect the torque-preload relationship. Spring washers add axial compliance that may require slightly increased torque to achieve the same preload as a joint with flat washers. Lubricated washers reduce the nut factor K, requiring adjusted torque values. When using washers on both sides of the joint - one under the bolt head and one under the nut - the clamping length increases slightly, which may affect preload in short-grip joints. For critical assemblies, torque-tension testing with the actual washer configuration is recommended.

ManufacturerPipe's Washer Supply Capabilities

ManufacturerPipe supplies all washer categories including flat, spring, tooth, square taper, Belleville, and sealing washers. Materials include carbon steel, stainless steel 304/316, and alloy steel to match any fastener assembly. We produce to ASME, ISO, DIN, and GB standards, and accept custom drawings for non-standard washer configurations. As a one-stop fastener supplier, we offer complete bolt + nut + washer matched packages that simplify procurement and ensure component compatibility.

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