Washers look simple, but they play a critical role in every bolted joint. They spread the load under a bolt head or nut, protect surfaces from damage, prevent loosening and can even seal against leakage. Choosing the wrong washer — or skipping one entirely — is a common cause of joint failure. This guide covers the main washer types and how to select the right one.
What Does a Washer Do?
A washer is a thin plate with a hole, placed under a bolt head or nut. Its main functions are: load distribution — spreading the clamping force over a larger area to avoid damaging soft materials; surface protection — preventing scratches and corrosion at the contact point; vibration resistance — with spring and lock washers; and spacing or sealing in specific designs.
Main Types of Washers
Flat washers (plain washers) are the most common. They distribute load and are used with bolts, screws and nuts in almost every application. Spring washers are split or wavy and provide tension to resist loosening under vibration. Lock washers (e.g. tooth washers) bite into the surface to prevent rotation. Fender washers have a large outer diameter for oversized holes or soft materials.
Materials and Finishes
Washers are available in carbon steel (zinc plated or plain), stainless steel (304/316), brass, copper and nylon. Choose stainless steel for marine and outdoor use, zinc-plated carbon steel for general indoor applications, and nylon for electrical insulation or where non-magnetic properties are required.
How to Select the Right Washer
First, match the washer inside diameter to the bolt or screw diameter (e.g. an M8 washer fits an M8 bolt). Second, choose the outer diameter based on the load and material — soft materials need a larger bearing surface. Third, decide whether vibration resistance is needed. When in doubt, use a flat washer plus a spring or lock washer in vibrating assemblies.
Browse the washer range at Fastener Hall, or send an RFQ with your bolt sizes and applications — our engineers will recommend the correct washer type and grade. You can also reach us on WhatsApp.
Washer Sizes and Standards
Washers are standardized to match bolt diameters. Metric flat washers follow ISO 7089 (normal series), ISO 7090 (normal, chamfered) and ISO 7093 (large series). DIN 125 and DIN 9021 are the older German standards still widely quoted. The inside diameter of the washer must clear the bolt threads (typically bolt size + 1 mm for M6–M24), while the outside diameter determines the bearing surface. For metric sizes, an M8 washer has an inside diameter of 8.4 mm and outside diameter of 16 mm (normal series) or 24 mm (large series).
Inch flat washers follow SAE and USS series. SAE washers have a smaller outside diameter and are preferred for higher-strength bolts; USS washers are larger and suited to lower-strength general applications. When a drawing calls for a specific washer standard, always confirm it — swapping a USS washer for an SAE washer changes the bearing area and can affect joint performance.
Spring Washers Explained
A spring washer is a split ring that acts as a spring under the nut or bolt head. It provides a small amount of elastic tension that helps resist loosening from vibration. The split spring washer (DIN 127, ASME B18.21.1) is the classic design. Wave and curved spring washers (DIN 137) are used where a light, continuous preload is needed without a pronounced split edge. Note that spring washers are effective in many static and light-vibration joints, but for heavy vibration, prevailing-torque lock nuts or adhesive are more reliable.
Lock Washers: Toothed and Internal/External
Toothed lock washers have teeth that bite into both the fastener head and the work surface, providing high resistance to rotation. Internal-tooth lock washers are used with small fasteners and soft materials; external-tooth lock washers provide a larger biting area and are common in electrical grounding applications because the teeth cut through oxide layers. Countersunk (flat-top) lock washers suit countersunk screws. When appearance and electrical continuity matter, tin-plated or zinc-plated toothed washers are available.
Special-Purpose Washers
Fender washers have a very large outside diameter relative to the hole and are used on sheet metal, thin wood, drywall and oversized holes to prevent pull-through. Sealing washers combine a metal washer with a rubber or Neoprene seal for plumbing and roofing. Belleville washers (conical) provide a high spring rate in a small space and are used for thermal expansion compensation and preload maintenance. Tab washers have a tab that bends up against a nut for permanent locking. Insulating washers in nylon or fibre prevent electrical contact and galvanic corrosion.
How Many Washers Should a Joint Use?
A typical bolted joint uses one flat washer under the nut (where the load is applied) and, in many designs, one under the head as well. For structural steel, hardened flat washers are required under both head and nut, sized to the bolt. Adding washers beyond the design intent changes the grip length and clamp load — follow the engineering drawing rather than adding washers arbitrarily. Where a plain washer and a lock washer are both specified, place the plain washer against the surface and the lock washer directly under the nut.
Quality Checks for Washers
Critical quality parameters include hardness (hardened washers for high-strength bolts must meet HRC requirements), inside/outside diameter tolerances, thickness, flatness, and coating thickness for zinc or Dacromet finishes. Salt spray testing verifies corrosion resistance, and dimensional sampling per batch confirms conformity to the specified standard. Fastener Hall supplies washers with material certificates and test reports on request.
Frequently Asked Questions
- Can I reuse washers? Flat washers can generally be reused if undeformed; spring and lock washers lose tension after first use and should be replaced.
- Do I need a washer with a flange nut? Flange nuts already integrate a wide bearing face, so a separate washer is often unnecessary.
- What material washer is best for outdoor use? Stainless steel or hot-dip galvanized washers for coastal and outdoor environments.
- What is a hardened washer? A washer heat-treated to HRC 38–45, used under high-strength structural bolts to prevent indentation and loss of preload.
- Do washers increase clamping force? They do not increase clamp force directly, but they distribute it over a larger area, protecting the material and preserving preload.
