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What Do Washers Do?

Washers are disc-shaped fasteners placed under the head of a bolt or nut to distribute the clamping load over a larger bearing area. Without washers, the concentrated load from a bolt head or nut would bear directly on a small area, potentially damaging the bearing surface (especially in soft materials like wood, aluminum, or thin steel). The larger the washer diameter, the greater the bearing area and the lower the surface pressure. For structural steel connections, washers also prevent the bolt head or nut from pulling through the steel plate under tension loading.

Plain Washers (Flat Washers)

Plain washers (also called flat washers) are the most common type. They have a central hole sized to fit the bolt or screw, with a flat bearing surface. The standard series (USS and SAE in imperial; ISO 7089/7090 in metric) specifies outer diameter and thickness for each bolt size. For general applications, standard flat washers are used. Heavy-duty (or 'fender') washers have a larger OD relative to the hole size, providing greater bearing area for soft materials.

Fender Washers: When You Need More Bearing Area

Fender washers (also called mudder washers or repair washers) have an oversized outer diameter relative to the hole, providing extra bearing area for: Thin sheet metal — prevents the bolt head or nut from pulling through the thin material; repair and retrofit applications where the existing hole is oversized; soft materials like wood, plastic, aluminum. A standard M10 flat washer has an OD of 20mm. An M10 fender washer might have an OD of 40-50mm — providing 4-6× the bearing area. For African construction with thin metal roofing and cladding, fender washers are essential for preventing pull-through.

Spring Washers (Split Lock Washers)

Spring washers (split lock washers) are designed to provide a spring preload that resists bolt loosening under vibration. The split (diagonal cut) creates spring tension that maintains clamp load even when the joint settles. However, the locking effectiveness of split washers is debated — studies show they can actually reduce effective clamp load in some conditions. For critical vibrating joints, use Nord-Lock wedge-locking washers or proper lock nuts rather than relying on split washers alone. Spring washers are most effective when used as a secondary measure with a primary locking method.

Washer Materials and Coatings

Washer material must match or be compatible with the bolt and the bearing surface: Zinc-plated steel — standard for most indoor and protected outdoor applications; Hot-dip galvanized (HDG) — for outdoor and corrosive environments, typically used with HDG bolts; Stainless steel 304/316 — for marine/coastal environments and food processing; Hardened steel (through-hardened) — for high-strength structural connections to prevent embedding; Nylon — for electrical insulation and non-marring applications. For African projects, always match the washer material to the bolt and the application environment.

Applications in African Construction

Washers are used in virtually every bolted connection in African construction: Roofing sheet attachment — fender washers with sealing washer (EPDM) under roofing screws to prevent pull-through and water ingress; Timber connections — large flat washers (or plate washers) under bolt heads and nuts to prevent embedding into soft wood; Bridge structural connections — hardened washers under bolt heads bearing on structural steel; Solar panel mounting — stainless fender washers for marine/coastal solar installations; Water tank stand construction — HDG washers for hot-dip galvanized bolt assemblies on water tank supports.

Frequently Asked Questions