Washer functions: DISTRIBUTE LOAD (spreads clamping force over larger area; prevents bolt head/nut from sinking into softer materials like wood, plastic, aluminum, painted steel; without washer, point loading can crush material); PREVENT SURFACE DAMAGE (protects finish from being marred by rotation of bolt/nut during tightening); PROVIDE BEARING SURFACE (clean smooth surface for nut to bear against; especially important on rough surfaces); PREVENT LOOSENING (lock washers add vibration resistance); FILL SPACE (when bolt is too long, washers or spacers take up the difference); ELECTRICAL ISOLATION (special washers like nylon, fiber electrically isolate parts).
Washer types: FLAT WASHER is standard, round, flat, for general load distribution. FENDER WASHER has large outer diameter for soft materials, thin sheet metal, distributing load over wide area; common in automotive body panels. LOCK WASHER prevents nut loosening from vibration; types: SPLIT-RING (helical spring, most common; one end of split twists creating friction and bite into surface), STAR (TOOTH, external or internal teeth bite into surfaces), DOUBLE-COIL; modern bolted joints often use Nord-Lock washers (wedge-locking, very effective), prevailing-torque nuts (Nyloc with nylon insert), or thread-locking compound (Loctite). BELLEVILLE WASHER (conical spring) provides constant spring force for high-vibration applications. SHOULDER or DOUBLE-D WASHER has special shape for specific applications. SEALING WASHER has bonded rubber to seal against fluid leaks; common in plumbing, hoses, oil drain plugs. WAVE or CRESCENT WASHER has wavy form with mild spring tension.
Washer materials: zinc-plated steel (common, rust-resistant for indoor use); stainless steel (corrosion-resistant for outdoor, marine); brass (non-magnetic, decorative); nylon/plastic (non-conductive, light); rubber (sealing, vibration damping); bonded (rubber-to-metal for sealing under pressure); copper (sealing on certain mating surfaces). INSTALLATION ORDER (typical): bolt head, flat washer, material, material, flat washer, lock washer, nut. Lock washer goes against the nut (or rotating element); flat washer between material and lock washer to protect material. CRITICAL applications use torque wrench to apply specified torque; under-tight equals clamping force insufficient; over-tight equals bolt stretches or breaks. Some assemblies use STRETCH BOLTS or TORQUE-TO-YIELD (TTY) bolts where measured rotation past initial torque deliberately stretches bolt to a specific clamp load; one-time-use, must be replaced. THREAD ENGAGEMENT rule of thumb: 1.5 times bolt diameter of thread engagement for normal applications; 2 times or more for aluminum or other softer materials; thread engagement past full threads of nut doesn't add strength.