Washers are small but directly affect joint reliability. Flat, spring and disc washers each serve a purpose; wrong combination causes plastic crushing, locking failure or stress concentration.
Flat washers distribute clamping force and protect the workpiece surface. They are essential on plastic, aluminum and other soft materials to prevent the bolt head from embedding. Flat washers do not lock.
Spring washers provide auxiliary locking via elastic tension in general service, but their benefit is limited under high vibration or high-strength bolting. Hardened spring washers carry hydrogen embrittlement risk. They are not recommended on plastic substrates due to crushing.
Belleville disc washers maintain preload under thermal expansion/contraction or creep, good for flanged joints with temperature variation and sealing needs. In bolt+flat+spring combinations, place the spring washer against the nut/head and flat washer against the joint surface. High-strength joints should use dacromet or black oxide and follow torque specs.
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