1. Definition
A flat‑washer is a flat circular gasket with a central hole. It is fitted onto bolt shanks between nuts and workpieces together with bolts and nuts and stamped from steel plates. Main domestic standards: GB/T97.1 Grade‑A precision flat washers, GB/T95 Grade‑C coarse flat washers; European standards: DIN125 and ISO7089.
2. Core Functions
Pressure dispersion (primary function): It enlarges bearing area and prevents work‑pieces such as aluminum parts, plastic components, timber and painted surfaces from being crushed under localized high compression while tightening nuts. Work‑piece surface protection: It avoids scratches on work‑piece surfaces during bolt tightening. Anti‑corrosion isolation: It separates bolts from connected parts to reduce galvanic corrosion and compensates slight uneven gaps on mating planes. Note: Flat washers cannot prevent loosening on their own. Split lock washers are required for anti‑loosening purposes.
3. Common Materials and Surface Treatments
Ordinary carbon‑steel (Q235): Electro‑galvanized, hot‑dip galvanized or black‑oxide finished, applied for general machinery and steel structures. Stainless‑steel (304/316): Corrosion‑resistant for outdoor and humid‑condition applications. 45# medium‑carbon steel: High‑strength flat washers matched with Grade‑8.8 and Grade‑10.9 high‑tensile bolts. Copper or nylon washers: Adopted for insulation and vibration‑damping scenarios.
4. Classification by Dimensions
Standard flat washer GB/T97.1: Most widely‑used type. An M8 bolt matches a washer with an inner diameter of 8.4 mm and outer diameter of 16 mm. Small‑size washer GB/T848: Reduced outer diameter for installation‑limited narrow spaces. Oversized washer GB/T96: Larger outer diameter for improved load distribution, extensively used in steel‑structure projects. Chamfered flat washer: With a 45‑degree chamfer on one side to fit fillets of hex‑head bolts for better contact.
5. Standard Assembly Sequence
Bolt → Flat washer → Work‑piece → Flat washer → Nut. Installing a flat washer next to the nut side is recommended for even load distribution.
6. Flat Washer versus Split Lock Washer
Flat washer: Solid ring without cut‑offs; Functions: Distribute pressure and protect work‑pieces for static‑state fastening. Split lock washer: Designed with a split opening; Function: Stop nut loosening by spring force.
Main Functions of Flat Washers
Dispense clamping force of bolts to stop thin plates, cast‑iron parts, aluminum‑alloy parts and painted layers from being crushed.
Isolate bolts from base materials and avoid surface scratches during tightening.
Offset minor surface irregularities so that loads can be evenly applied.
Main functions and disadvantages of spring washers
It prevents nut loosening by spring back force. Disadvantages:
① Its spring force is insufficient. Split lock washers perform poorly against loosening under heavy‑vibration conditions including vehicles, construction machinery and fans.
② Sharp split edges of lock washers may dent work‑piece surfaces.
③ If pre‑load is excessively high, the split lock washer will be fully flattened and lose its spring‑loaded anti‑loosening capacity.
④ Standard industrial practices prohibit regular split lock washers for Grade‑10.9 and Grade‑12.9 high‑strength bolts.