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Which washer, and in what order? Flat, spring, flange-face and wedge-locking washers explained
2026-10-26
Compiled by JIALI Industrial (164.hk) — a Hong Kong fastener supplier specialising in screws, bolts, nuts and selection know-how.
A washer is not a spare part - it does four jobs at once
On a drawing a washer often looks like an optional disc, yet in a real joint it performs four jobs: spreading bearing pressure, protecting the base material surface, isolating dissimilar metals from galvanic corrosion, and contributing to locking. All four have quantified limits. Pressure spreading is about bearing stress: an 8.8 grade M10 bolt preloaded to 0.7 of yield carries roughly 26 kN; bearing directly on an aluminium extrusion or a thin concrete embed plate that can push local pressure above 200 MPa - right at the edge of the local bearing limit of 6061-T6 (about 240 MPa) - and the base material collapses, bleeding preload on the spot. Standard flat washers to GB/T 97.1 come in A and B grades with thickness ratios around 1 : 1.5 (roughly 2 mm versus 3 mm at M10); a thin washer under a 10.9 or 12.9 joint simply dishes, a failure point many buyers overlook. Understanding these four jobs is what stops selection from becoming a price-only decision.
Four families compared: flat, spring, flange-face and wedge-locking
Family one, flat washers (GB/T 97.1 / DIN 125, GB/T 848 small series, GB/T 5287 large series): they spread load and protect the surface but do not lock, with hardness typically 100-200 HV - the economical choice up to grade 8.8. Under 10.9/12.9 bolts use the thicker GB/T 97.1 or hardened GB/T 1230 washers (300 HV and above), otherwise the washer yields first and preload follows it down. Family two, spring washers (GB/T 93 / DIN 127): elastic deflection compensates slight thermal expansion and mild vibration, but high preload flattens them and GB/T 94.1 itself limits their compensation capability - not recommended above grade 10.9. Family three, integrated flange faces (GB/T 6177 / DIN 6923 flange nuts, GB/T 5789 flange bolts): washer and nut combined, cutting part count and eliminating missing-washer errors; the bearing diameter typically runs 30% larger than the hex width across flats, ideal for automated lines and thin sheet. Family four, wedge-locking washers (DIN 25201, verified to GB/T 10431): two washers with radial cams mated cam-to-cam, the cam angle exceeding the thread lead angle so any loosening rotation must climb the cam and locks solid - among the most reliable purely mechanical anti-loosening devices for vibration, at roughly 8-15 times the price of a plain washer.
Installation order, hardness matching and acceptance: three common traps
Trap one is stacking order: the spring washer must sit against the nut or bolt head with the flat washer against the base material; reversed (flat washer outside, spring washer buried) the split will scar the base material and lose its compensating action, and wedge-locking washers must be used in matched pairs, cam face to cam face - a single washer is just an expensive flat washer. Trap two is clearance-hole fit: GB/T 5277 gives a medium-fit hole of 10.5 mm for M10, leaving 0.5 mm clearance; open it to 12 mm and the effective bearing area drops by more than 30%, concentrating stress rather than spreading it. Trap three is hardness matching: 8.8 bolts pair with 200 HV washers and 10.9+ with 300 HV, and note that A2-70 stainless bolts against carbon-steel washers produce fretting marks and rust bloom - coastal projects should use matching material or an insulating washer to break the galvanic couple. For acceptance, sample per GB/T 2828.1 (AQL 1.0-2.5) checking inner and outer diameter, thickness and hardness to GB/T 4340.1 Vickers, request EN 10204 3.1 certificates for hardened washers, and verify critical joints by torque-clamp-load testing per GB/T 16823.3 to confirm the assembled preload meets design. Need flat, spring, flange-face or DIN 25201 wedge-locking washers? Contact Chiali Industrial (164.hk) with sizes, bolt grade and service conditions for a matching proposal and quotation.
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