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Spring washers, flat washers or lock nuts? Choosing the right locking solution

2026-08-29

Compiled by JIALI Industrial (164.hk) — a Hong Kong fastener supplier specialising in screws, bolts, nuts and selection know-how.

Locking principle and applications

A spring washer relies on the elasticity of its split opening to maintain continuous tension and take up play, which suits light-load, low-vibration joints. A flat washer mainly spreads clamping load and protects the workpiece surface — it does not provide locking by itself, a point that is responsible for a great many loosened joints on machinery. Lock nuts (nylon insert, flange face) lock through friction or controlled deformation and offer the best vibration resistance. The physics behind the difference is worth stating plainly. Every bolted joint loses preload over time as the contact surfaces bed down and the bolt relaxes, and a joint that carries vibration loses it faster because the nut can ratchet against the thread. A spring washer supplies a small amount of stored elastic energy that compensates for the first part of that relaxation. A nylon insert nut instead creates a continuous friction drag on the thread, so the nut must overcome that drag before it can rotate at all — a much more robust mechanism against reversing loads. A flange nut adds a serrated bearing face and a wider contact area, which increases friction under the head as well as on the thread. Understanding which mechanism is doing the work tells you immediately why a spring washer is the weakest of the three and why the choice should follow the vibration level, not the habit of the assembly bench. Use a spring washer for light, low-vibration connections; for critical or highly vibrating joints, prefer lock nuts.

Common sizes, materials and a comparison across solutions

Spring washers are commonly made of zinc-plated 65Mn spring steel, covering M3–M30, with stainless 304 and 316 versions available for corrosive service. Flat washers follow GB/T 97.1 (grade A) and DIN 125 in large and small outer-diameter series, and hardened washers per DIN 6916 are used under high-strength structural bolts. Nylon insert lock nuts are typically DIN 985, while flange lock nuts correspond to GB/T 6177 (DIN 6923). Selection by vibration level and consequence of failure is the cleanest way to decide: static, dry, low-consequence joints such as shelving and furniture — a flat washer with a standard hex nut is sufficient and a spring washer adds little. Light vibration with intermittent running, such as fans and small pumps — a spring washer under the nut gives useful compensation at minimum cost. Continuous vibration, such as motors, compressors, conveyors and vehicle-mounted equipment — use a nylon insert lock nut (DIN 985 / GB/T 889) or a flange lock nut, with a flat washer under the nut to protect the surface. High preload structural joints on 8.8 grade and above — use hardened washers (DIN 6916) with a controlled tightening method, and consider a direct-tension-indicating washer where the joint cannot be re-inspected. Safety-critical joints and joints that will never be inspected again — a lock nut plus anaerobic threadlocker, with the torque recorded. Match the washer material to the bolt so the pair does not become a galvanic couple in a wet environment, and never fit a stainless nut to a galvanised bolt on an outdoor joint.

Combinations, cost and the high-preload limit

Washers and nuts are usually combined rather than chosen against each other, and the combination order matters. The normal arrangement is a flat washer directly under the nut to spread the clamping load, with the locking device doing the anti-loosening work: flat washer plus spring washer plus standard nut for light vibration; flat washer plus nylon insert lock nut for continuous vibration, which is the most cost-effective reliable combination in general machinery; flat washer plus flange lock nut where the surface is soft or the hole is slotted. Two combinations should be avoided. Fitting a spring washer directly onto a painted or soft surface, where it embeds into the material and loses its preload. And stacking two flat washers where one thicker hardened washer is specified, which reduces the bearing area instead of increasing it. Cost is rarely the deciding factor at the sizes used on machinery: a spring washer costs a few cents, a nylon insert nut a few tens of cents, and the difference over an assembly with a hundred joints is trivial against the cost of a line stoppage caused by a loosened fixing. Under high preload, spring washers are less effective, and this is a technical limit rather than a preference: once the joint is tightened well beyond the washer's elastic range the washer simply flattens and stops contributing. At that point lock nuts or thread-locking adhesive (anaerobic) are more reliable, and for 10.9 and 12.9 grade bolts the correct answer is usually a controlled torque plus a mechanical locking element rather than a spring washer.

Common mistakes and what to check at acceptance

A common mistake is treating the flat washer as a locking device — it handles load spreading and surface protection, while locking should come from a spring washer or lock nut, and the two can be combined when needed. Four further mistakes are worth flagging. Reusing deformed spring washers, which have already lost the elasticity that makes them work. Fitting a lock nut the wrong way round, so the nylon insert sits away from the bolt and provides no drag. Using a plain nut on a vibrating joint and compensating with extra torque, which over-stresses the bolt without addressing the loosening mechanism. And mixing grades, so a 4.8 nut sits on an 8.8 bolt and the thread strips first under load. At acceptance, check the washer and nut part numbers against the drawing, confirm the locking element is the specified type and is fitted in the right orientation, verify the torque on a sample of joints with a calibrated wrench, and apply a visible loosening mark to critical joints so a later inspection can spot movement at a glance. On equipment that has already given trouble, the useful diagnostic step is to record which joints loosen and under what duty cycle, since that pattern points directly at whether the locking method or the preload is at fault. Need a locking solution, samples or advice on a joint that keeps working loose? Contact JIALI Industrial (164.hk) with your vibration conditions and size requirements for fast advice.

Spring washers, flat washers or lock nuts? Choosing the right locking solution

Locking principle and applications

Common sizes, materials and a comparison across solutions

Combinations, cost and the high-preload limit

Common mistakes and what to check at acceptance

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