Plain Washers vs Spring Washers: What’s the Real Difference?

Ask a site supervisor why a joint has a spring washer under the nut and the answer is almost always some version of “to stop it coming loose.” It’s the most widely repeated piece of fastener folklore in construction and mechanical assembly — and it’s more complicated than the folklore suggests. Structural design codes in several countries actively discourage relying on spring (lock) washers to prevent loosening in critical connections, for reasons that are worth understanding before specifying one out of habit.

This guide separates what plain washers and spring washers actually do mechanically, where each genuinely belongs, and why “spring washer = anti-loosening” isn’t the reliable rule of thumb it’s often treated as.

What a Plain Washer Does

A plain (flat) washer is exactly what it sounds like — a flat, round (or sometimes square) disc with a central hole, placed under a bolt head or nut before tightening. Its job is almost entirely about load distribution and surface protection, not about preventing rotation:

• Spreads clamping force over a wider area, reducing the bearing pressure (force per unit area) on the material being clamped — important when the base material is softer than the bolt/nut, or when the clearance hole is oversized relative to the bolt.
• Protects the surface finish of the clamped material from galling, scoring, or gouging as the nut or bolt head rotates during final tightening.
• Provides a smooth, flat bearing surface where the underlying material is uneven, coated, or otherwise unsuitable for direct nut/bolt-head contact.
• Compensates for oversized clearance holes, ensuring the nut or bolt head has adequate bearing area even when the hole is larger than strictly necessary for the bolt diameter.

Plain washers are manufactured to ISO 7089/7090 (metric flat washers, normal and chamfered series) and IS 2016 in Indian practice, in mild steel, HDG, stainless steel, and high-tensile grades to match the bolt they’re paired with. Shop MS/HDG/SS/HT/GI plain washers across the full material range.

What a Spring Washer Does

A spring (or “lock”) washer is a split or curved-profile washer designed to compress under the clamping load and exert continuous spring force between the bolt head or nut and the clamped surface. The most common type — a split ring washer with the ends offset in different planes — is designed so its sharp cut ends can also bite slightly into both the fastener and the mating surface, in theory adding a mechanical resistance to rotation on top of the spring force itself.

The traditional (and widely repeated) claim is that this continuous spring pressure keeps the joint under some clamping tension even if the bolt loosens slightly under vibration, and that the biting edges resist the nut rotating loose in the first place. Spring washers are manufactured to DIN 127 and similar standards, available in the same material range as plain washers — mild steel, HDG, stainless steel, and heat-treated grades for higher spring force retention. Shop MS/HDG/SS/HT spring washers to match your bolt specification.

The Engineering Debate: Do Spring Washers Actually Prevent Loosening?

This is where it’s worth being straightforward rather than repeating the folklore: the effectiveness of split spring washers at genuinely preventing vibration-induced loosening has been questioned by engineering research and is treated with caution in several structural design codes. A few key points worth understanding:

• Spring force alone is a small fraction of total bolt preload. In a properly tightened joint, the axial tension in the bolt itself is vastly greater than the spring force a lock washer can add — the washer’s spring effect is a very minor contributor to the joint’s overall clamping force compared to correct bolt tension in the first place.
• Once a joint has lost significant preload (through vibration, embedment, or relaxation), a spring washer typically cannot restore enough tension to meaningfully prevent further loosening or actual separation of the joint — its travel and force range are limited.
• The biting/locking action depends on the washer’s edges actually engaging both surfaces, which isn’t guaranteed on hardened, coated, or smooth mating surfaces, and can be defeated entirely if the washer rotates with the nut rather than staying fixed against the clamped surface.
• Some structural steel design codes and specifications explicitly exclude or discourage spring/lock washers in structural connections, particularly friction-grip (slip-critical) joints, precisely because the connection’s resistance to loosening is meant to come from correctly achieved and verified bolt tension — not from a washer’s spring force. Our guide on DTI washers covers the verification method actually specified for these critical connections.

None of this means spring washers are useless — it means their actual mechanical contribution is smaller and more situational than the common trade assumption, and for genuinely vibration-critical or safety-critical connections, engineers increasingly specify other solutions instead: correct preload verified by torque or tension-indication methods, thread-locking compounds, prevailing-torque (nylon-insert) locknuts, or mechanical locking devices specifically engineered and tested for vibration resistance.

Plain Washer vs Spring Washer: Side-by-Side

Feature	Plain Washer	Spring Washer
Primary function	Load distribution, surface protection	Adds limited spring force and (theoretically) mechanical bite against rotation
Contribution to preventing loosening	None directly — relies on correct initial bolt tension	Modest and situational — not a substitute for correct preload
Structural/slip-critical connections	Standard, widely specified	Often discouraged or excluded by design code
General light-duty assembly	Standard	Common, traditional choice, reasonable secondary measure
Governing standards	ISO 7089/7090, IS 2016	DIN 127 and similar
Best paired with	Any bolted connection needing load spreading or surface protection	Lower-criticality assemblies where some vibration resistance is a “nice to have,” not the primary safeguard
What actually prevents loosening in critical joints	Correct, verified bolt preload (torque control, turn-of-nut, or DTI verification)	Same — spring washers are a secondary measure at best

When to Use a Plain Washer

• Under any nut or bolt head bearing on a softer material (timber, aluminium, thin sheet), to spread load and avoid crushing or gouging the surface.
• Where clearance holes are oversized relative to the bolt diameter, to ensure adequate bearing area.
• Under the head of a carriage bolt through timber, often specified with a wider square washer rather than a standard round plain washer for extra bearing area — see our taper washers vs square washers guide for that related decision.
• Wherever a coated, galvanized, or finished surface needs protection from direct contact with a rotating nut or bolt head during tightening.
• As standard practice under most structural and mechanical bolted connections, essentially by default, unless the design specifically calls for something else.

When to Use a Spring Washer

• General light-duty mechanical and equipment assembly where some vibration resistance is desirable but not safety-critical, and where periodic maintenance/re-inspection is practical.
• Applications with an established, traditional specification calling for spring washers, where changing the specification isn’t practical or necessary.
• As a secondary measure alongside correct torque control, not as a substitute for it — spring washers can be a reasonable belt-and-suspenders addition to a properly tensioned joint, just not a substitute for getting the tension right in the first place.

When to Avoid Spring Washers and Use Something Else Instead

• Structural steel friction-grip (slip-critical) connections — use verified preload methods (torque control, turn-of-nut, or DTI washers) instead, per the applicable design code.
• Genuinely vibration-critical, safety-critical connections — automotive, aerospace, or heavy machinery applications where loosening has serious consequences typically specify engineered locking solutions (nylon-insert locknuts, thread-locking compounds, or mechanical locking fasteners specifically tested for the application) rather than relying on a spring washer.
• High-preload, high-tensile bolted connections — spring washers can actually interfere with achieving accurate, consistent preload in high-tensile connections, since their compression behaviour adds a variable that torque-tension calculations don’t always account for cleanly.

A Practical Way to Decide Without Relying on Habit

When a connection specification doesn’t explicitly call out a washer type, it’s worth running through a short mental checklist rather than defaulting to whatever’s traditionally used on similar jobs. First, does the clamped material need load spreading or surface protection — softer materials, oversized holes, or finished/coated surfaces all point toward a plain washer as the baseline, essentially non-negotiable component. Second, is there a genuine vibration or loosening concern, and if so, is this a safety-critical or slip-critical connection — if yes, the answer is verified preload (torque control, turn-of-nut, or a DTI washer) rather than a spring washer, and potentially an engineered locking fastener solution as well. Third, if the loosening concern is present but the connection is lower-consequence and periodically inspected/maintained, a spring washer as a secondary, traditional measure alongside correct initial tightening is a reasonable, low-cost addition — just not one to rely on as the primary safeguard. This sequence — plain washer as default, verified preload for anything critical, spring washer only as a secondary measure on lower-stakes joints — reflects how modern fastening practice has moved away from the older assumption that a spring washer alone solves a loosening problem.

Material and Finish

Both washer types are available in mild steel, hot-dip galvanized, stainless steel, and heat-treated/high-tensile grades. For spring washers specifically, heat treatment matters more than it does for plain washers, since the washer’s ability to maintain spring force over time depends on the material’s elastic properties holding up under sustained compression — a poorly heat-treated spring washer can lose its spring tension (take a “permanent set”) relatively quickly, at which point it functions as little more than a plain washer with a slightly different shape.

Other Washer Types Worth Knowing (So You Don’t Default to Spring by Habit)

Because “spring washer” is often reached for by default rather than by deliberate selection, it’s worth knowing what else exists in the broader anti-loosening washer category, so the choice is at least an informed one:

• Tooth/serrated lock washers — internal or external toothed washers that bite into both the fastener and the clamped surface, offering a different (and in some tests, more consistent) mechanical resistance to rotation than a split spring washer, though with similar caveats about not substituting for correct preload in critical connections.
• Wave washers — a smoother, wave-profile spring washer offering a softer, more distributed spring force than a split ring design, more common in applications needing to absorb thermal expansion or take up assembly tolerance than in anti-loosening roles specifically.
• Belleville (conical spring) washers — high spring force in a compact axial space, used where a significant, controlled spring preload is genuinely needed (such as maintaining consistent clamping force across thermal cycling), rather than as a general-purpose anti-loosening measure.
• Prevailing-torque locknuts (nyloc) — not a washer at all, but frequently the more effective alternative to a spring washer where genuine vibration resistance is the goal, since the friction mechanism is more consistent and doesn’t rely on the washer maintaining contact with two specific surfaces.

None of these is a universal “better” choice — each solves a specific mechanical problem, and picking the right one (or determining that correct preload alone is sufficient) depends on the actual failure mode the design is trying to prevent, not on which washer happens to be in the bin closest to the workbench.

Common Mistakes

• Assuming a spring washer alone will keep a critical joint from loosening. As covered above, this is exactly the assumption structural codes and mechanical engineering practice increasingly push back against — correct, verified preload is what actually keeps a joint tight.
• Using a plain washer where load spreading genuinely matters, and skipping it entirely. Especially on softer materials or oversized clearance holes, omitting a plain washer can lead to crushed material or reduced effective clamping area.
• Mixing washer types inconsistently across an assembly without a specification reason. If a project specification calls for a particular washer type on structural connections, deviating without engineering sign-off risks non-compliance with the design intent, even if the deviation seems minor.
• Reusing a spring washer that’s taken a permanent set. A spring washer that’s been compressed flat and no longer springs back has lost its functional purpose and should be replaced, not reused.

Sizing and Standards Reference

Plain washers are sized primarily by inner diameter (matched to the bolt/screw it’s paired with), outer diameter, and thickness, with “normal” and “large” series available under ISO 7089/7090 to suit different bearing area requirements for a given bolt size — the large series is common wherever oversized clearance holes or softer clamped materials call for extra bearing area beyond the standard series. Spring washers are similarly sized to the bolt diameter, with the working spring characteristics (free height, compressed height, and resulting spring force) defined by the relevant standard (commonly DIN 127) rather than left to individual manufacturer discretion — confirming a spring washer is manufactured to a recognized standard, rather than a generic unspecified “lock washer,” is worth doing if the spring force characteristics genuinely matter to your application.

Frequently Asked Questions

Do spring washers actually stop bolts from coming loose? Their contribution is real but limited — spring force from a lock washer is small relative to proper bolt preload, and several structural codes discourage relying on them for critical, vibration-prone, or slip-critical connections. Correct, verified bolt tension (through torque control, turn-of-nut method, or direct tension indicators) is the primary defense against loosening; spring washers are, at best, a secondary measure.

Should I use a plain washer, a spring washer, or both? It depends on the application. Structural and load-spreading needs call for a plain washer. If a project specification or established practice calls for a spring washer as a secondary vibration-resistance measure in a non-critical joint, using both (plain washer against the surface, spring washer under the nut) is common. For safety-critical or slip-critical connections, neither substitutes for correctly verified bolt tension.

What’s the difference between a lock washer and a spring washer? “Lock washer” is often used as a broader term covering several washer designs intended to resist loosening, including split spring washers, but also tooth/serrated lock washers and other variants. “Spring washer” specifically refers to the split-ring, spring-profile design discussed in this guide, which is the most common lock washer type in general construction and mechanical use.

Why do some structural codes exclude spring washers from bolted connections? Because the connection’s resistance to slip and loosening is meant to come from a verified, calculated minimum bolt tension, and spring washers introduce a variable compression element that can interfere with accurately achieving and measuring that tension — particularly in high-preload, torque-controlled, or turn-of-nut tightening methods.

Can a spring washer replace a nylon-insert (nyloc) locknut? Not reliably for applications where the nyloc locknut is specified — nylon-insert locknuts create prevailing torque resistance through friction between the nylon insert and the bolt threads, an entirely different (and generally more effective for anti-vibration purposes) mechanism than a spring washer’s limited spring force.

Are spring washers required under Indian Standard practice for general construction? There’s no universal requirement — specification depends on the project, the applicable design code, and the specific connection type. For general light-duty assembly, spring washers remain common practice by convention; for structural and slip-critical connections, follow the project’s structural design specification, which increasingly favours verified preload methods over reliance on spring washers.

What’s a better anti-loosening solution than a spring washer for a vibration-critical joint? Depends on the application, but common engineered alternatives include prevailing-torque (nylon-insert) locknuts, thread-locking compounds, or mechanical locking fasteners specifically tested for vibration resistance — combined, always, with correctly achieved and verified initial bolt preload, which remains the foundation of any anti-loosening strategy.

Can plain and spring washers be stacked together under the same nut? Yes, this is common practice — a plain washer against the clamped surface for load distribution and surface protection, with a spring washer between the plain washer and the nut for whatever secondary spring-force benefit is wanted in a non-critical connection.

Do washer thickness and diameter need to match the bolt grade, not just the bolt diameter? Generally the washer needs to match the bolt diameter for a proper fit, but for higher-tensile bolts developing significant clamping force, using a washer with adequate thickness and hardness for that load matters too — an undersized or soft washer under a high-tensile bolt can deform or dish under load, which affects both the load distribution the washer is meant to provide and, indirectly, the accuracy of torque-based tension control. When in doubt on a high-tensile connection, confirm the washer’s hardness rating against the bolt’s property class rather than assuming any washer of the right diameter will perform equivalently.

Environmental Factors That Affect Washer Choice

Beyond the core mechanical question of load spreading versus spring force, environmental exposure plays a real role in washer selection that’s easy to overlook. In outdoor or high-humidity environments, a spring washer’s split-ring design creates a crevice where moisture and contaminants can collect against the clamped surface, which — on mild steel or inadequately coated washers — can accelerate localized corrosion at exactly the point where the washer contacts the structure, sometimes faster than the surrounding flat surfaces corrode. Plain washers, having a simpler, more uniform contact profile, generally present less of this crevice-corrosion risk, though the material and finish choice (HDG vs stainless vs mild steel) still needs to match the exposure environment regardless of washer shape. For coastal, marine, or heavy-industrial-atmosphere applications using spring washers, specifying stainless steel or a robust HDG finish is worth the modest extra cost specifically because the crevice geometry makes these washers somewhat more corrosion-prone than an equivalent plain washer in the same environment.

Sourcing Both Washer Types

Shree OSR Enterprises supplies plain washers and spring washers across mild steel, HDG, stainless steel, and heat-treated grades. For structural connections requiring verified bolt tension rather than reliance on washer spring force, see our guide on DTI washers, and for angled or sloped bearing surfaces, see taper washers vs square washers. Contact us through our contact page for grade and quantity requirements.