Types of Washers: Flat, Spring, Lock, Structural & Sealing Explained

Washers are among the most commonly used fastening components in Australian industry — and among the most poorly understood. A washer is not just a spacer. Depending on type, it distributes load, protects surfaces, seals against leaks, or actively prevents bolt loosening under vibration. Choosing the wrong washer for the application is a genuine failure mode: it can cause preload loss, fastener loosening, surface damage, or joint failure.

This guide covers every washer type used in Australian industrial, engineering, and construction applications — flat, spring, lock, wedge-lock, conical, sealing, and structural — with sizing tables, material selection guidance, installation order, and a frank assessment of which washers actually work under vibration. If you have ever wondered whether your washer grade needs to match your bolt grade, or whether split lock washers actually do anything useful, both questions are answered in full below.

What Does a Washer Actually Do?

A washer is a thin disc with a central hole, placed under a bolt head, nut, or similar fastener. Depending on type, a washer performs one or more of the following functions:

• Load distribution: Spreads the bearing stress from a bolt head or nut over a larger area, preventing the fastener from pulling through soft or thin materials, and reducing surface compression and embedding.
• Surface protection: Protects painted, anodised or finished surfaces from damage caused by a rotating bolt head or nut during installation.
• Bridging oversized holes: A washer with a larger outer diameter covers holes that are oversized for the bolt, ensuring full bearing contact around the fastener.
• Sealing: Sealing washers (copper, bonded, nylon) create a leak-proof seal at hydraulic fittings, drain plugs, and pipe joints by deforming under load to conform to the mating surface.
• Vibration resistance: Spring, lock, conical, and wedge-lock washers maintain clamp load and resist fastener loosening under vibration, dynamic loading, and thermal cycling.
• Electrical isolation: Nylon and PTFE washers electrically isolate the fastener from the assembly — used in panel wiring, enclosures, and electronics.

Not every washer performs all of these functions. A plain flat washer distributes load and protects surfaces; it does not prevent loosening. A split lock washer is intended to resist loosening; it does not seal. Selecting the correct washer type requires understanding which functions are actually required for the application.

Types of Flat Washers

Flat washers are plain, unsprung discs with no special locking or sealing function. They are the most widely used washer type and come in several configurations.

Standard Flat Washer (DIN 125 / ISO 7089)

The standard metric flat washer is manufactured to DIN 125 (Form A) or the equivalent ISO 7089 specification. These are sized by bolt diameter — an M10 washer has an inner diameter of 10.5 mm, an outer diameter of 20 mm, and a thickness of 2.0 mm. Standard flat washers are used in the vast majority of general engineering, fabrication, and maintenance applications where the bolt hole is correctly sized and the surface is not soft or damaged.

Wide Pattern Flat Washer (DIN 9021)

DIN 9021 washers have a larger outer diameter than the equivalent DIN 125 washer for the same bolt size, providing a greater bearing area. They are used where additional load distribution is needed — on softer materials, thinner sections, or where higher clamping loads are expected.

Mudguard Washer (Repair Washer)

Mudguard washers have a significantly oversized outer diameter relative to their inner hole — often four to six times the inner diameter. This large bearing area is used where the fastening surface is soft (rubber, plastic, fibreglass, thin sheet metal), thin, damaged, or oversized, and a standard flat washer would cut through or fail to provide adequate bearing area. Common applications include sheet metal repair, rubber panel fastening, plastic enclosure assembly, and roofing and cladding fixings. In Australian trade, these are almost universally called mudguard washers or repair washers. The US equivalent term is fender washer; the UK equivalent is penny washer. All refer to the same product — see Australian Terminology below.

Structural Flat Washer (AS/NZS 1252)

Structural washers are heavy, hardened and tempered flat washers specified for use in high-strength structural bolt assemblies under AS/NZS 1252.1. They are dimensionally and mechanically different from standard DIN 125 washers — heavier in section and harder in material — and they are required to be used as part of a complete Grade 8.8 structural bolt assembly. See Does Washer Grade Need to Match Bolt Grade? and the Structural Washers section below for full detail.

Square Washer

Square (or plate) washers have a square outer profile rather than round. They are used in timber framing, structural timber connections, and channel fixings where a round washer would rotate within the fastening surface or where a larger bearing area over a rectangular slot is required.

Types of Lock Washers: Split, Star and Tab

Lock washers are designed to resist fastener loosening through mechanical means. There are several distinct types, each with different working mechanisms and effectiveness levels.

Split Spring Lock Washer (DIN 127)

The split spring lock washer — also called a helical spring lock washer — is the most common type of lock washer in general use. It is a single-coil spring ring with a helical cut that creates a spring-like action when compressed under a bolt head or nut. The intention is that the spring pressure maintains friction on the mating thread, and the sharp cut ends dig into the surface to resist rotation. Split lock washers are available in mild steel (zinc-plated or plain), stainless steel, and phosphor bronze. They are inexpensive and widely used. However, as discussed in detail below, their actual effectiveness against vibration loosening is limited. For non-critical, low-vibration applications, split lock washers are adequate. For critical or high-vibration applications, specify conical spring washers, Nord-Lock wedge-lock washers, or thread-locking compound instead.

Internal Star Washer (Shakeproof Washer)

Internal serrated (star) washers have serrations on the inside of the central hole. The serrated teeth bite into the bolt shank or fastened surface, providing anti-rotation through mechanical interference. They are used primarily in electrical panel wiring and electronic enclosures where both vibration resistance and electrical continuity (achieved by cutting through surface oxide) are required.

External Star Washer

External serrated washers have serrations on the outer edge of the disc. The extended teeth grip the material surface more aggressively than internal types and are used where the bolt hole size limits the use of internal serrations. Both internal and external star washers work best in soft materials and are not suitable for use on hard surfaces where the teeth cannot engage properly.

Tab Washer

Tab washers have one or more bent tabs that lock the washer — and thus the fastener — against rotation. One tab is bent down into a slot or against a shaft flat; another is bent up against the nut face once the nut is tightened to the correct position. Tab washers are used in specific mechanical assemblies (gearboxes, axle hubs, bearing retainers) where a positive, non-frictional locking mechanism is required and where disassembly is anticipated. They are single-use: once bent, the tab should not be straightened and reused.

Conical Spring Washers and Belleville Disc Washers

Conical spring washers and Belleville disc springs store elastic energy in their conical shape. When compressed flat under a bolt head or nut, they act as a spring that continuously pushes back against the fastener, maintaining clamping force even when thermal expansion, vibration, or material settling would otherwise cause preload loss. For a wider view of where disc springs fit in the broader spring family — alongside compression, extension, torsion, gas and leaf types — see our Types of Springs Guide.

Schnorr Safety Washer (DIN 6796)

Schnorr conical spring washers — manufactured to DIN 6796 — are specifically designed for bolted connections. Their conical disc spring action provides a controlled spring clamping force that maintains bolt preload under vibration and thermal cycling. Unlike split lock washers, Schnorr DIN 6796 washers are engineered to a tested spring constant, providing a predictable and measurable clamping contribution. They are reusable, operate in both metric and imperial fastener systems, and are widely specified in automotive, rail, agricultural machinery, and general engineering where vibration resistance better than a split lock washer is needed but the bulk and cost of Nord-Lock is not warranted. Schnorr washers are available from AIMS Industrial for key metric sizes.

Belleville Disc Spring

Belleville disc springs (sometimes called disc spring washers or cupped spring washers) are conical washers designed as mechanical springs rather than as anti-loosening devices specifically. They are used to provide a controlled spring force in mechanical assemblies — for example, to compensate for thermal expansion, provide consistent bearing preload, or act as a buffer in impact applications. Belleville springs are typically used in stacks (parallel for increased force, series for increased deflection) and are specified by spring constant and deflection rather than by washer size alone.

Wedge-Lock Washers: How Nord-Lock Works

Nord-Lock wedge-lock washers are a mechanically engineered vibration-resistant fastening solution that works on a fundamentally different principle from all other washer types. They are supplied in pairs and function as a system — never as individual washers.

The Wedge-Lock Principle

Each Nord-Lock washer pair has two distinct faces: a cam face (inside) with a radial pattern of wedges, and a cross-hatch serrated face (outside) that grips the bolt head or nut and the mating surface. When the bolt is tightened, the cross-hatch faces grip the bolt head and the joint surface. The cam faces of the two washers mate against each other. When vibration or dynamic loading attempts to loosen the bolt, any rotation of the bolt requires the cam faces to ride up over each other — which increases the effective bolt length and therefore the bolt tension. The system locks because the cam angle is greater than the thread pitch angle, making reverse rotation mechanically impossible without increasing clamp load. Nord-Lock washers do not rely on friction or spring force — the locking action is geometric and positive.

Nord-Lock Installation

Nord-Lock washers are installed in pairs, cam faces together (the smooth, wedge-patterned sides face each other), with the cross-hatch serrated faces outward against the bolt head and the joint surface. The pair is installed as a unit — separating the pair defeats the locking mechanism. No thread-locking compound is used with Nord-Lock. Standard bolt torque values apply; no special tooling is required.

Nord-Lock Applications

Nord-Lock wedge-lock washers are specified in heavy machinery, rail rolling stock, mining equipment, structural steel, bridges, wind turbines, and any application where bolt loosening under vibration is a critical failure risk. They are reusable (inspect the cross-hatch faces between uses), compatible with standard metric and imperial fasteners, and available in standard steel and stainless steel (A4 / 316 equivalent) from AIMS Industrial.

Browse Nord-Lock wedge-lock washers at AIMS Industrial

Sealing Washers

Sealing washers combine a washer's load distribution function with a leak-proof seal at hydraulic fittings, pipe connections, fuel systems, and pressure joints. The sealing action is achieved by using a soft, deformable material — either as the full washer or as a bonded ring on a metal backing — that yields under bolt load to conform tightly to the mating face.

Copper Sealing Washers

Copper is the standard material for metric hydraulic and fuel system sealing washers. Copper's low yield strength means it deforms readily under bolt load, conforming to minor irregularities in the fitting face to create a leak-proof metal-to-metal seal. Copper sealing washers are used at banjo fittings, hydraulic brake fittings, diesel injector connections, engine drain plugs, and high-pressure fuel union faces. They are sized to metric bolt/fitting standards and are generally single-use — once compressed, the copper work-hardens and will not seal as effectively if reused. AIMS Industrial supplies Champion CWC-series metric copper sealing washers across common metric sizes.

Browse copper sealing washers at AIMS Industrial

Bonded Sealing Washers

Bonded (or rubber-faced) sealing washers have a metal outer ring with a vulcanised rubber or EPDM sealing face. They are widely used with self-drilling (Tek) screws in roofing and cladding, and at plumbing connections where a combined seal and load spreader is needed. The metal backing provides the structural bearing area; the rubber face provides the seal.

Nylon and PTFE Sealing Washers

Nylon and PTFE washers provide light-duty sealing combined with electrical isolation. They are used in plumbing tap connections, garden fittings, and electrical enclosures where both a seal and isolation from the metal fastener are required. PTFE washers offer higher chemical resistance and broader temperature range than nylon.

Structural Washers (AS/NZS 1252)

Structural washers are hardened and tempered flat washers specified exclusively for use in high-strength structural bolt assemblies under the Australian/New Zealand standard AS/NZS 1252.1:2016.

AS/NZS 1252.1 covers Grade 8.8 bolt assemblies — bolts, nuts, and washers — in diameters M12 to M36 for structural steel connections. Critically, AS/NZS 1252.1 treats the bolt, nut, and washer as a tested assembly, not as interchangeable individual components. When a structural drawing specifies an "AS/NZS 1252.1 assembly", the entire set must comply — including the washer.

Structural washers under AS/NZS 1252.1 are heavier and harder than standard DIN 125 washers of the same bolt size. They must be used with Grade 8.8 structural bolt assemblies for one critical reason: a standard mild steel or stainless flat washer will brinel (embed) under the high preload of a tightened Grade 8.8 structural bolt. This embedding causes immediate preload loss — the joint loses clamp force before the bolt even enters service. Hardened structural washers resist this embedding and maintain the preload the bolt was torqued to.

Structural washers are used in structural steel connections in buildings, bridges, industrial platforms, crane rails, and any bolted connection designed and documented to AS/NZS structural standards. They are not required in general engineering applications using Grade 8.8 bolts unless the connection is specifically designed to the structural standard. For further detail on when the washer grade must match the bolt grade, see the section below.

Washer Materials Guide

The washer material must suit the environment, the fastener material, and the service conditions. Using the wrong material causes galvanic corrosion, premature failure, or contamination in sensitive environments.

Zinc-Plated Mild Steel

The most common washer material for general engineering, maintenance, and fabrication. Zinc plating provides light corrosion resistance suitable for indoor, sheltered, and dry environments. Not suitable for marine, coastal, chemical, or wet outdoor environments where corrosion is a genuine risk. Inexpensive and widely available across all metric and imperial sizes.

Hot-Dip Galvanised Steel

Hot-dip galvanised (HDG) washers have a significantly heavier zinc coating than electroplated washers, providing substantially better outdoor corrosion resistance. Used in structural steel connections (with HDG bolts and nuts), fencing, rural and agricultural structures, and outdoor installations in sheltered or inland environments. Note that HDG fasteners should not be used with stainless steel fasteners — galvanic coupling accelerates corrosion of the zinc coating.

304 Stainless Steel (A2)

304 stainless (also designated A2 under ISO 3506) is the standard stainless steel grade for general engineering, food-grade, and light marine applications. 304 stainless washers are used in food processing equipment, pharmaceutical machinery, indoor marine fittings, general outdoor structures in low-chloride environments, and alongside A2-grade stainless bolts and nuts. 304 stainless provides good general corrosion resistance but is not suitable for high-chloride environments (saltwater, coastal spray, bleach contact).

316 Stainless Steel (A4)

316 stainless (A4) contains molybdenum, which provides significantly better resistance to chloride corrosion than 304. 316 stainless washers are specified for marine fittings, coastal infrastructure, swimming pool equipment, chemical plant, food processing lines where chloride-based cleaning agents are used, and offshore applications. Never substitute 304 for 316 in saltwater, marine spray, or bleach-contact applications — pitting corrosion will occur. AIMS Industrial stocks 316 stainless flat and mudguard washers from Hobson, Inox World, and GJ Works.

Copper

Copper washers are used primarily as sealing washers at hydraulic fittings and fuel system connections. Copper's softness (low yield strength) is the functional property — it deforms under bolt load to seal. Copper washers are not used as structural flat washers. They are also used in some electrical applications for their conductivity.

Brass

Brass washers are used where non-sparking properties are required (explosion-risk environments), for decorative applications, and in some electrical and plumbing connections. Brass offers moderate corrosion resistance and is compatible with copper and bronze fasteners without galvanic risk.

Nylon

Nylon washers are used for electrical isolation (preventing electrical contact between the fastener and the assembly), vibration dampening, and light sealing in low-pressure plumbing fittings. Nylon is lightweight, non-conductive, and resistant to many chemicals. Not suitable for high temperature, high clamping force, or structural applications.

Metric Washer Sizes: Reading the Numbers

Metric flat washers are sized by the nominal bolt diameter they are designed to fit — an M8 washer is used with an M8 bolt. The inner diameter (ID) of the washer is slightly larger than the bolt shank to allow for clearance. The outer diameter (OD) and thickness vary by specification (DIN 125, DIN 9021, or manufacturer standard).

The table below shows standard DIN 125 (Form A) dimensions for common metric sizes:

For non-standard assemblies, always confirm the inner diameter (ID), outer diameter (OD), and thickness against the actual fastener and joint dimensions. Do not assume all M10 washers are dimensionally identical — there is variation between DIN 125, DIN 9021, and manufacturer standards, particularly in outer diameter.

Imperial washers: Imperial washers are sized in fractional inch (1/4", 5/16", 3/8", 1/2", and so on) and are available in SAE and USS standards (USS being slightly wider for the same bolt size). Imperial washers remain in use in Australian industry on older plant, imported equipment, and applications where imperial fasteners are still specified.

Does Washer Grade Need to Match Bolt Grade?

This is one of the most frequently asked — and least clearly answered — questions in fastener selection. The short answer is: it depends on the bolt grade and the type of application. Here is the full breakdown.

Commercial and General Engineering (Grade 4.6, 8.8 Non-Structural)

For general engineering and maintenance applications using commercial Grade 4.6 or general-purpose Grade 8.8 bolts, standard commercial washers — zinc-plated mild steel, 304 stainless, 316 stainless, or hot-dip galvanised — are appropriate. Grade matching is not required. The washer's role in these applications is load distribution and surface protection; the washer does not need to match the bolt's mechanical strength because the bolt is not being preloaded to levels that cause washer embedding. This covers the vast majority of industrial maintenance, fabrication, and general engineering work in Australia.

Structural Grade 8.8 Applications (AS/NZS 1252)

Where Grade 8.8 bolts are being used in structural steel connections to AS/NZS 1252.1, hardened structural washers are mandatory — not optional. The reason is mechanical: structural bolts are torqued to high preloads that exceed the yield strength of a standard commercial flat washer. A standard washer in this position will brinel — the washer embeds into the steel surface or the bolt head face — causing an immediate and significant loss of the preload that the structural design depended on. Hardened structural washers resist this embedding and maintain the designed preload.

AS/NZS 1252.1 treats the bolt, nut, and washer as a complete assembly — not as three separate components. When a drawing or specification calls for AS/NZS 1252.1 fasteners, all three elements of the assembly must comply. Substituting a standard flat washer into an AS/NZS 1252.1 structural assembly is a non-conformance.

High-Strength Stainless Assemblies (A4-80, Bumax)

Premium high-strength stainless bolts such as Bumax 88 or Bumax 109 have yield strengths that exceed the capacity of standard A4-70 stainless washers. In a high-strength stainless assembly, a standard A4-70 washer becomes the weakest component — it will embed or deform under the preload required to develop the bolt's rated performance. Bumax supplies matched high-strength stainless washers to pair with their high-performance bolts. If you are specifying Bumax or equivalent premium stainless fasteners, specify matched washers. AIMS Industrial stocks Bumax washers alongside Bumax bolt and nut ranges.

The Underlying Principle

The washer must not become the weakest element in the fastener assembly. In most general engineering applications, this is not a concern — standard commercial washers are more than strong enough for load distribution under the preloads involved. The rule becomes critical only when the bolt is being preloaded to structural levels (AS/NZS 1252.1 Grade 8.8 structural connections) or when the bolt grade is genuinely high-performance (Bumax, A4-80). In both cases, the washer grade must match the assembly.

Correct Washer Installation Order

When using a combination of flat and spring or lock washers on the same fastener, the installation order matters for both the function and effectiveness of the assembly.

Using Flat Washer and Split Lock Washer Together

The correct sequence from the bolt head outward is:

1. Bolt head
2. Flat washer — sits against the bolt head, distributes load, protects the surface
3. Split lock washer — sits between the flat washer and the nut (or between the flat washer and the material, on the nut side)
4. Material being fastened
5. Flat washer (optional — recommended on soft or finished surfaces on the nut side)
6. Nut

The flat washer goes under the bolt head (or under the nut) to distribute load across the surface. The spring or lock washer sits adjacent to the nut, where its spring action is preserved and its cut ends can engage properly. Placing the lock washer under the flat washer (between the flat washer and the bolt head) reduces its effectiveness because the flat washer prevents the lock washer ends from engaging the surface.

Using Flat Washer on Both Sides

On soft, painted, or finished surfaces where surface damage from a rotating nut is a concern, use a flat washer under both the bolt head and the nut. This is standard practice in automotive, furniture, and equipment assembly where the surface quality must be preserved.

Nord-Lock Installation Order

Nord-Lock wedge-lock washers are installed in pairs, with the cam faces (smooth wedge-patterned faces) facing each other inside the pair. The cross-hatch serrated faces face outward — one against the bolt head or nut face, one against the joint surface. Install the pair as a unit and tighten to standard bolt torque. Do not separate the pair. A flat washer may be used under the Nord-Lock pair if load distribution is also required, but the Nord-Lock washers must be closest to the bolt head and nut, not buried between flat washers.

On Soft or Thin Materials

When fastening through thin sheet metal, rubber, plastic, fibreglass or timber, use a flat washer (or preferably a mudguard washer for very soft materials) on both sides of the fastened material. This prevents the bolt head and nut from pulling through or crushing the material under clamp load.

Do Split Lock Washers Actually Work?

This question has a definitive answer in engineering literature, even if it has not filtered into common practice: split lock washers provide very limited vibration resistance, and in rigorous testing, their effectiveness against vibration-induced loosening is essentially negligible.

NASA's Fastener Design Manual — one of the most referenced documents in fastener engineering — concluded that the locking ability of the helical spring lock washer is "nonexistent" under the tested conditions. Multiple independent engineering studies have confirmed this finding. The specific failure mechanism is straightforward: when vibration begins to loosen the bolt and the joint loses a small amount of clamp force, the split ring flattens completely and its spring action disappears — the very condition it was designed to prevent now neutralises it entirely.

Why Split Lock Washers Persist

Despite the engineering evidence, split lock washers remain extremely common because they are very inexpensive, simple to install, and adequate for a large number of non-critical, low-vibration applications. A garden gate hinge, an electrical panel cover, a general fabrication bracket — in applications like these, vibration loosening is not a design concern and a split lock washer does the job of preventing accidental loosening during handling and assembly. The problem arises when split lock washers are specified in applications where genuine vibration resistance is needed: machinery bolts, motor mounts, vehicle components, agricultural equipment, mining plant.

When Split Lock Washers Are Acceptable

• Non-critical, low-vibration applications where accidental loosening during assembly is the primary concern
• General fabrication and electrical panel work where vibration exposure is minimal
• When combined with correct torque on a properly torqued joint (proper torque is far more important than the washer type)
• Where cost and simplicity outweigh the need for engineered vibration resistance

When to Use Something Better

• Machinery, motors, compressors, pumps — specify Schnorr DIN 6796 conical spring washers or Nord-Lock wedge-lock washers
• Mining equipment, rail, heavy vehicles — specify Nord-Lock (positive locking, reusable, proven)
• Where disassembly is not planned — specify Loctite 243 thread-locking compound (see the Loctite 243 Guide for full detail)
• Structural connections — specify hardened structural washers to AS/NZS 1252.1 and correct torque

Nord-Lock vs Split Lock vs Schnorr vs Loctite: Vibration Resistance Compared

The table below compares the four main approaches to vibration-resistant fastening used in Australian industrial applications:

For applications where vibration resistance genuinely matters, the choice is between Nord-Lock (mechanical, reusable, positive locking), Schnorr DIN 6796 (spring-loaded preload, good vibration resistance, lower cost than Nord-Lock), and Loctite (chemical, non-reusable, no additional bulk). Split lock washers should not be specified in applications where the table above shows the other options are required.

See the Loctite 243 Guide for a detailed comparison of thread-locking compound grades and applications.

Australian Washer Terminology: Mudguard, Penny and Fender

The oversized flat washer with a large outer diameter relative to its inner hole is known by three different names depending on region:

All three names refer to the same product: a flat washer with an outer diameter significantly larger than a standard DIN 125 or DIN 9021 washer for the same bolt size, providing a much greater bearing area. The AU trade term "mudguard washer" references the original automotive application — covering the oversized fixings on vehicle mudguards/fenders. "Repair washer" reflects the common use in sheet metal repair where a larger bearing area is needed to bridge damaged or oversized holes.

When sourcing from Australian suppliers, use "mudguard washer" or "repair washer". If sourcing from US suppliers or catalogues, search for "fender washer". UK sourcing — "penny washer". The product itself is identical in function and dimensional format.

Browse mudguard washers at AIMS Industrial — available in 304 and 316 stainless steel

How to Choose the Right Washer

Use the decision framework below to select the correct washer type for the application:

For application-specific advice or to confirm the correct washer specification for your assembly, contact the AIMS Industrial team. We stock washers from Nord-Lock, Schnorr, Hobson, Bremick, Bumax, Inox World, GJ Works, and Champion — across flat, spring, wedge-lock, structural, mudguard, and sealing types in metric and imperial sizes.

Shop all washers at AIMS Industrial

Wing nut + washer combinations

Wing nuts deserve a special note. The wing nut's small bearing surface concentrates load on the underlying material, so always pair it with a flat washer to spread the clamping force and protect the surface. For vibration-prone applications, add a spring washer between the wing nut and flat washer to maintain tension as the nut backs off slightly. For the full reference on wing nut types, sizes, materials, and the limitations that make washer choice critical, see our Wing Nut Guide.

Frequently Asked Questions

What is the difference between a flat washer and a lock washer?

A flat washer is a plain disc that distributes the load from a bolt head or nut over a larger bearing area and protects the surface from damage — it does not prevent loosening. A lock washer (split, star, conical, or wedge-lock) is designed to resist the bolt or nut from loosening due to vibration or dynamic loads. In many assemblies, both are used together: the flat washer distributes load, the lock washer resists loosening.

Do I need a washer with every bolt?

Not necessarily. In precision machined assemblies where the bolt hole diameter is correct, the mating surface is hard and flat, and the bolt is torqued to a controlled preload, a washer may not be required. In most general engineering, construction, and maintenance applications, a flat washer under the bolt head and nut is standard practice because it distributes load, protects surfaces, and bridges any minor hole oversizing. When in doubt, use a flat washer.

Does washer grade need to match bolt grade?

For most general engineering and commercial applications (Grade 4.6 or non-structural Grade 8.8 bolts), standard commercial washers — zinc-plated, stainless, or galvanised — are appropriate and grade matching is not required. However, for structural Grade 8.8 bolts under AS/NZS 1252.1, hardened structural washers are mandatory — the standard treats bolt, nut, and washer as a complete tested assembly. For premium high-strength stainless bolts (such as Bumax A4-80), matched high-strength stainless washers should be used to prevent the washer becoming the weakest element in the assembly.

What size washer do I need for an M10 bolt?

An M10 flat washer (DIN 125 / ISO 7089) has an inner diameter of 10.5 mm, an outer diameter of 20 mm, and a thickness of 2.0 mm. Always select a washer with the same M-number as your bolt — M10 bolt with M10 washer. If a larger bearing area is needed (soft material, oversized hole), use a DIN 9021 wide-pattern M10 washer or an M10 mudguard washer. Do not use an oversized washer (M12 or M16) on an M10 bolt — the inner diameter clearance will be excessive.

Which goes first — flat washer or spring washer?

The flat washer goes against the bolt head (and optionally the nut face), with the spring or lock washer adjacent to the nut. The correct sequence from bolt head outward is: bolt head → flat washer → spring/lock washer → material → (flat washer, if required on nut side) → nut. The flat washer distributes load; the spring washer sits adjacent to the nut where its spring or locking action is most effective. Placing the lock washer between the flat washer and the bolt head buries the lock washer and reduces its effectiveness.

Do split lock washers actually prevent loosening?

In rigorous engineering testing, split spring lock washers provide very limited vibration resistance. NASA's Fastener Design Manual concluded their locking ability is essentially nonexistent under vibration conditions. They are adequate for non-critical, low-vibration applications where accidental loosening during assembly is the primary concern. For genuine vibration resistance in machinery, mining equipment, rail, or vehicle components, specify Schnorr DIN 6796 conical spring washers, Nord-Lock wedge-lock washers, or Loctite thread-locking compound — all of which provide measurably better vibration resistance.

What is a mudguard washer used for?

A mudguard washer (also called a repair washer) has an oversized outer diameter relative to its inner hole, providing a much larger bearing area than a standard flat washer. Mudguard washers are used where the fastening surface is soft (rubber, plastic, fibreglass, thin sheet metal), where the hole is oversized or damaged, or where pull-through of the fastener is a risk. Common applications include sheet metal repair, rubber panel fastening, plastic enclosure assembly, roofing, and any application where a standard flat washer would not provide sufficient bearing area. In the US, the same product is called a fender washer; in the UK, a penny washer.

What is a Nord-Lock washer and how does it work?

Nord-Lock wedge-lock washers are a vibration-resistant fastening solution supplied in pairs. Each pair has cam faces (wedge-patterned) on the inside that mate with each other, and cross-hatch serrated faces on the outside that grip the bolt head/nut and the joint surface. When vibration attempts to loosen the bolt, the cam faces ride up over each other — increasing bolt tension rather than allowing the bolt to loosen. The cam angle is greater than the thread pitch angle, making spontaneous loosening geometrically impossible. Nord-Lock washers are reusable, work with standard bolt torque, and are specified in mining, rail, heavy machinery, and structural applications where vibration resistance is critical.

Can washers be reused?

Most flat washers can be reused if they are not damaged, deformed, or corroded. Conical spring washers (Schnorr DIN 6796) and Nord-Lock wedge-lock washers are designed to be reusable — inspect for damage to the serrated faces or loss of spring shape before reuse. Split lock washers can technically be reused but their spring action diminishes with each use. Copper sealing washers should not be reused — copper work-hardens under compression and will not seal reliably if reinstalled. Tab washers must not be reused — re-bending the tab weakens it. Structural washers (AS/NZS 1252.1) should not be reused once removed from a structural connection.

What is the difference between 304 and 316 stainless steel washers?

Both 304 (A2) and 316 (A4) are austenitic stainless steels with good general corrosion resistance, but 316 contains molybdenum which provides substantially better resistance to chloride corrosion. Use 304 stainless washers for food-grade, pharmaceutical, general engineering, and light marine applications in sheltered or low-chloride environments. Use 316 stainless washers in marine fittings, coastal structures, swimming pools, chemical plant, and food processing where chloride-based cleaning agents are used. Never substitute 304 for 316 in saltwater or marine spray environments — pitting corrosion will occur.

When is a hardened structural washer required?

A hardened structural washer is required in structural steel connections where the bolt assembly is specified to AS/NZS 1252.1 — Grade 8.8 high-strength bolt assemblies in structural steel construction. AS/NZS 1252.1 treats the bolt, nut, and washer as a tested assembly. The hardened washer is required because standard flat washers will embed (brinel) into the steel surface under the high preload of a structural bolt, causing preload loss. In general engineering applications using Grade 8.8 bolts where the connection is not designed to the structural standard, standard commercial washers are acceptable.

What are copper sealing washers used for?

Copper sealing washers are used at hydraulic fittings, fuel system connections, brake line banjo fittings, diesel injector unions, engine drain plugs, and other high-pressure fluid connections where a leak-proof metal-to-metal seal is required at the fastener face. Copper's low yield strength means it deforms under bolt load to conform to minor irregularities in the fitting face, creating a seal. Copper sealing washers are sized to metric bolt/fitting standards and are generally single-use — copper work-hardens under compression and does not seal reliably if reused.