Where do Washers Go on Bolts

When it comes to bolted connections, the humble washer is often overlooked, yet choosing the right washer and placing it correctly can mean the difference between a joint that lasts decades and one that fails in weeks. At KENENG, we manufacture high‑precision washers with strict tolerances, multiple surface finishes (zinc plating, dacromet, geometric coating), and proven anti‑vibration performance. This guide not only shows you where to place washers on bolts, but also why and which type of washers to choose for maximum reliability.

Common Types of Washers Used in Conjunction with Bolts

1. Flat Washers – Load Distribution & Surface Protection

These flat washers are the most common and have a simple, flat, round shape. Flat washers are used to distribute weight, minimize friction, and protect the surface. Flat washers are available in a variety of sizes to accommodate various bolt diameters.

Hole Clearance Control: KENENG flat washers are precision stamped, with an inner diameter tolerance controlled within ±0.1mm, far exceeding industry standards (typically ±0.25mm), effectively reducing eccentric loads.
Surface Treatment Options: Zinc plating (yellow zinc/blue-white zinc), Dacromet, Gumite, blackening, phosphating, etc. are available. Salt spray testing can reach over 720 hours (Dacromet treatment).
Hardness and Fatigue Resistance: Made of 65Mn, 304/316 stainless steel, or 45# steel, with a surface hardness of HV300~450 after heat treatment, providing strong resistance to indentation and preventing bolt heads from embedding into the substrate.
Edge Burr Control: Deburring process prevents scratching of the workpiece surface.

Typical Applications: Automotive chassis, steel structure connections, electronic equipment grounding (providing a conductive plane).

2. Split Lock Washers – Vibration Resistance (Helical Spring Washers)

When compressed, split lock washers, also known as spring washers, feature a split or helical cut that creates tension. They are used to keep nuts and bolts from becoming loose as a result of vibration or heat expansion and contraction.

In-depth performance analysis:

Vibration Resistance: The open washer generates axial elasticity after being flattened, compensating for thread loosening caused by vibration. In vibration tests at frequencies of 20~200Hz and amplitudes of 2mm, its anti-loosening life is 3 times longer than that of ordinary flat washers.
Material and Fatigue: Made of spring steel (65Mn/SK5), quenched and tempered, it has a high elastic limit; after 10,000 cycles of repeated compression, the residual deformation is <0.05mm.
Surface Treatment: Zinc plating or phosphating to avoid hydrogen embrittlement risk (passes ISO 15330 hydrogen embrittlement test).
Caution and Limitations: Not suitable for soft materials (such as aluminum alloys and plastics), as the open edge may embed into the surface.

Recommended applications: industrial fans, agricultural machinery, vibrating screens, motor bases.

3. Serrated Lock Washers (External/Internal Teeth) – High Friction Locking

Serrated lock washers with teeth bite into the material and fastener, giving a locking action. They are often utilized in applications that require high vibration resistance.

Advantages

Embedded Anti-Loosening: The tooth tips penetrate the bolt head/nut and workpiece surface, creating a micro-embedding effect. The loosening torque is 40% higher than that of ordinary locking washers.
Fatigue Resistance: Due to the uniform distribution of contact stress by the tooth structure, the fatigue life under cyclic loads is twice that of two-piece locking washers.
Surface Treatment and Precision: The tooth tip height consistency is controlled within ±0.03mm, ensuring all teeth are in contact simultaneously; silver or tin plating is optional to enhance conductivity and prevent seizing.
Hole Clearance Requirements: Precise matching with the bolt diameter is required; a clearance ≤0.2mm is recommended, otherwise the tooth tips cannot effectively engage.

Serrated Lock Washer With External Teeth

Typical Applications: Rail transit, heavy truck suspension, wind turbine tower bolts.

4. Wave Washers – Preload & Shock Absorption

Wave washers are used to absorb shock, maintain tension, and reduce vibration. They feature a wavy or corrugated shape. They are frequently used in the automotive and aerospace industries.

Parameters :

Waveform Quantity and Stiffness: 3-wave, 4-wave, and 6-wave options available; stiffness range 5~500 N/mm; preload can be precisely calculated (suitable for applications with limited axial space).
Fatigue Resistance: Made of 301 stainless steel or Inconel, dynamic fatigue life exceeds 2 million cycles (amplitude 0.5 mm).
Surface Treatment: Polished or passivated to eliminate stress concentration points.
In Contrast to Flat Washers: Not used for load distribution, but rather to provide elastic preload to compensate for thermal expansion or wear.

Application Examples: Bearing preload, valve seals, connector terminals, precision instruments.

5. Belleville Washers – High Load in Small Space

Belleville washers, also known as conical spring washers, are conical in shape and are intended to produce large spring loads in small places. They are utilized in situations that need a large level of axial load.

Key Parameters :

Load-Deformation Characteristics: Cone angle 8°~12°, a single washer can withstand hundreds to tens of thousands of Newtons; can be used in combination (buttress/stacked) to achieve different stiffness and stroke.
Vibration Resistance and Relaxation: Hysteresis loop characteristics effectively absorb impact energy; in a 35Hz vibration test, preload attenuation is <8% (compared to 15~20% for ordinary disc washers)
Accuracy Grades: Keneng offers two accuracy grades according to DIN 2093 standard (thickness tolerance ±0.02mm, cone height tolerance ±0.05mm)
Surface Treatment: Dacromet or zinc-aluminum coating, resistant to high-temperature corrosion (suitable for exhaust systems).

Typical Scenarios: Steel structure preload, oil pipeline flanges, injection molding machine mold locking.

Washer Type Comparison

Washer Type	Function	Key Features	Advantages	Limitations
Flat Washer	Distributes load, protects surface	Simple, flat disc; uniform thickness	Reduces surface damage, prevents loosening	Does not prevent rotation under vibration
Split Lock Washer	Prevents loosening due to vibration	Ring split with a slight helical shape	Adds spring tension, reduces loosening	Can damage soft surfaces, limited load capacity
Serrated Lock Washer	Prevents loosening and improves grip	Teeth or serrations on the contact surface	Strong grip, good for high vibration	Can mar the surface, may deform under high loads
Wave Washer	Provides axial load, absorbs shock	Wavy, curved shape	Absorbs shock, compensates for thermal expansion	Limited load capacity, can fatigue over time
Belleville Washer	High-load spring action, maintains tension	Conical disc shape	Handles heavy loads, allows precise preload	Requires careful stacking for desired tension

Selection Recommendations

For applications requiring pure load distribution and surface protection: Flat washers
For general vibration damping and low cost: Split locking washers
For high vibration resistance and acceptable surface marks: Toothed locking washers
For axial elastic preload and short stroke: Wave washers
For high preload and minimal space requirements: Disc washers

Proper Placement of Washers on Bolts

1. Between the Bolt Head and the Material

Washers are commonly used between the bolt head and the material to which it is fastened. This is the most common arrangement for fastening purposes. This is the standard configuration for most fastening applications, and it serves several important purposes:

Correct Order: Bolt head → Flat washer (optional) → Workpiece → Nut. If using both flat and locking washers, the order should be: Bolt head → Flat washer → Locking washer → Workpiece → Nut (or symmetrical ends).
Common Mistake: Placing the flat washer outside the locking washer (closer to the nut), causing the locking washer to not compress effectively.
Hole Clearance Effect: If the workpiece hole diameter is too large (> bolt diameter + 0.3mm), a larger outer diameter flat washer must be used; otherwise, the washer will sink into the hole and lose its load-distributing function. Keneng offers custom outer diameter washers, covering holes up to Φ50mm.
Surface Treatment Matching: When the bolt is stainless steel and the workpiece is aluminum, it is recommended that the washer be galvanized or zinc-aluminum coated to prevent galvanic corrosion (potential difference drops from ~0.9V to within 0.3V).

2. Between the Nut and the Material

Double Nut + Washer Configuration: In applications requiring extremely high anti-loosening properties (such as crane booms), the sequence is: bolt head → workpiece → flat washer → lower nut → upper nut (locking). The washer must be positioned between the nut and the workpiece to prevent the nut from directly pressing on the workpiece.
Precision Requirements: If the nut end face roughness Ra > 3.2μm, the flat washer increases the contact area, reduces surface pressure, and prevents the nut from loosening due to fretting wear.

3. With Locking Washers

The opening direction of split locking washers: The opening should face the nut or bolt head (i.e., the cut side faces the clamping surface). This way, the cut edge will embed into the mating surface during compression, generating maximum anti-loosening torque. Many people install them backwards, with the opening facing away from the clamping surface, which reduces the effectiveness by more than 50%.
Orientation of toothed locking washers: The tooth tips must face the locked part (nut or bolt head). If the tooth tips face the workpiece, it will severely damage the workpiece surface and result in poor anti-loosening effect.
Combined use: When load distribution and anti-loosening are required simultaneously, the sequence is: bolt head → flat washer → locking washer → workpiece → flat washer → locking washer → nut (symmetrical at both ends). This avoids the locking washer directly damaging the workpiece surface while retaining the anti-loosening effect.

Load Distribution: The flat washer, if present, distributes the load evenly, while the locking washer adds tension to prevent loosening.
Vibration Resistance: Locking washers create tension and friction, making it more challenging for the nut or bolt to loosen under vibrational or dynamic loads.
Proper Orientation: Ensure that the locking washer is oriented correctly, with the split or teeth facing the nut or bolt head. This allows them to engage effectively and provide locking action.

4. Specific Applications

Belleville Disc Washers: Typically used individually, with the convex side facing the bolt head or nut (concave side facing the workpiece) to achieve a stable load-deformation curve. When used in combination (stacked/paired), the stacking order must strictly follow DIN 2093, for example: bolt head → flat washer → (disc spring assembly) → flat washer → workpiece → nut.
Wave Washers: Primarily used for axial preload, usually placed directly between the bolt head and workpiece or the nut and workpiece, without the need for an additional flat washer (unless the workpiece surface is very soft). Note that the compression of the wave washer should be controlled within 30%~50% of its free height to ensure optimal fatigue life.

Final Thoughts

Proper placement of washers on bolts is only half the battle; choosing high‑quality washers with tight tolerances, proper surface finish and verified anti‑vibration performance is equally critical. At KENENG, every washer is manufactured to exceed international standards (DIN, ANSI, JIS). We offer:

Over 1000 standard sizes – M1.6 to M64, and custom sizes available
More than 6 surface treatments – zinc, Dacromet, Gumite, phosphating, anodizing, nickel plating
Third-party fatigue & vibration test reports – data support available
24-hour sample response – free assessment of your assembly environment, recommending the best gasket type and placement

Contact us today to get the right washer, placed correctly, for your most demanding bolted connections