Spring Force Versus Preload
-What the Split Lock Washer Provides
- The elastic elongation of the shank can give thousands of pounds of clamping power for a properly torqued bolt . The split lock washer adds a little spring force - but it's important. If the bolt preload is reduced by vibration caused micro-sliding, differential thermal expansion or embedment relaxation, the compressed split lock washer pushes outwards against the fastener bearing surface to bridge the gap between full preload and zero clamping. Residual tension increases the service window of the joint before full loosening, yet does not eliminate the need for torque application.
- Quality manufacturing makes the spring force reliable. Washer material which is not adequately toughened may permanently flatten under load and lose its springiness. The second locking mechanism is the mechanical bite . The mechanical bite can not engage if the split edges are not strong . Key-CNC quenches and tempers spring-steel washers to hardness in the necessary range (HRC 40–50 for typical carbon steel grades) and monitors stamping tooling for edge-wear to maintain the cutting-edge profile that facilitates surface interlock.
Helix Geometry and Edge Bite - Dual Mechanism Design
| ● Tension in a Helical Spring The ring of the split lock washer is cut at one place and coiled such that the two ends are at different heights. When the nut or bolt head flattens the washer , the elastic energy stored in the washer produces a constant upward force against both the fastener and the mating surface . This mechanism works best with soft-to-medium hardness substrates (aluminium, mild steel, brass) where some embedment is acceptable and the spring rate of the washer can sustain contact pressure as the primary bolt preload decays. |
● Mechanical Interlock Split-Edge The split ends are twisted, so that they have sharp, angled edges which, when squeezed, dig into the bearing surface and the bottom of the fastener. This interference raises the rotational resistance – the dominant torque required to back the fastener off – by an average of 15–30% in soft substrates, providing a secondary lock independent of the spring force. On hardened or polished surfaces (above HRC 35) this biting mechanism cannot engage and so split lock washers are not suggested for use on case hardened parts, ground shafts or anodized aluminium without an intermediate flat washer to give a softer contact plane. |
Correct installation sequence Performance-defining decision
The number one reason split lock washers fail in the field is wrong stacking order. In a through hole joint, the components are stacked in the order :
bolt → flat washer → workpiece material → flat washer → split lock washer → nut
The flat washer between the split lock washer and the workpiece surface serves two purposes: first, it protects the substrate from being gouged by the split edges; second, it provides a uniform bearing plane so that the helical spring force is distributed evenly over the surface rather than being concentrated at two discrete points of contact.
For blind hole applications where there is simply a bolt head, the order is:
workpiece → flat washer → split lock washer → bolt head
Never use a split lock washer directly against aluminium, magnesium or plastic substrates – the sharp edges will dig in unevenly causing local stress risers that promote fracture initiation under cyclic loading.
When to Choose the Split Lock Washer - and When Not To
|
Recommended Use Conditions |
Avoid Use Conditions |
|
Soft-to-medium substrate hardness (aluminum, mild steel, brass) |
Hardened surfaces (HRC 35+), ground finishes, case-hardened parts |
|
Moderate vibration environments (general machinery, enclosures) |
Severe or sustained transverse vibration (engine mounts, structural steel) |
|
Low-to-medium preload requirements (M3–M16 fasteners) |
Large-diameter high-preload bolts (M20+ or 3/4″+) |
|
Non-critical, non-safety-rated assemblies |
Safety-critical joints per NASA Fastener Design Guide |
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Assembly with paired flat washers for surface protection |
Direct contact with soft coatings, plastics, or anodized surfaces |
|
Cost-sensitive, high-volume production |
Applications requiring certified anti-vibration proof (Junker test) |
For safety-rated or high-vibration assemblies, Key-CNC recommends alternative locking strategies: nylon-patch screws for chemical-bond locking, serrated flange bolts for mechanical interlock on flat surfaces, or wedge-locking washer pairs for heavy transverse vibration environments.
Materials, Standards & Specifications
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Product Name |
Split Lock Washer |
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Carbon spring steel (65Mn, SAE 1070); Stainless steel 304 (A2-70), 316 (A4-80); Alloy steel; Brass; Phosphor bronze |
|
|
Metric Size Range |
M3 – M30 |
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Inch Size Range |
#6 – 1″ (Regular, Heavy, Extra Duty, Hi-Collar series) |
|
Standards Compliance |
ASME B18.21.1; DIN 127 (superseded by DIN 7980); DIN 7980; GB/T 93 |
|
Hardness (Carbon Steel) |
HRC 40–50 (quenched & tempered) |
|
Hardness (Stainless Steel) |
HV 300–400 per ASME B18.21.1 |
|
Zinc plated (clear/yellow/black); Black oxide; Hot-dip galvanized; Passivation (stainless); Phosphated |
|
|
Helix Angle Tolerance |
±2° controlled via CNC stamping die profile |
|
Cross-Section Thickness Tolerance |
±0.05 mm standard; ±0.02 mm precision optional |
|
Production Method |
CNC precision stamping + quench/temper heat treatment |
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Minimum Order |
200 pcs (prototype); 5,000+ pcs (volume production) |
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ISO 9001:2015; full material traceability; lot-specific test reports |
ps： 5–7 day sample turnaround
COMMON QUESTIONS
Q: What is the purpose of a split lock washer?
A: There are two ways a split lock washer prevents loosening. First, it has spring tension that keeps residual preload when bolt force goes down. Second, it has mechanical edge bite that enhances rotational resistance to back-off. It works best on soft to medium substrates and at moderate vibration levels.
Q: Can I reuse a split lock washer after it's been taken apart?
A: No, not at all. The helical spring structure of the washer is plastically deformed after the first installation when it is fully compressed. This reuse reduces the spring force considerably and compromises the edge geometry. Always use a replacement split lock washer when reassembling.
Q: Best material for corrossive enviorment?
A: Stainless steel 316 (A4-80) split lock washers provide the best resistance to chloride for marine and chemical exposure. For normal outdoor use, stainless 304 (A2-70) with passivation treatment is enough. All forms of carbon steel need corrosion protection by means of zinc or hot-dip galvanising.
Q: How does Key-CNC ensure crisp split edges?
A: We check the stamping die edge condition at specified intervals and replace the tooling before rounding occurs. Optical comparison and hardness verification are performed to check the edge profile of each production batch, ensuring that the material retains adequate spring memory following heat treatment.
Q: Does Key-CNC sell matching sets of flat washers?
A: Yes. We manufacture quality bespoke flat washers on the same production line allowing matched ID/OD/thickness tolerances. We use split lock washers with our flat washers so it's easier to buy and easier to get the same results every time.
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