Home » L-Type Double Washer Socket Cap Head SEMS Screws - High Tensile Stainless Steel (A2-70)

L-Type Double Washer Socket Cap Head SEMS Screws - High Tensile Stainless Steel (A2-70)

L-Type Double Washer Socket Cap Head SEMS Screws – High Tensile Stainless Steel (A2-70)

FULLERKREG L-Type Double Washer Socket Cap Head SEMS Screws integrate an ISO 4762 hex socket cap head screw with two permanently captivated washers — a DIN 9021 large flat washer and a lock washer — assembled onto the shank by thread rolling. The oversized DIN 9021 flat washer provides a significantly larger bearing surface than standard washers, making the L-Type ideal for soft substrates, oversized clearance holes, and applications requiring enhanced load distribution. Available in A2-70 (SUS304) stainless steel with passivated finish, these screws eliminate loose washer handling and improve fastening reliability across demanding industrial environments.

What are Socket Cap Head SEMS Screws?

SEMS (pre-assembled screw and washer) screws are integrated fasteners that permanently combine a socket cap head screw with one or more captive washers into a single handling unit. The washers are retained on the screw shank during manufacturing via a thread-rolling process: washers are loaded onto the screw blank before threads are rolled, and the resulting thread shoulder prevents washer removal while allowing free rotation.

The L-Type Double Washer configuration specifically pairs a DIN 9021 large-series flat washer with a lock washer. The DIN 9021 washer has an outer diameter approximately twice the nominal thread diameter, providing a much larger contact footprint than ISO standard or small washers. The hex socket (Allen key) drive head conforms to ISO 4762, delivering high torque capacity and compatibility with standard hex key tooling across M2 to M10 thread sizes.

This design is especially beneficial in electronics assembly, automotive body panels, sheet metal enclosures, and any application where load spreading over a larger area is critical to joint integrity.

Benefits of Using SEMS Screws

  • Eliminates loose washers — the DIN 9021 large washer and lock washer are permanently captivated; they cannot be lost during kitting, handling, or installation, eliminating assembly errors and line stoppages.
  • Superior load distribution — the DIN 9021 large-series flat washer spreads clamping load over a significantly greater area, reducing surface pressure on soft or thin materials and preventing pull-through or surface damage.
  • Faster assembly — one integrated component replaces three separate parts (screw + large washer + lock washer), reducing cycle time and operator handling steps.
  • Consistent washer positioning — the washer is always correctly seated and centred under the head, ensuring uniform clamping load and preventing misalignment or tilting.
  • Improved vibration resistance — the integrated lock washer provides reliable anti-loosening performance without additional components or thread-locking compounds.
  • Reduced inventory complexity — one SKU replaces three separate part numbers, simplifying procurement, storage, and BOM management.
  • Automation-friendly — the single-piece design is compatible with automated feeding and driving equipment, supporting high-volume production lines.
  • Corrosion resistance — A2-70 (SUS304) stainless steel with passivated finish provides excellent resistance to oxidation, moisture, and mild chemical environments.
  • RoHS and REACH compliant — all finishes meet environmental compliance requirements for global markets.

Limitations of Using SEMS Screws

  • Fixed washer configuration — the DIN 9021 washer type and size are determined at manufacture; they cannot be changed or substituted in the field.
  • Larger footprint — the DIN 9021 large washer outer diameter requires adequate clearance around the fastener hole; not suitable for confined or closely spaced hole patterns.
  • Higher unit cost — SEMS screws carry a premium over equivalent loose-component assemblies due to the additional manufacturing step.
  • Minimum plate thickness requirement — the captive washer stack height must be considered when selecting screw length for thin sheet metal applications.
  • Not suitable for all joint configurations — applications requiring non-standard or specialty washers must use separate components.
  • Washer cannot be removed for inspection — the captive design means washer condition cannot be independently inspected or replaced without replacing the entire fastener.

Material / Finish

Component Material Grade / Standard Surface Treatment
Screw Body Stainless Steel (SUS304 / A2) A2-70 Passivated (Natural)
Large Flat Washer (DIN 9021) Stainless Steel (SUS304) DIN 9021 Passivated
Lock Washer Stainless Steel (SUS304) ISO Standard Passivated

Finish Options:

  • 0 – Natural: As-manufactured, no additional surface treatment.
  • 1 – Passivated: Electrochemical passivation to enhance corrosion resistance and remove free iron from the surface.
  • 2 – Stayblack: Black oxide finish for aesthetic and mild corrosion protection.

All finishes are RoHS and REACH compliant.

Dimensions of Pre-Assembled Washers in SEMS Screws

The L-Type configuration uses DIN 9021 large-series flat washers. DIN 9021 specifies an outer diameter approximately 3× the nominal thread diameter, providing a substantially larger bearing surface than ISO standard or small washers. The following table shows the outer diameter (d) and thickness (t) of the captive DIN 9021 flat washers by thread size:

DIN 9021 Large Plain Washer (L-Type – used in L-Type SEMS)

Thread Size M2.5 M3 M4 M5 M6 M8 M10
d1 – Inner Diameter (mm) 2.7 3.2 4.3 5.3 6.4 8.4 10.5
d2 – Outside Diameter (mm) 8 9 12 15 18 24 30
t – Thickness (mm) 0.5 0.8 1.0 1.2 1.6 2.0 2.5

DIN 9021 large washer outer diameter is approximately 3× the nominal thread diameter. All dimensions in mm. Dimensions per DIN 9021 standard.

Comparison: DIN 9021 (L-Type) vs. ISO Standard Washer (S-Type)

Thread Size ISO Std OD (mm) DIN 9021 OD (mm) Bearing Area Increase
M3 7.0 9.0 ~65%
M4 9.0 12.0 ~78%
M5 10.0 15.0 ~125%
M6 12.5 18.0 ~104%
M8 17.0 24.0 ~99%
M10 21.0 30.0 ~102%

Dimensional Data – Metric Series (ISO 4762 Head, A2-70)

All dimensions in mm. Head dimensions conform to ISO 4762. Thread rolled to BS 3643, Class 6g. Washer diameter column (D) reflects the DIN 9021 large washer outer diameter.

Part Number Thread (Class 6g) A – Head Diam (mm) B – Head Ht (mm) C – A/F Recess (mm) D – Washer Diam (mm) E – Washers Thk (mm) ② F – ① (mm) Unit Weight (g)
115-09(X)-020-(XX) M2 × 0.4 3.8 2.0 1.5 — — 2.4 —
115-09(X)-025-(XX) M2.5 × 0.45 4.5 2.5 2.0 8.0 — 2.8 —
115-09(X)-030-(XX) M3 × 0.5 5.5 3.0 2.5 9.0 — 3.8 —
115-09(X)-040-(XX) M4 × 0.7 7.0 4.0 3.0 12.0 — 4.6 —
115-09(X)-050-(XX) M5 × 0.8 8.5 5.0 4.0 15.0 — 5.6 —
115-09(X)-060-(XX) M6 × 1.0 10.0 6.0 5.0 18.0 — 5.9 —
115-09(X)-080-(XX) M8 × 1.25 13.0 8.0 6.0 24.0 — 7.8 —
115-09(X)-100-(XX) M10 × 1.5 16.0 10.0 8.0 30.0 — 9.4 —

① Distance before full thread starts.   ② Washers in compressed state (to be confirmed per size).   All dimensions in mm.
Unit Weight (g): to be updated. Contact FULLERKREG for weight data sheets.
D column reflects DIN 9021 large washer outer diameter. Head dimensions per ISO 4762.

Part Number Call-Out Guide

Field Position Description Example
SEMS Series 115-09X Metric series with DIN 9021 large flat washer + lock washer (L-Type double washer) 115-091 = Hex socket, DIN 9021 large washer + lock washer
Finish Code 1 digit 0 = Natural, 1 = Passivated, 2 = Stayblack 1 = Passivated
Thread Code 3 digits Thread size code (see table) 060 = M6
Underhead Length 2 digits Length in mm under head (L ±0.5 mm) 16 = 16 mm
Patch Lock Option Suffix P = Patch lock applied (leave blank if not required) P = with patch lock

Example: 115-091-060-16 P = Metric SEMS L-Type, Passivated, M6, 16 mm long, with Patch Lock

Available Lengths (L ±0.5 mm): 6 mm · 8 mm · 10 mm · 12 mm · 16 mm · 20 mm · 25 mm · 30 mm

Product Showcase

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Application

FULLERKREG L-Type Double Washer Socket Cap Head SEMS Screws are engineered for applications where a large bearing surface, vibration resistance, and assembly efficiency are simultaneously required:

  • Sheet Metal & Thin Panel Fastening — the DIN 9021 large washer distributes clamping load over a wider area, preventing pull-through and surface deformation on thin or soft sheet metal panels.
  • Electronics & Semiconductor Equipment — PCB mounting, enclosure assembly, and precision instrument panels where loose washers pose contamination risks and load distribution is critical.
  • Automotive & Transportation — body panel fastening, interior trim, and underbody components subject to vibration and thermal cycling, where the large washer prevents panel distortion.
  • Industrial Machinery — general machine assembly, conveyor systems, pump and motor housings requiring secure, repeatable fastening with enhanced load spreading.
  • Offshore & Marine — corrosion-resistant A2-70 stainless steel makes these screws suitable for saltwater-exposed structures, offshore instruments, and marine enclosures.
  • Composite & Soft Material Assemblies — the large washer footprint is ideal for fastening into composite panels, plastics, wood, and other materials with lower compressive strength.
  • Cleanroom & Medical Devices — available with Cleanroom Wash & Packaging Service; eliminates particle generation from loose washer handling in controlled environments.
  • Renewable Energy — solar panel mounting frames, wind turbine nacelle components, and electrical cabinet assembly where large washers prevent panel surface damage.

Why FULLERKREG?

  • 🏭 Direct Manufacturer — domestic production capability with full quality control from raw material to finished fastener.
  • 📐 Engineering-Grade Data — complete dimensional data, material certifications, and test reports available on request.
  • 📦 Bulk & Wholesale Supply — competitive pricing for high-volume orders; custom packaging and kitting available.
  • 🌍 Global Distribution — experienced in international logistics and export documentation for worldwide customers.
  • ✅ Compliance — RoHS and REACH compliant; ISO and DIN standard dimensions; material traceability available.
  • 🔧 Custom Specifications — special dimensions, lengths, finishes, and patch lock options available on request.
  • 🧪 Related Services — Cleanroom Wash & Packaging Service and Vibration Resistant Treatment Service available.
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