Secure fastening solution for bolted joints

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  4. Screw lock Twin-Lock

Best fastening performance with strong vibrations

Böllhoff’s Twin-Lock fastener locking system uses the preload of the threaded connection to ensure that thread systems do not come undone. This offers maximum reliability compared to conventional, friction-actuated anti-rotation locks.


Secure fastening solution, even with strong vibrations and dynamic loads

Twin-Lock fastener locking system

The Twin-Lock fastener locking system consists of two identical washers, which feature radial cogs on one side and splined cogs on the other side.

Twin-Lock operating principle

Twin-Lock fastener locking system

When the nut/screw is tightened the radial outer cogs of the Twin-Lock lock washer presses positively into the contact surface. The tilt angle of the splined cogs is greater than the pitch ß of the screw thread. This means that if the threaded connection loosens, the preload force increases. In practice, the combination of the washer pairs results in reliable anti-rotation locking, even in the case of strong vibrations and dynamic loads.

Twin-Lock put to the test

The diagram below shows the high anti-rotation locking capacity of Twin-Lock washers when subjected to vibrations. The blue line in particular illustrates the increase in force required to loosen the connection after 2000 load changes (click effect).

Test parameters

  • Screw: M8 class 8.8
  • Preload: 9.1 kN
  • Clamping length: 14 mm
  • Surface coating: Delta Protekt® KL100+VH30
  • Lubrication: SAE 30 oil
Test certificate – Twin-Lock® fastener locking system


  • M8 screw with Twin-Lock with a preload of 50% of the yield point
  • M8 screw without lock washer
  • Progression of the preload force while the screw is loosening (click effect)

Range of products

Twin-Lock is available in the following materials and versions

Twin-Lock fastener locking system

The Twin-Lock wedge-locking washers are available in carbon steel with a zinc flake coating and stainless steel 1.4404, all complying with the RoHs/ELV guidelines and the Reach regulation.

Materials available

  • Carbon steel, hardness 465-550 HV10 after hardening and tempering, coating: Delta Protekt® KL100 + VH301 GZ Cr6-free, corrosion resistance min. 600 hours until red rust in salt spray test as per ISO 9227
  • Stainless steel AISI 316 as per EN 10088 – 1.4404
    surface hardness = 550 HV0.05, surface hardened (kolsterising)

Both materials are available in two versions

  • With standard diameter (normal version), also suitable for countersunk bolts
  • With larger outer diameter, for painted surfaces and soft materials
Steel, standard

Steel, standard

Twin-Lock fastener locking system – standard version
  • Also suitable for countersunk bolts
Article number Metric d D h
53077STZL6 6 6,5 10,8 1,8
53077STZL8 8 8,7 13,5 2,5
53077STZL10 10 10,7 16,6 2,5
53077STZL11 11 11,4 18,5 2,5
53077STZL12 12 13 19,5 2,5
53077STZL14 14 15,2 23 3,4
53077STZL16 16 17 25,4 3,4
53077STZL18 18 19,5 29 3,4
53077STZL20 20 21,4 30,7 3,4
Steel, larger outer diameter

Steel, larger outer diameter

Twin-Lock fastener locking system with larger outer diameter
  • For painted surfaces and soft materials
Article number Metric d D h
53077STSPZL6 6 6,5 13,5 2,5
53077STSPZl8 8 8,7 16,6 2,5
53077STSPZL10 10 10,7 21 2,5
53077STSPZL12 12 13 25,4 3,4
53077STSPZL14 14 15,2 30,7 3,4
53077STSPZL16 16 17 30,7 3,4
53077STSPZL18 18 19,5 34,5 3,4
53077STSPZL20 20 21,4 39 3,4
Stainless steel (1.4404), standard

Stainless steel (1.4404), standard

Twin-Lock fastener locking system – standard version
  • Also suitable for countersunk bolts
Article number Metric d D h
53077A46 6 6,5 10,8 2
53077A48 8 8,7 13,5 2
53077A410 10 10,7 16,6 2
53077A411 11 11,4 18,5 2,5
53077A412 12 13 19,5 2
53077A414 14 15,2 23 3
53077A416 16 17 25,4 3
53077A418 18 19,5 29 3
53077A420 20 21,4 30,7 3
Stainless steel (1.4404), larger outer diameter

Stainless steel (1.4404), larger outer diameter

Twin-Lock fastener locking system with larger outer diameter
  • For painted surfaces and soft materials
Article number Metric d D h
53077A4SP6 6 6,5 13,5 2
53077A4SP8 8 8,7 16,6 2
53077A4SP10 10 10,7 21 2
53077A4SP12 12 13 25,4 3
53077A4SP14 14 15,2 30,7 3
53077A4SP16 16 17 30,7 3
53077A4SP18 18 19,5 34,5 3
53077A4SP20 20 21,4 39 3

Assembly instructions

Twin-Lock can be used in various applications

Threaded holes Countersunks Through holes Studs
Assembly instructions – Twin-Lock fastener locking system

Washers with a larger outer diameter combined with flange head screws or nuts are recommended for applications with wide holes or slotted holes and soft contact surfaces (rubber, painted surfaces etc.) to reduce the load on the contact surface.

Twin-Lock fastener locking system

The washers as supplied are glued together in pairs, to avoid positioning errors during installation. If they are re-used, check that the washers are in the correct position, cam face to cam face.

Twin-Lock is not recommended in the following cases

Twin-Lock is not recommended in the following cases
  • For moving surfaces (e.g. washer between Twin-Lock washer and contact surface)
  • For surfaces that are too soft (e.g. wood, plastic etc.)
  • In the case of significant settlement
  • For contact surfaces with a greater surface hardness than the Twin-Lock washer
  • For self-locking nuts and thread seals
  • For anti-clockwise threads


Using the Twin-Lock lock washer offers significant benefits

  • Optimal locking solution for extreme vibrations and dynamic loads
  • Functional system whatever the type of lubricant used, guaranteeing a locking effect with both dry and lubricated surfaces
  • Reusable system without any reduced effectiveness
  • Secures fastening elements with both low and high preloads
  • Can be used with screws/nuts of all property classes (up to 12.9)
  • No retightening required
  • All products are Cr6-free and comply with the RoHs/ELV guidelines and the Reach regulation
  • No risk of hydrogen embrittlement thanks to zinc flake coating
  • Easy to assemble system thanks to pre-bonded washer pairs, that significantly reduce the risk of incorrect initial assembly (wedge surfaces on wedge surfaces)

Downloads Version Language Type/Size
Double security
Aug 2015 English PDF / 3.944 KB

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