Twin-Lock keeps fastenings tight

2 mins read

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

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.

Operating principle

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 (see Figure 1). 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 Figure 2 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: 9.1kN
-Clamping length: 14mm
-Surface coating: Delta Protekt ®
-Lubrication: SAE 30 oil

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 include 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

There is also a stainless steel version AISI 316 as per EN 10088 – 1.4404
surface hardness = 550 HV0.05, surface hardened (kolsterising)

Both materials are available in two versions, the first with standard diameter (normal version), also suitable for countersunk bolts. The second has a larger outer diameter, for painted surfaces and soft materials

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

Twin-Lock is not recommended for moving surfaces (for example 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.

Significant benefits

Twin-Lock offers:

  • 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)

Twin-Lock affords an inexpensive solution to a perennial problem.