Screw locking


Reliable secure screw connections, even under high-stress conditions.

Effective, efficient fastening

U-NUT® locknuts are used to guarantee secure screw connections even in applications with high vibrations, dynamic loads, and minimal pre-tensioning force.

Screws are an important safety-critical element of many typical applications. If the screw connection fails and the nut loosens, it can result in expensive repairs. It is possible that the entire machine may be damaged; there may also be injuries. The U-NUT®, with its carefully designed integrated locking element, avoids the difficulties associated with standard locknuts and can be used to create screw connections on metric threads quickly and securely.

Method / principle

The U‑NUT® incorporates a flexible metal ring which presses against the thread as the nut is screwed on. This creates a controlled friction lock on the thread strong enough to prevent the connection from coming loose under simultaneous constant screw torques.


Use a U-NUT® anywhere where failure of the screw connection would be critical and the screws must be secured. The versatility of the U-NUT® makes it suitable for either conventional bolted joints based on VDI 2230, or uses with shorter clamping lengths and unclear screw connection parameters.

Product benefits
  • Excellent anti-loss Retainer
  • Easy to install
  • Constant installation parameters, including for reuse
  • No additional locking element needed
  • Wide range of materials and variants

Product benefits

There are many good reasons for using our products.

Strength and options.
Point by point.

Secure – guaranteed!

The friction lock on the thread is highly effective and prevents the connection from loosening.

Extremely reliable

Suitable for high rotation speeds and alternating direction of rotation.

Variants and types

Numerous dimensions and types available as standard.


Even installation torque, high reusability for screw fastenings.

Choice of materials

Steel and high-grade steel.

Detailed description


Thirty years of reliable joints

U-NUT® locknuts have been in use around the world for over thirty years, in all kinds of industries and sectors from robotics to transport. These locknuts have the locking function integrated directly in the nut.
The standard design incorporates a 180° lockring made of high-grade A2 steel in the nut. where it functions as a spring on the thread flanks during and after installation, setting up a clamping force between the nut and the thread. The resulting friction torque generated in reaction to the pitch and reactive force is greater than the unscrewing torque generated during use. This means that the friction lock secures the screw connection effectively, even under highly dynamic load conditions.
There are two types of U‑NUT® available. The standard version is ideal for many common applications. The lockring guarantees a secure connection capable of passing the American National Aerospace Standard test NAS 3350. The nut comes in Böllhoff’s standard range.

An alternative design, the GU‑NUT®, has a 270° lockring. This increases the clamping factor to the thread.

Product variants

Every project deserves a tailor-made solution.

Learn more about the product types and find the right variant for your particular application.


The U-NUT® is available in a variety of dimensions.



Locknut with increased clamping force due to larger clamping ring.



Find out more: related downloads.

If you would like to know more, simply download the relevant brochures.

Title Language Version File Download
Maximum security for your screw connection
English 09/2021 657 kB / PDF
English 09/2021 657 kb / PDF
Böllhoff bulletin
Securing Threaded Connections
English 1627/11.02 1218 kB / PDF
English 1627/11.02 1218 kb / PDF

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