Screw locking


Locknut with integrated locking element

Secure elements to shafts effectively, even with high loads

FINE U‑NUT® nuts are reliable and secure fasteners for shaft bearings, gears, tension rollers and other machine parts.

When machine parts such bearings and gear need to be secured to shafts, we often reach for a locknut. However, standard locknuts can come loose under dynamic loads, and have to be secured to the shaft to prevent this. The innovative FINE U‑NUT® integrates this locking function into the nut, eliminating the need for an additional locking element or laboriously groove creation.

Method / principle

The FINE 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 constant screw torques.


The FINE U‑NUT® can be used anywhere where machine parts such as shaft bearings, gears or tension rollers must be secured against coming loose on the shaft. It can also be used for conventional screw connections.

Product benefits
  • Excellent anti-loss Retainer
  • Easy to install
  • Constant installation parameters
  • No additional locking element needed
  • No need to mill down a keyway
  • Reusable

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.

Easy to install

Installing a FINE U‑NUT® is easy. It’s also cost-effective as no special tools are required: just a standard C spanner or socket wrench (even for servicing). You don’t need an additional locking element.


No need for an additional locking element, no need to mill grooves. This saves labour, saves money, and – since you only need to stock one item – saves storage costs.

Increased service life

High quality nuts, combined with fewer error sources during installation.

Rotational equilibrium

Improved shaft rotation characteristics; can be installed on thin-walled shafts.

Detailed description


Locknuts are used when fastening bearings and other machine parts to shafts.

They belong to the mechanical installation methods and have the advantage that it is easy to replace the bearing during servicing, with no need for laborious preparatory work on the shaft.
However, rotating shafts are associated with the risk that the locknuts will come loose. To combat this, an additional locking device is used in combination with the locknut, securing the connection and guaranteeing reliability.
The FINE U‑NUT® was developed to reduce the overall costs when installing bearings and improve the quality of the connection.


Secure connections

The FINE U‑NUT® integrates the locking device in the locknut directly. A 270° lockring made of high-grade A2 steel is built into 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 is greater than the unscrewing torque generated by using the shaft. This means that the friction lock secures the machine part effectively against loosening, even at high rotation speeds and with alternating direction of rotation.
Two types of FINE U‑NUT® are available. The standard version is ideal for many common applications where elements are secured to shafts. The lockring secures the connection reliably and the nut comes in Böllhoff’s standard range.
The second type, the TWIN FINE U‑NUT® has two lockrings inserted, meaning it clamps even more securely to the thread. The TWIN FINE U‑NUT® is also designed with higher thread precision (ISO4H). This means greater precision for the contact surface, which makes it easier to produce the thread and bearing surface.


Advantages over standard locknuts

Standard locknuts usually have to be secured to the shaft with an extra locking element, especially for high rotational speeds. This locking element must prevent the nut from coming loose, but not compromise the accuracy of the joint or affect its resistance against the forces generated. Designing and installing extra locking elements is laborious and creates potential sources of error.
Jam nuts only provide temporary locking; circlip constructions and lock washers with locking tabs and grooves, or bow shackles and securing plates are laborious to install and often fail to provide adequate axial security. Removing them for servicing is equally laborious.
Using a FINE U‑NUT® eliminates the need for any additional locking elements or laborious milling and grinding to create keyways on the shaft. No time-consuming design or installation work is required.


Quick and easy installation

The FINE U‑NUT® is quick to install with a C spanner. The shaft bolt thread must comply with ISO 965, with a thread tolerance of ISO6g (JIS6g). The thread end should be bevelled so the nut can easily be screwed on. We recommend use of a lubricant, especially if the shaft is made of materials with lower hardness.
After installing the machine parts, screw the FINE U‑NUT® onto the shaft with the lockring facing outwards. The bolt must protrude by at least two threads.
The FINE U‑NUT® may not be used for shafts where a keyway has already been created, as this would reduce the clamping force and risk damaging the lockring.
When the equipment is serviced, the FINE U‑NUT® can be reused with only negligible reduction to the clamping force.

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 self-locking nut guarantees a secure lock for bearings with thread tolerance ISO6H.



Self-locking nut with double lockring for applications where a highly secure connection is required and demand on the thread is high.


Tool set


Installation tools for the two types of FINE U-NUT®

  • Two socket wrenches (M 10 to M 20 and M 25 to M 50)
  • Two ratchets
  • Two extension pieces
  • Box
Installation tool set for the FINE U-NUT®

Tool set in box

Installation tools for the FINE U‑NUT®


Find out more: related downloads.

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

Title Language Version File Download
Böllhoff bulletin
Securing Threaded Connections
English 1627/11.02 1218 kB / PDF
English 1627/11.02 1218 kb / PDF

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