Rivets count too!

Whether blind rivet nuts, blind rivets or lockbolts - millions of rivets are used every day in a wide range of industrial applications - whether aerospace, automotive, general mechanical engineering or hydraulic components (expanders).

But are they always really tight? The reliable assembly of rivet elements can be a challenge. But why is that the case?

Blind rivet nut

The blind rivet nut is used where a conventional screw thread cannot be used due to a lack of material thickness or an existing material mix, e.g. carbon fiber composites.

The challenge of the apparently simple setting process is to guarantee the same precise assembly of the rivet element with repeat accuracy and despite fluctuating influencing factors. Why is precision so important?

• If the rivet element is overloaded during the setting process, the internal thread is compressed. The screw-in resistance during assembly of the screw increases and the required clamping force to be achieved in the screw connection is not achieved. As a result, the components to be joined can move beyond the operating forces that occur, causing the subsequent screw connection as such to fail completely.
• If the riveting element is not set tightly enough, the blind rivet nut rotates during the subsequent fastening process and changes the starting point for the angle monitoring of the fastening tool, leading to an undefined fastening angle, i.e. an NOK result.

A repeatable and monitored assembly of the fastener is therefore always crucial for the downstream fastening process.

Whether the use of simple, unmonitored riveting tools, which are still widely used, is sensible and expedient must be assessed by each user on the basis of their own application.

Both scenarios described are extremely unfavorable and lead not only to considerable reworking costs but also, in the worst case, to liability problems.

ALL riveting elements should be set with process reliability. But what is process-safe?

A setting process is only reliable if the force (kN) and the path (mm) are controlled and monitored, ideally at the correct setting speed, adapted to the rivet element and material properties. The setting properties of the components installed in the joint are also compensated for.

In principle, the same applies as for a screwing process: a slower setting process towards the end optimizes the assembly process and makes it more manageable.

Documentation is particularly mandatory for safety-critical connections. Riveting technology is no exception here.

Blind rivets or lockbolts:

Blind rivets or lockbolts are generally used for non-detachable connections. During the setting process, a constantly increasing force is generated until the blind rivet mandrel or lockbolt breaks off.

Why should the force, displacement and speed of these riveting elements also be monitored?

• The setting process may only be started once the components to be assembled have been correctly joined by an axial force. This prevents expansion of the rivet element between or above the components. In addition to controlling and monitoring the force (kN) and path (mm), a pressure control by the tool helps to ensure that the components to be joined are close together.
• Faulty rivet elements can be detected with these monitoring parameters if the rivet mandrel breaks off but the clamping force has not yet been reached. The result of such faulty or loose rivets is, for example, rattling noises. And the intended connection task is unlikely to be fulfilled. If operating forces occur, the rivet element is also likely to fail.
• The route should always be monitored to prevent incorrect or incomplete components from being joined.
• For safety-critical connections, documentation of the results should be available (Product Liability Act).

Examples of set rivet elements:

Summary:

While clear specifications for monitoring and documenting all tightening parameters to ensure reliable assembly have been in place for many years in screwdriving technology and are based on a wide range of standards and guidelines, in practice the simplest equipment is used.

Key industries such as aerospace, automotive and hydraulics, on the other hand, have long relied on the advantages of force- and displacement-controlled riveting systems and hand tools in their assembly processes, which count, document and enable continuous optimization of the assembly process based on the results.

Only controlled and monitored is really (process) safe!

Are you really counting on your rivets?