Screws High Preload Force Retention for Demanding Applications

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EJOT SE & CO. KG

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Preload force retention under thermal and dynamic load: The Springhead screw from Ejot is predestined for direct screwing into metals.

In the design of a bolted connection, the clamping force achieved by the bolt is an important design parameter. It ensures that the intended connection properties are maintained even under mechanical or thermal stress. Mechanical stresses on the connection manifest as additional forces, which can be considered during the design process. Thermal stresses, on the other hand, can directly affect the existing preload force of the connection in various ways.

Insert and clamping parts—particularly in thread-forming screw connections—are typically not made from the same material as the screw, which is usually carbon steel. Commonly used materials for structural and clamping parts are aluminum and magnesium alloys, copper materials, or plastics. These materials share the characteristic of having a higher thermal expansion coefficient than the screw material. As a result, heating a screw joint with such material combinations initially leads to an increase in the preload force. However, this can also trigger relaxation effects, which collectively result in an accelerated reduction of the preload force over time.

When a bolted joint is cooled, the preload force decreases due to the effect of differing thermal expansion. When heated, the preload force increases again. To ensure that the preload force in a connection always corresponds to the design requirements during operation, it may be necessary to integrate a spring element into the connection to minimize preload force fluctuations. Ideally, this is incorporated in such a way that automated fastening and feeding are possible while also preventing high localized surface pressure, as this can lead to creep in sensitive materials.

Screw with a Pressed-In Spring Element

A connection system that combines these properties is the so-called Springhead screw with an integrated, non-detachable rolled-on washer from Ejot. This innovative fastening element is a screw with a pressed-on spring element that functions similarly to a disc spring. Due to its lower stiffness, it reduces preload force fluctuations in the event of thermal and mechanical loads. The additional washer distributes the screw forces more evenly across the clamping part, specifically preventing high localized surface pressures. During assembly, the screw is tightened to ensure that the integrated spring element is correctly preloaded but not flattened. This effectively utilizes the elastic properties of the element.

This involves a screwing strategy that moves away from purely relying on tightening torque, as the system's geometry produces a specific torque curve. The torque initially rises steeply, as is typical with standard screw connections when the screw head makes contact. It then transitions into a relatively constant range due to the spring effect, before rising again once the spring element is fully compressed. The goal of a correctly preloaded fastening is to screw into this relatively constant range of the torque curve. This can be achieved, for example, through an automated screwing strategy that uses either head contact detection or a threshold torque and then finalizes the tightening via angle control. The advantage of this screwing strategy is that the achieved preload force results from the geometry and material properties of the spring element integrated into the screw and, in the case of successful fastening, is not dependent on friction coefficients.

The apparent alternative, a combination of a screw, a slipped-on disc spring, and a washer, has clear disadvantages compared to the Springhead system. The individual components must be assembled separately, which significantly complicates handling and makes automatic assembly virtually impossible. The reason for this is that the non-detachable rolling of two elements (disc spring and washer) followed by surface coatings and possibly heat treatments can lead to dangerous hydrogen embrittlement of the spring element, as it is typically made of steel with an increased carbon content.

Such preload-preserving connections are particularly advantageous when certain system properties need to be ensured through the use of a screw. Examples include securing seals against an – even fluctuating – internal pressure or maintaining the electrical conductivity of a structure. The latter, in particular, suggests the use of screws with integrated spring elements, especially in the field of electromobility.

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*Director Technology Management Fasteners at Ejot