An expansion screw is a noticeably elastic, stretchable screw of a higher strength class . In a screw connection, it behaves like a pretensioned tension spring . The special resilient property is caused by its slim and particularly long shaft.
The created by tightening the elongated shaft tensile force decreases only slightly when the screw later by plastic deformation sets . The corresponding screw connection can do without additional anti-twist protection , so it is self-locking.
When the load changes due to a force that changes over time , the screw expands further (increased force) without being overstretched, or it contracts somewhat (reduced force) without loosening.
In order to accommodate additional elastic deformation work, the shaft (cylindrical part without thread ) is longer than that of a standard screw . At the same time, its diameter is smaller than the core diameter of the thread so that it is no longer the weak element of the screw.
The parts to be connected are often too thin for an expansion screw. Then a long sleeve is "placed under". This is welcome as another elastically deformable component. Such expansion screw connections with protruding screws are geometrically conspicuous.
The use of an expansion screw prevents changes in length during operation - e.g. B. by temperature changes with high different thermal expansion between the screw and the components to be fastened - lead to strong changes in force; In this case, screws of normal shaft length would:
- expand more / shrink less than the parts to be connected and thus less stretched / tensioned, whereby the pre-tensioning generated during assembly disappears and the screw becomes loose, or
- expand / shrink less than the parts to be connected and thus more stretched / tensioned, whereby plastic deformation occurs until the screw is loosened, or separating deformation in the thread or shaft and thus destruction of the connection.
An expansion screw connection is also advantageous in the case of alternating loads, i.e. H. when the load changes rapidly between zero and maximum value. Load shocks are softened by the stretch. The notch effect of the thread is no longer the limiting phenomenon for the alternating strength of the screw , but the slimmer shaft.
Expansion bolts are most commonly used in internal combustion engines. Out of operation and in continuous operation, all parts have different operating dimensions depending on the temperature. Expansion screws allow u. a. that the cylinder head gasket is pressed together sufficiently in both of the operating states mentioned. When using "rigid" standard screws would be because of the large thermal expansion of from aluminum existing cylinder head impossible.
An expansion screw connection must always be created with the intended preload. The assembly must be carried out in a carefully controlled manner. The cylinder head screws on the 1.6-liter gasoline engine in the VW Golf III are z. B. dressed according to the following scheme:
- Level 40 Nm
- Level 60 Nm
- Step + 90 ° (1/4 turn)
- Level + 90 ° (1/4 turn).
With very large engines and other large machines, assembly devices are used with which it is possible to keep the screws mechanically stretched during installation. Another possibility is to install the screws longer by heating them up. In both cases, they are only screwed in until the heads touch down without additional torque . The preload is set after removing the device or after the screws have cooled down.
Expansion screws are often designed up to the possible material limit, so that stress peaks in the plastic area initially arise at the shaft ends (thread, head) and are reduced by deformation during the first operation. These screws have to be exchanged for new ones during repairs and must not be re-fitted, as their material has been damaged by overstretching and this can break them.
In the broadest sense, bicycle spokes can also be viewed as expansion screws. They have a thread and are subject to constant alternating loads. Accordingly, thinner spokes are more flexible and last longer than thicker ones.
When designing a screw connection, the designer works advantageously with the tension diagram according to Felix Rötscher (Rötscher diagram). This complex diagram takes into account the resilient properties of all parts involved and clearly shows their effect in the connection. In summary, it contains a force versus displacement curve ( spring characteristic ) for the stretched and for the compressed parts. Both meet at a common point that is specified by the pre-tensioning force and is the reference point for the changes to be examined in operation. The screw connection is designed in such a way that the connected parts do not lift from one another under the maximum forces that occur and the permissible strength of the screw is not exceeded.
- Why expansion screws? Article about the areas of application of expansion screws (especially DIN 2510)
- Rötscher diagrams for standard and expansion screws