Expansion bolts

Screw connections with expansion shank (e.g. DIN 2510) are particularly suitable for constructions which are stressed by changing operating forces and temperatures and if the durability of the screw connection is to be ensured. The favourable behaviour of expansion bolt connections is based on their elasticity, in particular on the fact that they show a greater elastic length change than so-called rigid bolt connections at the same pretension.

This behaviour results in a number of advantages:

• Expansion bolts show a good durability, because the additional stresses of the bolts themselves due to changing operating forces and temperature expansions as well as bending moments are reduced
• The drop in preload, which contributes significantly to loosening of the screw connections - during operation due to creep of the material, settling in the force-transmitting surfaces of the connection and the braced parts, and bending of the threads - is lower.
• Under certain conditions, the amount of applied preload can be measured easily by the change in length of the bolt

The following points should be observed when designing expansion screws and expansion bolts:

• The minimum expansion shaft length should be twice, better four times the thread diameter in order to achieve sufficient elasticity
• The ratio of thread core diameter to expansion shaft diameter should be about 1.1 with regard to the load-bearing capacity and elasticity of the screw
• The transition angle between expansion shaft and thread should be relatively small. This results in a favorable force flow and a better load distribution in the screwed thread

Sources:

DIN 2510 Teil 1
DIN 2510 Teil 3
Wiegand / Illgner (1962) Berechnung und Gestaltung von Schraubenverbindungen, Springer-Verlag, Berlin/Göttingen/Heidelberg