Page 676 - Mastik®

Strength under dynamic load Fatigue resistance
according to VDI 2230

Screws are notched components; the The fatigue strength of fine threads de- Other constructive measures which
notching is provided by the thread. Un- creases with increasing rigidity and fine- can increase the fatigue strength:
der conditions of changing load, fatigue ness of thread. Basically, all measures which can reduce
fractures can occur in the screws. In The fatigue strength of fine threaded the effective peak stresses or prevent
90% of the cases the break occurs in the fasteners with a property class of 12.9, combined loading (loading along more
first load-bearing part of the thread, at it can be up to 30% lower than for coarse than one axis), are suitable for increas-
the entry into the internal (female) thread. threads. ing the fatigue strength of the screwed
In these cases the design must allows for connections. Long rather than short
the fatigue strength ± A of the screws; For hot-dip galvanized screws the fa- screws, screws with waisted shanks
this amounts to a fraction of the tensile tigue strength is about 20% lower than rather than screws with normal shanks,
strength, independent of the static load- for screws hardened and tempered at the pins or fitted shoulder screws to absorb
ing! end of the manufacturing process. lateral forces, adequate and above all
controlled prestressing of the screws.

[N/mm 2 ] 150

fatigue strength ± A 100 2

50

0 1 Graphic: VDI 2230, Edition 1986
0 6 8 10 20 40 [mm] Thread rolled then hardened and
tempered (standard practice)
thread diameter
Hardened and tempered, then thread rolled

a b c d e f g

Fatigue
failure
Fatigue
Fatigue failure
failure

through hole possible blind hole

a) Danger of fatigue failure in the internal thread as well
b) reduces the danger of fatigue failure
– in the internal thread through overlapping screw threads
– in the first load-bearing part of the thread, through design which allows flexibility in the reduced shank
c) reduces the danger of fatigue failure in the internal thread through rounded indentation and overlapping screw threads
d) Danger of fatigue failure in jammed thread run out of the screw thread
e) reduces the danger of fatigue failure compared with (d) through design which allows flexibility, overlapping internal thread and bracing the screw with the
starter head.
f) as for e) but here the centre belt serves to reduce bending stresses in the screw thread.
g) reduces the risk of fatigue failure through tensioning the belt against the bearing surfaces of the internal thread, leading to general release of the screw
thread from bending stresses.