Gestaltfestigkeit von Pressverbindungen II

Creation of a practical design specification for the design of interference fits FKM 2013
Issue number 320
Project no. 294

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Abstract:
The fatigue strength verification for interference fits is currently carried out in general mechanical engineering
in accordance with DIN 743 (load-bearing capacity of axles and shafts). The
notch effect numbers specified in this DIN have not yet been scientifically validated. Together
with the results from the preliminary project, the findings obtained here
form a comprehensive basis for the dimensioning and design of interference fits
(PV). The number of tests selected in each case now allows a reliable statement
on the 50 % probability of survival. With
few exceptions, the tests carried out confirm the specifications in DIN 743.
The notch coefficients determined for bending-stressed PV with a smooth shaft are
2.4 for C45E, for example, and .5 for torsion-stressed PV. The inclination exponent
for the fatigue strength line is k = 4.7 for bending and therefore corresponds to a
sharp notch. These values can also be transferred to the material GJS-400. Surprisingly,
no significant influence
of the interference on the dimensional stability could be detected in the tested practical range. The main reason for this is
the uptraining of the coefficient of friction (self-help effect) in the joint, which is particularly pronounced in torsion
.
The offset shaft variant frequently found in gearboxes with an undercut joint due to the gearing
has a multiple notch with undercut, undercut and
interference fit. This combination leads to a significantly lower dimensional stability
of the shaft compared to the undisturbed undercut calculated according to DIN 743. Apparently
due to the greater frictional stress
resulting from the interrupted joint, the shaft does not fail in the undercut but under the hub edge. The strength of the connection
is therefore lower than that of the (undisturbed) undercut.
When calculating the design strength of the undercut, the static preload resulting from
the shrinkage must always be taken into account in future. Equations for this
are given in the report.
In summary, it can be stated that verified results are now available for the most important
applications occurring in practice.
The objective of the research project has been achieved.