EXPERIMENTAL STUDY OF FATIGUE LIFE OF STEPPED SHAFTS

During operation, some structural elements are
subjected to cyclically varying stresses (impacts)
depending on time. As a result of this effect, mechanical
fatigue of structural elements, as well as machine parts,
occurs. The simplest example of this process is the
movement of the wagon wheel along with the axle (shaft).
Since the wheel axis is also stepped and rotates with
variable angular velocity, it is subject to cyclical change.
The ultimate limit of fatigue occurring in a material is
fracture. The occurrence of failure does not depend on the
number of cycles, but on the magnitude of the stresses. The
article experimentally investigated the issue of the
resistance (endurance) of a stepped shaft during fatigue.
Such approaches have always existed for metallic
materials, and they still exist today. But nonmetals,
modern composites, and polymers have not been studied
enough. For this reason, the issue discussed in the article
is both relevant and scientific. The aim of the work is to
study the effect of the radius of the transition surfaces and
surface quality parameters of stepped shafts on the fatigue
life. For this purpose, fatigue experiments were conducted
on stepped shafts with different junction radii and surface
quality parameters. Three types of shafts with a radius of
transition surfaces of 0.5 mm, surface quality parameter
Rz = 4 μm; radius of 2 mm, surface quality parameter a
Rz = 4 μm and radius of 2 mm, surface quality parameter
Rz = 20 μm were selected as test specimens. The
experiments were conducted on a WP140 fatigue testing
machine. The surface roughness parameter Rz was taken as
an indicator of the quality of the shaft surface. The Rz
parameter characterizes the height of the roughness
protrusions at the 10 lowest and highest points along the
shaft surface. The experiment was repeated 3 times for
each type of test specimen. During the experiment, the test
specimen rotates and a force F = 200 N is applied to one
end. The experiment on each type of sample is continued
until the sample breaks. It was determined that the
maximum value of the stress in the test samples occurs in
the zones where the transition surfaces have radii. For all
three types of test samples, cracks appear and fracture
occurs starting from the end point of the junction radius
(the surface with a smaller diameter). It was determined
that samples with small values of the junction radius (r=0.5
mm) and the surface roughness parameter (Rz = 4 μm)
fracture in a smaller number of cycles, while samples with
large junction radius (r=2 mm) and a small numerical
value of the surface roughness parameter (Rz = 4 μm)
fracture in a larger number of cycles. The stress
distribution area increases with the radius of the transition
surfaces, and thus the probability of crack formation as a
result of fatigue decreases. The effect of the junction radius
and roughness parameter of the shaft transition surfaces on
the fatigue resistance is shown in the form of tables and
histograms. The obtained results show that the numerical
value of the surface quality parameters formed during the
technological process of shaft manufacturing and the
correct selection of the radius of the transition surfaces can
increase the fatigue life of stepped shafts during operation.