345 / 392
decreasing presence of small amplitudes. By further increase of the severity of the
exploitation regime all amplitudes become equal to the highest. That is the regime with a
constant stress amplitude (
x
m
= 1). If Weibull’s distribution function of stress amplitude
F
(
σ
a
) is known, the appraisal of the severity of the exploitation regime may be made if
the amplitude with the highest occurrence frequency is determined. Another derivative of
the function
F
(
σ
a
) equated to zero determines the extreme of the function maximum
f
(
σ
a
),
where
( 1)
am
β
σ
η
β
β
= −
, i.e.
1
m am a
x
σ
σ
=
.
H
(
x
)
0,4
0,3
0,2
0,1
0
1,0
0,8
0,6
0,4
0,2
0
10
-5
0,
0001 0,001 0,01 0,1
1
10
5
10
4
10
3
10
2
10
1
10
0
x
=
σ
a
/
σ
x
=
σ
a
/
σ
a
n
=
H
(
x
)
n
b
f
(
x
)
(
l
) (
m
(
h
(
l
)
(
m
)
(
h
)
a)
b)
Figure 9. Characteristic representatives of light (
l
), medium (
m
) and heavy (
h
) exploitation regimes
Upon analysis of conditions of different machine systems, big differences have been
determined in the heaviness of the exploitation regime. It depends on exploitation condi-
tions, the aims of which the machine system was developed, its function. In vehicles for
example, the installed engine power is by rule great so as to enable acceleration within
short time, and so that this power could be used in sudden circumstances for the perfor-
mance of short lasting operations with big loads. In the stationary driving regime, a
relatively small part of the installed power is used. In the statistical sense, small stress
amplitudes are dominant, and the large ones are present only in exceptional circum-
stances. Thus the stress amplitudes, i.e. the work regime of vehicles, are a typical exam-
ple of the light exploitation regime. The other extreme is represented by machine systems
where it is endeavoured in the stationary work state to achieve maximum capacity and
productivity. Ore mills in flotation or in cement plants are typical representatives. In
filling these systems it is endeavoured in the stationary work regime to reach the
maximum and use the maximum installed power as well as the carrying capacity of the
system parts. The stress amplitudes are by rule close to the highest, and the exploitation
regime is heavy. The work regime in the mining plants is just somewhat lighter. Other
machine systems operate with regimes which are between these border cases.
341
The stress spectrum based on which it is possible to estimate the exploitation regime
may also be obtained in a simpler way. Instead of determining the time functions of stress
amplitudes as is given in the previous text, the approach may also be simplified. It is
possible to make estimates and tentative calculations so as to determine what part of the
work time will the machine system work at full power and then what time will it be using
less power expressed in percentages. Based on this, it is possible to obtain load blocks
which, when put in order by value, give an ordered set of stress levels similar to the
presentation in Fig. 8, the number of stress levels need not be eight, it may be any other