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measurement of the structure-borne
noise through vibration pick-ups on
the supporting frame of the lifting
gear drive units and on the central
column, made in parallel with it,
have shown the dominance of the
meshing frequency of the input gear
taken at approx. 500 Hz within the
range of the structure-borne sound
formation. The drive motors of the
lifting gear proved to be the main
source of airborne sound formation. The frequencies of the
airborne sound emitted by the motors are 2500 Hz or twice
the pulse rate of the frequency converters. The same goes
for the slewing drive gears. It is only the meshing frequency
of the planetary gearboxes which differs from that one of
the bevel wheel gearboxes belonging to the lifting gear.
The aim of the structure-borne sound measurements
is to establish the transmission paths of the sound, and to
correlate the respective shares with the airborne sound
measurements, in order to be able to quantify the shares
the individual sources of sound have in the overall noise.
The structure-borne sound of the lifting gear drive units
is lead into the crane’s structural steelwork through the
floor of the machine house, and its radiation takes place
over the large-surface steel structure of the crane’s upper
section, including the boom, and the portal that is arranged
underneath the machine house. The central column acts as
baffle for the structure-borne sound stimulus, transmitted
through the slewing drive units and the ball-race slewing
connection into the central column.
Actions intended to reduce noise should begin at the
noise source, if possible, before taking secondary ones.
That is the reason why the gearboxes have been equipped
with a noise-optimized tooth. But this action could only
contribute to a slight reduction of the sound power of
approx. 1.5 dB. So, secondary actions near the noise source
needed to be taken. To this end, the drive units belonging
Issue 04 | 2010
TRANSFER OF TECHNOLOGY
Fig. 10:
Slewing crane – General
arrangement drawing
to the lifting gear have been fixed on flexible supports to
the steel structure. The design of such flexible supports
requires particular care to obtain the best possible
insulation. An execution while basing on the standard VDI
2062 also requires the keeping of the maximum load factor
of Elastomers, given by the manufacturer. The airborne
sound emitted by the units get to the outside through the
ventilation apertures provided for in the machine house,
the opening to enable the rope passage and the hollow
central column.
All openings, including that one allowing passage of
rope, have been equipped with sound absorbers. The
ventilators have been designed in a noise-optimized way,
and the plane on which the central column goes outdoors
got completely closed by means of sound insulation
elements having a sound insulation factor of R=30dB. When
designing sound insulation elements, care must be taken
that the sound insulation factor given by the manufacturer
will not be reached on constructions, as it relates to a
single element while taking into account ideal conditions.
On constructions consisting of these elements the sound
insulation factor to be reached gets reduced to approx. 8
to 15 dB, dependent on the execution and the complexity
of the construction.
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