[0001] The present invention relates to a hermetically encased compressor, particularly
for domestic refrigerating appliances, of the type comprising a volumetric motor-compressor
unit supported within a hermetically sealed casing by means of a spring suspension
system.
[0002] As known for instance from the description of DE-A-3 006 316, it is general practice,
with a view to reducing the noise produced by the compressor, to provide a spring
suspension systemwith at least three points of support situated substantially in a
horizontal plane below the center of gravity of the motor-compressor unit. More specifically,
the points of support are usually distributed about the periphery of the base of the
motor-compressor unit and have each attached thereto the upper end of a cylindrical
spring the opposite (lower) end of which takes support on an associated vertical prop
member resting on the bottom of the casing of the compressor and preferably made of
steel. With a view to minimizing the vibratory and acoustic effects of the pulsive
forces generated during operation of the motor-compressor unit, the connection of
the suspension springs to the supporting prop members is of particular importance.
Generally each prop member has an upwards projecting portion of a reduced diameter,
including a first cylindrical section and a second narrower portion, and projecting
partially into the associated spring when the latter is in the relaxed state. The
narrower upper end portion of the support prop member is usually of frustoconical
shape, with its lateral surface inclined at an angle of about 6
o with respect to the vertical axis of the prop member and the spring, the latter being
usually made of steel wire having a diameter of about 1.6 mm.
[0003] As known, the vibratory stresses to which the anchoring system of the suspension
units is subjected during the transient phases of starting and stopping the operation
of the compressor may be interpreted as the impulsive response (i. e. the product
of the resultant of the excitation of the system by the inherent mode of vibration
of the structure) on the part of a system constituted by a spring having one end simply
supported (with finite impedance) on the associated prop member and its other end
fixedly secured to the motor-compressor unit. The dampening effect of the system is
determined by the dampening properties of the spring plus the dampening component
resulting from friction between the surface of the prop member and the windings of
the spring when these two components are in mutual contact.
[0004] In the case of a known compressor of the type described above, the dampened noise
produced by a suspension during the transient at the time of stopping the operation
of the compressor (this being generally the transient generating the strongest noise),
has a typical profile as represented in fig. 1 of the drawings. In particular, the
noise thus produced is represented by a curve in which for a period T there occurs
a succession of peaks of logarithmically decreasing amplitude, with a maximum peak
amplitude P. With the suitable scale transformations, this function, measured experimentally,
may be taken as the representation of a displacement, an acceleration, or a sonar
pressure. In any case, the noise generated by the suspension system in a compressor
of the known type is undesirably strong, with a peak amplitude P corresponding to
about 47 to 49 dB and a duration T of between 1.5 and 1.8 seconds.
[0005] It is therefor a desirable object of the present invention to provide a hermetically
encased compressor with a spring suspension system of simple and reliable construction
and capable of substantially dampening the noise emitted during operation of the compressor,
particularly during the transient periods on starting and stopping the operation of
the compressor.
[0006] According to the invention, this object is attained in a hermetically encased compressor
including a spring suspension system incorporating the characteristics of the appended
claims.
[0007] The characteristics and advantages of the invention will become more clearly evident
from the following description, given by way of example with reference to the accompanying
drawings, wherein:
- fig. 2
- shows a diagrammatic sectional view of a hermetically encased compressor according
to the invention,
- figs. 3 and 4
- show lateral views of two components of the compressor of fig. 2 according to a preferred
embodiment of the invention, and
- fig. 5
- shows a graph representing the noise produced during a stop-operation transient of
the hermetically encased compressor according to the invention.
[0008] With reference to fig. 2, the hermetically encased compressor shown is substantially
of the same type as the known compressor used for measuring the noise represented
in fig. 1. It is mainly composed of a hermetically sealed casing 6 enclosing a volumetric
compressor 7 adapted to be driven by an electric motor 8. As per se known, the motor-compressor
unit 7, 8 takes support on the bottom of casing 6, containing a liquid lubricant bath
9, by the intermediary of a suspension system comprising at least three (preferably
four) suspension units generally indicated at 10 and evenly distributed about the
base of the motor-compressor unit 7, 8 below the center of gravity thereof. With additional
reference to figs. 3 and 4, each suspension unit 10 comprises a cylindrical spring
11 made of steel and having its upper end fixedly secured to the base of the motor-compressor
unit in a per se known and therefore not shown manner. The lower end of each spring
11 (considered under rest conditions as in figs 2 and 4) is partially slipped in a
force-fit onto an associated prop member 12 preferably made of steel and mounted in
vertical position on the base of casing 6. More specifically, each prop member 12
comprises a lower first cylindrical section 13 of a major diameter and an upper second
cylindrical section 14 having a smaller diameter and terminating in a frustoconical
end portion 15. The second cylindrical section 14 including its frustoconical end
portion 15 are normally received within the associated spring 11, the lower end of
which takes support on the shoulder defining the upper end of the first cylindrical
section 13 of the prop member.
[0009] Each spring 11 is preferably made of a steel wire having a diameter of about 1.5
mm, the spring having an inner diameter of about 10.9 mm, an axial length (in the
relaxed state) of about 28.5 mm, and a constant pitch between windings of about 2
mm.
[0010] The second cylindrical section 14 of each prop member 12 correspondingly has a diameter
of about 10.9 mm, while its axial length, inclusive of its frustoconical end portion
15, is about 13 mm, that is to say, fractionally shorter than half the axial length
of the spring 11. In particular, and according to an important aspect of the invention,
the lateral surface of the frustoconical end portion 15 has an inclination of about
10
o to 12
o, preferably 11
o, relative to the axis 16 of the respective prop member 12. In addition, the frustoconical
end portion 15 preferably has a blunt end and an axial length of about 9.5 mm.
[0011] With the values as set forth above, and in particular thanks to the inclination of
the lateral surface of the frustoconical end portion 15, it has been found and also
experimentally verified that in the hermetically encased compressor with a spring
suspension system according to the invention, the supporting prop members 12 interact
to a surprisingly limited degree with the associated springs 11 over the major part
of their vibration range, so that the emission of transient noise during start and
stop operations of the compressor is substantially dampened. As a matter of fact,
and as shown by way of example in fig. 5, the noise produced at the location of each
suspension unit 10 of the compressor according to the invention in a compressor stop
transient period comprises a series of oscillations which is of substantially limited
duration and displays substantially low peak values. In particular, it has been verified
in the course of more than two thousand trial operations, that the duration T of the
oscillations depicted in fig. 5 is only about 1 sec, with a peak value P of about
34 dB. It will be recognized that these values are drastically reduced relative to
corresponding values obtained with corresponding compressors of conventional construction.
[0012] According to the invention, this surprising result of acoustic optimization has been
obtained in the first place by ensuring the maximum attrition dampening of the oscillations
of the springs 11, and at the same time by minimizing the number of possible impacts
between the components which would act to transmit the oscillation energy of the springs.
[0013] The hermetically encased compressor as described may of course undergo any suitable
modifications within the purview of the invention.
1. A hermetically encased compressor comprising a hermetically sealed casing in which
a volumetric motor-compressor unit is supported by spring suspension units each comprising
a support prop member extending vertically from the bottom of said casing, with a
first cylindrical section of a major diameter and a second cylindrical section having
a smaller diameter and terminating in a frustoconical end portion, said second cylindrical
section being axially inserted in a force fit into the lower end of a cylindrical
spring having one end axially abutting said first cylindrical section and its other,
upper end secured to the base of said motor-compresor unit, characterized in that
said frustoconical end portion (15) of each prop member (12) has a lateral surface
inclined at 10o to 12o, preferably 11o, relative to the axis (16) of said prop member (12).
2. A hermetically encased compressor according to claim 1, characterized in that in each
said prop member (12), said second cylindrical section (14),inclusive of its frustoconical
end portion (15), has an axial length of about 13 mm, the axial length of said frustoconical
end portion being about 9.5 mm.
3. A hermetically encased compressor according to claim 1 or 2, characterized in that
each of said springs (11) is made of a steel wire having a diameter of about 1.5 mm
and being wound in windings which in the relaxed state have a pitch of about 2 mm
and an inner diameter of about 10.9 mm.
