Hermetic compressor

A compressor equipped with a compressing unit comprising a compressing section and a motor section which are connected to each other and Which are encased in a closed vessel, the compressing section including a crank shaft rotatable so that a piston reciprocates within a cylinder and the motor section comprising a rotor and stator and disposed at a lower side of the compressing section, the rotor being connected to the crank shaft so that the crank shaft is rotatable in response to driving of the motor section. Springs are provided between the compressing unit and the closed vessel so as to elastically support the compressing section and the motor section, and first and second balance weights are respectively provided on upper and lower portions of a rotor of the motor section. The first and second balance weights are determined on the basis of the resultant load to be applied to an eccentric portion of the crank shaft, a con'rod and the piston, the loads of the first and second balance weights and distances among the first and second balance weights and the eccentric portion of the crank shaft.

BACKGROUND OF THE INVENTION 
The present invention relates to reciprocating and hermetic compressors 
applicable to refrigerating systems and others. 
One of the major problems arising in the use of hermetic compressors is 
occurrence of noises and vibrations. Recently, improvement for such 
hermetic compressors is being made for the noise- and vibration-reduction 
purposes. One known noise and vibration reduction technique is disclosed 
in the Japanese Utility Model Publication No. 49-18245 where a compressing 
unit is elastically supported by springs with respect to a closed vessel. 
This prior art technique will be described hereinbelow with reference to 
FIG. 1 which is a cross-sectional view showing a conventional hermetic 
compressor. In FIG. 1, designated at numeral 1 is a hermetic compressor 
which is equipped with a compressing unit comprising a compressing section 
5 and a motor section 6 which are encased in a closed vessel 4 Comprising 
an upper shell 2 and a lower shell 3. The compressing section 5 is 
constructed with a cylinder 9 formed in a block 8, a piston 10 associated 
with the cylinder 9, a crank shaft 11 having a crank eccentric portion 
(crank pin) 11a, a con'rod 12 coupled to the crank shaft 11, and a bearing 
13 provided on the block 8 to support the crank shaft 11. Further, the 
motor section 6 is constructed with a rotor 14 and stator 15, the rotor 14 
being fixedly shrinkage-fitted to the crank shaft 11 and the stator 15 
being fixedly secured through screws to the block 8. In addition, on a 
lower end portion of the stator 15 there is provided a spring-mounting 
plate 17 having studs 16. Springs 7 are placed between the studs 16 and 
the lower shell 3 so as to elastically support the compressing section 5 
and the motor section 6. 
With this arrangement, when vibrations of the compressing unit occur in 
response to the refrigerant-compressing operation of this compressor, the 
springs 7 can attenuate the vibrations before the propagation to the 
closed vessel 4 to thereby suppress the occurrence of noises due to the 
vibration of the closed vessel 4. 
There is a problem which arises with such a conventional compressor 
arrangement, however, in that a moment can be generated and applied to the 
crank shaft 11 due to the balance configuration of the compressing unit 
relative to the supporting position and hence difficulty is actually 
encountered to sufficiently suppress the vibration of the closed vessel. 
Further, the Japanese Patent Publication No. 58-24633 discloses a technique 
in which a balance weight is provided at the vicinity of the crank 
eccentric portion 11a of the crank shaft 11. However, this arrangement 
cannot also sufficiently suppress the generation of the moment in the 
crank shaft 11. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a hermetic 
compressor which is capable of sufficiently suppressing the occurrence of 
noises resulting from the generation of the vibrations of the compressing 
unit. 
In accordance with the present invention, there is provided a compressor 
comprising: a compressing section including a crank shaft having a crank 
eccentric portion, a con'rod and a piston, the piston being coupled 
through the con'rod to the crank eccentric portion of the crank shaft and 
inserted into a cylinder, and the crank eccentric portion being arranged 
to circularly move with respect to the axis of the crank shaft in 
accordance with rotation of the crank shaft to cause the piston to 
reciprocate within the cylinder; a motor section comprising a rotor and 
stator and disposed at a lower side of the compressing section, the rotor 
being connected to the crank shaft so that the crank shaft is rotatable in 
response to the drive of the motor section; and first and second balance 
weights respectively provided on upper and lower portion of the rotor of 
the motor section, the first balance weight being positioned in a plane II 
perpendicular to the axis of the crank shaft and separated by a distance X 
from a plane I perpendicular to the axis of the crank shaft and including 
the axis of the piston, and the second balance weight being positioned in 
a plane III perpendicular to the axis of the crank shaft and separated by 
a distance Y from the plane II, wherein, when a resultant force of an 
inertia force of the piston, an inertia force and centrifugal force of the 
con'rod, and a centrifugal force of the crank eccentric portion in the 
plane I is taken to be Fa, a centrifugal force of the first balance weight 
in the plane II is taken to be Fb, and a centrifugal force of the second 
balance weight in the plane III is taken as Fc, the first and second 
balance weights are determined so that all of coefficients C1 to C3 given 
by the following equations become below 3. 
EQU C1=Fc.multidot.(X+Y)/(Fb.multidot.X) (A) 
EQU C2=Fc.multidot.Y/(Fa.multidot.X) (B) 
EQU C3=Fb.multidot.Y/(Fa.multidot.(X+Y)) (C) 
The compressing section and the motor section are encased in a vessel and 
supported at the vicinity of the plane III through spring means with 
respect to the vessel, the first and second balance weights are determined 
so that the coefficient C1 given by the equation (A) is below 1.5 and the 
following equations are satisfied: 
EQU .vertline.C2MAX-1.vertline.&lt;.vertline.C2MIN-1.vertline. 
EQU .vertline.C3MAX-1.vertline.&lt;.vertline.C3MIN-1.vertline. 
where C2MAX and C2MIN respectively represent maximum and minimum values of 
the coefficient C2 given in accordance with the equation (B) during one 
revolution of the crank shaft, and C3MAX and C3MIN respectively designate 
maximum and minimum values of the coefficient C3 given in accordance with 
the equation (C) during one revolution of the crank shaft. It is 
appropriate that the compressing section and the motor section are encased 
in a vessel and supported at the vicinity of the plane II through spring 
means with respect to the vessel, the first and second balance weights are 
determined so that the coefficient C1 given by the equation (A) is 
1.+-.0.5 and the following equations are satisfied: 
EQU .vertline.C2MAX-1.vertline..apprxeq..vertline.C2MIN-1.vertline. 
EQU .vertline.C3MAX-1.vertline..apprxeq..vertline.C3MIN-1.vertline. 
It is also appropriate that the compressing section and the motor section 
are encased in a vessel and supported at the vicinity of the plane I 
through spring means with respect to the vessel, the first and second 
balance weights are determined so that the coefficient C1 given by the 
equation (A) is equal to or greater than 0.5 and the following equations 
are satisfied: 
EQU .vertline.C2MAX-1.vertline.&gt;.vertline.C2MIN-1.vertline. 
EQU .vertline.C3MAX-1.vertline.&gt;.vertline.C3MIN-1.vertline. 
Further, according to this invention, there is provided a compressor 
comprising: a compressing section including a crank shaft rotatable so 
that a piston reciprocates within a cylinder; a motor section comprising a 
rotor and stator and disposed at a lower side of the compressing section, 
the rotor being connected to the crank shaft so that the crank shaft is 
rotatable in response to the drive of the motor section, the stator having 
at its lower end portion a spring-mounting plate; spring means disposed 
between the spring plate and a closed vessel for encasing the compressing 
section and the motor section so as to elastically support the compressing 
section and the motor section; and first and second balance weights 
respectively provided on the upper and lower portions of a rotor of the 
motor section. 
In addition, according to this invention, there is provided a compressor 
comprising: a compressing section including a crank shaft having a crank 
pin, a con'rod and a piston, the piston being coupled through the con'rod 
to the crank pin of the crank shaft in a first plane I perpendicular to 
the axis of the crank shaft, the crank pin being arranged to circularly 
move with respect to the axis of the crank shaft in accordance with 
rotation of the crank shaft to cause the piston to reciprocate within a 
cylinder, a portion consisting of the crank pin, the con'rod and the 
piston has the center of gravity at a position separated by a distance rk 
from the axis of the crank shaft; a motor section connected to a lower 
side of the compressing section and comprising a rotor and a stator, the 
rotor being connected to the crank shaft so that the crank shaft is 
rotatable in response to the drive of the motor section; and first and 
second balance weights respectively provided on upper and lower portion of 
the rotor of the motor section, the first balance weight being positioned 
in a second plane II, perpendicular to the axis of the crank shaft and 
existing between the first plane I and a reference plane O, perpendicular 
to the axis of the crank shaft and passing through the center of gravity 
of a combination of the compressing section and the motor section, and 
first balance weight having the center of gravity at a position separated 
by a distance rb from the axis of the crank shaft, and the second balance 
weight being positioned in a third plane III perpendicular to the axis of 
the crank shaft and existing at the opposite side to the first plane I 
with respect to the reference plane O, and the second balance weight 
having the center of gravity at a position separated by a distance rc from 
the axis of the crank shaft; and spring means for elastically supporting 
the compressing section and the motor section in a fourth plane IV 
perpendicular to the axis of the crank shaft and existing at the opposite 
side to the first plane I with respect to the reference plane O, wherein, 
when a mass of the portion consisting of the crank pin, the con'rod and 
the piston is taken to be M, a mass of the first balance weight is taken 
as mb and a mass of the second balance weight is taken as mc, the first 
and second balance weights are determined on the basis of the product 
mb.multidot.rb, the product mc.multidot.rc and the product M.multidot.rk. 
More specifically, the first and second balance weights are determined so 
that values Nb and Nc given in accordance with the following equations are 
respectively in first and second predetermined ranges: 
EQU Nb=mb.multidot.rb/M.multidot.rk 
EQU Nc=mc.multidot.rc/M.multidot.rk 
Preferably, the first predetermined range is 0.8 to 1.2 and the second 
predetermined range is 0.1 to 0.5.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 2, a description will be made hereinbelow in terms of 
an arrangement of a hermetic compressor according to a first embodiment of 
the present invention. In FIG. 2, parts corresponding to those in FIG. 1 
are marked with the same numerals. The hermetic compressor, being 
designated at numeral 1, is similarly equipped with a compressing unit 
comprising a compressing section 5 and a motor section 6 which are encased 
in a closed vessel 4 comprising an upper shell 2 and a lower shell 3. The 
compressing section 5 is constructed with a cylinder 9 formed in a block 
8, a piston 10 associated with the cylinder 9, a crank shaft 11 having a 
crank eccentric portion 11a at its end portion, a con'rod 12 coupled to 
the crank eccentric portion 11a of the crank shaft 11 and further 
connected to the piston 10, and a bearing 13 provided on the block 8 to 
support the crank shaft 11. The piston 10 is arranged so that its central 
axis is substantially perpendicular to the axis of the crank shaft 11. The 
crank shaft 11 is arranged to be rotatable about its own axis so that the 
crank eccentric portion 11a rotationally moves with respect to the axis of 
the crank shaft 11. In response to the rotational movement of the crank 
eccentric portion 11a, the con'rod 12 is linearly moved so that the piston 
10 reciprocates in the cylinder 9. Further, the motor section 6 is 
constructed with a rotor 14 and stator 15, the rotor 14 being fixedly 
secured to the crank shaft 11 and the stator 15 being provided at the 
periphery of the rotor 14 to be fixedly secured through screws to the 
block 8. The crank shaft 11 is coaxially disposed at the center portion of 
the motor section 6. 
In this embodiment, an upper balance weight 18 is disposed at an upper 
portion of the rotor 14 and a lower balance weight 19 is disposed at a 
lower portion of the rotor 14, the upper and lower balance weights 18 and 
19 being different by 180.degree. in angular position and the lower 
balance weight 19 being substantially equal in angular position to each 
other. Further, studs 20 are integrally provided on the block 8, and stays 
21 are fixedly secured to the lower shell 3 and equipped with pins 21a. A 
spring 7 is set and disposed between the stud 20 and the pin 21a of the 
stay 21 so as to elastically support the compressing section 5 and the 
motor section 6 of the compressor 1. 
Here, a description will be made in terms of the determination of the 
weights and configurations (shapes) of the upper and lower balance weights 
18 and 19. First, we consider the plane I perpendicular to the axis of the 
crank shaft 11 and including (passing through) the central axis of the 
piston 10, the plane II perpendicular to the axis of the crank shaft 11 
and including the center of gravity of the upper balance weight 18 and the 
plane III perpendicular to the axis of the crank shaft 11 and including 
the center of gravity of the lower balance weight 19. The distance between 
the planes I and II is X and the distance between the planes II and III is 
Y. When the resultant force comprising components of the inertia force of 
the piston 10, the inertia force and centrifugal force of the con'rod 12 
and the centrifugal force of the crank eccentric portion 11a on the plane 
I is taken to be Fa, the centrifugal force of the upper balance weight 18 
on the plane II is taken to be Fb, and the centrifugal force of the lower 
balance weight 19 on the plane III is taken as Fc. The weights and 
configurations (shapes) of the upper and lower balance weights 18 and 19 
are determined so that all the coefficients C1 to C3 given by the 
following equations (1) to (3) becomes below 3. 
EQU C1=Fc.multidot.(X+Y)/(Fb.multidot.X) (1) 
EQU C2=Fc.multidot.Y/(Fa.multidot.X) (2) 
EQU C3=Fb.multidot.Y/(Fa.multidot.(X+Y)) (3) 
In operation, in response to driving the motor section 6, the crank shaft 
11 rotates so that the crank eccentric portion 11a circularly rotates with 
respect to the axis of the crank shaft 11. Thus, the piston 10 connected 
through the con'rod 12 to the crank eccentric portion 11a reciprocates 
within the cylinder 9 to compress the refrigerant. Accordingly, the 
resultant force Fa of the centrifugal force of the crank eccentric portion 
11a due to the rotation, the reciprocating inertia force of the piston 10 
and the centrifugal force and reciprocating inertia force of the con'rod 
12 is applied on the above-mentioned plane I. Because of variations of the 
reciprocating inertia force of the piston 10 and the centrifugal force and 
reciprocating force of the con'rod 12 during one revolution of the crank 
shaft 11, the resultant force Fa does not become a constant load but 
becomes a variable load having the periodicity corresponding to one 
revolution of the crank shaft 11. On the other hand, because the distances 
between the centers of gravity of the upper and lower balance weights 18 
and 19 and the rotation axis of the crank shaft 11 are fixed and the 
weights of the upper and lower balance weights 18 and 19 are also fixed, 
the centrifugal force Fb of the upper balance weight 18 and the 
centrifugal force Fc of the lower balance weight 19 do not vary during one 
revolution of the crank shaft 11. Thus, as shown in FIG. 3, only the 
coefficient C1 becomes constant during one revolution of the crank shaft 
11, and the other coefficients C2 and C3 respectively vary during one 
revolution of the crank shaft 11. 
Here, the dynamic (kinematic) balance of the compressor 1 can be kept to 
the most desirable state when all the coefficients C1 to C3 are 1. In 
other words, a moment due to the loads Fa to Fc is not generated with 
respect to the crank shaft 11 when taking that condition. However, in the 
case that the compressor is of the reciprocating type, in practice 
difficulty is encountered to perfectly assume the dynamic balance 
therebetween during one revolution of the crank shaft 11. But, according 
to tests of the inventors, it has been found that, when all the 
coefficients C1 to C3 are below 3, the moment to be generated due to the 
loads Fa to Fc and applied to the crank shaft 11 relatively becomes small. 
Thus, if setting the coefficients C1 to C3 to be below 3 in addition to 
elastically supporting the compressing section 5 and the motor section 6 
through the springs 7, it is possible to suppress the generation of noises 
resulting from the vibration of the closed vessel 4. According to the 
tests, the respective coefficients C1 to C3 become below 3 in the case 
that the cylinder volume is above 10 cm.sup.3, become below 2.5 when the 
cylinder volume is 6 to 10 cm.sup.3, and become below 2 when the cylinder 
volume is below 6 cm.sup.3. 
FIG. 4 is a cross-sectional view showing an arrangement of a hermetic 
compressor according to a second embodiment of this invention where parts 
corresponding to those in FIG. 2 are marked with the same numerals and the 
description will be omitted for brevity. In FIG. 4, one feature of this 
embodiment is that a holder 22 is fixedly secured through screws to the 
lower surface of the stator 15 and stays 23 are fixedly welded onto the 
lower shell 3, the holder 22 being equipped with studs 22a which are in 
turn inserted into springs 7 and the stay 23 being equipped with pins 23a 
which are also inserted into the springs 7. The springs 7 are set and 
disposed between the holder 22 and the stays 23 by means of the studs 22a 
and the pins 23a, whereby it is possible to elastically support the 
compressing section 5 and the motor section 6. The holder 22 is arranged 
such that the studs 22a are placed at the vicinity of the plane III 
perpendicular to the axis of the crank shaft 11 and passing through the 
center of gravity of the lower balance weight 19. 
In this embodiment, the weights and configurations of the upper and lower 
weight balances 18 and 19 are determined so as to satisfy the following 
equation (4) or (5) under the condition that the coefficient C1 given in 
accordance with the above-mentioned equation (1) is below 1.5. 
EQU .vertline.C2MAX-1.vertline.&lt;.vertline.C2MIN-1.vertline. (4) 
EQU .vertline.C3MAX-1.vertline.&lt;.vertline.C3MIN-1.vertline. (5) 
where C2MAX and C2MIN respectively represent the maximum value and minimum 
value of the coefficient C2 given in accordance with the above-mentioned 
equation (2) during one revolution of the crank shaft 11, and C3MAX and 
C3MIN respectively designate the maximum value and minimum value of the 
coefficient C3 given in accordance with the above-mentioned equation (3) 
during one revolution of the crank shaft 11. 
In operation, as well as in the above-described first embodiment, in 
response to driving the motor section 6, the crank shaft 11 rotates so 
that the crank eccentric portion 11a circularly rotates with respect to 
the axis of the crank shaft 11. Thus, the piston 10 connected through the 
con'rod 12 to the crank eccentric portion 11a reciprocates within the 
cylinder 9 to compress the refrigerant. Accordingly, the resultant force 
Fa of the centrifugal force of the crank eccentric portion 11a due to the 
rotation, the reciprocating inertia force of the piston 10 and the 
centrifugal force and reciprocating inertia force of the con'rod 12 is 
applied on the above-mentioned plane I. Because of variations of the 
reciprocating inertia force of the piston 10 and the centrifugal force and 
reciprocating force of the con'rod 12 during one revolution of the crank 
shaft 11, the resultant force Fa does not become a constant load but 
becomes a variable load having the periodicity corresponding to one 
revolution of the crank shaft 11. On the other hand, because the distances 
between the centers of gravity of the upper and lower balance weights 18 
and 19 and the rotation axis of the crank shaft 11 are fixed and the 
weights of the upper and lower balance weights 18 and 19 are also fixed, 
the centrifugal force Fb of the upper balance weight 18 and the 
centrifugal force Fc of the lower balance weight 19 do not vary during one 
revolution of the crank shaft 11. Thus, as shown in FIG. 5, only the 
coefficient C1 becomes constant during one revolution of the crank shaft 
11, and the other coefficients C2 and C3 respectively vary during one 
revolution of the crank shaft 11 so as to take the maximum values C2MAX, 
C3MAX and the minimum values C2MIN, C3MIN. 
Here, although as described above, the dynamic (kinematic) balance of the 
compressor 1 can be kept to the most desirable state when all the 
coefficients C1 to C3 are 1, that is, a moment due to the loads Fa to Fc 
is not generated with respect to the crank shaft 11 when taking that 
condition, in the case that the compressor is of the reciprocating type, 
in practice difficulty is encountered to perfectly assume the dynamic 
balance therebetween during one revolution of the crank shaft 11. However, 
if setting the coefficient C1 to below 1.5 and satisfying the 
aforementioned equations (4) and (5), the compressing unit takes a 
vibration mode where vibration becomes small at the vicinity of the plane 
III and becomes large at the upper portion of the crank shaft 11, because 
the force (moment) applied to the upper portion of the crank shaft 11 
becomes greater and the force (moment) applied to the lower portion of the 
crank shaft 11, i.e., the vicinity of the lower balance weight 19, become 
smaller. In this embodiment, since the compressing section 5 and the motor 
section 6 are supported through the springs 7 at the vicinity of the plane 
III in which the vibration is small, it becomes possible to further 
suppress the generation of noises due to the vibration from the 
compressing unit to the closed vessel 4 as compared with the 
above-described first embodiment. 
FIG. 6 is a cross-sectional view showing an arrangement of a hermetic 
compressor according to a third embodiment of this invention where parts 
corresponding to those in FIG. 2 or 4 are marked with the same numerals 
and the description thereof will be omitted for brevity. In FIG. 6, one 
feature of this embodiment is that studs 24 are provided on the block 8 
and positioned at the vicinity of the plane II perpendicular to the axis 
of the crank shaft 11 and passing through the center of gravity of the 
upper balance weight 18, and stays 25 are fixedly welded onto the lower 
shell 3 so that springs 7 are disposed between the studs 24 of the block 8 
and pins 25a of the stays 25 whereby the springs 7 elastically support the 
compressing section 5 and the motor section 6. 
In this embodiment, the weights and configurations of the upper and lower 
balance weights 18 and 19 are determined so as to satisfy the following 
equations (6) and (7) (showing that .vertline.C2MAX-1.vertline. is 
approximately equal to .vertline.C2MIN-1.vertline. and 
.vertline.C3MAX-1.vertline. are approximately equal to 
.vertline.C3MIN-1.vertline.) under the condition that the coefficient C1 
given by the above-mentioned equation (1) is 1.+-.0.5. 
EQU .vertline.C2MAX-1.vertline..apprxeq..vertline.C2MIN-1.vertline.(6) 
EQU .vertline.C3MAX-1.vertline..apprxeq..vertline.C3MIN-1.vertline.(7) 
where C2MAX and C2MIN respectively represent the maximum value and minimum 
value of the coefficient C2 given in accordance with the above-mentioned 
equation (2) during one revolution of the crank shaft 11, and C3MAX and 
C3MIN respectively designate the maximum value and minimum value of the 
coefficient C3 given in accordance with the above-mentioned equation (3) 
during one revolution of the crank shaft 11. 
In operation, as well as in the above-described first embodiment, when the 
motor section 6 is driven, the crank shaft 11 rotates so that the crank 
eccentric portion 11a circularly rotates with respect to the axis of the 
crank shaft 11. Thus, the piston 10 connected through the con'rod 12 to 
the crank eccentric portion 11a reciprocates within the cylinder 9 to 
compress the refrigerant. Accordingly, the resultant force Fa of the 
centrifugal force of the crank eccentric portion 11a due to the rotation, 
the reciprocating inertia force of the piston 10 and the centrifugal force 
and reciprocating inertia force of the con'rod 12 is applied on the 
above-mentioned plane I. Because of variations of the reciprocating 
inertia force of the piston 10 and the centrifugal force and reciprocating 
force of the con'rod 12 during one revolution of the crank shaft 11, the 
resultant force Fa does not become a constant load but becomes a variable 
load having the periodicity corresponding to one revolution of the crank 
shaft 11. On the other hand, because the distances between the centers of 
gravity of the upper and lower balance weights 18 and 19 and the rotation 
axis of the crank shaft 11 are fixed and the weights of the upper and 
lower balance weights 18 and 19 are also fixed, the centrifugal force Fb 
of the upper balance weight 18 and the centrifugal force Fc of the lower 
balance weight 19 do not vary during one revolution of the crank shaft 11. 
Thus, as shown in FIG. 7, only the coefficient C1 becomes constant during 
one revolution of the crank shaft 11, and the other coefficients C2 and C3 
respectively vary during one revolution of the crank shaft 11 so as to 
take the maximum values C2MAX, C3MAX and the minimum values C2MIN, C3MIN. 
Here, as described above, in the case that the compressor is of the 
reciprocating type, in practice difficulty is encountered to perfectly 
assume the dynamic balance therebetween during one revolution of the crank 
shaft 11. However, if setting the coefficient C1 to 1.+-.0.5 and 
satisfying the aforementioned equations (6) and (7), the compressing unit 
takes a vibration mode where the node of vibration appears at the vicinity 
of the plane II, because the moments substantially equal to each other and 
opposite in direction to each other are respectively applied to both the 
upper and lower portions of the crank shaft 11. In this embodiment, since 
the compressing section 5 and the motor section 6 are supported through 
the springs 7 at the vicinity of the plane II where vibration is small, it 
becomes possible to further suppress the generation of noises due to the 
vibration from the compressing unit to the closed vessel 4 as compared 
with the above-described first embodiment. 
FIG. 8 is a cross-sectional view showing an arrangement of a hermetic 
compressor according to a fourth embodiment of this invention where parts 
corresponding to those in FIG. 2, 4 or 6 are marked with the same numerals 
and the description thereof will be omitted for brevity. In FIG. 8, one 
feature of this embodiment is that studs 26 are provided on the block 8 
and positioned at the vicinity of the plane I perpendicular to the axis of 
the crank shaft 11 and passing through the central axis of the piston 10, 
and stays 27 are fixedly welded onto the lower shell 3 and equipped with 
pin 27a so that springs 7 are disposed between the studs 26 and the pins 
27a of the stays 27 so as to elastically support the compressing section 5 
and the motor section 6. 
In this embodiment, the weights and configurations of the upper and lower 
balance weights 18 and 19 are determined so as to satisfy the following 
equations (8) and (9) under the condition that the coefficient C1 given by 
the above-mentioned equation (1) is above 0.5. 
EQU .vertline.C2MAX-1.vertline.&gt;.vertline.C2MIN-1.vertline. (8) 
EQU .vertline.C3MAX-1.vertline.&gt;.vertline.C3MIN-1.vertline. (9) 
where C2MAX and C2MIN respectively represent the maximum value and minimum 
value of the coefficient C2 given in accordance with the above-mentioned 
equation (2) during one revolution of the crank shaft 11, and C3MAX and 
C3MIN respectively designate the maximum value and minimum value of the 
coefficient C3 given in accordance with the above-mentioned equation (3) 
during one revolution of the crank shaft 11. 
As described above, the resultant force Fa of the centrifugal force of the 
crank eccentric portion 11a due to the rotation, the reciprocating inertia 
force of the piston 10 and the centrifugal force and reciprocating inertia 
force of the con'rod 12 is applied on the above-mentioned plane I. Because 
of variations of the reciprocating inertia force of the piston 10 and the 
centrifugal force and reciprocating force of the con'rod 12 during one 
revolution of the crank shaft 11, the resultant force Fa does not become a 
constant load but becomes a variable load having the periodicity 
corresponding to one revolution of the crank shaft 11. On the other hand, 
because the distances between the centers of gravity of the upper and 
lower balance weights 18 and 19 and the rotation axis of the crank shaft 
11 are fixed and the weights of the upper and lower balance weights 18 and 
19 are also fixed, the centrifugal force Fb of the upper balance weight 18 
and the centrifugal force Fc of the lower balance weight 19 do not vary 
during one revolution of the crank shaft 11. Thus, as shown in FIG. 9, 
only the coefficient C1 becomes constant during one revolution of the 
crank shaft 11, and the other coefficients C2 and C3 respectively vary 
during one revolution of the crank shaft 11 so as to take the maximum 
values C2MAX, C3MAX and the minimum values C2MIN, C3MIN. 
Here, although in practice difficulty is similarly encountered to perfectly 
assume the dynamic balance therebetween during one revolution of the crank 
shaft 11, if setting the coefficient C1 to above 0.5 and satisfying the 
aforementioned equations (8) and (9), the compressing Unit takes a 
vibration mode where vibration becomes small at the vicinity of the plane 
I and becomes large at the lower portion of the crank shaft 11, because 
the moment applied to the lower portion of the crank shaft 11 becomes 
greater and the moment applied to the upper portion of the crank shaft 11, 
i.e., the vicinity of the crank eccentric portion 11a, become smaller. In 
this embodiment, since the compressing section 5 and the motor section 6 
are supported through the springs 7 at the vicinity of the plane I where 
vibration is small, it becomes possible to further suppress the generation 
of noises due to the vibration from the compressing unit to the closed 
vessel 4 as compared with the above-described first embodiment. 
Further, a description will be made hereinbelow with reference to FIG. 10 
in terms of a hermetic compressor according to a fifth embodiment of this 
invention. In FIG. 10, parts corresponding to those in the above-described 
embodiments are marked with the same numerals and the description thereof 
will be omitted for brevity. The hermetic compressor 1 similarly comprises 
a compressing section 5 at an upper portion and a motor section 6 at a 
lower portion which are encased in a closed vessel 4 comprising upper and 
lower shells 2 and 3, and includes upper and lower balance weights 18 and 
19 provided on the upper and lower surfaces of a rotor 14 of the motor 
section 6. On a lower end portion of the stator 15 there is fixedly 
provided a spring-mounting plate 22 which is connected through springs 7 
to stays 23. 
The compressing unit comprising the compressing section 5 and the motor 
section 6 is arranged to have the center S of gravity existing in a 
reference plane O, and the crank pin 39 (crank eccentric portion 11a) of 
the crank shaft 11 is coupled to the con'rod 12 in the first plane I 
perpendicular to the axis of the crank shaft 11 and separated by a 
distance c from the reference plane O. A portion consisting of the crank 
pin 39, con'rod 12 and piston 10 has a mass M and has the center of 
gravity at a position separated by an eccentric distance rk from the axis 
of the crank shaft 11. The upper balance weight 18 is disposed in the 
second plane II between the first plane I and the reference plane O, the 
second plane II being perpendicular to the axis of the crank shaft 11. 
This balance weight 18 has a mass mb, and is arranged to have the center 
of gravity at a position separated by a distance rb from the axis of the 
crank shaft 11 and disposed at a position separated by a distance d from 
the reference plane O. On the other hand, the lower balance weight 19 is 
disposed in the third plane III perpendicular to the axis of the crank 
shaft 11 and existing at the opposite side to the second plane II with 
respect to the reference plane O. Further, the lower balance weight 19 has 
a mass mc, and is arranged to have the center of gravity at a position 
separated by a distance rc from the axis of the crank shaft 11 and 
disposed at a position separated by e from the reference plane O. The 
springs 7 are arranged to elastically act or operate with respect to the 
compressing unit in a fourth plane IV between the reference plane O and 
the third plane III. This fourth plane IV is separated by a distance z 
from the reference plane O. 
Here, in this embodiment, the balance weights 18 and 19 are designed so 
that the ratios of the product of the mass mb and the distance rb and the 
product of the mass mc and the distance rc relative to the product of the 
mass M and the distance rk are in predetermined ranges whereby the radial 
vibration of the compressing unit in the fourth plane IV becomes extremely 
small. More specifically, the weights, position and others of the balance 
weights 18 and 19 are determined so that the value Nb obtained in 
accordance with the following equation (10) is in a range of 0.8 to 1.2 
and the value Nc obtained in accordance with the following equation (11 ) 
is in a range of 0.1 to 0.5. 
EQU Nb=mb.multidot.rb/M.multidot.rk (10) 
EQU Nc=mc.multidot.rc/M.multidot.rk (11) 
As shown in FIGS. 12 and 13, when the balance weights 18 and 19 are 
designed so as to satisfy these conditions, the vibration of a leg portion 
45 of the compressor 1 becomes below a desired value (10 .mu.m). 
According to tests, for example, in a compressor having a cylinder 9 volume 
of 4.8 cm.sup.3, the mass M of 73 g and the eccentric distance rk of 7.65 
mm, when the balance weights are not provided on the rotor 14, the 
vibration of the spring-mounting plate 22 is 300 to 400 .mu.m, and when 
the balance weights 18 and 19 are provided so that rb=19.5 mm, mb=32.4 g, 
rc=19.5 mm and mc=10.8 g, the vibration of the spring-mounting plate 22 
becomes 50 to 80 .mu.m smaller as compared with no provision of the 
balance weights, whereby the vibration of the leg portion 45 reduced 
through the springs 7 becomes about 8 .mu.m below a desired value (10 
.mu.m). 
According to this embodiment, since the third plane III passing through the 
balance weight 19 is disposed below the fourth plane IV in which the 
springs 7 act to the compressing unit, it is possible to sufficiently 
allow a space for the provision of the springs 7, thereby preventing the 
total height of the compressor 1 from increasing. As shown in FIG. 11 
which is an enlarged view illustrating a portion indicated by character A 
in FIG. 10, the balance weight 19, being provided on the lower surface of 
the rotor 14, comprises a horn (downwardly projecting portion) 47 formed 
integrally with an end ring 46 of the rotor 14 and a counter weight 48 
surrounding the horn 47. The horn 47 and the counter weight 48 are fixedly 
Connected through an adequate adhesive (for example, SW-2214 manufactured 
by Sumitomo 3M Co., Ltd.) 49 to each other. In a conventional balance 
weight, the horn 47 is constructed to be longer and a counter weight is 
fitted and caulked as indicated by a dotted line 50. Thus, according to 
this arrangement, the dimension of the balance weight 19 can be reduced by 
f, thereby reducing the total height of the compressor 1. In FIG. 11, 
numeral 51 represents an oil surface. It is also appropriate that the 
balance weight 19 is formed integrally with the end ring 46. 
In this embodiment, the distances c, d, e and z are determined by taking an 
adequate rotational balance. 
It should be understood that the foregoing relates to only preferred 
embodiments of the present invention, and that it is intended to cover all 
changes and modifications of the embodiments of the invention herein used 
for the purposes of the disclosure, which do not constitute departures 
from the spirit and scope of the invention.