Vibration isolator having magnetic springs

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a vibration isolator for use in, for example, an ambulance for carrying sick or wounded people to make them more comfortable.

2. Description of the Related Art

Hydraulic or air suspensions are employed in most of the vibration isolators, and various measures have been taken up to this time to improve the riding comfort. Vibration isolators that perform an active control to improve the riding comfort have been proposed.

The running of ambulances includes ordinary running, in which they run with the stream of cars, and special running when they are called in an emergency. The riding comfort differs between the ordinary running and the special running.

In order to confirm the effects of the vibration isolators on the riding comfort, actual running tests were carried out using a typical domestic car and a typical imported car. During the tests, longitudinal (back and forth), widthwise (right and left) and vertical accelerations of a floor of an ambulance and those of the waist of a subject lying on a stretcher were measured and analyzed. The tests revealed that the conventional vibration isolators could achieve effective isolation in a high-frequency region (10-20 Hz), but could not satisfactorily restrain low-frequency vibrations (0.1-10 Hz), particularly the longitudinal and vertical low-frequency vibrations. For this reason, there arose the problems that the condition of a sick or wounded person may become worse due to resonance of his or her internal organs in particular, a fluctuation in blood pressure (a sense in which blood concentrates on the head) may be caused by a nose dive, or he or she may get carsick. Such problems were, however, sometimes caused by an improper adjustment of a suspension system of the vibration isolator to the vehicle body.

SUMMARY OF THE INVENTION

The present invention has been developed to overcome the above-described disadvantages.

It is accordingly an objective of the present invention to provide a vibration isolator for use in an ambulance that can restrain unpleasant feeling of a sick or wounded person, which has been hitherto caused by a sudden stop or rapid speed reduction, by making use of magnetic springs and magnetic dampers in a suspension mechanism of the vibration isolator.

In accomplishing the above and other objectives, the vibration isolator according to the present invention includes a lower frame movably mounted on a floor, an upper frame vertically movably mounted on the lower frame, a link mechanism coupled to the upper and lower frames, operable to move the upper frame relative to the lower frame, and a plurality of magnetic springs interposed between the upper and lower frames and each having a plurality of permanent magnets with like magnetic poles opposed to each other. In this vibration isolator, a vertical vibration of the upper frame relative to the lower frame is restrained by the plurality of magnetic springs, and the front side of the vibration isolator is lifted upon receipt of a forward acceleration.

Furthermore, an acceleration inputted in a direction longitudinally of the vibration isolator is restrained by virtue of a single-sided pendulum motion about an instantaneous center of rotation of the lower frame.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objectives and features of the present invention will become more apparent from the following description of preferred embodiments thereof with reference to the accompanying drawings, throughout which like parts are designated by like reference numerals, and wherein:

FIG. 1

is a schematic side view of a vibration isolator according to a first embodiment of the present invention;

FIG. 2

is a schematic side view of the vibration isolator of

FIG. 1

on which a stretcher together with a sick or wounded person is placed, particularly showing the behavior of various portions of the vibration isolator;

FIG. 3

is a schematic side view of the vibration isolator of

FIG. 2

, particularly showing the case where an acceleration has been applied thereto by sudden braking;

FIG. 4

is a schematic diagram showing relationships between a component of the acceleration in the direction of advance of the vibration isolator and a component of the acceleration of gravity in the direction of advance of the vibration isolator during ordinary running and during sudden braking;

FIG. 5

is a schematic; side view of the vibration isolator of

FIG. 2

, particularly showing the initial angle of inclination thereof;

FIG. 6

is a graph showing a relationship between the displacement and spring forces of various springs mounted in the vibration isolator of

FIG. 1

;

FIG. 7

is a side view of the vibration isolator of

FIG. 1

, particularly showing the behavior of an upper frame when the vibration isolator rides over a projection;

FIG. 8

is a graph showing the PSD (Power Spectral Density) of the acceleration on a floor and the vibration transmissibility of the waist of a subject on the stretcher with respect to a longitudinal vibration during actual running;

FIG. 9

is a graph similar to

FIG. 8

, but showing the PSD of the acceleration on the floor and the vibration transmissibility of the waist of the subject on the stretcher with respect to a vertical vibration during actual running;

FIG. 10

is an exploded perspective view of a vibration isolator according to a second embodiment of the present invention;

FIG. 11A

is a schematic side view of a lower frame of the vibration isolator of

FIG. 10

during ordinary running;

FIG. 11

B is a fragmentary schematic plan view of a portion of the lower frame of

FIG. 11A

;

FIG. 12A

is a view similar to

FIG. 11A

, but during sudden braking; and

FIG. 12B

is a view similar to

FIG. 11B

, but during sudden braking.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This application is based on an application No. 10-299510 filed Oct. 21, 1998 in Japan, the content of which is incorporated hereinto by reference.

Referring now to the drawings, there is shown in

FIG. 1

a vibration isolator A according to a first embodiment of the present invention. The vibration isolator A includes two lower frames

2

a

,

2

b

secured to each other and mounted on a floor so as to be movable in the longitudinal direction thereof, an upper frame

4

vertically movably mounted on the lower frames

2

a

,

2

b

, and a top plate

6

longitudinally slidably mounted on the upper frame

4

. The two lower frames

2

a

,

2

b

are sometimes referred to simply as a lower frame

2

hereinafter.

Each of the lower frames

2

a

,

2

b

is provided with a conductor

8

a

,

8

b

such as, for example, aluminum secured to the lower surface thereof. The conductor

8

a

(

8

b

) is mounted, via levers

12

a

(

12

b

) disposed on respective sides thereof, on support plates

10

a

(

10

b

) extending upwardly from the floor so as to be movable in the longitudinal direction of the vibration isolator A. Two permanent magnets

14

a

(or

14

b

), which are spaced a predetermined distance from each other, are securely mounted on the floor on respective sides of each conductor

8

a

(or

8

b

).

The upper frame

4

is coupled to the lower frame

2

via X-shaped, links

16

a

,

16

b

and pantographs

18

a

,

18

b

, both disposed on respective sides thereof.

Each of the X-shaped links

16

a

,

16

b

includes two relatively long levers

20

,

22

, each of which is pivotally connected at one end thereof to the upper frame

4

or the lower frame

2

. The other end of the lever

20

is pivotally connected to one end of relatively short lever

26

, the other end of which is pivotally connected to an upper nd of a support member

30

extending upwardly from the lower frame

2

. Similarly, the other end of the lever

22

is pivotally connected to one end of a relatively short lever

24

, the other end of which is pivotally connected to a lower end of a support member

28

extending downwardly from the upper frame

4

. The two relatively long levers

20

,

22

are mutually pivotally connected at intermediate portions thereof.

Each of the pantographs

18

a

,

18

b

includes four levers

32

,

34

,

36

,

38

. Of these levers

32

,

34

,

36

,

38

, the two levers

32

,

34

are pivotally connected to each other in the form of “L”, while the other two levers

36

,

38

are similarly pivotally connected to each other in a symmetric fashion relative to the two levers

32

,

34

. The levers

32

,

36

are pivotally connected at upper ends thereof to the upper frame

4

, while the levers

34

,

38

are pivotally connected at lower ends thereof to the lower frame

2

. Furthermore, a coil spring

40

is connected at one end thereof to a connecting portion between the two levers

32

,

34

and the other end thereof to a connecting portion between the other two levers

36

,

38

, thereby generating a lifting force of the upper frame

4

.

Each of the lower frames

2

a

,

2

b

is provided with a permanent magnet

42

,

44

secured to the upper surface thereof at a center in the widthwise direction thereof. The permanent magnets

42

,

44

confront permanent magnets

46

,

48

secured to the lower surface of the upper frame

4

, respectively, with like magnetic poles opposed to each other. A repulsive force acting between the two permanent magnets

42

,

46

and that acting between the two permanent magnets

44

,

48

act as lifting forces of the upper frame

4

. The lower frame

2

is connected, via a shock absorber

52

and a coil spring

54

, to a support member

50

secured to the floor on the left-hand side as viewed in

FIG. 1

(this side is hereinafter referred to as the head side of a sick or wounded person, while the opposite side is hereinafter referred to as the leg side).

The top plate

6

is mounted on the upper frame

4

via sliders

56

a

,

56

b

disposed on respective sides thereof so as to be slidable in the longitudinal direction of the vibration isolator A. On each of the head and leg sides, the top plate

6

is provided with two projections

58

a

,

58

b

secured to the lower surface thereof, while the upper frame

4

is provided with a projection

60

secured to the upper surface thereof and interposed between the two projections

58

a

,

58

b

of the top plate

6

. Coil springs

62

a

,

62

b

and rubber dampers (not shown) are interposed between the projection

58

a

of the top plate

6

and the projection

60

of the upper frame

4

and between the projection

58

b

of the top plate

6

and the projection

60

of the upper frame

4

, respectively, thereby maintaining the top plate

6

at a predetermined position relative to the upper frame

4

.

In the above-described construction, of the component parts mounted on the upper and lower frames

4

,

2

and those on the top plate

6

, the permanent magnets

42

,

44

,

46

,

48

, the shock absorber

52

, the coil spring

54

and the like are disposed at the central portion of the vibration isolator A in the widthwise direction thereof while the other component parts are disposed on respective sides of the vibration isolator A, but only one side is shown in FIG.

1

.

The vibration isolator A of the above-described construction operates as follows.

As shown in

FIG. 2

, various physical values associated with the vibration isolator A are represented as follows.

m

0

: mass of a sick or wounded person;

m

1

: mass of a stretcher;

m

2

: mass of the vibration isolator A;

k

1

: spring constant of the coil spring

62

a

interposed between the projections

58

a

,

60

;

k

2

: spring constant of the coil spring

62

b

interposed between the projections

58

b

,

60

;

k

3

: spring constant of the coil spring

40

mounted in the pantographs

18

a

,

18

b

;

k

4

: spring constant of the magnetic spring made up of the two permanent magnets

42

,

46

or

44

,

48

;

k

5

: spring constant of the coil spring

54

having one end secured to the lower frame

2

a

;

C

1

: damping coefficient of the rubber damper interposed between the projections

58

a

,

60

;

C

2

: damping coefficient of the rubber damper interposed between the projections

58

b

,

60

;

C

3mg

: damping coefficient of the magnetic damper made up of the permanent magnets

14

a

(

14

b

) and the conductor

8

a

(

8

b

);

C

4

: damping coefficient of the shock absorber

52

having one end secured to the lower frame

2

a.

The vibration isolator A according to the present invention includes means for restraining pitching vibration of a living body, which may be caused by sudden braking or when running over a projection, or an acceleration created in the longitudinal direction. More specifically, the vibration isolating mechanism makes use of, as such means, two motions: a pseudo-single-sided pendulum motion about an instantaneous center of rotation of the lower frames

2

a

,

2

b

relative to the floor as means for restraining the motion of the head in the upward direction only, and a translation motion of the masses (m

0

+m

1

) occurring in the same direction as the travel of the vehicle. In conventional active control systems, an acceleration of about 0.5 G that is created in the event of sudden braking is reduced by making use of the acceleration of gravity. According to the present invention, however, as shown in

FIG. 3

, a reaction against the acceleration caused by the sudden braking is utilized to move the center of the masses (m

0

+m

1

) rearwards (toward the leg side), thereby activating the metal spring (k

3

) and the magnetic spring (k

4

) on the rear side. As a result, the angle &thgr; of inclination of the top plate

6

becomes large. At this moment, as shown in

FIG. 4

, a component, directed in the direction of advance of the vibration isolator A, of the acceleration caused by the sudden braking is reduced by a component of the acceleration of gravity in the direction of advance of the vibration isolator A. The remainder is attenuated by the metal springs (k

1

, k

2

), the rubber dampers (C

1

, C

2

), and the magnetic dampers (C

3

).

Further explanation is made hereinafter in detail.

FIG. 5

depicts an initial condition of the vibration isolator A on which a sick or wounded person together with a stretcher is placed. In this condition, the upper frame

4

is maintained at an initial angle &thgr;

0

of inclination by the metal spring

54

and the shock absorber

52

. At this moment, if an acceleration of, for example, about 0.5 G is inputted to the vibration isolator A by sudden braking, it moves towards the head side against the biasing forces of the coil springs

62

a

with a point O shown in

FIG. 2

as an instantaneous center of rotation. The center of gravity of the masses (m

0

+m

1

) is then moved rearwards by a reaction against the acceleration, i.e., the biasing forces of the coil springs

62

a

, and a large load is applied to the pantograph

18

b

and the magnetic spring

44

,

48

on the leg side, thereby increasing the angle &thgr; of inclination of the upper frame

4

, as shown in FIG.

3

. When the angle &thgr; of inclination increases, a forward component of the acceleration caused by the sudden braking is attenuated by a rearward component of the acceleration of gravity, as shown in

FIG. 4

, while the remaining component that has not been attenuated by the rearward component of the acceleration of gravity is attenuated by the spring forces of the coil springs

62

a

,

62

b

, the damping forces of the rubber dampers, and the damping forces caused by electromagnetic induction between the permanent magnets

14

a

,

14

b

and the conductors

8

a

,

8

b.

Furthermore, the vibration characteristics of the magnetic springs are utilized as means for restraining vertical vibration.

FIG. 6

depicts the spring characteristics of the vibration isolating mechanism in response to the vertical vibration and the response amplitude at several frequencies when the vibration isolating mechanism has been oscillated by a sine wave having an acceleration amplitude of 0.3 G. The relationship between the magnetic spring force and the displacement has been found from formulas (3) and (4), and the relationship between the metal spring force and the displacement has been found in view of the conversion of force by the link structure. The results fall within 5% variations with respect to actual measurements.

As shown in

FIGS. 6 and 7

, when the pantograph that has been designed at a balanced position P in advance is compressed by an impact from the floor that may be caused by a projection on the floor, the pantograph is further compressed by virtue of the negative damping characteristics thereof, and the top plate on the springs is depressed in the direction of gravity. When the speed has approached zero at a bottom dead point (this corresponds to the time when the vibration isolator has reached a position close to the top of the projection), an upward push-back is commenced by virtue of the positive damping characteristics of a combined spring of the static magnetic springs and the metal springs.

On the other hand, under the condition in which the top plate

6

is oscillating, the dynamic magnetic springs provide positive spring characteristics, for example, at point Q in

FIG. 6

, while the pantographs provide negative spring characteristics. The combined characteristics of the pantographs and the dynamic magnetic springs exhibit a pseudo-condition of k=0 and eliminate the resonant point, making it possible to reduce the vibration transmissibility.

More specifically, the frequency of a pendulum is given by:

f

=

1

2

⁢

π

⁢

g

l

(

1

)

The natural frequency of the metal spring (k

5

) is set to a value higher than f. Both the shock absorber (C

4

) and the magnetic dampers (C

3

) provide damping characteristics. The damping coefficient of the magnetic dampers is approximately given by:

C

3mg

=pB

2

hA&agr;/&sgr;

  (2)

p: number of magnetic fluxes

B: magnetic flux density

h: thickness of a conductor

A: area of magnetic fluxes

&agr;: experimental correction factor

&rgr;: electrical resistance of the conductor.

Furthermore, the force by the magnetic spring is given by:

F

=

k

(

m

)

z

+

F

0

(

3

)

Accordingly, any optimum spring constant can be set by selecting the distance (z) between the magnets at the balanced position with the loaded mass, k

m

and F

0

being constants. By way of example, the force produced by the magnetic spring in the vicinity of the balanced point and in the vicinity of the bottom dead point within a predetermined stroke are respectively given by:

F

r1

=

4.27

z

+

224

,

⁢

F

r2

=

7.31

z

-

24

(

4

)

The natural frequency of the magnetic spring is given by:

f

m

=

1

2

⁢

π

⁢

(

m

⁢

⁢

g

-

F

0

)

2

m

·

k

(

m

)

(

5

)

m: loaded mass.

In

FIG. 6

, actual measurements of the static spring constant and the dynamic spring constant of the magnetic spring used are indicated.

Longitudinal (back and forth), widthwise (right and left) and vertical accelerations of a floor (the support portion of the vibration isolator) on an axle of rear wheels and those of the waists of subjects lying on a stretcher were measured using a domestic car A having a relatively hard suspension and an imported car B having a relatively soft suspension. The weights of the subjects were 56 kg, 72 kg, and 82 kg.

Also, acceleration measurements and sensory evaluation were carried out using the car A having a floor of a large acceleration, the car B and a trial car C capable of reproducing the same acceleration as in the car A. All the cars A, B and C were caused to run on a paved road with the vibration isolator A according to the present invention mounted in the car C.

FIGS. 8 and 9

depict the PSD (Power Spectral Density) of the acceleration on the floor and the vibration transmissibility of the waist of the subject on the stretcher with respect to the longitudinal and vertical vibrations during actual running, respectively. As shown therein, the magnetic spring type vibration isolator A has a resonant frequency of about 2 Hz with respect to the longitudinal vibration and a resonant frequency of about 3 Hz with respect to the vertical vibration. These 10 resonant frequencies deviate from the resonant frequencies of 0.4 Hz and 2 Hz of the floor, respectively.

The graph of

FIG. 8

reveals that the performance has been improved at low-frequency regions of about 0.3-1 Hz and about 3 Hz, and it is unlikely that resonance occurs on the head or legs having a resonant point in the range of 0.6-4 Hz.

The n-th order natural frequency of a wheel base with respect to the vertical vibration is given by:

f

WB

=

n

×

V

L

⁢

⁢

V

:  vehicle speed

L

:  length of wheel base

n

:  order of road-surface shape component.

(

6

)

As can be seen from the graph of

FIG. 9

, the car C has been improved as compared with the cars A and B in that the resonance of internal organs has been relieved at frequencies of 4-8 Hz that correspond to a vehicle speed range of up to 80 km/h.

When the vibration energy ratio of the waist of the human body to the floor was evaluated by making use of the SEAT value (Seat Effective Amplitude Transmissibility) that was proposed by Mr. Griffin, the conventional articles exhibited 145-155. On the other hand, the article according to the present invention exhibited 135, which indicates that the vibration energy has been reduced by about 10%.

Furthermore, the sensory evaluation that was carried out using a trial car was as follows.

(1) The tendency of the head to fall down during sudden braking has been fairly reduced.

(2) The vibration of the vehicle body has not been transmitted to the stretcher.

(3) Upon receipt of an impact, the legs have not been caused to spring up as if they have got stuck to the floor.

(4) The performance has been improved, compared with the conventional articles.

(5) The rolling properties have been improved.

FIG. 10

depicts a vibration isolator Al according to a second embodiment of the present invention, which includes a lower frame

72

longitudinally movably mounted on the floor and an upper frame

74

vertically movably mounted on the lower frame

72

.

The lower frame

72

is mounted on the floor via a plurality of sliders

76

disposed at front and rear portions thereof so as to be slidable in the longitudinal direction of the vibration isolator Al. The lower frame

72

is coupled to the sliders

76

via a plurality of levers

78

,

80

to allow a rocking motion thereof. Each of the levers

78

has a lower end pivotally mounted on the lower frame

72

and an upper end pivotally mounted on an upper end of a support plate

82

extending upwardly from the front slider

76

. Each of the levers

80

has an upper end pivotally mounted on the lower frame

72

and a lower end pivotally mounted on an upper end of a support plate

84

extending upwardly from the rear slider

76

.

Also, the upper frame

74

is coupled to the lower frame

72

via a plurality of y-shaped links and v-shaped links, both disposed on respective sides of the lower frame

72

.

Each of the y-shaped links is made up of a relatively long lever

86

and a relatively short lever

88

. The long lever

86

has an upper end pivotally mounted on the upper frame

74

and a lower end pivotally mounted on a lower end of another lever

90

, an upper end of which is pivotally mounted on a bracket

92

secured to the upper surface of the lower frame

72

. On the other hand, the short lever

88

has an upper end pivotally mounted on an intermediate portion of the long lever

86

and a lower end pivotally mounted on the lower frame

72

. The lower ends of the two long levers

86

and those of the two short levers

88

are connected to each other via rods

94

,

96

, respectively. A plurality of coil springs (not shown) are connected at opposite ends thereof to the rods

94

,

96

, respectively, to generate a lifting force of the upper frame

74

.

Each of the v-shaped links is made up of two levers

98

,

100

pivotally connected to each other. An upper end of the upper lever

98

is pivotally connected to the upper frame

74

, while a lower end of the lower lever

100

is pivotally connected to the lower frame

72

. The connecting portion of the two levers

98

,

100

is connected, via a rod

102

, to the connecting portion of the long and short levers

86

,

88

constituting the y-shaped link, thereby interlocking the y-shaped links and the associated v-shaped links with each other to vertically move the upper frame

74

.

The lower frame

72

has generally rectangular openings

104

,

106

defined therein at front (head-side) and rear (leg-side) portions thereof, through which a projection

108

and permanent magnets

110

,

112

secured to the floor extend, respectively. A plurality of coil springs

114

,

116

are disposed in front of and behind the projection

108

, while two permanent magnets

118

,

120

are disposed in front of and behind the two permanent magnets

110

,

112

. The two permanent magnets

118

,

120

are secured to the lower frame

72

and spaced from the permanent magnets

110

,

112

, respectively, with like (repulsive) magnetic poles opposed to each other, making it possible to attenuate a longitudinal movement of the lower frame

72

.

The lower frame

72

also has two generally rectangular openings

122

,

124

defined therein in a side-by-side fashion at an intermediate portion thereof, through which permanent magnets

126

,

128

secured to the floor extend, respectively. A conductor

130

made of, for example, aluminum and secured to the lower frame

72

is interposed between the two permanent magnets

126

,

128

so that the longitudinal movement of the lower frame

72

may be attenuated by virtue of electromagnetic induction.

Moreover, a permanent magnet

132

and two permanent magnets

134

,

136

are secured to the lower frame

72

at locations in front of and behind the conductor

130

, respectively. The three permanent magnets

132

,

134

,

136

confront permanent magnets

138

,

140

,

142

secured to the upper frame

74

and are spaced a predetermined distance therefrom, respectively, with like magnetic poles opposed to each other. The two opposing permanent magnets constitute a magnetic spring to attenuate a vertical movement of the upper frame

74

. As shown in

FIG. 10

, the two permanent magnets

134

,

136

are inclined with respect to the lower frame

72

, while the two permanent magnets

140

,

142

are similarly inclined with respect to the upper frame

74

.

Two shock absorbers

144

each having an upper end pivotally connected to the lower frame

72

and a lower end pivotally connected to the floor are disposed on respective sides of the lower frame

72

generally at the center thereof.

It is to be noted here that in the vibration isolator A

1

as shown in

FIG. 10

, a stretcher together with a sick or wounded person is to be placed on the upper frame

74

.

The operation of the vibration isolator A

1

of the above-described construction is explained hereinafter with reference to

FIGS. 11A

,

11

B,

12

A and

12

B.

Under the condition in which a stretcher together with a sick or wounded person is placed on the vibration isolator A

1

, the lower frame

72

is held at a predetermined position by the coil springs

114

,

116

, the permanent magnets

110

,

112

,

118

,

120

, the shock absorbers

144

and the like, as shown in FIG.

11

A. At this moment, when an acceleration is inputted by, for example, sudden braking, the lower frame

72

is caused to rock to the head (front) side about the instantaneous center of rotation thereof against the biasing force of the coil springs

116

and the repulsive force of the permanent magnets

110

,

118

.

Because the front lever

78

is pivotally connected at the upper end thereof to the slider

76

and at the lower end thereof to the lower frame

72

, while the rear lever

80

is contrariwise pivotally connected at the upper end thereof to the lower frame

72

and at the lower end thereof to the slider

76

, the head side of the lower frame

72

is lifted, whereas the leg side of the lower frame

72

is caused to drop. As a result, a frontward component of the acceleration caused by the sudden braking is attenuated by a rearward component of the acceleration of gravity, while the remaining component that cannot be attenuated by the rearward component of the acceleration of gravity is attenuated by the spring forces of the coil springs

114

,

116

, the repulsive force of the permanent magnets

110

,

118

, and the damping force created by electromagnetic induction acting between the permanent magnets

126

,

128

and the conductor

130

.

It is to be noted that the vibration isolator A

1

shown in

FIG. 10

does not always require the rods

102

for connecting the y-shaped links and the v-shaped links. In the construction having no such rods, the y-shaped links and the v-shaped links operate independently. In that case, upon receipt of an acceleration caused by sudden braking, the angle of inclination of the upper frame

74

and the rearward component of the acceleration of gravity become larger, making it possible to effectively attenuate the forward component of the acceleration caused by the sudden braking.

As described hereinabove, according to the present invention, the magnetic springs made up of a plurality of permanent magnets with like magnetic poles opposed to each other act to restrain the vertical vibration, and the front side of the vibration isolator is lifted upon receipt of a forward acceleration caused by, for example, a rapid speed reduction. Accordingly, a component of the acceleration in the direction of advance of the vibration isolator is reduced by a component of the acceleration of gravity acting in the direction of advance of the vibration isolator, making it possible to improve the riding comfort of the vibration isolator.

Moreover, an acceleration inputted in the longitudinal direction of the vibration isolator is restrained by virtue of a single-sided pendulum motion about an instantaneous center of rotation of the lower frame. By so doing, the riding comfort can be improved with a simple structure.

Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted here that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications otherwise depart from the spirit and scope of the present invention, they should be construed as being included therein.