Push-button switch, and operation device and teaching pendant comprising the same

This application is a Continuation of International Application No. PCT/JP98/01943, filed Apr. 27, 1998, which claims priority based on Japanese Patent Application No. 9-125008, filed Apr. 28,1997; Japanese Patent Application No. 9-284434, filed Sep. 30,1997; and Japanese Patent Application No. 9-284435, filed Sep. 30,1997. The entire disclosures of the above applications are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a push-button switch and more particularly, to a push-button switch which is shifted from an initial or first OFF state to an ON state and then, to a second OFF state as the amount of depression of the push-button increases.

2. Description of the Background Art

In cases where, for example, a manual operation is performed on a numerically controlled machine such as a robot, an operator often enters a dangerous area to carry out his job. In such cases, a pendant with a push-button, such as called an enable switch (or deadman switch), is used for preventing the occurrence of an accident during the work.

This pendant is a portable unit which is enabled by connection with an operation device to teach a program to the robot or operate the robot. As shown in

FIG. 80

, the pendant

500

includes an input keyboard

501

disposed on a main surface and a push-button switch (enable switch)

502

disposed on one side surface thereof. Incidentally, the push-button switch

502

may be sometimes disposed on the rear side of the pendant

500

. The pendant

500

further includes a signal cable

503

for connection with the operation device not shown.

As shown in

FIG. 77

, a conventional push-button switch

502

includes a push button

505

and a microswitch

506

disposed opposite to the push button. Disposed on a lower surface of the push button

505

is a leaf spring

507

extended downwardly therefrom. Disposed on a top surface of the microswitch

506

are a resilient push plate

508

and an actuator

509

. A bent portion

507

a

is formed at a tip of the leaf spring

507

.

When the push-button switch

502

is used, the pendant

500

incorporating the push-button switch

502

is first connected, via the signal cable

503

, to a control panel of a machine to be manually operated. If the push-button switch is in the OFF state at this time, manipulating the keyboard

501

of the pendant

500

does not effect the key entry.

Upon subsequent depression of the push button

505

, the bent portion

507

a

of the leaf spring

507

moving along with the push button

505

engages the push plate

508

of the microswitch

506

, and the push plate

508

is resiliently deformed downward to press down the actuator

509

, as shown in FIG.

78

. This causes the actuator

509

to lower for establishing contact between contacts within the microswitch

506

, thereby shifting the microswitch

506

to the ON state.

The operator keys in through the keyboard

501

of the pendant while keeping the push button

505

depressed for maintaining the microswitch in the ON state. If, at this time, the operator releases the push button

505

, sensing the danger of contacting some moving part of the machine manually operated, the push button

505

returns to the state shown in

FIG. 77

for turning OFF the microswitch

506

. Thus, the machine is stopped.

In a case where the operator, who has panicked sensing imminent danger, further presses down the push button

505

, the bent portion

507

a

of the leaf spring

507

slides on the push plate

508

to disengage therefrom, as shown in

FIG. 79

, so that the push plate

508

is returned to its original position by its restoring force. This shifts the microswitch

506

to the OFF state for stopping the machine.

Thus, the push-button switch

502

is adapted to enable the keyboard

501

of the pendant

500

or permits the key entry through the keyboard

502

for manual operation only when the microswitch

506

is in the ON state. Therefore, the operator's intent at the manual operation can be made distinct and hence, the operator's safety is ensured.

However, the known push-button switch is arranged such that the switch is maintained in the ON state by the engagement of the leaf spring and shifted to the OFF state by disengagement thereof which results from increased elastic deformation thereof. Accordingly, precisions of the leaf springs significantly affect a timing of shift between the ON and OFF states.

Therefore, the switch may sometimes be quick to be shifted from the ON state to the OFF state or slow to be shifted depending upon the variations of the leaf springs. Thus, the switch suffers from unstable operation and poor switching accuracy.

SUMMARY OF THE INVENTION

It is therefore, an object of the present invention to provide a push-button switch adapted for stable operation.

Another object of the invention is to provide a push-button switch capable of forcibly separating the contacts for shifting the switch to the OFF state, even if they are fused to each other, thereby providing even more stable operation of the switch.

It is still another object of the invention to provide a push-button switch which provides good operability and a positive shift to the OFF state in the event of an emergency when used as the enable switch of a teaching pendant for industrial manipulating robots.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1

is a sectional front view showing a push-button switch according to a first embodiment of the invention;

FIG. 2

is a sectional top plan view taken on the line II—II in

FIG. 1

;

FIG. 3

is a sectional front view for illustration of operation of the push-button switch according to the first embodiment;

FIG. 4

is a sectional front view for illustration of the operation of the push-button switch according to the first embodiment;

FIG. 5

is a sectional front view for illustration of the operation of the push-button switch according to the first embodiment;

FIG. 6

is a sectional front view for illustration of the operation of the push-button switch according to the first embodiment;

FIG. 7

is a sectional front view for illustration of the operation of the push-button switch according to the first embodiment;

FIG. 8

is a sectional front view for illustration of the operation of the push-button switch according to the first embodiment;

FIG. 9

is a graph representing a relation between the operating load and the operation stroke of a push button according to the first embodiment;

FIG. 10

is a sectional front view showing a push-button switch according to a second embodiment hereof;

FIG. 11

is a sectional top plan view taken on the line XI—XI in

FIG. 10

;

FIG. 12

is a sectional front view for illustration of operation of the push-button switch according to the second embodiment;

FIG. 13

is a sectional front view for illustration of the operation of the push-button switch according to the second embodiment;

FIG. 14

is a sectional front view for illustration of the operation of the push-button switch according to the second embodiment;

FIG. 15

is a sectional front view for illustration of the operation of the push-button switch according to the second embodiment;

FIG. 16

is a sectional front view for illustration of the operation of the push-button switch according to the second embodiment;

FIG. 17

is a sectional front view for illustration of the operation of the push-button switch according to the second embodiment;

FIG. 18

is an enlarged view showing a state of a stationary terminal in the push-button switch according to the second embodiment;

FIG. 19

is an enlarged view showing a different state of the stationary terminal in the push-button switch according to the second embodiment;

FIG. 20

is a sectional front view showing a push-button switch according to a third embodiment hereof;

FIG. 21

is a sectional front view for illustration of operation of the push-button switch according to the third embodiment;

FIG. 22

is a sectional front view for illustration of the operation of the push-button switch according to the third embodiment;

FIG. 23

is a perspective view showing a portion of the push-button switch according to the third embodiment;

FIG. 24

is a plan view showing the portion of the push-button switch according to the third embodiment;

FIG. 25

is a sectional front view showing a push-button switch according to a fourth embodiment hereof;

FIG. 26

is a sectional front view for illustration of operation of the push-button switch according to the fourth embodiment;

FIG. 27

is a sectional front view for illustration of the operation of the push-button switch according to the fourth embodiment;

FIG. 28

is a perspective view showing a portion of the push-button switch according to the fourth embodiment;

FIG. 29

is a perspective view showing another portion, as a modification, of the push-button switch according to the fourth embodiment;

FIG. 30

is a sectional view showing the portion, as the modification, of the push-button switch according to the fourth embodiment;

FIG. 31

is a sectional front view showing a push-button switch according to a fifth embodiment hereof;

FIG. 32

is a sectional front view showing a push-button switch according to a sixth embodiment hereof;

FIG. 33

is a perspective view showing a portion of the push-button switch according to the sixth embodiment;

FIG. 34

is a perspective view for illustration of operation according to the sixth embodiment;

FIG. 35

is a perspective view for illustration of the operation according to the sixth embodiment;

FIG. 36

is a perspective view for illustration of the operation according to the sixth embodiment;

FIG. 37

is a sectional front view showing a push-button switch according to a seventh embodiment hereof;

FIG. 38

is a sectional front view for illustration of operation of the push-button switch according to the seventh embodiment;

FIG. 39

is a sectional front view for illustration of the operation of the push-button switch according to the seventh embodiment;

FIG. 40

is an exploded perspective view showing a portion of the push-button switch according to the seventh embodiment;

FIG. 41

is an exploded perspective view showing a modification of the portion of the push-button switch according to the seventh embodiment;

FIG. 42

is a sectional side view showing a push-button switch according to an eighth embodiment hereof;

FIG. 43

is a sectional side view for illustration of operation of the push-button switch according to the eighth embodiment;

FIG. 44

is a sectional side view for illustration of the operation of the push-button switch according to the eighth embodiment;

FIG. 45

is a sectional front view showing a push-button switch according to a ninth embodiment hereof;

FIG. 46

is a perspective view showing a portion of the push-button switch according to the ninth embodiment;

FIG. 47

is an enlarged sectional view showing a portion of the push-button switch according to the ninth embodiment;

FIG. 48

is a sectional side view showing a sate of a portion of a push-button switch according to a tenth embodiment hereof;

FIG. 49

is a sectional side view showing a different state of the portion of the push-button switch according to the tenth embodiment;

FIG. 50

is a sectional side view showing a schematic construction of a push-button switch according to an eleventh embodiment hereof;

FIG. 51

is a sectional rear view showing the push-button switch according to the eleventh embodiment;

FIG. 52

is a sectional rear view showing a push-button switch according to a twelfth embodiment hereof;

FIG. 53

is a sectional side view showing a push-button switch according to a thirteenth embodiment hereof;

FIG. 54

is a schematic diagram showing a portion of the push-button switch according to the thirteenth embodiment;

FIG. 55

is a sectional front view showing a push-button switch according to a fourteenth embodiment hereof;

FIG. 56

is a sectional top plan view showing the push-button switch according to the fourteenth embodiment;

FIG. 57

is a sectional front view showing an emergency stop switch according to a fifteenth embodiment hereof;

FIG. 58

is a sectional front view taken on the line Y—Y in

FIG. 57

;

FIG. 59

is a sectional front view for illustration of operation of the emergency stop switch according to the fifteenth embodiment hereof;

FIG. 60

is a diagram for illustration of a working-effect of the fifteenth embodiment;

FIG. 61

is a sectional front view showing an emergency stop switch according to a sixteenth embodiment hereof;

FIG. 62

is a sectional front view for illustration of operation of the emergency stop switch according to the sixteenth embodiment;

FIG. 63

is a diagram for illustration of a working-effect of the emergency stop switch according to the sixteenth embodiment;

FIG. 64

is an enlarged view showing a state of a stationary contact in the emergency stop switch according to the sixteenth embodiment;

FIG. 65

is an enlarged view showing a different state of the stationary contact in the emergency stop switch according to the sixteenth embodiment;

FIG. 66

is a front view showing a teaching pendant according to a seventeenth embodiment hereof;

FIG. 67

is a perspective view of the teaching pendant of the seventeenth embodiment as seen from the rear side thereof;

FIG. 68

is a rear view showing a portion of the teaching pendant according to the seventeenth embodiment;

FIG. 69

is a side view showing the portion of the teaching pendant according to the seventeenth embodiment;

FIG. 70

is a perspective view showing a state of a teaching pendant according to an eighteenth embodiment hereof as seen from the rear side thereof;

FIG. 71

is a perspective view showing a different state of the teaching pendant according to the eighteenth embodiment as seen from the rear side thereof;

FIG. 72

is a plan view showing a state of the teaching pendant of the eighteenth embodiment with its right half portion cut off;

FIG. 73

is a fragmentary perspective view of the eighteenth embodiment;

FIG. 74

is a perspective view showing a portion of a teaching pendant according to a nineteenth embodiment hereof;

FIG. 75

is a perspective view showing another portion of the teaching pendant according to the nineteenth embodiment;

FIG. 76

is a group of diagrams illustrating operations of the teaching pendant according to the nineteenth embodiment;

FIG. 77

is a schematic diagram showing a construction of a prior-art push-button switch;

FIG. 78

is a diagram for illustration of operations of the prior-art push-button switch;

FIG. 79

is a diagram for illustration of the operations of the prior-art push-button switch; and

FIG. 80

is a perspective view showing a pendant including the prior-art push-button switch.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(First Embodiment)

A first embodiment of the invention will be described with reference to

FIGS. 1

to

9

.

FIG. 1

is a sectional front view showing a push-button switch according to the first embodiment;

FIG. 2

a sectional top plan view taken on the line II—II in

FIG. 1

;

FIGS. 3

to

8

sectional front views for illustration of operations of the push-button switch; and

FIG. 9

a graph representing a relation between the operating load and the operation stroke of a push button.

As seen in

FIG. 1

, a push-button switch

1

includes a hollow push button

2

of a substantially rectangular parallelepiped shape, a case

3

for supporting the push button

2

, and a switching mechanism

6

having an electrically conductive stationary terminal

4

fixed to a bottom

31

of the case

3

and an electrically conductive movable terminal

5

disposed above the stationary terminal

4

.

The push button

2

is formed with a concave hole

2

a

on its lower side so as to be hollowed, and is stepped substantially at midportions on the right and left sides of the hole

2

a.

Both the stepped portions of the hole

2

a

are formed with slopes

2

b,

2

b,

respectively. Projected downwardly of a bottom of the push button

2

are a plurality of support shafts

21

, each of which carries thereabout a coiled spring

7

having a greater length than the shaft. An upper end of each coiled spring

7

is locked to a lower surface

2

c

of the push button

2

whereas a lower end thereof is locked to a bottom surface

31

a

of the bottom

31

. The push button

2

is constantly urged upward by a spring force of each coiled spring

7

.

The stationary terminal

4

is comprised of a bent member disposed in the case

3

and substantially shaped like “T” as viewed in plan (see FIG.

2

). Such a bent portion

4

a

has resilience or spring characteristics with respect to vertical directions. A first contact

41

is affixed to a distal end of the bent portion

4

a.

An upper part of the switching mechanism

6

is inserted in the hole

2

a

of the push button

2

. The inserted portion

61

of the switching mechanism is formed with a pair of lateral holes

61

a,

61

a

extending in a transverse direction which is orthogonal to a direction of depression of the push button

2

.

Slide blocks

8

are transversely slidably inserted in the holes

61

a,

61

a,

respectively. The slide blocks

8

,

8

are formed with slopes

8

a,

8

a

capable of engaging the respective slopes

2

b

of the hole

2

a

of the push button

2

. Inserted in the respective holes

61

a,

61

a

are coiled springs

9

,

9

, which urge the slide blocks

8

in directions to project the slide blocks from the holes

61

a,

respectively.

A shaft

62

extending downward is disposed at a lower part of the switching mechanism

6

. The movable terminal

5

is vertically slidably carried by an upper end portion of the shaft

62

. Second contacts

51

are affixed to lower surfaces of opposite ends of the movable terminal

5

, respectively. A truncated cone-shaped coiled spring

10

applies a downward spring force to an upper portion of the movable terminal

5

. The coiled spring

10

is disposed to ensure a contact pressure when the second contacts

51

of the movable terminal

5

come into contact with the first contacts of the stationary terminal

4

.

A bottom portion of the shaft

62

is inserted in a hole

31

b

defined in the bottom

31

of the case

3

. Disposed in the hole

31

b

is a coiled spring

12

serving as a return spring. An upper portion of the coiled spring

12

is mounted on a boss-shaped portion as wound thereabout, the boss-shaped portion formed in a smaller diameter at a lower end of the shaft

62

. The shaft

62

is constantly urged upward by a spring force of the coiled spring

12

. Within the hole

31

b,

there is formed a stopper surface

31

c,

against which a stepped portion

62

a,

a base of the boss-shaped portion of the shaft

62

, is to abut.

Respective pairs of projections

22

extend downward from forward and backward places of the bottom of the push button

2

. These projections are equivalent to forcible separation means. As shown in

FIG. 2

in particular, the respective pairs of projections

22

are so located as to sandwich the movable terminal

5

therebetween as allowed to abut against respective ends of the stationary terminal

4

without touching the movable terminal

5

.

In a first OFF state or initial state in which the push button

2

is not depressed, as shown in

FIG. 1

, the push button

2

is placed at an initial position by the spring force of the coiled springs

7

so that the first and second contacts

41

,

51

are spaced from each other to define a gap therebetween. On the other hand, the slopes

8

a

of both slide blocks

8

,

8

are engaged with the slopes

2

b

of the hole

2

a

of the push button

2

. The switching mechanism

6

is interlocked with the depression of the push button

2

via this engagement.

Next, operations of the push-button switch

1

according to this embodiment will be described with reference to

FIGS. 3

to

8

.

If the push button

2

is depressed in the first OFF state shown in

FIG. 1

, because of the engagement between the slopes

8

a

of the slide blocks

8

and the slopes

2

b

of the push button

2

, the switching mechanism

6

is lowered along with the push button

2

thereby bringing the second contacts

51

of the movable terminal

5

of the switching mechanism

6

into contact with the first contacts

41

of the stationary terminal

4

of the case

3

, as shown in FIG.

3

. Thus, the switch is shifted to an ON state.

In this ON state, the slopes

8

a

of the slide blocks are subject to a pressing force from the slopes

2

a

of the hole

2

a

of the push button

2

, the force acting to retract the slide blocks

8

inwardly. However, the spring force of the coiled springs acting to project the slide blocks outwardly dominates this pressing force and hence, the slide blocks

8

are not retracted into the holes

61

a.

At this time, within the hole

31

b

of the bottom

31

of the case

3

, a gap t is defined between the stepped portion

62

a

of the shaft

62

of the switching mechanism

6

and the stopper surface

31

c

in.

Subsequently, if the push button

2

is further pressed down in the ON state shown in

FIG. 3

, the stepped portion

62

a

of the shaft

62

of the switching mechanism

6

abuts against the stopper surface

31

a

of the case bottom

31

, thereby to reduce the gap t to zero, while the second contacts

51

of the movable terminal

5

stay in contact with the first contacts

41

of the stationary terminal

4

, as shown in FIG.

4

. At this time, as indicated by a broken line in

FIG. 4

, the projections

22

at the bottom of the push button

2

overlap with the movable terminal

5

with respect to a direction perpendicular to the drawing sheet.

If the push button

2

in this state is further pressed down, the pushing force applied by the push button

2

to the slopes

8

z

of the slide blocks

8

becomes dominant over the spring force of the coiled springs

9

so that the slopes

8

a

of the slide blocks

8

start sliding on the slopes

2

b

of the push button

2

and the slide blocks

8

start to slide into the holes

61

a,

as shown in FIG.

5

. Eventually, the slide blocks

8

are completely retracted into the holes

61

a

whereby the slopes

8

a

of the slide blocks

8

are disengaged from the slopes

2

b

of the push button

2

. This permits the upper part of the switching mechanism

6

to move up and down in the hole

2

a

of the push button

2

thereby releasing the switching mechanism

6

from the interlocked relation with the depression of the push button

2

.

At this time, on the other hand, the coiled spring

12

in the hole

31

b

of the case bottom

31

is compressed so that the stepped portion

62

a

of the shaft

62

is subject to the spring force of the coiled spring

12

, the force pushing the shaft

62

upward. Therefore, when the slopes

2

b

of the push button

2

are disengaged from the slopes

8

a

of the slide blocks

8

, as mentioned supra, the spring force of the coiled spring

12

causes the upper part of the switching mechanism

6

to move upward in the hole

2

a

of the push button

2

and also the whole body of the switching mechanism

6

to move upward, as shown in FIG.

6

. This separates the second contacts

51

of the movable terminal

5

from the first contacts

41

of the stationary terminal

4

, shifting the switch to a second OFF state.

Thus, the switch is adapted for shifting from the ON state to the second OFF state in conjunction with the disengagement of the slopes

8

a

of the slide blocks

8

from the slopes

2

b

of the push button

2

. Therefore, the switch is stably shifted from the ON state to the second OFF state, accomplishing stable switching operations.

Next, if the push button

2

is further pressed down in the second OFF state shown in

FIG. 6

, the projections

22

at the bottom of the push button

2

are pressed against the bent portions

4

a

of the stationary terminal

4

to push down the first contacts

41

, thereby forcibly separating the first contacts

41

from the second contacts

51

. Thus, the first and second contacts

41

,

51

are forcibly brought out of contact even if the first and second contacts are fused to each other. This contributes to an even more positive switch shifting from the ON state to the second OFF state.

It is noted that instead of providing the projections

22

at the bottom of the push button

2

, the whole lower end portion of the push button

2

may be used to push down the first contacts

41

of the stationary terminal

4

. Otherwise, the projections may be disposed at the bent portions

4

a

of the stationary terminal

4

.

In the state of

FIG. 5

, on the other hand, even if a breakage of the coiled spring

12

disables the coiled spring

12

to apply its spring force to the shaft

62

of the switching mechanism

6

, the depression of the push button

2

permits the projections

22

at the bottom of the push button

2

to forcibly push down the first contacts

41

of the stationary terminals

4

, thereby positively shifting the switch from the ON state to the second OFF state (see FIG.

8

).

Now referring to

FIG. 9

, description will be made on a relation between the operating load applied to the push button

2

for manipulation of the push-button switch

1

and the operation stroke. It is noted that circled figures in the graph correspond to the drawing numbers, respectively.

Until the switch is shifted from the first OFF state {circle around (

1

)}, or an initial state shown in

FIG. 1

, through the ON state to a state {circle around (

4

)} shown in

FIG. 4

, the operating load progressively increases with increase in the operation stroke. In the subsequent shift from the state {circle around (

4

)} of

FIG. 4

to a state {circle around (

5

)} shown in

FIG. 5

, the operation stroke increases little while the operating load increases sharply. This is because a great load is required for plunging the slide blocks

8

inwardly.

In the subsequent shift from the state {circle around (

5

)} of

FIG. 5

to a state {circle around (

6

)} shown in

FIG. 6

, the operation load drops abruptly. This is because the push button

2

is disengaged from the slide blocks

8

. It is preferred that the push button

2

is operable with light touch when the operator, manipulating the switch in the ON state, panics to press down the push button forcefully. Hence, the switch is designed to shift smoothly from the ON state to the second OFF state by setting the operating load at a small value. At this time, the operator is also provided with a tactile click-touch (tactile feedback to the operation of the switch).

In the subsequent shift from the state {circle around (

6

)} of

FIG. 6

to a state {circle around (

7

)} shown in

FIG. 7

, the operating load progressively increases with the increase in the operation stroke. At this time, the projections

22

of the push button

2

progressively press down the contacts

41

of the stationary terminal

4

.

According to the first embodiment, the switch is adapted for shifting from the ON state to the second OFF state in conjunction with the disengagement of the slopes

8

a

of the slide blocks

8

from the slopes

2

b

of the push button

2

. Therefore, the switch is stably shifted from the ON state to the second OFF state for accomplishing the stable switching operations.

Further, when the switch is shifted from the ON state to the second OFF state, the upward movement of the switching mechanism

6

brings the second contacts

51

of the movable terminal

5

out of contact with the first contacts

41

of the stationary terminal

4

while the first contacts

41

are forcibly separated from the second contacts

51

by the projections

22

of the push button

2

pushing down the contacts

41

. This ensures that the first and second contacts

41

,

51

are separated from each other even if the contacts are fused to each other. Thus, the switch is positively shifted from the ON state to the second OFF state, accomplishing even more stable switching operations.

Further according to the first embodiment, the stationary terminal

4

is comprised of a single strap-like member. This contributes to a reduced number of components and a simplified construction of the switch.

Although the description of the first embodiment mentioned the projections

22

, as the forcible separation means, which are integrally formed with the push button, the projections are not particularly required to be integrally formed. As a matter of course, the forcible separation means, such as the projections

22

, may be formed independently from the push button

2

and affixed to the push button.

(Second Embodiment)

Next, a second embodiment of the invention will be described with reference to

FIGS. 10

to

19

.

FIG. 10

is a sectional front view showing a push-button switch according to the second embodiment of the invention;

FIG. 11

a sectional top plan view taken on the line XI—XI in

FIG. 10

;

FIGS. 12

to

17

sectional front views for illustration of operations of the push-button switch; and

FIGS. 18 and 19

enlarged views showing the stationary terminal in the push-button switch.

FIGS. 10

to

17

correspond to

FIGS. 1

to

8

of the first embodiment, respectively. In the figures, the same reference characters with those of the first embodiment represent the same or equivalent portions, respectively.

The second embodiment differs from the first embodiment only in the construction of the stationary terminal. Therefore, this description focuses solely on the stationary terminal and a detailed explanation of the other portions is dispensed with.

In

FIGS. 10

to

17

, a stationary terminal

40

disposed at the bottom

31

of the case

3

essentially consists of a fixed metal piece

42

fixed to the bottom

31

and a movable metal piece

43

pivotally supported by the fixed metal piece

42

.

An upright plate

42

extends upward from one end of the fixed metal piece

42

. One end

43

a

of the movable metal piece

43

engages a lower end of the upright plate

42

a.

This arrangement permits the movable metal piece

43

to vertically pivot about the lower end of the upright plate

42

a.

As shown in

FIGS. 11 and 18

, restriction plates

42

b

are disposed at opposite ends of the upright plate

42

a

for restriction of the upward pivotal movement of the movable metal piece

43

. Incidentally,

FIGS. 10

,

12

to

17

omit the restriction plates

42

b

for convenience of depiction.

A coiled spring

44

is stretched between the upright plate

42

a

and the movable metal piece

43

. The coiled spring

44

has one end thereof locked to the upright plate

42

a

while the other end thereof locked substantially to a midportion of the movable metal piece

43

. The movable metal piece

43

is constantly urged into an upward pivotal movement by a spring force of this coiled spring

44

.

As shown in

FIG. 11

, the movable metal piece