JPH07125641A - Mechanical fuse device - Google Patents

Abstract

PURPOSE:To normally actuate a mechanical fuse device for a long period of time even in the state where a comparatively high load repeatedly works on a key. CONSTITUTION:This device has a shaft 10 having key grooves 28 and 29, a gear having key grooves 30 and 31 engaged with the shaft 10, a flat plate type key 32 inserted into the key grooves 28 and 30 and a flat plate type key 33 inserted into the key grooves 29 and 31. The keys 32 and 33 are constituted of ceramics, and when their respective loads exceed a specified value, they break. A clearance between the side surface of the key 32 and the side surface of their key grooves is set smaller than a clearance between the side surface of the key 33 and the side surface of their key grooves, and at the time of normal actuation, only the key 32 holds the loads.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a connecting structure of a substantially rod-shaped member and a member fitted thereto, and more particularly to a mechanical fuse device for controlling the connecting state of such two members. Pertain.

[0002]

2. Description of the Related Art As one of mechanical fuse devices,
For example, as described in JP-A-61-220968, in an electric power steering device for a vehicle such as an automobile, an auxiliary member that rotates integrally with a steering shaft and an output gear driven by a motor Is connected via a shear pin, and an excessive load acts between the auxiliary member and the output gear, causing a shear pin to break when a load greater than a predetermined value is applied to the share pin and driving between the auxiliary member and the output gear. 2. Description of the Related Art A mechanical fuse device configured to release a connection state has been conventionally known.

According to such a mechanical fuse device,
For example, even if the motor becomes unusable due to an abnormality such as a lock, the vehicle driver operates the steering wheel to rotationally drive the steering shaft to apply a high load to the shear pin, causing the shear pin to break, As a result, it is possible to reliably prevent the steering from becoming impossible due to the motor being locked or the like.

[0004]

A connecting member such as a shear pin is generally made of a metal or resin having excellent impact resistance. However, the strength of metals and resins decreases comparatively significantly due to fatigue when repeatedly loaded. Therefore, in the conventional mechanical fuse device as described above, the strength of the shear pin decreases due to fatigue during the process in which the power steering device is used for a long period of time.
As a result, even if the actual load acting on the shear pin is less than the designed breaking load, the shear pin will be broken, which will assist the electric power steering device in the situation where it should maintain its original operating state. There is a problem that the torque is not transmitted between the member and the output gear, and the power steering device unnecessarily switches to the manual steering device.

Further, in order to prevent the shear pin from being broken when the actual load acting on the shear pin due to the fatigue-induced strength reduction of the shear pin is less than the set breaking load, the shear pin is expected to be reduced in strength due to fatigue. Attempting to set a lower breaking load for the shear pin results in a lower breaking load on the shear pin than the load that the shear pin must withstand in order for the power steering system to operate properly, and so again in this case In such a situation, if the transmission torque between the auxiliary member and the output gear becomes high, the share pin will be broken and the power steering device will fail even though the power steering device should maintain its original operating state. If necessary, switch to a manual steering device.

On the other hand, since ceramics are inferior in impact resistance to metals and resins, it is conventionally considered unsuitable as a constituent material of a connecting member such as a key for connecting two members. Ceramics have an excellent feature that strength reduction due to fatigue is extremely small as compared with metals and resins.

However, even if the connecting member is made of ceramics, if the mechanical fuse device is used for a long period of time in a situation where a relatively high load is repeatedly applied to the connecting member, the strength of the connecting member will be reduced. Therefore, as in the case where the connecting member is made of metal or resin, the connecting member may break under a set load.

In view of the above-mentioned problems in the conventional mechanical fuse device, the present invention allows the mechanical fuse device to be used for a long period of time in a situation where a relatively high load is repeatedly applied to a key as a connecting member. With the purpose of providing an improved mechanical fuse device, the key does not break under a set load due to strength reduction due to its fatigue, and thereby can operate normally for a long period of time. There is.

[0009]

SUMMARY OF THE INVENTION According to the present invention, the above object is to provide a substantially round bar-shaped first member having first and second keyways, and the first member. A second member having a third and a fourth key groove that are respectively aligned with the first and the second key groove, and the first and second key members that are inserted into the first and the third key groove. A flat plate-shaped first key for connecting the members so that torque can be transmitted, and a flat plate-shaped second key that is inserted into the second and fourth key grooves and connects the first and second members for transmitting the torque. A first key and a second key are made of ceramics, and break when the load transmitted between the first member and the second member exceeds a predetermined value. In a mechanical fuse device configured to release the connected state of the first and second members, The clearance between the side surface of the third keyway and the side surface of the first key is set smaller than the clearance between the side surface of the second or fourth keyway and the side surface of the second key. A mechanical fuse device, or a substantially rod-shaped first member having a first key groove, and a first member that fits into the first member and is aligned with the first key groove. A key having a second member having two key grooves and a flat plate-shaped key inserted into the first and second key grooves to connect the first and second members to each other so that torque can be transmitted. Is made of ceramics, and breaks when the load transmitted between the first member and the second member exceeds a predetermined value to release the connection state of the first and second members. In the constructed mechanical fuse device, the key has a thick wall and a thin wall. It is accomplished by a mechanical fuse device, characterized in that it has and.

[0010]

[Function] FIG. 14 shows the delayed fracture strength and fatigue strength of ceramics (silicon nitride) as metal (alloy steel, JIS standard SCM).
415) is a graph showing comparison with the fatigue strength. From FIG. 14, the strength decrease due to metal fatigue is relatively large, whereas the fatigue strength after 10 ⁸ times of ceramics is about 80% of its initial strength, and the strength decrease due to fatigue of ceramics is extremely small. Understand.

According to the former configuration (claim 1) described above,
The first and second keys that control the connection state of the first and second members are made of ceramics whose strength is reduced very little by fatigue. The clearance between the side surface of the key and the side surface of the second or fourth keyway is set smaller than the clearance between the side surface of the second key and the side surface of the second key. Only the second key, not the second key,
Therefore, since the strength of the second key does not substantially decrease at all, even if the mechanical fuse device is used for a long period of time in a situation where a relatively high load is repeatedly applied to the first key, the second key is It does not break when the load acting on it is less than the set load, and it breaks when the load reaches the set load accurately, so the mechanical fuse device operates as designed for a long period of time. It is possible to let

According to the latter construction (claim 2) described above, the key is made of a ceramic material whose strength is not significantly reduced by fatigue, and has a thick portion and a thin portion. Since the load acts only on the thick wall portion and not on the thin wall portion, and the strength of the thin wall portion is not substantially reduced at all, a relatively high load is repeatedly applied to the thick wall portion of the key. Even if the mechanical fuse device is used for a long time, the thin-walled part will not break when the load acting on it is less than the set load, and will break when the load reaches the set load accurately. Therefore, the mechanical fuse device can be operated for a long period of time as designed.

[0013]

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

FIG. 1 is a schematic structural view showing an electric power steering device to which a mechanical fuse device according to the present invention is applied, and FIG. 2 is a first mechanical fuse device applied between a gear and a shaft in FIG. FIG. 3 is an enlarged partial vertical sectional view showing an embodiment, and FIG. 3 is a plan sectional view of the first embodiment shown in FIG.

In FIG. 1, reference numeral 10 denotes a steering shaft having a steering wheel 12 fixed to the upper end. The steering shaft 10 has a torque sensor 14 for detecting steering torque in the middle thereof, and is connected at its lower end to a pinion shaft of a rack-and-pinion steering device 16. In addition, the steering shaft 12
A driven gear 18 is fixed between the torque sensor 14 and the lower end of the motor 20 as will be described later.
It meshes with the drive gear 22 driven by. The rotation of the motor 20 is based on the detection result of the torque sensor 14 and other sensors not shown in the figure, and the electronic control unit 24
The auxiliary steering force is generated according to at least the steering torque.

As shown in FIGS. 2 and 3, the steering shaft 10 is provided with half-moon shaped recesses 28 and 29 along the axis 26 thereof as first and second keyways. Further, the driven gear 18 is fitted to the steering shaft 10 at its boss portion 18A, and the inner surface of the boss portion is aligned with the recesses 28 and 29 and extends along the axis 26 so as to extend along the axis 26. Key grooves 30 and 31 are provided. A half-moon shaped flat ceramic key 32 is inserted in the recess 28 and the key groove 30, and a half-moon shaped flat ceramic key 33 is inserted in the recess 29 and the key groove 31. Accordingly, the driven gear 18 is fixedly connected to the steering shaft 10 by the keys 32 and 33.

In the illustrated first embodiment, as shown in FIGS. 4 and 5, both sides 32A and 3 of the key 32 are shown.
2B respectively have notch grooves 34 and 36 aligned with each other.
Is provided. The cutout grooves 34 and 36, when the key 32 is properly inserted into the recess 28 and the key groove 30, face the connecting portion of the steering shaft 10 and the driven gear 18, that is, the surface 38 of the steering shaft and the boss of the driven gear. It extends at a constant depth parallel to the flat end face 32C so as to extend in alignment with the inner surface 40 of the portion 18A. Further, the notch grooves 34 and 36 are composed of a portion having a constant width and a wedge-shaped portion, and the center planes 34A and 36 of the respective notch grooves.
A is the corresponding side surface 32A and 32B of the key 32, respectively.
Extend vertically aligned with each other.

Similarly, as shown in FIGS. 6 and 7,
The both side surfaces 33A and 33B of the key 33 are provided with notch grooves 35 and 37 which are aligned with each other. In the notch grooves 35 and 37, the key 33 has a depression 29 and a key groove 31.
When properly inserted therein, the opposing surfaces of the connecting portion of the steering shaft 10 and the driven gear 18, that is, the surface 38 of the steering shaft and the inner surface 40 of the boss portion 18A of the driven gear.
To extend in parallel with the flat end surface 33C so as to extend in a constant depth. Notch grooves 35 and 37
Has a constant width portion and a wedge-shaped portion, and the central planes 35A and 37A of the respective cutout grooves extend perpendicularly to the corresponding side surfaces 33A and 33B of the key 33, respectively.

As shown in FIGS. 5 and 7, the depression 2
The width of 8 and 29 is W28 and W29 respectively, and the keyway 3
The widths of 0 and 31 are W30 and W31 respectively, and the key 32
Assuming that the thicknesses of and 33 are T32 and T33, the depths of the cutout grooves 34 and 36 are D34 and D36, and the depths of the cutout grooves 35 and 37 are D35 and D37, respectively, these dimensions are as follows: Are set to satisfy the relationship.

W28 = W29 <W30 = W31 T32 ≦ W28 T33 ≦ W29 T33 <T32 D34 = D36 D35 = D37 D34> D35 Therefore, the clearance between the side surface of the key 32 and the recess 28 and the side surface of the key groove 30 is the key 33. Smaller than the clearance between the side surface of the dent 29 and the side surface of the depression 29 and the key groove 31,
Therefore, when the torque is transmitted between the steering shaft 10 and the driven gear 18 in the state where the key 32 is not broken, the load of the key is entirely carried by the key 32.

FIG. 8 shows the mechanical fuse device of the first embodiment shown in FIG. 8 in which the steering shaft 10 and the driven gear 1 are broken by breaking the key (indicated by a mark X in the drawing).
8 shows the relationship between the load P on the key and the stress intensity factor Ki at the bottom of the notch groove of each key until the connection state of No. 8 is released. As can be seen from FIG. 8, the depths D34 and D36 of the cutout grooves 34 and 36 of the key 32 are such that the stress intensity factor Ki is the fracture toughness Kic when the load P of the key 32 slightly exceeds the maximum allowable load Pmax. Is set to be equal to. On the other hand, for the depths D35 and D37 of the cutout grooves 35 and 37 of the key 33, the load P of the key 33 is set to the breaking load P.
The stress intensity factor Ki is set to be equal to the fracture toughness Kic when it becomes equal to f.

The "maximum permissible load Pmax" means the key 3 when the power steering device is operating normally.
2 and 33 means the maximum load that must be withstood, and the "set breaking load Pf" means that the mechanical fuse device connects the steering shaft 10 and the driven gear 18 when an abnormality occurs in the power steering device. The key 33, which is broken after the key 32 is broken to release, means the load to be broken.

In the illustrated first embodiment, the power steering device is operating normally, and the torque transmitted between the steering shaft 10 and the driven gear 18 is less than a predetermined value, which results in steering. When the load P applied to the keys 32 and 33 by the shaft and the driven gear is less than or equal to the maximum allowable load Pmax, none of the keys breaks, which keeps the driven gear integrally connected to the steering shaft. It

In this case, since the load caused by the torque transmission between the steering shaft 10 and the driven gear 18 is all carried by the key 32 and the load does not act on the key 33, the strength of the key 33 is reduced due to fatigue. There is no.
In addition, even if the strength of the key 32 is slightly reduced due to the repeated load acting on it, the key 32 is not shown in FIG.
It only breaks at a load slightly lower than the load at the position indicated by the mark, and does not affect the strength of the key 33 at all.

On the other hand, an abnormality such as a motor lock occurs in the power steering device, and the steering shaft 10
When the transmission torque between the driven gear 18 and the driven gear 18 exceeds a predetermined value, first, the load P of the key 32 is increased to the maximum allowable load Pmax.
When the key 32 is slightly exceeded, the key 32 breaks at a position corresponding to the cutout grooves 34 and 36, and as a result, the load is applied to the key 3
When the load is accurately set to the set breaking load Pf, the key 33 breaks at a position corresponding to the cutout grooves 35 and 37, whereby the steering shaft and the driven shaft are driven. The connection of the gears is released and the two members are free to rotate relative to each other about the axis 26, whereby the power steering device is switched to a manual steering device.

In the illustrated embodiment, the keys 32 and 33 are used.
The corresponding depressions and keyways are spaced 180 ° from each other about the axis 26 of the steering shaft 10, but the two sets of keys make an angle of about 1 about the axis 26.
The angle may be other than 80 °, the two sets of keys and the like may be offset from each other in the axial direction of the steering shaft, and further three or more sets of keys and their corresponding depressions and keyways may be provided. May be.

FIG. 9 is an enlarged partial vertical sectional view showing a second embodiment of the mechanical fuse device applied between the gear and the shaft, and FIG. 10 is a plan sectional view of the second embodiment shown in FIG. 11 and 12 are a front view (A) and a plan view (B) showing the key of the second embodiment shown in FIG. 9 and 10, parts corresponding to those in FIGS. 2 and 3 are designated by the same reference numerals as those in these figures.

In the second embodiment, the steering shaft 10 is provided with one half-moon shaped recess 48 as a first key groove along the axis 26 thereof, and is fitted to the steering shaft. Boss 18 of driven gear 18
On the inner surface of A, there is provided one key groove 50 as a second key groove that extends along the axis 26 in alignment with the recess 48. A semi-moon shaped flat plate-shaped ceramic key 52 is inserted in the recess 48 and the key groove 50, whereby the driven gear is moved by the key 52 to the steering shaft 1.
Fixed at 0.

As shown, the key 52 comprises a thick portion 54 and a thin portion 56, which are in the form of a quarter circle. The thick portion 54 and the thin portion 56 are integrally joined to each other at a joint portion 57, for example, by an adhesive,
As a result, it is easily broken along the joint 57. The key 52 may be formed as one member having a relatively deep notch groove at a position corresponding to the joint portion 57.

Notched grooves 58 and 60, which are aligned with each other, are provided on both side surfaces 54A and 54B of the thick portion 54, respectively. The key 52 is recessed in the notch grooves 58 and 60.
When properly inserted into the key groove 50, the surface 38 of the steering shaft 10 and the boss portion 18A of the driven gear 18 are inserted.
End face 54C that extends to align with the inner surface 40 of the
It extends parallel to and at a certain depth. Notch groove 5
Reference numerals 8 and 60 are each composed of a portion having a constant width and a wedge-shaped portion, and the central planes of the respective cutout grooves extend perpendicularly to the corresponding side surfaces 54A and 54B of the thick portion.

Similarly, as shown in FIGS. 11 and 13, notched grooves 62 and 64 are provided on both side surfaces 56A and 56B of the thin portion 56 so as to be aligned with each other. The cutout grooves 62 and 64 are aligned with the cutout grooves 58 and 60, respectively, and extend at a constant depth parallel to the planar end surface 56C. The notch grooves 62 and 64 are also composed of a portion having a constant width and a wedge-shaped portion.
They extend perpendicular to B to each other.

As shown in FIGS. 12 and 13, the width of the recess 48 is W48, the width of the key groove 50 is W50, and the thickness of the thick portion 54 and the thin portion 56 of the key 52 is T, respectively.
54 and T56, the depths of the cutout grooves 58 and 60 are D58 and D60, and the depths of the cutout grooves 62 and 64 are D62 and D64, respectively, these dimensions are set so as to satisfy the following relationship. Has been done.

W48 <W50 T54 ≦ W48 T56 ≦ W48 T54> T56 D58 = D60 D62 = D64 D58> D62 Therefore, the clearance between the side surface of the thick portion 54 of the key 52 and the side surface of the recess 48 and the key groove 50 is thin. The clearance is smaller than the clearance between the side surface of the portion 56 and the side surface of the recess 48 and the key groove 50, so that torque is transmitted between the steering shaft 10 and the driven gear 18 in a state where the key 52 is not broken. At this time, the load of the key is entirely carried by the thick portion.

As shown in FIG. 8, also in this embodiment, the depth D58 and D60 of the cutout grooves 58 and 60 of the thick portion 54 of the key 52 is such that the load P of the key 52 is the maximum allowable load Pma.
The stress intensity factor Ki is set to be equal to the fracture toughness Kic at a stage slightly exceeding x. On the other hand, thin part 5
The depths D62 and D64 of the notch grooves 62 and 64 of No. 6 are set so that the stress intensity factor Ki becomes equal to the fracture toughness Kic when the load P of the key 53 becomes equal to the set fracture load Pf. .

The "maximum allowable load Pmax" means that the key 5 is pressed when the power steering device is operating normally.
2 means the maximum value of the load that the thick wall portion 54 and the thin wall portion 56 must withstand, and the "set breaking load Pf" means that the mechanical fuse device causes the steering shaft 10 to operate when an abnormality occurs in the power steering device. Also, the thin portion 56 that breaks after the thick portion 54 breaks in order to release the connected state of the driven gear 18 means the load to be broken.

In the second embodiment, the power steering device is operating normally, and the torque transmitted between the steering shaft 10 and the driven gear 18 is less than the predetermined value, which results in the steering shaft. When the load P applied to the key 52 by the driven gear is less than or equal to the maximum allowable load Pmax, the key does not break in either the thick wall portion 54 or the thin wall portion 56, so that the driven gear is integrated with the steering shaft. Be maintained in a physically connected state.

In this case, since the load due to the torque transmission between the steering shaft 10 and the driven gear 18 is carried by the thick portion 54 and the thin portion 56 does not act, the strength of the thin portion is reduced due to fatigue. It never drops. In addition, even if the strength of the thick-walled part is slightly reduced due to the repeated load acting on it, the thick-walled part is
It only fractures at a load slightly lower than the load at the position indicated by the mark, and does not affect the strength of the thin portion at all.

On the other hand, an abnormality such as a motor lock occurs in the power steering device, and the steering shaft 10
When the transmission torque between the driven gear 18 and the driven gear 18 exceeds a predetermined value, first, the load P of the key 52 is increased to the maximum allowable load Pmax.
In the stage slightly exceeding the above, the thick portion 54 breaks at the positions corresponding to the joint portion 57 and the cutout grooves 58 and 60, so that the load is carried only by the thin portion 56.
When the load reaches the set breaking load Pf accurately, the thin-walled part breaks at the positions corresponding to the notch grooves 62 and 64, whereby the connected state of the steering shaft and the driven gear is released, and the two The member is free to rotate relative to one another about axis 26, which causes the power steering system to be switched to a manual steering system.

FIG. 15 shows, for example, the load P acting on the key and the notch groove in the mechanical fuse device of the comparative example in which only the key 32 of the first embodiment and the corresponding depression 28 and key groove 30 are provided. The relationship between the stress intensity factor and the stress intensity factor at the bottom of the key is shown. The solid line shows the characteristic at the start of use of the mechanical fuse device, and the broken line shows the characteristic after a relatively high load is repeatedly applied to the key. Shows.

From FIG. 15, even if the depth of the notch groove is set so that the key breaks when the load P acting on the key becomes equal to the set breaking load Pf, even if the depth of the notch groove is set, a relatively high load is applied to the key. When the mechanical fuse device is used for a long period of time in a situation where the key repeatedly operates, the strength of the key decreases due to fatigue, and as a result, the key is in a stage where the load P acting on the key becomes the maximum allowable load Pmax or less. It turns out that it will start to break.

On the other hand, as described above, in the first embodiment, no load acts on the key 33, and in the second embodiment, no load acts on the thin portion 56. Since these strengths do not decrease, the connection between the steering shaft and the driven gear is released by the mechanical fuse device only when the load acting on the key 33 or the thin portion 56 reaches the set breaking load Pf accurately. Can be made.

Although the present invention has been described above in detail with reference to specific embodiments, the present invention is not limited to the embodiments described above, and various other embodiments are also possible within the scope of the present invention. It will be apparent to those skilled in the art that

For example, the dimensions of the keys 32 and 33 and the corresponding depressions and keyways in the first embodiment and the dimensions of the keys 52 and the corresponding depressions and keyways in the second embodiment are as follows: Although set to the above-mentioned specific relationship,
In the first embodiment, the clearance between the side surface of the key 32 and the side surface of the recess 28 and the key groove 30 is set smaller than the clearance between the side surface of the key 33 and the side surface of the recess 29 and the key groove 31. In the second embodiment, the key 52
As long as the clearance between the side surface of the thick portion 54 and the side surface of the recess 48 and the key groove 50 is set smaller than the clearance between the side surface of the thin portion 56 and the side surface of the recess 48 and the key groove 50, It may be set to a relationship other than the dimensional relationship of. Further, the thick wall portion 54 and the thin wall portion 56 in the second embodiment are used as keys independent of each other without being integrally joined to each other, whereby the mechanical fuse device has the mechanical structure according to claim 2. It may be used as a fuse device.

The cross-sectional shapes of the notch grooves 34 and 36 provided in the key 32 of each illustrated embodiment are V-shaped or rectangular and V-shaped.
However, the notch groove provided in the key has an arbitrary cross-sectional shape including, for example, a double V shape having a root portion with a large groove angle and a tip portion with a small groove angle. Good.

[0045]

As is apparent from the above description, according to the structure of claim 1 of the present invention, the first and second keys for controlling the connection state of the first and second members have the strength due to fatigue. It is composed of ceramics with a very small decrease,
The clearance between the side surface of the first or third key groove and the side surface of the first key is set smaller than the clearance between the side surface of the second or fourth key groove and the side surface of the second key. Since the repeated load acts only on the first key and not on the second key, and therefore the strength of the second key is not reduced at all, it is compared to the first key. Even if the mechanical fuse device is used for a long period of time under the condition that a relatively high load is repeatedly applied, the second key does not break when the load acting on it is less than the set load,
When the load reaches the set load accurately, the load is broken, so that the mechanical fuse device can be operated for a long time as designed.

According to the above-mentioned structure of claim 2, the key is made of ceramics whose strength is hardly reduced by fatigue, and has a thick portion and a thin portion.
The repeated load acts only on the thick wall portion and not on the thin wall portion, and therefore the strength of the thin wall portion does not substantially decrease at all.Therefore, a relatively high load is repeatedly applied to the thick wall portion of the key. Even if the mechanical fuse device is used for a long time, the thin-walled part does not break when the load acting on it is less than the set load, and it breaks when the load reaches the set load accurately. Therefore, the mechanical fuse device can be operated for a long period of time as designed.

According to the structure of claim 2, the number of keys and key grooves can be reduced as compared with the case of the structure of claim 1, whereby the structure of the mechanical fuse device can be simplified. In addition, it is possible to easily assemble the mechanical fuse device.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an electric power steering device to which a mechanical fuse device according to the present invention is applied.

FIG. 2 is an enlarged partial vertical cross-sectional view showing a first embodiment of the mechanical fuse device applied between the gear and the shaft of FIG.

FIG. 3 is a plan sectional view of the first embodiment shown in FIG.

FIG. 4 is a front view showing a first key of the first embodiment shown in FIG.

5 is an enlarged partial vertical cross-sectional view showing a main part of the first key shown in FIG.

6 is a front view showing the second key of the first embodiment shown in FIG. 2. FIG.

FIG. 7 is an enlarged partial vertical cross-sectional view showing the main parts of the second key shown in FIG.

FIG. 8 is a graph showing stress intensity factors at the bottoms of the cutout grooves of the first and second keys of the first embodiment.

FIG. 9 is an enlarged partial vertical cross-sectional view showing a second embodiment of the mechanical fuse device applied between the gear and the shaft.

10 is a plan sectional view of the second embodiment shown in FIG. 9. FIG.

11 is a front view (A) and a plan view (B) showing the key of the second embodiment shown in FIG. 9. FIG.

12 is an enlarged partial vertical cross-sectional view taken along the line XII-XII in FIG.

FIG. 13 is an enlarged partial vertical sectional view taken along the line XIII-XIII in FIG.

FIG. 14 is a graph showing the fatigue strength of ceramics in comparison with the fatigue strength of metals.

FIG. 15 is a graph showing changes in the stress intensity factor at the bottom of the cutout groove of the key of the mechanical fuse device of the comparative example.

[Explanation of symbols]

 10 ... Steering shaft 16 ... Rack and pinion type steering device 18 ... Driven gear 22 ... Drive gears 28, 29 ... Recesses 30, 31 ... Key groove 32, 33 ... Keys 34, 35, 36, 37 ... Notch groove 48 ... Dimple 50 ... Key groove 52 ... Key 54 ... Thick portion 56 ... Thin portion 58, 60, 62, 64 ... Notch groove

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Nao Iwasaki 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation (72) Inventor Kazunori Kagawa 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation

Claims (2)

[Claims] 1. A substantially round bar-shaped first member having first and second keyways, and a first member fitted to the first member and aligned with the first and second keyways, respectively. A second member having three and fourth key grooves, and a flat plate-shaped first key that is inserted into the first and third key grooves and connects the first and second members so that torque can be transmitted. A flat plate-shaped second key that is inserted into the second and fourth key grooves and connects the first and second members to each other so that torque can be transmitted, and the first and second keys are ceramics. And is configured to break when the load transmitted between the first member and the second member exceeds a predetermined value and release the connected state of the first and second members. In the mechanical fuse device, between the side surface of the first or third key groove and the side surface of the first key. Clearance is a mechanical fuse device, characterized in that is smaller than the clearance between the side surfaces and the second key of the second or fourth keyway. 2. A first member having a substantially round bar shape having a first key groove, and a second key groove fitted to the first member and aligned with the first key groove. A second member, and a flat plate-shaped key that is inserted into the first and second key grooves and connects the first and second members so that torque can be transmitted, and the key is made of ceramics, A mechanical fuse device configured to break when the load transmitted between the first member and the second member exceeds a predetermined value and release the connected state of the first and second members. Where
The mechanical fuse device, wherein the key has a thick portion and a thin portion.