Power steering system having a mechanical safety breaker

A mechanical safety breaker for disengaging a mechanical connection between two mechanical members at a time when a force transmitted through the connection of the two members exceeds a predetermined value is provided by employing an overload breaker element made of a ceramic. A ceramic breaker element provides a long time stabilized fracture performance particularly when the load applied thereto is of a repetitive nature.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a mechanical safety breaker to be 
incorporated in a mechanical connection to sever the connection when a 
force or torque transmitted therethrough increases beyond a predetermined 
value. 
2. Prior Art 
The mechanical safety breaker adapted to operate normally to transmit a 
force or torque therethrough but to get broken when the magnitude of the 
force or torque increases beyond a predetermined limit value so as thereby 
to cease the transmission of the force of torque for the safety purpose is 
known and used in various mechanical devices. Conventionally, a breaker 
element which forms an essential part of a force or torque transmitting 
construction of such a mechanical safety breaker and is fractured by an 
overloading is generally made of metal or resin generally known as having 
high impact resistance. However, there is a problem in making the breaker 
element by metal or resin, because the fracture strength of metal or resin 
generally lowers much due to fatigue when it has been subjected to 
repetitive loadings. Therefore, when the breaker element of the mechanical 
safety breaker is made of metal or resin, there occurs a great 
inconvenience that the breaker is undesirably actuated after a period of 
use even when the mechanical load to be transmitted is still below a limit 
value predetermined for its actuation. 
In Japanese Patent Laid-open Publication 61-220968 it is shown to 
incorporate a mechanical safety breaker in an electric motor type power 
steering system of a vehicle such as an automobile such that an electric 
motor is drivingly connected with the steering shaft via a breaker element 
(sheer pin) adapted to be broken when the sheering force applied thereto 
increases beyond a predetermined value, considering that, in the electric 
motor type power steering system which generally includes an electronic 
control system and controls supply of electric power to the electric motor 
to be generally increased in accordance with increase of the manual force 
for operating the steering wheel, the electronic control system is not so 
free from any troubles as the simply mechanical steering shaft system, and 
further, since a reduction gear train is generally incorporated between 
the electric motor and the steering shaft driven thereby, if the supply of 
the electric power to the motor fails or improperly lowers, a high 
resistance is applied to the manual operation of the steering shaft. 
However, such a breaker element is subjected to highly frequently 
repetitive applications of alternating load due to the steering operation 
of the vehicle, rendering the fracture strength thereof to quickly fall 
due to fatigue. Nevertheless, it is highly required that the limit value 
of the steering torque at which the mechanical safety breaker of the power 
steering system is actuated is stably maintained within a relatively 
narrow range for a long time such that the mechanical safety breaker 
should not respond to such a temporal increase of the steering torque as 
will be caused by a bumping on of a steering vehicle wheel on a curbstone 
or the like, while ensuring that the steering shaft is released from the 
electric motor for free manual operation of the steering any time when a 
trouble has occurred in the assisting power steering system. 
SUMMARY OF THE INVENTION 
In view of the above-mentioned problems and requirements, it is a primary 
object of the present invention to provide an improved mechanical safety 
breaker which keeps a highly stabilized actuation point over a long period 
of operation without being affected by repetitive applications of load and 
aging of the material. 
According to the present invention, the above-mentioned primary object is 
accomplished by a power steering system for assisting steering operation 
of a manual steering system having a rack and pinion steering means, a 
steering shaft connected with the rack-and-pinion means for steering 
operation thereof, and a steering wheel connected with the steering shaft 
for driving the steering shaft in either of opposite steering directions 
by a driver. The power steering system comprises a motor and means for 
torque transmittingly connecting the motor with the manual steering 
system. The torque transmitting connection means includes a mechanical 
safety breaker element for breaking the torque transmitting connection of 
the motor with the manual steering system when the torque transmitted 
exceeds a predetermined limit value. The breaker element is made 
substantially of a ceramic material. 
FIG. 49 shows the performance of fracture strength of a ceramic (silicon 
nitride) and a metal (alloyed steel, JIS SCM 415) in comparison. As will 
be appreciated from this figure, the fracture strength of the ceramic is 
much stabilized against fatigue than that of the metal. For example, the 
fracture strength of the ceramic remains at about 80% of its initial value 
even after 10.sup.8 times repetitive loadings. In this figure, the 
fracture strength is shown in the stress by megapascal. The aging 
performance of the fracture strength of the ceramic is also stabilized. as 
will be appreciated in this figure. 
Therefore, by making the breaker element of the mechanical safety breaker 
by a ceramic material, the fracture performance of the breaker element can 
be highly stabilized for a long time of use under repetitive applications 
of load. 
Thus, based upon the long term stabilized performance accomplished by the 
basic concept of the present invention, it is a further object of the 
present invention to provide an improved mechanical safety breaker which 
has a high precision with respect to the setting of the actuation point 
thereof. 
In order to accomplish the further object said breaker element may be a 
plate element adapted to transmit said force via a sheering along a cross 
sectional region at a middle portion thereof in a plane extension thereof. 
In this case, said cross sectional region of said plate element may be 
defined by a pair of notches formed in opposite surfaces of said plate 
element to be substantially aligned to one another across the thickness 
thereof. 
Further, said plate element may have a sectoral configuration with said 
notches being formed in parallel with a straight edge thereof to traverse 
a middle portion of said sectoral configuration, the depth of said notches 
being changed along a length thereof so as to be smallest at a central 
portion of the length and to be largest at opposite ends of the length. In 
such a construction, the depth of said notches may be changed arcuately so 
as to define arcuately convex opposite edges of said cross sectional 
region. Or the depth of said notches may be changed straightly so as to 
define straightly convex opposite edges of said cross sectional region. 
Further, said plate element may have a sectoral configuration with said 
notches being formed in parallel with a straight edge thereof to traverse 
a middle portion of said sectoral configuration, the depth of said notches 
being substantially constant along a length thereof. 
When the mechanical safety breaker is constructed in such a construction 
that said first mechanical member has cylindrical outside surface and a 
substantially radial first key groove opening to said cylindrical outside 
surface thereof, while said second mechanical member has a cylindrical 
inside surface to slidably engage with said cylindrical outside surface of 
said first mechanical member and a substantially radial second key groove 
opening to said cylindrical inside surface thereof and adapted to align 
with said first key groove, said plate element being mounted half by half 
in said first and second key grooves at opposite half portions thereof so 
as to be subjected to a sheering in the direction of thickness thereof 
along said cross sectional region by a relative rotational movement of 
said first and second mechanical members, said pair of notches are 
arranged such that central planes of said pair of notches join 
tangentially to a phantom cylindrical curve centred at a central axis of 
said cylindrical outside surface of said first mechanical member at 
bottoms of said notches so as thereby to induce a sheering fracture of 
said plate element along the phantom cylindrical curve. 
According to another embodiment, said cross sectional region of said plate 
element may be defined by at least a pair of Knoop indents accompanied by 
corresponding sectoral cracks formed in opposite surfaces of said plate 
element to be substantially aligned to one another across the thickness 
thereof. 
In this case, said cross sectional region of said plate element may be 
defined by at least three pairs of Knoop indents accompanied by 
corresponding sectoral cracks formed in opposite surfaces of said plate 
element, each said pair of Knoop indents and the corresponding sectoral 
cracks being substantially aligned to one another across the thickness 
thereof, while said at least three pairs of Knoop indents and the 
corresponding sectoral cracks being consistent to define said cross 
sectional region. 
Alternatively, said cross sectional region of said plate element may be 
defined by a pair of linear scratches formed by a Knoop head to be 
accompanied by corresponding linear cracks formed in opposite surfaces of 
said plate element to be substantially aligned to one another across the 
thickness thereof. 
According to a further embodiment, said plate element may be an assembly of 
at least first, second and third plate members, at least said first plate 
member being made of a ceramic, said second and third plate members being 
bonded to a surface of said first plate member so as to define a slit 
therebetween to induce a fracture of said first plate member along a 
portion thereof aligned with said slit. 
Further, said ceramic breaker element may be coated with a layer of a 
resin. 
The mechanical safety breaker according to the present invention may be 
constructed such that said first mechanical member has a cylindrical 
outside surface and a substantially radial first key groove opening to 
said cylindrical outside surface thereof, while said second mechanical 
member has a cylindrical inside surface to slidably engage with said 
cylindrical outside surface of said first mechanical member and a 
substantially radial second key groove opening to said cylindrical inside 
surface thereof and adapted to align with said first key groove, said 
breaker element being a key mounted half by half in said first and second 
key grooves at opposite half portions thereof so as to transmit a torque 
between said first and second mechanical members around an axis 
corresponding to a central axis of said cylindrical outside surface of 
said first mechanical member. 
In the above-mentioned construction, said first and second key grooves may 
each be formed to define an edge at an opening end thereof which functions 
as a sheering edge against said key across a middle cross sectional region 
thereof. 
Alternatively, at least one of said first and second key grooves may be 
formed with a port space for widening an opening end thereof so as to 
provide a space in which said key is substantially subjected to a bending 
stress by a torque transmitted therethrough. 
Further, the mechanical safety breaker according to the present invention 
may be constructed such that said first mechanical member has an annular 
surface adapted to turn about a rotation axis and a first key groove 
opening to said annular surface thereof, while said second mechanical 
member has an annular surface opposing said annular surface of said first 
mechanical member with a substantial space left therebetween and a second 
key groove opening to said annular surface thereof and adapted to align 
with said first key groove, said breaker element being a key mounted half 
by half in said first and second key grooves at opposite half portions 
thereof with a middle portion thereof traversing said space so as to 
transmit a torque between said first and second mechanical members around 
said axis of rotation. 
Said first and second key grooves and said key received therein may be 
provided in duplicate around said central axis such that when a first set 
of said key grooves and key operates to substantially transmit a torque 
between said first and second mechanical members, a second set of said key 
grooves and key idles, and when said key of said first set of said key 
grooves and key has been fractured, said second set of said key grooves 
and key operates to substantially transmit a torque between said first and 
second mechanical members. 
Alternatively, a single key may be constructed to have a stepped thickness 
so as to present a relatively thicker half portion and a relatively 
thinner half portion such that when said thicker half portion operates to 
substantially transmit a torque between said first and second mechanical 
members, said thinner half portion idles, and when said thicker half 
portion has been fractured, said thinner half portion operates to 
substantially transmit a torque between said first and second mechanical 
members. 
Further, the mechanical safety breaker according to the present invention 
may be constructed such that said first and second mechanical members are 
each a rotary member adapted to rotate about a common axis of rotation as 
axially spaced from one another along said axis of rotation, and said 
breaker element is a member to connect axially opposing end portions of 
said first and second mechanical members with one another so as to 
transmit a torque therebetween by bearing a twisting load applied thereto.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the following the present invention will be described in more detail in 
respect of the preferred embodiments thereof with reference to the 
accompanying drawings. All through the figures, corresponding portions are 
designated by corresponding reference numerals with or without particular 
descriptions thereabout for the convenience of review. 
Referring to FIG. 1, a power steering system herein shown in a somewhat 
schematic fashion comprises a steering shaft 10, a steering wheel 12 and a 
rack-and-pinion steering means 16 arranged according to a well known 
conventional construction, and further comprises a torque sensor 14 for 
detecting a torque applied to the steering shaft 10 through the steering 
wheel 12, a bevel type driven gear 18 mounted on the steering shaft 10 
through the means described in detail hereinbelow, an electric motor 20, a 
bevel type drive gear 22 mounted on an output shaft of the motor 20 and 
meshing with the driven gear 18, and an electronic control means 24 
adapted to supply a controlled electric power to the motor 20 according to 
input signals including a signal indicating a manual steering torque 
detected by the torque sensor 14. The electronic control means 24 is 
generally so constructed as to operate the motor 20 in a manner of 
generating a power generally proportional to the manual steering torque 
detected by the torque sensor 14. 
The driven gear 18 mounted on the steering shaft 10 is torque 
transmittingly connected therewith by such a key construction as shown in 
FIG. 2 in an axial section along a central axis 26 of the steering shaft 
10 as well as in FIG. 3 in a cross section perpendicular to the central 
axis 26. As shown in these figures, the steering shaft 10 is formed with a 
sectoral key groove 28, while the gear 18 is formed with a linear key 
groove 30 at a hub portion 18A to align with the sectoral key groove. 
Then, there is mounted a sectoral key 32 such that a half portion thereof 
is closely received in the sectoral key groove 28 of the steering shaft 
10, while a remaining half portion thereof is closely received in the 
linear key groove 30 of the gear 18, so as to provide a torque 
transmitting connection between the steering shaft 10 and the gear 18. 
This key 32 is made of a ceramic such as silicon nitride. 
Referring to FIGS. 4 and 5, in more detail, the key 32 is formed with a 
pair of notches 34 and 36 in opposite surface portions 32A and 32B so as 
to extend in parallel with a linear edge 32C of a sectoral configuration 
thereof and to be in alignment with a clearance between an outside surface 
38 of the steering shaft 10 and an inside surface 40 of the gear 18 
defining a bore thereof through which the steering shaft 10 extends. As 
shown in FIG. 4, the width of the notches 34 and 36 at the opening end 
thereof is substantially wider than the clearance between the surfaces 38 
and 40 such that, even when some relative shifting occurs in the leftward 
or rightward direction as viewed in FIG. 4 among the steering shaft 10, 
the gear 18 and the key 32 due to shifting of the clearance or thermal 
expansion, the edges defined by the outer surface 38 and the opposite side 
surfaces of the sectoral key groove 28 as well as the inside surface 40 
and the opposite side surfaces of the linear key groove 30 still remain as 
exposed in the notches 34 and 36. 
The above-mentioned key construction provides a mechanical safety breaker 
which normally torque transmittingly connects the steering shaft 10 with 
the gear 18, i.e. the assisting steering power source provided by the 
motor 20, as long as the key 32 remains integral, but ceases the torque 
transmitting connection so that the steering shaft 10 can be rotated free 
of the gear 18, i.e. the motor 20 and its driving gear train (generally 
reduction gear train) schematically illustrated by the bevel gears 18 and 
20, when the key 32 has been fractured along the notches 34 and 36. 
A limit value of the force, or more precisely the sharing force in the 
above-mentioned constructions, applied to the key 32 according to a torque 
transmitted therethrough between the steering shaft 10 and the gear 18 
which causes the fracture of the key 32 along the cross sectional region 
defined between the notches 34 and 36 will stably remain with a very small 
reduction such as 20% for a large number of repetitive applications of 
load such as 10.sup.8 times as will be appreciated in view of FIG. 49. 
Further, since in the above-mentioned constructions the sheering edges 
which causes the fracture of the key 32 is substantially determined by the 
notches 34 and 36, without being affected by the edges of the steering 
shaft 10 and the gear 18 bordering the respective key grooves 28 and 30, a 
high precision is available in the setting of the torque limit at which 
the mechanical safety breaker is actuated. FIG. 6 shows a portion around 
the notch 34 in FIG. 4 at a larger scale, wherein the points of contact 
between the steering shaft 10 and the key 32 and between the gear 18 and 
the key 32 crucial in determining the fracture of the key 32 are shown by 
white triangles. By the opening width of the notch 34 being selected to be 
large enough to accept the opposing edges of the steering shaft 10 and the 
gear 18 therein in spite of any probable relative shifting therebetween 
according to a clearance between the key and the corresponding key groove, 
thermal expansion, etc., the above-mentioned crucial contact points always 
remain along the opposite edge portions of the notch regardless of such 
relative shiftings, more stabilizing the fracture performance of the key 
32. 
The cross sectional shape of the notch 34' (and also notch 36') may be 
modified as shown in FIG. 7 to increase the sharpness of the notch at the 
bottom portion of notch 34' of key 32' thereof relative to the wideness of 
the key groove at its open end so that a relatively large distance is 
available between the opposite points of contact. 
The depth of the notch 34" and/or the notch 36" defined by the bottoms 34B" 
and 36B" may be constant along the length thereof as shown in FIG. 8(B). 
In this case, although the larger half portion of the key 32" bordered by 
the notches 34" and 36" has a substantially rectangular plan configuration 
to contact with the gear 18, the smaller half portion of the key 32" 
bordered by the notches 34" and 36" has a genuine sectoral plan 
configuration to contact with the gear 18, with an extension thereof 
perpendicular to the notches 34" and 36" gradually decreasing from a 
central maximum extension toward zero at opposite side ends, as shown by 
hatching in FIG. 8(A), and therefore, a stress factor (defined as the rate 
of stress generated in the breaker element to the load applied thereto) 
which provides the magnitude of the sheering stress generated at each 
portion of the cross sectional region of the key 32" left between the 
opposing notches 34" and 36" along the length thereof according to a 
torque load applied between the steering shaft 10 and the gear 18 changes 
as shown in FIG. 8(C), along the length of said cross sectional region. In 
this case, therefore, when the torque load applied between the steering 
shaft 10 and the gear 18 exceeds a limit value, the fracture of the key 
32" will start at a lengthwise central portion of the sectional region 
left between the notches 34" and 36". This will provide an advantage that 
the limit torque to start the fracture of the key 32" is sharply 
determined to be the value of the torque corresponding to the crest of the 
convex curve shown in FIG. 8(C). 
FIG. 9(A)-FIG. 9(C) show a modification with respect to the depth of the 
notches 34" and 36" in the same manner as in FIGS. 8(A)-8(C). In this 
embodiment, the depth of the notches 34'" and 36'" is changed according to 
an arcuate contour such that the notches are shallowest at the lengthwise 
central portion thereof and gradually get deeper toward opposite side ends 
thereof. By the curvature of the depth contour being appropriately 
determined, the stress factor can be adjusted to be substantially constant 
along the length of the cross sectional region defined between the notches 
34'" and 36'" as shown in FIG. 9(C). This embodiment will provide an 
advantage that the fracture of the key 32'" is sharpened with respect to 
the lapse of time, or in other words, when the fracture of the key 32'" 
occurs, the fracture is completed in a short time after the start. 
FIGS. 10(A)-10(C) show still another modification with respect to the depth 
of the notches 34' and 36' in the same manner of illustration as in FIGS. 
8(A)-8(C) and 9(A)-9(C). In this embodiment, the depth of the notches 34"" 
and 36"" is changed linearly such that it is shallowest at the lengthwise 
central portion thereof and increases linearly gradually toward opposite 
side ends thereof. In this case, the stress factor shows a concave 
performance as shown in FIG. 10(C). Therefore, when the torque load 
applied between the steering shaft 10 and the gear 19 has reached a limit 
value, the fracture of the key 32"" will start at the lengthwise opposite 
ends thereof so as to gradually proceed toward the lengthwise central 
portion thereof. This embodiment will provide an advantage that the 
fracture performance is sharpened with respect to the magnitude of the 
torque load so that the fracture of the key starts at the lengthwise 
opposite. ends of the sectional region left between the notches 34"" and 
36"" at which the stress factor becomes highest and the fracture 
performance is also sharpened with respect to the lapse of the time as the 
fracture proceeds from the opposite ends toward the central portion in 
parallel. 
With respect to the cross sectional configuration of the notches 34 and 36, 
it is desirable that they are symmetrically aligned with one another as 
diagrammatically shown in FIG. 11 (which applies to all embodiments but 
for convenience depicts the first embodiment) so that the highest 
concentration of the sheering stress occurs along a phantom plane extended 
between the bottoms 34B and 36B of the notches 34 and 36 which coincides 
with a common. central plane 34A and 36A of the notches 34 and 36, thereby 
sharply defining the section of fracture. 
However, as an alternative, the notches 34 and 36 may be arranged as 
diagrammatically shown in FIG. 12 such that the central planes 34A and 36A 
of the notches 34 and 36 join tangentially to. a phantom cylindrical curve 
42 centred at the central axis 26 of the steering shaft 10 at the bottoms 
34B and 36B of the notches 34 and 36 so as thereby to induce a sheering 
fracture of the key 32 along the phantom cylindrical curve 42, and thereby 
to obtain an advantage that the sheer fractured surfaces of the opposite 
halves of the key 32 generated by the fracture thereof present a contour 
which follows more faithfully the cylindrical configuration of the annular 
space between the steering shaft 10 and the gear 18, thus presenting less 
obstacle to the free rotation of the steering shaft 10 relative to the 
gear 18 after the fracture of the key 32. 
Although the fracture performance of the mechanical safety breaker 
according to the present invention against repetitive applications of load 
to be transmitted and the aging is substantially stabilized by employing a 
ceramic breaker element such as the keys 32 in the embodiments described 
above, the ceramic key is of course not completely free from the change of 
fracture performance due to repetitive application of load and the aging, 
as will be apparent from the right-downwardly inclined performance curves 
shown in FIG. 49. A lowering of the fracture strength of the ceramic 
breaker element according to the repetitive applications of load and the 
aging corresponds to an increase of the ratio of the stress factor to the 
load transmitted by the mechanical safety breaker as shown in FIG. 13, in 
which the performance curve shown by a solid line represents the relation 
between the stress factor K and the load P transmitted by the mechanical 
safety breaker at the new starting condition of a ceramic breaker element, 
such an initial performance curve being shifted toward the performance 
curve shown by a broken line reached after a period of operation, such 
that the breaker which will be activated at a load value Ps which provides 
a fracture stress factor Kf will be actuated at a lower load Pn when it 
has been used for a certain period. 
FIGS. 14 and 15 are views similar to FIGS. 2 and 3, respectively, showing 
still another embodiment of the mechanical safety breaker according to the 
present invention. In this embodiment, however, a one more set of key 
grooves and key is provided in addition to the former set of the key 
grooves 28 and 30 and the key 32. However, it is to be noted that the key 
32 of FIGS. 14 and 15 also represents the keys 32', 32", 32'" and 33"" of 
the similar embodiments. In the shown embodiment, the new set of a 
sectoral key groove 29 formed in the steering shaft 10 and having a 
similar sectoral configuration as the key groove 28, a linear key groove 
31 formed in the gear 18' and a ceramic key 33 having a similar sectoral 
configuration as the key 32 and half by half received in the key grooves 
29 and 31, is provided as just diametrically opposed to the set of key 
grooves 28 and 30 and the key 32 with respect to the central axis 26 of 
the steering shaft 10. However, it is not essential that these two sets of 
key grooves and key are positioned as diametrically opposed to one 
another. 
FIG. 16 is a sectional view similar to FIG. 11 or 12 showing 
diagrammatically the cross sectional configuration of the key 32 in the 
embodiment shown in FIGS. 14 and 15 with the cooperating key grooves 28 
and 30 shown in phantom lines. Similarly, FIG. 17 shows diagrammatically 
the cross section of the key 33 and the cooperating key grooves 29 and 31 
in the same manner as in FIG. 16, wherein 33A and 33B designate the 
opposite side surfaces of the key 33 in the same manner as 32A and 32B 
with respect to the key 32, and 35 and 37 designate notches formed in 
opposite side surfaces of the key 33 in the same manner as the notches 34 
and 36 with respect to the key 33. 
In the above constructions of the two sets of key grooves and key, denoting 
the width of the respective key grooves, the thickness of the respective 
keys and the depth of the respective notches as shown in FIGS. 16 and 17, 
the condition to let the key 32 first operate while letting the key 33 
idle in the meanwhile, and when the key 32 has been fractured, to let the 
key 33 newly operate to provide the same fracture performance as the key 
32, will most simply be as follows, provided that, when the center of the 
key 32 is aligned with the center of the key grooves 28 and 30, the center 
of the key 33 is aligned with the center of the key grooves 29 and 31: 
W28=W28&lt;W30=W31 
T32=T32 
D34=D36=D35=D37 
Thus, when the torque transmittance between the steering shaft 10 and the 
gear 18 is intermittently repeated in either rotational direction with an 
increasing lapse of time, it is only the key 32 which is subjected to a 
lowering of the fracture strength due to fatigue, while the idling key 33 
remains almost new. 
Referring again to FIG. 13, according to the progress of operation the 
performance curve of the key 32 gradually shifts from that shown by the 
solid line to turn upward toward that shown by the broken line, and after 
a substantial lapse of operation, when the performance curve of the key 32 
was shifted to the curve shown by the broken line, if the load applied to 
the key 32 increases beyond the limit value Pn, there occurs the fracture 
of the key 32. When the key 32 has been fractured, the key 33 is now put 
into its operation to provide a new mechanical safety breaker operative 
according to the new performance curve shown by the solid line, provided 
that the key 33 is of the same material and the same dimensions of 
configuration as the key 32. Therefore, according to the embodiment shown 
in FIGS. 14-17, the lifetime of the mechanical safety breaker is doubled. 
FIGS. 18-22 show still another embodiment which is, in fact, a modification 
of the technical concept embodied by the double key construction shown in 
FIGS. 14-17. In this embodiment, the double keys 32 and 33 in the 
preceding embodiment are provided by a single stepped key 52 which has a 
thicker half portion 54 and a thinner half portion 56. These two half 
portions 54 and 56 may be made as separate ceramic elements and bonded 
together along a border surface 57 as best shown in FIG. 20, or they may 
be formed as respective parts of an integral ceramic key element with a 
bordering step being formed in one surface thereof or two bordering steps 
being formed in opposite surfaces thereof. The essential point in this 
respect is that the thinner half portion 56 is adapted to be readily 
separated from the thicker half portion 54 when the thicker half portion 
54 has been fractured as described hereinbelow. 
As is clear from FIGS. 18 and 20(A) and 20(B), the ostensibly single 
ceramic key element 52 having substantially the same sectoral overall 
configuration as the key elements 32 and 33 in the preceding embodiment is 
also mounted half by half in the sectoral key groove 48 formed in the 
steering shaft 10 and a linear key groove 50 formed in the gear 18 such 
that a pair of notches 58 and 60 formed in opposite surfaces 54A and 54B 
of the sectoral half portion 54 and a pair of notches 62 and 64 formed in 
opposite surfaces 56A and 56B of the sectoral half portion 56 are aligned 
with the clearance between the outer surface of the steering shaft 10 and 
the inner surface of the annular gear 18 in the same manner as in the 
former embodiments. 
According to the same technical concept as in the preceding embodiment 
shown in FIGS. 14-17, the thicker half portion 54 of the ceramic key 
element 52 first operates substantially to transmit a torque between the 
steering shaft 10 and the gear 18, while in the meantime the thinner half 
potion 56 idles. 
Denoting various dimensions with respect to the key and the key grooves as 
shown in FIGS. 21 and 22, when the key grooves 48 and 50 are each formed 
to have a uniform width such as W48 and W50 against the thicker half 
portion 54 and the thinner half portion 56, since the thickness T56 of the 
thinner half portion 56 is smaller than the thickness T54 of the thicker 
half portion 54, if the thinner half portion 56 should show substantially 
the same fracture performance as the thicker half portion 54 so that, as 
viewed in FIG. 13, the fracture performance of the key is renewed to 
substantially the same condition as the new starting of the mechanical 
safety breaker as shown by the solid performance curve in FIG. 13 after 
the thicker half portion 54 has been fractured according to the fatigued 
performance curve as shown by the broken line in FIG. 13, the depth D62 
and D64 of the notches 62 and 64 will have to be designed to be smaller 
than the depth D58 and D60 of the notches 58 and 60, provided that each 
pair of these notches are formed symmetrically. 
It will be appreciated that according to the embodiment shown in FIGS. 
18-22, when the ostensibly single ceramic key element 52 is so constructed 
that, when the fracture has occurred in the thicker half portion 54 along 
the notches 58 and 60, the thinner half portion 56 is separated from the 
debris of the thicker half portion 54, the same operation to duplicate the 
lifetime of the ceramic key element is available as in the preceding 
embodiment. 
In the above-mentioned embodiments, the sectoral ceramic key 32, 33 or 52 
was formed with relatively deep notches such as 34 and 36, 35 and 37 or 
58, 60, 62 and 64 to define the sheering section of the breaker element 
along which the fracture of the breaker element should occur. However, 
considering the matter that the key is applied with a sheering force along 
a simple phantom plane or a cylindrical curve such as 42 in FIG. 12 
extending through a pair of minute parallel clearances defined between the 
pair of opposing edges defining the opening of a key groove such as 28, 29 
or 48 of the steering shaft 10 and the opening of a key groove such as 30, 
31 or 50 of the gear 18, it is expected that the key would fracture 
substantially along said phantom plane or curve if the fracture is once 
initiated at least at a portion thereof positioned in said phantom plane 
or curve, without being so definitely delimited by such relatively deep 
notches as formed in the above-mentioned embodiments. If such expectation 
is really available, the substantial and difficult work of forming a 
relatively deep notch such as 34, etc. is substantially obviated by 
reducing the total thickness of the key to a dimension corresponding to 
the thickness between the bottom ends of the notches 34 and 36, etc. It is 
in fact a difficult and time-consuming process to form such a relatively 
deep notch as 34 and 36, etc. in the above-mentioned embodiments, because 
the ceramic is a very hard material to be ground away by a grinder blade, 
and since it gets difficult to keep a high precision about re-positioning 
of the grinder blade if the grinder blade is exchanged with a new one 
during forming of one notch, it is generally compelled to use a single 
grinder blade to form one notch until it is finished. In this case, as the 
edge of the grinder blade wears along with the progress of grinding 
operation, here is also bound another difficult problem that the accuracy 
of the cross sectional contour of the notch is limited by the wearing 
performance of the grinder blade. 
FIGS. 23-25 show an embodiment in which it was tried to provide a sectoral 
ceramic key 132 having substantially the same outer configuration as the 
key 32 or 33 in the former embodiments. However, the key 132 in this 
embodiment has no such notches as 34 and 36 in the former embodiments. 
Instead, the key 132 is formed with a pair of dot like indents 134 and 136 
each accompanied by a sectoral crack 138 or 140 induced at the time of 
forming the corresponding indent by a Knoop head as explained hereinbelow. 
When a punching head 146 is pressed against a ceramic key 132 as shown in 
FIG. 26, if the punching head 146 is a Vickers head having a tip of a 
regular square pyramid, two cracks crossing one another along the axis 147 
of the punching head are induced by the indent, while if the punching head 
146 is a Knoop head having a tip of a diamond pyramid (generally having 
angles of 130.degree. vs. 172.degree.30'), a single crack 138 is induced 
to align with the longer axis of the diamond to accompany an indent 134 
reflecting the shape of the diamond pyramid of the Knoop head left as 
permanently deformed. 
It was experimented if the sectoral ceramic key 132 formed with the Knoop 
indent 134 and the sectoral crack 138 as shown in FIGS. 23-25 such that 
the longer axis of the diamond pyramid is aligned with a straight line 
corresponding to the notch 34 or 36 in the former embodiments, with 
formation of a corresponding similar Knoop indent 136 accompanied by a 
corresponding sectoral crack 140 formed in the opposite surface 132B, is 
fractured along the line corresponding to the notch 34 or 36 when it is 
mounted between the steering shaft 10 and the gear 18 in the same manner 
as shown in the former embodiments with one half being received in the key 
groove 28 while the other half being received in the key groove 30. As a 
result, it was confirmed that the fracture is always initiated at either 
the Knoop indent 134 or 136 according to the direction of the sheering 
force applied thereto such that the fracture propagates approximately 
along the line corresponding to the notch 34 or 36. It was also confirmed 
that the fracture strength can be controlled within a fluctuation of about 
3% by the control of the depth of the Knoop indent. Thus, it was confirmed 
that the key construction shown in FIGS. 23-25 is also an operative 
embodiment of the mechanical safety breaker according to the present 
invention. 
The same experiment was continued by providing three pairs of Knoop indents 
134 and 136 with the accompanying sectoral cracks 138 and 140 along a line 
142 along which the fracture of the key should occur, as shown in FIGS. 
27-29. As a result, it was confirmed that the fracture of the key occurs 
more arcuately along the line 142. It was also confirmed that the fracture 
strength can also be controlled within a fluctuation of about 3% by the 
control of the depth of the Knoop indents. Therefore, the construction of 
a key 132' shown in FIGS. 27-29 is also still another operative embodiment 
of the mechanical safety breaker according to the present invention. 
Further, it is presumed based upon the constructions shown in FIGS. 23-25 
and 27-29 that other numbers of Knoop indents also effectively operative, 
provided that they are positioned to be in consistent with one another so 
as to define a desired fracture section. 
Instead of pressing the Knoop head 146 perpendicularly into the surface of 
the ceramic key as shown in FIG. 26, the Knoop head 146 was shifted along 
the surface of the ceramic key as pressed therein at a lesser depth, 
starting from an edge portion of the surface as shown in FIG. 30, A to 
proceed as shown in FIG. 30(B) and FIG. 30(C), so that a linear scratch 
135 less deep than the above-mentioned Knoop indent 134 is formed with an 
accompanying linear crack 137. FIGS. 31-33 are views similar to FIGS. 
23-25 or 27-29, showing a ceramic key 132" formed with a pair of linear 
scratches 135 and 137 with the corresponding linear cracks 139 and 141 
along the line 142. As a result of the same performance experiments it was 
confirmed that these linear scratches and cracks are effective to 
precisely define the fracture section of the ceramic key. Therefore, the 
construction shown in FIGS. 31-33 is also an effectively embodiment of the 
mechanical safety breaker according to the present invention. 
With respect to those constructions shown in FIGS. 23-25, 27-29 and 31-33 
wherein the fracture section of the ceramic key is defined by a Knoop 
notch accompanied by a sectoral crack formed therearound or a linear 
scratch formed by a Knoop head and accompanied by a linear crack, there 
would be an apprehension that the fracture strength of the key decreases 
relatively rapidly due to a growth of the sectoral crack or the linear 
crack under the application of loading. The keys of these constructions 
were tested with respect to change of fracture strength according to the 
period of application of load or aging. The results are shown in FIG. 34, 
wherein the broken line shows the performance of the keys formed with the 
Knoop notches accompanied by the sectoral cracks as shown in FIGS. 23-25 
or 27-29, while the solid line shows the performance of the key shown in 
FIGS. 31-33 formed with the linear scratch by the Knoop head accompanied 
by the linear crack. From these test results it will be appreciated that 
in both constructions the reduction of fracture strength along with 
loading period is within a practically acceptable range. Herein it will 
also be appreciated that the reduction rate of the fracture strength along 
with loading period is slightly lower in the construction of the linear 
Knoop scratch with the linear crack than in the construction of the Knoop 
notch with the sectoral crack. 
In consideration that the reduction of fracture strength along with the 
lapse of loading time will be at least partly caused by chemical reactions 
of the ceramic crystals with the moisture or halogen elements contained in 
the atmospheric air, particularly at the portion thereof placed under a 
stress concentration generated around the Knoop indent, Knoop scrathe and 
cracks, the ceramic key element having the construction of FIGS. 23-25 was 
encased. as shown in FIGS. 35-37, wherein 148 shows a resin layer formed 
to extend over the entire outside surface of the key 132"" and also into 
the Knoop indents 134"" and 136"" as well as into the sectoral cracks 
138'" and 140'". Such a resin coating was provided in a manner shown in 
FIG. 38, wherein a liquidized resin 154 was contained in a vessel 156, and 
the key 132'" was supported on a base 158 placed on the bottom of the 
vessel as completely immersed in the liquidized resin. In this condition, 
the Knoop head 146 was pressed into the upper surface of the key 132'". 
FIG. 39 shows a result of experiments carried out to confirm the effect of 
the resin coating provided around the key element as shown in FIGS. 35-37 
according to the process shown in FIG. 38. From the result shown in FIG. 
39 it will be appreciated that the resin coating contributes to improving 
the mechanical safety breaker according to the present invention in 
suppressing the reduction of fracture strength at least due to aging. 
In view of the matter that the reduction of fracture strength of the 
ceramic key due to chemical reactions of the ceramic crystals with the 
moisture or halogen elements contained in the atmospheric air will be more 
substantial at portions where the ceramic crystals are subjected to the 
stress concentration generated around the Knoop indents, the Knoop 
scratches and the cracks, the resin coating construction may be modified 
as shown in FIGS. 40 and 41, wherein the resin coating is provided only 
around the Knoop indents and the cracks. In this embodiment a pair of 
shallow grooves 150 and 152 of a rectangular cross section are formed in 
the opposite surfaces 132A"" and 132B"" of the ceramic key element 132"" 
before the knoop indents 134 and 136"" are formed together with the 
corresponding sectoral cracks 138"" and 140"", such that, when the resin 
layers have been formed around the Knoop indents 134"" and 136"" to fill 
the concave portions formed therearound, the resin layers are formed to 
have reinforcing rib portions 148' serving to stably support the resin 
layers covering the Knoop indents 134"" and 136"" and the sectoral cracks 
138"" and 140"". 
It will be apparent for those skilled in the art that the art of covering 
the key element as a whole or particularly the Knoop indents and the 
sectorally cracked portions by a resin coating may of course be applied to 
the key element formed with the linear scrathes formed by the Knoop head 
and the accompanying linear cracks to obtain the same improvement. 
Further, it will also be apparent that this art may also be applied to the 
key elements formed with the notches 34, 36, etc. for the same purpose. 
In order to induce the fracture of the ceramic key element along the border 
between the outside surface of the steering shaft and the inside surface 
of the annular gear as in the various embodiments described above, still 
another embodiment is possible as shown in FIGS. 42 and 43. In this 
embodiment, a sectoral ceramic key construction generally shown by 232 and 
having substantially the same outer configuration as the keys 32 in the 
former embodiments is assembled of a sectoral ceramic plate element 260 of 
the same sectoral contour as the keys 32 and two pairs of plate elements 
262, 264 and 266, 268 attached to opposite surfaces of the plate element 
260 to provide the same sectoral outside contour as the plate element 260 
thereby defining linear borders 270 and 272 at opposite sides thereof 
along a line 242 corresponding to the linear notches 34 and 36 of the key 
32 in the former embodiments. The plate elements 262, 264, 266 and 268 may 
desirably be also made of a ceramic material and each bonded to the plate 
element 260 by a bonding material having the same composition as the 
sintering material of the ceramic material constructing the plate element 
260, the bonding being carried out by a hot static pressing process or the 
like. It was also confirmed that the key element of this type, when used 
in place of the key element 32 in the former embodiments with the border 
lines 270 and 272 being aligned with the clearance between the outside 
surface 38 of the steering shaft 10 and the inside surface 40 of the gear 
18 so that the half portion sandwiched by the plate elements 262 and 264 
is received in the groove 28 while the half portion sandwiched by the 
plate elements 266 and 268 is received in the groove 30, is fractured at a 
section of the plate element 260 corresponding to the border lines 270 and 
272 when the torque load applied between the steering shaft 10 and the 
gear 18 increases beyond a determinate limit value. 
The mechanical safety breaker employing a ceramic breaker according to the 
present invention may be incorporated in the power steering system having 
the construction schematically shown in FIG. 1, according to a modified 
construction slightly different from the above-mentioned embodiments, as 
shown in FIGS. 44-46. In this embodiment, a ceramic key is incorporated 
within a gear itself corresponding to the gear 18. As is shown in these 
figures, such a gear 318 is constructed to include an inner annular member 
320 having a cylindrical outside surface 322 and a central bore 324 
adapted to receive the steering shaft 10 therethrough so as to be 
torque-transmittingly mounted thereon, and an outer annular member 326 
having a cylindrical inside surface 328 slidably engaged around the 
cylindrical outside surface 322 and outside gear teeth 330. The inner 
annular member 320 is formed with a radial key groove 332 and a port space 
334 extending around the key groove 332 at an opening region thereof so as 
to widen the opening end portion of the key groove 332. On the other hand, 
the outer annular member 326 is formed with a radial key groove 336 
adapted to radially oppose the key groove 332 of the inner annular member 
320, the key groove 336 being also axially extended to open at one axial 
end of the outer annular member 326 for the convenience of assemblying the 
device. A ceramic key 338 having a shape of a square bar is mounted half 
by half in the key groove 332 of the inner annular member and the key 
groove 336 of the outer annular member so as to traverse the port space 
334. 
In this embodiment, when a torque load is applied to the gear 318', the key 
338 is applied with a corresponding load which is principally a bending 
load concentrated at a portion thereof extending through the port space 
334. The ceramic material also shows a fracture strength performance 
against bending which is substantially less lowered according to 
repetitive applications of the load than metallic material just as in the 
fracture strength against sheering. Therefore, by incorporating the 
ceramic key in a manner of being subjected to the bending load as in this 
embodiment, there is also obtained a mechanical safety breaker improved in 
the stability performance for a long period of repetitively loaded 
operation. 
It will be apparent for those skilled in the art that a port space similar 
to the port space 334 may be provided at an opening end portion of the key 
groove 336 of the outer annular member 326 instead of or in addition to 
the port space 334, in order for the key 338 to operate as a bending 
fracture element. 
FIG. 47 is a view similar to FIG. 46, showing another embodiment in which a 
ceramic key is also used in the substantially bending mode. In this 
embodiment an inner annular member 320 and an outer annular member 326 
corresponding to those shown in FIG. 46 are formed with axially 
overlapping annular portions 320A and 326A, respectively. These 
overlapping annular portions are substantially spaced from one another in 
the axial direction by an annular projection 320B to leave a substantially 
annular space 340 therebetween. The axially overlapping annular portions 
320A and 326A are respectively formed with axial key grooves 342 and 344 
adapted to oppose one another. The key grooves 342 and 344 are each 
cylindrical bores in this embodiment and a ceramic key 346 having a 
cylindrical bar configuration is mounted half by half in the key grooves 
342 and 344 while traversing the space 340 at a middle portion thereof. 
Also in this embodiment, when a torque load is applied to the gear 318, 
the ceramic key 346 is subjected to a substantially bending stress 
concentration at the middle portion extending through the space 340, and 
when the torque load exceeds a determinate value, the ceramic key 346 is 
fractured at the portion extending through the space 340. 
It will be apparent for those skilled in the art that the bending key 
constructions shown in FIGS. 45-47 may be modified to provide a double key 
construction similar to that shown in FIGS. 14-17 or a stepped key 
construction similar to that shown in FIGS. 18-22 such that a 
substantially doubled lifetime of the mechanical safety breaker is 
available. 
The mechanical safety breaker employing the ceramic breaker element 
according to the present invention may further be constructed to load the 
ceramic breaker element in a manner of twisting. One such embodiment is 
shown in FIG. 48 in a construction incorporated in the output shaft of the 
motor 20 of the power steering system shown in FIG. 1. According to this 
embodiment, the output shaft of the motor 20 is constructed to include a 
first shaft portion 348 directly connected with a rotor of the motor not 
shown in the figure, a second shaft portion 350 directly connected with 
the bevel gear 22 as coaxially aligned with the first shaft portion 348, 
and a ceramic key 352 torquetransmittingly connecting axially opposed end 
portions of the first and second shaft portions 348 and 350. The key 352 
has a configuration of a rectangular plate element, and is received at 
opposite end portions thereof in corresponding key grooves 354 and 356 
formed at the opposing end portions of the first and second shaft portions 
348 and 350, respectively, so as to transmit a torque therethrough between 
the first and second shaft portions. The shaft portions 348 and 350 are 
rotationally supported by bearings 358 and 360, respectively. 
When a torque load is applied between the first and second shaft portions 
348 and 350 in either direction, the ceramic key 352 is twisted about a 
common central axis through the first and second shaft portions 348 and 
350 and the gear 22, and when the torque load exceeds a determinate limit 
value, the ceramic key 352 is fractured by twisting. It was also confirmed 
that the ceramic key 352 in this construction shows a well stabilized 
fracture performance against repetitive applications of load as in the 
former embodiments in which the ceramic keys are principally subjected to 
sheering or bending. 
Although the present invention has been described in detail in the above in 
the form of some preferred embodiments thereof, it will be apparent for 
those skilled in the art that various modifications of the shown 
embodiments and other embodiments are possible within the technical scope 
of the present invention.