Electromagnetic sliding shoe brake

An electromagnetic sliding shoe brake also referred to as a slipper shoe ke is constructed especially for use in or on rail vehicles, especially high speed rail vehicles. The magnetic core, with its energizing coil, has at least two pole pieces. The sliding brake shoe comprises ferromagnetic pole plates alternating with interconnecting magnetically insulating members. The pole plates, or rather the brake shoe, is connected to the magnetic core substantially in a frictionless manner. The brake shoe is connected to the magnetic core by parallelogram type pivoting elements and biased against the chassis by one or two springs (7) which will assure that in operation the brake shoe is pressed against a brake force take-up member at least with a predetermined brake force which will be substantially constant and independent of the friction coefficient between the brake shoe and the brake force take-up member such as a rail.

BACKGROUND OF THE INVENTION 
The present invention relates to an electromagnetic sliding shoe brake 
comprising a brake shoe which is pulled against a ferromagnetic take-up 
member for applying a predetermined brake force to the take-up member. 
Such ferromagnetic take-up member may, for example, be a rail when this 
type of brake is installed in or on a rail vehicle. 
Such a brake is, for example, necessary where it is not possible to apply a 
sufficient brake force to a rail vehicle solely by means of the brake 
force applied to the wheels to thereby achieve a sufficient deceleration 
for example in an emergency. In such situations it is conventional to 
generate additional braking forces by means of a sliding shoe brake also 
referred to as a slipper shoe brake. Such additional brake forces are 
produced by friction between the brake or slipper shoe and the top surface 
of the rail caused by corresponding electromagnetic attraction forces 
between the sliding shoe and the rail. In such a use the electromagnetic 
slide shoe brake actually functions as an emergency brake which must 
satisfy several requirements. On the one hand an emergency brake must 
assure the application of a certain minimum brake force. Normally, such 
minimum brake force is larger than the maximum brake force applied to the 
rail vehicle by the service brake such as an electrical service brake of 
the rail vehicle. On the other hand, it is required that the maximum brake 
force of the emergency brake or of the sliding shoe brake is only 
insignificantly larger than its minimum brake force. However, this minimum 
brake force must be determined in accordance with a minimum friction 
coefficient between the sliding shoe and the rail in order to achieve the 
minimum brake force which is required for obtaining the necessary 
deceleration of the rail vehicle for any friction coefficient. As a 
result, the normally present friction coefficients are at least four times 
larger than the minimal friction coefficient, whereby correspondingly 
larger brake forces are caused. These larger brake forces result in a 
respectively larger, undesired vehicle deceleration which substantially 
reduce the passenger safety. Additionally, these undesirably large 
decelerations result in an unnecessarily large loading of the rail track 
body or of the roadway or roadbed. 
Heretofore, such loading of the rail track or roadbed required meeting, 
among other conditions, the condition that the road construction is 
over-dimensioned. The over-dimensioning of the track was especially 
required for elevated tracks which are capable of taking up longitudinal 
loads only to a limited extent. Such over-dimensioning was necessary 
heretofore especially for tracks constructed for high speed vehicles, for 
example, magnetic levitation vehicles. 
In view of the foregoing it is clear that a sliding shoe brake which is 
designed to meet the first requirement, namely to provide a certain 
minimum braking force, will not satisfy the second requirement that the 
maximum braking force resulting from normal frictional coefficients will 
not be substantially larger than the minimum braking force so that vehicle 
decelerations will always correspond to decelerations resulting from the 
application of the minimum braking force by the sliding shoe brake. 
OBJECTS OF THE INVENTION 
In view of the above it is the aim of the invention to achieve the 
following objects singly or in combination: 
to construct a sliding shoe brake of the type described above which is 
capable to compensate for the variations of the friction coefficient 
between the sliding shoe and the cooperating brake surface such as the top 
of a rail so that the effect of the braking action on the respective 
vehicle will substantially be constant at all times and under all 
operating conditions; 
to construct a sliding shoe brake in such a manner that it will 
simultaneously satisfy the above stated conditions, namely, that the 
minimum brake force and the maximum brake force are substantially the 
same; 
to limit the maximum brake force that may be applied by the sliding shoe 
brake; 
to construct a sliding shoe brake in such a manner that the electromagnetic 
attraction forces are automatically adjusted in response to friction 
coefficient variations between the sliding shoe and the surface to which 
the brake force is applied; 
to make sure that the reaction time of the sliding shoe brake becomes the 
smaller the larger the relative speed is between the sliding shoe and the 
surface to which the brake force is applied, whereby the present brake is 
particularly suitable for use in high speed rail vehicles, particularly 
magnetic levitation vehicles; and 
to provide a sliding shoe brake which is passively adjusted in its brake 
pressure so that the brake becomes absolutely failsafe. 
SUMMARY OF THE INVENTION 
According to the invention there is provided an electromagnetic sliding 
shoe brake having a magnetic energizing coil which may be energized for 
pulling the sliding shoe against a ferromagnetic braking surface 
constituted by a member for taking up the braking force such as the 
surface of a rail forming the track. The magnetic coil magnetizes a 
magnetic core having at least two pole pieces of alternating or rather 
opposite polarity. Each pole piece has a respective pole face. The sliding 
brake shoe is operatively held between the pole faces and the surface to 
which the brake force is to be applied. The brake shoe is constructed to 
comprise ferromagnetic pole plates alternating with magnetically 
insulating spacer members. The pole plates of the sliding brake shoe 
contact the magnetic core or rather the pole faces of the pole pieces of 
the magnetic core in a manner substantially free of friction. The sliding 
brake shoe is held in a rated position relative to the magnetic core under 
the effect of a biasing spring in response to the friction between the 
sliding brake shoe and the surface to which the brake force is applied, 
such as the top of a rail. The effect of the biasing spring is adjustable 
in accordance with a predetermined brake force. Stated differently, the 
sliding brake shoe is held in a rated position relative to the magnetic 
core by means of a spring only until a minimum brake force is achieved, 
preferably the above mentioned minimum brake force which is determined by 
the respective spring biasing. In this rated position of the sliding brake 
shoe relative to the magnetic core the electromagnetic field, which is 
closed through the member taking up the braking force, is substantially 
undistorted. To keep the magnetic field substantially undistorted is 
particularly suitable for determining or calculating the minimum brake 
force which is determined by the minimum friction value between the 
sliding brake shoe and the surface taking up the braking force. When the 
minimum friction coefficient is exceeded, the correspondingly larger 
longitudinal force applied by the brake shoe to the spring will exceed the 
spring biasing so that the sliding brake shoe is moved out of the above 
mentioned rated position relative to the magnetic core or rather its pole 
faces in response to the friction at the surface taking up the brake 
force. As a result, the magnetic field is distorted, whereby the 
electromagnetic stray flux is increased between the magnetic energizing 
coil and the surface taking up the brake force depending on the extent of 
the displacement of the sliding brake shoe which in turn depends on the 
selected spring stiffness. On the other hand, the displacement of the 
sliding brake shoe results in a corresponding reduction of the 
cross-sectional areas between the pole faces of the magnetic core and the 
pole plates of the sliding brake shoe through which the effective 
electromagnetic flux flows. As a result, the effective air gap between the 
magnetic coil and the surface taking up the braking force is increased. 
This air gap increase has the advantage that it occurs independently of 
the position of the magnetic core relative to the direction of the 
relative movement between the magnetic core and the surface taking up the 
braking force. 
Both effects, namely the distortion of the magnetic field and the increase 
of the effective air gap result in a reduction of the electromagnetic 
attraction forces between the sliding brake shoe and the surface taking up 
the braking force, whereby the latter is limited accordingly. Thus, in a 
brake according to the invention the electromagnetic attraction forces are 
automatically adjusted in response to the friction values or in response 
to the friction value change between the sliding brake shoe and the 
surface or member which takes up the braking force. Therefore, according 
to the invention it is possible to limit the braking forces in accordance 
with a predetermined value for the minimum braking force. Another 
advantage of the invention is seen in that the reaction time of the 
sliding shoe brake becomes the smaller, the larger the relative speed 
between the sliding brake shoe and the surface taking up the brake force 
is. Thus, the present brake is particularly suitable for use in connection 
with high speed rail bound vehicles, particularly magnetically levitated 
vehicles. 
The foregoing adjustment of the brake force may be considered to be a 
passive adjustment of the sliding brake shoe against the surface taking up 
the brake force. Such passive adjustment has the advantage, as compared to 
a brake with an active control of the brake force causing pressures, that 
it is absolutely failsafe.

DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BEST MODE 
OF THE INVENTION 
FIG. 1 shows an emergency brake 1 secured to the chassis 2 of a rail bound 
vehicle. The brake 1 is an electromagnetic sliding shoe brake secured to 
the chassis 2 by conventional support means 3 which suspend the brake 1 
from the chassis 2 above the rails 4 of a track. The support means 3 
comprise compression springs 3.1 guided in respective cylinders which in 
turn are pivoted to the chassis 2 at the upper end thereof and to the 
magnetic core 1.3 at the lower end thereof. When the brake is not 
energized, the compression springs 3.1 suspend the entire brake structure 
vertically. The magnetic core 1.3 with its energizing coil 1.1 is held 
against displacement in the horizontal direction by a rod 5 also pivoted 
to the chassis 2 and to the magnetic core 1.3. However, the brake shoe 1.2 
is displaceable substantially horizontally. The vehicle travels in the 
direction indicated by the arrow F. The slide brake shoe 1.2 is suspended 
by parallel bars 6, the upper ends of which are pivoted to the upper end 
of the magnetic core 1.3. The lower ends of the parallel bars 6 are 
pivoted to the brake shoe 1.2. Thus, the horizontal portion of the 
magnetic core 1.3, the parallel bars 6, and the brake shoe 1.2 form a 
parallelogram of which the brake shoe portion is movable to the extent 
permitted by the parallel bars 6 and the spring 7 as will be described in 
more detail below. 
In order to operate the sliding shoe brake 1 its magnetic coil 1.1 is 
energized by a d.c. source not shown. As a result, and as shown in FIG. 1, 
the brake shoe 1.2 is pulled against the rail 4 acting as a brake force 
take up member. Thus, the brake shoe 1.2 slides with friction on the top 
surface 4.1 of the rail 4 acting as a brake surface. As a result, the 
brake force or the resulting brake force of the sliding shoe brake 1 is 
automatically adjusted depending on the variation of the coefficient of 
friction between the sliding shoe 1.2 and the top surface 4.1 of the rail 
4. For this purpose the magnetic core 1.3 which carries the magnetic core 
1.1 on its upper leg, comprises at least two magnetic pole pieces 1.4 
having opposite polarities. The pole pieces 1.4 have downwardly facing 
pole faces 1.5, the surface area of which is enlarged by the lateral feet 
forming these pole faces 1.5. Additionally, the sliding shoe 1.2 is made 
of ferromagnetic pole plates 1.6 alternating with magnetically insulating, 
nonmagnetic spacer members 1.7, for example, made of aluminum. In the 
example embodiment two pole plates 1.6 of ferromagnetic material are 
spaced from each other by a central spacer member 1.7 of nonmagnetic 
material and the ends of the shoe are also formed by nonmagnetic members 
1.7. Due to the arrangement of the above described parallelogram, the pole 
plates 1.6 are movable relative to the pole faces 1.5 substantially 
without any friction. 
Further, due to this suspension of the sliding shoe 1.2 it is assured that 
the shiftability or displacement of the sliding shoe 1.2 depends on the 
friction of the sliding shoe 1.2 on the surface 4.1 of the rail 4. This 
friction is limited only by the further connection of the sliding shoe 1.2 
with the support means such as the chassis 2 through the spring means 7. 
The spring means 7 are providing a biasing force which is determined in 
accordance with a desired minimal brake force of the sliding shoe brake 1 
in response to a minimum friction value between the sliding shoe 1.2 and 
the top surface 4.1 of the rail 4. Thus, only when this minimum friction 
value is exceeded so that the longitudinal force of the sliding shoe 1.2 
effective on the spring 7 overcomes the biasing of the spring, will the 
sliding shoe 1.2 be displaced relative to the magnetic core 1.3 or rather 
relative to the pole faces 1.5 as shown in FIG. 2. 
Depending on the extent of the displacement of the brake shoe, which extent 
is determined by the respective value of the friction coefficient, the 
electromagnetic attraction forces will be reduced between the sliding shoe 
1.2 or rather the pole plates 1.6 thereof and the rail surface 4.1. As a 
result, the longitudinal force of the sliding shoe 1.2 effective on the 
spring 7 or rather the available or effective brake force of the sliding 
shoe brake 1 is limited to the above mentioned minimum brake force. 
In order to assure this just described effect or function of the sliding 
shoe brake according to the invention, it is necessary to provide an air 
gap 8 shown in FIG. 1 so that the magnetic attraction forces between the 
brake shoe 1.2 and the rail surface 4.1 are automatically reduced as the 
coefficient of friction increases. Thus, it is automatically accomplished 
that the effective or rather the sufficient brake force or vehicle 
deceleration is not substantially exceeded. This air gap 8, which is 
present in the rated position of the sliding brake shoe 1.2, also provides 
the advantage that there is substantially no friction present between the 
pole plates 1.6 of the brake shoe 1.2 and the pole faces 1.5 of the pole 
pieces 1.4. 
FIG. 3 shows that the air gap 8 may be kept as small as possible in the 
rated position of the brake shoe 1.2 if the pole plates 1.6 of the brake 
shoe 1.2 extend laterally upwardly along the longitudinal sides of the 
pole pieces 1.4 of the magnetic core 1.3. The entire brake shoe may be 
formed to extend laterally upwardly along the pole pieces 1.4. 
If the present brake is to be used in vehicles constructed for travelling 
in one or the other direction without turning, a further biasing spring 
corresponding to spring 7 will be arranged exactly as spring 7, but in a 
mirror-symmetrical position thereto at the opposite end of the brake shoe 
1.2. Thus, one spring 7 will be provided for each travelling direction and 
one spring will be effective at a time depending on the travelling 
direction. 
The above example embodiment relates to the construction of the present 
brake for use primarily on a rail vehicle. However, it will be appreciated 
that the present brake is not limited to such a use, for example, the 
present brake may also be constructed as a disk brake, whereby the brake 
components would be stationary relative to a moving component which 
cooperates with the stationary brake shoe for closing the magnetic 
circuit. 
Although the invention has been described with reference to specific 
example embodiments, it will be appreciated, that it is intended to cover 
all modifications and equivalents within the scope of the appended claims.