Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
MAGNET IC BRAKE S HOE
FIELD OF THE_INVENTION
The present invention relates to an eLectromagnetic
brake shoe, especially for a braking system for railway ve-
hicles by employing the steel running rail to brake thevehicle.
Norma'.ly, the type of brake shoe is composed of a series
of elementary friction shoes constituted by a ferromagnetic
core having a pair of pole faces~ namely, north and south
poles, which are separated by a vertical clearance. During
braking, the ferromagnetic core is magnetically excited by at
least one electromagnetic coil which carries a D.C. electric
current for causLng a ma~netic attraction between the polar
aces and the running surface o the rail which closes the
lS maCJnetic circuit.
The attractive force between the polar faces and the
~t:eel rail is relatively independent rom the rallway ve-
hicles wheel's adherence on the running rail, with the excep-
tion of the initiaL application effect force of the magnetic
~0 brake shoe on the rail which is generally greater than twice
the action of gravity on the magnetic shoe, and ltS suspension
and return system from an idle position remote from the rail.
The attractive force between the shce and the rail may
reach very high values which could be more than 20 times
the gravational force on the shoe and its suspension and lift-
in~ system which may be in the vicinity of the application
force upon the rail by the wheels of a railway truck frame.
~&~
For example, it's possible to show that the two magnetic shoes
of the railway truck weigh l/2 ton wlth their movable appli-
cation and lifting equipment, but they can be applied on the
rail by electromagnetic attraction with a maximum attractive
force of approximately 14,000 decanewtons (daN). The re-
tarding forces obtained with such shoes during braking tests
do not usually exceed 1,000 to 1,200 daN values which may be
interpreted as a low friction coefficient between the friction
surface of the shoes and the rail as well as a decrease in
the induction going through the surface of the rail as the
shoe travels on the rall. In spite of these apparently mo-
dest braking performances, the magnetic brake shoes are in-
creasingly used on rapid railway vehicles which are destined
for passenger transportation. The use of magnetic brakes, for
~x~mpLe, requires almost no adhesion between the wheels and
th~ rail, and the stopping distance will not be decreased
~v~n itl the worst adhesive conditions. On the other hand,
a magnetic brake might be considered as a safety device since
the shoes may be automatically applied in the event of emer-
gency braking by being supplied with D.C. current from thevehicle's batteries. The use of the magnetic brake shoes on
the light railway vehicles for passenger transportation, such
as streetcars, has always been preferred since it is possible
to feed a great amount of electric power to the exciting coil
~5 by directly or almost directly deriving 600V D.C. current
fxom an overhead contact wire, which is sufficient power to
attain a 3m/sec deceleration rate for emergency braking of
trucks.
- 2 -
~s~
It will be appreciated ~hat numerous improvements have
been proposed and/or applied in ordex to attempt to attain
an improved braking performance of the magnetic shoes. The
proposed increase in the electric excitation has been con-
sidered, but has n ot been effective since the magnetic cir-
cuit is almost totally saturated. Further, the suggested
increase of dimensions, namely, the width and the length,
which contact the limi~ed width of the head of the rail and
also the limited available space between both wheels of the
truck, would make it impossible to transmit to the vehicle
the required friction forces applied by the rail to the shoe.
Another previous improvement consisted of adding, between
the magnetlc poles of the shoe, specifically in the clear-
ance between the two poles, some frictional lining posses-
L~ ~ing a high coefficient of friction. Such a lining has beenEound to be practical]y ineffective, either because the appli
cation force on the rail is low, or because the permeability
between the magnetic shoe poles and the rail is seriously
affected so that the gain on the friction coefficient is al-
~O mo-qt cancelled by the decrease of the magnetic application
force of the shoe on the rail.
OBJECTS AND SUMMARY OF THE INVENTION
One of the objects of this invention is to provide an
improved magnetic brake shoe which is compatible with the
actual weight, space, electric excitation power requirements,
and in keeping with the price of the railway vehicle's truck
meant for passenger transportation and, in turn, to provide
a magnetic brake shoe which has effective braking performance
on the track which is considerably improved.
Effectively, the electromagnetic shoe is utilized for
the railway vehicle braking by employing the steel running
rail. The brake includes at leas~ one elementary friction
shoe formed by a ferromagnetic core having north and south
polar faces which are separa-ted by a vertical clearance or
air gap. The polar faces of the ferromagnetic core are cap-
able of engaging the running surface of the rail. The corecooperates with at least one excitation coil which is cap-
able of causing the magnetic attraction of the polar faces
to the running surface of the rail which closes the magnetic
circuit~ This is characterized by the fact that the ferro-
lrj magnetic core and its poLar faces form a closed magnetic fluxpa~h with the rails which are made of a high rnagnetic per-
meabLe material, such as soft steel. A plate made of a good
magnetic permeability material, with a high friction coeffic-
ient and good mechanical resistance, such as, a conglomerated
and sintered powder with a smooth iron base or of ferrites,
is added and rigidly fastened on each polar face. Thus, the
plate becomes interposed between the rail surface and the
polar face which i9 made of high magnetic permeable material
when the shoe ls applied to the running rail. The friction
~5 material, the most generally used until now, to realize the
polar faces, was a friction cast iron with good magnetic
permeability but which did not have as high inductive
characteristics as soft steel. In the invention, it is pos-
sible to realize improved magnetic circuits comparable to
soft steel with a high permeability and also to benefit from
the high friction coefficient between the pole surfaces and
the rail. There are sintered materials in existence which
possess a high permeability and have a coefficient of friction
between 0.3 and 0.5. The interposition of plates of such a
material between the polar faces and the rail should enable
the tripling of braking force attainable with the magn~tic
shoes without increasing their weight, space raquirement
and electric excitation.
~ ccording to another characteristic of the invention,
the material with good magnetic permeability and high fric-
tLon coeEfi.cient also possesses remanent magnetism proper-
1~
t.i~s which is capable of intensifying the attraction force
a~ t~d on the running rail without adversely affecting the
re~u.rn of t.he magnetic shoe to idle position after cutting
o~ the current to the excitating coil. A shoe having a
slight amount of remanent magnetism may be applied to the
~0
rail without electric excitation of the coil during the ser-
vice bra~ing of the vehicle in order to clean either a wet
rail or one polluted by greasy substances. The application
of such a shoe upon the rail may be controlled by the opera-
tion of the brake pressure of the vehicle which causes the
25operation of one or several application pistons. Thus, the
frictional materiaL of high coefficient of friction on -the
poles causes an appreciable braking effect and an efficient
cleaning of the rail surface results. When desired, the
shoe may be returned to an idle positlon away from the rail
by providing a return spring greater than that of returning
force which is normally equal to twice the weight of the
shoe and the movable portion of lts suspension and return
system.
In another form, the material with good magnetic per-
meability and high coefficient of friction may possess frag-
mentation quality to abrasion due to a rubbing effect such
as, that of cast iron in railroad brake shoes which permits
the removal of a large portion of the frictional heat which
is developed during the engagement of the shoe with the rail~
According to another important characteristic of the
1~ Lnvention, the material with good magnetic permeability and
high coeEficient of friction also has a great abrasive re-
~is~ance and is laid in thin layers on the polar faces by a
m~ans of metallization spattering.
Another accomplishment of this invention consists of
moLding the material with good magnetic permeability and
high coefficient of friction into a massive block of friction
shoe for attachment to each of the polar faces. Each of the
shoe blocks is removably attached to the polar faces in such
a-manner for insuring the magnetic circuit continuity through
the ferromagnetic core. In practice, the two friction shoe
blocks which correspond to the two poles, namely~ north and
south poles, of an elementary friction shoe may be assembled
-- 6 --
as a unique monoblock, on one hand constituted by two shoe
blocks separated by a continuous band of nonmagnetic mate-
rial. The band may comprise a heat sensitive Material,
such as, fusible wax, or the like. The wax block is a
funnel-shaped section having a widened portion facing toward
the excitation coil in order to create a minimum width clear-
ance at the bottom end of both poles. The unitary monoblock
is located between the two poles and the top side of the
rail.
According to a particularly compact and innovating
arrangement, the continuous band of nonmagnetic material is
solidified by fritting to both friction shoe blocks which
correspond to the respective poles. The unitary monoblock
is clamped and assembled in a detachable way by any suitable
L~ m~ans, such as, screws between ~he opposite lateraL polar
f~c~ provided on each magnetic core pole. Further, it is
~upported by each o these side faces on another polar face
par~lleL1y to the surface of the tail on each core pole. The
opposite lateral polar ~aces provided on each magnetic core
pole preferabLy offer a limited height which enables, in the
idle position of the magnetic shoe mounted on the railroad
vehicle, to become disengaged from the rail. Thus, the uni-
que monoblock is disposed between the magnetic core poles.
In another application, the unique monoblock may be later-
~5 ally pushed between both opposite lateral polar races pro-
vided on each pole of the core until it reaches a lateral
position to permit the seating and/or unseating of the
monoblock in parallel to the rall. For the assembling on
the nonmay~etic core, the two opposite lateral faces pro-
vided on each of the poles of the core may be symmetrically
slanted in relation to the longitudinal axis of the shoe and
may form two retaining surfaces for the unique monoblock
being assembled in dovetail between both polar faces of the
ferromagnetic core.
The central continuous band of nonmagnetic material of
the unique monoblock may be provided with teeth and/or
flanges capable of working with teeth and/or flanges pro-
vided on a rigid caslng of the exciting coil in order to
transmit to this rigid casing the fric~ion forces being de-
veloped on the rail to the part of the unique monoblock when
lt ~i~ in contact with the rail.
1.5 BRIEF DFSCRIPTION OF THE DRAWINGS
Other ob~ects, advantages and characteristics of this
inv~ntion will appear more readily evident by reading the
dascription which follows, which is supplied as non-limitat-
ive and opposite to the drawing where:
~O E'igs. 1 and 2 are cross-sectional views showing the
invention, two types of different embodiments of the electro-
magnetic shoe as applied to the running rail in a braking
position for a railroad vehicle.
Fig. 3 is a partial elevational view of a railroad ve-
hicle body equi~ped with electromagnetic shoes as disclosedin the present invention.
S~,39~
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawings, and in particular to
Fig~ 3, there is shown a shoe in service on a vehicle. As
shown, the reference numeral 1 indicates an electromagnetic
shoe which is supported from the truck 2 of a railway vehicle
by piston rods 4a and 4b of two suitable raising or lifting
cylinders having a direction of motion as indicated by arrows
4. A plurality of annular excitation coils 6 are located in-
side a rigid casing 5. The coils are set in line along the
longitudinal axis of the running rail 30 These coils have
a vertical line of symmetry 7 (see Fig. 1 and 2) extending
along the entire height of the shoe which coincides with the
ver-tical line of symmetry of the longitudinal rail 3. The
shoe 1 comprises a series of rubbing contact elementary shoes
mounted in line and positioned between two headpiece mem-
b~r~ 9. The rlyid casing 5 forms the supporting frame so
khat each elementary shoe may be connected with its respect-
i.ve coil 6. The two elementary end shoes 9 are each pro-
vLded with a beveled edge 18 for permitting them to gradually
engage the rail 3 so as to clear and prevent a hangup when a
sudden discontinuity occurs along the running surface, such
as, at a rail joint or at switch points.
Referring now to Fig. 1, there is shown the details of
one of the shoes 8 as applied on the running surface of the
head 3a of the rail 3. Each of the shoes comprises an in-
verted U-shaped ferromagnetic core. The core includes two
depending legs or branches LO which form a central opening 6a
g _
~5~
facing toward the rail 3. The coil 6 surrounds the bight of
the U-shaped core which has formed on the ends of the legs 10
a pair of poles or polar faces 11. The pole faces 11 are
disposed opposite the upper surface of the rail 3 and are
perpendicular to its longitudinal axis of the rail 3. As
mentioned above, the upper end of the two branches of the U-
shaped core pass through the cen~ral opening of -the coil 6
and are contiguous to one another as indicated by a surface
line 12. The contiguous surfaces are flat without clearance
to form a closed magnetic circuit of each elementary shoe 8.
Thus, the magnetic branches form t~o easily detachable parts
13 and 14 on each side of the vertical line of symmetry 7.
As shown, the parts 13 and 14 each include a lower base
port.ion which comprises a friction tip 15. The tip portions
1~ .15 part.ially encompass the bottom side of the coil 6, and
ar~ v~rti.cally directed toward the rail 3. The homologous
~i.p oE one part 13 is separated from the homologous tip of
kh~ other part 14 by means of a separation band 16 made of
st.rip or nonmagnetic material, such as, brass or sta nless
steel. The two detachable parts 13 and 14 constitute the
ferromagnetic core member of each elementary shoe 8 and are
pressed against contiguous surface 12 and the separator 16 by
a plurality of traverse bolts 170 The upper bolt which
passes through apertures in the bight of the core and sur-
~5 face 12 may be made of a highly magnetic permeable materialwhile the lower bolt, which passes through a space located
at the base of the coil 6 and holes formed in the parts 15
-- 10 --
' ?~
~5~
of the branches 10 and the dividing band 16 should be made
of nonmagnetic material, such as, stainless steel, in order
to prevent a magnetic short circuit from occurring across
the clearance constituted by band 16.
In accordance with the invention, the pole faces 11 of
the parts 13 and 14 are covered by a thin plate 19 which has
a good magnetic permeability as well as a high coefflcient
of friction, a good mechanic resictance and a good abrasive
resistance. These plates are manufactured by powder metal-
lurgy, specifically by sintering; however, the manner of
fastening, which is usually achieved by screws, presents
some dificulties when the plates are thin. In instances
where the material used between the rail surface and the po-
lar faces shows a high abrasive resistance, it is possible
to consider attaching the material to the pole faces in thin
layers by metal depositing or plating, i.e., by spattering
the material at high impact speed when it is in the form of
melted or micro-melted particles.
When the magnetic shoe assumes an active position as
shown in Fig. 1 the shoes engage and slide on running sur-
face of the rail 3. That is, when the piston cylinders allow
the rods ~a and 4b to move downwardly the shoe will come to
rest on top of the rail 3. At the same time, a D.~. current
is caused to flow through the coil 6 which results in a high
number of ampere-turns of excitation which is capable of al-
most saturating the magnetic circuit. The magnetic flux
flows through a closed magnetic circuit made up of parts 13
-- 11 --
and 14 and head 3a of the rail 3. The magnetic flux enters
-the rail head from one of the pole faces 11 and exits the
rail head into the other pole face and causes concentric
circles of flux to flow through rail head 3a as shown by
fine lines 20. The separation band 16 and the nonmagnetic
rod of the lower bolt 17 compels the rnagnetic flux to more
readily flow through the rail head which has a relatively
higher permeability to lines of force and causes the linking
of the north and south poles of the polar faces 11. The
high magnetic induction which passes through the head of
the running surface 3a of the rail 3 causes a very large
magnetic attractive force between plate 19 and the surface
of the rail~ The application force which is created between
~he plate 1.9 and the rail by very long magnetic shoes may
.LS rt-~ach statlc values of 7 to 10 thousand daN~ The fast dis-
placement o the magnetic shoe on the rail induces Foucault
~r ed~y current into the rail Foucault or eddy currents
which may tend to decrease the magnetic induction passing
khrough the rail surface, but the higher coefficient of
~0 rictiorl exhibited by the plates 19 causes a higher braking
force to be applied by the head 3a of the rail to the pLates
19 and to be transmitted to the parts 13 and 14 of the ferro-
magnetic core which then supports the lateral faces of the
opening 6a o the coil 6 located in the rigid casing 5 which
is ixed to the frame of the truck of the vehicle.
In previously-known magnetic shoes, the ferromagnetic
cores either are made of massi~e cast iron which has a rela-
t:ively good magnetic permeability or are made of molded steel
- ~2 -
1 .1 8~i9~;4
which has a very good magnetic permeability with cast iron
friction shoes which possess only averag~ magnetic permea-
bility. The permeability of magnetic circuit of such cores
is relativeLy lower than that attained with magnetic shoes
as shown in Fig. 1. Such an advantage permits a considerable
increase of the magnetic induction which passes through the
surface of the rail and, accordingly, results in a consider-
ably greater attraction between the shoe and rail since the
attraction force is proportional to the square of the in-
duction. In such a shoe arrangement, the bucking force pro-
duced by the Foucault or eddy current is decreased as much
as po~ssible in order to increase the friction action between
the plates 19 which have high coefficient of friction (0.2 ~o
0.~) and the surface of rail 3. The roughness of the surface
e the rail is augmented after the abrasive passing of the
in.itiaL magnatic shoe of the first truck of the train so
~ha~ the shoe efficiency increases the following ones.
The magnetic brake shoe as shown in ~ig. 2 employs some
of the same elements which are shown on Fig. 1. It will be
~0 observed that the same elements have identical references
and deal with the excitation coil 6, the circuit branches 10
of the ferromagnetic core in two parts 13 and 14 and the
common supporting surface 12, etc.
For the purpose of simplifying the disassembly of the
~S pole shoes from the outside of the truck frame 2~ the bolts
and nuts are replaced by hexagonal cap screws 17 which have
the heads exposed to the outer side of part L4 while part 13
~5~ ~
is provided with threaded holes for receiving the threaded
ends of screws 17. The lower tip portions 15 of parts 13
and 14 of the magnetic core are not completely extended -to
the top of the rail, but includes two bearing surfaces 21 and
22, which are perpendicular one another. The surface 21 is
parallel to the li~e of symmetry 7 while surface 22 is paral-
lel to the upper surface of the running rail.
According to the invention, a massive friction block 23
is made up of two massive poles 24 and 25 of a good magnetic
permeable material having a high coefficient of friction and
good mechanical resistance. The pole pieces may be cast by
a powder metalurgy and sintering process into a single or
unitary monobLock 23 which includes a separation band 26.
The separation band establishes the clearance between both
15 pol~s 24 and 25. The monoblock is assembled on the magnetic
co.r~ and is adapt0d to engage the lateral bearing faces of
eac~s 2L and 22 of parts L3 and L4 of the magnetic core.
'rh~ monobLoclc is cLamped between both faces 21 by the Lower
nonmagnetic screw 17 whc h passes through a clearance bore
~0 27 provided in the bLock. The separation band is made of a
suitabLe nonma~netic material by any acceptable method, and
specificaLly by powder metallurgy process. The nonmagnetic
band includes a wide section near the top adjacent the coil
6, as shown in Fig. 2. This wide section, at the upper end
o the band, provides a great distance in the vicinity of the
coil 6 which is the most susceptlble to magnetic short cir-
cuit due to induction leakage while a small width at the
~s~
lower end of the band provides little clearance between both
poles around the fric-tion surface on the rail. This tapered
configuration causes an increase in the contact surface be-
tween the frictiorl shoes and the rail. It also lncreases the
S induction field passing through contacting surfaces as well
as increasing the attraction force between the magnetic shoe
and the rail. The monoblock 23, which is mounted on the
front contacting shoe 9, is provided with beveled edge 18
as previously described in reference to Fig. 3. As shown in
Fig. 2, an exterior laterally extending fin 28 is provided
on the monoblock to reposition it on the rail in the event
that transversal lateral shock offsets the magnetic shoes
a~sembly of a truck connected kogether by a frontal and late-
ral thrust on the truck frame.
L5 L~t us suppose the magnetic shoe is applied on therrail
and assumes its active position as shown in Fig. 2 so that
it ~lides on the upper surface of the rail 3. Further, let
u~ a~sume D.C. current passes through the coil 6 so that mag-
n~tic 1ux readily passes through intimate contacting faces
~ 2L and 22, into the massive poles 24 and 25 which have good
magnetic permeability. The magnPtic flux flows through one
of the polar faces ll and enters the rail. The magnetic
lines of force pass through the rail head 3a as shown by the
concentric lines 20. The flux lines emerge from the rail
and go back through the massive pole 24 into part 14 of the
erromagnetic core without passing through the separation
band 26 which exhibits a variable reluctance clearance. The
1 5
, ~ ~
magnetic attraction developed between poles 24 and 25 and the
rail produces a high rubbing force due to the high coefficient
of friction of the sintered materlal of the poles and non-
magnetic material of the separation band 26. In this vers-
ion, it is possihle -to construct the massive block 23 of
material which is less resistant to wear and abrasion than
in the case of Fig. 1 since the block 23 may be more easily
replaced on the shoe while still mounted on the truck 2. To
accomplish this, it is only necessary to unscrew the lower
stainless screws 17 and to drop the blocks 23 on the rail
with the lever when the magnetic shoe is in idle position
ag shown in Fig. 3. It is then possible to laterally remove
thc blocks 23 since the clearance between the lower edge 22
and the rail is greater than the height of edge 21. In the
L~ ~v~nt that the face 21 is higher than the available space
b~twe~n the pole face 11 and the rail, it's possible to re-
mov~ the blocks 23 by sliding them, longitudinally, between
~he faces 21 after releasing the lower screws 17 until
reaching an open end of the end shoe 9. It is understood
that the massive poles 24 and 25 may be separated from the
band 26 if they are only held together by the lower screw 17.
The separation band 26 may have various cross-sectional con-
figurations, specificallyt a rectangular cross~section of a
known type which could facilitate the tightening of the mas-
sive poles between two faces 21. Further, the nonmagneticband may be equipped with teeth and/or projections which
- 16 -
~35~
would cooperate with matching teeth and/or conjugated de-
pressions, which are provided on the rigid casin~ 5 which
is preferably made of stainless steel sheet metal in order
to minimize magnetic leakage.
It is understood that the gap between the bore 27 and
the intermediate portion of the screws 17 allows the massive
poles 24 and 25 to be securely clamped between the faces 21
yet permits the free vertical expansion of these poles when
the temperature rises during their frictional contact with
the rail. In order to reduce the overheating of the rail and
the pole pieces during the braking operation, it i5 highly
advantageous to provide an additive to the pole piece such as
the phosphorous in cast iron of brake shoes for railways, to
di~ipate a part of the friction heat caused by the sliding
.L5 abra~ion or to construct the separation band 26 o~ a heat
accumulation b:Loclc of nonmagnetic material, such as, fusible
w~
It is understood that this invention is not limited to
only the specific types of embodiments shown and disclosed.
It is apparent that numerous changes and variations are avail-
able to the state of the art, without deviating or departing
from the invention. For example, in the event the magnetic
brake shoe which includes the massive poles 24 and 25 might
be heated excessively by the rubbing friction of the rail,
2S it is possible to make symmetrlcally inclined faces 21 in re-
lat.ion to the axis 7 in order to form two retaining surfaces
toward the bottom for the monoblock 23. Thus, a dovetail
~s~
will exist between both faces 21 ~ith a slight clearance for
permitting it to rest on parts 13 and 14 of faces ~2 during
braking. In such an arrangement, the magnetic circuit will
have a slight clearance bet~een poles 24 and 25 and faces 22
but a good contact between those poles 24 and 25 and the sup-
port surface 22. The block 23 and the poles 24 and 25 may
freely expand in every direction with regard to the soft steel
magnetic core. Such an arrangemant appears partlcularly ad-
vantageous when the coefficient of expansion of the cores and
block 23 are dramatically different, but it is evident that
lower screws 17 must be extremely strong in order to meet the
torsional forces and working forces applied to this core when
the magnetic shoe i5 in service on the rail. The use o~ mono-
block 23 having a dovetail requires a lateral sliding inter-
L~ connection for these monoblocks at one of the ends of the magnetic 5hoe.
The use of magnetic shoes with friction parts, either
th~ highly resistant to abrasion type, such as the thin
plates 19 o Fig. 1, or the easily replaceable on the shoe
~0 in service type, such as monoblock 23 on Fig. 2, permits the
use o such shoes in service braking. In an arrangement
where the regulating braking is attempted without excitating
current modulation is very difficult and expensive to attainn
However, as previously described, controlled braking could
~5 be achieved by causing the shoe to be applied on the rail by
a fluid pressure acting on at least one of the application
pistons of the lifting mechanism~ The shoes are biased up-
wardly by springs and other return means to lift the shoes
18 -
to their nonbrake or idle position. The disadvantage of us-
ing permanent magnets in railway environmen~ is that they have
a tendency to pick up any ferromagnetic objects which they
pass.
Another variation consists of employlng a material for
the pole pieces 24 and 25 and also for the separation band
26 which has a relatively elastic characteristic in which the
desired quality might be obtained by leaving voids between
the grains of sintered material.
Sucn an elastic structure would permit the polar faces
11 to readily assume the shape of the running rail surface
under the action of the magnetic attraction force which would
increase the magnetic induction as well as the braking force
of the shoe on the rail.
1 9
