Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Magnetic Rail Brake
The present invention relates to a magnetic rail brake for a rail vehicle and
more particularly to a
magnetic rail brake with improved stability from tipping. It also relates to a
method of controlling
the maximum permitted tilt of a magnet in a magnetic rail brake.
Modern rail vehicles may brake in a number of different ways including
electrodynamic braking
via the driving motor, eddy current braking via the rails, pneumatic or
hydraulic braking acting
on the wheels or axles and aerodynamic braking. Particularly in the case of
trams or streetcars
and other light rail vehicles, it is commonplace to use magnetic brake shoes
which act directly on
the rails. Such devices generally comprise a magnet suspended from the bogie
or vehicle frame a
small distance above the track. The magnet is supported by springs causing it
to be biased away
from contact with the track. On actuation of the magnetic rail brake, the
magnet is pulled down
into contact with the rails against the restorative spring force. The
frictional contact force
between the magnet and the track provides the necessary braking force. This
braking force is
transmitted to the rail vehicle through interacting elements arranged on the
magnet and on the
bogie which allow the longitudinal braking force to be transmitted.
In the present context, longitudinal is used to denote the lengthways
direction of the rail vehicle
and corresponds to the direction of the rails and track. It 'is accepted that
this direction may of
course also be considered as circumferential, when the .vehicle negotiates a
curved portion of
track. Similarly, lateral, is used to denote the direction perpendicular to
the rails across the track,
which may or may not be horizontal, depending upon the presence of camber.
In order to follow the track, the magnet must also be laterally guided with
respect to the rails. To
this end, guides are usually provided which constrain the magnets to remain
above the rail. These
guides must however ensure a certain degree of lateral freedom in order for
the magnet to
optimally adapt to the track e.g. when negotiating curves. The guides ensure
vertical motion of
CONFIRMATION COPY
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the magnet above the rail and may allow lateral motion of in the order of 8mm.
As a consequence
of this lateral freedom, the magnets can also tilt about a longitudinal axis
lying parallel to the rail.
Because of the nature of the upper surface of the track, it is also desirable
that the magnet can tilt
in order to optimise the magnetic attraction between the rail and the magnet.
A typical
electromagnet may, on activation, exert in the order of 80 KN of force on the
rail. When
compared with the weight of the magnet itself, which may be in the order of 2 -
3 KN, it can be
appreciated that the magnet will tend to pull itself perpendicular to the
largest portion of the rail
surface, which under conditions of wear may not be horizontal. In general it
is desirable that the
magnet can tilt by between 2° and 3° either side of vertical.
The lateral freedom mentioned above has however been found to allow excess
tilting of the
magnets. Such excess tilting, which may be in the order of between 12°
and 15°, is highly
undesirable as it can present the lowermost side edge of the magnet to the
rail. When crossing
points or other rails, particularly under conditions of adverse wear of the
rails or the brake, this
lowermost edge of the magnet may snag on the side of the rail to be crossed
and be caused to ride
along it rather than over. Such action causes additional wear and in the worst
case may lead to
complete skewing of the magnet causing considerable damage to the magnetic
brake, guides and
bogie and even to derailment of the rail vehicle.
A prior art magnetic rail brake is known from document DE1780100 A. According
to this
arrangement, the individual brakes located on opposite sides of the bogie are
connected by cross
members, which link the magnets together but allow both to swing with respect
to the bogie. A
further mechanism is provided for bringing the magnets into alignment above
the rail on
25. actuation. Other similar arrangements are also known using cross members
to allow lateral
motion while preventing tilting. A problem which is common to all of these
arrangements is the
need for adequate space between the magnets for location of the cross members.
In the case of
modern low floor rail vehicles such as trams or streetcars, this space is no
longer available. The
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use of such cross members also substantially prevents any tilting, which for
the reasons given
above is usually undesirable.
An alternative arrangement is known from DE4140056 A which relates to the
parallel guiding of
a brake magnet for maintaining the magnet parallel to the rail. The
arrangement provides a
bracket with rubber buffers at each end of the magnet, which permit lateral
motion of each end
by about 2mrn whereby the magnet can align with the rail. The disclosure is
silent with respect to
the problems of tilting.
There is therefore a need for an improved arrangement of magnetic rail brakes
which ensures
lateral freedom of movement of the magnet to follow the rail, while preventing
excessive tilting,
without the need for cross members extending across the rail vehicle. In
particular there is a need
to prevent magnetic brakes from skewing when activated during the crossing of
points.
According to the present invention, there is provided a magnetic rail brake
for a rail vehicle
comprising a magnet supported from the rail vehicle, a guide assembly to guide
the magnet for
movement in a substantially vertical plane while allowing limited lateral
movement and an
extension arm. The extension arm is arranged to contact the rail vehicle at a
distance from the
magnet which is substantially greater than either the maximum extent of
vertical movement or
the maximum extent of lateral movement of the magnet to thereby reduce tilting
of the magnet.
In a first embodiment of the present invention, the guide extension is
provided by an arm rigidly
attached to the magnet and extending a suff cient distance therefrom such that
on tilting of the
magnet by the maximum desired amount, the end of the arm swings through a
distance
equivalent to either the maximum lateral movement of the magnet or the maximum
vertical
movement of the magnet. By providing a stop, arranged on the body of the
vehicle to stop the
movement of the arm, further tilting of the magnet is prevented.
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In this context, reference to the stop being .on the body of the vehicle is
intended to refer to the
location of the stop as being fixed with respect to the magnet. Usually, the
stop will be located on
the bogie frame which carries the wheels and the magnetic rail brake.
Alternatively, it may be
arranged to be attached on the wheel axles themselves. Of importance, is that
the stop is in fixed
vertical relation to the rail and to the mounting for the magnet i.e. on the
rail side of any
suspension system.
If the arm extends generally vertically, the stop should limit horizontal
motion of the arm and it
is the lateral freedom of movement of the magnet in relation to the length of
the arm which
determines the maximum angle of tilt. If the arm extends generally
horizontally, the stop should
limit vertical motion of the arm and it is the vertical freedom of movement of
the magnet in
relation to the length of the arm which determines the maximum angle of tilt.
In an alternative embodiment of the magnetic rail brake, the guide means may
comprise a
laterally sliding pivot arranged on the rail vehicle and the guide extension
may comprises an arm,
rigidly attached to the magnet and extending laterally to the sliding pivot
whereby the magnet
and arm can rotate around the pivot and slide laterally with respect to the
rail vehicle. Such an
arrangement is advantageous in that the magnet itself need no longer be
laterally guided by
vertical guides arranged on either side of it as is has been common in the
past. Furthermore, the
magnet may be set such that on contacting the rails it is substantially
vertical with tilting
occurnng only on lifting. In this way it is assured that the magnet is always
untilted when in
contact with the rails, regardless of lateral displacement.
In a further alternative, the sliding pivot may allow a degree of movement of
the pivot in the
vertical direction whereby the precise degree of tilting of the magnet is
determined.
Advantageously, in the above embodiments, the point of contact or pivot is
arranged laterally
outboard of the brake magnet. This allows full utilisation of the space
between the wheels which
is particularly important in the case of modern low floor designs.
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Additionally, it may be desirable that the tilting of the magnet be used to
provide a signal to a
brake control unit whereby actuation of the brake may be momentarily
terminated to allow
realignment of the magnet prior to reactuation.
5
The present invention also provides a method of controlling the maximum
permitted tilt of a
magnet in a magnetic rail brake, the magnet being arranged for vertical and
lateral movement,
comprising rigidly attaching an extension arm to the magnet and controlling
the movement of the
end of the extension arm distant from the magnet, the length of the extension
arm being
substantially greater than either the maximum extent of vertical movement or
the maximum
extent of lateral movement of the magnet, to thereby control tilting of the
magnet.
In general, it is envisaged that the present invention be used in the contest
of electromagnets. It is
however also possible that other forms of magnet may be employed.
Further aspects and advantages of the present invention are embodied in the
dependent claims.
Embodiments of the present invention will now be described, by way of example
only, having
reference to the accompanying figures, in which:
Figure 1 is a side view of a rail vehicle bogie incorporating a prior art
magnetic rail brake;
Figure 2 is an end view of a prior axt magnetic rail brake;
Figure 3 is an end view of a first embodiment of a magnetic rail brake
according to the present
invention;
Figure 4 is an end view of a second embodiment of a magnetic rail brake
according to the present
invention including an adjustable stop;
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Figure 5 is an end view of the embodiment of Figure 4, tilted into engagement
with the
counterstop;
Figure 6 is an end view of a third embodiment of a magnetic rail brake
according to the present
invention including a stop adjustable for tilting both inwards and outwards;
Figure 7 is an end view of a fourth embodiment of a magnetic rail brake
according to the present
invention including a vertically extending arm;
Figure ~ is an end view of a fifth embodiment of a magnetic rail brake
according to the present
invention including two laterally extending arms;
Figure 9 is an end view of a sixth embodiment of a magnetic rail brake
according to the present
invention including a sliding pivot; and
Figure 10 is an end view of a seventh embodiment of a magnetic rail brake
according to the
present invention including a profiled pivot plate.
Figure 1 shows a bogie of a typical rail vehicle in side view illustrating
bogie frame l, wheels 2
and magnetic rail brake 3 arranged to follow a rail 4. The magnetic rail brake
3 comprises a
magnet 5, which is supported from the bogie frame 1 a height h above the rail
4 by a pair of
tension springs 7. Although tension springs are illustrated, it is well known
that alternative
suspension devices may also be used including compression springs arranged
e.g. above the
bogie frame, hydraulic or pneumatic cylinders or actuators, servo actuators,
electromagnets and
the like.
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The magnet 5 is an electromagnet comprising an internal coil, which may be
actuated to generate
a pair of magnetic poles. It is also conceivable that instead of an
electromagnet, a conventional
magnet could also be used.
Actuation means (not shown) causes actuation of the electromagnet on receiving
a braking signal
from the driver. On actuation, the magnet is attracted to the metal rail 4
with such a force that
elongation of the springs 7 occurs and the spring is drawn down onto the rail.
The attractive force
between the magnet and the rail and the coefficient of friction between them
determine the
resulting braking force.
In order to ensure that the rail vehicle is brought to rest by this braking
force, it must be
effectively transmitted from the magnet t'o the bogie. .To achieve this,
.transmitting members 10
are rigidly mounted to the magnet. These transmitting members 10 interact with
supporting
members 12 rigidly attached to the bogie frame 1. In the device according to
Figure 1 and as
better disclosed in Figure 2, these supporting members 12 also serve to
laterally support the
magnet and guide it to follow the rail 4.
Figure 2 is a view along line 2-2 in Figure 1 and shows the magnet 5 supported
between
supporting members 12. As can be seen from Figure 2, a gap between the magnet
5 and the
supporting members 12 ensures a limited freedom of lateral movement whereby
the magnet 5 can
better align with the rail 4 on negotiating a curve or the like. The gap has
the maximum value g
which represents the allowable lateral movement of the magnet. Figure 2 also
illustrates how the
gap g also allows the magnet to tilt to adapt to the upper surface of the rail
but how this tilting
can also lead to snagging and over-tilting when braking during negotiating of
points or crossing
of other rails.
Figure 3 shows a similar view to Figure 2 of a first embodiment of the
invention in which the
magnet 5 is provided with a guiding extension in the form of an arm 20 rigidly
attached to the
magnet 5. In this embodiment the arm 20 comprises an extension of one of the
transmitting
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members 10. It is also within the scope of the present invention that the arm
be a separate
element attached directly to the ~ magnet 5. The arm 20 extends laterally a
distance d from the
magnet. In order to limit rotation of the magnet 5, the arm 20 is provided
with a stop surface 24
which interacts with a counterstop 26 to prevent the arm from rotating anti-
clockwise more than
the distance h corresponding to the distance descended by the magnet 5 in
order to contact the
rail 4. Since the distance d is substantially greater than h, the maximum
angle of tilt in this
direction is small and corresponds approximately to Arctan h/d.
The counterstop 26 may be provided on the bogie frame 1 or any other suitable
part of the
vehicle body which serves as a fixed reference to determine and limit the
tilting of the magnet.
Advantageously, in Figure 3, the stop is located at an outboard region of the
train and does not
therefore impinge on the limited space in the region between the wheels as is
the case when a
cross member is used.
Generally, rail vehicles are susceptible to wear. In particular, as the wheels
2 and the magnet 5
wear down, the settings of various parameters including the height of the
brake magnet 5 must be
adjusted. In order to allow for this and also to permit adjusting of the
sensitivity of the tilt
limitation to meet different track conditions, it is desirable for the stop
surface to be adjustable.
Figures 4 and 5 illustrates a second embodiment of the present invention in
which details of the
suspension of the magnet 5 have been omitted for clarity. The arm 20 is
provided with an
adjustable stop surface in the form of a set screw 2~. By adjusting the set
screw 2~, the point at
which the set screw 28 contacts the counterstop 26 may be adjusted. It is
noted, that it is possible
to set the distance between the set screw 28 and the counterstop 26 to less
than the height h. In
this case, the tilting of the lowered brake is further reduced, but on
deactivating the brake, the
magnet returns vertically through the height h. During this movement, set
screw 2~ will contact
counterstop 26 causing the arm 20 and magnet 5 to tilt in a clockwise
direction.
While in many circumstances it is sufficient to stop tilting only in one
direction e.g. outwards of
the track, there are also circumstances where it is necessary to prevent
tilting in either direction.
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Figure 6 illustrates a third embodiment of the present invention which uses a
second set screw 30
and a second counterstop 32 to limit tilting in the clockwise direction too.
It is noted that while
the adjustment according to these embodiments utilises set screws attached to
the arm, it is
equally possible to use set screws mounted as part of the counterstop.
Equally, the skilled person
will be well aware of many variations for the arrangement of the counterstops
to achieve the
desired effect. For instance, the second counterstop 32 may be adjustably
mounted from the first
counterstop 26 by further set screws, with the arm 20 and first set screw 28
captured in between.
An alternative arrangement of guiding extension is illustrated in Figure 7
which discloses a
generally vertically extending arm 40. This embodiment illustrates the
strength of the present
invention in providing the control of tilting at any point where sufficient
space is available for
such control. If the space beneath the vehicle is insufficient, or the
required distance d cannot be
achieved, the arm may follow a different extension. The arm 40 has two stop
surfaces 42 and 44
and is constrained between a pair of counterstops which in this embodiment are
provided by set
screws 46 and 48. The sum of the gaps between the set screws 46, 48 and their
respective stop
surfaces 42, 48 must correspond to the gap g. The maximum angle of tilt is now
however reduced
in proportion to the difference in height between the arm 40 and the portion
of the magnet 5
which is guided between the supporting members 12.
A further and fifth embodiment of the present invention is disclosed in Figure
8 and shows the
magnet 5 provided with two arms 50 and 52. Each arm is provided with its own
set screw
whereby the maximum allowed tilt in either direction can be independently set.
An alternative principle of guiding is disclosed according to the sixth and
seventh embodiments
as disclosed in Figures 9 and 10. According to Figure 9, the magnet 5 is
provided with an arm 60
extending laterally in a similar way to earlier embodiments: Unlike the
earlier embodiments
however, the end of the arm 60 is provided with a slot 62 in which slides a
pin 64 located on the
bogie frame 1 or other fixed reference point, to form a sliding pivot. The
slot 62 allows the pin
64 to move laterally a distance g and thus serves the function of the
supporting means 12, which
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in this embodiment are no longer required for lateral guiding. The whole arm
60 and magnet 5
may thus pivot about the pin 64 from a vertical position of the magnet when in
engagement with
the rail 1 to a slightly tilted position once raised.
5 The embodiment of Figure 9 does not allow for any tilting of the magnet when
in contact with
the rail. If this is nevertheless required, the slot must adopt a different
form. According to Figure
10 there is provided an alternative form of arm 70 having a slot 72 which at
its extremity closest
to the magnet is wider than the pin 64. In this region of the slot, the magnet
is free to tilt within
the width of the slot. Clearly, other forms of slot may be provided with
profiles which match or
10 compensate for the upper surface of the rail 4 in order to encourage
appropriate and controlled
tilting of the magnet.
The embodiments of Figures 9 and 10 do not disclose any form of adjustment to
compensate for
wear of the wheels and brakes or for adjustment of the settings. It is evident
to the skilled person
however that such adjustment could be provided at various positions within the
system, either at
the pin 64 or at the slots 62 or 72 or elsewhere on the magnet itself. Clearly
too, if the pin is to
serve the function of pivot and lateral guide for the brake magnet 5, it
should be of sufficiently
solid construction to withstand the forces encountered during braking.
A particularly advantageous aspect of the present invention is the possibility
of including a tilt
sensor to detect the movement of e.g. the arm 20 or 40. By providing an
appropriate motion or
pressure type sensor, tilting beyond a given angle may be detected. This may
be used to
deactivate the magnet allowing momentary disengagement from the rail. This may
be sufficient
to prevent the brake magnet from departing on another rail and restore it to
its untilted position
on resuming braking.
While the above examples illustrate preferred examples of the present
invention it is noted that
various other arrangements may also be considered which fall within the spirit
and scope ~of the
appended claims.
