Spring brake unit and its application to a braking system

Braking unit for vehicle, of the type comprising an axial actuating helical spring maintained in release position, against its elastic reaction force, through a screw-nut system rotatively driven by an electric motor, the spring acting upon the brake through the intermediary of a hydraulic master cylinder of which the piston is axially connected to an abutment ring of the spring also mechanically connected, via a screw-nut system to the rotor of an electric motor, such an unit being applied to a spring braking system for a vehicle possibly with a built-in park brake.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The present invention relates to a vehicle braking unit, of the type 
comprising at least one spring axially actuating the brake, such as a 
helical spring maintained in the brake release position against its 
reaction force through a screw-nut system adapted to be driven in rotation 
by an electric motor the electrical feeding current of which is adapted to 
adjust the braking force of the unit, constituted by the axial reaction 
force of the spring reduced from the subtractive force provoked by the 
screw-nut system rotatively driven by the electric motor. The invention 
also concerns the application of this braking unit to a service park, 
and/or emergency braking system fitted, where necessary, with anti-skid 
means. 
In order to dispose of a spring-brake of which the release does not require 
the use of a pressurized fluid such as compressed air or pressurized oil, 
braking units have already been proposed and produced in which the brake 
application force is supplied by a prestressed spring such as a helical 
spring and the adjustable release effort is performed by an electric motor 
which delivers, for example, an axial release effort via a screw 
presenting reversibility characteristics, such as a ball screw (ball 
circulation worm gearing). In such a braking unit, the motor which is 
generally a direct current motor, with separated and constant excitation 
by permanent magnets, is maintained under full voltage in brake release 
position. The progressive brake application is achieved through 
progressive decrease of the motor voltage, i.e. according to the most 
widely used adjustment method, of the direct current flowing through the 
rotor windings of the motor. 
The brake application force is thus the result of the difference between on 
the one hand, the reaction force of the spring considered as constant but 
which decreases slightly with the return stroke of the spring, and 
theoretically slightly decreases in time as a consequence of a relaxation 
effect of the metal of the spring and, on the other hand, the adjustable 
release force (through adjustment of the motor current), transmitted by 
the ball screw which presents a restricted and acceptable hysteresis rate 
of about 10%. 
These braking units which present the advantages of spring brakes with 
respect to operating security due to the fact that it is always possible 
to interrupt the release effort and has the facility of placing in long 
term park position through cutting out of the release current, are used on 
vehicles where, for reasons of bulkiness ad facility of maintenance, it is 
desired to suppress the need of a pressurized fluid source for the 
braking, while changing to "all electric" piloting of the brakes. However, 
the braking units must be placed immediately adjacent to the braking 
surfaces upon which they act and which are constituted generally by the 
lateral faces of a brake disc and the important volume of electric motor, 
spring and slack adjusting means between the braking surfaces and the 
brake linings generally lead to a bulky assembly which is very difficult 
to house within the bogie chassis of light railway stock such as street 
cars, or rapid vehicles such as high speed trains, which are more adapted 
to receive high security braking units with "all electric piloting". 
One of the objects of the present invention is specifically to offer a 
braking unit which allows to retain the qualities of the spring brake 
released by an electric motor and which can be housed more easily in the 
chassis or the bogie of the vehicle while acting simultaneously on several 
braking surfaces such as those of multiple discs brake of a high-speed 
railroad bogie. 
SUMMARY OF THE INVENTION 
According to the invention, the spring is connected to the brake through 
the intermediary of at least a hydraulic master cylinder of which the 
piston is axially actuated by an abutment ring of the spring, this ring 
being mechanically connected via the screw-nut system to the rotor of the 
electric motor and at least a hydraulic fluid connector of the master 
cylinder is connected to at least one receiving cylinder of which the 
piston is adapted to apply at least one brake lining of the brakes on a 
braking surface of the vehicle wherby there us a remote connection between 
the actuating spring and the brake lining by means of a hydraulic fluid 
column which is adapted to transmit and to transform the axial reaction 
force of the actuating spring and which presents, where necessary, a 
variable volumn adapted to adjust the application slack of the brake 
lining. 
The brake unit can be constituted, on the one hand, by a 
mechanical-hydraulic assembly containing the screw-nut system and the 
operating spring and at one end of which is mounted the master cylinder 
and, on the other hand, of an electrical assembly constituted by a rotary 
electric motor of which the rotor is connected to a driving member for 
during the nut of the screw of the screw-nut system. 
The mechanical-hydraulic assembly and the electrical assembly are mounted 
parallely with respect to each other on a transmission bracket in which is 
disposed a flexible transmission member, comprising where necessary a 
demultiplication, between the driving member and the screw or the nut of 
the screw-nut system such as a chain or a belt, in such a manner as to 
reduce the length of the control unit, or in an alternative, the 
mechanical-hydraulic assembly and the electrical assembly are mounted in 
series in such a manner that the rotor of the electric motor is directly 
coupled to the nut or to the screw of the screw-nut system, so as to 
reduce the bulkiness diameter required for the braking unit. When the 
operating spring is a helical spring, the screw-nut system is housed at 
least partially within the helical spring. 
According to one embodiment of the invention ensuring supplementary safety, 
the abutment ring is mechanically connected to a mechanism for actuating 
an emergency and/or park brake of the vehicle adapted to replace at least 
provisionally the master cylinder in the case of it being defective. The 
mechanical connection between the abutment and the actuating mechanism is 
constituted by a lever articulated on an axle integral with the unit body 
and of which one end rests upon the abutment ring of the actuating spring 
while the other end is connected to a cable of a cable and sheath system 
for actuating the emergency and/or park brake of which the sheath is 
adapted, respectively, in the application position of the emergency brake, 
to be in abutment on a rigid abutment and, in the inactive position of the 
emergency and/or park brake, to be released in order to allow the service 
brake to act freely via the hydraulic master cylinder. The rigid abutment 
is movable and adapted, either to be returned in the active position, or 
to be brought into the inactive position, releasing the sheath, by a 
remotely controlled member such as a fluid cylinder or an electric motor. 
According to another embodiment, the hydraulic master cylinder is provided 
with pre-stroke means adapted to provoke an initial emission of hydraulic 
fluid at a high flow-rate under reduced pressure in order to carry out the 
initial approach of the brake linings on the braking surfaces and to seal 
up by a checkvalve the braking circuit once the application pressure on 
the linings exceeds the maximal pressure possible for the high flow-rate 
circuit, in such a manner as to improvedly use the energy of the actuating 
spring to apply the brake linings on the braking surfaces. 
When the brake control block according to the invention is applied to a 
park braking system, the electric motor is also adapted to be electrically 
powered in order to be driven in rotation in counter direction prior to a 
parking operation, i.e. in the direction in which the screw-nut system 
provokes on the abutment ring of the operating spring a supplementary 
force on the master cylinder which is added to the elastic reaction force 
of the actuating spring, in such a manner as to provoke a supplementary 
application force on the brakes and to reverse in parking situation the 
hysteresis of the screw-nut system after cutting out the reverse electric 
current on the electric motor. 
In an alternative, the electric motor is also adapted to be driven in 
rotation in the counter direction in emergency braking situation of the 
vehicle, in such a manner as to provoke a supplementary braking 
application force corresponding to the emergency braking. 
When the brake unit is included within a braking system equipped with 
anti-skid means, i.e. temporary releasing of the brakes in the case where 
there is a trend of blocking at least one wheel of the vehicle braked by 
this unit, the electric motor is adapted to be fed during the temporary 
release or "anti-skid" phases by a release current greater than the normal 
total release current of the brakes, in such a manner as to accelerate the 
release rotation of the screw-nut system during the "anti-skid" release 
phases of the brakes. 
According to another alternative of the method of use of the braking blocks 
according to the invention, in order to obtain a total release of the 
brakes, the electric motor is fed during the release phase per se with an 
electric current greater than that necessary to obtain the compression of 
the actuating spring in release position of the brakes, then after this 
release, is fed with an electric current lower than that necessary for 
ensuring this compression of the spring but sufficient to maintain the 
brakes in full release position by using the hysteresis friction of the 
screw-nut system, in such a manner as to decrease the heat or thermal 
charge of the electric motor in the released brake position.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The brake unit 1 represented at FIG. 1 is intended principally to actuate 
hydraulically controlled brake calipers 2 and 3 of which the pistons 4 and 
5 apply, under the effect of a hydraulic pressure, brake linings 6, 7 and 
respectively 8, 9 on the braking surfaces of brake discs 10 and 11. The 
hydraulic pressure is generated by a master cylinder 12 of a known type, 
for example with a plunger 13 and which is adapted to be supplied in 
hydraulic fluid from a fluid reservoir 14 through a check valve 15 
maintained in open position by a flange 16 of the plunger 13, when this 
latter is in inactive or withdrawn position. In order to perform automatic 
slack adjusting, the pistons 4 and 5 of the calipers 2 and 3 cooperate, 
for example with rubbing and springy means 17 and 18, such as resilient 
segments and/or washers, that are opposed to the withdrawal of the pistons 
4 and 5 beyond the normal slack value when these pistons have advanced due 
to wear of the brake linings or of the friction surfaces of the discs 10 
and 11. The hydraulic or pressure chambers 19 and 20 of the brake calipers 
2 and 3 are connected to the pressure chamber 21 of the master cylinder by 
connecting lines 22 and 23 which present flexible portions allowing a 
displacement between the master cylinder 12 and the brake calipers that 
can be of any number, corresponding for example to the number of brake 
discs of a railroad bogie, number that can reach eight on the bogies of 
high-speed-train (HST) but can be limited to two in the case of a motor 
bogie intended for a rapid transit vehicle of the metro or street car 
type. 
Air drain means for bleeding the hydraulic circuit are provided on the 
upper parts of the circuit, especially the hydraulic chamber 19 and 20 of 
the calipers and the pressure chamber 21 of the master cylinder 12 (only 
the bleeding screw 24 of the master cylinder 12 has been represented). 
According to the invention, the plunger 13 of the master cylinder 12 is 
connected by an end abutment ring 25 to an axial actuating spring 26 
constituted, in the present example, by a preset helical spring abuting by 
its other end via a setting ring 27 on a abutment tube 28 itself abuting 
on a support body 29 of the braking unit 1. The abutment tube 28 is 
extended in the direction of the end abutment ring 25 by guiding fingers 
30 (a single finger is represented in cross-section) which are housed in 
guiding grooves 31 (a single groove is represented in cross-section) of a 
return tube 32 which carries, at its other end on the side of the plunger 
13, the abutment ring 25. The other end of the return tube 32 carries a 
ball nut 33 (recirculation ball worm gearing) schematically represented on 
FIG. 1 and which cooperates with a ball screw 34 (a worm presenting a 
helical groove in which roll the balls in continuous rolling via a return 
rolling track) which is extended over the entire length of the return tube 
32. The ball screw 34 is rotatively mounted between two bearings one of 
which 35 is carried by the guiding fingers 30 (being at minimum number of 
three in order to ensure correct centering) and maintained in place by a 
blocking nut 36 of the ball screw 34. 
The other guide bearing 37 of the ball screw 34 is carried by a flange 38 
of the body support 29, beyond which the ball screw 34 is rendered 
integral, for example by keying, with a sprocket wheel 34 rotation of the 
screw 34. The wheel 39 is coupled by a chain 40, to the outlet sprocket 41 
of an electric motor 42. The connection between the sprocket 41 and the 
shaft 43 of the electric motor is obtained, for example, by a 
unidirectional free-wheel 44 whose function will be fully described 
herein-below. The electric motor, which in the present example is a 
commutator direct current motor 45, is flangedly mounted on the support 
body 29 with its axis parallel to the axis of the ball screw 34. 
FIG. 1 shows that the chain connection mechanism between the ball screw 34 
and the electric motor 42 is covered in a sealed manner by a cover 47 
applied to an edge 29a (FIG. 2) of the support body 29 through the 
intermediary of screws 48 attached onto protrudings 49 of this body. On 
the side view of FIG. 2, where the cover 47 has been removed, the 
electrical connections 50 of the electric motor 42 and the chain drawing 
device 51 are represented. It will be understood from FIGS. 1 and 2 that 
when the voltage is applied onto the electric motor 42 (constituted in the 
present example by a permanent magnets stator 42a and a rotor 42b fed with 
adjustable direct current through the intermediary of the commutator 45 
and of brushes 52 only one of which is represented) it allows to have the 
ball screw 34 rotated in the direction that compresses the spring 26 in 
order to release the brakes. 
The axial displacement mechanism constituted by the spring 26, the ball nut 
33, the ball screw 34, the return tube 32, the abutment ring 25 and the 
plunger 13 that crosses a sealed bearing 53, is housed in a cylindrical 
guard 54 flanged on the support 29 and which is extended by a closing 
flange 55 carrying the sealed bearing 53 of the plunger 13 and on which is 
rigidly attached by screws 56 the master cylinder 12. In the vicinity of 
the flange 55, the guard 54 carries an external bracket 57 on which is 
pivoted an emergency or park braking lever 58 which is bearing through a 
fork 59 on a bearing 60 of the abutment ring 25. The lever 58 crosses in a 
sealed manner the guard 54 by means of an elastic bellow 61 and its end 
external relative to the guard 54 is hinged to a cable 62 adapted to slide 
in a sheath 63 flexible in bending and relatively rigid in compression and 
which abuts, on the side of the lever 58 on an abutment 64 rotating around 
an axle 65 (in the plane of FIG. 1) and controlled by a piston 66 (its 
axis is perpendicular to the plane of FIG. 1) and respectively, on the 
opposite side, on a fixed abutment 67. The sheath is rushed onto the fixed 
abutment 67 by return springs or similar 63a. 
The cable 62 is connected to the application lever 68 of a park brake 
caliper 69 combined to a threaded connecting rod 70, to a release spring 
71 and to brake linings 72 and 73 in order to constitute, with a brake 
disc 74, an emergency or park brake. In the case where it is desired to 
use a high power park braking for a railway bogie, it is possible to use a 
disc 74 mechanically connected to the axle transmission by a high 
demultiplication (for example connected directly to the axle of the 
driving electric motor) and brake linings 72 and 73 having a high rubbing 
coefficient such as brake linings made of elastomer. When the park brake 
does not have to be actuated, the piston 66 is subjected to the action of 
a pressurized fluid or a spring (safety solution) and causes the support 
64 to pivot around the axle 65, in such a manner that the sheath 63 turned 
by the springs 63a does not have an abutment point and that the cable 62 
is not actuated against the release spring 71 when the fork 59 of the 
lever 58 is driven towards the right-hand side of the figure. 
The relatively heavy breaking unit 1 is attached onto the chassis of the 
bogie or of the vehicle and is connected by flexible conduits to the brake 
calipers (through brake hoses) and respectively by the flexible cable and 
sheath system to emergency brake calipers. 
The operating of the brake unit and system which is described herein-above 
will now be explained. In normal service of the vehicle on which is 
mounted the braking unit, the service and park or emergency brake is 
released by putting under voltage the motor 42 which, from a position of 
applied brake, drives in rotation via the free wheel 44, the sprocket 41 
and in counter direction, through the intermediary of the chain 40, the 
wheel 39. The ball screw 34 is thus driven in rotation with a torque 
multiplied by the ratio of the diameters of the sprocket wheel 39 and of 
the sprocket 41, in the direction which pushes the ball nut 33 towards the 
left-hand side of the figure by compressing the spring 26 until abutment 
position (the abutment has not been represented on the figure) 
corresponding to that represented in FIG. 1. In this release position of 
the brakes, the flange 16 of the plunger 13 opens the check valve 15, 
thereby connecting the hydraulic chambers 19 and 20 of the brake calipers 
to the hydraulic fluid tank 14 and puts the hydraulic circuit at 
atmospheric pressure. The brake linings 6-9 of the brake calipers 2 to 3 
are maintained close to the braking surfaces of discs 10 and 11 with a 
small slack due to the high friction of the rubbing and springy means 17 
and 18. As long as sufficient electric intensity flows through the motor 
42 to oppose, with the rubbing forces, to the rotation of the sprocket 41 
in counter direction under the effect of the reaction force of the spring 
26, this spring 26 remains compressed and the brake is released. 
In order to obtain a brake application, the electronic and electric control 
circuit for the electric supply of the motor 42 provokes a decrease in the 
power supply of the rotor of this motor up to a value lower than that 
sufficient to maintain, with the friction forces the spring 26 in the 
compressed position represented in the figure. The spring 26 which can no 
longer be retained by the torque of the motor 42 is thus driving the screw 
34 in rotation in the counter or reverse direction relative to the motor 
torque and moves towards the right-hand side of the figure by pushing the 
plunger 13 inside the pressure chamber 21 of the master cylinder 12. The 
check valve 15 closes, isolating the hydraulic circuit that rises in 
pressure and applies, through the intermediary of pistons 4 and 5 of the 
calipers, the brake linings 6 to 9 on the braking surfaces of the brake 
discs 10 and 11. The pistons 4 and 5 usually have diameters greater than 
that of the plunger 13, which allows to amplify several times the reaction 
force of the spring 26 on each of the brake linings. When the pressure 
prevailing in the hydraulic circuit is balanced, with the torque remaining 
on the shaft 43 of the motor 42, by the reaction force of the spring 26 
that decreases slightly with the expansion of said spring, the 
displacement of the plunger 13 is interrupted in a balanced position. 
During the displacement of the spring 26, the abutment 60 of the abutment 
ring 25 is applied on the fork 59 and causes the lever 58 to pivot, but 
this does not generate the displacement of the lever 68 against the 
release spring 71 of the parking brake, since the support 64 is removed 
and a traction on the cable 62 is converted into a displacement of the 
sheath 63 against the return springs 63a. 
In order to increase the braking forces, a further reduction of the release 
intensity that flows through the windings of the rotor 42b, at a rate 
sufficient to overcome the hysteresis rubbings of the ball nutscrew 
assembly provokes a fresh displacement towards the right of the plunger 
13. In order to provoke effective release of the brakes, it is necessary 
to increase the current intensity that flows through the windings of the 
rotor of the motor by a value great enough to overcome the mechanical 
hysteresis of the spring system and of the ball screw-nut assembly. The 
spring 26 is thus compressed by a supplementary value and the plunger 13 
is displaced towards the left of the figure, thereby increasing the volume 
of the hydraulic circuit and provoking by expansion a decrease of its 
pressure. It will be noted that the hydraulic circuit is isolated in a 
conventional manner, on the side of the receivers constituted by the brake 
calipers, by a residual pressure valve 75 which maintains the receiver 
circuit in slight overpressure when it is released in order to prevent 
stray air from re-entering, by pressing the lips of the seal linings on 
their bearings. 
In order to carry out a park braking, the parking brake is first of all 
activated by bringing, through the piston 66, the abutment support 64 into 
the position represented in FIG. 1, which is possible in released brake 
position since the springs 63a return the sheath 63 of the cable 62 and 
the face of the support 64 on which abuts the sheath can present a slight 
slope which allows the abutment 64 to act as a wedge which pushes the 
sheath 63 through the pivoting effect imposed by the piston 66. The cable 
62 is housed in a slot 64a of the abutment. In order to complete the park 
braking action, the electric current is cut-out in the winding of the 
rotor 42b and the completely freed spring 26 pushes the plunger 13 towards 
the right of the figure by provoking the full application of the disc 
brakes by the calipers 3 and 4 and a slight tightening of the disc 74 by 
the lever 68 since the sheath 63 arrives in abutment on the abutment 
support 64, once the fork 59 of the lever 58 pivots towards the right. It 
should be understood, in fact, that taking into consideration the small 
slack that exists between the service brake linings 6 to 9 and the 
corresponding brake discs 10 and 11, it is sufficient to have a small 
travel of the plunger 13 towards the right of the figure in order to 
provoke a full application of the service brake. 
If an oil leak occurs during the parking of the vehicle, most often through 
the check valve 15, the plunger 13 is displaced very slowly towards the 
right until the rubber abutments 33a of the ball nut 33 abuts on the 
abutment ring 35a of the ball bearing 35 of the screw 34, or until the 
abutment of the plunger 13 on the bottom of the hydraulic chamber 21. With 
a right adjustment of the threaded connecting rod 70 of the park brake 
caliper 69, the park brake is applied completely by the cable 62 and it is 
the fork 59 that stops the spring 26 in its displacement towards the right 
of the figure by transmitting to the park brake having a high rubbing 
coefficient, the whole of the reaction effort of the spring 26. A 
mechanical force of transmission by levers and cable and sheath system 
thus replaces the hydraulic transmission in the case of its deficiency 
during park braking. In the case where a complete service braking is 
actuated by cutting out the electric current of the rotor while the 
hydraulic circuit is defective, for example following the rupture of a 
connecting hose to a brake caliper, the spring 26 expands suddenly driving 
the rotor 42b of the motor 42 in rotation at high speed and at the moment 
of the sudden contact between the rubber abutments 33a and the ring 35a, 
the interruption of the rotation of the rotor 42b would provoke the 
rupture of the chain 40 if the unidirectional free wheel 44 was not 
interposed between the shaft 43 of the motor and the sprocket 41. Due to 
this free wheel 44, upon the sudden halt of the rotation of the screw 34 
and of the sprocket 41, the rotor 42b of the electric motor continues to 
rotate thus preventing the rupture of the chain 40. 
Numerous adaption and applications alternatives of the braking unit 
according to the invention are possible. The chain can be replaced by a 
belt in order to provide more elasticy to the transmission and to ensure 
thereupon a kind of torque limitation allowing to suppress the free wheel 
44. The motor 42 can be a motor of a type other than a direct current 
motor with shunt excitation by magnets, especially an asynchronous or 
synchronous-asynchronous induction motor or step by step motor. The 
hydraulic receivers can be of various and mixed types, for example for 
shoe brake and disc brake with intermediate pressure proportioning valves. 
The braking unit 1 comprises two assemblies having different functions : on 
the one hand, a mechanical-hydraulic assembly containing the screw-nut 
system which is not necessarily the ball type and the actuating or 
operating spring 26 as well as the hydraulic master cylinder 12 that can 
be simple or multiple, in series or parallel mounting and on the other 
hand, an electric assembly of which the rotor 42b is mechanically 
connected to the nut or to the screw of the screw-nut assembly. The 
hydraulic mechanical assembly and the electrical assembly can be mounted 
parallely with respect to each other such as represented on FIG. 1, in 
such a manner as to reduce the length of the control block. With the same 
purpose, the screw-nut system is housed as far as possible inside the 
helical spring 26 as possible which can be constituted by several stacked 
springs or springs in parallel, or be replaced by a stack of Belleville 
washers. 
According to another embodiment (not represented), the hydraulic mechanical 
assembly and the electrical assembly are mounted in series in order to 
reduce the transversal bulkiness of the unit which has thus the general 
form of an elongated cylinder. The mechanical connection between the rotor 
of the electric motor and the screw-nut assembly is obtained through 
direct coupling of the motor or via a reducing gear, preferably 
epicyloidal, in order to obtain increased compactness. 
According to one particular advantageous embodiment, the hydraulic master 
cylinder is for instance of the type disclosed in French Pat. published 
under no. 2,562,957, i.e. is provided with prestroke means adapted to 
provoke an initial emission of the hydraulic fluid at a high flow-rate 
under reduced pressure, in order to carry out the initial approach of the 
brake linings 6, 9, on the braking surfaces 10, 11 and to seal by a check 
valve the braking circuit once the application pressure of the linings 6-9 
exceeds the maximal pressure possible for the high flow-rate circuit, in 
such a manner to improvedly use the energy of the actuating spring 26 for 
application of the brake linings 6-9 on their braking surfaces 10, 11. It 
is thus possible to allow more slack between the brake linings 6-9 and the 
braking surfaces without having to increase the stroke of the actuating 
spring 26. 
By suppressing in one way or another the free wheel 44, the electric motor 
42 can be powered in order to be driven in rotation in counter direction 
prior to a parking operation, i.e. in the direction where the screw-nut 
system 34, 33 provokes on the end abutment ring 25 of the actuating spring 
26 a supplementary application force on the master cylinder 12, which adds 
to the resilient reaction effort of the actuating spring 26, in such a 
manner as to provoke a supplementary application force of the brakes and 
to reverse, in parking situation, the hysteresis of the screw-nut system 
34, 33 after cut-out of the inverse electric motor on the electric motor 
42. The electric motor 42 is also adapted to be rotated in counter 
direction in a situation of emergency braking of the vehicle, in such a 
manner as to provoke a supplementary braking force corresponding to the 
emergency braking. When the braking system is fitted with anti-skid means 
allowing a temporary release of the brakes in case of a blocking trend of 
at least one vehicle wheel braked by this braking unit 1, the electric 
motor 42 is adapted to be supplied during the temporary release phases or 
"anti-skid" phases by a release current higher than the normal current of 
total release current of the brakes, in such a manner as to accelerate the 
release rotation of the screw-nut system 34, 33 during these "anti-skid" 
release phases of the brakes. 
In order to ensure total release of the brakes, the electric motor 42 can 
be supplied during the release phase with an electric current higher than 
that necessary for ensuring the compression of the operating spring 26 in 
total release position of the brakes, then after this release, can be 
supplied with an electric current lower than that necessary to ensure this 
compression of the spring, but sufficient to maintain the brakes in total 
release position by using the hysteresis friction of the screw-nut system 
34, 33 in such a manner as to reduce the permanent heat charge of the 
electric motor in released brake position. 
The application methods of the braking unit to the various service 
conditions that have been described herein-above, are also applicable to a 
unit with a spring brake that is released by an electric motor where the 
transmission of the forces is carried out by direct mechanical 
transmission without use of a transmission and slack adjusting hydraulic 
circuit. the hydraulic transmission braking unit according to the 
invention is relatively heavy and bulky, but presents a high power 
sufficient to actuate all the brake shoes of a railroad bogie. According 
to the most advantageous embodiment, the braking unit is housed in the 
bogie chassis in a space disposing of sufficient place and the hydraulic 
conduits presenting hose portions connect it to the various relatively 
light weight pressure receivers that are disposed on the various brake 
actuating devices and that are all adapted to be connected to the unit by 
flexible transmissions including the park and emergency brake that is 
connected, as seen herein-above, by a cable and sheath system adapted to 
match curves. 
It is well understood that the present invention is in no way limited to 
the embodiments described and represented herein-above, but can be adapted 
to numerous variants available to the man skilled in the art without 
departing from the scope and spirit of the invention.