Abstract:
A brake actuator for delivering a brake force through a brake block or br pad to a wheel or disc to be braked comprises in a housing ( 1 ) a service brake arrangement ( 3, 5 ). The actuator also comprises a safety brake arrangement ( 4, 9 ) including at least one powerful spring ( 9 ), acting in the brake application direction, means for tensioning the spring by the reaction force at a service brake application, and mechanical means ( 11, 12, 13 ) for locking the spring to the housing in a tension state. These locking means can be released at will for accomplishing a safety brake application.

Description:
TECHNICAL FIELD 
     This invention relates to a brake actuator for delivering a brake force through a brake block or brake pad to a wheel or disc to be braked, the actuator comprising in a housing a service brake arrangement. 
     TECHNICAL BACKGROUND 
     Brake actuators for service braking of vehicles, both rail vehicles and automotive vehicles, are well known. Such actuators are normally pneumatically or hydraulically operated, but also electro-mechanical actuators are known. Service braking of a vehicle may occur as block braking or disc braking, as is well known in the art. 
     It is in many cases desirable that the vehicle is braked if the supply of the normal brake medium, normally the pneumatic or hydraulic pressure, fails. The normal way of achieving this is by means of a so called spring brake, in which a powerful compression spring is held compressed and inactive by means of pneumatic or hydraulic pressure in a separate chamber; if this pressure drops for any reason the spring will expand and accomplish spring braking. 
     In certain cases, however, a normal spring brake is not the right solution. Especially if the service brake is of the electro-mechanical type, such a spring brake is not applicable at all. 
     THE INVENTION 
     A brake actuator of the kind defined above, fulfilling all requirements and specifications, is according to the invention attained in that the actuator also comprises a safety brake arrangement including at least one powerful spring, acting in the brake application direction, means for tensioning the spring by the reaction force at a service brake application, and mechanical means for locking the spring to the housing in a tensioned state, which locking means can be released at will for accomplishing a safety brake application. 
     It will be noted that in this solution still a powerful spring (or several powerful springs) is used for the safety brake. However, this spring is tensioned at the first service brake application after former use and is mechanically locked in this tensioned state, until it is to be used for safety braking, when for example electrical control power to the locking means is cut off. 
     In a first practical embodiment of the invention, in which the service brake arrangement is actuated by a pressurized brake fluidum, preferably compressed air, an inlet for the brake fluidum is arranged between a service brake piston and a safety brake piston, the safety brake spring being arranged between the safety brake piston and the actuator housing, and releasable locking means are arranged in the housing for releasably holding the safety brake piston. 
     In this arrangement the two brake pistons are pressed apart at a first application by the brake fluidum, and the safety brake piston is mechanically locked to the housing with the safety brake spring compressed and ready for action, whereupon normal service braking can occur without any interference with the safety brake arrangement. 
     In this embodiment a locking rod may be rotatably journalled to the safety brake piston and be in thread engagement with the housing, and means can be provided to releasably lock the locking rod to the housing. 
     Preferably the locking means include an electro-magnet attached in the housing and a locking washer, which is connected to the locking rod and is arranged in the vicinity of the electro-magnet for locking thereby. 
     In a specific variation hereof a locking spring is arranged between the locking washer and the locking rod for obtaining a locking servo action. 
     In another version of the locking means a connecting spring ring is arranged in corresponding grooves in a tubular piston rod of the safety brake piston and a sleeve or extension in the housing, and control means are provided on the housing for changing the diameter of the spring ring between positions for engagement and disengagement between the piston rod and the housing extension. 
     In a second practical embodiment of the invention, in which the service brake arrangement still is actuated by a pressurized brake fluidum, the safety brake spring is arranged between the housing and a safety brake piston, which is placed in front of the service brake piston as counted in the brake application direction and is provided with attachments extending out through slits in the housing, and releasable locking means are provided between the housing and a tubular safety brake piston, which encloses a push rod for the service brake. 
     This embodiment is rather close to the previous embodiment in certain respects. However, the safety brake arrangement is arranged in front of the service brake arrangement, and attachments on the safety brake piston serve as mountings for the actuator in the vehicle. 
     The locking means in this embodiment preferably consist of a connecting spring ring, which is arranged in corresponding grooves in a sleeve-shaped housing extension and the safety brake piston rod, and the control means are provided on the housing for changing the diameter of the spring ring between positions for engagement and disengagement between the extension and the piston rod. 
     The control means hereby include an electro-magnet, a pneumatic or hydraulic actuator or the like. 
     In a third embodiment of the invention, in which the service brake arrangement is actuated by a pressurized brake fluidum, preferably hydraulic fluid acting on a service brake piston, at least one safety brake spring is arranged—in a disc brake caliper, generally comprising a caliper frame and a caliper housing—between a caliper frame flange and a caliper housing flange, and a locking rod is rotatably journalled in the caliper frame and is in thread engagement with the caliper housing, means being provided to releasably lock the locking rod to the caliper housing. 
     The locking means here preferably include an electro-magnet attached in the caliper housing and a locking washer, which is connected to the locking rod and is arranged in the vicinity of the electro-magnet for locking thereby. 
     In a fourth practical embodiment, in which the service brake arrangement is actuated by an electric motor via means for transforming its rotary movement into an axial movement of a service brake push rod, i.e. an electro-mechanical service brake, the safety brake spring is arranged between the housing and a safety brake piston, which is provided with attachments extending out through slits in the housing, and releasably locking means are provided between the housing and a tubular safety brake piston rod, which encloses the service brake push rod. 
     Here, a connecting spring ring is arranged between a sleeve shaped housing extension and the safety brake piston rod, and control means are provided on the housing for changing the diameter of the spring ring between positions for engagement and disengagement between the housing extension and the piston rod. 
     Also in a fifth embodiment the service brake arrangement is actuated by an electric motor via means for transforming its rotary movement into an axial movement of a service brake push rod or force transmitting member. Accordingly, this arrangement is an electro-mechanical service brake, and in fact this is a practical and preferred embodiment. 
     Here, in a disc brake caliper, generally comprising a caliper frame and a caliper housing, at least one safety brake spring is arranged between a caliper frame flange and a caliper housing flange; a locking rod is further rotatably journalled in the caliper frame and is in thread engagement with the caliper housing, and means are provided to releasably lock the locking rod to the caliper housing. 
     These locking means may preferably include an electro-magnet attached in the caliper housing and a locking washer, which is connected to the locking rod and is arranged in the vicinity of the electro-magnet for locking thereby. 
     In one modification hereof a locking spring is arranged between the locking washer and the locking rod for obtaining a locking servo action. This locking spring may constitute a slitted end portion of the locking rod. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in further detail below reference being made to the accompanying drawings, in which 
     FIG. 1 is a schematic section through a first embodiment of a block brake actuator according to the invention, 
     FIG. 2 is a corresponding section through a second embodiment, 
     FIGS. 3A-C are cross sections along the line III—III in FIG. 2 of different design alternatives, 
     FIG. 4 is a section through a modification of the embodiment of FIG. 2, 
     FIG. 5 is a cross section along the line V—V in FIG. 4, 
     FIG. 6 is a view in the direction of the arrow VI in FIG. 4, 
     FIG. 7 is a partly sectional side view of a third embodiment of the invention, 
     FIG. 8 is a cross section along the line VIII—VIII in FIG. 7, 
     FIG. 9 is a section corresponding to FIG. 1 through a fourth embodiment, 
     FIG. 10 is a cross section along the line X—X in FIG. 9, 
     FIG. 11 is a shematic section through a first embodiment of a disc brake actuator according to the invention, 
     FIG. 12 is a section corresponding to FIG. 11 of a second and preferred embodiment of a disc brake actuator, 
     FIG. 13 is a schematic section through a fourth embodiment of a block brake actuator according to the invention, and 
     FIG. 14 is a section through a portion of the second embodiment according to FIG. 12 in a slightly modified version in line with FIG.  13 . 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In a simple version a block brake actuator according to the invention may be of the design shown in FIG.  1 . 
     In a common cylinder  1 , which is provided with attachments  2  for its mounting for example to the under-frame of a vehicle, two sealed pistons  3  and  4  are axially movable. The first piston  3  is a service piston connected to a push rod  5 , preferably comprising a mechanical slack adjuster of any type, and further to a brake block  6  for braking engagement with a wheel  7  to be braked. A return spring  8  for the service piston  3  is arranged in the cylinder  1 . 
     The second piston  4  is a spring brake or safety brake piston, which is acted on in the brake application direction by a powerful safety brake spring  9  of compression type. An inlet  10  for pressurized air is arranged on the cylinder  1  between the two pistons  3  and  4 , which accordingly are pressed apart at the admission of air under such pressure that the force of the safety brake spring  9  is overcome. 
     Means are provided for releasably locking the safety brake piston  4  with the safety brake spring  9  compressed. In the shown case these means include a locking rod  11  rotatably attached to the safety brake piston  4  and in thread engagement with a thread in the cylinder  1 . A washer  12  provided with teeth at its inner circumference is axially movable on splines at the end of the locking rod  10  remote from the piston  4 . A powerful electro-magnet  13  is attached to the cylinder  1  in the vicinity of the washer  12 . 
     When energized as shown, the electro-magnet  13  will attract the washer  12  and keep the locking rod  11  from rotating, so that the safety brake piston  4  is held in an inactive position with the safety brake spring  9  compressed. 
     If the electro-magnet  13  is deenergized on the other hand, the washer  12  and thus the locking rod  11  is allowed to rotate, so that the safety piston  4  moves to the left in the drawing under the action of the safety brake spring  9  and applies the brake via the service piston  3  and the push rod  5 . 
     When the safety brake piston  4  is held inactive, the brake actuator functions as a normal service brake actuator at the admission of pressurized air through the inlet  10  to the cylinder  1 . 
     A variation of the block brake actuator shown in FIG. 1 is shown in FIGS. 2 and 3. 
     In a cylinder  15  a service piston  16  is axially movable in a brake application direction to the left in the drawing under the action of pressurized air admitted through an inlet  17  therefore. In the opposite direction the piston  16  is movable under the action of a return spring  18 . The service piston  16  is connected to a push rod  19 , preferably containing a suitable slack adjuster, and further to a brake block  20  for brakingly engaging a vehicle wheel  21 . 
     A safety brake piston  22  is also axially movable in the cylinder  15 , but in this case in front of the service piston  16 . This safety brake piston  22  is provided with attachments  23  for mounting of the brake actuator for example to the underframe of the vehicle; these attachments extend out through axial slits in the cylinder  15 . A tubular safety brake piston rod  24  connected to the safety brake piston  22  extends forward or to the left in the drawing around the push rod  19 . A powerful safety brake spring  25  of compression type is arranged between the cylinder  15  and the safety brake piston  22 . 
     Means are provided for releasably locking the safety brake piston  22  with the safety brake spring  25  compressed, as shown in FIG.  2 . In this case these means include a spring ring  26 , shown both in FIG.  2  and in FIGS. 3A-C. This spring ring  26  is of the type with a bias towards an expanded position and may be held in a compressed position shown in FIGS.  2  and  3 A-C by for example any of the arrangements shown in FIGS. 3A-C. 
     The free ends of the spring ring  26  are slanted and cooperate with corresponding inner slanted surfaces in an operating block  27 , which in the alternative shown in FIG. 3A is axially movable by an arrangement including an electro-magnet  28 , a push rod  29 , and two levers  30  and  31  forming a toggle joint. 
     In the position shown in FIG. 3A, the electro-magnet  28  is energized, pushing the push rod  29  downwards and—through the cooperation with the toggle joint  30 ,  31 —the operating block  27  to the left in the drawing, so that the spring ring  26  is held compressed. Hereby the spring ring  26  will rest in a circumferential, external groove in the safety brake piston rod  24  but partly in a deeper, circumferential, internal groove in a tubular extension  15 ′ of the cylinder  15 . In this position the spring ring  26  locks the safety brake piston rod  24  and accordingly the safety brake piston  22  to the cylinder  15 . Hereby the brake actuator can work as a normal service brake actuator under the action of pressurized air on the service piston  16 . 
     When a safety braking is to be performed, for example if no pressurized air is available, the electro-magnet  28  is deenergized, so that the operating block  27  is free to move to the right in the drawing under the force from the spring ring  26 , which accordingly expands entirely into the groove in the cylinder extension  15 ′ leaving the safety brake piston rod  24  and the safety brake piston  22  freedom to move in relation to the cylinder  15 . As the actuator is mounted (to the vehicle underframe) by means of the attachments  23  on the safety brake piston  22 , the force from the spring  25  acts on the cylinder  15  and accordingly on the service piston  16  to the left in the drawing or in the brake application direction. 
     Two alternative designs for actuating the operating block  27  are shown in FIGS. 3B and C. In FIG. 3B the block  27  is directly connected to an electro-magnet or a pneumatic or hydraulic actuator  32 . In FIG. 3C the end of the operating block  27  is wedge-shaped and cooperates with a wedge  33  connected to an electro-magnet or a pneumatic or hydraulic actuator  32 . 
     When the safety brake has been used and the arrangement is to be returned to the position shown in FIG. 2 ready for new action, pressurized air is admitted through the inlet  17 , moving the cylinder  15  to the right in the drawing (against the force of the safety brake spring  25 ). When the grooves in the safety brake piston rod  24  and the cylinder extension  15 ′ are opposite each other, the electro-magnet  28  or the actuator  32  can be energized, so that the spring ring  26  is locked in the shown position. 
     FIG. 4 illustrates a variation of the embodiment of FIG. 2, and only parts which differ from those in FIG. 2 are shown and described. 
     In this case the spring ring, which connects the tubular cylinder extension  15 ′ and the tubular brake piston rod  24  and which here has received the numeral  35 , is of the type with a bias towards the compressed position and may be held in an expanded position shown in FIGS. 4-6 by an arrangement to be described. 
     The free ends of the spring ring  35  are chamfered and cooperate with corresponding chamfered surfaces or a wedge of an operating member  36 , which is axially movably arranged in the brake piston rod  24 . The end of this operating member  36  is wedge-shaped and cooperates with a wedge  37  actuated by an electro-magnet or a pneumatic or hydraulic actuator  38  in the same way as has been described with reference to FIGS.  2  and  3 A-C. 
     The embodiment shown in FIGS. 7 and 8 has similarities with the embodiment according to FIG. 1 but is more specifically intended for automotive use. Especially, the pistons thereof are of the diaphragm type. 
     Two deep-drawn cylinder halves  40  and  41  held together by a band  42  form a brake cylinder, as is well known in the art. The first cylinder half  40  is provided with screws  43  for the mounting of the brake cylinder to a suitable part of a vehicle underframe. 
     A service brake piston  44  and a safety brake piston  45 , both of the diaphragm type, are clamped between the two cylinder halves  40  and  41  together with an intermediary ring  46 , which also provides access for pressurized air through an inlet  46 ′ therefore, by means of the band  42 . 
     The service brake piston  44  has a piston rod  47  with a disc  48  engaging the diaphragm piston  44 . This piston rod  47  may be provided with a suitable slack adjuster, but in most cases the slack adjusting function is performed elsewhere in the brake arrangement. A return spring  49  of the compression type is arranged between the cylinder half  40  and the disc  48 . 
     The safety brake piston  45  has a tubular piston rod  50  and a disc  51  engaging the diaphragm piston  45 . A powerful safety brake spring  52  of compression type is arranged around the piston rod  50 , between the second cylinder half  41  and the disc  51 . 
     The second cylinder half  41  is provided with an extension or sleeve  53  extending into the tubular piston rod  50 . Means are provided for releasably locking the safety brake piston  45 , i e the tubular piston rod  50 , to the second cylinder half  41 , i e the extension  53 , with the spring  52  compressed. In this case these means include a spring ring  54 , which is of the type with a bias towards a compressed position and may be held in an expanded position by means not shown in detail in FIG. 7 but for example corresponding to those shown in FIGS. 4 and 6 and described above. Only an actuator  55  for this purpose is shown in FIG.  7 . 
     The function of the embodiment shown in FIG. 7 should be clear from the description of the previous embodiments. 
     FIGS. 9 and 10 show a safety brake arrangement according to the invention applied to an electro-mechanical block brake actuator. This electro-mechanical service brake is already known and will not be dealt with in detail, although it is comparatively complicated. 
     In this case the service brake force is generated by an electric motor  60 , which is mounted on a cylinder  62  together with a brake control clutch unit  61 . A rotary shaft from the unit  61  is provided with a small gear wheel  63  engaging a larger gear wheel  64  on a spindle  65 . This spindle  65  is journalled in the cylinder  62  by means of a bearing  66  and an intermediary member  67  (to be described). 
     The rotary motion in a brake applying direction of the spindle  65  is transformed into an axial movement to the left in the drawing of a push rod  68  connected to a brake block  69  for braking engagement with a wheel  70 . The mechanism for accomplishing the transformation from rotary to axial movement is not shown but may for example be a conventional ball screw. 
     When the brake force has attained a certain value, a reaction force is sensed in an elastic ring  71  arranged between the cylinder  62  and the intermediary member  67 . This force is sensed by a force transducer  72 , which may emit a signal for terminating the rotation of the electric motor  60  and thus the service brake application. 
     The actuator according to FIG. 9 is provided with a safety brake arrangement of the same general kind as the actuator according to FIG. 2. A safety brake piston  73  is axially movable in the cylinder  62 . It is provided with a tubular safety brake piston rod  74  extending forward around the push rod  68  and guided in a cylinder extension  62 ′. A powerful safety brake spring  75  is arranged between the cylinder  62  and the safety brake piston  73 . The safety brake piston  73  is provided with attachments  76  for mounting of the brake actuator for example to the underframe of the vehicle; these attachments extend out through axial slits in the cylinder  62 . 
     Means are provided for releasably locking the safety brake piston  73  with the safety brake spring  75  compressed, as shown in FIG.  9 . In the same way as with the arrangement shown in FIGS. 2 and 3A and described with reference thereto, these means include an expanding spring ring  77 , cooperating over slanted surfaces with an operating block  78 . The arrangement also includes an electro-magnet  79 , a push rod  80 , and two levers  81  and  82  forming a toggle joint. 
     The operation of the embodiment according to FIGS. 9 and 10 corresponds so closely to that of the embodiment according to FIGS. 2 and 3A that reference is made to the description above thereof. 
     It is to be noted that variations as shown in FIGS. 3B and C and in FIG. 4 may also be used for the embodiment according to FIG.  9 . 
     In FIG. 11 a hydraulic disc brake actuator with a safety brake arrangement according to the invention is shown. 
     A caliper frame  85  is arranged around a rotating disc  86  to be braked. A first brake pad  87  is mounted in the frame  85 , whereas a second brake pad  88  is mounted at the end of a hydraulic service brake piston  89 . 
     The hydraulic brake piston  89  is axially movable substantially perpendicular to the brake disc  86  in an axial bore in a caliper housing  90 . The caliper frame  85  and the caliper housing  90  are connected to each other by means of safety brake arrangements to be described. At service braking they may be regarded as a unit, whereas they are axially movable in relation to each other at safety braking. 
     Hydraulic fluid under pressure is admitted to the piston  89  through an inlet  91  at service braking. 
     The hydraulic service piston  89  is provided with a slack adjuster arrangement, which does not form any part of the invention and will only be briefly described without reference to its function. A non-rotatable spindle  92  is elastically mounted to the caliper housing  90 . A tubular adjuster nut  93  is in non-self-locking thread engagement with the spindle  92 . The end of the adjuster nut  93  is in releasable clutch engagement with the hydraulic piston  89 . The adjuster nut  93  is urged into this engagement by means of a compression spring  94  between a piston shoulder and an adjuster nut bearing  95 . 
     As already stated, a safety brake arrangement now to be described connects the caliper frame  85  and the caliper housing  90 . In the shown case the arrangement contains two identical devices, and for the sake of simplicity only one such device—the one to the right in FIG.  11 —is provided with reference numerals. 
     The caliper frame  85  forms a sleeve  96  with an inwardly directed flange  97 , whereas the caliper housing  90  has a tube  98 , which extends into the sleeve  96  and has an outwardly directed flange  99 . A powerful safety brake spring  100  of compression type is arranged between the flanges  97  and  99 , the arrangement thus being such that the spring bias tends to push the two members (caliper and housing) together and applying brake force from the brake pads  87  and  88  on the brake disc 
     The tube  98  is provided with internal threads for cooperation with external threads  101  on a locking rod  102 , which is rotatably arranged in the the caliper frame  85 . 
     Towards its lower end in the drawing the locking rod  102  is splined, and a washer  103  with internal teeth is axially movable on this splined end of the locking rod  102 . A powerful electro-magnet  104  is arranged in the caliper housing  90  close to the washer  103 . When energized the electro-magnet  104  attracts the washer  103  and accordingly locks the locking rod  102  against rotation, so that the spring  100  is held in its shown, compressed condition ready for safety braking, if and when the electro-magnet  104  is deenergized. 
     Also FIG. 12 shows a disc brake actuator, which—like the actuator shown in FIG.  9 —contains a service brake of an electro-mechanical type. However, the general construction of the actuator has similarities with that of the disc brake actuator of FIG.  11 . 
     A caliper frame  110  is arranged around a rotating disc  111  to be braked. A first brake pad  112  is mounted in the frame  110 , whereas a second brake pad  113  is mounted at the end of a force transmitting member  114 . 
     This force transmitting member  114  is axially movable perpendicular to the brake disc  111  in an axial bore in a caliper housing  115 . The caliper frame  110  and the caliper housing  115  are connected to each other by means of safety brake arrangements to be described. At service braking they may be regarded as a unit, whereas they are axially movable in relation to each other at safety braking. 
     The electro-mechanical service brake arrangement, which will now be briefly described and which does not per se form any part of the invention, is placed entirely in the caliper housing  115 . The arrangement has already been shown and disclosed in EP-B-0 334 435, and for further explanations reference is made to this publication, especially the embodiment according to FIG. 3 therein. 
     The power source for the service brake is an electric motor  116 , the shaft  117  of which being journalled for rotation in the housing  115 . From this shaft  117  the rotation is transmitted via gearings to a brake control clutch unit  118 , which also is journalled for rotation in the housing  115  in parallel with the motor shaft  117 . 
     The rotation of the brake control clutch unit  118  is ultimately transmitted to a gear wheel  119  on a spindle  120  via an intermediate gear wheel  121  rotatably journalled in the housing  115 . This spindle  120  is journalled in the housing  115  by means of a bearing  122  and an intermediary member  123  (to be described). 
     The rotatable spindle  120  is in thread engagement with the force transmitting member  114 , which is guided for axial movements only in its bore in the housing  115 . This means that rotational movements of the spindle  120  will be transformed into axial movements of the force transmitting member  114 , so that the brake pad  113  will be brought into braking engagement with the rotating brake disc  111 . 
     Other means of transforming the rotating movement into an axial movement are feasible. 
     When the brake force has attained a certain value, a reaction force is sensed in a elastic ring  124  arranged between the housing  115  and the intermediary member  123 . This force is sensed by a force transducer  125 , which may emit a signal for terminating the rotation of the electric motor  116  and thus the service brake application. 
     The force transducer  125  may be mounted on a control card  101  arranged under a cover  127  on the housing  115 . Also mounted on the control card is a position detector  128  for the motor shaft  117 . The control card  126  may contain all electronic equipment for control of the brake actuator. 
     As already stated, a safety brake arrangement now to be described connects the caliper frame  110  and the caliper housing  115 . This arrangement contains in this case two identical devices, but only one such device is visible in the section chosen in FIG. 12, where the service brake arrangement is placed “above” the second safety brake device. It should be noted that the number of safety brake devices can vary depending on the circumstances. 
     The caliper frame  110  forms a sleeve  129  with an inwardly directed flange  130 , whereas the caliper housing  115  has a tube  131 , which extends into the sleeve  129  and has an outwardly directed flange  132 . A powerful safety brake spring  133  of compression type is arranged between the flanges  130  and  132 , the arrangement thus being such that the spring bias tends to push the two members (caliper and housing) together and applying brake force from the brake pads  112  and  113  on the brake disc  111 . 
     The tube  131  is provided with internal threads for cooperation with external threads  134  on a locking rod  135 , which is rotatably arranged in the caliper frame  110 . 
     Towards its lower end in the drawing the locking rod  135  is splined, and a washer  136  with internal teeth is axially movable on this splined end of the locking rod  135 . A powerful electro-magnet  137  is arranged in the caliper housing  115  close to the washer  136 . When energized the electro-magnet  137  attracts the washer  103  and accordingly locks the locking rod  135  against rotation, so that the spring  133  is held in its shown, compressed condition ready for safety braking, if and when the electro-magnet  137  is deenergized. 
     The fourth embodiment of a block brake actuator according to the invention as shown in FIG. 13 has obvious similarities with the first embodiment shown in FIG.  1 . 
     A cylinder  150  is provided with attachments  151 . A service piston  152  and a spring brake or safety brake piston  153  are axially movable therein. 
     The service piston  152  is connected to a push rod  154 , preferably comprising a mechanical slack adjuster of any type, and further to a brake block  155  for braking engagement with a wheel  156  to be braked. 
     The safety brake piston  153  is acted on in the brake application direction by a powerful safety brake spring  156  of compression type. An inlet  157  for pressurized air is arranged on the cylinder  150  between the two pistons  152  and  153 . 
     Means are provided for releasably locking the safety brake piston  153  with the safety brake spring  156  compressed, when compressed air has been admitted through the inlet  157 . A locking rod  158  is rotatably attached to the safety brake piston  153  and in thread engagement with a thread in the cylinder  150 . A locking spring  159  is arranged around the locking rod  158 , and its end to the left in the drawing is connected to the rod  158  (having an axial groove for this purpose). Its other end is in a similar manner connected to a washer  160  in the vicinity of a powerful electro-magnet  161 , fixed in the cylinder  150 . 
     The nominal inner diameter of the locking spring  159  corresponds to the outer diameter of the locking rod  158 , so that when the electro-magnet  161  is deenergized and the washer  160  is left free, the locking rod  158 , the locking spring  159  and the washer  160  are free to rotate as a unit. 
     When on the other hand the electro-magnet  161  is energized and locks the washer  160  to it, a minor further rotation of the locking rod  158  (under the action of the safety brake spring  156 ) in the opening direction of the locking spring  159  will effect a locking against the inner periphery of the cylinder bore in which the locking rod  158  is arranged. The locking will be effected with a servo action. 
     FIG. 14 is a section through a practical and modified version of the embodiment according to FIG. 12, the portion shown being the one to the right in FIG.  12 . 
     The general design and function of this embodiment will not be repeated for the FIG. 14 version, reference instead being made to the description of FIG.  12 . 
     The caliper frame sleeve has received the reference numeral  170  in FIG. 14, the caliper housing  171 , the housing cover  172  and the caliper housing tube  173 . Safety brake springs  174  are arranged between a sleeve ring  170 ′ and the end of the tube  173 . 
     A locking rod  175  is journalled in the caliper frame by means of an axial bearing  176 . By means of external threads  177  the locking rod  175  is in thread engagement with internal threads in the tube  173 . A sleeve-shaped end of the locking rod  175  extending downwards in the drawing into the tube  173  is circumferentially slitted to form a locking spring  175 ′ with an outer diameter slightly smaller than the inner diameter of the tube  173 . 
     The lower end of the locking spring  175 ′ is extended into two fingers  175 ″, which are in engagement with corresponding notches or the like in the upper end of a locking sleeve  178 . 
     The lower end of the locking sleeve  178  in the form of a washer bears on an electro-magnet  179 , attached to the housing cover  172 . When energized, the electro-magnet  179  will hold the locking sleeve  178  against rotation. 
     The locking rod  175  is under a certain bias upwards in the drawing by means of a compression spring  180  arranged inside the locking spring  175 ′ and the locking sleeve  178 . This spring  180  is arranged between a flange on a spring sleeve  181 , bearing against the housing cover  172 , and a flange on a spring guide rod  182 , which is axially movable within the spring sleeve  181 . The rod  182  has a pointed end in engagement with a corresponding cup in the locking rod  175  for allowing free mutal rotational movements. The purpose of the spring bias on the locking rod  175  is to ensure a proper position of the locking rod relative to the caliper frame. 
     When the electro-magnet  179  is deenergized, the locking sleeve  178  and the locking rod  175  with its locking spring  175 ′ will be free to rotate (under the action of the expanding safety brake springs  174 ). When on the other hand the electro-magnet  179  is energized, the locking spring  175 ′ will be expanded into firm and locking engagement with the tube  173  by a small rotation under the action of the safety brake springs  174 . A locking servo action is accordingly obtained by the use of the locking design with the locking spring  175 ′. 
     It will be understood that modifications of different kinds are possible to all embodiments. Especially, pneumatic actuation can be replaced by hydraulic actuation, and vice versa. When reference has been made to block braking, it can in most cases be replaced by disc braking, and vice versa. Only slight modifications are necessary to replace a spring ring with a bias to expand with one with a bias to compress, as will be readily understood by anyone skilled in the art.