Patent Publication Number: US-2020300319-A1

Title: Silent electromagnetic brake

Description:
FIELD OF THE INVENTION 
     The present invention relates to a silent electromagnetic brake which can be used in a hoisting machine of an elevator, for example. 
     RELATED BACKGROUND ART 
       FIG. 8  shows a cross-sectional view of an electromagnetic brake  101  for a hoisting machine of an elevator according to the prior art. This electromagnetic brake  101  comprises an electromagnet having a coil  105  provided in a flange  102 , and a spring  106  arranged in the center of the coil  105 . The spring  106  presses a brake pad  107  against a braking surface (not shown) which is part of a hoisting motor, for example. The brake pad  107  is mounted to an armature  103  which is movable with respect to the flange  102  along a shaft  108  mounted to the flange  102 . In the state of the brake pad  107  being pressed against the braking surface, there is an air gap  104  between the flange  102  and the armature  103 . The electromagnet can provide an electromagnetic force when the coil  105  is excited so as to attract the armature  103  against the force of the spring  106 . As a result, the brake  101  is opened with the air gap  104  becoming smaller. Finally, the armature  103  will abut on the flange  102  with the air gap  104  disappearing. The abutting action of the armature  103  on the flange  102  generates a noise which can be noticed by persons travelling in the elevator as disturbing. 
     One means for reducing this noise is to provide a damper spring which ensures a slower motion of the armature  103  resulting in that the abutting action of the armature  103  generates less noise. However, the provision of the damper spring requires that the magnetic force overcomes a higher spring force for opening the brake, i.e. the force of the damper spring in addition to force of the spring  106 . 
     Thus, there is a need to provide an electromagnetic brake which can be operated without generating the above explained noise. 
     According to the present invention, the above need is achieved with an electromagnetic brake having the features of claim  1 . 
     An electromagnetic brake according to the invention comprises an electromagnet arranged about a central axis; a slide movable with respect to the electromagnet in a slide actuation direction upon actuation of the electromagnet, and an air gap provided between the electromagnet and the slide through which a magnetic field passes when the electromagnet is actuated. The slide is arranged radially inside or outside of the electromagnet with respect to the central axis. 
     The above-mentioned central axis can be understood as being the central axis of the electromagnet. 
     Due to this arrangement, the electromagnet is arranged about a central axis. For example, the electromagnet can be ring-shaped about the central axis. However, the shape of the electromagnet is not limited to the ring shape and can have any shape which has a center such as a parallelogram, elliptic, a polygon and so on. Thus, the air gap, which is formed between the electromagnet and the slide, can extend in a direction substantially parallel to the central axis. As a result, when the slide moves in the slide actuation direction, the distance between the slide and the electromagnet with respect to a direction perpendicular to the center axis remains constant. As a result, although the slide can move, the slide will never abut on the electromagnet in a manner explained above with reference to  FIG. 8  such that a noise accompanied by the abutment of the slide with the electromagnet can be avoided. Also, the electromagnetic brake can be configured in a manner that the slide will not abut on other structural members of the electromagnetic brake. Hence, an electromagnetic brake which is silent by nature can be obtained. 
     In one embodiment, the slide is arranged radially inside the electromagnet and the electromagnet is arranged radially outside the slide. Alternatively, the slide can be arranged radially outside the electromagnet and the electromagnet can be arranged radially inside the slide. 
     Preferably, the slide comprises ferromagnetic material, such as iron, for example. This allows the slide or a part of the slide being used as a part of a magnetic circuit of the electromagnet. 
     Preferably, the electromagnet and the slide are ring-shaped, respectively. This allows a simple configuration which is easy to manufacture. 
     Preferably, the electromagnet comprises a coil wound about the central axis and adapted to generate a magnetic field upon excitation. Also preferably, the electromagnetic brake comprises magnetic field adaptation means for adapting the magnetic field so as to generate a magnetic force acting on the slide in the slide actuation direction. 
     The magnetic field adaptation means adapts the magnetic field in a manner that the slide can be moved with respect to the electromagnet in a manner not moving the electromagnet but in parallel to the electromagnet. 
     Preferably, the magnetic field adaptation means comprises at least one electromagnet projection projecting from the electromagnet into the air gap, and at least one slide projection projecting from the slide into the air gap. The provision of the projections allows to have a suitable structure for providing the magnetic field adaptation means which can be easily manufactured. 
     Preferably, the electromagnet projection and the slide projection are offset from each other in the slide actuation direction in the state of the electromagnet not being actuated. Since the projections are offset from each other in the state of the electromagnet not being actuated, when the magnetic field is generated due to actuation of the electromagnet, electromagnetic forces are generated between the projections which are offset from each other such that the projection of the slide will be attracted by the projection of the electromagnet with a magnetic force corresponding to the amount of electric current supplied to the electromagnet. As a result, the slide can move in the slide actuation direction. 
     Preferably, the electromagnet projection comprises an inclined surface on the side facing in the slide actuation direction, and the slide projection comprises an inclined surface on the side facing opposite to the slide actuation direction. The inclined surfaces are inclined with respect to the direction perpendicular to the slide actuation direction. The inclined surfaces improve the adaptation of the magnetic field in a manner to more suitable generate the magnetic forces which make the projections attract each other. 
     Preferably, a permanent magnet is provided parallel to the electromagnet. The permanent magnet provides a permanent magnetic field which combines with the magnetic field of the electromagnet when the latter is actuated. The permanent magnet decreases the needed magnetization/actuation of the electromagnet for making the slide move. Hence, the permanent magnet ensures a magnetic force which is sufficient for moving the slide. As a result, the size of the coil of the electromagnet can be reduced such that the electromagnetic brake can be manufactured at lower cost. 
     Preferably, the electromagnetic brake comprises guiding means for guiding the slide in the slide actuation direction. The guiding means secure that the slide moves along the slide actuation direction in a manner without decreasing the distance between the slide and the electromagnet with respect to the direction perpendicular to the center axis. As a result, it can be ensured that the slide will never come into contact with the electromagnet at any position. This ensures the natural noiselessness of the electromagnetic brake. 
     Preferably, the electromagnetic brake further comprises at least one spring for urging the slide opposite to the slide actuation direction. The at least one spring allows to bring the slide either into the position of the brake being closed or into the position of the brake being opened. 
     Preferably, the electromagnetic brake comprises a friction member connected to the slide and being adapted to be pressed against a braking surface. Preferably, the slide actuation direction is the direction in which the slide is moved so as to press the friction member against the braking surface. This allows to have a spring loaded electromagnetic brake, which is closed when the electromagnet is not activated. Alternatively, the slide actuation direction is the direction in which the slide is moved so as to move the friction member away from the braking surface. This allows to have a spring loaded electromagnetic brake, which is opened when the electromagnet is not activated. 
     Preferably, the electromagnetic brake is adapted to be applied to a transportation system such as an elevator, an escalator or a moving walkway. Preferably, the electromagnetic brake is adapted to be applied to a hoisting machine, wherein the electromagnetic brake may be attached to a framework of the hoisting machine to brake a shaft and/or a traction sheave of the hoisting machine. However, the electromagnetic brake is not limited to these applications and it can be applied in any technical field in which it is desired to an electromagnetic brake which is silent by nature. 
     Preferably, the slide comprises a plurality of blind holes each for receiving one of a plurality of springs. 
     Preferably, the slide comprises a plate-shaped slide portion and a spring is coaxially provided with respect to the electromagnet so as to urge the plate-shaped slide portion opposite to the slide actuation direction. 
     Preferably, at least two sets of an electromagnet and a slides are provided. In this case, each electromagnet is arranged about its own central axis such that the electromagnetic brake comprises as many central axis as sets of electromagnet and slide. 
    
    
     
       DESCRIPTION OF THE EMBODIMENTS 
       These and other objects, features and advantages will become more fully apparent from the following detailed description of embodiments of the present invention which is to be taken in conjunction with the appended drawings, in which: 
         FIG. 1  is a schematic cross-sectional view of an electromagnetic brake according to a first embodiment of the invention, 
         FIG. 2  shows the magnetic field when the coil is not excited, 
         FIG. 3  shows the magnetic field when the coil is excited, 
         FIG. 4  is a chart showing the relation between a moving position of the slide and forces of a spring and an electromagnet, respectively, and 
         FIG. 5  is a schematic cross-sectional view of an electromagnetic brake according to a second embodiment of the invention. 
         FIG. 6A  is a schematic cross-sectional view of an electromagnetic brake according to a third embodiment of the invention in a state where the brake is engaged. 
         FIG. 6B  is a schematic cross-sectional view of the electromagnetic brake according to the third embodiment of the invention in a state where the brake is open. 
         FIG. 7  is a schematic cross-sectional view of an electromagnetic brake according to a fourth embodiment of the invention. 
         FIG. 8  is a schematic cross-sectional view of a conventional electromagnetic brake. 
     
    
    
     FIRST EMBODIMENT 
     An electromagnetic brake  1  according to the first embodiment of the invention, as is shown in  FIG. 1 , comprises a ring-shaped flange  2  made from steel such as S 355 , a ring-shaped coil  5  provided inside the flange  2 , and a slide  3  provided radially inside of the flange  2 . 
     The flange  2  is mounted to a support  12 , for example a housing of the electromagnetic brake  1 , which can be mounted to a hoisting machine (not shown) of an elevator, for example. The flange  2  is ring-shaped about a central axis SA. 
     The coil  5  is placed inside the flange  2  and is also ring-shaped about the central axis SA. The coil  5  comprises windings (not shown) which are oriented on a substantially circular path about the central axis SA. Since the coil  5  is the main constituent member of the electromagnet, the electromagnet according to the present invention is arranged about the central axis SA. In other words, the central axis is defined by the coil  5  and thus by the electromagnet. 
     A ring-shaped permanent magnet  8  is provided inside the flange  2  at a position between the coil  5  and the slide  3  and thus parallel to the electromagnetic magnet  2 . 
     In this embodiment, the flange  2  is constituted by an upper part  2   a  and a lower part  2   b  between which the coil  5  and that permanent magnet  8  are provided. 
     The slide  3  has a ring-shaped slide portion  3   a  and a plate-shaped slide portion  3   b,  which is connected to a bottom portion of the ring-shaped flange portion  3   a.  The slide  3  is substantially symmetric with respect to the center axis SA. 
     A friction pad  10  is mounted to the bottom portion of the plate-shaped slide portion  3   b.  The friction pad  10  can be brought into contact with a braking surface of a brake disc  11  when the slide  3  is moved downwardly in  FIG. 1  and thus towards the brake disc  11 . 
     A plurality of springs  9  are provided between the support  12  and the slide  3  so as to urge the slide  3  towards the brake disc  11 . The springs  9  can be coil springs and can be evenly distributed along the circumferential extension of the slide  3 . Preferably, four springs  9  are provided but the number is not limited to four. 
     Projections  7  are formed on a radially outer side of the ring-shaped slide portion  3   a.  Each of the projections  7  is ring-shaped and has a cross-sectional shape with an upper straight line perpendicular to the center axis SA, an outer straight line parallel to the center axis SA and a lower straight line extending in an inclined manner downwardly towards the ring-shaped slide portion  3   a.  The lower straight line in the cross-sectional view corresponds to an inclined surface  7   a  of the projection  7  which is inclined with respect to the direction perpendicular to the slide actuation direction. The upper straight line and the radially outer straight line correspond to ring-shaped surfaces of the projections  7 . In the first embodiment, the slide  3  comprises ten projections  7 . 
     Thus, a recess  15  is formed between two adjacent projections  7 . With ten projections  7 , the slide  3  comprises nine recesses. The upper four recesses  15  and the lower four recesses  15  have the same depth in the direction perpendicular to the center axis SA. A middle recess  16  is formed between the two projections  7  which are placed in the middle part of the slide  3  with respect to the direction of the center axis SA. In other words, when numbering the projections  7  from the bottom side to the top side of the slide  3 , the middle recess  16  is formed between the projections  7  with numbers five and six, and is deeper that the other recesses  15  with respect to the direction perpendicular to the center axis SA and larger than the other recesses  15  in the direction of the center axis SA. Further, the middle recess  16  is longer than the other recesses in the direction of the center axis SA. 
     Projections  6  are formed on a radially inner side of the flange  2 . Each of the projections  6  is ring-shaped and has a cross-sectional shape with a lower straight line perpendicular to the center axis SA, an outer straight line parallel to the center axis SA and an upper straight line extending in an inclined manner upwardly towards the flange  2 . The upper straight line in the cross-sectional view corresponds to an inclined surface  6   a  of the projection  6  which is inclined with respect to the direction perpendicular to the slide actuation direction. The lower straight line and the radially inner straight line correspond to ring-shaped surfaces of the projections  7 . In the first embodiment, the flange  2  comprises ten projections  6 . 
     Thus, a recess  13  is formed between two adjacent projections  6 . With ten projections  6 , the flange  2  comprises nine recesses. The upper four recesses  13  and the lower four recesses  13  have the same depth in the direction perpendicular to the center axis SA. A middle recess  14  is formed between the two projections  6  which are placed in the middle part of the flange  2  with respect to the direction of the center axis SA. In other words, when numbering the projections  6  from the bottom side to the top side of the flange  2 , the middle recess  14  is formed between the projections  6  with numbers five and six, and is deeper than the other recesses  13  in the direction perpendicular to the center axis SA. Further, the middle recess  14  is deeper longer than the other recesses  13  in the direction of the center axis SA. The bottom of the middle recess  14  of the flange  2  is formed by the radially inner surface of the permanent magnet  8 . 
     An air gap  4  is provided between the radial inner side of the flange  2  and the radial outer side of the slide  3 . In the cross-sectional view of  FIG. 1 , the air gap substantially extends in the direction parallel to the center axis SA. In fact, the air gap  4  is defined by the surfaces of the projections  6  of the flange  2 , the surfaces of the recesses  13 ,  14  between these projections  6 , the surfaces of the projections  7  and the surfaces of the recesses  15 ,  16  between these projections  7 . Hence, the length of the air gap  4  in the direction perpendicular to the center axis SA is larger at a first section perpendicular to the center axis SA, which goes through a recess  13  of the flange and a recess  15  of the slide  3 , than that at a second section perpendicular to the center axis SA, which goes through a projection  6  of the flange  2  and a projection  7  of the slide  3 . 
       FIG. 2  shows the magnetic field in the flange  2  and the ring-shaped portion of the slide  3  in the state of the coil  5  not being excited. In this state, the magnetic field is generated only by the permanent magnet  8  and the magnetic field is going around the coil  5 . Hence, the magnetic flux density in the region of the air gap is as small that it does not generate a magnetic force which would overcome the spring force of the spring  9 . As a result, the electromagnetic brake  1  is in the closed state in which the friction pad  10  is pressed against the braking surface of the disc  11  by the spring force of the spring  9 . In this closed position, the slide  3  is offset from the flange  2  in the direction of the center axis SA, towards the braking surface of the brake disc, such that the projections  7  of the slide  3  are accordingly offset from the projections  6  of the flange  2 . 
     When the coil  5  is excited, the magnetic field of the permanent magnet  8  and that of the excited coil  5  go through the ring-shaped portion of the slide  3 . As is shown in  FIG. 3 , the magnetic flux density is especially high in the region where the projections  6  and  7  are close to each other where a portion of the air gap  4  formed between the flange  2  and the slide  3  is narrow. The magnetic field generated in the projections  6  and  7  generate attraction forces between the flange  2  and the slide  3  which have a force component in a slide actuation direction which is directed away from the brake disk  11 . Since the slide  3  is guided by guiding means to slide in the slide actuation direction, the slide  3  is hindered from being moved in the direction perpendicular to the slide actuation direction. The guiding means can be guiding rods on which the slide  3  is mounted via sliding bearings, for example. 
       FIG. 4  shows a spring force curve  20  and magnet force curves  21  to  24  which curves indicate relations between the respective forces (vertical axis) and the associated moving position (horizontal axis) of the slide  3 . The moving position −2.00 mm corresponds to the position in which the slide  3  is offset with respect to the flange  2  to the largest amount in a manner that the projections do not overlap each other at all. In the present embodiment, the brake is closed at moving position −1.25 mm. The moving position 0.00 mm corresponds to the position in which the brake  1  is fully open, i.e. projections  6  of the slide  3  are aligned with the projections  7  of the flange  2 . In this position, the slide  3  has been moved in the slide actuation direction away from the brake disc  11  to the largest amount. 
     The spring force curve  20  is substantially linear. The magnet force curves  21  to  24  are non-linear because the magnetic force is exponentially proportional to the air gap length in the magnetic circuit. The air gap length in the magnetic circuit changes when the position of the slide changes with the result that the offset between the projections  6 ,  7  decreases such that the projections  6 ,  7  align with each other to a larger extent. 
     Magnet force curve  21  shows the state that no current is supplied to the coil  5 . In this state, the current density (Finnish term “Virrantiheys” in  FIG. 4 ) is zero and the generated magnet force is substantially zero. 
     Magnet force curve  22  shows the state that the coil  5  is excited with a current density of 5 000 000 A/m 2 . The magnet force curve  22  intersects the spring force curve  20  at position −1.125 mm. At this position, the projections  6 ,  7  are aligned to a larger extent compared to the state of the coil  5  not being excited and the slide  3  being positioned at −2.00 mm. Hence, the air gap  4  has become smaller. 
     Magnet force curve  23  shows the state that the coil  5  is excited with a current density of 10 000 000 A/m 2 . The magnet force curve  23  intersects the spring force curve  20  at position −0.8 mm. At this position, the projections  6 ,  7  are aligned to a larger extent compared to the state of the coil  5  being excited with a current density of 5 000 000 A/m 2  and the slide  3  being positioned at −1.125 mm. Hence, the air gap  4  has become even smaller. 
     Magnet force curve  24  shows the state that the coil  5  is excited with a current density of 15 000 000 A/m 2 . The magnet force curve  24  intersects the spring force curve  20  at position −0.75 mm. At this position, the projections  6 ,  7  are aligned to larger extent compared to the state of the coil  5  being excited with a current density of 10 000 000 A/m 2  and the slide  3  being positioned at −0.8 mm. Hence, the air gap  4  has become even smaller. 
     Thus, it can be understood that the position of the slide  3  will be on the crossing point of the electromagnetic force corresponding to the applied current density and spring force, and by controlling the current density, i.e. the current supplied to the coil  5 , the slide  3  will move. During the movement of the slide  3 , the projections  6 ,  7  become more and more aligned with each other. As has been explained above, the air gap  4  is formed between the projections  6 ,  7  and the recesses  13 ,  14 ,  15 ,  16 , respectively. In the region where recesses  13 ,  14 ,  15 ,  16  oppose each other, the air gap is larger than in the region where the projections  6 ,  7  oppose each other. Hence, when the slide  3  is moving in the slide actuation direction, the proportion of the projections  6 ,  7  opposing each other with respect to proportion of the recesses  13 ,  14 ,  15 ,  16  opposing each other becomes larger. As a result, the air gap  4  becomes smaller when the slide  3  moves in the slide direction to open the brake  1 . 
     Second Embodiment 
       FIG. 5  shows an electromagnetic brake  101  according a second embodiment of the present invention. The members of this electromagnetic brake have been referenced with reference signs which are obtained by adding  100  to the reference signs used in the first embodiment. In the following, only the major differences with respect to the first embodiment as described. 
     In the second embodiment, the slide  103  is provided radially outside of the flange  102  and comprises blind holes  117  for receiving springs  109 . Although only two blind holes  117  are shown in  FIG. 5 , a plurality of blind holes  117  and a corresponding number of springs  109  can be provided over the circumferential extension of the slide  103 . Preferably, there are four blind holes  117  and four springs  109  equally distributed over the circumferential extension. 
     Furthermore, in the present embodiment, the flange  102  is formed by an upper part  102   a  and a lower part  102   b  with the coil  105  and the permanent magnet  108  provided between these parts  102   a,    102   b.  In this embodiment, the upper part  102   a  is integrally formed with the support  112 . 
     Also, the distribution of the projections  106 ,  107  is different from that of the projections  6 ,  7  of the first embodiment. In the present embodiment, seven projections  106  having an inclined surface  106   a  are formed in the lower part  102   b  of the flange  102  and one projection  106  having an inclined surface  106   a  formed in the upper part  102   a.  A corresponding number of projections  107  is formed in the slide  103  below and above the recess  116 , respectively. Apart from that, the mode of operation is substantially the same as that of the first embodiment. 
     Third Embodiment 
       FIGS. 6A and 6B  show an electromagnetic brake  201  according a third embodiment of the present invention. The members of this electromagnetic brake have been referenced with reference signs which are obtained by adding  200  to the reference signs used in the first embodiment. In the following, only the major differences with respect to the first embodiment as described. 
     The electromagnetic brake  201  according to this embodiment has a more compact structure compared to that of the first embodiment. While the plate-shaped slide portion  3   b  according to the first embodiment has a central bore, the plate-shaped slide portion  203   b  according to the third embodiment does not have such a bore but is formed like a continuous plate. 
     Furthermore, the electromagnetic brake  201  according to this embodiment comprises only a single spring  209  which is provided between the support  212  and the plate-shaped slide portion  203   b  for urging the slide  203  away from the support  212 , i.e. opposite to the slide actuation direction. This single spring  209  is provided substantially coaxial to the ring-shaped slide portion  203   a.  In other words, the central axis of the spring  209  corresponds to the central axis of the slide and that of the electromagnet. 
     Also, the electromagnetic brake  201  according to this embodiment is configured to apply a braking force to the outer circumferential surface of a traction sheave of a hoisting machinery. For this reason, the friction pad  210  has a curved surface on the side facing away from the plate-shaped slide portion  203   b  with a curvature corresponding to the curvature of the outer circumferential surface of the traction sheave of the hoisting machinery. 
     Fourth Embodiment 
       FIG. 7  shows an electromagnetic brake  301  according a third embodiment of the present invention. The members of this electromagnetic brake have been referenced with reference signs which are obtained by adding  300  to the reference signs used in the first embodiment. In the following, only the major differences with respect to the third embodiment as described. 
     The electromagnetic brake  301  according to this embodiment comprises two sets of a flange  302  and a slide  303  having the structures of those of the third embodiment, respectively. Furthermore, each of the slides  303  is urged by a single spring  312  as in the third embodiment. Other than the curved surface of the friction pad  210  of the third embodiment, each of the friction pads  310  of the fourth embodiment has a flat surface on the side facing away from the plate-shaped slide portion  303   b  since this electromagnetic brake  301  is applied to a brake disc  311 . 
     With the electromagnetic brakes according to the embodiments, the slide can be moved against the spring force of the spring while the air gap remains always between the slide and the flange. Hence, since the slide never comes into abutment with the flange, the noise generated by abutment between the flange and the armature as in the conventional electromagnet brake shown in  FIG. 8 , will not be generated. Thus, the electromagnetic brake according to the embodiments is silent by nature. 
     Dropping/closing of the electromagnetic brake is also silent by nature because the slide moves in proportion to the current density of the coil. Making reference to  FIG. 4 , the slide position can be set by the crossing points of the magnetic force curve and of the spring force curve assumed that there would be no friction losses and that the mass of the slide is considered negligible. Thus, by decreasing the current of the coil accordingly, the brake may be closed silently. 
     Furthermore, in the embodiments, there is also an air gap between the upper portion of the slide and the support. When the electromagnetic brake is operated, this air gap above the slide changes with in the direction of the center axis as can best be understood from  FIGS. 6A and 6B . However, this air gap can be dimensioned such that, when the electromagnetic brake is operated (see for example  FIG. 6B ), the slide does not abut the support. Also for this reason, the electromagnetic brake according to the embodiments is silent by nature. 
     Furthermore, the structure of the electromagnet brake according to the embodiments is simpler and thus requires less maintenance. 
     The present invention is not limited to the above embodiments and can be modified as follows. 
     The electromagnetic brake according to the embodiments is of the configuration in which the slide is urged by the spring against the brake disc into the closed position. However, the invention can be applied to an electromagnetic brake in which the slide is urged by a spring away from the brake disc into the opened position and the electromagnet is configured to move the slide against the spring force in a slide actuation direction towards the brake disc upon actuation of the electromagnet. 
     The electromagnetic brake according to the embodiments has the permanent magnet which is provided between the coil and the slide. The permanent magnet ensures an opening force of the electromagnet and allows to decrease the size of the coil. However, the permanent magnet can be omitted for example for applications which do not require a large brake force such that the spring force is smaller and thus the required magnetic force for opening the brake is smaller. Alternatively, the coil can be made larger in case the permanent magnet is omitted. 
     In the first, second and fourth embodiment, the invention is applied to a brake disc. However, the invention can be applied to other types of brakes which require that a friction pad is moved for providing a braking action such as the brake of the third embodiment. Hence, the configuration of the first, second and fourth embodiment can be applied to a brake in which a curved friction pad is applied to the circumference of a rope disc, and the configuration of the third embodiment can be applied to a brake disc. 
     In the embodiments, the inclined surfaces of the electromagnet projections and of the slide projections are straight surfaces. However, these surfaces may instead be curved surfaces.