Abstract:
The present invention provides a safety device for elevators, which belongs to the field of elevator safety technologies. The safety device for elevators includes a housing; a safety piece having a guide rail groove, the safety piece being disposed in the housing; and asymmetric active and counter wedges that are slidably disposed on the safety piece at both sides of the guide rail groove, respectively. Moreover, the device further includes a U-shaped elastic element and a blocking piece that are disposed on the safety piece. The safety device for elevators can provide a relatively stable arresting force, is reliable in repetitive work, achieves high safety, is relatively easy as well as fast and efficient in restoration, and is especially suitable for high-speed elevators.

Description:
PRIORITY 
       [0001]    This application claims priority to Chinese Patent Application No. CN201510564637.8, filed Sep. 8, 2015, and all the benefits accruing therefrom under 35 U.S.C. §119, the contents of which in its entirety are herein incorporated by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention belongs to the field of elevator safety technologies and relates to a safety device for elevators for decelerating or braking elevators. 
       BACKGROUND ART 
       [0003]    A safety device for elevators may also be referred to as a “safety arrester”, which is an indispensable component of an elevator to guarantee safe operation of the elevator. With increasing requirements on safety and reliability of the elevator, requirements on deceleration or braking performance of the safety device for elevators are also increased. 
         [0004]    The safety device for elevators is generally provided with a wedge, and in a normal operation of a common elevator, the wedge and a guide rail of the elevator are not in contact (there is a gap distance between the two), and in a deceleration or braking process, the arrestment similar to braking is caused by a frictional force between the wedge and the guide rail of the elevator, where the magnitude of the frictional force reflects the magnitude of an arresting force exerted on the guide rail. For example, when the elevator is in an abnormal state such as fast dropping, a speed limiter disposed in the elevator is used to judge whether a current dropping speed exceeds a predetermined speed value; if the current dropping speed exceeds the predetermined speed value, the speed limiter triggers an action, and further triggers a pulling transmission component of the elevator to act on the wedge of the safety device for elevators, so that a frictional force is generated between the wedge and the guide rail. The frictional force further pulls the wedge to move upward; therefore, the frictional force is increased rapidly, the wedge clamps the guide rail in a self-locking manner, and an elevator car stops moving, thus guaranteeing operation safety of the elevator. 
         [0005]    When classification is carried out according to wedge structures, safety devices for elevators can be classified as symmetric arresters and asymmetric arresters. The U.S. Pat. No. 481,965, which is entitled “Arrester Device for Elevators” and belongs to the prior art, discloses an asymmetric arrester device, including an active wedge and a counter wedge that are asymmetrically disposed on both sides of a guide rail. In a deceleration or braking process, a downward acting force is exerted on the counter wedge through an elastic force of multiple disc springs disposed above the counter wedge, thereby obtaining a desired stable frictional force (that is, an arresting force) that can arrest an elevator car. However, such an asymmetric arrester device has at least the following disadvantages: (1) the force value repeatability of the elastic force generated by the multiple disc springs is poor, and therefore, the working stability of the safety device is easily affected; (2) a force value of the elastic force that can be exerted by the multiple disc springs depends on the number of disc springs superposed, and due to restrictions such as space, the force value of the elastic force that can be generated by the disc springs is usually limited, and a braking effect on a high-speed elevator may be undesirable; (3) due to an excessively high stiffness and an excessively small deformation amount, the disc springs are extremely sensitive to wear of the wedge; as the wear of the wedge changes, the elastic force that is generated by the disc springs when the active wedge moves upward to a predetermined position decreases significantly, the desired frictional force (that is, the arresting force) is hard to achieve, and therefore, there exists a potential safety hazard. 
       SUMMARY OF THE INVENTION 
       [0006]    To solve one or more aspects of the foregoing problems, the present invention provides a safety device for elevators, including: a housing; a safety piece having a guide rail groove, the safety piece being disposed in the housing; asymmetric active and counter wedges that are slidably disposed on the safety piece at both sides of the guide rail groove, respectively; and 
         [0007]    the safety device for elevators further including a U-shaped elastic element and a blocking piece that are disposed on the safety piece; 
         [0008]    wherein a guide groove is disposed in the safety piece, the blocking piece is capable of moving approximately upward along the guide groove during at least part of a braking process, and the guide groove and the blocking piece are configured to be capable of stopping, during at least a restoration process, a pre-tightening force generated by the U-shaped elastic element from being transferred to the counter wedge; and 
         [0009]    a lower U-shaped end of the U-shaped elastic element fixedly acts on a lower end surface of the safety piece, and an upper U-shaped end of the U-shaped elastic element elastically acts on an upper end surface of the blocking piece, and transfers, through the blocking piece during the at least part of the braking process, at least part of an elastic force of the U-shaped elastic element to the counter wedge that interacts with a lower end surface of the blocking piece. 
         [0010]    Through the following detailed description with reference to the accompanying drawings, the foregoing features and operations of the present invention will become evident, and advantages of the present invention will also become more complete and clearer. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  is a 3D schematic structural front view of a safety device for elevators according to an embodiment of the present invention; 
           [0012]      FIG. 2  is a 3D schematic structural rear view of a safety device for elevators according to an embodiment of the present invention; 
           [0013]      FIG. 3  is a 3D schematic structural front view of a safety piece in the safety device for elevators of the embodiment shown in  FIG. 1 ; 
           [0014]      FIG. 4  is a 3D schematic structural top view of a safety piece in the safety device for elevators of the embodiment shown in  FIG. 1 ; 
           [0015]      FIG. 5  is a plot of acceleration vs. time of a safety device for elevators; and 
           [0016]      FIG. 6  is a plot of acceleration vs. friction coefficient of a safety device for elevators. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    The present invention will be described more completely with reference to the accompanying drawings. Exemplary embodiments of the present invention are shown in the accompanying drawings. However, the present invention may be implemented according to many different forms, and should not be construed as being limited to the embodiments illustrated herein. On the contrary, these embodiments are provided to make the disclosure of the present invention thorough and complete, and convey the conception of the present invention to those skilled in the art completely. In the accompanying drawings, same reference numerals refer to same elements or components, and therefore, the description thereof is omitted. 
         [0018]    Herein, the orientation terms: “upper”, “lower”, “front”, “rear”, “left” and “right” are defined in the directions shown in  FIG. 1 , where  FIG. 1  shows a 3D structural diagram, viewed approximately from the front, of a safety device for elevators in normal use according to the present application; it should be understood that, these directional terms are relative concepts, and they are used for relative description and clarity, and may change accordingly as the placement orientation of the safety device for elevators changes. 
         [0019]      FIG. 1  shows a 3D schematic structural front view of a safety device for elevators according to an embodiment of the present invention;  FIG. 2  shows a 3D schematic structural rear view of a safety device for elevators according to an embodiment of the present invention;  FIG. 3  shows a 3D schematic structural front view of a safety piece in the safety device for elevators of the embodiment shown in  FIG. 1 ; and  FIG. 4  shows a 3D schematic structural top view of a safety piece in the safety device for elevators of the embodiment shown in  FIG. 1 . In  FIG. 1  to  FIG. 4 , a movement direction of the elevator, that is, a direction of the guide rail, is defined as a z-axis direction, and a vertically upward direction is defined as a positive direction of the z-axis; a direction horizontally perpendicular to the guide rail is defined as an x-axis direction, and a horizontally rightward direction is defined as a positive direction of the x-axis; a direction horizontally perpendicular to the wedge is defined as a y-axis direction, and a direction perpendicularly pointing to the safety piece from the wedge is defined as a positive direction of the y-axis. 
         [0020]    Referring to  FIG. 1  and  FIG. 2 , a safety device  10  for elevators mainly includes a housing  110 , a safety piece  120 , an active wedge  130 , a counter wedge  140 , a U-shaped elastic element  150 , and a blocking piece  160 . The housing  110  is approximately set as a cuboid structure, and may be made of a high-strength material; the safety piece  120 , the active wedge  130 , the counter wedge  140 , the U-shaped elastic element  150 , the blocking piece  160 , and the like are disposed in an inner space of the housing  110 . 
         [0021]    The safety piece  120  is disposed in the housing  110  via a pin column  170  that is approximately disposed along the x-direction, and the movement of the safety piece  120  along the z-direction is limited by means of the pin column  170 . A spring  171  disposed on the pin column  170  is located between the housing  110  and the left side of the safety piece  120 , and can exert a pressure on a side surface of the left side of the safety piece  120 , thereby limiting the movement of the safety piece  120  along the x-direction. For a specific structure of the safety piece  120 , refer to  FIG. 3  and  FIG. 4 . A middle portion of the safety piece  120  is provided with a guide rail groove  121  along the z-direction, which is used to receive a guide rail of an elevator, and the guide rail groove  120  is correspondingly aligned with a notch of the housing  110 , so that in normal operation, the guide rail can move up and down freely with respect to the safety device  10  for elevators. 
         [0022]    Referring to  FIG. 1  and  FIG. 2  continuously, both sides of the guide rail groove  121  of the safety piece  120  are provided with the active wedge  130  and the counter wedge  140  respectively. In this embodiment, the active wedge  130  is disposed on the left side of the guide rail groove  121 , and the counter wedge  140  is disposed on the right side of the guide rail groove  121 . However, it should be understood that, by symmetrically transforming the structure of the safety piece  120  with respect to the guide rail groove  121 , the active wedge  130  and the counter wedge  140  may also be disposed on the right side and the left side of the guide rail groove  121  respectively. In this embodiment, the active wedge  130  and the counter wedge  140  are respectively disposed on slide rail grooves  124  and  123  that are on the left and right sides of the safety piece  120 , and the active wedge  130  and the counter wedge  140  may be provided rollers or similar elements respectively, so that under the effect of an external force, they can slide up and down along the slide rail grooves  124  and  123  respectively. Therefore, the active wedge  130  and the counter wedge  140  are movable wedges, and the arrangement of specific sliding structures thereof with respect to the safety piece  120  is not limited. 
         [0023]    It will be understood that, as the slide rail grooves  124  and  123  are integrally formed with the safety piece  120 , it is sure that the slide rail grooves  124  and  123  are completely fixed with respect to the safety piece  120 , and they can also be regarded as “fixed wedges” as opposed to the movable wedge. Moreover, in this embodiment, a left cover plate  125  and a right cover plate  126  (as shown in  FIG. 1 ) are further provided corresponding to the active wedge  130  and the counter wedge  140  respectively. The left cover plate  125  and the right cover plate  126  are specifically fixed on the safety piece  120  via bolts. The left cover plate  125  and the right cover plate  126  may also be regarded as a part of the “fixed wedges” respectively. 
         [0024]    In this embodiment, the active wedge  130  is a right-trapezoid block, and an xy cross section thereof is approximately a right trapezoid. As shown in  FIG. 1 , the active wedge  130  has an upper end surface  132 , and a friction surface  131  toward the guide rail (not shown in the figure) in the guide rail groove  121 , where a self-locking angle α, that is, a base angle of the trapezoid, is formed between a lower bottom surface and a trapezoid inclined surface on the left side. The self-locking angle α also reflects angle setting of an inclined surface where the slide rail groove  124  is located, that is, the slide rail groove  124  has an angle of inclination substantially the same as that of the trapezoid inclined surface (the inclined surface on the left side) of the active wedge  130 . In a braking process, the active wedge  130  moves upward along the slide rail groove  124 , and therefore the friction surface  131  moves leftwards to get closer to the guide rail in the guide rail groove  121 ; meanwhile, the active wedge  130  presses the slide rail groove  124  of the safety piece  120  leftwards, and the slide rail groove  124  exerts a rightward counter force on the active wedge  130 , that is, a positive pressure F exerted by the active wedge  130  on the guide rail is increased, thus increasing a frictional force. Therefore, in the braking process, the active wedge  130  has an effect of actively implementing braking, thus being referred to as an “active” wedge. 
         [0025]    In case of normal operation of the elevator (when the safety device  10  for elevators does not work), the active wedge  130  is located at a lowermost end and is in direct contact with the housing  110  (as shown in  FIG. 1 ), and upon detecting that the speed of an elevator car exceeds a predetermined value, a speed limiter of the elevator triggers a pulling transmission component of the elevator to pull the active wedge  130  to start to move upward. A travel distance of the active wedge  130  in the slide rail groove  124  is configurable, that is, a travel distance of the upward movement of the active wedge  130  is configurable, and may be configured by using the height of the active wedge  130  and/or the height of an inner top surface  128  of the safety piece  120  (as shown in  FIG. 3 ); when the active wedge  130  moves to an uppermost end, the upper end surface  132  of the active wedge  130  contacts the inner top surface  128  of the safety piece  120 , thus being blocked. In this case, an x-direction component of the force exerted by the safety piece  120  on the active wedge  130 , that is, the positive pressure F exerted by the active wedge  130  on the guide rail, substantially reaches a maximum value. 
         [0026]    Referring to  FIG. 1  continuously, the counter wedge  140  is an upside-down right-trapezoid block, and an xy cross section thereof is approximately an upside-down right trapezoid. As shown in  FIG. 1 , the counter wedge  140  also as a relatively wide upper end surface, a friction surface  141  toward the guide rail (not shown in the figure) of the guide rail groove  121 , and a lower bottom surface and a trapezoid inclined surface that are relatively narrow, where a self-locking angle β is formed between the upper end surface and the trapezoid inclined surface on the right side. The self-locking angle β also reflects angle setting of an inclined surface where the slide rail groove  123  is located, that is, the slide rail groove  123  has an angle of inclination substantially the same that of as the trapezoid inclined surface (the inclined surface on the right side) of the counter wedge  140 . Because the upper end surface of the counter wedge  140  is wider than the lower bottom surface, when the counter wedge  140  is driven to move upward under the effect of the frictional force with the guide rail, the friction surface  141  will move rightward to be away from the guide rail in the guide rail groove  121 , which therefore helps increase a distance between the friction surface  131  and the friction surface  141 , thereby facilitating reduction of the positive pressure F exerted by the friction surface on the guide rail. Therefore, in the braking process, when the active wedge  130  and the counter wedge  140  move upward simultaneously, the counter wedge  140  generates a counter effect with respect to the active wedge  130 , and therefore is referred to as a “counter” wedge. 
         [0027]    By setting the self-locking angle α of the active wedge  130  and the self-locking angle β of the counter wedge  140 , the distance between the two opposite friction surfaces  131  and  141  can be reduced when the active wedge  130  and the counter wedge  140  are moving upward simultaneously. Exemplarily, the self-locking angle α is set within a range of 5°-11°, the self-locking angle β is set within a range of 4°-10°, and the self-locking angle β is 0.5°-1.5° smaller than the self-locking angle α. In this way, even when the counter wedge  140  moves upward simultaneously with the active wedge  130 , the positive pressure F exerted by the two wedges on the guide rail still increases, realizing a self-locking effect. 
         [0028]    Referring to  FIG. 1  and  FIG. 2  continuously, a U-shaped surface of the U-shaped elastic element  150  is approximately vertically disposed, and a U-shape opening thereof faces towards a negative direction of the y-direction, so that at least the counter wedge  140  and the blocking piece  160  can be disposed within the U-shape opening of the U-shaped elastic element  150 . In this embodiment, above the counter wedge  140 , the safety piece  120  is correspondingly provided with a guide groove  122  (referring to  FIG. 3  and  FIG. 4 ) that is at least used to receive the blocking piece  160 . Specifically, left and right inner sides of the guide groove  122  are each provided with a guide rail groove  1221 , and left and right external sides of the blocking piece  160  are each correspondingly provided with a pin  163  that protrudes outward. In this way, machining is relatively easy to implement and the pin  163  is limited in the guide rail groove  1221  to slide along the guide rail groove  1221 . For example, when the counter wedge  140  acts upwardly on the lower end surface  162  of the blocking piece  160 , the blocking piece  160  can move upward, in the guide groove  122 , approximately simultaneously with the counter wedge  140 . An angle of inclination of the guide groove  122  may be set to be the same as the angle of inclination of the slide rail groove  123 , that is, having a same size as β; in this way, the U-shaped surface of the U-shaped elastic element  150  also has the same angle of inclination, that is, an angle of inclination with respect to the xy plane also has an approximately same size as β. 
         [0029]    A U-shaped bottom portion of the U-shaped elastic element  150  is disposed in the rear of the safety device  10  for elevators (as shown in  FIG. 2 ). The U-shaped opening end of the U-shaped elastic element  150  includes a lower U-shaped end  150   a  and an upper U-shaped end  150   b , the lower U-shaped end  150   a  fixedly acts on a lower end surface  129  of the safety piece  120 , and the upper U-shaped end  150   b  acts on an upper end surface  161  of the blocking piece  160 . Therefore, an inward contraction elastic force of the U-shaped elastic element  150  can be transferred to the counter wedge  140  through the blocking piece  160 . 
         [0030]    In the normal operation of the elevator, the counter wedge  140  falls at a lower position, the lower bottom surface of the counter wedge  140  may be seated on a support elastic element (which is not shown in the figure) that is located below the counter wedge  140  and between the counter wedge  140  and the safety piece  120 , and the upper end surface of the counter wedge  140  is in contact with the blocking piece  160 , but the counter wedge  140  substantially exerts no upward acting force on the blocking piece  160 . To relatively fixedly dispose the U-shaped elastic element  150  on the safety piece  120 , pre-tightening forces need to be respectively biased on the lower end surface  129  and the upper end surface  161  of the blocking piece  160  through the lower U-shaped end  150   a  and the upper U-shaped end  150   b  of the U-shaped elastic element  150 . Therefore, the “pre-tightening force” defines an elastic force generated when the U-shaped elastic element  150  is initially installed on the safety device  10 . 
         [0031]    In this embodiment, a bottom portion of the guide rail groove  1221  is provided with a blocking portion (not shown in  FIG. 3  and  FIG. 4 ). When the counter wedge  140  exerts no acting force upwardly, the blocking portion blocks the pin  163 , to implement blocking the downward movement of the blocking piece  160 , so that almost all the pre-tightening force generated by the U-shaped elastic element  150  is exerted on the blocking portion (that is, on the safety piece  120 ), which can realize a function of stopping or even preventing the pre-tightening force generated by the U-shaped elastic element  150  from being transferred to the counter wedge  140 . In the following description about the working principle of the safety device  10  for elevators, advantages and effects brought by the function can be understood. 
         [0032]    The U-shaped elastic element  150  may be, for example, a U-shaped spring, and the amount of deformation thereof is mainly embodied by a change of distance between the lower U-shaped end  150   a  and the upper U-shaped end  150   b . Parameters such as stiffness and a U-shaped opening width of the U-shaped elastic element  150  may be set according to parameters such as a stable frictional force (predetermined maximum frictional force) desired by the safety device  10  for elevators, and a distance by which the counter wedge  140  is capable of moving upward. Compared with that of a disc spring, an elastic force generated by the U-shaped elastic element  150  under an amount of deformation is stable in magnitude and fully repeatable. 
         [0033]    The width of the blocking piece  160  is substantially equal to the width of the guide groove  122 , and the height and/or stiffness of the blocking piece  160  can be determined according to parameters such as the opening width of the U-shaped elastic element  150 , the stable frictional force desired by the safety device  10  for elevators, and the distance by which the counter wedge  140  is capable of moving upward. 
         [0034]    The safety device  10  for elevators according to the embodiment of the present invention is installed under an elevator car, and provides an arresting force for the elevator car. The basic working principle of the safety device  10  for elevators according to the embodiment of the present invention is further described below. 
         [0035]    Normal Operation of the Elevator 
         [0036]    In the normal operation of the elevator, the safety device  10  for elevators does not need to provide any arresting force for the elevator car. As shown in  FIG. 1 , the active wedge  130  falls at a lowest position, that is, falls on the safety piece  120 ; the counter wedge  140  also falls at a lowest position, and it falls on the support elastic element. In this case, a distance between the friction surface  131  and the friction surface  141  is maximum, and neither friction surface  131  nor friction surface  141  contacts the guide rail of the elevator, so that the operation of the elevator is not affected substantially. 
         [0037]    Braking Process 
         [0038]    In the braking process, the safety device  10  for elevators needs to provide an arresting force for the elevator car immediately. The pulling transmission component triggers the active wedge  130  to start to move upward. As the self-locking angle α is set, when the active wedge  130  ascends to a particular position, the friction surface  131  of the active wedge  130  starts to contact the guide rail, and a frictional force generated between the two continues to drive the active wedge  130  to move upward. Further, the distance between the friction surface  131  and the friction surface  141  becomes shorter, the friction surface  141  also starts to contact the guide rail, and driven by the frictional force, the counter wedge  140  also starts to tend to move upward. However, under the effect of the blocking piece  160 , the counter wedge  140  firstly needs to overcome the pre-tightening force exerted by the U-shaped elastic element  150  on the blocking piece  160 , and thus can move upward. In other words, at least part of the frictional force generated by the guide rail with respect to the counter wedge  140  can be transferred to the upper U-shaped end  150   b  of the U-shaped elastic element  150  through the blocking piece  160 , and the elastic force generated by the U-shaped elastic element  150  can be transferred to the counter wedge  140  through the blocking piece  160 , only when the frictional force generated by the guide rail with respect to the counter wedge  140  is greater than the pre-tightening force exerted by the U-shaped elastic element  150  on the blocking piece  160 . 
         [0039]    It will be understood that, the frictional force between the guide rail and the friction surface  131  or  141  is substantially equal to the friction coefficient multiplied by the positive pressure F (that is, a pressure vertically exerted on the guide rail). As the active wedge  130  continues to move upward, the active wedge  130  and the counter wedge  140  respectively press the safety piece  120  leftward and rightward more vigorously, parts toward the guide rail (that is, the positive pressure F) of counter forces that are exerted by the safety piece  120  respectively on the active wedge  130  and the counter wedge  140  increase, and the frictional force continues to increase. The blocking piece  160  and the counter wedge  140  start to move upward only when the frictional force between the guide rail and the counter wedge  140  can overcome the pre-tightening force generated by the U-shaped elastic element  150  and the gravity generated by the blocking piece  160 . Meanwhile, the amount of deformation of the U-shaped elastic element  150  increases, and the contraction elastic force of the U-shaped elastic element  150  also increases; moreover, the elastic force can be at least partially transferred to the counter wedge  140  through the blocking piece  160 , thereby increasing the positive pressure F. Meanwhile, it should be noted that, on the other hand, the upward movement of the counter wedge  140  also causes the friction surface  141  to move leftward, which also reduces the positive pressure F. In this process, because the active wedge  130  still moves upward continuously and the distance between the friction surface  131  and the  141  still decreases continuously, although the friction surface  141  moves leftward, the overall positive pressure F still increases. 
         [0040]    After the active wedge  130  moves upward to a top end and is fixed, that is, after the active wedge  130  slides upward to the upper end surface  132  of the active wedge  130  to contact the inner top surface  128  of the safety piece  120 , and be blocked and fixed, the active wedge  130  no longer contributes to increasing the positive pressure F. In this case, a transient dynamic equilibrium point is formed between the counter wedge  140  and the U-shaped elastic element  150 . In other words, the counter wedge  140  is enabled to move to a position point (where the position point is not fixed, and may vary as the friction coefficient or the like changes), so that the magnitude of the frictional force between the counter wedge  140  and the guide rail substantially corresponds to an elastic force, which has a particular value, of the U-shaped elastic element  150  and substantially remains stable, the frictional force does not change significantly with the relative movement or the frictional coefficient between the guide rail and the friction surface  141 , and the magnitude of the friction is the desired stable frictional force or arresting force. For example, if the frictional force cannot reach the desired magnitude because the positive pressure F is not large enough, the counter wedge  140  continues to move upward; therefore the elastic force of the U-shaped elastic element  150  increases, and a positive feedback helps increase the positive pressure F, till the frictional force reaches the desired magnitude. Further, for another example, if the frictional force cannot reach the desired magnitude because the friction coefficient changes (the friction coefficient between the friction surface  141  and the guide rail is variable, and may change with different working conditions), the counter wedge  140  continues to move upward; therefore the elastic force of the U-shaped elastic element  150  increases, and a positive feedback helps increase the positive pressure F, till the frictional force reaches the desired magnitude. Therefore, in this structure, the positive pressure F is fully self-adjustable with respect to the change of the friction coefficient. 
         [0041]    After the dynamic equilibrium is reached, the magnitude of the frictional force is substantially stable, so that a substantially stable acceleration condition can be generated for the elevator car, achieving a desirable braking effect. 
         [0042]      FIG. 5  shows a plot of acceleration vs. time of the safety device for elevators according to an embodiment of the present invention. As shown in  FIG. 5, 51  is a plot of acceleration vs. time of an existing safety device for elevators,  52  is a plot of acceleration vs. time of the safety device  10  for elevators, and the braking working process begins at the third second, where the friction coefficient fluctuates. It can be found by comparison that the safety device  10  for elevators in the embodiment of the present invention can obtain a stable acceleration condition in an arresting process (for example, an acceleration value is substantially stabilized at approximately 0.9 g), and a phenomenon of sudden acceleration climbing will not occur even when an arresting time increases. 
         [0043]    It should be understood that, herein, the “stable” frictional force, arresting force or acceleration condition does not refer to a fixed numerical value without any change; instead, the frictional force, arresting force or acceleration condition may remain relatively stable within an interval range, and therefore, they are relative concepts. 
         [0044]      FIG. 6  shows a plot of acceleration vs. friction coefficient of the safety device for elevators according to an embodiment of the present invention. As shown in  FIG. 6, 61  is a plot of acceleration vs. friction coefficient of an existing safety device for elevators, and  62  is a plot of acceleration vs. the friction coefficient of the safety device  10  for elevators, where it is reflected that the acceleration of the safety device  10  for elevators is more stable on the condition that the friction coefficient fluctuates. 
         [0045]    It can be learned from the foregoing braking principle analysis that, in case where other parameter conditions are absolutely determined, at the foregoing dynamic equilibrium point, when the counter wedge  140  moves to a particular position point, a corresponding elastic force that the U-shaped elastic element  150  is capable of generating can be absolutely determined through calculation. Therefore, the corresponding elastic force that the U-shaped elastic element  150  is capable of generating at this position point may be set and determined in advance, to roughly determine the magnitude of the frictional force, so that the acceleration condition, which can be generated by the safety device  10  for elevators, is stable as desired. Specifically, the relatively stable frictional force or arresting force desired by the safety device  10  for elevators may be roughly obtained by setting the stiffness and/or opening width of the U-shaped elastic element  150 . Therefore, the U-shaped elastic element  150  is one of crucial components of the safety device  10  for elevators. 
         [0046]    The safety device  10  for elevators of this embodiment fully combines and utilizes performance features of the U-shaped elastic element  150 . The elastic force generated by the U-shaped elastic element  150  under an amount of deformation is stable in magnitude and fully repeatable. Therefore, the acceleration condition that is desired to be generated after the dynamic equilibrium can be relatively stable; moreover, the U-shaped elastic element  150  has a relatively large amount of deformation, and the desired frictional force or acceleration condition can be easily set in an expanded range, which is flexible in design and is fully applicable to high-speed elevators requiring relatively high arresting acceleration. More importantly, even if the counter wedge  140  or the like is worn, the U-shaped elastic element  150  is relatively insensitive to the wear because the structure of the U-shaped elastic element  150  determines that it has smaller stiffness compared with a disc spring. Although the amount of deformation of the U-shaped elastic element  150  increases in the dynamic equilibrium condition due to the wear, and the desired frictional force changes, that is, the desired acceleration condition changes, the amount of deformation is still in a range relatively easy to accept, and the phenomenon that no arresting force can be generated will not occur at all, achieving desirable safety and reliability. 
         [0047]    Moreover, it should be further understood that, especially in case where the blocking piece  160  is disposed to stop the pre-tightening force from being exerted on the counter wedge  160 , in the foregoing braking process, while the counter wedge  140  is overcoming the pre-tightening force exerted by the U-shaped elastic element  150  on the blocking piece  160 , the blocking piece  160  does not move upward, and the amount of deformation of the U-shaped elastic element  150  does not change, and the upper U-shaped end  150   b  does not move upward either, which helps reduce the amount of deformation of the U-shaped elastic element  150  in the dynamic equilibrium condition, and further helps expand a setting range of the desired acceleration condition. 
         [0048]    Restoration Process 
         [0049]    In the restoration process, the safety device  10  for elevators needs to restore a normal operation state from a braking state. An elevator control system drives the elevator car and the safety device  10  for elevators to move upward with respect to the guide rail, and the guide rail generates a downward frictional force against the active wedge  130  and the counter wedge  140  in contact with the guide rail on both sides, to drive the active wedge  130  and the counter wedge  140  to move downward. The active wedge  130  slides downward as being driven by the frictional force, causing the positive pressure F to decrease, and the counter wedge  140  also slides downward as being driven by the frictional force, causing the positive pressure F to increase. The decreasing speed of the positive pressure F is greater than the increasing speed thereof, and after the blocking piece  160  is restored to the original position as shown in  FIG. 1 , the pin  163  is blocked, stopping the pre-tightening force generated by the U-shaped elastic element  150  from being transferred to the counter wedge  140 , which helps reduce the descending movement of the counter wedge  140 , and thereby helps make the restoration process smoother. 
         [0050]    Besides, it should be understood that, the safety device  10  for elevators of the embodiment of the present invention can ultimately generate a frictional force and acceleration of a relatively stable magnitude (as shown in  FIG. 5 ) in the braking process, and will not generate an excessively large frictional force due to changes of the friction coefficient or the like; therefore, the active wedge  130  and the counter wedge  140  will not clamp the guide rail excessively tightly either, so that the restoration is easier and faster. 
         [0051]    The examples above mainly illustrate the safety device for elevators of the present invention. Although only some implementation manners of the present invention are described, those of ordinary skill in the art should understand that the present invention can be implemented in many other forms without departing from the subject matter and scope of the present invention. Therefore, the demonstrated examples and implementation manners are regarded as being illustrative rather than limitative, and the present invention may cover various modifications and replacements without departing from the spirit and scope of the present invention as defined in the appended claims.