Abstract:
A safety mechanism for a lift system, the safety mechanism including a first motion retarder which operates frictionally between a load-carrying platform and a hoistway and a second motion retarder which operates by breaking a frangible element. The two motion retarders may act together so that when the frictionally engaging element is depleted, the frangible elements are then broken. In an embodiment, the frictional element is disengaged to engage the breaking of the frangible elements when the car is a predetermined distance above a floor of the hoistway, thereby ensuring that the load-carrying platform can be stopped prior to a collision with the floor or ceiling of the hoistway.

Description:
BACKGROUND 
     Technical Field 
     The present disclosure relates to a mechanism for improving the safety of lifts. 
     Description of the Related Art 
     Lifts transport people and goods from place to place, usually in a vertical direction. Safety has always been a major concern due to the catastrophic consequences of a failure in a lift. One of the major contributors to the commercialization of passenger lifts was the invention by Elisha Otis in 1852 of a practical safety mechanism for passenger lifts. 
     A number of different safety mechanisms are known. For example, DE19833772 discloses two locking bars arranged below a passenger car of a lift. The locking bars are kept in a retracted position by a load cable so that when the load cable snaps the locking bars extend to engage with rungs situated in the hoistway. The rungs may rupture to provide a breaking force. 
     U.S. Pat. No. 6,131,703 discloses a safety mechanism for a lift which includes brake pads which may be brought into engagement with braces. To provide additional braking power, the pads are provided with protrusions which engage with corrugations, flattening these corrugations as the protrusions pass thereover. 
     BRIEF SUMMARY 
     According to an embodiment, there is provided a safety mechanism for a lift, the lift comprising a load-carrying platform arranged for movement relative to a hoistway, the safety mechanism comprising first and second motion retarders acting between the load-carrying platform and the hoistway for reducing a speed of the load-carrying platform relative to the hoistway wherein the first motion retarder includes a friction brake and the second motion retarder includes a frangible element. 
     The first safety mechanism may include a pad which engages frictionally with the hoistway to retard a speed of the load-carrying platform, the frictional engagement of the pad with the hoistway causing depletion of the pad, and the second motion retarder may be mounted relative to the first motion retarder so that depletion of the pad causes engagement of the second motion retarder. 
     The safety mechanism may further comprise a hook wherein engagement of the second motion retarder occurs when the hook engages with the frangible element so that a tearing of the frangible element by the hook reduces a speed of the load-carrying platform. The hook may be attached to the load-carrying platform and the frangible element may be attached to the hoistway. 
     The second motion retarder may further comprise a biasing means for encouraging the hook into engagement with the frangible element. 
     The safety mechanism may further comprise retention means for preventing engagement between the hook and the frangible element, wherein the retention means is adapted to be operational during normal operation of the associated lift. The retention mechanism may be further adapted to disengage during an emergency situation. In an embodiment, the retention mechanism is an electromagnet. 
     The safety mechanism may further comprise swapping means for disengaging the first motion retarder and engaging the second motion retarder. 
     The first motion retarder may have an operational element pivotally moveable relative to the second motion retarder and wherein the swapping means causes pivoting of the operational element to thereby cause disengagement of the first motion retarder and engagement of the second motion retarder. The operational element may include a pad which is frictionally engaged with an element mounted on a hoistway. 
     The swapping element may be adapted to be mounted a predetermined distance from a base of the hoistway. 
     The frangible element may be mounted relative to the hoistway. 
     The second motion retarder may include a plurality of frangible elements mounted relative to the hoistway, and in this case each frangible element may comprise a recess adapted to receive a hook which, when the second motion retarder is engaged, causes tearing of the frangible element. 
     In an embodiment, a method of retarding the motion of a lift, wherein the lift comprises a load-carrying platform arranged for movement relative to a hoistway, includes reducing a speed of the lift by applying friction between the load-carrying platform and the hoistway and reducing the speed of the lift by breaking a frangible element. 
     The applying friction may include engaging a pad frictionally with the hoistway to retard a speed of the load-carrying platform, wherein the frictional engagement of the pad with the hoistway causes depletion of the pad, and the method further comprising engaging said breaking said frangible element in response to said depletion of said pad. 
     The breaking said frangible element may include bringing a hook into engagement with said frangible element. 
     The hook may be attached the load-carrying platform and the frangible element may be attached to the hoistway. 
     The method may further comprise biasing the hook into engagement with the frangible element. 
     The method may further comprise retaining the hook relative to the frangible element, with a retaining means, to prevent engagement between the hook and the frangible element, during normal operation of the associated lift. 
     The method may further comprise disengaging the retaining means during an emergency situation. 
     The retention mechanism may be an electromagnet. 
     The method may further include disengaging the friction and engaging means to break the frangible element. 
     The method may include pivotally moving a friction engaging element relative to said means to break the frangible element. 
     The friction engaging element may include a pad which is frictionally engaged with an element mounted on a hoistway. 
     The pivotally moving the friction engaging element may occur when the load-carrying platform is located a determined distance from a bottom of the hoistway. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Example embodiments are described with reference to the accompanying schematic diagrams where: 
         FIG. 1  is a schematic diagram of a lift safety mechanism according to an embodiment; 
         FIG. 2  is a schematic diagram of a detail of the lift safety mechanism of  FIG. 1  according to an embodiment; 
         FIG. 3  is a schematic diagram of a detail of the lift safety mechanism of  FIG. 1  according to an embodiment; 
         FIG. 4A  is a schematic diagram of a lift safety mechanism according to an embodiment in a first example configuration; 
         FIG. 4B  is a schematic diagram of the lift safety mechanism of  FIG. 4A  in a second example configuration; 
         FIG. 5  is a schematic diagram of an embodiment of a portion of a lift safety mechanism of  FIGS. 4A and 4B ; 
         FIG. 6  is a schematic diagram of an embodiment of a portion of a lift safety mechanism of  FIGS. 4A and 4B ; and 
         FIG. 7  is a schematic diagram of a track according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments are described hereafter with reference to the accompanying diagrams. 
       FIG. 1  illustrates a lift system  2  comprising a passenger car  12  supported by a hoist cable  14 . In this embodiment, the passenger car  12  includes a load-carrying platform  13  on which the passengers stand during transport in the car  12 . The passenger car  12  moves vertically within a hoistway  8  under the action of a motor and counterweight, neither of which are illustrated in the accompanying drawings, in a known manner. The hoistway  8  is a vertical void formed in a structure such as a building defined by walls  18 . Tracks  16  run on either side of the hoistway  8  and define a path along which the car  12  travels. 
     It is to be realised that embodiments are not limited to lifts which operate vertically or to those which operate in buildings. 
     The lift system  2  includes a safety mechanism  10  which includes a first motion retarder  20  and a second motion retarder  50 . Embodiments therefore include two, or more, safety devices which may act in concert. In general, multiple safety devices have not hitherto been utilized due to the cost and complexity which this adds to lift construction. Using two or more such safety devices has the advantage that one safety mechanism may act as a backup to the other. This increases the likelihood that, even in the situation where one of the safety devices fails, the other will be able to act as a backup. 
     The safety mechanism  10  including both the first motion retarder  20  and the second motion retarder  50  is provided on one side of the passenger car  12 . In the embodiment illustrated, a further safety mechanism  40 , comprising two similar motion retarders, is also provided on the other side of the passenger car. In the description which follows, the operation of the motion retarders  20  and  50  will be described. However, it is to be realised that the same considerations and descriptions apply in respect of the other safety mechanism  40 . 
     The first motion retarder  20  and the second motion retarder  50  are brought into engagement when an emergency situation is detected, as discussed in further detail below. The lift system  6  includes an electromagnet  70  which disengages when such an emergency situation is detected. The electromagnet  70  includes a base  72  attached to the passenger car  12  and a mount  74  engaged with the base  72 . The power cable for the electromagnet  70  is embedded in the hoist cable  14 . 
     The first motion retarder  20  and the second motion retarder  50  are connected to the mount  74 . When the electromagnet  70  is engaged, motion of the mount  74  relative to the base  72  is prevented. However, when the electromagnet  70  is turned off or disengaged, movement of the mount  74  relative to the base  72  is permitted. 
     In an embodiment, the electromagnet  70  continues to be engaged until it is purposively disengaged (e.g. when an emergency situation is detected) or when the electrical power supply to the magnet is interrupted. Therefore, if electrical power supplied to the lift system  6  is interrupted, the electromagnet  70  will disengage and thereby engage the first and second motion retarders. This facilitates embodiments operating as safety mechanisms even in the absence of power being supplied to the lift system  6 . 
     Turning again to  FIG. 1 , the first motion retarder  20  comprises a lever  24  connected, near its base, to a spring  26 . The electromagnet  70  fastens the position of the lever  24  and prevents it from moving. When the electromagnet is released, the lever is allowed to move under the influence of the spring  26  which pulls the base of the lever  24  towards the passenger car  12 . 
     The first motion retarder also includes a brake pad  22  attached to the lever  24  at the location of the mount  74  of electromagnet  70 . When the lever  24  moves under the influence of the spring  26 , the top of the brake pad  22  is brought into engagement with the track  16  (discussed below with reference to  FIG. 3 ) formed on a wall  18  of the hoistway  8 . Friction between the brake pad  22  and the track  80  will counter the downward motion of the passenger car  12  when the hoist cable  14  no longer supports the car. Therefore, the brake pad  22  can be brought into engagement with the track  16  when an emergency is detected. For example, when the hoist cable has been severed. 
     However, as the brake pad  22  engages with the track  16 , friction between the pad and the track will cause wear on the pad. As the pad  22  wears, the force with which the spring  26  acts on the lever  24  will diminish. Therefore the force with which the pad  22  is brought into contact with the track  16  will likewise diminish. 
     The second motion retarder  50  comprises a hook  52  attached to the end of the lever  24  opposite to the base where the spring  26  is attached. As the brake pad is worn, the hook will move closer to the wall  18  of the hoistway  8 . 
       FIG. 2  illustrates the configuration where the pad  22  has worn away sufficiently that the hook  52  (illustrated here in greater detail) is brought into engagement with an aperture  54  formed in the track  16  provided on the wall  18  of the hoistway  8 . The hook  52  comprises a base  60  connected by a substantially thinner neck  62  to a thicker head  64 . 
       FIG. 3  illustrates the track  16  provided on the side wall  18  of the hoistway  8 . Track  16  includes a braking track  80  against which the brake pad  22  engages. The track  16  further includes a plurality of keyhole apertures  70  provided adjacent to the brake track  80 . Each keyhole aperture  70  comprises a larger upper void  72  and a smaller, elongate lower void  76  to provide a keyhole shape. Each aperture  70  is formed in a frame  76  comprised of a frangible material. Therefore each aperture  70  together with each frangible material  76  forms a frangible element. 
     The apertures  70  are keyhole-shaped. When the brake pad  22  ( FIG. 2 ) has worn down sufficiently the hook  52  is in a position that it may engage with one of the apertures  70 . The head  64  of the hook  54  is shaped to be located in the upper, larger void  72  of an aperture  70 . When this occurs, the shape of the head, which tapers down to the neck  62  of the hook  52 , encourages the neck  62  to be located in the lower, elongate portion  74  of the aperture  70 . 
     As the passenger car  12  continues its downward movement (with reference to the direction of  FIG. 1 ), the neck  62  will cause the frangible material of frame  76  to tear. The energy needed to tear this material will provide a stopping force to the passenger car  12 . 
     Therefore, the second motion retarder  50  will provide a further stopping force to the passenger car  12  when the first motion retarder  20  becomes ineffectual due to wear on the brake pad  22 . Therefore, in embodiments, two motion retarders are provided; configured and arranged so that when the first retarder is ineffectual, the second retarder is engaged. Therefore, the second motion retarder acts as a backup to the first motion retarder. 
     It is to be realised however, that the first motion retarder will serve to stop the passenger car in almost all emergency situations. It is only in those cases where the initial speed, height and weight of the passenger car when the emergency occurs combine to render the first motion retarder insufficient to stop the car completely. 
       FIGS. 4A and 4B  illustrate a lift safety mechanism  100  according to an embodiment. The lift safety mechanism  100  includes a first motion retarder  120  and a second motion retarder  150 . A lever  124  is connected to a passenger car  120  and arranged to pivot in two planes about a pivot point  155  (as described below in greater detail). The first motion retarder  120  is similar to the first motion retarder  20  of the embodiment illustrated in  FIGS. 1 to 3 , and includes a brake pad  122  arranged on the lever  124  so that when the first motion retarder  120  is engaged, the brake pad  122  engages with the track  116  provided on the hoistway wall. 
     As the brake pad  122  engages with the track  116  the brake pad will wear.  FIG. 4B  illustrates the configuration of the safety mechanism  100  after a period of wear on the brake pad  122  has occurred. 
     The safety mechanism  100  of this embodiment includes a pivot guide  200  which serves to locate the hook  152  in an aperture  170  (see  FIG. 5 ) in the track  116 . 
       FIG. 5  illustrates the track  116  of an embodiment of the safety mechanism. With reference to  FIGS. 4 and 5 , the track  116  includes a brake track  180  with which the brake pad  122  engages to provide a first means for retarding the downwards motion of the passenger car, in a manner similar to that described above with reference to  FIGS. 1 to 3 . In addition, the track  116  includes a plurality of keyhole apertures  170  provided in a frangible frame having frangible material  176 . The keyhole apertures therefore provide a second means of retarding the downward motion of the passenger car and operate in the same manner as the keyhole apertures  70  of  FIG. 3 , the operation of which is described above. 
     In an embodiment, the keyhole apertures  170  are located so that when the passenger lift does come to an emergency stop, the lift will be located so that passengers are able to alight onto a floor of the building in which the lift is operational, and are not stuck between floors. 
     In the embodiments illustrated in  FIGS. 4 and 5 , the brake tracks are located to one side of the line of apertures with which the hooks may engage. In some embodiments, the brake tracks are located in line with the apertures and/or on both sides thereof. 
     The embodiments of  FIGS. 4 to 6  differ from those of  FIGS. 1 to 3  in that the arrangement illustrated in  FIG. 5  includes a wedge or prism-like protrusion  182  formed at a determined height, in-line with the keyhole apertures  170  of this embodiment. The lever  124  includes a co-operating nub  186 . The wedge protrusion  182  is shaped so that, when the nub  186  engages with the wedge protrusion, the entire assembly comprising the first and second motion retarders will be laterally displaced to the orientation shown in  FIG. 6  in dotted outline. This lateral displacement is shown by dashed arrow  192 . In this orientation, the hook  152  engages with a laterally displaced tearing strip  188 . The tearing strip  188  comprises second frangible material  184  with reinforcing  190 . The wedge protrusion  182  therefore acts as a swapping means to swap between the frangible material  170  and the second frangible material  184 . 
     The reinforcing  190  in the frangible material  184  provides the tearing strip  188  with greater stopping power than the apertures  170 . The swapping means, in the form of the wedge protrusion and the co-operating nub  186 , together with the tearing strip  188 , mean that an embodiment is able to provide increased stopping power at a determined height. In an embodiment, when the passenger car is nearing the bottom  187  of the hoistway, and it is unlikely that the frangible material  170  will provide sufficient retarding action to bring the passenger car to a stop, the more resilient frangible material  184  can be engaged. The reinforcing on tearing strip  188  will then facilitate ensuring that the car will be brought to a stop. Although this may increase the risk of injury by providing a more abrupt stop that is ideally desirably, it reduces the risks of very serious or critical injury by preventing a collision between the bottom, or top, of the hoistway and the passenger car. 
       FIG. 7  is a schematic diagram of a track  216  according to an embodiment. This track  216 , in a manner similar to the tracks  16  and  116  described above, is designed to be attached to the side wall of a hoistway. The track  216  may be used with any of the safety mechanisms illustrated in  FIGS. 1 to 6 . 
     The track  216  includes a number of apertures  272 , only two of which are illustrated in this Figure. Each aperture  272  includes a central enlarged portion  270  above and below which are located respective elongated portions  274  and  278 . The central enlarged portion  270  is adapted to engage with a corresponding hook in the manner described above. The track  216  has the advantage that it is able to slow and stop lift cars travelling upwards as well as those travelling downwards. 
     Similarly, the arrangement illustrated in  FIGS. 5 and 6  whereby the swapping means causes the lateral displacement of the retarding means may be mirrored at the top of the hoistway to be engaged in those situations where the upwards movement of the lift car need be retarded to avoid injury to the occupants. 
     It is to be realised that embodiments are applicable to all sorts of vertically arranged lifting mechanisms. Certain embodiments relate to passenger lifts such as those discussed above, whereas further embodiments relate to hoists comprising a load-bearing platform having no retaining walls and no upper covering such as a ceiling. Yet further embodiments relate to hoists comprising a load-bearing platform with one or more retaining walls and ceiling. 
     Either or both of the first and second retarding means described above, or according to further embodiments, may be engaged in dependence on an output of a sensor, preferably as interpreted by appropriate logic. In an embodiment, the passenger car is fitted with a sensor which determines the speed of the passenger car relative to the hoistway. (Various sensors can be applied to achieve the same end result: a signal to engage the safety mechanisms). When this speed exceeds a determined amount, an emergency situation is declared and the electromagnet ( 74  or  174 ) disengaged to cause the brake pad ( 22  or  122 ) to engage, to thereby slow the passenger car down, and eventually stop. As previously described, this action also, in an embodiment, causes engaging of the hook and aperture retarding means, if the brake pad wears away past a determined point. 
     Appropriate speed sensors are, in an embodiment, based on laser distance measures such as the DLS-C or FLS-C sold by Dimetix AS of Herisau, Switzerland. 
     Arrangements according to embodiments may be relatively simple when compared to many known safety mechanisms as they do not rely on complex electronics and associated software. Therefore, they may be cheaper to implement and maintain and may be particularly suited to cheaper lift installations such as hoists.