Design method for anti-falling unit for elevator Field of the Invention The invention relates to a design method for the elevator, especially to a design method for the anti-falling unit suitable for the elevator. 5 Description of prior art When the elevator is in an accident under some condition, the elevator may speedily fall down or rise up, such movement may cause serious harm to the passager in the elevator, especially when 10 rescuers evacuate passagers from the elevator stuck between two floors, the elevator may speedily move down, so the moving cabin combined with the static wall or floor may cut anything therebetween, such that some parts of the passager or rescuer may be cut down by the moving elevator. 15 How to avoid the elevator from moving down or falling unexpectedly so as to reduce the chance of injury, it is a problem waiting to be solved. 20 Summary of the present invention The object of the invention is to provide a design method for the anti-falling unit suitable for the elevator so as to solve the above problem. 25 Therefore, the invention provides a design method for the anti-falling unit for the elevator, the design method comprises providing a trestle fixedly connected with the elevator cabin and a guide rail arranged along the elevator shaft; providing the anti-falling unit with a first gear, a second gear, a 30 trolley wheel, a triggering unit, a connecting rod, an elastic 1/8 component and a supporting shaft; mounting the triggering unit on the trestle; freely rotatably mounting the trolley wheel on the trestle by the supporting shaft; 5 freely rotatably mounting the first gear on the trestle by the supporting shaft; arranging the guide rail between the first gear and the trolley wheel; arranging for the first gear to engage with the gear surface of 10 the guide rail; arranging for the trolley wheel to roll on the smooth surface of the guide rail; arranging for the connecting rod to make the second gear revolve around the axis of the first gear, mean while the second gear 15 freely rotates with respect to the connecting rod; providing each side of the up and down sides of the first gear with one second gear engaging with the first gear; providing the connecting rod with a triggering bore matting with the triggering end of the triggering unit; 20 arranging for the triggering end to pluggably mate with the triggering bore. Specifically, when the triggering end inserts into the triggering bore, holding the second gear arranged on the connecting rod at the 25 position where is below the first gear and away from the gear surface of the guide rail, that is arranging for the triggering end to make the second gear not contact with the gear surface; when the triggering end withdraws from the triggering bore, arranging for the elastic force from the elastic component to 30 make the second gear on the connecting rod locate near to the gear surface of the guide rail and make the second gear contact and engage with the gear surface of the guide rail. Similarly, when the triggering end inserts into the triggering bore, holding the second gear arranged on the connecting rod at the 2/8 position where is above the first gear and away from the gear surface of the guide rail, that is arranging for the triggering end to make the second gear not contact with the gear surface; when the triggering end withdraws from the triggering bore, 5 arranging for the elastic force from the elastic component to make the second gear on the connecting rod locate near to the gear surface of the guide rail and make the second gear contact and engage with the gear surface of the guide rail. 10 The following will describe how the elevator can not move down or fall when the elevator is in fault state. When the second gear do not get involved, the second gear rotates in an engagement with the first gear, which rotates in an engagement with the guide rail, the elevator works normally. When 15 the second gear gets involved, the tooth of the second gear withstands the tooth of the guide rail due to each motion direction of respective tooth of the second gear and the guide rail, the limiting action of the trolley wheel arranged behind the guide rail (smooth surface) avoids the guide rail from deformation caused by 20 the force when the second gear gets involved, thus the first gear, the second gear and the guide rail are tightly stuck together. Because the withstanding action between the second gear and the guide rail stops the first gear rotating in an engagement with the second gear so as to stop the first gear rotating, that is the first gear 25 stops rotating in an engagement with the guide rail so as to stop the elevator in fault state fall or moving down unexpectedly. Advantageous Effects When the elevator is in a state of normal opeartion, the triggering 30 unit make the second gear hold in the position where is away from the gear surface of the guide rail, the first gear engages with the gear surface of the guide rail, the first gear rotates to drive the second gear rotating, the trolley wheel rolls on the smooth surface of the guide rail, the mechanism consisting of the first gear, the 3/8 second gear, the connecting rod, the guide rail and the trolley wheel has one freedom degree, so the elevator opeartes normally. When the elevator is in fault state, the triggering unit is triggered, the triggering end withdraws from the triggering bore, the elastic 5 force from the elastic component makes the second gear on the connecting rod locate near to the gear surface of the guide rail and makes the second gear contact and engage with the gear surface of the guide rail, at this moment, the first gear, the guide rail and the second gear engage with one another, the mechanism consisting of 10 the first gear, the second gear, the connecting rod, the guide rail and the trolley wheel has zero freedom degree, so the elevator do not move down. Especailly, because the second gear is located below the first gear, 15 the tendency of the elevator moving downwards makes the first gear engage downward with the guide rail, while the second gear located below the first gear is bound to exert a force with opposite direction to stop the first gear from trying to rotate. So the anti-falling process of the elevator is such, the triggering end is 20 triggered, the elastic force from the elasic component make the first gear, the second gear and the guide rail build and hold a stuck engagement relationship between one another, such stuck engagement relationship stops the elevator from trying to fall in fault state. 25 The gear engagement makes the mechanism have zero freedom degree, which is different from that the wheel makes the mechanism have zero freedom degree, the wheel is easy to slip between the first gear and the guide rail, such that the elevator may 30 fall or raise; while the gear makes the first gear, the second gear and the guide rail tightly stuck together for avoiding the elevator from falling. Brief description of the drawing 4/8 In the following, the invention will be described in greater detail by means of some embodiments with reference to the accompanying drawings, in which Fig.1 is a 3d-drawing of the anti-falling unit of the invention; 5 Fig.2 is from Fig.1, wherein trestle of one side is hidden; Fig.3 is a front view of the Fig.1; Fig.4 is a front view of the Fig.2; Fig.5 is from Fig.4, wherein the second gear is to get involved; Fig.6-7 are drawings of the connecting rod connecting with the 10 triggering unit; Fig.8 is a 3d-drawing of the trestle; Fig.9 is a 3d-drawing of the connecting rod; Fig.10 is a secontional view of the connecting rod in Fig.9; Fig.11 is an exploded 3d-drawing of the anti-falling unit in Fig.1; 15 Fig.12-13 are 3d-drawings of the triggering unit; Fig.14 is a drawing of the anti-falling unit in embodiment 2; Fig.15 is a 3d-drawing of the anti-falling elevator in embodiment 1. 1.the first gear; 2. the guide rail; 3. the trolley wheel; 4. the second gear; 5. the supporting shaft; 20 6. the trestle; 7. the connecting rod; 8. the spring; 9. the triggering unit; 10. the bushing; 601. the through hole; 602. the screw hole; 701. the triggering bore; 901. the triggering end. 25 Detailed description of the preferred embodiment Embodiment 1 Referrign to Fig.1, Fig.2, Fig.11 and Fig.12, it is an anti-falling unit suitable for the elevator of the invention, the anti-falling unit includes a trestle 6 fixedly connected with elevator cabin, a first gear 30 1, a second gear 4, a guide rail 2, a trolley wheel 3, a triggering unit 9, a connecting rod 7, a spring 8, a supporting shaft 5, a nut, a washer, a split pin and a bushing 10. The triggering unit 9 is removably mounted to the trestle 6 by screw, the trolley wheel 3 is 5/8 freely rotatably mounted between two trestles 6 by the supproting shaft 5, the first gear 1 is freely rotatably mounted between two trestles 6 by the supproting shaft 5, the guide rail is configured between the first gear 1 and the trolley wheel 3, the first gear 1 5 engages with the gear surface of the guide rail 2, the trolley wheel 3 rolls on the smooth surface of the guide rail 2, the connecting rod 7 makes the second gear 4 revolve around the axis of the first gear 1, mean while the second gear 4 freely rotates with respect to the connecting rod 7. The second gear 4 engages with the first gear 1, 10 the connecting rod 7 is provided with a triggering bore 701 matting with the triggering end 901 of the triggering unit 9, the triggering end 901 pluggably mates with the triggering bore 701. The bushing 10 adjusts the position of the second gear4 to align the central surface of the first gear 1 with the central surface of the second gear 15 4, the trestles 6 on both sides of the first gear 1 are connected by the nut and washer, then the split pin avoids the nut from looseness and drop out. Referring to Fig.3 and Fig.4, when the triggering end 901 inserts into 20 the triggering bore 701, the second gear 4 on the connecting rod 7 is held at the present position where is below the first gear 1 and away from the gear surface of the guide rail 2, that is the triggering end 901 makes the second gear 4 not contact with the gear surface, at this moment, the spring 8 is in tension state. 25 Referring to Fig.6, the triggering end 901 of the triggering unit 9 inserts into the triggering bore 701, the connecting rod 7 is limited at the present position, that is the connecting rod 7 is straight down. 30 Referring to Fig.7,the triggering end 901 withdraws from the triggering bore 701, the elastic force from the spring 8 makes the second gear 4 on the connecting rod 7 locate near to the gear surface of the guide rail 2 and makes the second gear 4 contact and 6/8 engage with the gear surface of the guide rail 2, referring to Fig.5. Referring to Fig.8, the trestle 6 is provided with a through hole 601 matching the gap of the supporting shaft 5 and a screw hole 602 for 5 mounting the triggering unit 9. Referring to Fig.9 and Fig.10, each end of the head and tail ends of the connecting rod 7 is provided with a through hole 601 matching the gap of the supporting shaft 5, the side surface of the connecting 10 rod 7 is provided with a triggering bore 701 pluggably matting with the triggering end 901 of the triggering unit 9, the triggering bore 701 is a blind bore matching the gap of the triggering end 901, the front end of the triggering bore 701 is provided with chamfer convenient to receive the triggering end 901. 15 Referring to Fig .12 and Fig.13, the front end of the triggering unit 9 is provided with a scalable triggering end 901 controlled by the electromagnetic coil. The triggering end 901 retracts when the triggering signal exerts on the triggering unit 9. 20 When the second gear 4 do not get involved, the second gear 4 rotates in an engagement with the first gear 1, which rotates in an engagement with the guide rail 2. When the second gear 4 gets involved, the tooth of the second gear 4 withstands the tooth of the 25 guide rail 2 due to each motion direction of respective tooth of the second gear 4 and the guide rail 2, the limiting action of the trolley wheel 3 arranged behind the guide rail 2 (smooth surface) avoids the guide rail 2 from deformation caused by the force when the second gear 4 gets involved, thus the first gear 1, the second gear 4 30 and the guide rail 2 are tightly stuck together. Because the withstanding action between the second gear 4 and the guide rail 2 stops the first gear 1 rotating in an engagement with the second gear 4 so as to stop the first gear 1 rotating, that is the first gear 1 stops rotating in an engagement with the guide rail 2 so as to stop 7/8 the elevator in fault state moving down or falling unexpectedly for guaranteeing safe evacuation. Embodiment 2 5 Arranging the second gear 4 arranged below the trestle 6 in embodiment 1 above the trestle 6, when the triggering end 901 inserts into the triggering bore 701, the second gear 4 on the connecting rod 7 is held at the present position where is above the first gear 1 and away from the gear surface of the guide rail 2, that is 10 the triggering end 901 makes the second gear 4 not contact with the gear surface. When the triggering end 901 withdraws from the triggering bore 701, the elastic force from the spring 8 makes the second gear 4 on the connecting rod 7 locate near to the gear surface of the guide rail 2 and makes the second gear 4 contact and 15 engage with the gear surface of the guide rail 2, such arrangement can also achieve the anti-falling function. 8/8