Device for preventing overload of a winch
DESCRIPTION 
     1. Technical Field 
     The present invention relates to a device for preventing overload of a winch mounted on a vehicle, for instance. The winch includes a line drum, a motor, a planetary gearing between the motor and the drum and a sensing means coupled to a device which interrupts further operation of the winch when overloaded. 
     2. Background Art 
     To prevent overload of winches it has been suggested to use mechanical devices such as safety clutches and breakpins which prevent the winch motor from being overheated or prevent the motor or other elements of the winch from being damaged or destroyed. In the case the motor is electric it has also been suggested to use so called thermo switches which break the current to the motor when the winch motor becomes too hot when overloaded. 
     The above described devices suffer from certain disadvantages. The mechanical devices are often difficult to set so that they are released at an exactly predetermined pulling force or are difficult to reset in working order after release. Further, they have complicated construction and/or are not especially reliable. The electric thermo switches suffer from several disadvantages and the main disadvantage is that they are not released at a predetermined pulling force but are released only when the winch motor has reached a certain temperature. 
     DISCLOSURE OF INVENTION 
     The object of the present invention is to provide a device with which the disadvantages with previously known devices are removed. This object is fulfilled by giving the invention the characteristics stated in the claims. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a side view, partly in section, of a winch including the device according to the invention, and 
     FIG. 2 is a section along line II-II in FIG. 1. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The winch shown in the Figures, which preferably is mounted on the front bumper of a lorry, includes a line drum 1, an electric continuous-current motor 2 and a gearing which transfers the driving force of the motor to the drum. The gearing is divided into a first gearing 3 between the motor 2 and a shaft 4 extending through the drum and a second gearing 5 between the shaft 4 and the drum. The gearing 5 is a known planetary gearing and will not be described further. The gearing 3 also is a known planetary gearing but has been modified for use in the device according to the invention. 
     The gearing 3 includes a sun gear 6 driven by the driving shaft of the motor 2. The sun gear is rotatably supported relatively to the winch frame R. The gearing 3 also includes two planet wheels 7 engaging the sun gear and rotatably supported by a planet carrier 8, which is rotatably supported on the sun gear via a slide bearing 16, and a ring wheel 9 having an inner tooth path 9a engaging the planet wheels 7. The ring wheel 9 is mounted to the shaft 4 by means of a wedge 10. 
     It is evident from FIG. 2 that the planet carrier 8 is rotatably connected to a rod 11 which may be displaced axially in a hole 24 in the frame R. The upper portion of the rod 11 is surrounded by a package 12 of cup springs. The lower end of the package abuts against the winch frame and its upper end abuts against a locking nut 13 fastened to the rod. The package 12 is prestressed and presses the rod 11 upwards to its normal position shown in FIG. 2. The movement upwards is limited by an enlarged portion 11a of the rod. This portion abuts against the lower surface of the winch frame. The upper portion of the rod 11 is provided with a screw 14. A contact tongue 15a of an electric switch 15 mounted on the winch frame is in contact with the under surface of the screw. The tongue 15a assumes the position shown in FIG. 2 during normal pulling work of the winch and in this position it breaks the current in an electric circuit to which the switch is connected. 
     During the normal pulling work of the winch an electric switch 17 preferably located in the cab of the lorry is set to one of its three possible positions, namely the position wherein a current source is connected to the point of contact a. A line 18 connected to one pole of the switch 15 also connects the point of contact a to a point of contact b of a relay 19 mounted on the winch adjacent its motor 2 and shown with dashed lines in FIG. 2. The other pole of the switch 15 is connected to the point of contact c at the switch 17 via a line 20 and is also connected to a reversing relay B for the winch motor and to a point of contact d in the relay 19. The relay 19 also includes a magnet 21 connected to two tongues 22 and 23. The connection is indicated by means of dashed lines within the relay 19. 
     If the relay 19 was not present and the switch 15 was directly connected to the winch motor 2 the current to the motor--when the winch is overloaded--would be switched off and thereafter immediately switched on again when the switch 15 switches off the current again at the interruption of the overload. When the switch 15 switches off the current to the motor it is understood that the motor will stop whereby the tension in the line wound around the drum 1 will decrease a little which has the effect that the switch again switches on the current to the motor which starts again and rotates until the switch switches off the current again. By connecting the relay 19 to the switch 15 and the motor the repeated starting and stopping of the motor is avoided during overload. 
     The course of events during winch overload and the function of the relay 19 will now be described further. 
     The winch is started by setting the switch 17 to the point of contact a so that current via the line 18, the point of contact b and the tongue 22 is conducted to the relay F which starts the winch motor 2 and rotates it forwards. Because the tongue 15a of the switch is uneffected and the current between between the two poles of the switch is switched off no current is flowing through line 20. 
     When the pulling force in the line of the winch reaches a predetermined value which is the overload limit for the winch the planet carrier 8 will be rotated clockwise in FIG. 2 around the sun gear in the same direction as the latter is rotated by the motor 2. The reason for this relative rotational movement is that the ring wheel 9 greatly resists rotational movement (the pulling force of the line is transmitted to the ring wheel) exerted thereon by the sun gear 6 via the planet wheels 7, whereby a part of the rotational movement is taken up by the planet carrier via the planet wheels. 
     The rotation of the planet carrier 8 takes place against the action of the pressure force in the package 12. If the rotational force on the carrier 8 is sufficiently great the package will be compressed so that the rod 11 is moved downwards in FIG. 2 and the screw 14 on the rod engages the tongue 15a so that it switches on the current between the two poles of the switch 15. Current will then flow through line 20 to the reversing relay B of the motor 2, to the point d of the relay 19 and to the magnet 21 which resets the tongues 22 and 23 so that tongue 22 breaks the connection between point b and relay F, whereby the motor stops, and tongue 23 closes the connection between point d and relay F. However, when the tongue 23 is reset the relay F will not be actuated, i.e. switch on the current to the motor, because the current from point d will be connected to ground via the magnet 21. Nor will the relay B be activated for the same reason. 
     When the cause for the overload has been set aside, i.e. when the pulling force in the line has been decreased to a permissible value, the planet carrier 8 is returned to the position shown in FIG. 2 by means of the package 12, and the current between the poles of the switch 15 is switched off again, but this will not affect the motor because the tongues 22 and 23 are retained in their last described positions. In order to start the winch again the switch 17 first must be reset for a short moment to contact the point of contact c whereby current flows to the point of contact d in the relay 19, so that the magnet 21 is activated and returns the tongues 22 and 23 to the positions shown in FIG. 2, and whereby current also flows to the reversing relay B so that the motor reverses its direction of rotation for a short moment. Then the switch 17 is reset to contact point a and the previous described operation, during which the winch is rotating forwards, will be repeated. 
     When it is desired to rotate the winch in the backward direction the switch 17 is set to contact the point of contact c so that current is conducted to the relay B independently of whether the switch 15 is actuated or not. 
     Even though only one embodiment of the invention has been described above and shown on the drawings it should be understood that the invention is not limited to this embodiment but only to what is stated in the claims.