Abstract:
A transmission member for a worm gear having a thrust ring placed over the shaft of a motor. The worm gear is slip-fit over the shaft and has a coupling head with a pair of projections facing away from the motor. A coupling washer is press-fit onto the motor shaft and has a pair of projections as well, each of which is fitted with an O-ring. The coupling washer is mated to the worm gear so both sets of projections intermesh and form a torque-transmitting arrangement, with the O-rings acting as a buffer between the two sets of projections and allowing the motor shaft to turn relative to the worm for a fraction of a revolution to gain inertia and speed helpful to break loose a jammed gear train.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to the field of transmission members for worm gears and, more particularly, to a compact damping coupling mechanism for worms in geared actuators. 
     2. Description of the Related Art 
     A wide range of couplings and dampeners are known in the art. Dampeners are normally employed to reduce the shock loads when an actuator runs into a hard stop, and are made from rubber-like material that absorbs some of the shock energy. Couplings are used to allow the motor to run freely in the opposite direction after running into the hard stop, thereby building up some energy to break loose from a stall condition as the gear train, especially with worm gears, may have locked up due to the stall load. 
     Couplings and dampeners are normally employed between the worm and the motor, thus forcing the worm to be at a certain minimal distance from the motor, moving out the mating helical gear with the worm and wasting some space. In addition, couplings and damping elements are typically purchased components which thereafter require assembly to the motor shaft and the worm. A further disadvantage of such prior art elements lies in the fact that since the coupling/dampener is situated between the worm and the motor, the worm must be on a separate shaft, increasing the number of parts required for a complete assembly. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, one object of the present invention is to overcome the difficulties of high impact loads and break-free problems associated with a worm mounted to the shaft of a DC motor. 
     Another object of the invention is to provide a damping coupling mechanism that requires a minimal number of parts, simplifying manufacturing and reducing cost. 
     A further object of the invention is a damping coupling mechanism for worms in geared actuators in which the worm gearing is located adjacent the motor, reducing space requirements. 
     Yet a further object of the invention is an integrated damping and coupling assembly that utilizes many standard parts to ease manufacturing requirements and minimize cost. 
     In accordance with this and other objects, the present invention is directed to a space-saving damping coupling assembly for use in geared actuators. The damping coupling assembly includes a worm, a thrust ring, a coupling washer and O-rings. The thrust ring is press-fitted to the motor shaft closest to the motor, followed by the slip-fit worm. The worm is held in place on the shaft by the coupling washer. Coupling elements on the worm fit cooperatively with torque-transmitting portions on the washer, with the interface between the worm and washer being cushioned by the O-rings fitted to the torque-transmitting portions. Torque from rotation of the motor shaft is transmitted to the worm through the washer, with the O-rings compressing to absorb shock. Before transmitting torque, the motor shaft is allowed to turn relative to the worm for a fraction of a revolution to gain inertia and speed to break loose a jammed gear train. 
     These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a coupling assembly in accordance with the present invention; 
     FIG. 2 is an exploded view of the coupling assembly of FIG. 1; 
     FIG. 3 is a cross-sectional view of the coupling assembly of FIG. 1; 
     FIG. 4 is a perspective view of the worm as mounted on the motor shaft in accordance with the present invention; 
     FIG. 5 is a perspective view of the inside face of the coupling washer, relative to the motor, of the coupling assembly of FIG. 1; 
     FIG. 6 is a perspective view of the coupling washer of FIG. 5, as mounted on the motor shaft with O-rings assembled therewith; 
     FIG. 7 is a block diagram of a conventional coupling and damping arrangement; and 
     FIG. 8 is a block diagram showing the space-saving coupling assembly according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Although only one preferred embodiment of the invention is explained in detail, it is to be understood that the embodiment is given by way of illustration only. It is not intended that the invention be limited in its scope to the details of construction and arrangement of components set forth in the following description or illustrated in the drawings. Also, in describing the preferred embodiments, specific terminology will be resorted to for the sake of clarity. It is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. 
     As shown in FIGS. 1-4, the present invention is directed to a coupling assembly, generally designated by the reference numeral  10 , which is mounted immediately adjacent a motor  12  on a motor shaft  14 . The assembly  10  includes a worm  20 , generally designated by the reference numeral  20 , an annular thrust ring  30 , a coupling washer, generally designated by the reference numeral  40 , and O-rings  50 . As can be seen in FIG. 3, the inner diameter of the worm  20  is larger than the outer diameter of the shaft  14  so that the worm rides on the shaft with a slip fit. The worm is held in place axially by means of the thrust ring  30  on one side, and the coupling washer  40  on the other side. Each of the thrust ring  30  and the coupling washer  40  are press fit onto the motor shaft  14 . 
     The worm  20  includes a head portion  22  and a helical geared portion  24 . The head portion  22  includes coupling elements  26  which are raised relative to an outer planar surface  28  of the head portion. The outer planar surface  28  is substantially perpendicular to the longitudinal axis of the helical geared portion  24  and generally cylindrical. The coupling elements  26  fit cooperatively with mating portions of the washer  40 , transmitting torque from the motor shaft  14  to the worm  20 . 
     As shown in FIG. 4, the coupling elements  26  have a generally triangular shape as viewed perpendicular to the outer surface  28 , with an inner truncated vertex  26 A and two outer vertices  26 B. The peripheral convex outside edge  26 C of each element  26  between the two outer vertices  26 B is flush with the circumference of the head portion  22 . The truncated inner vertex  26 A is concave such that the coupling element has an inner curvature that approximates that of the shaft but does not contact the motor shaft  14 . Alternatively, the inner vertex  26 A may be constructed without truncation so as to have a structure like that of the outer vertices  26 B. Two concave sides  26 D join the inner vertex  26 A with each of the two outer vertices  26 B, respectively. The coupling elements  26  are preferably machined into the head portion  22  of the worm. 
     As shown in FIGS. 1,  2  and  5 , the washer  40  has a flat outer face  44 A generally parallel with an inner face  44 B, and two straight, generally parallel, sides  40 A joined by two slightly convex sides  40 B. The outer face  44 A may meet the sides  40 A,  40 B at substantially a right angle or the edge therebetween may be beveled. Protruding from each of the straight sides  40 A is a torque-transmitting element, generally designated by the reference numeral  42 , which also extends inwardly so as to project beyond the otherwise planar inner face  44 B of the washer  40 . In addition, the washer has a cylindrical central opening  46  to receive the motor shaft  14 . 
     Each of the torque-transmitting elements  42  has a substantially planar inside face  42 A, and side faces including a generally concave inner portion  42 B nearest the central opening  46 , and a generally convex portion  42 C. An angular portion  42 D joins each end of the concave portion  42 B to an adjoining end of the convex portion  42 C. The resulting structure is convexo-concave, i.e., the convex portion  42 C of the side face has greater curvature than the concave portion  42 B. The inside face  42 A may meet the side face portions  42 B,  42 C,  42 D at substantially a right angle or the edges therebetween may be beveled. The shape of the washer enables the torque-transmitting elements  42  to fit cooperatively between the coupling elements  26 , with the inside faces  42 A against the outer surface  28  of the head portion  22  of the worm  20 . 
     As illustrated in FIG. 6, standard annular O-rings are fitted on the torque-transmitting elements  42 . The angular portions  42 D of the torque-transmitting elements  42  orient corresponding portions of the O-rings advantageously to complement the curvature of the concave sides  26 D of the coupling elements  26 . Thus, the geometries of the torque-transmitting elements  42  and the coupling elements  26  are adapted to each other to ensure the O-rings are only compressed between flat or slightly curved faces, and thus not pinched and possibly damaged by an edge of these torque-transmitting features. 
     During assembly, the thrust ring  30  is pressed onto the motor shaft  14  toward, but not in contact with, the motor housing. The worm  20  is then slipped onto the shaft  14  with the coupling elements  26  facing away from the thrust ring  30 . The O-rings  50  are fitted onto the torque-transmitting elements  42  of the coupling washer  40 , and the washer  40  is then pressed onto the motor shaft, leaving sufficient axial clearance to allow the worm to turn within the limits of the coupling geometry. Additional wear washers (not shown), such as between the worm  20  and the coupling washer  40 , may be added during this assembly sequence depending upon the expected life of the product. 
     In operation, the O-rings are compressed when torque is transmitted to the worm. In case of shock load, the O-rings are increasingly compressed as the torque becomes higher and higher. Due to the forgiveness of the O-rings, the peak torque, which determines the loading of adjacent components, e.g., the gears, bearings, motor shaft, casing and the coupling itself, is reduced. 
     The present invention is advantageous whether the motor shaft is running against a point contact outside the motor or internally within the motor housing. If the thrust from the worm is taken up by the ends of the motor shaft thrusting against an outside housing or casing, the diameter of the thrust face can be low, virtually a point contact, and will typically be no more than the outer diameter of the motor shaft. The inventive arrangement is particularly effective with such a configuration, i.e., with the motor is running against point contacts or small flat faces on its end, i.e., thrusting with the motor shaft ends, rather than against an internal contact inside the motor housing, because the least amount of force (voltage) is necessary to effectively break free, and friction generated by the thrust forces of the worm is minimized, maximizing thrust transference. 
     If, on the other hand, point contacts are not used and the motor takes up the thrust internally within the motor housing, a thrust ring mounted on the motor shaft is needed to transmit the force. Since such a thrust ring must be larger than the diameter of the motor shaft, more friction is generated. Nevertheless, the geometries of the torque-transmitting elements  42  and the coupling elements  26  still allow the motor shaft  14  to turn relative to the worm  20  for a fraction of a revolution to gain inertia and speed before the washer begins to transmit torque to drive the worm. This allowed movement, and the resulting inertia and speed gained thereby, facilitates breaking free from a stalled condition, e.g., a jammed gear train. 
     Another advantage realized through the inventive design is reduced space requirements. With conventional worm gears, representatively depicted in FIG. 7, the worm  20 ′ and mating helical gear  17  are at a distance from the motor  12 . The motor shaft  14 ′ holds part of the coupling  8 A, while the worm  20 ′ and a mating coupling portion  8 B are mounted on an auxiliary shaft  15 . As can be seen, the spacing of the worm  20 ′ and helical gear  17  from the motor  12  creates wasted space  55 . 
     By contrast, the design of the present invention, shown in FIG. 8, positions the coupling and damping arrangement on the far end of the motor shaft  14 , with the worm  20  then being as close to the motor  12  as the dimensions of the mating helical gear  17  will allow. The result is a very compact arrangement with no wasted space. 
     The coupling washer  40  is preferably made of sintered metal which allows a large design freedom in the geometry of the torque-transmitting elements  42  that transmit the torque and hold the O-rings. The worm is machined such that the mating geometry of the coupling elements  26  that receive the torque is easily manufactured in a shape that requires only a simple milling operation from two sides to achieve such coupling elements  26 . 
     In summary, the present invention represents a space-saving arrangement that provides safe break-free capability from a jammed gear condition with a minimal amount of power. No separate is worm bearing system is necessary, and manufacture is simplified through machining of the worm with coupling features and the use of standard O-rings which obviates the need for custom molded damping elements. 
     The foregoing descriptions and drawings should be considered as illustrative only of the principles of the invention. The invention may be configured in a variety of shapes and sizes and is not limited by the dimensions of the preferred embodiment. Numerous applications of the present invention will readily occur to those skilled in the art. Therefore, it is not desired to limit the invention to the specific examples disclosed or the exact construction and operation shown and described. Rather, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.