Abstract:
A motor assembly includes a working part, a synchronous motor having a shaft, and a mechanical coupling joining the working part to the motor. The coupling has two driving teeth fixed relative to the shaft, two driven teeth fixed relative to the working part, and two middle members. The coupling provides a predetermined range of angular movement between the motor and the working part. The middle members are circumferentially distributed between the two driving teeth and between the two driven teeth, thereby each of the middle members is movable by the driving teeth to contact the driven teeth and provides an interference between a driving tooth and a driven tooth to drive the working part.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This non-provisional patent application claims priority under 35 U.S.C. §119(a) from Patent Application No. 201010539038.8 filed in The People&#39;s Republic of China on Nov. 10, 2010. 
     FIELD OF THE INVENTION 
     This invention relates to a motor assembly and in particular, to a single phase synchronous motor assembly having a mechanical coupling connecting the rotor of the synchronous motor with a working part. 
     BACKGROUND OF THE INVENTION 
     Small electric pumps, for example as used for circulating water in fish tanks and for draining water from washing machines, are well known. Such pumps are usually driven by sub-fractional horsepower single phase synchronous motors which have a long life and a simple construction allowing the stator to be fully sealed from the pump chamber and from the rotor of the motor. 
     Being a single phase synchronous motor, the motor can start in either direction and thus, the impeller of the pump is designed to operate in both directions. However, the starting torque is relatively low and the motor has difficulty starting under a full load. To overcome this problem, it is known to allow a limited range of free rotation between the rotor and impeller, usually up to about 360 degrees. This allows the motor to initially oscillate until it gains sufficient initial momentum before driving the impeller and its load. This is achieved by using a mechanical coupling, commonly referred to as a lost motion clutch, in which two spigots or teeth are arranged to interact. However, as the coupling allows free movement of slightly less than 360 degrees there is considerable noise and strain as the spigots crash together. 
     U.S. Pat. No. 4,861,240, sought to overcome this problem by having the spigots rotate within a sealed chamber containing a viscous fluid, e.g., an oil or grease, to absorb the knocking noise. As the volume of viscous fluid is relatively large and the pressure on the viscous fluid is also relatively large, this placed a great strain on the seal and introduced a possible source of contamination should the seal of the chamber fail during use, allowing the viscous fluid to mix with the fluid being pumped. 
     Thus, there is a desire for an improved coupling between a rotor of a synchronous motor and a working part such as an impeller of a pump. 
     SUMMARY OF THE INVENTION 
     Accordingly, in one aspect thereof, the present invention provides a motor assembly comprising: a synchronous motor having a shaft; a working part with a hollow body; and a mechanical coupling comprising a driving member having at least one driving tooth, at least one driven tooth and at least one middle member; wherein the at least one driving tooth projects outwardly from a cylindrical hub of the driving member fixed relative to the shaft; the at least one driven tooth projects inwardly from the hollow body and is radially spaced from the at least one driving tooth; and the at least one middle member is disposed between the cylindrical hub and the hollow body and to arranged to allow up to a predetermined range of relative angular motion between the driving member and the working part before causing an interference between the at least one driving tooth and the at least one driven tooth whereby the driving member is able to drive the working part. 
     Preferably, the at least one middle member is made of elastic material. 
     Preferably, there are two driving teeth, diametrically opposed to each other and each projecting from the cylindrical hub, there are two driven teeth, diametrically opposed to each other and each projecting from a hollow body of the working part, and there are two middle members each arranged between the cylindrical hub and the hollow body in a radial direction of the shaft and respectively disposed between the two driving teeth and respectively disposed between the two driven teeth. 
     Preferably, the radial width of each middle member is less than the radial distance between the cylindrical hub and the hollow body. 
     Preferably, the two middle members are arcuate in shape. 
     Preferably, the at least one middle member is configured to make an initial line contact with its corresponding driving tooth. 
     Preferably, a surface of the at least one middle member and an opposing surface of its corresponding driving tooth form an angle of between 10 to 80 degrees when the middle member is in line contact with said corresponding driving tooth. 
     Preferably, the at least one middle member is configured to make planar contact with its corresponding driven tooth. 
     Preferably, the cylindrical hub is connected to the shaft by a sleeve, and the sleeve and the cylindrical hub are made of plastic material. 
     Preferably, the driving member is captured within the hollow body of the working part by a disc, the disc being sealed to the hollow body and to the shaft by O-rings and axial play between the driving member and the working part is reduced by a spring disposed between two washers disposed about the shaft and between the disc and the driving member and each driving tooth has a ridge formed on a surface facing an end wall of the hollow body and the spring resiliently urging the ridge into contact with the end wall. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred embodiment of the invention will now be described, by way of example only, with reference to figures of the accompanying drawings. In the figures, identical structures, elements or parts that appear in more than one figure are generally labeled with a same reference numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are generally chosen for convenience and clarity of presentation and are not necessarily shown to scale. The figures are listed below. 
         FIG. 1  shows a motor assembly in accordance with the preferred embodiment of the present invention; 
         FIG. 2  is partial view of a mechanical coupling, being a part of the motor assembly of  FIG. 1 ; 
         FIG. 3  shows a driving member and middle members being parts of the coupling of  FIG. 2 ; 
         FIG. 4  shows an impeller forming a part of the coupling of  FIG. 2 ; and 
         FIG. 5  is a sectional view of the coupling of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in the drawings, a motor assembly  10  in accordance with a preferred embodiment of the present invention includes a single phase synchronous motor  11  having a shaft  16 , a working part  20  and a mechanical coupling connecting working part  20  to the shaft  16 . The mechanical coupling is of the lost motion clutch type, meaning that the coupling allows a certain maximum angular movement in either direction before the driving part drives the driven part. 
     The synchronous motor  11  includes a stator  12  with an electromagnet and a rotor with permanent magnets. The electromagnet comprises a coil wound about a stator core forming two stator poles  13  and  15 . The rotor is received in a rotor housing  14  between two stator poles. The rotor housing  14  is preferably made of plastic. In this embodiment, the working part  20  is an impeller of a small pump, which has a hollow central hub  22 , a plurality of blades  24  outwardly extending from the central hub  22 , and a hollow body  26  extending downwardly from the central hub  22 . The hollow body  26  forms an annular ring which is concentric with the shaft  16 . It should be understood that the working part  20  may be another kind of load which is suitable to be driven by a synchronous motor. 
     The mechanical coupling includes a driving member  30 , two driven teeth  40 , and two middle members  50 . 
     The driving member  30  is monolithic and includes a cylindrical hub  32  fixed to the shaft  16  so as to rotate with the shaft  16 , a base portion  34  outwardly extending from the lower end of the cylindrical hub  32 , and two driving teeth  36  projecting radially from the outer surface of the cylindrical hub  32 . The two driven teeth  40  project radially inwardly from the inner surface of the hollow body  26 . Preferably, the two driving teeth  36  are diametrically opposite to each other and the two driven teeth  40  are diametrically opposite to each other. The driven teeth are radially spaced from the driving teeth such that with the middle members removed, the driving member would freely rotate within the driven member. That is, the driving teeth and the driven teeth are axially aligned but radially spaced with the driving teeth driving the driven teeth through the middle members. 
     In this embodiment, in the rotational direction of the rotor, each of the driving teeth  36  and the driven teeth  40  has two end surfaces which are respectively defined as a front end surface and a rear end surface. For example, if, as shown in  FIG. 2 , the direction of rotation of the rotor is counter-clockwise, as indicated by the arrow, then the first end surface  38  of each driving tooth  36  is a front end surface, the second end surface  39  of each driving tooth  36  is a rear end surface, the first end surface  42  of each driven tooth  40  is a front end surface, and the second end surface  43  of each driven tooth  40  is a rear end surface. 
     The two middle members  50  are separate and circumferentially spaced so as to be able to move freely relative to each other. The base portion  34  is received in the hollow body  26 . The cylindrical hub  32  is received in the hollow body  26  and the central hub  22 . The volume bounded by the upper surface  35  of the base portion  34 , the cylindrical hub  32 , the inner cylindrical wall of the hollow body  26  and an end wall  23  of the hollow body forms a cavity  52  in which the driving teeth, driven teeth and middle members are located. Each middle member  50  is supported on the upper surface  35  of the base portion  34  thereby being limited between the outer surface of the cylindrical hub  32  and the inner surface of the hollow body  26  in the radial direction of the shaft  16 , and limited between the base portion  34  and the end wall  23  of the hollow body in the axial direction. The radial width of each middle member  50  is less than the radial distance between the outer surface of the cylindrical hub  32  and the inner surface of the hollow body  26  so that the middle member  50  is arranged between the cylindrical hub  32  and the hollow body  26  even when the middle member  50  is immersed in hot water, which causes the middle member  50  to expand. Thus, the situation that the middle member  50  is jammed between the cylindrical hub  32  and the hollow body  26  can be avoided. 
     The two middle members  50  are alternately distributed with the two driving teeth  36  and the two driven teeth  40  in the circumferential direction of the shaft  16 . That is, each middle member  50  is arranged between and able to be in contact with the front end surface  38  of its corresponding driving tooth  36  and the rear end surface  43  of its corresponding driven tooth  40  so that the mechanical coupling can rotate the working part  20  once the rotor starts rotating. By this configuration, the motor assembly can have a better dynamic balance. Each middle member  50  is respectively arranged between the two driving teeth  36  and between the two driven teeth  40 , thus the rotor is able to freely oscillate through a certain angular distance during the starting phase. 
     Preferably, the middle members  50  are made of an elastic material such as rubber and are arcuate in shape so as to slide along the inner surface of the hollow body  26 . Each middle member  50  may be configured to be in planar contact with the rear end surface  43  of its corresponding driven tooth  40  so as to prevent the middle member  50  from sliding into the gap between the inner surface of the driven tooth  40  and the outer surface of the cylindrical hub  32 . On the other hand, each middle member  50  may be configured to be in line contact with the front end surface  38  of its corresponding driving tooth  36 . When the middle member  50  is in line contact with its corresponding driving tooth  36 , two opposing surfaces of the middle member  50  and its corresponding driving tooth  36  may form an angle α of between 10 to 80 degrees, preferably between 30 to 60 degrees. Thus, the cylindrical hub  32  helps to absorb the shear force applied to the middle member  50  when the middle member  50  is pressed by the driving tooth  36  against the driven tooth  40 , avoiding breaking the middle member  50 . Also, by the initial line contact occurring at the root of the driving teeth, the middle member is more effectively pressed against the driven tooth by the slightly outwardly resultant force applied to the middle member help to keep the middle member engaged or pressed against the circumferential wall of the hollow body  26 . By the above configurations, the mechanical coupling can achieve a higher reliability and a longer life. 
     Although the cylindrical hub  32  of the driving member  30  may be pressed directly on to the shaft  16 , the driving member preferably has a further sleeve  31  disposed between the cylindrical member  32  and the shaft  16 . The shaft  16  has a plurality of axially extending knurls  17 . When the sleeve  31  is pressed on to the shaft the knurls bite into the sleeve to rigidly fix the sleeve to the shaft. The cylindrical hub  32  is pressed onto the sleeve  31  with a high friction or tight fit so as to rotate with the sleeve  31  and shaft  16 . However, this additional part, sleeve  31 , allows the driving member to slip on the shaft should the impact on the driving member be greater than a preset value, thereby preventing the impact force from causing twisting of the driving member on the shaft over the knurls which would quickly destroy the connection between the driving member and the shaft. This arrangement allows the use of a plastic driving member compared with typically brass driving members of the prior art due to the greater holding forces which can be achieved with brass on the shaft. 
     As shown in  FIG. 5 , the driving member  30  is held captive within the hollow body of the driven member  20  by a disc  28 . The shaft  16  passes through the disc and the radially outer edge of the disc has a groove which mates with a ridge  25  formed on the inner circumferential surface of the hollow body  26 . Optionally, to seal the cavity  52  in order to prevent contamination and loss of lubricant, if used, an O-ring  27  may be disposed in the groove to seal the disc to the hollow body  26  and a further O-ring  29  used to seal the disc to the shaft  16 . 
     Optionally, to limit axial movement between the working part  20  (such as a pump impeller) and the driving member  30 , a spring  46 , such as a disc spring, may be used to reduce axial play. The spring is disposed between two washers  48 , one in contact with the disc  28  and the other in contact with the driving member  30 . the spring and washers are disposed about the shaft and press the driving member into axial contact with the working part. In particular, the driving teeth  36 , each have a ridge  37  on their upper surface which makes contact with the end wall  23  of the hollow body. The ridges  37  significantly reduce the contact surface area between the driving member and the end wall compared to full contact between the driving teeth and the end wall. Alternatively, the cylindrical hub may contact the upper end of the central hub, but this is less preferred. 
     It should be understood that the mechanical coupling may have more than two driving teeth, more than two driven teeth, and more than two middle members which are alternately distributed between the driving teeth and between the driven teeth in the circumferential direction of the shaft. In another case, the mechanical coupling may have a single driving tooth, a single driven tooth, and a single middle member or two middle members disposed on respective sides of the driving tooth in the circumferential direction. 
     The motor assembly in accordance with the embodiment of the present invention is particularly suitable for a centrifugal pump, such as a drain pump for a dishwasher. 
     In the description and claims of the present application, each of the verbs “comprise”, “include”, “contain” and “have”, and variations thereof, are used in an inclusive sense, to specify the presence of the stated item but not to exclude the presence of additional items. 
     Although the invention is described with reference to one or more preferred embodiments, it should be appreciated by those skilled in the art that various modifications are possible. Therefore, the scope of the invention is to be determined by reference to the claims that follow.