Patent Publication Number: US-6698933-B2

Title: Thrust cap

Description:
This application is a divisional of application Ser. No. 09/679,548, filed on Oct. 6, 2000, the entire contents of which are hereby incorporated by reference and for which priority is claimed under 35 U.S.C. §120; and this application claims priority of application Ser. No. 9923857.8 filed in GREAT BRITAIN on Oct. 9, 1999 under 35 U.S.C. §119. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     This invention relates to an electric motor and in particular, to a miniature electric motor with a combined bearing cover/thrust plate. 
     Small electric motors are very common and are often used with pinions fitted to the output shaft. Usually, such motors will have a thrust bearing of some form to limit the axial movement of the shaft. However, often the pinion is fitted to the output shaft after the motor has been fully assembled by the purchaser of the motor. As a result, the thrust bearing is damaged when the pinion is fitted due to the relatively large force required to press the pinion on to the shaft which is bearing directly onto the thrust bearing. 
     One solution to this problem is to use stronger thrust plates but while this reduces the damage to the thrust plate, it does not eliminate the damage and it does add significantly to the cost of the motor. Alternatively, the force required to fit the pinion can be reduced but this can lead to other problems such as loose pinions. 
     Hence, there is a need for a thrust plate which can avoid being damaged during fitting of a pinion to the output shaft of a miniature electric motor. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a thrust cap for an electric motor comprising: a thrust cap body; a thrust bearing surface supported by the body; and securing means for securing the thrust cap body to the electric motor wherein the thrust bearing surface is sprung and arranged to be in continuous contact with an end of a shaft of the motor to urge the shaft axially away from the thrust bearing cap. 
     The present invention also provides an electric motor incorporating such a thrust cap. 
     Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a part sectional view of an electric motor incorporating a thrust cap according to a first embodiment; 
     FIG. 2 is an end view of the motor of FIG. 1; 
     FIG. 3 is an enlarged sectional view of a part of the motor of FIG. 1 showing the thrust cap; 
     FIG. 4 is a view similar to FIG. 3 showing a modified thrust cap according to a second embodiment; 
     FIG. 5 is a sectional view of a modified thrust cap according to a third embodiment; and 
     FIGS. 6 and 7 are schematic diagrams used to explain the function of the modified thrust caps. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A miniature PMDC motor  10  is shown in FIG.  1 . The motor has a housing, a permanent magnet stator  14  and a rotor  18 . The housing has a rear housing  12  and an end cap  30 . The rear housing  12  is a deep drawn can-like metal part having an open end and a closed end. The rear housing  12  supports the magnets of the stator  14 . The closed end of the rear housing supports a sintered bronze busing  16 . The rotor  18  has a shaft  20  journalled in the bushing  16 , an armature  22  and a commutator  24 . 
     The open end of the rear housing is closed by the end cap  30 . The end cap has two parts, a plastics material part  32  and a metal part  34 . The plastics part  32  supports and insulates the brush gear, of which only a single fingerleaf brush  36  is shown, and motor terminals  38 , from the metal part  34 . Posts  40  on the plastics part  32  are used to connect the two parts of the end cap together by being plastically deformed after being passed through corresponding holes in the metal part  34 . The metal part  34  has a bearing retainer  42  accommodating a second sintered bronze bushing  44  which supports one end of the shaft  20 . The bearing retainer has a central opening  46  through which access to the end of the shaft can be made. A thrust cap  50  closes this opening. 
     FIG. 2 is an end view of the motor of FIG. 1 showing the thrust cap  50  fitted to the end cap  30 . Also visible are the four posts  40  and two motor terminals  38 . The connection between the end cap  30  and the rear housing  12  is by way of crimps at the four locations labeled  48 . 
     The thrust cap  50  and the connection between the thrust cap and the bearing retainer  42  is more clearly shown in FIG.  3 . The thrust cap has a bearing surface  52  which is flat and arranged to be born against by the end of the shaft which is rounded to reduce friction. The thrust cap has four fingers  54  which have tapered heads  56  with shoulders  58  forming an abutment surface. The fingers are resiliently deformable to allow the heads to pass through the opening  46  and spring back to engage the shoulders with the inner surface of the bearing retainer  42 , thus forming a snap fit connection with the bearing retainer. The shoulders are sized and shaped to retain the thrust cap in position against the maximum or greatest expected axial thrust developed by the shaft in normal use. 
     The thrust cap is preferably formed from a moldable low friction material such as PTFE (TEFLON) to reduce friction as the thrust cap is in direct contact with the end of the shaft. 
     FIG. 4 shows a thrust cap  50  according to a second embodiment. This thrust cap is constructed and fitted to the motor in the same manner as the thrust cap of the first embodiment with the exception that the bearing surface  52  is formed on an arm  60  integrally formed with the thrust cap. The arm extends from the body of the thrust cap offset from the shaft axis and at an angle to the shaft axis in its free state. The arm, which is resiliently deformable provides a spring like effect on the bearing surface which contacts the rounded end of the shaft offset from the shaft axis. The spring like effect allows the bearing surface to remain in contact with the shaft as the shaft moves axially and provides an increasing resistance to axial movement as the shaft moves axially toward the thrust cap. 
     FIG. 5 shows a thrust cap  50  according to a third embodiment. This cap is similar in construction to the thrust cap of the first embodiment with the exception that the bearing surface  52  is provided by a U-shaped spring  62  fastened by rivet  64  to the body of the thrust cap which is very similar to the thrust cap of the first embodiment. As the body of the thrust cap is not in direct contact with the shaft  20 , it is not necessary for it to be made of low friction material and cheaper suitable materials can be used with the spring  62  being of suitable low friction spring material such as beryllium copper or stainless steel strips. 
     The thrust cap of the invention as illustrated by the embodiments allows a pinion or cog to be fitted to the shaft of the motor without damaging the motor&#39;s thrust bearing by providing a thrust bearing which can be fitted to the motor after the pinion of cog has been fitted. Before the thrust cap is fitted, access to the end of the shaft can be gained through the opening  46  in the bearing retainer to support the shaft during fitting of the pinion or cog. After the pinion or cog has been fitted, the thrust cap of the preferred embodiment is snapped into place to provide the thrust bearing to limit end play (axial movement of the shaft) during use. The thrust cap provides a further feature of spring loading the thrust bearing surface. This avoids the knocking noise which is generated as the motor shaft strikes a hard thrust surface. The second and third embodiments add the option of providing a transverse loading onto the end of the shaft to reduce bearing rattle. 
     FIGS. 6 and 7 are schematics showing the effect of the offset spring force and illustrate a further advantage of providing a sprung thrust face in which the point of contact is offset from the axial centre of the shaft. As the shaft end is rounded, an axial force offset from the axis will have a radial component as well as an axial component. The axial component limits the end play while the radial component will be arranged to assist gravity and/or other radial force to urge the shaft to contact the bushing&#39;s bearing surface at a predetermined location. This helps to reduce bearing rattle which occurs when the shaft, as it rotates, tries to crawl around the bearing surface and then falls. 
     Variations and modifications will be evident to the skilled addressee without departing from the spirit of the invention described and it is intended that all such variations and modifications are covered by this application.