Abstract:
The present invention is organized about the concept of providing an improved pawl brake assembly for use in electromechanical devices such as all electric motors or solenoids. For example, the improved pawl brake assembly may be used for braking the rotor assembly upon de-energization of the motor. The pawl brake assembly comprises a pawl arm and a brake portion. The pawl brake assembly employs electromagnetic principles to reduce the noise and metal fatigue caused by the undesired excess vibrational motion of a pawl arm wen a motor is energized. The novel use of a magnet disposed upon the pawl arm of a pawl brake assembly provides a single, elegant, and inexpensive solution to the identified noise and metal fatigue problems.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present invention is based on and claims priority to U.S. Provisional Patent Application Serial No. 60/127,790 filed on Apr. 5, 1999 and is a national stage application of PCT International Application No. PCT/US00/08869 filed Apr. 4, 2000 published in English on Oct. 03, 2000 as Publication No. WO 00/60722. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to electric motors and solenoids, and in particular to an improved pawl brake assembly for any electric motor or solenoid. 
     BACKGROUND OF THE INVENTION 
     Many electric motors manufactured today use a pawl brake assembly to stop the movement of the rotor assembly when the motor is de-energized. Typically, in prior art brake designs when a motor is de-energized, the magnetic field generated in the stator core diminishes causing a spring in cooperative contact with the pawl brake assembly to pull the pawl brake into frictional engagement with the rotor assembly. 
     A disadvantage with prior art pawl brake assembly designs is that when the motor is energized, the portion of the pawl arm proximate to the motor lamination stack has excess vibrational movement. This excess movement causes the pawl arm to rattle and bump against the motor lamination stack, resulting in undesirable noise levels. Thus, there is a need in the art for a solution to the noise and metal fatigue caused by the excessive movement of the pawl arm of a pawl brake assembly. 
     SUMMARY OF THE INVENTION 
     The present invention is organized about the concept of providing an improved pawl brake assembly for use in electromechanical devices such as all electric motors or solenoids. For example, the improved pawl brake assembly may be used for braking the rotor assembly upon de-energization of the motor. The pawl brake assembly employs electromagnetic principles to reduce the noise and metal fatigue caused by the undesired excess motion of a pawl arm when a motor is energized. 
     In an exemplary embodiment according to the invention, the pawl brake assembly comprises a pawl arm and a brake portion in pivotal engagement with one another. The pawl arm may be composed of any metal alloy subject to the influence of a magnetic field, and the brake portion may be composed of any material such as plastic. The pawl arm may comprise a first end, a second end, and an extending trigger portion radially disposed between the first end and the second end of the pawl arm. The movement of the first end of the pawl arm is governed by pivotal contact with a fastener used to secure a bearing retainer to the stator assembly of an electric motor. The radially extending trigger portion of the pawl arm is in pivotal contact with the brake portion of the pawl brake assembly, e.g. via a pivotal groove. 
     To reduce noise and metal fatigue caused by excess vibration of the pawl brake assembly, a permanent magnet, e.g. a magnetic strip, is disposed on the second end of the pawl arm. When the motor is energized, sufficient magnetic attraction is established between the stator assembly and the magnetic strip to force and hold the second end of the pawl brake arm against the motor lamination stack. Significantly, the vibration common to prior art is eliminated. The absence of excess vibrational movement on this portion of the pawl brake assembly results in quiet operation of the motor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     For a better understanding of the present invention, together with other objects, features and advantages, reference should be made to the following description of the preferred embodiment which should be read in conjunction with the following figures wherein like numerals represent like parts: 
     FIG.  1 : is a side view of an exemplary motor wherein the motor is de-energized and a prior art pawl brake assembly is engaged; 
     FIG.  2 : is a front view of the motor shown in FIG. 1; 
     FIG.  3 : is a side view of an exemplary motor wherein the motor is energized and a prior art pawl brake assembly is disengaged; 
     FIG.  4 : is a front view of the motor shown in FIG. 3; 
     FIG.  5 : is a side view of a motor assembly including an exemplary embodiment of the present invention wherein the motor is de-energized and the pawl brake assembly is engaged; 
     FIG.  6 : is a front view of the motor assembly of FIG. 5; 
     FIG.  7 : is a side view of a motor assembly including an exemplary embodiment of the present invention wherein the motor assembly is energized and the pawl brake assembly is disengaged; and 
     FIG.  8 : is a front view of the motor assembly of FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In an exemplary embodiment, the present invention is useful in connection with an AC C-frame motor whose design possesses components required for operation via electromagnetic principles well known to those skilled in the art. It should be noted that because the present invention operates via electromagnetic principles, it may be adapted for use in all electric motors and solenoid devices. 
     FIGS. 1 and 2 illustrate an exemplary embodiment of a prior art design wherein the AC C-frame motor is de-energized (i.e. no electric input to the motor) and the pawl brake assembly is in the braking position. FIGS. 3 and 4 illustrate an exemplary embodiment of a prior art design wherein the AC C-frame motor is energized (i.e. electric input to the motor) and the pawl brake assembly is not engaged. The main components of the illustrated AC C-frame motor assembly  2  are: a rotor assembly  40 ; and a stator assembly  10  having a C-frame portion  12 , and I-bar portion  14 , and a coil assembly  16 . The C-frame portion  12  and the I-bar portion  14  may be made up of stacks of thin, steel laminations held together in intimate contact by welding, adhesive bonding, or rivets. The combined laminations of the C-frame portion and the I-Bar portion of the stator shall be referred to as the motor lamination stack  22 . As is known, the size and power of the motor  2  maybe determined in part by the number of laminations included in the stator assembly  10 . The coil assembly  16  may be made up of a plastic bobbin  18  with a specific number of turns of wire; i.e. a coil (not shown) wrapped around the bobbin  18 . The size of the wire and the number of turns on the bobbin  18  are calculated to provide the desired performance characteristics of the motor  2 . 
     The rotor assembly  40  may include a drive shaft  24 , a two-pole permanent magnet  26 , and a bearing  28 . The magnet  26  is positioned circumferentially around the drive shaft  24 , and the bearing  28  is disposed circumferentially around the end of the drive shaft  24  in a manner familiar to those skilled in the art. Finally, a bearing retainer  30  secures the rotor assembly  40  in position relative to the stator assembly  10  via two mounting posts  42  and  44 . The bearing retainer  30  may have at least two posts  42 , and  44  with bores formed therein which align with corresponding bores (not shown) formed through the stator assembly  10 . At least two fasteners (i.e. bolts or screws)  36 , and  38  pass through the at least two bores (not shown) in the bearing retainer  30 , and the corresponding bores (not shown) in the stator assembly  10 . The bearing retainer  30  thereby secures the rotor assembly  40  to the stator assembly  10  in a manner well known to those skilled in the art. In turn, the two posts  42 , and  44  provide pivot points for the motion crucial to the operation of the pawl brake assembly of the present invention. The rotor drive shaft  24  may extend to a gearbox  46  for driving an output shaft through a gear train (not shown). 
     It is well known that the pawl brake assembly  50  may be mounted underneath the bearing  28  of the rotor assembly  40  in such an orientation so as to permit the pawl brake assembly  50  unimpeded cooperative contact with the rotor assembly  40  when the motor  2  is de-energized. Such cooperative contact between the pawl brake assembly  50  and the rotor assembly  40  facilitates the braking function of the pawl brake assembly  50 . 
     As illustrated in FIGS. 1-4, the pawl brake assembly  50  consists of a pawl arm  52  with an extending trigger portion  54  and a brake portion  56  in cooperative contact with the extending trigger portion  54 . A first end  58  of the pawl arm  52  is pivotally disposed around a first post  42  of the bearing retainer  30 . A second end  60  of the pawl arm  52  is disposed adjacent to the motor lamination stack  22  for removing the brake portion  56  of the pawl arm assembly  50  from frictional contact with the rotor assembly  40  upon energization of the motor  2 . The pawl arm  52  may be composed of any metal that would be subject to the influence of a magnetic field. 
     The extending trigger portion  54  of the pawl arm  52  is adapted to interact with the brake portion  56  of the pawl brake assembly  50  via a pivotal groove in the brake portion (not shown). The brake portion  56  of the pawl brake assembly  50  may be composed of a variety of materials, e.g. plastic. 
     As can be seen in FIGS. 1-4, the brake portion  56  of the pawl brake assembly  50  as well as the spring  70  in which it is in cooperative contact, are positioned to pivotally engage a second post  44  of the bearing retainer  30 . The steel spring  70  is cooperatively engaged with the brake portion  56  of the pawl brake assembly  50  in a manner readily apparent to those skilled in the art, to bias the head  64  of the brake portion  56  into frictional engagement with the rotor assembly  40 . 
     As is well known, when electric current creates a magnetic field in the stator assembly  10 , the second end  60  of the pawl arm  52  is attracted to the stator assembly  10 , as shown in FIGS. 3 and 4. As a result, the distal end (not shown) of the extending trigger portion  54  of the pawl arm  52  forces the head  64  of the brake portion  56  of the pawl brake assembly  50  out of engagement with the rotor assembly  40  against the bias of the spring force. Conversely, when the motor  2  is de-energized, as shown in FIGS. 1 and 2, the magnetic force in the stator assembly  10  is diminished, the second end  60  of the pawl arm  52  falls away from the motor lamination stack  22 , and the spring force is sufficient to return the head  64  of the brake portion  56  of the pawl brake assembly  50  into frictional engagement with the rotor assembly  40 . 
     Operation of the exemplary embodiment of the prior art has been problematic. To maintain free and smooth operation of the motor  2 , the pawl arm  52  is mounted with considerable shaft clearance. Consequently it vibrates and is noisy during the motor operation. Additionally, this vibration of the pawl arm  52  may cause metal fatigue and eventual malfunction. In the prior art, the problems of noise and metal fatigue have been ameliorated somewhat by affixing a rubber bumper  74  onto the surface of the second end  60  of the pawl arm  52  in order to cushion the interaction between the pawl arm  52  and the motor lamination stack  22 . The prior art has also attempted to solve this problem by reducing the shaft clearance. However, it has been found that if the clearances are reduced, malfunction of the motor may occur. 
     FIGS. 5-8 illustrate an exemplary embodiment of the present invention that addresses the identified problems. FIGS. 5 and 6 illustrate an exemplary embodiment of the present invention wherein the AC C-frame motor  2  is de-energized (i.e. no electric input to the motor) and the pawl brake assembly  50  is engaged. FIGS. 7 and 8 illustrate an exemplary embodiment of the present wherein the AC C-frame motor  2  is energized (i.e. electric input to the motor) and the pawl brake assembly  50  is not engaged. In the present invention, a permanent magnet  80  is disposed upon the surface of the second end  60  of the pawl arm  52 , replacing the inefficient bumper  74 . The magnet  80  may be a magnetic strip or may be configured in any appropriate size and shape that will not interfere with the necessary spatial clearance between the pawl arm  52  and the motor lamination stack  22 . The magnet  80  may be secured to the pawl arm  52  in a variety of ways, e.g. by an adhesive. When the motor  2  is energized, the magnet  80  is attracted to the motor lamination stack  22  with just enough magnetic force to hold and stabilize the second end of the pawl arm  60  against the motor lamination stack  22 . Advantageously, the vibration of the second end  60  of the pawl arm  52  and the noise associated therewith are eliminated. When the motor  2  is de-energized, the pawl arm  52  moves away from the motor lamination stack  22 , and the head of the brake portion  64  engages the rotor assembly  40  under the force of the spring  70 . 
     The novel use of a magnet  80  in cooperation with a pawl brake assembly  50  provides a simple, elegant, and inexpensive solution to the identified noise and metal fatigue problems. Employing electromagnetic principles to improve the operation of all electric motors by including a permanent magnet on a pawl brake assembly has the additional advantage of solving the identified problems without having to significantly re-fashion the physical design of the motors. 
     The embodiments which have been described herein, however, are but some of the several which utilize this invention and are set forth here by way of illustration but not of limitation. For example, the novel use of a magnetic strip to stabilize a pawl arm during the energizing of a motor would apply to all solenoid devices. It is obvious that many other embodiments, which will be readily apparent to those skilled in the art, may be made without departing materially from the spirit and scope of this invention. Accordingly, the foregoing description is by way of example only, and is not intended to be limiting. The invention is limited only as defined in the following claims and the equivalents thereto.