Electrical generator or motor structure – Dynamoelectric – Rotary
Utility Patent
1997-07-10
2001-01-02
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S242000, C310S245000, C310S246000, C310S249000
Utility Patent
active
06169351
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a brush assembly for a dynamoelectric machine, preferably a universal fractional horsepower motor.
BACKGROUND OF THE INVENTION
In fractional horsepower universal motors commonly used for power tools and appliances, electrical contact is made to the rotating armature coils through the engagement of carbon brushes with a rotating commutator. Electrical contact from the brush to the motor circuit may be provided through a shunt wire embedded in the brush or through a spring biased arm engaged with the brush. The brushes, which are made carbon, are softer than the commutator and are worn away. The brush is typically slidably mounted in a holder and is biased by a spring or a spring biased arm toward the commutator to maintain electrical contact as the brush wears with use of the motor. Typically, for cost and simplicity, there is no specific means to identify when the brush is worn and should be replaced. The need for replacement is typically identified by the user when the tool stops working. This method for detecting when the brushes should be replaced has led to some significant problems. The travel of the spring or arm must be limited when the brush is worn to prevent contact of the arm or spring with the commutator which would damage the commutator. In addition if a brush with a shunt wire is used, the travel of the brush must be limited to prevent engagement between the shunt wire embedded in the brush and the commutator. Use of a shunt wire thus reduces the usable length of the brush because of the presence of the wire within a portion of the brush.
Shunted brushes have other disadvantages compared to shuntless brushes including higher manufacturing cost, high operating temperature and lower reliability. The manufacturing cost is higher because of the cost of forming the brush with the shunt wire embedded. A shunted brush has a high operating temperature because the shunt wire is normally electrically insulated and is therefore not easily cooled. Heat buildup increases the resistance of the brush assembly and therefore increases electrical losses. The shunted brushes have lower reliability because the wire is subject to snagging during assembly and operation of the motor.
To extend brush life and to mitigate the other disadvantages, shuntless brushes are commonly used for some applications. However, in the development of shuntless brushes, damage to the brush has been observed as the brush reaches an end of life condition. Such problems have been found to occur particularly in high current power tool motor applications having ratings of 6 to 8 amps. Thus there is a need to develop a shuntless brush assembly that is less subject to damage at the end of life of the brush.
SUMMARY OF THE INVENTION
According to the present invention a brush assembly comprises a tubular brush holder, a carbon brush, an arm for biasing the brush and a spring for biasing the arm. The holder has a cavity extending in a first direction, defined by the sidewalls of the holder and having an inner and outer open ends. The brush is slidably mounted in the cavity of the holder. A stop is formed between the brush and the holder and has a disengaged position to permit travel of the brush partially through the inner end of the holder and an engaged position to limit travel of the brush through the inner end of the holder. The electrically conductive arm is pivotally mounted on the support and extends through a slot of the holder. A distal end of the arm engages the outer end of the brush and biases the brush toward the inner end of the cavity. The spring is connected between the support and the arm for biasing the arm into engagement with the brush.
A significant feature of the invention is that the arm is spaced outwardly of the end of the slot to apply a bias to the brush when the stop is in an engaged position. As a result the brush when worn out is firmly held between the arm and the stop. This has resulted in a reduction in degradation of the components of the brush assembly.
The stop is preferably constituted by first and second grooves formed respectively in first and second sidewalls of the brush and mating first and second ribs formed in sidewalls of the holder. The grooves extend from the inner end of the brush toward the outer end of the brush and slidably receive the ribs for guiding movement of the brush in the cavity of the holder as the brush wears. First and second end walls are formed, respectively, in the first and second grooves inwardly of the brush outer end. The stop engaged position is formed when the end walls of the grooves engage the ribs and limit travel of the brush through the inner end of the holder.
The arm is preferably connected to the support through a low friction pivot formed by a base of the arm pivotally biased against a sharp edge of a post integrally formed with the holder.
The spring provides essentially a constant bias on the brush through the travel of arm as the brush is worn. The spring preferably has one end anchored to the post, an intermediate section coiled about the post and the other end engaged with the midsection of the arm.
Other aspects and advantages of the invention will be apparent from the detailed description of the preferred embodiment, the appended claims and the accompanying drawings or may be learned by practice of the invention.
REFERENCES:
patent: 1307028 (1919-06-01), Sweet
patent: 2194620 (1940-03-01), Sekyra
patent: 3656018 (1972-04-01), Maher
patent: 4354128 (1982-10-01), Chew
patent: 5895995 (1999-04-01), Soh
patent: 5939812 (1999-08-01), Wetzel
patent: 5952763 (1999-09-01), Bruhn
patent: 3328683 (1985-02-01), None
patent: 19523896 (1997-01-01), None
Bohart Russell J.
Wheeler Dale K.
Black & Decker Inc.
Del Ponti John D.
Dinh Le Dang
Ramirez Nestor
Shapiro Bruce S.
LandOfFree
Brush assembly for dynamoelectric machine does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Brush assembly for dynamoelectric machine, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Brush assembly for dynamoelectric machine will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2515377