Abstract:
A small-size motor of a type comprising brushes and a commutator slidably engaged with the brushes, characterized in that a motor case is filled with an atmosphere containing vapor of at least one organic compound selected from the group consisting of paraffins, mono- or poly-hydric alcohols, ethers, cyclic ethers, esters, ketones, ether alcohols, ester alcohols, aminoalcohols, carboxylic acids, amides, primary, secondary and tertiary amines, imidazoles, imidazolines and monocyclic oxyterpenes, which is stably operated and has a prolonged duration of life.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a small-size motor. More particularly, it relates to a small-size motor which is filled with an atmosphere containing an organic compound vapor so as to prolong its duration of life.  
         BACKGROUND OF THE INVENTION  
         [0002]    A small-size motor of a type comprising brushes and commutator elements slidably engaged with the brushes is, for example, used for driving a recording tape of a tape recorder, video tape recorder and the like.  
           [0003]    In case of a small-size motor in which make and break of contact most frequently occur, two kinds of brushes are typically used. One of them comprises a sheet or a wire rod of an electrically conductive resilient material (e.g., copper, beryllium-copper alloy, copper-titanium alloy, phosphor bronze and nickel silver) which is cladded with or partially spot welded by a noble metal or its alloy (e.g., gold, palladium, platinum, silver and copper or their alloys). The other comprises the above sheet or wire rod a part of which is provided with a brush element made of sintered carbon (e.g., carbon, Ag-carbon, Cu-carbon, AgCu-carbon, which optionally contain an additive such as SiC, MoS 2 , Pb and an epoxy resin) by means of an electrically conductive adhesive, spot welding or calking.  
           [0004]    The conventional small-size motor has several drawbacks such that when the commutator rotates with sliding on the brushes, sliding parts of the brushes unavoidably vibrate to an unacceptable extent, which results in abnormal welding of contact surfaces of the sliding parts of the brushes contacting with the commutator segments, and that spark often generates a black material, which is a mixture of carbon and worn powder formed between the brushes and the commutator segment and induces and worsens abnormal phenomena.  
           [0005]    In one type of the small-size motor, the sliding part of the brush, which is slidably engaged with the commutator segment, is integrally molded with a brush base supported by a supporting member provided on, for example, a cover of a motor case. A contacting state between the brushes and the commutator segments of such the motor is shown in FIG. 1.  
           [0006]    In FIG. 1, bent sliding parts  3 , 3  of brushes are mounted on respective brush bases  2 , 2  supported by respective brush supporting means  1 , 1 . The brush bases and the sliding parts of the brushes are inherently resilient so that they are pressed against and in contact with commutator segments  4 . The pressure to be exerted by the brushes are adjusted by selecting the angle between the brush base  2  and the sliding part of the brush  3 . The commutator segment  4  is fixed on a revolving shaft  6  of the motor via a commutator base  5  and rotates in accordance with the rotation of the motor shaft.  
           [0007]    During commutation and on make and break of the contact, spark is generated between the commutator segment  4  and the sliding part of the brush  3  and undesirably causes electric noise, and abrasion or welding of the commutator segment  4  and/or the sliding part of the brush  3 .  
           [0008]    For instance, when the welding occurs at a part A of the brush where the contact with the commutator begins and ends, the welded material scratches and damages the surface of the commutator segment  4 , and the thus abrased chips are trapped in gap B between the adjacent commutator segments to form of circuits between them. This results in unstable rotation of the motor. The rough surface of the commutator segment induces vibration of the brushes, enhances the spark generation and/or causes abnormal abrasion of the brushes, and further induces noise and/or dislocation of the commutation point. These result in the deterioration of the performance of the motor.  
           [0009]    The spark is most frequently generated at the part A of the sliding part of the brush  3  with which the commutator segment  4  contacts. Needless to say, the mechanical abrasion due to other causes than the spark is generated on the commutator segment  4  and the sliding part of the brush  3 .  
           [0010]    The sliding part of the brush is commonly made of the noble metal alloys or the sintered carbon as described in the above, while the commutator segment is made of the noble metal alloy (e.g., AgCu, AgCd, AgCuCd, AgCuTi, etc.). In case of the noble metal brush, the sliding part of the brush and the commutator segment are made of substantially the same material having high surface energy as follows (calculated from the sublimation heats at 20° C. cf. Chem. Rev., 52, 417 (1953)):  
           [0011]    Ag (100)=1,920 dyne/cm  
           [0012]    Ag (111)=1,650 dyne/cm  
           [0013]    Au (100)=2,516 dyne/cm  
           [0014]    Au (111)=2,175 dyne/cm  
           [0015]    Cu (100)=2,892 dyne/cm  
           [0016]    Cu (111)=2,499 dyne/cm  
           [0017]    Pt (100)=3,747 dyne/cm  
           [0018]    Pt (111)=3,248 dyne/cm.  
           [0019]    Thus, the sliding surfaces are disadvantageously scratched. In case of the sintered carbon brush, although it has smaller dynamic resistance than the noble metal alloy, it may scratch the surface of the commutator segment since it contains a small amount of impurities such as SiO 2 .  
           [0020]    Further, the noble metal-carbon brush is not desirable to be used in combination with the noble metal commutator segment because of the same reason as in case of the noble metal brush.  
           [0021]    [0021]FIG. 2A shows a brush not having a mechanical means for preventing spark, namely means for damping vibration. Since the sliding part of such brush vibrates with a large amplitude, it frequently induces spark and consequently abrasion and welding of the commutator segment.  
           [0022]    [0022]FIG. 2B shows a brush having means for damping vibration comprising a rubber vibration insulator  9  adhered on the sliding part of the brush  3  with an adhesive  8 . Although the rubber vibration insulator absorbs vibration and prevents generation of the spark at a room temperature, adhesivity of the adhesive is deteriorated at a temperature around 60° C. so that bonding between the insulator  9  and the sliding part  3  is weakened. As the result, the vibration insulating property is degraded and therefore spark is vigorously generated. This means that a motor comprising such brush does not have a long duration of life.  
           [0023]    [0023]FIG. 2C also shows a brush having another means for damping vibration comprising a vibration insulating sheet  12  adhered on the sliding part of the brush  3  with an adhesive sheet  11  comprising a substrate  10  both surfaces of which are coated with an adhesive  8 . The sheet  12  prevents deterioration of the adhesivity of the adhesive at a high temperature to some extent. However, it does not satisfy requirements for a long duration of life and high quality at a high temperature. The small-size motor is often used in a temperature range between −10° C. and +60° C.  
           [0024]    Some literatures and experiments have been analyzed conditions under which the welding between the sliding part of the brush and the commutator segment or abnormal abrasion or circuit is caused. In such conditions, an atmosphere surrounding the sliding part of the brush contains, or the commutator segment itself adsorbs an unsaturated cyclic hydrocarbon (e.g., styrene, toluene, etc.), which will form the black material (probably a decomposed material or a carbon-like polymer produced by a mechanochemical reaction or their mixture with powder abrased from the brush and the commutator segment).  
           [0025]    The amount of the black material increases when the unsaturated cyclic hydrocarbon is burnt by spark between the sliding part of the brush and the commutator segment. Probably because of incomplete combustion and/or its chemical structure, the material has dense bonding structure to form a hard carbonaceous mass.  
           [0026]    It has been found that, although a hydrocarbon other than the unsaturated cyclic hydrocarbon forms a black material, the material is not hard but soft and does not induce the above described abnormal conditions of the motor. According to Examples described below, following results were obtained:  
           [0027]    1. At a higher temperature and lower humidity, the commutator segment is more often abnormally abrased.  
           [0028]    2. When the commutator segment adsorbs as little as 10 μg of the unsaturated cyclic hydrocarbon, the malfunction of the commutator segment is decreased and the duration of life of the motor is prolonged, but the wow and flutter are worse.  
         SUMMARY OF THE INVENTION  
         [0029]    One object of the present invention is to provide a small-size motor in which slidability between the brushes and the commutator segment is improved so that wear and welding of them are decreased and thus the abrasion or mechanical loss of them is prevented, which results in a long duration of life of the motor.  
           [0030]    Another object of the invention is to provide a small-size motor which improves characteristics such as wow and flutter and a duration of life of a tape recorder and the like in that the motor is installed.  
           [0031]    These and other objects are achieved by a small-size motor according to the present invention of a type comprising brushes and a commutator slidably engaged with the brushes, characterized in that the interior is filled with an atmosphere containing vapor of at least one organic compound selected from the group consisting of paraffins, mono- or poly-hydric alcohols, ethers, cyclic ethers, esters, ketones, ether alcohols, ester alcohols, aminoalcohols, carboxylic acids, amides, primary, secondary and tertiary amines, imidazoles, imidazolines and monocyclic oxyterpenes.  
       
    
    
     BRIEF EXPLANATION OF THE DRAWINGS  
       [0032]    [0032]FIG. 1 shows a contact state of brushes and commutator segments of a small-size motor,  
         [0033]    [0033]FIGS. 2A, 2B and  2 C show three embodiments of brushes used in the small-size motor,  
         [0034]    [0034]FIGS. 3 and 4 show structures of preferred embodiments of the small-size motor of the invention,  
         [0035]    [0035]FIG. 5 is a cross section of one embodiment of the small-size motor of the invention,  
         [0036]    FIGS.  6 - 1  to  6 - 22  are graphs of commutating waves of small-size motors used in Examples. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]    The organic compound is impregnated in a suitable material such as felt, a synthetic or natural plastic or rubber open cell foam and installed in the motor case interior. Alternatively or additionally, it can be directly applied to an element of the motor such as the brush or the sliding part of the brush for simplicity of the motor construction.  
         [0038]    The organic compound is vaporized at an ambient temperature so as to provide an atmosphere of its vapor in the motor case interior The evaporated compound is adsorbed by the surfaces of the brush and/or the commutator segment to form a mono- or poly-molecular layer, so that the lubricity of the surface is improved and the abrasion of the surfaces is significantly reduced.  
         [0039]    [0039]FIGS. 3, 4 and  5  show embodiments of the small-size motor of the present invention.  
         [0040]    In FIG. 5, the small-size motor comprises an upper motor case  22  and a lower motor case  15 . The upper motor case  22  comprises a cylinder shape case having a top covering at one end and an opening at the other end in which a bearing  18  for supporting a shaft  6  is provided at the center of the top covering in a thrust direction. On an inner wall surface of the upper motor case  22 , a ring-form permanent magnet  21  is fixed.  
         [0041]    The plane lower motor case  15  is installed at the opening of the upper motor case  22  to sealingly close said opening and to form a motor case defining a motor case interior. On the inner surface of the lower motor case  15 , installed is a plate-shape plastic bracket  14  for supporting a lower bearing  7  which supports an unloaded end of the shaft  6 . The plastic bracket  14  also supports the brushes  13 .  
         [0042]    To the shaft  6  positioned in the motor case, a rotor core  20  is attached, which faces an inner surface of the magnet  21  with a small air gap therebetween. Around the rotor core  20 , coils  19  consisting of magnet wires are wound. In addition, commutator segments  4  and plastic commutator bases  5  are attached to the shaft  6 , and with the commutator segments  4 , the brushes  3  slidingly contacts. Between the end surface of the magnet  21  in the axis direction and the inner surface of the top covering, there is provided a felt  17  impregnated with the organic compound, and the vapor of the organic compound evaporates from the felt and fills the motor case interior. The evaporation rate of the organic compound can be controlled by selecting a thickness and/or apparent density of the felt.  
         [0043]    [0043]FIGS. 3 and 4 illustrate other embodiments of the small-size motor of the present invention. In the embodiment of FIG. 3, a felt  16  impregnated with the organic compound is placed on the bracket  14  close to the sliding part of the brush  3 , and in the embodiment of FIG. 4, the felt  16  impregnated with the organic compound is placed on the brush  3  together with the rubber vibration insulator  9 .  
         [0044]    The place where the organic compound-impregnated material is placed or the element in which the organic compound is impregnated is not limited in so far as the place or the element does not interfere or deteriorate the operation of the motor. For example, the organic compound may be blended in the plastic bracket  14  or the plastic commutator base  5 , impregnated in the magnetic wire of the coils  19 , blended in a lubrication oil of the upper bearing  18  and/or the lower bearing  17 , placed in the gap between the magnet  21  and the upper motor case  22 , blended in the rubber vibration insulator  9  or the adhesive  8 . The organic compound may be contained in a gas permeable film or case made of, for example, polyethylene and placed in the small-size motor.  
         [0045]    The organic compound used according to the present invention does not form such hard black material as formed from the unsaturated cyclic hydrocarbon and reduces the sliding resistance between the sliding part of the brush and the commutator segment.  
         [0046]    At a high temperature and low humidity at which the abnormal conditions often occur in the motor, the motor case is provided with an atmosphere containing the organic compound. Since the organic material used according to the present invention has at least one polar group and is strongly adsorbed on a metal surface by van der Waals binding, its absorption on the metal surface is stable.  
         [0047]    The organic compound has preferably a boiling or sublimation point of 40 to 350° C., particularly 100 to 300° C. under atmospheric pressure. When the boiling or sublimation point is too low, the compound is too quickly evaporated and its effect does not last long. When the boiling point is too high, it is hardly evaporated and the motor case is not quickly filled with the atmosphere containing the compound vapor.  
         [0048]    Specific examples of the organic compound used according to the present invention are listed below together with a boiling point under 1 atm. unless otherwise indicated:  
         [0049]    Paraffins  
         [0050]    2,2,4-trimethylpentane (99.2° C.), n-decane (174.1° C.), n-dodecane (216.3° C.), tetradecane (253.6° C.).  
         [0051]    Monohydric alcohols  
         [0052]    n-propanol (97.2° C.), isopropanol (81.5-83.0° C.), isobutanol (106-109° C.), isoamyl alcohol (128-132° C.), cyclohexanol (160-162.4° C.), 2,4-dimethyl-3-pentanol (138.4° C.), 3-methyl-3-pentanol (122.4° C.), 3-ethyl-3-pentanol (143.1° C.), diacetone alcohol (167.4° C.), n-octanol (193-196.0° C.), 2-ethylhexanol (183.5° C.), tetrahydrofurfuryl alcohol (177-178.0° C./743 mmHg), 1,3-dimethoxy-2-propanol (169.0° C.), 1-methyl-1-cyclohexanol (168.0° C.), 7-menthanol (236.0° C.), isopropylbenzyl alcohol (246.0° C.), isobutylcarbinol (176-184° C.), benzyl alcohol (205.4° C.).  
         [0053]    Polyhydric alcohols  
         [0054]    propylene glycol (187° C.), ethylene glycol (197° C.), 1,3-butanediol (205-210° C.), 2,3-butanediol (182° C.), 1,4-butanediol (228° C.), 4-methyl-1,2-prpanediol (182° C.), 2-methyl-2,3-butanediol (177.5° C./750 mmHg), 1,5-pentanediol (242.5° C.), 1,6-hexanediol (134° C./10 mmHg), 2,5-hexanediol (218° C.), diethylene glycol (250° C.), 2-methyl-2,4-pentanediol (197.4° C.), 2-ethyl-1,3-hexanediol (240-250° C.), dipropylene glycol (215-245° C.), glycerol (290° C.).  
         [0055]    Ethers  
         [0056]    n-butyl ether (143° C.), isoamyl ether (173.2° C.).  
         [0057]    Cyclic ethers  
         [0058]    1,4-dioxane (101.4° C.), 1,3-dioxane (106° C.).  
         [0059]    Esters  
         [0060]    dimethyl succinate (195° C.), ethyl octanoate (208° C.), diethyl fumarate (214° C.), diethyl decanoate (242° C.), diethyl adipate (245° C.), methyl tartrate (280° C.), dietyl tartrate (288° C.).  
         [0061]    Ketones  
         [0062]    cyclopentanone (130.7° C.), 2-octanone (174° C.), 2-nonanone (195° C.), isophorone (215.2° C.), 2-undecanone (229° C.).  
         [0063]    Ether alcohols  
         [0064]    ethylene glycol monoethyl ether (134.8° C.), ethylene glycol monobutyl ether (167-173° C.), ethylene glycol dibutyl ether (203.6° C.), ethylene glycol monohexyl ether (208.1° C.), 1,3-dimethoxy-2-propanol (169° C.), diethylene glycol monomethyl ether (194.2° C.), diethylene glycol monoethyl ether (195° C.), diethylene glycol monobutyl ether (230° C.), tripropylene glycol monomethyl ether (242.5° C.), mono(polyoxyalkylene)ether of C 6 -C 24  alkane- or alkene-diol such as an adduct of hexylene glycol and 1 mole of ethylene oxide or propylene oxide (Adduct 1 or 2), an adduct of hexylene glycol and 2 moles of ethylene oxide or propylene oxide (Adduct 3 or 4), an adduct of hexylene glycol and 4 mole of propylene oxide (Adduct 5) and an adduct of hexylene glycol and 2 moles of ethylene oxide which is further added with 1 mole of propylene oxide (Adduct 6) having properties as follows:  
                                                                                   Thermogravimetric analysis 2)                  Water absorbance 1)     Half value   Final temp.           (%)   temp. (° C.)   (° C.)                        Adduct 1   8   164   215       Adduct 2   7.5   175   205       Adduct 3   20   185   220       Adduct 4   10   187   216       Adduct 5   8   200   225       Adduct 6   13   207   227                                          
 
         [0065]    These adducts may contain a small amount of diether used as a raw material or other unreacted compounds.  
         [0066]    Ester alcohols  
         [0067]    ethyl lactate (154° C.), ethyl glycolate (160° C.), 2-hydroxylethyl acetate (188° C.), diethyl L-maleate (253° C.).  
         [0068]    Aminoalcohols  
         [0069]    diethylethanolamine (163° C.), dibutylethanolamine (199° C.).  
         [0070]    Carboxylic acids  
         [0071]    pivalic acid (164° C.), isocrotonic acid (169° C.), isovaleric acid (177° C.), adipic acid (205.5° C.), hexanoic acid (205.8° C.), n-octanoic acid (239° C.), crotonic acid (189° C.).  
         [0072]    Amides  
         [0073]    n-ethylacetoamide (205° C.).  
         [0074]    Primary, secondary and tertiary alcohols  
         [0075]    2-octaneamine (163-164° C.), octylamine (188° C.), hexylamine (130° C./762 mmHg), 1,6-hexanediamine (81.5° C./10 mmHg), dibutylamine (160° C.), dicyclohexylammonium nitrite (255.8° C.), tripropylamine (157° C.), tributylamine (212° C.), 3-methyl-2-oxazolidinone (87-90° C./1 mmHg).  
         [0076]    Imidazoles and Imidazolines  
         [0077]    2-ethyl-4-methylimidazole (melting point, 44-55° C.), 2-ethyl-4-methylimidazoline (111° C./15 mmHg), 2-ethylimidazoline (110° C./15 mmHg), 2,4-dimethylimidazoline (108° C./15 mmHg), 1,2,3-benzotriazole (melting point, 93° C.).  
         [0078]    Oxazolidinones  
         [0079]    3-methyl-2-oxazolidinone (87-90° C./1 mmHg).  
         [0080]    Monocyclic oxyterpenes  
         [0081]    Υ-terpineol (114-115° C.), β-terpineol (209-210° C./752 mmHg), α-terpineol (219-221° C.), terpinen-4-ol (209-212° C.), maltol (sublimation at 93° C.), l-menthol (sublimation at 216.5° C.), d-camphor (sublimation at 209° C.).  
         [0082]    Among these organic compounds, the poly- and mono-hydric alcohols and the ether alcohols are preferred.  
         [0083]    The present invention will be hereinafter explained further in detail by following Examples.  
       EXAMPLES 1-22  
       [0084]    In Examples, used was a small-size DC motor of FIG. 5 having an outer diameter of 30 mm, a height of 25 mm and normal power of 0.1 W and comprising brushes made of sintered AgC and commutators made of phosphor bronze.  
         [0085]    The motor contained a sheet of polyester felt 17 which had been washed and defatted with polyfluoroethylene and impregnated with each organic compound.  
         [0086]    In each Example, following organic compound was used and the motor was operated on 4.2 V at 25 mA. The maximum period of time in which the motor was normally operated is shown in following Table 1. However, the operation was terminated at 1,500 hours.  
                               TABLE 1                                   Example No.   Organic compound   Operation time                            1 (comp.)   No organic compound     300 hours            2 (comp.)   Styrene     100 hours            3 (comp.)   Toluene     300 hours            4   n-Dodecane   1,500 hours            5   Benzyl alcohol   1,500 hours            6   2-Methyl-2,4-pentanediol   1,500 hours            7   Isoamyl ether   1,500 hours            8   1,3-Dioxane   1,500 hours            9   2-Nonanone   1,500 hours           10   2-Methyl-2,4-pentanediol/   1,500 hours               1 mole of propylene oxide           11   2-Hydroxyethyl acetate   1,500 hours           12   Diethyl fumarate   1,500 hours           13   Dibutylethanolamine   1,500 hours           14   Hexanoic acid   1,500 hours           15   n-Ethylacetoamide   1,500 hours           16   Octylamine   1,500 hours           17   Dibutylamine   1,500 hours           18   Tributylamine   1,500 hours           19   2-Ethyl-4-methylimidazole   1,500 hours           20   α-Terpineol   1,500 hours           21   2-Methyl-2,4-pentanediol   1,500 hours               containing 50% of styrene           22   3-Methyl-2-oxazolidinone   1,500 hours                      
 
         [0087]    A commutating wave properly reflects the abnormality of the motor. An abnormal wave indicates the abnormal operation of the motor such as unstable revolution, increase of demand power, failure of starting and abnormal revolution. The abnormal wave means that the noise is added on the normal wave or the abnormal part of the wave falls to the ground (GND) level (current=0) as seen in FIGS.  6 - 1  to  6 - 3 .  
         [0088]    The commutating waves before and after operation are shown in FIGS.  6 - 1  to  6 - 22 , in which bold and thin curves represent waves before and after operation, respectively.  
         [0089]    As understood from these waves, the motors containing the organic compounds used according to the present invention are stably operated after 1,500 hours while the conventional motors containing no organic compound or the unsaturated cyclic compound cannot be stably operated only after 100 or 300 hours.  
         [0090]    A motor having the same structure as that of FIG. 5, normal power of 0.04 W and comprising brushes cladded with AgPd and commutator made of a cladding material of AuAg and AdCd was operated on 12 V at 100 mA at a temperature of −10° C. and RH of 20-30%. The abrased amount of the brush, wow and flutter measured according to JIS C 5551, initial no-load current and initial no-load noise are shown in following Table 2.  
                                                     TABLE 2                           Abrased   Wow and                   amount   flutter                   of brush (μm)   (WTD)   Initial   Initial       Example   (after   (after   no-load   no-load       No.   1,000 hrs)   300 hrs)   current (mA)   noise (dB A )                                1   25   0.1   30   28       2   2   0.33   28   27       3   3   0.25   28   27       4   10   0.088   27   26       5   15   0.075   28   27.5       6   5   0.07   27   26       7   8   0.09   28   26       8   10   0.1   27   28       9   7.5   0.08   27   26       10   8   0.07   28   27       11   7   0.08   28   27       12   7   0.07   29   28       13   5   0.095   29   27       14   6   0.08   28   26       15   6   0.08   27   27       16   7.5   0.085   28   27       17   10   0.07   28   27       18   9   0.075   28   28       19   8.5   0.08   29   27       20   10   0.07   28   27       21   10   0.088   27   26       22   8   0.09   27   26                  
 
         [0091]    According to the present invention, the organic compound is adsorbed on the surface of the brushes and the commutator segments and prevents the abrasion and welding of the sliding surfaces of them.  
         [0092]    Even if the motor interior contains the vapor of the unsaturated cyclic compound such as styrene and toluene, the sliding surfaces of the brushes and the commutator segments are protected by the organic compound used according to the present invention so that the formation of the hard black material is prevented. Further, the imperfect contact between the brush and the commutator segment is prevented so that the electric noise is decreased and the motor is stably operated. This results in decrease of wow and flutter.  
         [0093]    When the small-size motor with operating voltage of 1-30 V and operating current of 100 mA is operated in an atmosphere containing the vapor of the organic compound used according to the present invention, the current is decrease by 5-10% since the organic compound has a lubricating property.  
         [0094]    In addition, the organic compound adsorbed on the surface of the brushes and the commutator segments have a damping effect to prevent vibration of the brushes caused by the sliding between the brushes and the commutator segments and thus decrease the mechanical noise by 1-10%.