Abstract:
A compact geared motor unit, capable of properly protecting an electric motor without disengaging gears or suspending the unit, even when a thrust force larger than an allowable value is generated. The geared motor unit  10  has an electric motor and a gear configured to generate a thrust force applied to the electric motor when transmitting a torque, and a control section for controlling the electric motor. The control section has a thrust load obtaining part which obtains a thrust load of the electric motor at predetermined timings or time intervals; a critical thrust load storing part which stores a critical thrust load of the electric motor; and a torque limiting part which electrically limits the torque of the electric motor when the obtained thrust load exceeds the stored critical thrust load.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a geared motor unit having a function for limiting a thrust load applied to a motor when transmitting power to the motor via a gear. 
         [0003]    2. Description of the Related Art 
         [0004]    In an electric motor having a structure for applying a constant preload to a bearing for supporting a rotor, when the preload is not applied to the bearing, the rotor may fluctuate and as a result the motor may be damaged or broken. In a geared motor in which a gear is attached to an electric motor, a thrust load may be generated in a direction so as to cancel the preload, due to the rotation of the motor, depending on the type of gear. When the generated thrust load is larger than an allowable torque, the stiffness of the motor may be lowered and the motor may fluctuate due to disturbance, whereby the motor may be damaged. 
         [0005]    In the prior art, a technique for protecting a geared motor from a thrust load larger than an allowable torque has been proposed. For example, JP 2003-322223 A discloses a gear unit wherein an annular projection formed on an inner periphery of a shaft hole of a helical gear is engaged with a groove formed on an outer periphery of a drive shaft to which the helical gear is attached, and the annular projection is disengaged from the groove when an axial thrust force exceeds a certain limit value. 
         [0006]    Further, JP S60-129453 A discloses a gear transmission for transmitting torque by engaging helical gears, wherein the gear transmission includes a load cell configured to detect an excess thrust force due to an excess torque and transmit an operation stop signal. 
         [0007]    The geared unit of JP 2003-322223 A can be prevented from being damaged by disengaging the helical gear from the drive shaft. Therefore, it is necessary to fit the helical gear with the drive shaft once again, which is troublesome. On the other hand, in the gear transmission of JP S60-129453 A, the operation of a machine including the gear transmission is stopped each time a thrust force larger than a limit value is generated, which may reduce operation efficiency of the machine. 
         [0008]    Alternatively, a torque limiter may be used for a geared unit, by which a torque of an electric motor is limited so that a load applied thereto does not exceed an allowable torque. However, in this case, it is necessary to prepare a space for positioning the torque limiter, between the motor and a transmission or between the transmission and a driven object, resulting in an increase in size of the unit. 
       SUMMARY OF THE INVENTION 
       [0009]    An object of the present invention is to provide a compact geared motor unit, capable of properly protecting an electric motor without disengaging gears or suspending the unit, even when a thrust force larger than an allowable value is generated. 
         [0010]    According to the present invention, a geared motor unit, comprising: an electric motor; a gear connected to the electric motor so that a thrust load is applied to the electric motor when transmitting a torque; and a control section which controls the electric motor, wherein the control section comprises: a thrust load obtaining part which obtains a thrust load of the electric motor; a critical thrust load storing part which stores a critical thrust load of the electric motor; and a torque limiting part which electrically limits the torque of the electric motor when the thrust load of the electric motor obtained by the thrust load obtaining part exceeds the critical thrust load of the electric motor stored in the critical thrust load storing part, is provided. 
         [0011]    In a preferred embodiment, the thrust load obtaining part calculates the thrust load of the electric motor based on the torque of the electric motor obtained by the control section and information regarding a dimension of the gear. 
         [0012]    Alternatively, the thrust load obtaining part may obtain the thrust load of the electric motor as a detected value of a thrust load detector which detects the thrust load of the electric motor. 
         [0013]    In a preferred embodiment, the critical thrust load storing part stores a value of the critical thrust load which is predetermined depending on a type of the electric motor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The above and other objects, features and advantages of the present invention will be made more apparent by the following description of the preferred embodiments thereof with reference to the accompanying drawings wherein: 
           [0015]      FIG. 1  shows a schematic configuration of a geared motor unit according to a preferred embodiment of the present invention; and 
           [0016]      FIG. 2  shows various types of electric motors used for the geared motor unit of  FIG. 1 , and shows a table representing a critical thrust load of each electric motor depending on the type thereof. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  shows a schematic configuration of a geared motor unit  10  according to a preferred embodiment of the present invention. Geared motor unit  10  has an electric motor  12  and a gear  16  connected to a drive shaft  14  of electric motor  12 . Gear  16  may be a helical gear, a straight bevel gear, a worm gear or a screw gear, etc., and is configured to generate a force in the axial direction (or a thrust force) applied to electric motor  12  when a torque of electric motor  12  is transmitted to a driven object (not shown) engaged with gear  16 . 
         [0018]    Geared motor unit  10  has a control section  18  for controlling electric motor  12 . Control section  18  has a thrust load obtaining part  20  which obtains a thrust load of electric motor  12  at predetermined timings or time intervals; a critical thrust load storing part  22  which stores a critical thrust load of electric motor  12 ; and a torque limiting part  24  which electrically limits the torque of electric motor  12  when the thrust load of electric motor  12  obtained by thrust load obtaining part  20  exceeds the critical thrust load of electric motor  12  stored in critical thrust load storing part  22 . In the embodiment of  FIG. 1 , torque limiting part  24  transmits a (normal) torque command to an amplifier  26  of electric motor  12  at predetermined time intervals or transmits a limited torque command to amplifier  26  when the thrust load exceeds the critical thrust load of electric motor  12  stored in thrust load storing part  22 . 
         [0019]    Also, geared motor unit  10  may have an input section  28 , such as a keyboard or a touch panel, etc., capable of being operated by an operator. In this case, the operator can input various information (for example, a type of the electric motor as explained below) and/or commands to control section  18  via input section  28 . 
         [0020]    Next, a working example, in which thrust load obtaining part  20  obtains the thrust load of electric motor  12 , will be explained. As shown in  FIG. 1 , by using a thrust load detector  30  (in the drawing, detector  30  is attached to drive shaft  14 ) for measuring the thrust load of electric motor  12 , thrust load obtaining part  20  may obtain a detected value (or an output) of thrust load detector  30  as the thrust load applied to the motor. In addition, a commercially available detector may be used as thrust load detector  30 . 
         [0021]    Alternatively, thrust load obtaining part  20  may calculate thrust load Fx based on information (characteristic values) regarding the dimension of gear  16  and torque T of electric motor  12  obtained by control section  18 , and may obtain calculated thrust load Fx as the thrust load of electric motor  12 . In this regard, the information of gear  16  may include the type of the gear and characteristic values such as a dimension of each portion of the gear. Concretely, regarding the helical gear, the straight bevel gear, the worm gear and the screw gear as described above, thrust load Fx can be calculated by using following equations (1) to (4), respectively. As torque T, a feedback value (or a torque detected value) used to control electric motor  12  may be used, or, a calculated value by using necessary information, including a current value, a rotational frequency and an amount of slip, may be used, or, a torque command value for amplifier  26  may be used. 
         [0022]    The meanings of characters in equations (1) to (4) are as follows. In this regard, a sign (or a direction) of the thrust load is determined by a helix direction of the gear. 
         [0023]    Friction coefficient: μ 
         [0024]    Diameter of reference circle: d [mm] 
         [0025]    Face width: b [mm] 
         [0026]    Teeth number: z 
         [0027]    Pressure angle: α 
         [0028]    Helix angle: β 
         [0029]    Normal pressure angle: αn 
         [0030]    Module: m [mm] 
         [0031]    Torque: T [N·m] 
         [0032]    When gear  16  is a helical gear, equation (1) is true. 
         [0000]        Fx =(2000 ×T/d )×tan(β)   (1)
 
         [0033]    When gear  16  is a straight bevel gear, equation (2) is true. 
         [0000]        Fx =(2000 ×T /( d−b· sin(tan −1 ( z 1 /z 2)))×tan(α)×sin(tan −1 ( z 1 /z 2))   (2)
 
         [0034]    Regarding equation (2), character “z1” means a teeth number of a driving gear (i.e., gear  16  attached to electric motor  12 ), and character “z2” means a teeth number of a driven gear (not shown). 
         [0035]    When gear  16  is a worm gear, equation (3) is true. 
         [0000]    
       
         
           
             
               
                 
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         [0036]    When gear  16  is a screw gear, equation (4) is true. 
         [0000]        Fx =(2000 ×T/d )×(cos(α n )×sin(β)−μ·cos(β))/(cos(α n )×cos(β)+μ·sin(β))   (4)
 
         [0037]    As explained above, thrust load obtaining part  20  can obtain (or calculate) the thrust load of electric motor  12 , based on the information of gear  16  and the torque of electric motor  12 , without using thrust load detector  30  as shown in  FIG. 1 . The operator may input the information of gear  16  to control section  18  via input section  28 , etc., as described above. Alternatively, the information of gear  16  may be previously stored in a proper storing device such as a memory, etc., and when the operator designates (or inputs) the type of the gear, thrust load obtaining part  20  may extract the information of the gear corresponding to the designated type from the storing device so as to calculate the thrust load. 
         [0038]    The critical thrust load stored in critical thrust load storing part  22  means a threshold value predetermined depending on the type of electric motor  12 , wherein electric motor  12  may be damaged or broken when the thrust load applied to the electric motor exceeds the threshold value. For example, as shown in  FIG. 2 , when four motors  32 ,  34 ,  36  and  38  are specified as motors capable of being applied to electric motor  12  of  FIG. 1 , a relationship between a model name (such as “Model A” or “Model B”), a parameter number and a critical thrust load with respect to each motor may be stored in critical thrust load storing part  22 . For example, the relationship may be represented by a table  40  corresponding to the type of each motor. By virtue of this, when the operator selects and inputs the type (the model name or the parameter number) of the electric motor by using input section  28 , etc., the critical thrust load corresponding to the type of the selected motor can be automatically set or specified. For example, model “B” or parameter number “2” is selected or input, the critical thrust load is automatically set to 10 [N]. 
         [0039]    As described above, when selecting the type of the electric motor, the model name of the electric motor (such as “Model A” or “Model B”) may be selected. Alternatively, the model number (or the parameter number) may be assigned to each type of the electric motor, and the model number may be set or selected so as to select the type of the electric motor. In addition, the operator may directly input a value of the critical thrust load so as to store the critical thrust load in critical thrust load storing part  22 . 
         [0040]    Torque limiting part  24  compares the thrust load of electric motor obtained by thrust load obtaining part  20  and the critical thrust load of electric motor  12  stored in critical thrust load storing part  22  at predetermined timing or time intervals. Then, when the obtained thrust load exceeds the stored critical thrust load, torque limiting part  24  electrically limits the torque of electric motor  12  (in the illustrated embodiment, torque limiting part  24  transmits a torque command to amplifier  26  for limiting the torque of electric motor  12 ) so that the obtained thrust load does not exceeds (or falls below) the critical thrust load. In addition, torque limiting part  24  may be constituted as a part of control section  18 , and thus the torque of electric motor  12  can be rapidly lowered without using a torque limiter, etc. Therefore, in the present invention, electric motor  12  can be appropriately protected from overload without disengaging gear  16  from drive shaft  14  and without stopping electric motor  12 . 
         [0041]    According to the present invention, it is not necessary to disengage specific members from each other in order to prevent the motor being damaged due to excessive thrust load, and thus a recovery operation for the motor is not necessary. Since the torque of the electric motor is limited so that the thrust load does not exceed the critical thrust load, it is not necessary to stop or suspend the unit (or the electric motor). Therefore, the motor unit can be continuously operated, whereby productivity of the unit may be improved. Further, the torque of the electric motor can be electrically limited without using a torque limiter, etc., the number of components of the unit may be reduced and the unit may be compact. 
         [0042]    While the invention has been described with reference to specific embodiments chosen for the purpose of illustration, it should be apparent that numerous modifications could be made thereto, by one skilled in the art, without departing from the basic concept and scope of the invention.