Hereinafter, using FIGS.  19-22, an example of a conventional power transmission device in which an object with a weight is attached to an output shaft will be described.  FIG.  19 is a perspective view showing an example of the conventional power transmission device.  In the example shown in FIG.  19, a power transmission device 1501 transmits power to a vehicle-mounted display panel (liquid crystal display panel) 102 set on a ceiling surface in an automobile (not shown) (or example, see Patent Document 1). 
As shown in FIG.  19, the power transmission device 1501 mainly is composed of a display panel 102, an output shaft 103 to which the display panel 102 is attached, a motor 108 for rotating the output shaft 103, and a power supply unit 1503 for supplying electric power to the motor 108.  The motor 108 and the power supply unit 1503 are connected to each other via a wire 1502. 
Furthermore, a transmission gear 106 is attached to an output shaft (not shown) of the motor 108, and an output gear 104 is attached to one end of the output shaft 103 in such a manner that the transmission gears 103 and 106 are engaged with each other.  Consequently, the driving force of the motor 108 is transmitted from the transmission gear 106 to the output shaft 103 via the output gear 104.  Herein, in the example shown in FIG.  19, the transmission gear 106 is a worm gear, and the output gear 104 is a helical gear (worm wheel).  Therefore, when the output gear 104 is engaged with the transmission gear 106 to rotate, a rotation axis direction is changed by 90°from a motor shaft direction to an output shaft direction. 
Hereinafter, the operation of the power transmission device 1501 shown in FIG.  19 further will be described specifically with reference to FIGS.  20-22.  FIG.  20 shows an external force applied to a display panel set on a ceiling surface in an automobile.  FIG.  20A is a diagram illustrating an external force applied to the display panel and a direction thereof, and FIG.  20B is a graph showing a relationship between the rotation angle of the display panel and the torque applied to the output shaft due to the weight of the display panel.  FIG.  21 is a cross-sectional view showing a state in which the output gear and the transmission gear are engaged with each other in the case where the rotation angle of the display panel is less than 90°.  FIG.  22 is a cross-sectional view showing a state in which the output gear and the transmission gear are engaged with each other in the case where the rotation angle of the display panel exceeds 90°. 
In FIGS.  20A and 20B, the rotation angle of the display panel 102 in the case where the display panel 102 is accommodated on a ceiling surface is defined as 0°.  Furthermore, the rotation angle of the display panel in the case where the display panel 102 descends from the ceiling surface while rotating around the output shaft 103 is defined as a “display panel rotation angle θ”.  Typically, the stop position of the display panel 102 is set at a place where the display panel rotation angle θ is about 90°. 
Furthermore, in FIG.  20B, a torque T1[N·m] applied to the output shaft 103 due to the weight of the display panel 102 is taken on a vertical axis, and the display panel rotation angle θ is taken on a horizontal axis.  In FIGS.  20A and 20B, a “CW (clockwise) direction” indicates a clockwise direction (direction in which the display panel 102 shifts from an open state to an accommodated state) when the display panel 102 is seen from the right side of a viewer who watches the display panel 102.  A “CCW (counterclockwise) direction” indicates a counterclockwise direction (direction in which the display panel 102 shifts from the accommodated state to the open state) when the display panel 102 is seen from the right side of a viewer who watches the display panel 102. 
First, the case where the display panel 102 is rotated around the output shaft 103 by the motor 108, and the display panel rotation angle θ changes from 0° to 90° will be studied.  In this study, as shown in FIG.  20A, the output shaft 103 is supplied with the torque T1 in the CCW direction due to the weight of the display panel 102. 
Herein, when a mass of the display panel is defined as M, a gravitational acceleration is defined as g, and a distance from the output shaft 103 to the center of gravity of the display panel is defined as r, the torque T1 applied to the output shaft 103 due to the weight of the display panel 102 is represented by the following Expression (1).  Furthermore, as shown in the following Expression (1) and FIG.  20B, the torque T1 becomes maximum when the display panel rotation angle θ is 0°, and becomes 0 when the display panel rotation angle θ is 90°.T1=rMg cos θ  (Expression 1)
Furthermore, as shown in FIG.  21, in the case where the display panel rotation angle θ changes from 0° to 90°, in order to rotate the output gear 104 and the output shaft 103 in the CCW direction, the transmission gear 106 rotates so that its tooth surfaces move in the CCW direction. 
It should be noted that, in this case, the motor 108 functions as a brake suppressing the display panel 102 from rotating immediately in the CCW direction due to the torque T1 caused by the weight.  Thus, in this case, as shown in FIG.  21, the tooth surfaces of the output gear 104 on a rotation direction front side and the tooth surfaces of the transmission gear 106 on a rotation direction back side come into contact with each other. 
Next, the case where the display panel 102 rotates further, and the display panel rotation angle θ exceeds 90° will be studied.  In this case, as shown in FIG.  20B, the direction of the torque T1 caused by the weight changes from the CCW direction to an opposite direction thereof (i.e., the CW direction) at a time when the display panel rotation angle θ reaches 90°.  A point at which the torque direction is changed from the previous direction to an opposite direction thereof is referred to as a “torque change point”. 
On the other hand, the rotation direction of the transmission gear 106 does not change, so that the torque T1 functions as a brake with respect to the display panel 102 that rotates in the CCW direction when the display panel rotation angle θ exceeds 90°.  Thus, in this case, the display panel 102 rotates in the CCW direction only with the driving force of the motor 108 via the transmission gear 106, and as shown in FIG.  22, the tooth surfaces of the output gear 104 on the rotation direction back side and the tooth surfaces of the transmission gear 106 on the rotation direction front side come into contact with each other. 
As described above, in the power transmission device 1501, when the direction of the torque T1 is switched, the tooth surfaces that come into contact with each other are switched between the output gear 104 and the transmission gear 106.  Then, during a period from a time when the switching of tooth surfaces starts to a time when the switching of tooth surfaces ends, the tooth surfaces of the output gear 104 do not come into contact with the tooth surfaces of the transmission gear 106 due to a backlash.  Consequently, play occurs in the output gear 104, and the smoothness is lost from the rotation movement of the display panel 102. 
Therefore, conventionally, a power transmission device (hereinafter, referred to as a “backlash-less power transmission device”) has been proposed, in which the occurrence of play is suppressed by removing a backlash apparently for example, see Patent Documents 2 and 3). 
In the conventional backlash-less power transmission device disclosed by Patent Document 2, an output gear attached to an output shaft is driven with a plurality of transmission gears, and at that time, the rotation speed of each gear for driving is varied.  In the backlash-less power transmission device in Patent Document 2, tooth surfaces come into contact at all times, so that the occurrence of play due to a backlash in the output gear can be prevented. 
Furthermore, even in a conventional backlash-less power transmission device disclosed by Patent Document 3, a backlash is removed by the same principle as that in the example of Patent Document 2.  According to the example of Patent Document 3, a motor is provided for each transmission gear in the backlash-less power transmission device. 
Hereinafter, using FIGS.  23 and 24, the backlash-less power transmission device disclosed by Patent Document 3 will be described.  FIG.  23 is a perspective view showing an example of a conventional backlash-less power transmission device.  FIG.  24 is a cross-sectional view showing a state in which an output gear and a transmission gear are engaged with each other in the backlash-less power transmission device shown in FIG.  23, and FIGS.  24A and 24B respectively show engagement states between different gears. 
As shown in FIG.  23, in the same way as in the power transmission device 1501 shown in FIG.  19, a backlash-less power transmission device 1801 includes an output shaft 103 and a motor 108 that rotates the output shaft 103.  Furthermore, a transmission gear 106 is attached to an output shaft (not shown) of the motor 108, and an output gear 104 is attached to one end of the output shaft 103.  When the transmission gear 106 and the output gear 104 are engaged with each other, the driving force of the motor 108 is transmitted to the output shaft 103. 
As described above, although the backlash-less power transmission device 1801 has the same configuration as that of the power transmission device 1501 shown in FIG.  19, it further includes a motor 109 as a power source.  The motor 109 has the same characteristics as those of the motor 108, and a transmission gear 107 is attached to a tip end of an output shaft (not shown) of the motor 109.  Furthermore, an output gear 105 newly is attached to the other end of the output shaft 103 separately from the output gear 104. 
The output gear 105 is fixed at the output shaft 103 in the same way as in the output gear 104, and rotates together with the output gear 104 integrally with the output shaft 103.  Furthermore, the output gear 105 is set so that its tooth surfaces and the tooth surfaces of the output gear 104 rotate in the same phase, i.e., the tooth surfaces of the output gear 105 and the tooth surfaces of the output gear 104 are aligned in the shaft direction of the output shaft 103. 
Furthermore, the transmission gear 107 of the motor 109 is a worm gear in the same way as in the transmission gear 106 of the motor 108, and the output gear 105 is a helical gear (worm gear) in the same way as in the output gear 104.  Furthermore, the output gear 105 and the transmission gear 107 are engaged with each other in the same way as in the output gear 104 and the transmission gear 106.  Thus, when electric power is supplied to the motors 108 and 109 by a power supply unit 1804, the output shaft 103 is driven by both the motors 108 and 109. 
In the backlash-less power transmission device 1801, a speed reducer 1802 is set between the output shaft of the motor 108 and the transmission gear 106.  Furthermore, a speed reducer 1803 having a speed reducing ratio different from that of the speed reducer 1802 is set between the output shaft of the motor 109 and the transmission gear 107.  Thus, when the motors 108 and 109 are rotated at the same rotation speed in the same direction, the output shaft 103 is driven while the rotation speed of the transmission gear 106 is different from that of the transmission gear 107. 
Herein, the case where the output shaft 103 rotates in the CCW direction, and the rotation speed of the transmission gear 107 is set to be higher than the rotation speed of the transmission gear 106, will be studied.  In this case, as shown in FIGS.  24A and 24B, since the output shaft 103 rotates in the CCW direction, the output gears 104 and 105 also rotate in the CCW direction shown in FIGS.  24A and 24B. Furthermore, the transmission gears 106 and 107 that are worm gears rotate around a shaft perpendicular to the output shaft 103 so that the tooth surfaces move in the CCW direction, and rotate the output gears 104 and 105 in the CCW direction. 
As shown in FIG.  24A, the rotation speed of the transmission gear 107 is higher than that of the transmission gear 106, and the movement speed in the CCW direction of the tooth surfaces of the transmission gear 107 becomes higher than that of the transmission gear 106.  Therefore, the tooth surfaces of the output gear 105 on the rotation direction back side come into contact with the tooth surfaces of the transmission gear 107 on the rotation direction front side.  On the other hand, as shown in FIG.  24B, the rotation speed of the transmission gear 107 is lower than that of the transmission gear 107, so that the tooth surfaces of the transmission gear 106 on the rotation direction back side come into contact with the tooth surfaces of the output gear 104 on the rotation direction front side. 
Thus, as is understood from FIGS.  24A and 24B, in the case where the output shaft 103 rotates in the CCW direction, the output shaft 103 is supplied with not only a load torque for rotating the output shaft 103 in the CCW direction but also a load torque for rotating the output shaft 103 in the CW direction opposite to the CCW direction.  That is, the output shaft 103 rotates in one direction at all times, and simultaneously is braked in the opposite direction.  The “load torque” as used herein refers to a torque that is provided to the output gear of the output shaft by the transmission gear of the motor. 
Consequently, the contact of tooth surfaces between the transmission gear and the output gear is not interrupted, and a backlash is removed apparently wherever the output gear is placed, whereby the occurrence of play in the output gear due to the backlash is prevented.     Patent Document 1: JP 2002-200941 A    Patent Document 2: JP 2003-343704 A    Patent Document 3: JP 61(1986)-197847 A