You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a tension mechanism in a power slide device for vehicle sliding door, and in particular, it relates to an improved tension roller for tension mechanism. 
     2. Description of the Related Art 
     In general, a conventional power slide device for vehicle sliding door comprises a wire drum rotated by motor, and a door-opening cable connected to the wire drum, and a door-closing cable connected to the wire drum. When the wire drum is rotated in a door-opening direction, the door-opening cable is wound up, and at the same time, the door-closing cable is pulled out, so that the sliding door is slid in the door-opening direction. On the contrary, when the wire drum is rotated in a door-closing direction, the door-opening cable is pulled out, and at the same time, the door-closing cable is wound up, so that the sliding door is slid in the door-closing direction. 
     The wire cable is retained at an appropriate tension by a tension mechanism. The tension mechanism comprises a pair of tension rollers biased in such a manner as to be adjacent to each other by elasticity of the springs. 
       FIG. 1  is a schematic view showing a relation among a wire drum A, a wire cable B, and a tension roller C. A typical thickness of the wire drum A along a drum shaft D is approximately 20 mm, and when the wire drum A rotates, the wire cable B is wound up (or pulled out), moving upward or downward guided by a helical engaging groove E of the wire drum A. 
     Since the typical tension roller C is a bobbin type roller with a short diameter in center, even if the wire cable B moves upward or downward in response to rotation of the wire drum A, the tension roller C substantially keeps the wire cable B at the center. The problem of this structure has been that the wire cable B is unmovable upward and downward relatively to the tension roller C. Hence, when the wire cable B moves upward or downward for the wire drum A, the wire cable B between the wire drum A and the tension roller C deviates widely from a right angle with the drum shaft D of the wire drum A, thereby often causing an engaging trouble between the wire cable B and the engaging groove E. The engaging trouble becomes serious as the distance between the wire drum A and the tension roller C becomes shorter. Consequently, the bobbin type tension roller C has been disposed at a place away from the wire drum A, thereby inviting a large size of the power slide device. 
     In contrast to this, as shown in  FIG. 2 , a tension roller C′ which is formed into a cylindrical roller having the same diameter from the top to the bottom is also publicly known. A cylindrical tension roller C′ attempts at miniaturization of the power slide device by forming the tension roller C′ at approximately 20 mm in accordance with the thickness of the wire drum A so that the space between the tension roller C′ and the wire drum A is made short. 
     The device of  FIG. 2  has a problem in that a “cable rubbing noise” is generated when the wire drum A is rotated. A cause of the noise generation will be described below. 
     By the rotation of the wire drum A, when the cable B, for example, moves upward for the wire drum A, the upward movement of the cable B relatively to the tension roller C′ is slightly delayed. Hence, an angle X between the cable B and the lower side surface of the roller C′ exceeds more than 90 degrees. Then, a downward external force as shown by an arrow a is applied to the cable B in the vicinity of the roller C′ so that the upward movement of the cable B in the vicinity of the roller C′ is further delayed. As a result, the wire cable B rubs against an angular portion of the engaging groove E of the wire drum A, thereby generating the noise. 
     Further, the cable B in the vicinity of the roller C′ abruptly moves in order to catch up on the delay of the upward movement, thereby causing the noise. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a tension roller of the power slide device having controlled the generation of noises by allowing the upward or downward movement of the cable for the tension roller to be smoothly performed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view showing a relation among a wire drum, tension roller, and wire cable of a conventional device; 
         FIG. 2  is a schematic view showing a relation among another conventional wire drum, tension roller, and wire cable; 
         FIG. 3  is a side view of a vehicle including a power slide device of the present invention; 
         FIG. 4  is a development of the power slide device and the slide door; 
         FIG. 5  is a partially abbreviated sectional view of a tension mechanism of the power slide device; 
         FIG. 6  is a partially abbreviated sectional view of the tension mechanism; 
         FIG. 7  is an explanatory view of the operation of the tension mechanism; and 
         FIG. 8  is a table showing the experiment result of the tension mechanism according to the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A first embodiment of the present invention will be described below with reference to the drawings.  FIG. 3  shows a rough relation between a power slide device  10  of the present invention and a slide door  11  for vehicle which slides in a door-closing direction and a door-opening direction by the power slide device  10 .  FIG. 4  shows a developed relation between both the device  10  and the door  11 . 
     The slide door  11  is slidably attached to a vehicle body  12 , and slides in the backward and forward direction of the vehicle body  12  along a guide rail  13  provided on the vehicle body  12 . The slide device  10  has a motor  14  and a wire drum  15  rotated by the motor  14 , and these elements are attached to a base plate  16  fixed to the vehicle body  12 . 
     The wire-drum  15  is connected with a pair of wire cables  17 , that is, tip end sides of the door-opening cable  17 A and the door-closing cable  17 B, respectively. When the wire drum  15  rotates in the door-opening direction, the door-opening cable  17 A is wound up, and the door-closing cable  17 B is pulled out, and the slide door  11  is slid in the door-opening direction. When the wire drum  15  is rotated in the door-closing direction, the door-opening cable  17 A is pulled out, and the door-closing cable  17 B is wound up, and the slide door  11  is slid in the door-closing direction. 
     The other end of the door-opening cable  17 A is connected to a bracket  19  of the slide door  11  through a rear side pulley  18  pivotally mounted on the vehicle body  12 . Similarly, the other side of the door-closing cable  17 B is connected to the bracket  19  through a front side pulley  20  pivotally mounted on the vehicle  12 . 
     The base plate  16  is provided with a tension mechanism  21  retaining a tension of the wire cable  17  at an appropriate pressure. The tension mechanism  21  has a pair of tension rollers  22  and  23  abutted against by the cables  17 A and  17 B. The tension shafts  24  and  25  of the tension rollers  22  and  23  are slidably attached to elongated slots  26  and  27  formed on the base plate  16 . The tension rollers  22  and  23  are biased in such a manner as to be mutually approached by elastic force of the tension spring  28 . Reference numeral  29  denotes a cover case of the tension mechanism  21 . 
     Both upper and lower ends of the tension roller  22 , as shown in  FIG. 6 , are formed with flanges  22 A and  22 A, and a cable abutting surface  22 B between the flanges  22 A and  22 A is formed on an inclined surface which becomes gradually shorter in diameter from the bottom to the top. Similarly, both upper and lower ends of the tension roller  23  are formed with flanges  23 A and  23 A, and the cable abutting surface  23 B between the flanges  23 A and  23 A is formed on an inclined surface which becomes gradually longer in diameter from the bottom to the top. In the present embodiment, the cable abutting surface  22 B and the cable abutting surface  23 B are inclined mutually in a reverse direction, and when the upper portion of one abutting surface becomes long in diameter, the upper portion of the other abutting surface becomes short in diameter. This depends on an attachment relation among the wire drum  15 , the door-opening cable  17 A, and the door-closing cable  17 B to be described later. 
       FIG. 7  shows a relation between the wire drum  15  and the tension rollers  22  and  23  in the present invention. The door-opening cable  17 A and the door-closing cable  17 B are wound around the engaging groove  30  of the wire drum  15 . When the slide door  11  is in a door-closed position, the door-opening cable  17 A is pulled out from the engaging groove  30  of the wire drum  15 , and the door-closing cable  17 B is wound up by the engaging groove  30  of the wire drum  15 . At this time, in the embodiment of  FIG. 7 , the relation is established such that both the door-opening cable  17 A and the door-closing cable  17 B are positioned at the bottom side of the wire drum  15 . When the wire drum  15  is rotated in a door-opening direction about the drum shaft  31  as a center, the door-opening cable  17 A, while being wounded, is guided to the engaging groove  30 , and moves upward as shown by an arrow b, and the door-closing cable  17 B already wounded around the engaging groove  30 , while being pulled out, similarly moves upward as shown by an arrow c (the relation is reversed at the door-closing rotation of the wire drum). Further, both the door-opening cable  17 A and the door-closing cable  17 B move from the long diameter side to the short diameter side of the tension rolls  22  and  23  when wound up by the drum  15 . Hence, in the present embodiment, the cable abutting surface  22 B and the cable abutting surface  23 B incline in a mutually reversed direction. 
     However, the relation between the wire drum  15  and the cables  17 A and  17 B becomes sometimes such that, when the wire drum  15  rotates, depending on the shape of the engaging groove  30  of the wire drum  15 , one of the cables moves upward and the other moves downward. In this case, inclinations of the cable abutting surface  22 B and the cable abutting surface  23 B are set in the same direction, and the cable to be wound up is changed so as to move toward the short diameter of the abutting surface. 
     (Operation) 
     When the slide door  11  is at the door-closed position, the door-opening cable  17 A has been pulled out from the engaging grove  30  of the wire drum  15 , and the door-closing cable  17 B has been wound up by the engaging groove  30  of the wire drum  15 . In this state, when the wire drum  15  is rotated in the door-opening direction, in the embodiment of  FIG. 7 , the door-opening cable  17 A, while being wound up, is guided to the engaging groove  30 , and moves upward, and further, the door-closing cable  17 B already wound up by the engaging groove  30 , while being pulled out, similarly moves upward. 
     At this time, since the door-opening cable  17 A moves toward the upper side (short diameter side) of the cable abutting surface  22 B when wound up, and the angle X between the lower side surface of the cable abutting surface  22 B of the tension roller  22  and the cable  17 A is below 90 degrees due to the inclination of the abutting surface  22 B, when the door-opening cable  17 A moves upward relatively to the tension roller  22 , the delay of the upward movement for the tension roller  22  is controlled, and the door-opening cable  17 A to be wound up is smoothly wound up. 
       FIG. 8  shows a measurement result of the “cable rubbing noise” according to the angle of the cable abutting surface  22 B of the tension roller  22 . At the cable pulling out side, no matter whatever angle the cable abutting surface has, no generation of sounds that becomes a problem has been recognized. However, at the cable winding side, when the angle of the cable abutting surface is zero degree (when the cable abutting surface  22 B is in parallel with the drum shaft  31 ), the generation of sounds has been recognized. When the inclination is made three degrees, the noise has been considerably controlled. In case the angle is five degrees, no generation of noises has been substantially recognized. The suitable angles of the cable abutting surfaces  22 B and  23 B, in spite of the slight fluctuation depending on the factor such as the distance and the like between the tension rollers  22  and  23  and the wire drum  15 , are desirable to be three to seven degrees.

Summary:
A power slide device includes a first cable connected to a wire drum and moving in a first direction when wound up by the wire drum, a second cable connected to the wire drum and moving in a second direction when wound up by the wire drum, a first tension roller having a first abutting surface abutting against the first cable, and a second tension roller having a second abutting surface abutting against the second cable. The first abutting surface is taken as an inclined surface becoming gradually shorter in diameter toward the first direction, and the second abutting surface is taken as an inclined surface becoming gradually shorter in diameter toward the second direction.