Patent Publication Number: US-7213479-B2

Title: Vibratory mechanism and vibratory roller

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a vibratory mechanism and a vibratory roller. 
   2. Description of the Relevant Art 
   A vibratory roller is mainly used for a compaction of an embankment in a construction site, such as a highway or a dam, or an asphalt pavement of a road. 
   The compaction using the vibratory roller is performed while vibrating a vibratory roll (roll). Thus, the ground to be compacted is densified in a very dense state. As an example of a vibratory mechanism that is provided within the vibratory roll and causes a vibration of the vibratory roll, the mechanism that causes vibration by rotating a vibratory shaft provided with an eccentric weight has been known. 
   Here, as an example of a vibration state of vibratory roll, two types of vibration state have been known. One is “standard vibration” which is a vibration that the vibratory roll vibrates in all radial directions thereof. The other is “horizontal vibration”, which is the vibration that the vibratory roll vibrates in the direction tangential to the circumference of the vibratory roll. 
   In the mechanism disclosed in U.S. Pat. No. 4,647,247, is a switching unit, by which the vibration state of the vibratory roll is changed to/from the standard vibration from/to the horizontal vibration. 
   In FIGS. 10A and 10B of U.S. Pat. No. 4,647,247, a total of two vibratory shafts are provided within the vibratory roll. One of the vibratory shafts is provided at an opposite position across the center of the vibratory roll with respect to the other vibratory shaft. Each of the vibratory shafts is provided with an eccentric weight, and the eccentric weight of at least one of the vibratory shafts is rotatably attached to the vibratory shaft. 
   In this U.S. patent, if the relative phase angle between eccentric weights in case of rotation in one direction of the vibratory shaft is denoted by 0°, the relative phase angle between the eccentric weights in case of rotation in the other direction of the vibratory shaft is 180°. 
   When vibrating the vibratory roll under standard vibration or horizontal vibration, the vibratory roll should be vibrated at the suitable amplitude for respective vibration states. 
     FIG. 4  is an explanatory view showing the vibration of a vibratory roll equipped with a pair of vibratory shafts in case of standard vibration. 
   In this vibratory roll, an eccentric weight of the same shape is provided to respective vibratory shafts, which are rotated in accordance with a rotational torque supplied from a power supply mechanism (not shown). Thus, respective eccentric weights are rotated in the same direction at the same angular position. 
   In this occasion, the vibratory force directed away from the center of the vibratory roll is caused, and the direction thereof changes sequentially according to the angular position of eccentric weights. Here, if it is focused on the element vertical to a ground from among all elements of the vibratory force, and the vibratory force thereof is denoted by F, the vibratory force F is indicated by a following formula.
 
 F= 2· m·r·ω   2 ·sinω t  
 
where
 
   m is a mass of an eccentric weight 
   r is a distance between the center of the vibratory shaft and the center of gravity of the eccentric weight 
   ω is an angular velocity of vibratory shaft. 
   Here, m·r is defined as eccentric moment (hereinafter m·r is indicated as “mr”). 
   Thus, a ground can be indicated as a model of spring, which has a predetermined spring constant K and which acts in a perpendicular direction with respect to the contact surface between the vibratory roll and a ground. 
   When vibratory force F is periodically working on the vibratory roll whose mass is M 0 , if spring constant K is regarded as a negligibly small value by assuming that a ground is quite loose, the equation of motion is shown by a following formula.
 
2 ·mr·ω   2 ·sin ω t=M   0   ·d   2   y/dt   2  
 
where
 
   y is a displacement in ups-and-downs directions. 
   Then, the following formula is obtained from this formula.
 
 y =(−2 ·mr/M   0 )·sin ω t  
 
   Thus, the amplitude a 1  in the ups-and-downs directions of the vibratory roll in case of standard vibration can be shown by a following formula (1).
 
 a   1 =2 ·mr (standard vibration)/ M   0   (1)
 
   In this formula, “mr (standard vibration)” means that the eccentric moment in case of standard vibration. 
     FIG. 5  is an explanatory view showing the vibration of a vibratory roll equipped with a pair of vibratory shafts in case of horizontal vibration. 
   A vibration proof rubber provided between the vibratory roll and a frame (not shown) of the vibratory roller can be indicated as a model of a spring, which has a predetermined spring constant K 1  and which acts in a horizontal direction with respect to a shaft center O′ of the vibratory roll. 
   A ground can be indicated as a model of a spring, which has a predetermined spring constant K 2  and which acts in a horizontal direction with respect to the contact surface between the vibratory roll and a ground. 
   When a periodic torque T is acting on a moment of inertia I around the shaft center O′ of the vibratory roll, which is supported by the spring of spring constant K 1  and the spring of spring constant K 2 , the equation of motion of this case is as follows.
 
 p· 2 ·mr·ω   2 ·sin ω t=I·d   2   θ/dt   2  
 
where
 
   p is a distance between the shaft center O′ of the vibratory roll and the center of the vibratory shaft. 
   Here, respective spring constant K 1  and K 2  are regarded as a negligibly small values by assuming respective springs are quite loose. 
   If the radius of the vibratory roll is denoted by R, a displacement y in a horizontal direction with respect to the contact surface between the vibratory roll and a ground can be indicated as y=R·θ, on regarding θ as a slight angular displacement. Thus, a following formula can be obtained.
 
 p· 2 ·mr·ω   2 ·sin ω t= ( I/R )· d   2   y/d t   2  
 
   Then, by performing a formula translation based on y, a following formula is obtained from this formula.
 
 y =−(( R·p· 2· mr )/ I )·sin ω t  
 
   Thus, the amplitude a 2  in a horizontal direction with respect to the contact surface between the vibratory roll and a ground in case of horizontal vibration can be shown by a following formula.
 
 a   2   =R· 2· p·mr (horizontal vibration)/ I   (2)
 
   In this formula (2), “mr (horizontal vibration)” means that the eccentric moment in case of horizontal vibration. 
   Here, a mass M 0  of a vibratory roll, a radius R of the vibratory roll, and a moment of inertia I around the shaft center O′ of the vibratory roll are determined depending on a dimension of the vibratory roll. Therefore, it is required that the eccentric moment mr (standard vibration) can be determined freely for controlling the amplitude a 1  in case of standard vibration to the desired value. 
   Additionally, it is required that at least one of the distance p and the eccentric moment mr (horizontal vibration) can be determined freely for controlling the amplitude a 2  in case of horizontal vibration to the desired value. Here, the distance p is a distance between the shaft center O′ of the vibratory roll and the center of the vibratory shaft. 
   In the vibratory roll, however, since the vibratory shaft is provided within the vibratory roll, there is a limitation of the distance p (see  FIG. 5 ). Thus, the eccentric moment mr (horizontal vibration) has a great influence on the amplitude a 2  in case of horizontal vibration. 
   Therefore, it is preferable that the eccentric moment in case of standard vibration is different from the eccentric moment in case of horizontal vibration, for establishing the amplitude a 1  of standard vibration and the amplitude a 2  of horizontal vibration at respective suitable values. 
   In U.S. Pat. No. 4,647,247, as described above, a total of two vibratory shafts, each of which is provided with an eccentric weight, are stored within the vibratory roll, and the eccentric weight of one of the vibratory shafts is rotatably attached to the vibratory shaft. Therefore, the angular position between eccentric weights varies depending on the rotation direction of the vibratory shaft, but the eccentric moment in case of standard vibration is the same as the eccentric moment in case of horizontal vibration. Therefore, it has been difficult to control the amplitude of the eccentric moment to respective suitable amplitudes for the standard vibration and the horizontal vibration. 
   Therefore, the vibratory roller that can control the amplitude of the vibratory roll to the desired value for the standard vibration or the desired value of the horizontal vibration has been required. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a vibratory mechanism. This vibratory mechanism includes vibratory shafts, which are stored within a roll and are arranged symmetrically across a rotation axis of the roll, a fixed eccentric weight fixed to respective vibratory shafts, a rotatable eccentric weight rotatably attached to respective vibratory shafts, a rotation controller controlling a range of movement of the rotatable eccentric weight, and an eccentric moment controller which changes an eccentric moment around the vibratory shafts depending on a rotation direction of the vibratory shafts. 
   In this vibratory mechanism, the roll vibrates in all radial directions when respective vibratory shafts rotate in one direction, and the roll vibrates in a direction tangential to the circumference of the roll when respective vibratory shafts rotate in a reverse direction. 
   In the vibratory mechanism, a total of two vibratory shafts, that is, a first vibratory shaft and a second vibratory shaft are stored in the roll, and the first vibratory shaft is arranged at a 180° opposite position across a rotation axis of the roll with respect to the second the vibratory shaft. 
   In this vibratory mechanism, a total eccentric moment around the first vibratory shaft is substantially the same as a total eccentric moment around the second vibratory shaft, when the first vibratory shaft and the second vibratory shaft are rotated in one direction. Additionally, a total eccentric moment around the first vibratory shaft is substantially the same as a total eccentric moment around the second vibratory shaft, when the first vibratory shaft and the second vibratory shaft are rotated in the reverse direction. 
   Here, the total eccentric moment around the first vibratory shaft is obtained by subtracting an eccentric moment of the fixed eccentric weight from an eccentric moment of the rotatable eccentric weight and the total eccentric moment around the second vibratory shaft is obtained by subtracting an eccentric moment of the rotatable eccentric weight from an eccentric moment of the fixed eccentric weight, when the first vibratory shaft and the second vibratory shaft are rotated in one direction. Additionally, the total eccentric moment around the first vibratory shaft is obtained by adding an eccentric moment of the fixed eccentric weight to an eccentric moment of the rotatable eccentric weight and the total eccentric moment around the second vibratory shaft is obtained by adding an eccentric moment of the rotatable eccentric weight to an eccentric moment of the fixed eccentric weight, when the first vibratory shaft and the second vibratory shaft are rotated in the reverse direction. 
   In the vibratory mechanism, respective rotatable eccentric weights of the first vibratory shaft and the second vibratory shaft are allowed to rotate around the first vibratory shaft and the second vibratory shaft, respectively, within limits of 0 to 180°. In this vibratory mechanism, the eccentric moment around the first vibratory shaft of the fixed eccentric weight is substantially the same as the eccentric moment around the second vibratory shaft of the rotatable eccentric weight, and the eccentric moment around the first vibratory shaft of the rotatable eccentric weight is substantially the same as the eccentric moment around the second vibratory shaft of the fixed eccentric weight. 
   The vibratory mechanism of the present invention is suitable for use in the roll of the vibratory roller. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an axial sectional view of the vibratory roll equipped with a vibratory mechanism according to the present invention. 
       FIG. 2A  is a sectional view along the line E—E in  FIG. 1 , wherein the vibratory roll is causing standard vibration. 
       FIG. 2B  is a sectional view along a line E—E in  FIG. 1 , wherein the vibratory roll is causing horizontal vibration. 
       FIGS. 3A–3D  are side sectional views explaining a vibratory force caused under horizontal vibration. 
       FIG. 4  is a schematic view used for computing amplitude of the vibratory roll in case of standard vibration. 
       FIG. 5  is a schematic view used for computing amplitude of the vibratory roll in case of horizontal vibration. 
   

   DETAILED DESCRIPTION OF THE PRESENT EMBODIMENT 
   As shown in  FIG. 1 , a vibratory roll  1  is rotatably supported by support boards  2 , which are fixed to a frame of a vibratory roller (not shown), respectively. 
   The vibratory roll  1  has a shape of a hollow cylinder, and a first plate  3  provided with a central aperture  3   a  and a second plate  4  provided with a central aperture  4   a  are provided therein. In this vibratory roll  1 , a predetermined interval is provided between the first plate  3  and the second plate  4 . A housing case  5 , which stores a vibratory mechanism and has a shape of a hollow cylinder, is sandwiched between fringes of respective central apertures  3   a  and  4   a  at both sides thereof so that the housing case  5  is coaxially arranged with respect to a shaft center of the vibratory roll  1 . 
   An axle shaft  6  is attached to the first plate  3  by fixing a flange  6   a  of the axle shaft  6  to the fringe of the first plate  3  using bolts  8 . An axle shaft  7  is attached to the second plate  4  by fixing a flange  7   a  of the axle shaft  7  to the fringe of the second plate  4  using bolts  8 . Thereby, the central aperture  3   a  and the central aperture  4   a  are closed by the axle shaft  6  and the axle shaft  7 , respectively. 
   Each of the bearings  10 , for example roller bearing and the like, located within a bearing-housing  9  rotatably supports the axle shaft  6  on the bearing-housing  9 . The bearing-housing  9  is connected to the support board  2  through a vibration proof rubber  11  and a mounting plate  12 . 
   The axle shaft  7  is connected to a power transmission unit  14   a  of a drive motor  14  through a mounting plate  13 . A stationary part  14   b  of the drive motor  14  is fixed to the support board  2  through a mounting plate  15  and a vibration proof rubber  16 . In this embodiment, a motor, such as an hydraulic motor, is used as the drive motor  14 . 
   A reversible motor  18 , which is used for generating a vibration on the vibratory roll, is connected to the bearing-housing  9 , and a rotation axis thereof is connected to a gear shaft  20  through a coupling  19 . 
   Each of bearings  21 , such as roller bearings, located within the axle shaft  6  rotatably supports the gear shaft  20  so that the gear shaft  20  becomes parallel and coaxial with respect to the shaft center of the vibratory roll  1 . The gear shaft  20  is provided with a drive gear  23 , such as a spur gear, at an end part thereof so that the drive gear  23  is positioned within the housing case  5 . 
   In this embodiment, a motor, such as an hydraulic motor, is used as the reversible motor  18 , and the rotation axis thereof is allowed to rotate in both clockwise and anticlockwise directions. 
   Both ends of respective vibratory shafts  24  and  25  are supported by bearings  22 , respectively, so that the vibratory shaft  24  becomes parallel with respect to the vibratory shaft  25 . The vibratory shaft  24  is placed at the position opposite across the rotation shaft of the vibratory roll  1  with respect to the vibratory shaft  25 . 
   A driven gear  26  provided on one end of vibratory shaft  24  and a driven gear  27  provided on one end of vibratory shaft  25  are engaged with the drive gear  23  of gear shaft  20 . Here, the diameter of the driven gear  26  is the same as that of the driven gear  27 , and the respective driven gears  26  and  27  are provided with the same number of teeth. 
   According to the vibratory roll  1  having these constructions, when the power transmission unit  14   a  of the drive motor  14  begins to rotate, since the axle shaft  6  is rotatably supported by the bearing-housing  9 , the vibratory roll  1  begins to rotate. 
   In this occasion, if the reversible motor  18  is turned on and is operated, this causes the rotation of the drive gear  23 . Thereby, the rotative force caused by the reversible motor  18  is transmitted to vibratory shafts  24  and  25  through driven gears  26  and  27 , and causes the synchronous rotation in the same direction of vibratory shafts  24  and  25 . 
   The vibratory mechanism  31  according to the present invention includes vibratory shafts  24  and  25 , fixed eccentric weights  32  and  33 , which are fixed to vibratory shafts  24  and  25 , respectively, rotatable eccentric weights  34  and  35 , which are rotatably attached to vibratory shafts  24  and  25 , respectively, and a rotation controller  30 , which is composed with stoppers  36  and  37 , and which are rotated together with vibratory shafts  24  and  25  and controls the angular position of rotatable eccentric weights  34  and  35  with respect to respective fixed eccentric weights  32  and  33 . 
   Firstly, explanations about vibratory shaft  24  will be given. The vibratory shaft  24  is provided with fixed eccentric weights  32 , which are spaced apart from each other and are fixed on the vibratory shaft  24  by welding, etc. 
   As shown in  FIGS. 2A ,  2 B, the fixed eccentric weight  32  is composed of an arch part  32   a  and an eccentric part  32   b . The arch part  32   a  surrounds part of the circumference of the vibratory shaft  24  and fixed thereon. The eccentric part  32   b  having an approximately half-round shape surrounds the remainder of the circumference of the vibratory shaft  24  and is eccentrically fixed thereon. 
   A stopper  36  constituting the rotation controller  30  is a pole-shaped object. This stopper  36  is inserted into a through-hole provided on respective fixed eccentric weights  32  and is welded to them. Thereby, as shown in  FIG. 1 , the stopper  36  (shown by dot-dash line) is provided across fixed eccentric weights  32  and  32  so that the stopper  36  becomes parallel with respect to the vibratory shaft  24 . This stopper  36  is fixed on respective fixed eccentric weights  32  by welding. 
   The rotatable eccentric weight  34  is composed of an arch part  34   a  and an eccentric part  34   b . The arch part  34   a  surrounds part of the circumference of the vibratory shaft  24 . The eccentric part  34   b  having a half-round shape surrounds the remainder of the circumference of the vibratory shaft  24  and is eccentrically attached to the vibratory shaft  24 . In this embodiment, the rotatable eccentric weight  34  is mounted rotatably about the vibratory shaft  24 . 
   A shoulder to be touched with the stopper  36  is provided at opposing ends across the vibratory shaft  24  of the eccentric part  34   b , respectively. That is, a total of two shoulders are provided on the eccentric part  34   b.    
   In the case of  FIG. 2A , one of shoulders of the rotatable  20  eccentric weight  34  and the stopper  36  are in contact. Therefore, if the vibratory shaft  24  rotates anti-clockwise by 180° from this state, since the rotatable eccentric weight  34  turns around the vibratory shaft  24 , the other of the shoulders comes in contact with the stopper  36 . 
   Next, explanations about vibratory shaft  25  will be given. As can be seen from  FIG. 1  through  FIG. 2B , the vibratory shaft  25  has almost the same construction as the vibratory shaft  24 . 
   That is, the vibratory shaft  25  is provided with fixed eccentric weights  33 , which are spaced apart from each other. In other words, one of fixed eccentric weights  33  is fixed to the vibratory shaft  25  and is positioned apart from the other of the fixed eccentric weights  33 . 
   As shown in  FIGS. 2A ,  2 B, the fixed eccentric weight  33  is composed of an arch part  33   a  and an eccentric part  33   b . The arch part  33   a  surrounds part of the circumference of the vibratory shaft  25  and is fixed thereon. The eccentric part  33   b  having an approximately half-round shape surrounds the remainder of the circumference of the vibratory shaft  25  and is eccentrically fixed thereon. 
   A stopper  37  constituting the rotation controller  30  is a pole-shaped object. This stopper  37  (shown by dot-dash line in  FIG. 1 ) is inserted into a through-hole provided on respective fixed eccentric weights  33 . Thereby, as shown in  FIG. 1 , the stopper  37  (shown by dot-dash line) is provided across fixed eccentric weights  33  and  33  so that the stopper  36  becomes parallel with respect to the vibratory shaft  25 . 
   The rotatable eccentric weight  35  is composed of an arch part  35   a  and an eccentric part  35   b . The arch part  35   a  surrounds part of the circumference of the vibratory shaft  25 . The eccentric part  35   b  having a half-round shape surrounds the remainder of the circumference of the vibratory shaft  25  and is eccentrically attached to the vibratory shaft  25 . In this embodiment, the rotatable eccentric weight  35  is mounted rotatably about the vibratory shaft  25 . 
   A shoulder to be touched with the stopper  37  is provided at opposing-ends across the vibratory shaft  25  of the eccentric part  35   b , respectively. That is, a total of two shoulders are provided on the eccentric part  35   b.    
   In the case of  FIG. 2A , one of shoulders of the rotatable eccentric weight  35  and the stopper  37  are in contact. Therefore, if the vibratory shaft  25  rotates anticlockwise by 180° from this state, since the rotatable eccentric weight  35  turns around the vibratory shaft  25 , the other of the shoulders comes in contact with the stopper  37 . 
   Here, the positional relationship between fixed eccentric weights  32  and  33  will be explained with reference to  FIG. 2A , in which the vibratory shaft  24  is positioned upside with respect to the shaft center O and the vibratory shaft  25  is positioned downside with respect to the shaft center O. 
   In this embodiment, respective fixed eccentric weights  32  and  33  are fixed to respective vibratory shafts  24  and  25  so that the eccentric part  33   b  of the fixed eccentric weight  33  is positioned in the right side with respect to a center line  38  connecting the shaft centers of respective vibratory shafts  24  and  25 , if the eccentric part  32   b  of the fixed eccentric weight  32  is positioned in the left side with respect to the center line  38 . 
   The vibratory mechanism  31  has an eccentric moment controller  40 , which changes the eccentric moment depending on the rotation direction of respective vibratory shafts  24  and  25 . By providing the eccentric moment controller  40 , the vibration mode of the vibratory roll  1  can be switched between “standard vibration” and “horizontal vibration”. 
   Here, in the following explanations, a total eccentric moment around the vibratory shaft  24  that is caused by fixed eccentric weights  32  is denoted by “m 1 r 1 ”, an eccentric moment around the vibratory shaft  24  that is caused by the rotatable eccentric weight  34  is denoted by “m 2 r 2 ”, a total eccentric moment around the vibratory shaft  25  that is caused by fixed eccentric weights  33  is denoted by “m 3 r 3 ”, and an eccentric moment around the vibratory shaft  25  that is caused by the rotatable eccentric weight  35  is denoted by “m 4 r 4 ”. 
   Here, m 1 , m 2 , m 3 , and m 4  are mass of respective eccentric weights, r 1  and r 2  are the distance from the center of the vibratory shaft  24  to the center of gravity of respective eccentric weights  32  and  34 , and r 3  and r 4  are the distance from the center of the vibratory shaft  25  to the center of gravity of respective eccentric weights  33  and  35 . 
   The eccentric moment due to the rotation controller  30  (the stopper  36  and the stopper  37 ) is vanishingly small in comparison to the eccentric moment due to respective eccentric weights. Thus, in the present embodiment, it is considered that the eccentric moment caused by the rotation controller  30  is included in the eccentric moment due to the fixed eccentric weights. 
   Therefore, respective eccentric moments caused by the stopper  36  and the stopper  37  are included in the eccentric moment (m 1 r 1 ) caused by fixed eccentric weights  32  and the eccentric moment (m 3 r 3 ) caused by fixed eccentric weights  33 , respectively. 
   As shown in  FIG. 2A , when each of vibratory shafts  24  and  25  rotates clockwise due to the anti-clockwise rotation of the drive gear  23 , each of stoppers  36  and  37  rotates around the vibratory shafts  24  and  25 , respectively, while pushing one of shoulders of respective rotatable eccentric weights  34  and  35 . 
   In this case, the center of the gravity of the fixed eccentric weights  32  ( 33 ) is in the opposite side across the vibratory shaft  24  ( 25 ) with respect to the center of gravity of the rotatable eccentric weights  34  ( 35 ). 
   On the contrary, as shown in  FIG. 2B , when each of the vibratory shafts  24  and  25  rotates anti-clockwise due to the clockwise rotation of the drive gear  23 , each of stoppers  36  and  37  rotates around vibratory shafts  24  and  25 , respectively, while pushing the other of shoulders of respective rotatable eccentric weights  34  and  35 . That is, the angular position of the rotatable eccentric weight  34  ( 35 ) with respect to the fixed eccentric weight  32  ( 33 ) differs by 180° compared to the case of  FIG. 2A . 
   In this case, as shown in  FIG. 2B , the fixed eccentric weights  32  ( 33 ) and the rotatable eccentric weight  34  ( 35 ) are rotated in the same angular position, when the vibratory shaft  24  ( 25 ) rotates anti-clockwise. That is, the phase difference between the fixed eccentric weights  32  ( 33 ) and the rotatable eccentric weight  34  ( 35 ) is zero. 
   In the present embodiment, as for the vibratory shaft  24 , the eccentric moment (m 2 r 2 ) of the rotatable eccentric weight  34  is larger than the eccentric moment (m 1 r 1 ) of the fixed eccentric weights  32 , m 2 r 2 &gt;m 1 r 1 . As for the vibratory shaft  25 , the eccentric moment (m 4 r 4 ) of the movable eccentric weight  35  is smaller than the eccentric moment (m 3 r 3 ) of the fixed eccentric weights  33 , m 3 r 3 &gt;m 4 r 4 . 
   In the present embodiment, as can be seen from  FIG. 1 , these conditions are achieved by changing the thickness (the width in the left-and-right directions in  FIG. 1 ) of respective eccentric weights. 
   In the case of  FIG. 2A , the total eccentric moment to the vibratory shaft  24  of eccentric weights, that is, the eccentric moment caused by the rotatable eccentric weight  34  and fixed eccentric weights  32  is denoted by “m 2 r 2 −m 1 r 1 ”. Thus, the vibratory force directed from the vibratory shaft  24  to the right side in  FIG. 1A , shown by vector, is caused. 
   Also, the total eccentric moment to the vibratory shaft  25  of eccentric weights, that is, the eccentric moment caused by the rotatable eccentric weight  35  and fixed eccentric weights  33  is denoted by “m 3 r 3 −m 4 r 4 ”. Thus, the vibratory force directed from the vibratory shaft  25  to the right side in  FIG. 1A , shown by vector, is caused. 
   In the case of  FIG. 2B , the total eccentric moment to the vibratory shaft  24  of eccentric weights, that is, the eccentric moment caused by the rotatable eccentric weight  34  and fixed eccentric weights  32  is denoted by “m 1 r 1 +m 2 r 2 ”. Thus, the force that makes the vibratory roll rotate in a left-side direction along the circumference of the vibratory roll is caused on the vibratory shaft  24 . In other words, the force that makes the vibratory roll rotate in anticlockwise is caused on the vibratory shaft  24 . 
   Also, the total eccentric moment to the vibratory shaft  25  of eccentric weight is denoted by “m 3 r 3 +m 4 r 4 ”. Thus, the force that makes the vibratory roll rotate in a right-side direction along the circumference of the vibratory roll is caused on the vibratory shaft  25 . That is, the force that makes the vibratory roll rotate in anticlockwise is caused on the vibratory shaft  25 . 
   In the case of  FIG. 2A , if the moment around the shaft center O of the vibratory roll  1  exists, the force directed in a circumference direction with respect to the vibratory roll is applied to vibratory shafts  24  and  25 . Thereby, the slight horizontal vibration is caused. 
   In the present embodiment, the total eccentric moment around the vibratory shaft  24  and the total eccentric moment around the vibratory shaft  25  should be established at equal value, in order to cancel the moment around the shaft center (axis) O of the vibratory roll. That is, (m 2 r 2 −m 1 r 1 )=(m 3 r 3 −m 4 r 4 ). 
   Thereby, a vibratory force directed to the same direction of the same value is caused on vibratory shafts  24  and  25 , respectively. 
   In the present embodiment, since respective vibratory shafts  24  and  25  synchronously rotate in the same direction, the slight horizontal vibration is cancelled. But, the vibratory force due to the eccentric rotation of respective vibratory shafts that is caused in conventional vibratory roll is acting on the vibratory roll. 
   To be more precise, in the present embodiment, respective vibratory shafts  24  and  25  synchronously rotate in the same direction. Thus, the direction of the vibratory force to be caused from the vibratory shaft  24  becomes the same direction as the direction of the vibratory force to be caused from the vibratory shaft  25 . That is, if the direction of the vibratory force to be caused from the vibratory shaft  24  is a left direction, the direction of the vibratory force to be caused from the vibratory shaft  25  is a left direction. If the direction of the vibratory force to be caused from the vibratory shaft  24  is an upper direction and a lower direction, the direction of the vibratory force to be caused from the vibratory shaft  25  is an upper direction and lower direction, respectively. 
   Thereby, the vibratory roll  1  receives the vibratory force, which is the sum of vibratory forces that are caused from respective vibratory shafts  24  and  25  and that have the same value, and is vibrated in 360° directions (in all radial directions). 
   In the case of  FIG. 2B , if a resultant force of vibratory force around the shaft center (axis) O of the vibratory roll exists, the slight standard vibration is caused on the vibratory roll. The total eccentric moment around the vibratory shaft  24  is established at the same value as the total eccentric moment around the vibratory shaft  25  in order to prevent the occurrence of the standard vibration. That is, (m 1 r 1 +m 2 r 2 )=(m 3 r 3 +m 4 r 4 ) 
   Thereby, if it is assumed that a ground exists in a lower-side in  FIG. 2B , the horizontal force directed from left to right in figure is applied to the contact surface between the vibratory roll and a ground. 
     FIG. 3A  through  FIG. 3D  illustrates eccentric weights in four different angular positions. The angular position shown in  FIG. 2B  is the same as that shown in  FIG. 3D . 
   When respective vibratory shafts  24  and  25  rotate anti-clockwise, each of stoppers  36  and  37  rotates around the vibratory shafts  24  and  25 , respectively, while pushing one of the shoulders of respective rotatable eccentric weights  34  and  35 . In this occasion, the angular position of the eccentric weights is changed in order of:  FIG. 3A ,  FIG. 3B ,  FIG. 3C , and  FIG. 3D . In each angular position, respective eccentric weights are rotated in the same angular position. That is, the relative phase difference of them is 0°. 
   In the case of  FIG. 3A , the force directed to the center of the vibratory roll  1  is caused on the vibratory shaft  24 , and the force directed to the center of the vibratory roll  1  is also caused on the vibratory shaft  25 , which is positioned in the opposite position across the shaft center O with respect to the vibratory shaft  24 . Therefore, as can be seen from  FIG. 3A , since these forces have the same value, these forces are canceled by each other. 
   In the case of  FIG. 3B , the force, which causes a rotative torque at the top of the vibratory roll that is directed in a right-side direction along the circumference of the vibratory roll, is caused on the vibratory shaft  24 . On the contrary, the force, which causes a rotative torque at the bottom of the vibratory roll that is directed in a left-side direction along the circumference of the vibratory roll, is also caused on the vibratory shaft  25 . That is, the force that makes the vibratory roll  1  rotate in clockwise is caused on vibratory shafts  24  and  25 . 
   Thereby, if it is assumed that a ground exists in a lower-side in  FIG. 3B , the horizontal force directed to the left side from the right side in this figure is applied to the contact surface between the vibratory roll  1  and a ground. 
   In the case of  FIG. 3C , the force directed away from the center of the vibratory roll  1  is applied to the vibratory shaft  24 , and the force directed away from the center of the vibratory roll  1  is applied to the vibratory shaft  25 . Thereby, these forces are canceled by each other. 
   In the case of  FIG. 3D , the force, which causes a rotative torque at the top of the vibratory roll  1  that is directed in a left-side direction along the circumference of the vibratory roll  1 , is caused on the vibratory shaft  24 . On the contrary, the force, which causes a rotative torque at the bottom of the vibratory roll that is directed in a right-side direction along the circumference of the vibratory roll, is also caused on the vibratory shaft  25 . That is, the force that makes the vibratory roll  1  rotate in anticlockwise is caused on vibratory shafts  24  and  25 . 
   Thereby, if it is assumed that a ground exists in a lower-side in  FIG. 3D , the horizontal force directed to the right side from the left side in this figure is applied to the contact surface between the vibratory roll  1  and a ground. 
   Therefore, since the relative position between the eccentric weights of  FIG. 3B  and that of  FIG. 3D  are repeated alternately, the torque directed in a horizontal direction is applied to the contact surface between the vibratory roll  1  and a ground. 
   Therefore, the relation of eccentric moments is denoted by the following formula (3) and formula (4).
 
 m   2   r   2   −m   1   r   1   =m   3   r   3   −m   4   r   4   (3)
 
 m   1   r   1   +m   2   r   2   =m   3   r   3   +m   4   r   4   (4)
 
   Based on these formulas (3) and (4), following formulas are obtained.
 
m 2 r 2 =m 3 r 3   (5)
 
m 1 r 1 =m 4 r 4   (6)
 
   That is, the eccentric moment of the rotatable eccentric weight  34  and that of the fixed eccentric weight  33  are equal (see formula (5)). Additionally, the eccentric moment of the fixed eccentric weight  32  and that of the rotatable eccentric weight  35  are equal (see formula (6)). 
   In the present embodiment, if the total eccentric moment around the vibratory shaft  24  in case of rotation in one direction of the vibratory shaft  24  (in case of standard vibration) is denoted by “m 2 r 2 −m 1 r 1 ” and the total eccentric moment around the vibratory shaft  24  in case of rotation in the other direction of the vibratory shaft  24  (in case of horizontal vibration) is denoted by “m 1 r 1 +m 2 r 2 ”, this greatly expands the possibility of the selection of the amplitude of the vibratory roll. This is because of following-reasons. 
   Here, if the total eccentric moment around the vibratory shaft  24  in case of standard vibration is denoted by “mr (standard vibration)” instead of “m 2 r 2 −m 1 r 1 ” and the total eccentric moment around the vibratory shaft  24  in case of horizontal vibration is denoted by “mr (horizontal vibration)” instead of “m 1 r 1 +m 2 r 2 ”, the following formulas can be obtained.
 
 m   2   r   2 =( mr (standard vibration)+ mr (horizontal vibration))/2  (7)
 
 m   1   r   1 =( mr (standard vibration)− mr (horizontal vibration))/2  (8)
 
   EXAMPLE 
   As for  FIG. 1 , if it is assumed that the vibratory roll has a dimension of 1 meter and has about 15 millimeters (hereinafter indicated as “mm”) thickness, the drum weights M 0  is about 720 kg and the eccentric moment around center axis O of the vibratory roll  1  is about 155 kgm 2 . 
   Here, if the amplitude a 1  in the ups-and-downs directions of the vibratory roll  1  in case of operation of the vibratory roll under the standard vibration is determined as 0.3 mm, which corresponds to one of suitable amplitude for the compaction of the asphalt mixture, a following formula is obtained from formula (1).
 
0.0003=(2× mr (standard vibration))/720 ∴ mr (standard vibration)=(0.0003×720)/2=0.11
 
   Thus, 0.11 kgm is obtained as the value of mr(standard vibration). 
   In the case of U.S. Pat. No. 4,647,247, the eccentric moment around the vibratory shaft caused by the eccentric weight in case of standard vibration is the same as that in case of the horizontal vibration. Thus, the value of mr(horizontal vibration) is the same as the value of mr(standard vibration). Thereby, the value of 0.11 kgm is also the value of mr(horizontal vibration). 
   Then, if the distance between the rotational axis O of the vibratory roll  1  and the respective vibratory shafts  24  and  25  is denoted by “p”, since the maximum (limit) value of p is 0.25 m due to the limitation in the size of the vibratory roll  1 , the amplitude a 2  in case of horizontal vibration is obtained from formula (2).
 
 a   2 =(0.5×2×0.25×0.11)/155=0.18 mm
 
That is, the value of a 2  is 0.18 mm.
 
   Generally, the amplitude a 2  suitable for the compaction of asphalt mixture under horizontal vibration is about 0.5 mm. But, in the case of the vibratory roll disclosed in U.S. Pat. No. 4,647,247, since limit of the amplitude a 2  of the vibratory roll is 0.18 mm, the amplitude suitable for horizontal vibration of the vibratory roll is not obtained. 
   In the present invention, on the contrary, the value of mr in case of horizontal vibration differs from the value in case of standard vibration. If the amplitude a 2  in case of horizontal vibration is determined as 0.5 mm, mr(horizontal vibration)=0.31 kgm is obtained from formula (2).
 
0.0005=(0.5×2×0.25× mr (horizontal vibration))/155 ∴ mr (horizontal vibration))=0.31 kg·m
 
   Thus, the eccentric moment (m 2 r 2 ) around the vibratory shaft  24  of the rotatable eccentric weight  34  is computed from formula (7) based on these computed values. That is, m 2 r 2 =(0.11+0.31)/2=0.21 kg·m. Additionally, the eccentric moment (m 1 r 1 ) around the vibratory shaft  24  of the fixed eccentric weight  32  is computed from formula (8) based on these computed values. That is, m 1 r 1 =(0.31−0.11)/2=0.10 kg·m. 
   Accordingly, the eccentric moment (m 2 r 2 ) around the vibratory shaft  24  of the rotatable eccentric weight  34  is 0.21 kgm. The eccentric moment (m 1 r 1 ) around the vibratory shaft  24  of the fixed eccentric weight  32  is 0.10 kgm. 
   Here, as can be seen from formula (5) and formula (6), if the eccentric moment m 2 r 2  around the vibratory shaft  24  of the rotatable eccentric weight  34  and the eccentric moment m 3 r 3  around the vibratory shaft  25  of the fixed eccentric weight  33  are set at 0.21 kgm and if the eccentric moment m 1 r 1  around the vibratory shaft  24  of the fixed eccentric weight  32  and the eccentric moment m 4 r 4  around the vibratory shaft  25  of the rotatable eccentric weight  35  are set at 0.10 kgm, the amplitude of 0.3 mm suitable for standard vibration and amplitude of 0.5 mm suitable for horizontal vibration are obtained. 
   In other words, if the eccentric moment m 2 r 2  and the eccentric moment m 3 r 3  are 0.21 kgm and the eccentric moment m 1 r 1  and the eccentric moment m 4 r 4  are 0.10 kgm, 0.3 mm and 0.5 mm are computed using formula (5) and the formula (6) as the amplitude suitable for standard vibration and the amplitude suitable for horizontal vibration, respectively. 
   In the present invention, as described above, the vibratory mechanism includes vibratory shafts, which are stored within a roll and are arranged symmetrically across a rotation axis of the roll (vibratory roll), a fixed eccentric weight fixed to respective vibratory shafts, a rotatable eccentric weight rotatably attached to respective vibratory shafts, a rotation controller controlling a range of movement of the rotatable eccentric weight, and an eccentric moment controller which changes an eccentric moment around the vibratory shaft depending on a rotation direction of the vibratory shafts. 
   According to this vibratory mechanism having these constructions, the roll vibrates in all radial directions when respective vibratory shafts rotate in one direction, and the roll vibrates in a direction tangential to the circumference of the roll when respective vibratory shafts rotate in the reverse direction. Thereby, the amplitude of the vibratory roller can be controlled for the use in standard vibration or horizontal vibration. 
   In the present invention, as described above, a first vibratory shaft  24  and a second vibratory shaft  25  are stored in the roll (vibratory roll  1 ), and the first vibratory shaft  24  is arranged at 180° opposite position across a rotation axis O of the roll  1  with respect to the second vibratory shaft  25 . 
   In this occasion, a total eccentric moment around the first vibratory shaft  24  is substantially the same as a total eccentric moment around the second vibratory shaft  25 , when the first vibratory shaft  24  and the second vibratory shaft  25  are rotated in one direction (for example, anti-clockwise), and a total eccentric moment around the first vibratory shaft  24  is substantially the same as a total eccentric moment around the second vibratory shaft  25 , when the first vibratory shaft  24  and the second vibratory shaft  25  are rotated in the reverse direction (for example, clockwise). 
   Here, the total eccentric moment around the first vibratory shaft  24  is obtained by subtracting an eccentric moment (m 1 r 1 ) of fixed eccentric weights  32  from an eccentric moment (m 2 r 2 ) of rotatable eccentric weight  34  and the total eccentric moment around the second vibratory shaft  25  is obtained by subtracting an eccentric moment (m 4 r 4 ) of rotatable eccentric weight  35  from an eccentric moment (m 3 r 3 ) of fixed eccentric weights  33 , when the first vibratory shaft  24  and the second vibratory shaft  25  are rotated in one direction (for example, anti-clockwise), and the total eccentric moment around the first vibratory shaft  24  is obtained by adding an eccentric moment of fixed eccentric weights  32  to an eccentric moment of rotatable eccentric weight  34  and the total eccentric moment around the second vibratory shaft  25  is obtained by adding an eccentric moment of rotatable eccentric weight  35  to an eccentric moment of fixed eccentric weights  33  when the first vibratory shaft  24  and the second vibratory shaft  25  are rotated in the reverse direction (for example, clockwise). 
   According to the vibratory mechanism having these constructions, the switching of the amplitude of the vibratory roll equipped with a pair of vibratory shafts can be achieved with simple construction. Thereby, amplitude suitable for standard vibration and amplitude suitable for horizontal vibration can be selected. 
   As an example of the movable eccentric weight, the mechanism disclosed in Japanese Unexamined Patent publication No. S61-40905 (equivalent to U.S. Pat. No. 4,586,847) can be cited. In this patent publication, the vibratory roll, in which inner walls and liquidity weights are provided, is disclosed. In this vibratory roll, liquidity weights, which are stored in the vibratory roll and which move along the inside-circumference of the roll when the vibratory roll is rotated, correspond to the rotatable eccentric weight. Inner walls which restrict the range of the movement of the liquidity weights correspond to the rotation controller. 
   In the present invention, as described above, respective rotatable eccentric weights  34  and  35  of the first vibratory shaft  24  and the second vibratory shaft  25  are allowed to rotate around the first vibratory shaft  24  and the second vibratory shaft  25 , respectively, within limits of 0 to 180°. 
   Here, the eccentric moment m 1 r 1  around the first vibratory shaft  24  of the fixed eccentric weights  32  is substantially the same as the eccentric moment m 4 r 4  around the second vibratory shaft  25  of the rotatable weight  35 , and the eccentric moment m 2 r 2  around the first vibratory shaft  24  of the rotatable eccentric weight  34  is substantially the same as the eccentric moment m 3 r 3  around the second vibratory shaft  25  of the fixed eccentric weights  33 . 
   According to the vibratory mechanism having these constructions, the design of rotatable eccentric weights  34  and  35  can be achieved with ease. Thereby, amplitude suitable for standard vibration and amplitude suitable for horizontal vibration can be selected. 
   If the vibratory roll equipped with the vibratory mechanism according to the present invention is adopted by the vibratory roller, the vibratory roller, which can meet various needs of compaction operation, can be obtained. This is because the amplitude of the vibratory roll can be adjusted to the suitable value for standard vibration and horizontal vibration. 
   Here, the vibration of the vibratory roll between standard vibration and horizontal vibration can be suitably changed depending on a quality (condition) of the ground to be compacted. 
   In the above described embodiment, a total of two vibratory shafts are provided within the vibratory roll. But the numbers of the vibratory shaft is not restricted to this. For example, the vibratory roll which includes a total of four vibratory shafts may be adoptable. In this vibratory roll, vibratory rolls having the same construction are provided around the rotation shaft of the vibratory roll at a phase difference of 90°. 
   In the present invention, additionally, each of the fixed eccentric weights is provided separately from the vibratory roll. But these fixed eccentric weights may be provided as a single unit with the corresponding vibratory shafts. 
   According to the present invention, since the amplitude of the vibratory roll can be controlled to the suitable value for standard vibration and horizontal vibration, the satisfactory compaction result can be obtained. 
   Although there have been disclosed what is the present embodiment of the invention, it will be understood by persons skilled in the art that variations and modifications may be made thereto without departing from the scope of the invention, which is indicated by the appended claims.