Abstract:
A vibration generator includes a weight member, a shaft rotatable about an axis of rotation extending longitudinally therethrough and having a mounting area on which the weight member is mounted, and a positioning arm disposed in engagement with the weight member on the shaft and rotatable about the axis of rotation relative to both the shaft and the weight member. The mounting area engages the weight member and prevents movement of the weight member relative to the shaft in a circumferential direction about the axis of rotation but permits movement of the weight member relative to the shaft in a radial direction orthogonal to the axis of rotation and coplanar with the circumferential direction. The weight member also defines an elongate positioning slot and the positioning arm includes an axially extending portion disposed through the positioning slot for slidable movement within the positioning slot during rotation of the positioning member relative to the shaft. The axially extending portion in fact defines a cam surface disposed in engagement with the weight member within the positioning slot. Rotation of the positioning arm relative to both the shaft and the weight member moves the weight member relative to the shaft in a radial direction orthogonal to the axis of rotation thereby altering the moment of inertia of the weight member about the axis of rotation and resulting in a different amplitude of vibration during rotation of the weight member.

Description:
FIELD OF THE PRESENT INVENTION 
     The present invention relates to a vibration generator for a compaction machine and, in particular, to a variable amplitude vibration generator for a compaction machine used in the road construction industry. 
     BACKGROUND OF THE PRESENT INVENTION 
     It is well known to use a compaction machine having a compaction drum in leveling a road surface in the road construction industry. Furthermore, it is also well known that better results and better efficiency are achieved by causing small high-frequency vibrations in the compaction drum during such leveling. Vibrations are often generated by rotating an eccentric weight within the compaction drum. Moreover, the amplitude of vibration is dependent upon the rotational rate of the eccentric weight; however, the amplitude of vibration is also dependent upon the radial spacing of the center of mass of the weight to the axis of rotation, i.e., the eccentricity of the weight. 
     In Schmelzer et al. U.S. Pat. No. 4,830,534 (the &#39;534 Patent), vibrations in the compaction drum are generated by rotation of an eccentric weight mounted on a rotor shaft. A high amplitude of vibration or, alternatively, a low amplitude of vibration is produced depending upon the radial position of the eccentric weight with regard to the axis of rotation of the shaft. Springs are provided in the mounting of the eccentric weight and, when the eccentric weight is not undergoing rotation, the springs urge the eccentric weight into a default radial position in abutment with the shaft. A latch fixedly mounted to the shaft controls the radial positioning of the eccentric weight as well as drives the rotation of the eccentric weight. In particular, the latch includes a slot and the eccentric weight, which is rotatably mounted on the shaft, includes a pin that extends axially through the slot. Rotation of the latch in a first direction causes the pin to move to a first end of the slot which, in turn, moves the weight into a low radial position with respect to the axis of rotation, thereby generating a low amplitude of vibration. Furthermore, the slot is C-shaped or L-shaped and a side of the slot engages the pin and thereby restrains the weight from moving into a higher radial position. Rotation of the latch in the reverse direction causes the pin to move to the other end of the slot and causes the eccentric weight to move into a high radial position, thereby generating a high amplitude of vibration. 
     An object of the present invention is to provide a vibration generator for a compaction machine which exhibits both high and low amplitude of vibration states without utilizing the vibration generator of the &#39;534 Patent. 
     SUMMARY OF THE PRESENT INVENTION 
     The vibration generator for a compaction machine of the present invention includes a weight member, a shaft rotatable about an axis of rotation extending longitudinally through the weight member and having a mounting area on which the weight member is mounted, and a positioning arm disposed in engagement with the weight member and rotatable about the axis of rotation relative to both the shaft and the weight member. Rotation of the positioning arm relative to both the shaft and the weight member moves the weight member relative to the shaft in a radial direction orthogonal to the axis of rotation thereby altering the moment of inertia of the weight member about the axis of rotation and resulting in a different amplitude of vibration during rotation of the weight member. 
     In a feature of the present invention, the mounting area engages the weight member and prevents movement of the weight member relative to the shaft in a circumferential direction about the axis of rotation but permits movement of the weight member relative to the shaft in a radial direction orthogonal to the axis of rotation and coplanar with the circumferential direction. In a preferred embodiment of the present invention, the weight member includes an elongate mounting slot having a pair of opposed parallel planar sides between which the mounting area of the shaft extends, and the mounting area includes parallel planar surfaces disposed in sliding abutment with the planar sides. 
     In a further feature of the present invention, the weight member defines an elongate positioning slot and the positioning arm includes an axially extending portion disposed through the positioning slot for slidable movement within the positioning slot during rotation of the positioning member relative to the shaft. Furthermore, the axially extending portion in fact defines a cam surface disposed in engagement with the weight member within the positioning slot. In a preferred embodiment of the present invention, the weight member includes a weighted portion and an arm portion extending from the weighted portion, with the weighted portion defining the positioning slot and the arm portion defining the elongate mounting slot. In an alternative preferred embodiment including this feature, the weight member includes a weighted portion, an offsetting portion disposed opposite the weighted portion relative to the axis of rotation, and an arm portion extending between and connecting the weighted portion and the offsetting portion, with the offsetting portion defining the positioning slot and the arm portion defining the elongate mounting slot. 
     In a further feature of the present invention, the positioning slot extends along its length from an end thereof away from the axis of rotation and then extends in closer proximity to the axis of rotation. In one preferred embodiment including this feature, the positioning slot is generally checkmark shaped. In an alternative preferred embodiment, the positioning slot extends parallel to a plane orthogonal to the axis of rotation and perpendicularly intersects a radial line orthogonal to the axis of rotation. 
     Preferably, in each embodiment of the present invention the center of mass of the weight member is located in a pie-wedged weighted portion thereof. 
     The present invention also includes a compaction machine including the vibration generator of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features, embodiments, and advantages of the present invention will become apparent form the following detailed description with reference to the drawings, wherein: 
     FIG. 1 is a perspective view of a compaction machine used in the road construction industry in which the variable amplitude vibration generator of the present invention is preferably utilized; 
     FIG. 2 is a partially broken-away perspective view a compaction roller of the compaction machine of FIG. 1 showing an embodiment of the variable amplitude vibration generator of the present invention; 
     FIG. 3 is an exploded view in partial cross-section of the variable amplitude vibration generator and a bearing housing as shown in FIG. 2; 
     FIG. 4 is a perspective view of part of the variable amplitude vibration generator of FIG. 2 in a high-amplitude position; 
     FIG. 5 is a perspective view of part of the variable amplitude vibration generator of FIG. 2 in a low-amplitude position; 
     FIG. 6 is a cross-sectional elevational view of the variable amplitude generator of FIG. 2 in a high-amplitude position; 
     FIG. 7 is a cross-sectional elevational view of the variable amplitude generator of FIG. 2 in a first intermediate position during the transition thereof from a high-amplitude position to a stable low-amplitude position; 
     FIG. 8 is a cross-sectional elevational view of the variable amplitude generator of FIG. 2 in a second intermediate position during the transition thereof to a stable low-amplitude position; 
     FIG. 9 is a cross-sectional elevational view of the variable amplitude generator of FIG. 2 in a low-amplitude position; 
     FIG. 10 is a very general graphical illustration of the radial spacing of rotation of the center of mass of the weight member as a function of rotation of the positioning arm of the variable amplitude vibration generator of FIG. 2; 
     FIG. 11 is a perspective view of another embodiment of the variable amplitude vibration generator of the present invention in a high-amplitude position; 
     FIG. 12 is a perspective view of the variable amplitude vibration generator of FIG. 11 in a low-amplitude position; 
     FIG. 13 is a cross-sectional elevational view of the variable amplitude generator of FIG. 11 in a high-amplitude position; 
     FIG. 14 is a cross-sectional elevational view of the variable amplitude generator of FIG. 11 in a first intermediate position during the transition thereof from a high-amplitude position to a stable low-amplitude position; 
     FIG. 15 is a cross-sectional elevational view of the variable amplitude generator of FIG. 11 in a second intermediate position during the transition thereof from a high amplitude position to a stable low-amplitude position; 
     FIG. 16 is a cross-sectional elevational view of the variable amplitude generator of FIG. 11 in a low-amplitude position; and 
     FIG. 17 is a very general graphical illustration of the radial spacing of the center of mass of the weight member as a function of rotation of the positioning arm of the variable amplitude vibration generator of FIG.  11 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A compaction machine  20  used in the road construction industry is generally shown in FIG. 1, and the variable amplitude vibration generator of the present invention (hereinafter simply referred to as “vibration generator” for brevity) is preferably used in this type of compactor for transmitting variable amplitude vibrations to a compaction drum  24  during leveling of a recently paved road surface  26 . Two preferred embodiments of the present invention are shown in the drawings. A first embodiment is shown in FIGS. 2-9 and a second embodiment is shown in FIGS. 11-16. Each of these two preferred embodiments includes a shaft, two weight members, and two positioning arms. However, the basic construction of the preferred vibration generator of the present invention includes only a shaft having a single weight member and a single positioning arm mounted thereon and, as will be apparent to one having ordinary skill in the art, any number of pairs of a weight member and a positioning arm can be provided on the shaft as desired, with two pairs being preferred. Consequently, each pair of a weight member and a positioning arm in the illustrated preferred embodiments, and identical parts thereof, will be identified by identical reference numerals in the Figures. 
     As shown in FIG. 2, a vibration generator  22  is disposed within the compaction drum  24  itself and is contained therein within a bearing housing  28  as shown in FIG.  3 . The vibration generator  22  is not fixed directly to the compaction drum  24  and therefore does not necessarily rotate in direct correlation with the compaction drum  24 . Instead, the bearing housing  28  is secured by a flange  30  to the compaction drum  24  and the vibration generator  22  is supported within the bearing housing  28  in slidable engagement therewith. Lubrication for this slidable engagement is provided by circulation of oil through passages  32  in the bearing housing  28  as shown in FIG. 3 or, alternatively, by packing grease within an enclosed area  34  of the bearing housing  128  surrounding the vibration generator  122  as shown in FIG.  11 . Vibrations that are generated by the vibration generator  22 , 122  as discussed in greater detail below are transmitted to the compaction drum  24  and road surface  26  through contact between the vibration generator  22 , 122  and the bearing housing  28 , 128  of the compaction drum  24 . 
     With specific regard first to the preferred embodiment illustrated in FIGS. 2-9, the vibration generator  22  includes a shaft  36  that is rotatable within the bearing housing  28  along an axis of rotation  42  with reference to which a radial direction ρ, an axial direction Z, and a circumferential direction θ are defined. The axis of rotation  42  extends longitudinally along the center of the shaft  36 , and the radial direction ρ, axial direction Z, and circumferential direction θ are orthogonal to one another and define a cylindrical coordinate system. 
     The vibration generator  22  also includes a weight member  38 . The weight member  38  includes an arm portion  44  which defines a mounting slot  46  having a pair of opposed parallel planar sides  48  between which a mounting area  50  of the shaft  36  extends. The mounting area  50  of the shaft  36  includes two parallel planar surfaces  52  which are disposed in sliding abutment with the planar sides  48  when the weight member  38  is mounted to the shaft  36 , whereby the weight member  38  is movable relative to the shaft  36  in the radial direction ρ but is precluded from movement relative to the shaft  36  in the circumferential direction θ. The weight member  38  also includes a pie-shaped weighted portion  54  which defines a positioning slot  56  having a general checkmark configuration. Furthermore, a center of mass CM of the entire weight member  38  is located within the weighted portion  54 . 
     The vibration generator  22  also includes a positioning arm  40  mounted on the shaft  36  adjacent the weight member  38  and rotatable about the axis of rotation  42  relative to both the shaft  36  and the weight member  38 . A bolt  58  and washer  60  are secured to the end of the shaft  36  and retain the positioning arm  40  on the shaft  36 . A bearing ring  62  is also mounted on the shaft  36  adjacent the other side of the weight member  38  whereby the weight member  38  is retained adjacent the positioning arm  40  and prevented from axial movement. The bearing ring  62  also represents the portion of the vibration generator  22  that slidably engages the bearing housing  28  and, thus, is the element that directly supports the vibration generator  22  within the bearing housing  28 . 
     The positioning arm  40  includes an axially extending portion that extends through the positioning slot  56  defined by the weight member  38 . The axially extending portion preferably comprises a cylindrical pin  64  which is slidable along the length of the positioning slot  56  during rotation of the positioning arm  40 . The surface of the pin  64  engages the weight member  38  within the positioning slot  56  and acts as a cam surface  65  during rotation of the positioning arm  40  relative to the shaft  36  and weight member  38 . The radial distance R from the axis of rotation  42  to the cylindrical pin  64  is constant. 
     As shown in FIGS.  2  and  4 - 5 , but omitted in FIG. 3 for clarity, an actuating rod  66  is disposed coaxial with the shaft  36  of the vibration generator  22  and is mounted to the positioning arm  40  through a coupling member  68 . The actuating rod  66  is driven in rotation by a motor arrangement (not shown) of the compaction machine  20 , with driven rotation of the actuating rod  66  causing rotation of the positioning arm  40  about the axis of rotation  42 . Preferably, the actuating rod  66  is linked to the compaction drum  24 , whereby rotation of the compaction drum  24  drives rotation of the actuating rod  66 . The direction of rotation of the actuating shaft  36  can be in either a clockwise or counterclockwise direction as desired. In such an arrangement, movement of the compaction drum  24  results initially in rotation of the actuating rod  66  and positioning arm  40 . During rotation of the positioning arm  40 , there is a sufficient lack of frictional force between the cylindrical pin  64  and weight member  38  to permit the cylindrical pin  64  extending through the positioning slot  56  to slide within the positioning slot  56  to an end thereof without causing any initial rotation of the weight member  38 . Then, once the cylindrical pin  64  engages an end of the positioning slot  56 , continued rotation of the positioning arm  40  by the actuating rod  66  results in corresponding rotation of the weight member  38  and shaft  36 ; hence, clockwise rotation of the actuating rod  66  results in clockwise rotation of the weight member  38  as shown in FIG. 4, and counterclockwise rotation of the actuating rod  66  results in counterclockwise rotation of the weight member  38  as shown in FIG.  5 . 
     Different radial dispositions of the center of mass CM of the weight member  38  relative to the axis of rotation  42  results in different moments of inertia of the weight member  38  about the axis of rotation  42 . Rotation of the weight member  38  in each different disposition therefore results in different amplitudes of vibration in the shaft  36  which, in turn, are transmitted through the bearing rings  62  to the bearing housing  28  and to the compaction drum  24 . 
     In the vibration generator  22  of the present invention, the weight member  38  is selectively disposed relative to the axis of rotation  42  to generate different amplitudes of vibration during rotation thereof. Selective disposition of the weight member  38  preferably results from the configuration of the positioning slot  56  and direction of rotation of the positioning arm  40 . In particular, the selective disposition of the pin  64  of the positioning arm  40  in each of the two opposed ends of the positioning slot  56  results in different radial dispositions of the weight member  38  and, thus, different amplitudes of vibration. Indeed, the disposition of the weight member  38  in FIG. 4 is shown in cross-sectional elevational view in FIG. 6, wherein the center of mass CM of the weight member  38  is disposed at a radial distance of D 1  to the axis of rotation  42 . On the other hand, the disposition of the weight member  38  in FIG. 5 is shown in cross-sectional elevational view in FIG. 9, wherein the center of mass CM is disposed at a different radial distance D 4  to the axis of rotation  42 , with D 4  being less than D 1 . Consequently, the disposition of the weight member  38  shown in FIGS. 4 and 6 is a high-amplitude position (greater eccentricity of the weight member  38 ) which results in a higher amplitude of vibration than the amplitude of vibration generated by the disposition of the weight member  38  shown in FIGS. 5 and 9, which is a low-amplitude position (lower eccentricity of the weight member  38 ). 
     Furthermore, both the high-amplitude position and the low-amplitude position are stable in the vibration generator  22  of the present invention. This stability also results from the configuration of the positioning slot  56 . As a result of so-called “centrifugal” force, the weight member  38  will naturally tend toward the greatest radial disposition of its center of mass CM during rotation. When the weight member  38  is rotated in the clockwise direction as shown in FIG. 4, the weight member  38  is in the high-amplitude position with the greatest radial distance to the axis of rotation  42  and, therefore, will remain in this disposition during rotation. In order to retain the weight member  38  in a stable low-amplitude position at a radial spacing less than that of the high-amplitude position, however, it is necessary to configured the positioning slot  56  so that a local minimum radial spacing of the center of mass CM of the weight member  38  is obtained during the transition of the weight member  38  from the high-amplitude position to the stable low-amplitude position. This is accomplished by configuring the positioning slot  56  to extend along its length from an end thereof away from the axis of rotation  42  and then in closer proximity to the axis of rotation  42 . Consequently, rotation of the positioning arm  40  relative to both the shaft  36  and the weight member  38 , which are locked together in the circumferential direction θ, results in a center of mass CM of the weight member  38  moving relative to the shaft  36  first in a radial direction −ρ toward the axis of rotation  42  and then in a radial direction +ρ away from the axis of rotation  42 . The movement of the cylindrical pin  64  between opposite ends of the slot results in the center of mass CM of the weight member  38  passing through a local minimum radial distance to the axis of rotation  42 . 
     With reference to the sequence of the transition of the weight member  38  from the high-amplitude position as shown in FIG. 6 to the stable low-amplitude position as shown in FIG. 9, the radial distance of the center of mass CM first decreases from D 1  to D 2  (FIG. 7) and then to a minimum value of D 3  (FIG.  8 ), and then increases to D 4  (FIG.  9 ). While D 4  is less than D 1 , D 4  is greater than D 3  and, therefore, the weight member  38  when in the stable low-amplitude position of FIG. 9 will nevertheless still be at a greater radial distance than D 3 , will remain in such position, and will not tend toward the high-amplitude position of FIG. 6 as it would first have to pass through the even lower but unstable amplitude position of FIG.  8 . 
     The radial distance of the center of mass CM to the axis of rotation  42  is very generally illustrated in FIG. 10 for the sole purpose of comparing the relative values D 1 ,D 2 ,D 3 , and D 4 . As will be apparent, radial spacings D 1  and D 4  represent equilibrium positions of the weight member  38  while D 3 , as a local minimum radial spacing, represents the turning point between these two equilibrium positions. 
     With specific regard now to the preferred embodiment illustrated in FIGS. 11-16, the vibration generator  122  is very similar to the vibration generator  22  of the preferred embodiment illustrated in FIGS. 2-9 and described in detail above, and common elements between the two are identified by the same reference numerals. 
     The second illustrated preferred embodiment of the vibration generator  122  includes a shaft  36  that is rotatable within a bearing housing  128  along an axis of rotation  42  with reference to which a radial direction ρ, axial direction Z, and a circumferential direction θ are defined. The axis of rotation  42  extends longitudinally along the center of the shaft  36 , and the radial direction ρ, axial direction Z, and a circumferential direction θ are orthogonal to one another and define a cylindrical coordinate system. 
     The vibration generator  122  also includes a weight member  138 . The weight member  138  includes an arm portion  44  which defines a mounting slot  46  having a pair of opposed parallel planar sides  48  between which a mounting area of the shaft  36  extends. The mounting area of the shaft  36  includes two parallel planar surfaces  52  which are disposed in sliding abutment with the planar sides when the weight member  138  is mounted to the shaft  36 , whereby the weight member  138  is movable relative to the shaft  36  in the radial direction but is precluded from movement relative to the shaft  36  in the circumferential direction. The weight member  138  also includes a pie-shaped weighted portion  154  and a center of mass CM of the entire weight member  138  is located within the weighted portion  154 . However, unlike in the first illustrated preferred embodiment, the weight member  138  in the second preferred embodiment also includes an offsetting portion  170  disposed opposite the weighted portion  154  about the axis of rotation  42 , with the arm portion  44  connecting the offsetting portion  170  and the weighted portion  154  together. In the second embodiment the offsetting portion  170  defines the positioning slot  156  rather than the weighted portion  54  as in the first embodiment. Furthermore, the positioning slot  156  is not checkmark shaped, but rather, linear and disposed so that it extends tangential to an arc φ having a radius equal to the radial extent R of the positioning arm  40  as shown in FIG.  15 . 
     The vibration generator  122  also includes a positioning arm  40  mounted on the shaft  36  adjacent the weight member  138  and rotatable about the axis of rotation  42  relative to both the shaft  36  and the weight member  138 . A bearing ring  62  is also mounted on the shaft  36  adjacent the other side of the weight member  138  whereby the weight member  138  is retained adjacent the positioning arm  40  and prevented from axial movement. The bearing ring  62  also represents the portion of the vibration generator  122  that slidably engages the bearing housing  128  and, thus, is the element that directly supports the vibration generator  122  within the bearing housing  128 . The positioning arm  40  includes an axially extending portion that extends through the positioning slot  156  defined by the weight member  138 . The axially extending portion preferably comprises a cylindrical pin  64  which is slidable along the length of the positioning slot  156 . The surface of the pin  64  engages the weight member  138  within the slot and acts as a cam surface  65  during rotation of the positioning arm  40  relative to the shaft  36  and weight member  138 . 
     As in the first illustrated embodiment, an actuating rod  66  is disposed coaxial with the shaft  36  of the vibration generator  122  and is mounted to the positioning arm  40  through a coupling member  68 . The actuating rod  66  is driven in rotation by a motor arrangement (not shown) of the compaction machine  20 , with driven rotation of the actuating rod  66  causing rotation of the positioning arm  40  about the axis of rotation  42 . Preferably, the actuating rod  66  is linked to the compaction drum  24 , whereby rotation of the compaction drum  24  drives rotation of the actuating rod  66 . The direction of rotation of the actuating shaft  36  can be in either a clockwise or counterclockwise direction as desired. In such an arrangement, movement of the compaction drum  24  results initially in rotation of the actuating rod  66  and positioning arm  40 . During rotation of the positioning arm  40 , there is a sufficient lack of frictional force between the cylindrical pin  64  and weight member  138  to permit the cylindrical pin  64  extending through the positioning slot  156  to slide within the positioning slot  156  to an end thereof without causing any initial rotation of the weight member  138 . Then, once the cylindrical pin  64  engages an end of the positioning slot  156 , continued rotation of the positioning arm  40  by the actuating rod  66  results in corresponding rotation of the weight member  138  and shaft  36 ; hence, counterclockwise rotation of the actuating rod  66  results in counterclockwise rotation of the weight member  138  as shown in FIG. 11, and clockwise rotation of the actuating rod  66  results in clockwise rotation of the weight member  138  as shown in FIG.  12 . 
     As in the first embodiment, different radial dispositions of the center of mass CM of the weight member  138  relative to the axis of rotation  42  results in different moments of inertia of the weight member  138  about the axis of rotation  42 . Rotation of the weight member  138  in each different disposition therefore results in different amplitudes of vibration in the shaft  36  which, in turn, are transmitted through the bearing rings  62  to the bearing housing  128  and to the compaction drum  24 . The weight member  138  is selectively disposed relative to the axis of rotation  42  to generate different amplitudes of vibration during rotation thereof. Selective disposition of the weight member  138  preferably results from the configuration of the positioning slot  156  and direction of rotation of the positioning arm  40 . In particular, the selective disposition of the pin  64  of the positioning arm  40  in each of the two opposed ends of the positioning slot  156  results in different radial dispositions of the weight member  138  and, thus, different amplitudes of vibration. 
     The disposition of the weight member  138  in FIG. 11 is shown in cross-sectional elevational view in FIG. 13, wherein the center of mass CM of the weight member  138  is disposed at a radial distance of D 5  to the axis of rotation  42 . On the other hand, the disposition of the weight member  138  in FIG. 12 is shown in cross-sectional elevational view in FIG. 16, wherein the center of mass CM is disposed at a different radial distance D 8  to the axis of rotation  42 , with D 5  being less than D 8 . Consequently, the disposition of the weight member  138  shown in FIGS. 11 and 13 is a high-amplitude position (greater eccentricity of the weight member  138 ) which results in a higher amplitude of vibration than the amplitude of vibration generated by the disposition of the weight member  138  shown in FIGS. 12 and 16, which is a low-amplitude position (lower eccentricity of the weight member  138 ). 
     Furthermore, both the high-amplitude position and the low-amplitude position are stable in the vibration generator  122  of the present invention. This stability also results from the configuration of the positioning slot  156 . As a result of centrifugal force during rotation of the weight member  138 , the weight member  138  will naturally tend toward the greatest radial disposition of its center of mass CM. When the weight member  138  is rotated in the counterclockwise direction as shown in FIG. 11, the weight member  138  is in the high-amplitude position with the greatest radial distance to the axis of rotation  42  and, therefore, will remain in this disposition during rotation. In order to retain the weight member  138  in a stable low-amplitude position at a radial spacing less than that of the high-amplitude position, however, it is necessary to configured the positioning slot  156  so that a local minimum radial spacing of the center of mass CM of the weight member  138  is obtained during the transition of the weight member  138  from the high-amplitude position to the low-amplitude position. This is accomplished by configuring the positioning slot  156  to extend along its from an end thereof first in closer proximity to the axis of rotation  42  and then away from the axis of rotation  42 . (One of ordinary skill in the art will note that this is opposite to the first illustrated embodiment since the positioning slot  156  is disposed opposite the weighted portion  154  relative to the axis of rotation  42  in the second embodiment.) Thus, rotation of the positioning arm  40  relative to both the shaft  36  and the weight member  138 , which are locked together in the circumferential direction, results in a center of mass CM of the weight member  138  moving relative to the shaft  36  first in a radial direction −ρ toward the axis of rotation  42  and then in a radial direction +ρ away from the axis of rotation  42 . The movement of the cylindrical pin  64  between opposite ends of the positioning slot  156  to the other end results in the center of mass CM of the weight member  138  reaching a local minimum (but unstable) radial distance to the axis of rotation  42 . 
     With reference to the sequence of the transition of the weight member  138  from the high-amplitude position as shown in FIG.  11  and FIG. 13 to the low-amplitude position as shown in FIG.  12  and FIG. 16, the radial distance of the center of mass CM first decreases from D 5  (FIG. 13) to D 6  (FIG. 14) and then to a minimum value of D 7  (FIG.  15 ), and then finally increases to D 8  (FIG.  16 ). While D 8  is less than D 5 , D 8  is greater than D 7  and, therefore, the weight member  138  will nevertheless still be at a relatively greater radial distance when in the stable low-amplitude position of FIG. 16, will remain in such position, and will not tend toward the high-amplitude position of FIG. 13 as it would first have to pass through the even lower but unstable amplitude position of FIG.  15 . 
     The radial distance of the center of mass CM to the axis of rotation  42  is very generally illustrated in FIG. 17 for the sole purpose of comparing the relative values of D 5 , D 6 , D 7 , and D 8 . As will be apparent, radial spacings D 5  and D 8  represent equilibrium positions of the weight member  138  while D 7 , as a local minimum radial spacing, represents the turning point between these two equilibrium positions. The commonality between the first illustrated embodiment and the second illustrated embodiment of the present invention is clearly established by comparison between the graph of FIG.  10  and that of FIG.  17 . 
     It will therefore be readily understood by those persons skilled in the art that the present invention is susceptible of broad utility and application. Many embodiments and adaptations of the present invention other than those herein described, as well as many variations, modifications and equivalent arrangements, will be apparent from or reasonably suggested by the present invention and the foregoing description thereof, without departing from the substance or scope of the present invention. Accordingly, while the present invention has been described herein in detail in relation to preferred embodiments, it is to be understood that this disclosure is only illustrative and exemplary of the present invention and is made merely for purposes of providing a full and enabling disclosure of the invention. The foregoing disclosure is not intended or to be construed to limit the present invention or otherwise to exclude any such other embodiments, adaptations, variations, modifications and equivalent arrangements, the present invention being limited only by the claims appended hereto and the equivalents thereof. 
     Consequently, it will be obvious that a checkmark shape slot could be provided in an offsetting portion and a linear slot in a weighted portion so long as a local minimum value of the radial distance of the center of mass CM of the weight member is obtained during the transition of the cylindrical pin between the ends of the positioning slot. Furthermore, it should be noted that the location of the center of mass of the weight member on the axis of rotation would result in no vibrations being generated by the rotation of the weight member which would, in such position, then not be eccentric. The positioning slot can therefore be configured to substantially eliminate vibrations by the vibration generator when in a minimal vibratory state by orienting the positioning slot or forming the positioning slot so that the radial distance in the low-amplitude position is minimized to its smallest practical value which accommodates stability in this low-to-no-amplitude position. 
     Legend 
       20  compaction machine 
       22  vibration generator 
       24  compaction drum 
       26  road surface 
       28  bearing housing 
       30  flange 
       32  passages 
       34  enclosed area 
       36  shaft 
       38  weight member 
       40  positioning arm 
       42  axis of rotation 
     ρ radial direction 
     Z axial direction 
     θ circumferential direction 
       44  arm portion 
       46  mounting slot 
       48  opposed parallel planar sides 
       50  mounting area 
       52  parallel planar surface 
       54  weighted portion 
       56  positioning slot 
     CM center of mass 
       58  bolt 
       60  washer 
       62  bearing ring 
       64  cylindrical pin 
       65  cam surface 
       66  actuating rod 
       68  coupling member 
       122  vibration generator (second embodiment) 
       128  bearing housing 
       138  weight member 
       154  weighted portion 
       156  positioning slot 
       170  offsetting portion