EP0034914A1

EP0034914A1 - Vibratory compaction system

Info

Publication number: EP0034914A1
Application number: EP81300665A
Authority: EP
Inventors: Kenneth E. c/o Shovel Supply Co. Inc. Brooks; Lynn A. Schmelzer; W. Thomas Fouser
Original assignee: Hyster Co
Current assignee: Hyster Co
Priority date: 1980-02-22
Filing date: 1981-02-18
Publication date: 1981-09-02
Also published as: AU6748981A; BR8101060A

Abstract

A vibratory compactor vehicle (10) includes a roller drum (12) provided with internal eccentric weights (38, 82) which are rotated for imparting vibration to the drum. The weights (38, 82) are mounted upon concentric shafts (34, 68) having mating, helically splined surfaces (66) whereby longitudinal movement of one of the shafts (34, 68) with respect to the other brings relative rotation of the weights (38, 82) thereby changing the amplitude of the resulting vibration.

Description

THE PRESENT INVENTION relates to a vibratory compaction system and particularly to such a system having a simplified and rugged construction and an adjustable amplitude of vibration.
Compactor vehicles provided with vibratory rollers are used in compacting road surfaces and the like including dirt or asphalt. The vibration is suitably brought about by means of eccentric weights attached to the roller in some manner and rotated comparatively rapidly for imparting a vibratory force to the roller.. On course the extent of vibration desired may depend upon the surface materials employed and the degree of compaction desired, and therefore the amplitude of vibration is preferably adjustable. Various systems have been proposed heretofore for adjusting the degree or amplitude of vibration, but many have suffered from certain difficulties. For instance, a common drawback relates to a requirement for stopping a vehicle in order to change or adjust the positioning of weights and therefore the extent of vibration. Other systems allow for change in vibratory amplitude during operation of the vehicle, but the adjustment is found to be somewhat limited or produces only a change from maximum to minimum vibration amplitude. Some vehicles require a rotating hydraulic connection between the moving drum and the control apparatus.
Other systems have been proposed which are relatively complex in their organization and lack structural durability. For example, a pin and camming slot arrangement has been suggested for producing relative rotation of eccentric weights. However, such a system does not provide for the ease of operation and ruggedness required in a compactor vehicle.
In accordance with the present invention, a vibratory compaction system includes a pair of concentric shafts positioned coaxially with the compactor drum and adapted for respectively rotating eccentric weights positioned within the drum. The shafts have a driving connection therebetween, preferably comprising mating helically grooved and ribbed or splined surfaces extending along an appreciable length thereof to provide a structurally reliable connection while facilitating relative rotation between the shafts. Means, preferably a linear actuator, moves one shaft axially with respect to the other and the mating helically splined surfaces cause relative rotation between the shafts and consequent relative rotation of the eccentric weights within the drum, and a change in the amplitude of vibration produced by the eccentric weights.
In a preferred embodiment, the mating grooved and ribbed surfaces are polygonal in cross-section, i.e. the exterior of the inner concentric shaft takes the form of a polygon in cross-section for mating with a similarly configured internal cross-section on the outer concentric shaft. This configuration is found to have improved constructional ruggedness and reliability, while at the same time enabling adjustment of vibration amplitude.
In a preferred embodiment, a straight splined connection between a second of the shafts and a second eccentric weight enables relative longitudinal movement between said second shaft and said second eccentric weight, as well as relative rotation of said second eccentric weight with respect to a first concentric weight. However, the latter splined connection is not a necessity. Alternatively, a straight splined connection can be employed between first and second'shafts with a helically splined connection relating the second shaft and the second eccentric weight. Alternatively, both splined connections can be helical if so desired.
Thus, the present invention ofers the advantages of an improved vibratory compaction system wherein adjustment of the amplitude of vibration during operation is facilitated, which is simpler in construction and more reliable in operation than those available heretofore, and which has improved structural ruggedness and reliability, enabling infinite variation of the amplitude of vibration between a maximum and minimum vibration condition, enabling such variation of amplitude during rotation of the compaction drum, and providing reversibility of the direction of rotation of the vibration generating mechanism.
The invention, however, both as to organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements.

FIGURE 1 is a side view of a compactor vehicle with which the system according to the present invention is employed;
FIGURE 2 is a partially exploded, perspective view of a first vibratory compaction system according to the present invention;
FIGURE 3 is a longitudinal cross-sectional view, partially broken away, through the drum and compaction system of Figure 2.
FIGURE 4 is a longitudinal cross section, partially broken away, through the drum and compaction system according to a second embodiment of the present invention; and
FIGURES 5 to 12 are cross-sectional and side views of phasing shafts as suitably employed according to the Figure 4 embodiment.

Referring to Figure 1, a compactor vehicle with which the present invention is designed to be employed suitably comprises a main frame 10 supported at its rearward end by a hollow, steel roller drum 12, and at its forward end by a second drum or tyre 14 steerable by a steering wheel 16. The drum 12 is internally provided with a pair of rotatable eccentric weights, as hereinafter more fully described. These impart a vibration to the drum, actually causing the same to rotate about an epicentre between the resultant centre of mass for the drum and the centre of the drum shaft. The distance from the centre of mass of the drum and the epicentre of rotation is called amplitude, and this amplitude is conveniently varied in accordance with the apparatus of the present invention. The ability to vary the vibratory effect is useful for the compaction of a variety of base materials, dirt and various road construction materials including asphalt.
Referring more particularly to Figures 2 and 3, an example of a vibratory system according to the present invention is illustrated in greater detail. Drum 12 is ultimately attached to frame 10 via four rubber isolators (two of which are illustrated in Figure 3), these isolators supporting the shaft system about which a drum 12 rotates. Isolator bracket or plate 20 extends between a pair of the isolators 18 at the left-hand side of the drum and carries a reversible vibrator drive motor 22 as well as a linear actuator in the form of double-acting hydraulic cylinder 24. The outer end of the shaft of motor 22 rotates in bearings 26 provided in bracket 20, said shaft having drive gear 28 mounted thereupon for engaging vibrator driven gear 30. Gear 30 is secured upon a first hollow eccentric weight shaft 34 which rotates within bearings 36 carried by bracket 20 as motor 22 imparts rotation to gears 28 and 30. Axially inward of the drum, shaft 34 is integral with a first end disk 40 of eccentric weight 38 further including remote end disk 40' and a hollow cylinder 42 positioned between the disks and secured to the disks by end bolts 44. The eccentric weight 38 is off-centre with respect to shaft 34 whereby rotation thereof as caused by rotation of shaft 34 imparts a first component of vibration to the drum. It will be observed that the cylinder 42 is mounted between disks 40 and 40' such that it extends radially farther from the axis of shaft 34 in a direction downward and to the right in Figure 2, or downward in Figure 3.
Between gear 30 and first end disk 40 of weight 38, the shaft 34 is enlarged in diameter providing a shoulder or enlargement 46 supporting drum hub 48 via bearings 50, the drum hub being attached to inner struts 52 of the drum by way of screws 55 and having an inwardly extending axial flange portion holding the outer race for bearings 50. The drum 12 itself thus rotates with respect to shaft 34 on bearings 50, with an oil seal ring 54 being positioned between the drum hub and bracket 20.
At the opposite end of eccentric weight 38, end disk 40' is integral with shaft extension 34' journaled in bearings 36'. The bearings are disposed within ring 58 welded to plate 56, wherein the plate 56 is supported from a pair of isolators 18. Shoulder or enlargement 46' of shaft extension 34' carries bearings 50' upon which drum hub 48' turns, and the drum hub 48' is attached to drum strut 52' employing screws 55'. An oil seal ring 54' separates the drum hub and the aforementioned ring 58. A cap 64 closes the right-hand end of shaft 34', acting as a retainer for bearings 36', and a cover plate 60 is secured to the plate 56 over the shaft end cap 64 with screws 62 so as to cover a central aperture in plate 56. The two sides of apparatus are thus quite similar providing bearing support for the shaft 34-34' which rotates eccentric weight 38, and further providing bearing support for drum 12 upon the shaft 34-34'.
Shaft 34 is hollow and is provided with an internal helically grooved surface 66 receiving a mating phasing shaft 68 which is coaxial with shaft 34 and helically ribbed for engaging the grooved surface 66. These engaged surfaces provide a driving connection between shaft 34 and shaft 68 and normally cause rotation of shaft 68 at the same speed as shaft 34. The surface 66 and the exterior surface 68 are helically configured over an appreciable length thereof to bring about a screwing effect of some force such that longitudinal movement of shaft 68 also causes rotational movement thereof with respect to shaft 34, or vice versa. The actuating arm 32 of cylinder 24 extends through central aperture 70 in gear 30, and is attached to high speed thrust bearing 72 secured to the end of phasing shaft 68 by means of retaining ring 74. This construction is depicted in exploded fashion in Figure 2 for clarity of illustration. Thus, the hydraulic cylinder 24 is adapted to move phasing shaft 68 in a longitudinal direction whereby the phasing shaft is also forced to rotate relative to shaft 34.
The grooved surface 66 and the ribbed exterior of phasing shaft 68 take the form of involute helical splines in the embodiment illustrated in Figures 2 and 3. These splines can be substantially square in a cross-section taken along the centre line of the shaft, but are preferably involute. Other forms of mating grooved and ribbed helical surfaces are described in connection with subsequent embodiments: It is to be understood that the term "spline" in a broad sense will comprehend various forms of irregular or indented male and female surfaces adapted for sliding relation. In the present embodiment, mating helical splines bring about rotation in conjunction with such sliding movement. The helical splines each describe a simple helix of constant ramp.
On the remote end of phasing shaft 68, farthest from actuating rod 32, the exterior of the phasing shaft is provided with straight exterior splines 76 having an exterior diameter less than that of the interior splines of surface 66. These straight splines matingly engage internal straight splines 78 in a hub 80 and end opening of a second eccentric weight 82, located within cylinder 42 of weight 38, to provide a driving connection to such weight. The hub 80 axially extends within an aperture in end disk 40 of the first eccentric weight and is rotatable with respect thereto on bearings 84 disposed within such aperture. The main body of the second eccentric weight 82 comprises a solid cylinder 86 joined to the hub 80 with screws 88 and 90 respectively extending through an outer portion of hub 80 and through an end member 92 secured within the splined recess of hub 80. A similar hub 80' is joined in substantially the same manner to solid cylinder 86 at the remote end thereof and turns in bearings 84 in end disk 40' of the first eccentric weight. The second eccentric weight 82 is also off-centre with respect to the coaxial shafts 34 and 68 whereby the rotation thereof brought about by the rotation of shaft 68 imparts a second component of vibration to the drum 12. The resultant eccentric moment and therefore the resultant vibration is dependent on the relative angular orientation between the concentric weights 38 and 82.
Operation of the hydraulic cylinder 24 for moving actuating rod 32 inwardly or outwardly brings about similar longitudinal movement of phasing shaft 68 with respect to shaft 34. As a consequence of the intermeshing splined or grooved and ribbed surfaces, axial movement of phasing shaft 68 also causes rotation thereof with respect to shaft 34 and consequent rotation of splined end 76 of shaft 68 with respect to disk 40. The splined surfaces 76 and 78 allow relative axial movement between shaft 68 and eccentric weight 82, whereby inward and outward movement of the shaft 68 is permitted with respect to eccentric weight 82. However, since such inward and outward movement causes rotation of the shaft 68 with respect to shaft 34, similar relative rotation is effected for eccentric weight 82 with respect to eccentric weight 38. When the two eccentric weights are relatively aligned, so their peripheral positions farthest from their axes are aligned as illustrated in Figures 2 and 3, the maximum amplitude vibration effect on drum 12 will be produced. However, when actuator shaft 32 is driven in a longitudinal direction by a sufficient distance to cause the peripheral positions farthest from the weight axes to be opposed, i.e. in opposite radial directions, the amplitude of vibration is at its minimum level. The operation of the hydraulic cylinder provides for this adjustment under the control of the vehicle operator. The change in the position of the eccentric weight 82 with respect to eccentric weight 38 is thus easily accomplished by the operator when the vehicle is in operation, the mating splined surfaces providing an adjustment feature of enhanced durability and reliability.
When the phasing shaft 68 is in a particular longitudinal position, it is held in such position by the hydraulic cylinder 24, thereby establishing a selected relative positioning of weights 38 and 82 for determining the amplitude of vibration as the weights are rotated by motor 22. The relative positioning of the weights and therefore the amplitude of vibration, is selected through the positioning of the hydraulic cylinder.
A preferred embodiment of the present invention is illustrated in Figure 4 which is substantially similar to the previous embodiment in a number of respects. An iso-' lator bracket 120 is mounted from isolators 118 (only one of which is illustrated in Figure 4) and supports vibrator drive motor 122 by way of shroud support 196. Motor shaft 198 is keyed to gear shaft 200, the remote end of which is rotatable in bearings 126 and upon which is mounted a drive gear 128 disposed in mating engagement with vibrator driven gear 130. Gear 130 is secured upon hollow eccentric weight shaft 134, e.g. by means of screws 202, said shaft rotating within bearings 136 carried by annular member 121 which is secured to bracket 120. A bearing retaining ring 204 is also positioned against the exterior side of member 121.
Axially inward of drum 112, the shaft 134 is integral with a first eccentric weight 138 which in this embodiment is sector shaped, i.e. extending radially outward from shaft 134 in an area on one side of the shaft. This sector shaped eccentric weight is hollow to provide a cavity 206 therewith for receiving and permitting relative, rotation of a second, solid, sector shaped eccentric weight 182. On the opposite side of the cavity 206, weight 138 joins a shaft extension or stub shaft 134' which rotates in bearings 136' positioned in a tubular section 210 of a bearing carrier 212, the latter being joined to a forward end or flange 214 secured between drum hub 148 and the drum by means of screws 155. The bearing carrier is bell shaped extending axially inwardly in surrounding relation to weights 138 and 182 and supplying the support for bearings 136'. An interior bearing retaining ring 218 is secured to the inner end of tubular section 210.
Drum hub 148 is provided with an inner axial flange member 149 within which bearings 150 are received adapting the drum hub and the drum, as well as bearing carrier 212, to rotate with respect to shaft 134. Flange member 149 is also secured to a disk shaped inner bearing carrier wall 216 which has a sealing relation with the interior walls of the bearing carrier. An oil seal ring 154 is disposed between the drum hub 148 and member 121 to provide for sealing of bearing oil.
A phasing shaft 168 is positioned coaxially within hollow shaft 134 and has an exterior helical ribbed surface 169 which mates with a grooved interior helical surface provided on insert 220 secured within gear 130. Considering the gear 130 as an extension of shaft 134, the grooved surface of insert 220 is considered to extend for an appreciable length along shaft 134, while the ribbed outer surface 169 of phasing shaft 168 extends for an appreciable distance along the length of the phasing shaft, namely through the insert 220 to a position in the specific embodiment about halfway between the insert and the eccentric weights 138 and 182. The nature of the ribbed and grooved mating surfaces on the phasing shaft 168 and insert 220 will be hereinafter more fully discussed. The remainder of the phasing shaft is necked down at 167 toward the eccentric weights and is provided with exterior straight splines 176 which mate with interior straight splines provided on weight 182 including axial flanged portions 222 thereof. The axial portions 222 are rotatably carried by bearings 224 disposed within the hollow interior of shaft 134. The resulting splined connection between the phasing shaft and the weight 182 permits slidable movement while constraining the weight 182 for simultaneous rotation with phasing shaft 168. A counterweight member 226 is affixed to the weight l82, opposite the main body thereof, with screws 228.
The actuator arm 132 of cylinder 124 extends through a shroud member 230 and terminates in a yoke 232 having a pivotal connection with a bearing arm 234 supporting thrust bearings l72 in which the end of phasing shaft 168 is journaled. Movement of the actuator arm 132 through operation of hydraulic cylinder 124 causes axially inward and outward movement of the phasing shaft 168 with respect to insert 220, and since the two are provided with mating helical ribs and grooves, the phasing shaft is constrained to rotate with respect to shaft 134. Rotation of the phasing shaft brings about simultaneous rotation of eccentric weight 182 with respect to eccentric weight 138. As a consequence, the relative positions of weights 138 and 182 are altered and the amplitude of the resulting vibration can be adjusted. For instance, when the weights 138 and 182 are substantially aligned as shown in Figure 4, the amplitude of vibration is maximum, but if hydraulic cylinder 124 is operated to withdraw the phasing shaft 168 to the left, eccentric weight 182 can be rotated to a position substantially diametrically opposite weight 138 in balancing relation thereto for minimizing or cancelling the vibration. When the phasing shaft is in any particular position and is held in such position by the hydraulic cylinder 124, the relative positioning of weights 138 and 182 is maintained constant at a location bringing about a selected value of vibration amplitude. Of course, both weights are rotated from motor 122 as a result of the driving connection between the phasing shaft 168 and the mating insert 220 wherein the speed of rotation will determine the frequency of vibration.
The maximum "throw" of phasing shaft 168 is adjustable by means of adjusting stud 236 engaging a nut 238 welded to shroud 230. Lock nut 240 secures the stud in a given position. Stud 236 extends through the shroud and carries a bracket 242 for engaging bearing arm 234 and limiting the travel of phasing shaft 168 toward the left in Figure 4. In a two-value amplitude system, i.e. where hydraulic cylinder 124 moves the phasing shaft between maximum amplitude and minimum amplitude positions, the extent of the difference in vibration amplitude between these two positions can be adjusted by means of stud 236.
The construction of Figure 4 is repeated on each side of the vehicle, and a central shaft 244 suitably extends from the left side of the drum to a similar structure on the right (not shown). Central shaft 244 is splined at its left-hand end in a manner to mate with the straight splines in axial portion 222 of weight 182. The remote end of central shaft 244 similarly mates with the straight splines on the opposite side of the drum for bringing about simultaneous rotation of the weights on the right-hand side of the vehicle.
The phasing shaft 168 in the embodiment of Figure 4 is polygonal in external transverse cross-section, with the transverse cross-section of insert 220 being internally polygonal to match. Polygonal is herein taken to mean having having the outline of a polygon, broadly encompassing a figure having three or more sides. Thus, the present definition encompasses the triangular cross-section of Figure 5 and the square cross-section of Figure 7 as well as the pentagon and hexagon cross-sections of Figures 9 and 11 respectively. As illustrated in Figures 6, 8, 10 and 12, the polygonal portions 169a-169d extend along an appreciable length of the phasing shaft, while the shaft is necked down at 167a-167d and provided with straight splines 176a-176d along an appreciable length thereof at the opposite end of the shaft for internally engaging the second eccentric weight in sliding relation. Phasing shafts constructed in this manner are found to have superior strength in bringing about relative rotational movement of the second or interior eccentric weight with respect to the first, and shafts constructed in this manner are also relatively simple in construction. Each of the polygonal surfaces of sections 169a through 169d proceed helically along the length thereof in a manner to describe a simple helix of constant ramp, and it is understood the internal configuration of insert 220 is similarly helical. The sides of the polygonal shapes 168a-l68d need not be flat, but can be slightly concave, or preferably slightly convex.
While helical splines are herein described as employed between the hollow first shaft and the phasing shaft, and straight splines are utilized between the phasing shaft and the second eccentric weight, it is understood the reverse can be true. That is, straight splines may be employed at the end of the phasing shaft that engages the interior of hollow shaft 34, with helical splines being employed between the phasing shaft and the interior of second eccentric weight 80, 82. As another alternative, both mating connections can be helical in configuration, bringing about a more rapid rotation of the weight 82 for a given axial movement of the phasing shaft. In any case, the phasing shaft configuration is desirably helically splined over a substantial length thereof, while the shaft 34 is also helically splined over a substantial length thereof to bring about a simplified and heavy duty mechanism for relatively rotating the second eccentric weight even while the vehicle is in rolling operation.

Claims

1. A vibrator system for a drum (12) employed in earth compaction or the like, said system comprising: first and second eccentric weights (38, 82) positioned within said drum (12) and adapted to be rotated within said drum to impart vibration thereto; a first shaft (34) extending coaxially with respect to said drum, a first of said eccentric weights (38) being mounted for rotation by said first shaft; means including a second shaft (68) positioned coaxially with said first shaft for providing a connection comprising helical ribs and mating grooves between said first shaft (34) and said second eccentric weight (82); means (22) for rotating one of said shafts; and means (24) for causing relative axial movement between said shafts such that said helical connection produces concurrent rotation of said second eccentric weight (82) with respect to said first eccentric weight (38) and a change in the amplitude of vibration produced with rotation of said eccentric weights.

2. The system according to claim 1 wherein said helical ribs and mating grooves comprise male and female splines.

3. The system according to claim 1 wherein said helical ribs and mating grooves comprise polygonal external and internal surfaces.

4. A vibratory compaction system comprising: a frame (10), a drum (12) and means rotatably mounting said drum to said frame, said drum having at least one end hub; a first shaft (34) coaxial with said drum and extending through said hub, and means (22) external to said drum for rotating said first shaft; first and second eccentric weights (38, 82) positioned within said drum and adapted to be rotated within said drum to impart vibration thereto, said first of said eccentric weights (38) being mounted for rotation with said first shaft (34); means including a second shaft (68) coaxial with said first shaft for providing a helically splined connection between said first shaft and said second eccentric weight (82); and means (24) for imparting axial movement to said second shaft such that said helically splined connection produces concurrent rotation of said second eccentric weight with respect to said first eccentric weight and a change in the amplitude of vibration produced by said eccentric weights.

5. A vibrator system for a drum (12) employed in earth compaction or the like, said system comprising: first and second eccentric weights (38, 82) positioned within said drum and adapted to be rotated within said drum to impart vibration thereto; a first shaft (34) extending coaxially with respect to said- drum, the first of said eccentric weights (38) being mounted for rotation by said first shaft; a second shaft (68) disposed coaxially with respect to said first shaft; means (22) for rotating one of said shafts; and a linear actuator (24) axially engaging one of said shafts for bringing about relative longitudinal movement of said second shaft with respect to the first shaft; said second shaft (68) having splined connections with said first shaft (34) and with said second eccentric weight (82) wherein at least one such splined connection is helical in form to provide relative rotation of said second eccentric weight (82) with respect to said first shaft (34) and said first eccentric weight (38) for controlling the amplitude of vibration caused by said eccentric weights in response to relative longitudinal movement of said second shaft.

6. A vibrator system for a drum (12) employed in earth compaction or the like, said system comprising: first and second eccentric weights (38, 82) positioned within said drum and adapted to be rotated within said drum to impart vibration thereto; a first shaft (34) extending coaxially through the hub of said drum (12) the first of said eccentric weights (38) being mounted for rotation by said first shaft, and means (22) external to said drum for rotating said first shaft; a second shaft (68) disposed coaxially with respect to said first shaft (34) and having a driving connection with said second eccentric weight (82) for rotating said second eccentric weight, said connection including axially slidable means allowing axial movement of said second shaft with respect to said second eccentric weight while constraining said second shaft and said second eccentric weight for simultaneous rotation; said first shaft (34) having a driving connection with said second shaft (68) for bringing about concurrent rotation of said shafts and said first and second eccentric weights, said last mentioned driving connection comprising a first internal helically grooved surface on one of said shafts extending along an appreciable length thereof and a mating externally helically ribbed surface on the other shaft extending along an appreciable length thereof in engaging relation with said first grooved surface; and a linear actuator (24) external to said drum and axially engaging the said second shaft (68) for bringing about longitudinal movement of the said second shaft with respect to said first shaft (34) and second eccentric weight (82), said helically ribbed and grooved shaft surfaces causing rotation of the said second shaft (68) with respect to said first shaft (34) bringing about rotation of said second eccentric weight (82) for altering the angular position of said second eccentric weight with respect to said first eccentric weight and thereby altering the amplitude of the vibration caused by simultaneous rotation of said eccentric weights.

7. The system according to claim 6 wherein said eccentric weights (38, 82) are concentrically positioned.

8. A vibratory compaction system comprising: a frame (10), a drum (12) and means rotatably mounting said drum to said frame, said drum having at least one end hub; a pair of shafts (34, 68) coaxial with said drum at least a first (34) of which extends through said hub, and means (22) external to said drum for rotating said first of said shafts; first and second eccentric weights (38, 82) positioned within said drum and adapted to be rotated within said drum to impart vibration thereto, the first of said eccentric weights being mounted for rotation with the first of said shafts and the second (82) of said eccentric weights being mounted for rotation with the second (68) of said shafts; said first (34) of said shafts having a driving connection with the second of said shafts for bringing about concurrent rotation of said shafts and said first and second eccentric weights (38, 82), said driving connection comprising helical ribs and mating grooves on said shafts along an appreciable length thereof whereby relative translational movement between said shafts also brings about relative rotational movement therebetween; and actuator means (24) mounted with respect to said frame for axially engaging the second of said shafts for selectively causing longitudinal movement thereof and consequent rotation with respect to the first of said shafts together with relative rotation of said first and second eccentric weights for thereby altering the amplitude, of vibration caused by rotation of said first and second eccentric weights for thereby altering the amplitude of vibration caused by rotation of said eccentric weights.

9. The system according to claim 8 including bearing means (50, 50') for mounting said drum (12) for rotation on the exterior of said pair of coaxial shafts (34, 68).

10. The system according to claim 8 further including a splined connection (76, 78) between the second of said shafts (68) and the eccentric weight (82) rotated thereby to permit longitudinal movement of said second of said shafts while constraining said second of said shafts to turn the corresponding eccentric weight.