US2284671A

US2284671A - Shaking device

Info

Publication number: US2284671A
Application number: US288549A
Authority: US
Inventors: Gotthold H Meinzer
Original assignee: Individual
Current assignee: Individual
Priority date: 1939-08-05
Filing date: 1939-08-05
Publication date: 1942-06-02
Anticipated expiration: 1959-06-02

Description

June 2, 1942. G. H MEINZER 2,284,671

SHAKING DEVICE 3 Sheets-Sheet 1 Filed Aug. 5, 1959 June 2, 1942. cs. H. MEINZER SHAKING DEVICE Filed Aug. 5, 1959 3 Sheets-Sheet 2 June 2, l942 MEINZER 2,284,671

SHAKING DEVICE Filed Aug. 5, 1939 3 Sheets-Sheet 3 Goff/mid 17 JWQL/ZZQF Patented June 2, 1942 UNITED STATES PATENT OFFICE snsxmo navron Gotthold n. Melnzer, Glendale, cam. Application August 5, 1939, Serial No. 288,549 19 Claims. (01. 209-325) The present invention relates to shakers and more particularly to sieve shakers used in separating granular materials. This application is a continuation in part of my earlier application, filed April 15, 1938, for improvements in Shaking machines, reference to which is hereby made.

In the present nvention I provide an improved sieve shaker comprising a gyratory vibrating system suspended on elastic supports wherein a resulting motion having both horizontal and vertical components is developed by a centrifugal driving force applied at right angles to the vertical axis passing through the center of mass of the system.

The elastic supports permit the shaking table to have a motion determined by relative distribution of masses and points of application of centrifugal driving forces which are internally generated to produce a table motion wherein every.

point on the table describes a circular or elliptical path and every point simultaneously vibrates in a vertical direction through an amplitude proportional to the radial distance of the point on the table from the center of mass.

I provide a sieve shaker wherein a novel combination of high speed gyratory and oscillatory motion is imparted to every point on the screening surface with the oscillatory component of the motion increasing from zero at the center of the screen to a maximum at the outer edge of the screen to produce a positive and uniform distribution of the material on the screen.

Moreover, a novel method is provided, by the present invention, of periodically bumping or obtaining impacts to keep the screen from blind Another object of the inventionis to provide a dynamically isolated system, i. e.,za system in which the internal dynamic forces are balanced and not reactively transmitted to the support, which imparts to a shaker table a combined motion having a horizontal gyratory component and a superimposed vertical oscillatory component.

Another object of my invention is to provide a sieve shaker wherein low power consumption may be had due to balancing of inertial forces and an absence of reaction to the supports.

These being among the objects of the present invention, other and further objects will become apparent from the drawings, the description relating thereto and the appended claims.

Referring now to the drawings:

x in Fig. 2 showing one of the supports of the table; d

Fig. 6-is a side view of one end of the table showing a vertical and horizontal projection of the motion of the table when the shaker is in operation;

Fig. '7 is a diagrammatical view similar to Fig.

1 showing six screens mounted upon the table with certain dimensions of an actual working model included; v

Fig. 8 is a static force diagram showing the distribution of masses and points of application of centrifugal forces as included for a better understanding of the invention; and

Fig. 9 is a diagrammatical representation of another form of screen load attachment.

Referring now to the drawings in further detail, the invention will be best understood by referring first, to the general organization of the elements present in the assembly; second, to the structural details and characteristics of the elements; and finally to the theory and operation of the invention.

With reference to the general organization of the elements making up the preferred form of the system, as illustrated in Fig. 1, the sieve supporting table In is mounted at all four corners upon the base II by elastic supports l2, located one at each corner. The power unit I3 is secured to the bottom side of the table l0 and a sieve container I4 is held in place on top of the table by clamps IS.

The table l0 comprises a die-cast plate l6 made substantially square and provided with depending flanges I1, around the edge thereof. The central axis 20 of the system passes perpendicular thereto through the center of the table. Around the axis 20 a circular upstanding flange 2| is provided to hold the sieve container 22 or nested sieves loosely in place as centered upon the table. In the construction of the table and the support for the sieves, I provide a means for keeping the screen surfaces from blinding and the table. For this purpose transversely apertured lugs 23 are bolted to the table with the openings 2d of both lugs disposed coaxial with each other upon a horizontal line M (Fig. 2) intersecting the axis 20, at right angles. Gontainer or sieve hold-down rods 21, having crooked lower ends, engage in the openings 24 to permit the top of the container to move back and forth about the line 19.

At the bottom of the container I provide a fulcrum which permits the sieves to rock back and forth in a way allowing the bottom of the container to strike the table surface twice each cycle of the power unit l3. This fulcrum comprises a metal strip 25 aligned with the line 19 and extending beyond the guides 2| where they are cut away at 22 to receive the ends of the strip 25 to keep the strip from changing its relative position on the table top. The strip 25 is preferably located under a rubber pad 26 which cushions the container and the impacts. The vertical oscillatory component of the table motion supplies the force for the impacts.

The hold-down rods 21 receive winged nuts 3| at their upper ends which permit adjustment of clamping tension under cross bar 32 that has bolted to it the lid 33 of the container. The bolt 34 employed in securing the elastic lid 33 to the cross bar 32 provides a pivot pin for the top of the container upon the axis 20 of the system to permit axial rotation of the screens. The screens 38 are mounted as trays one upon the other inside the container 22, whenever a container is used.

The base II comprises a box-like cabinet 35 which has a protective fence on the top thereof secured by machine screws 31. The top of the fence 36 terminates well short of the bottom of the table I6. Vacuum cup feet 40 are provided upon the bottom of the cabinet 35.

As best shown in Fig. 3 a main switch 4| and a rheostat 42 are connected in series with the power lead-in 43 to regulate the power input to motor iii.

The power unit l3 comprises a motor 44 whose shaft 45 is alined with the axis 20. The shaft 45 extends beyond both ends of the motor. Upon the upper end 46 of the shaft a large eccentric flywheel 41 is mounted while upon the lower end 48 a smaller eccentric flywheel 50 is mounted. The

eccentric flywheels

41 and 50 may be adjusted independent of the motor position and their phase relation is also adjusted by set screws .55 (Fig. 3).

v The power unit is rigidly secured to the bottom of the table l6 by means of straps 5| bolted thereto as at 52 with depending ends 53 of the straps 5| welded to a clamp collar 54 that encircles and supports the motor. 56 is provided and secured to the motor housing in order to lower the center of gravity of the loaded system for a purpose which will be discussed in further detail in describing the operation of the power unit.

The elastic supports I2 comprise preferably light coil springs secured at their bottom ends by lugs 51 on the top of the cabinet 35 and at their top ends by spring lugs 58 located upon the bottom of the table I6. Although an aperiodic support such as sponge rubber, may be used instead of coil springs, I prefer to use coil springs for obvious reasons.

This method of suspension allows freedom of movement both horizontally and vertically, mak- A counterweight at the four corners of the table.

2,284,671 by causing a periodic/hnpact between the screens ing possible a simultaneous gyratory and oscillatory motion of the shaking table.

The elastic properties of the supports are so designed that the constraining forces on the shaking system are negligible within the limits of motion of the system. The moduli of the supports is preferably substantially equal, with a naturalperiod of vibration vertically and horizontally which does not coincide with the period of the shaker oscillation. If the two periods were in resonance, energy would be transmitted to the support on which the shaker rests and motion would be damped. Hence the motion of the shaking system is not conditioned by the elastic constants of the supports.

It is true that the elastic supports exert some dampening effect on the table motion, but it will be appreciated that like the swing of a pendulum, this damping afiects only the amplitude and not the period of the vibration, or the path of the motion. The design and elastic moduli of my shaker are such that no reactive dynamic forces are transmitted from. the shaking system through the supports to the stationary base II.

As mentioned above. the supports are arranged Any other arrangement will be satisfactory as long as the distribution of points of support is symmetrical about the center, whether arranged in the form of a cross or a square it does not matter.

Thus, the shaking system, comprising the screen load [4, the surface of the table II, the

motor 44, the counterweight 56 and the driving weights or eccentric masses 4'! and 50, forms a dynamically isolated system wherein the centrifugal driving forces are generated within the system and are absorbed, or balanced within it without being transmitted through the supports to the base.

Referring now to the static force diagram shown in Fig. 8 by way of preliminary explanation, T, in Fig. 8, represents a table such as the table ID of the shaker supported on the springs l2 to permit the table a limited free movement horizontally and vertically. The point M1 (l3) represents the mass of the motor and the counterweight, and m represents the mass of the sieve load. M1 and M2, are shown rigidly and centrally attached to the table by a hypothetical rod R representing the line 20 in relation to the rigid mounting of the power unit Hi. In this diagram the resulting center of mass of the system is hypothetical y at point C.

If a centrifugal force is applied in a horizontal plane, at the center of mass C as by the eccentric flywheel 41, a horizontal gyratory force component will be generated in which the system will rotate on its elastic supports to define a surface of revolution whose axis is the normal resting position of the rod R and any point on the table will describe a circle whose radius is proportional to the centrifugal force F1. In short the radial force-vector rotates about the center of mass.

By applying a secondary centrifugal force F2, representing a construction such as flywheel 50, to the rod R at a point above or preferably below the center of mass C, a torque or couple about the point C is developed which imposes a vertical oscillatory component upon the horizontal motion and results in an angular displacement of the table surface about the center of mass of the shaking system.

Since the angular displacement is produced by a centrifugal force F: whose radial vector rotates about the central vertical axis of the sys- .trifugal force of 15.7 lbs.

' tem represented by R, the rod It would. move in an inclined orbit. In the diagrammatical representation, made in Fig. 8, the orbit of the rod R would describe the figure of a double opposed cone, one above and one below the center of mass C with the adjacent apiceslying in the center of mass.

The precession of the vertical axis results in a progressive vertical oscillatory vibration of the table surface about the center of mass of the system. The vertical amplitude of this vibration is at a minimum at the center and at a maximum at the edge of the table. It follows that any point on the table surface will move horizontally in gyratory path and simultaneously vibrate vertically through an amplitude depending on its distance from the center of mass of the system. The resultant orbit of the vertical axis or rod R will describe a figure made up of two opposed truncated cones, the two small sections being adjacent and passing through the center of mass. The precessional velocity and the gyratory velocity of the rod R are identical and in phase since the eccentric weights producing the two force components are rotated by the same shaft.

From this it will be observed that the resultant motion of the table is such that every point upon its surface describes a circular or elliptical path and simultaneously vibrates in a vertical direction through an amplitude proportional to its radial distance from'the center of gyration and oscillation depending upon the degree of displacement of the secondary centrifugal force F2.

To explain the purpose and effectiveness of this combined gyratory-oscillatory motion it may be helpful to consider the action of each compcnent separately. A simple horizontal gyratory screening motion is known in the art to be effective, but is open to the serious objection, that when used by itself, it results in a segregation or K piling up, of the material being screened, at the outer edge of the screen surface due to the centrifugal action of the gyratory motion; thus greatly reducing the active screen area. The oscillatory or progressive vibratory motion by itself, produces the opposite effectthe material on the screen surface tending to collect at the center, since the vertical amplitude is a minimum in that area. Hence by combining a horizontal gyratory motion with a vertical oscillation about the center in the proper proportion and phase, the gyratory forces tending to move the material to the periphery of the screen and the oscillatory forces tending to move it toward the center are balanced, and a uniform distribution 1 of the material over the whole screening area is attained. In actual operation of the shaker, a continuous circulation of the material results. The bottom layer, immediately in contact with the screen surface, flows out from the center toward the periphery of the screen, and returns to the center along the surface layers of the material.

In Fig. 8 the center of mass is shown as located at C while in Fig. 7 a dimensioned sketch is shown wherein the resultant center of mass of the whole system is located at C1.

Referring to Fig. 7, a shaker is shown wherein six sieves are mounted upon the table. In Fig. '1, the point C1 is approximately Ti; of an inch above the table. The force F1 generated by revolution of the upper eccentric mass is the primary centrifugal force. The motor runs at approximately 1000 R. P. M. and develops a cen- The main function of force F1 is to produce the horizontal gyratory motion, as already mentioned, since its largest component is horizontal. However, with the loading specified in the six sieve embodiment illustrated in Fig. 7, the force F1 also contributes to the vertical component since it acts at a point spaced approximately one inch from the center of mass C1, thus also producing an angular displacement with a resulting vertical component.

'In Fig. 7, force F: generating a centrifugal force of 3 lbs. at 1000 R. P. M. is spaced approximately 7 inches from the center C1. The main function of force F2 is to produce the angular displacement or vertical component in the motion of the table, It will be seen from this that the driving force for the shaking motion is supplied by two rotating eccentric flywheels or weights, which generate horizontal centrifugal forces. These forces act radially through and around the central vertical axis of the system and at right angles to it. The primary centrifugal force F1 is applied substantially at, or near the center of mass, thus producing the horizontal gyratory component of the shaking motion. Thesecondary centrifugal force F2 is applied at a vertical distance from the center of mass, thus producing a torque, which results in the angular displacement or oscillatory component of the motion, about the center of mass. Force F2 is made variable to effect an adjustment of the vertical component for the purpose of balancing the in-flowingand outflowing tendency of the material, as explained above.

By varying the phase angle of F2 with respect to F1 the magnitude of the vertical component can be made greater or less depending on whether F1 or F2 are working in the same direction or in opposite directions. These measurements of weight are related to a screen load of approximately 7 lbs. with a power input of 66 watts, R. P. M. 1000.

Further consideration of Fig. 7 enables a prediction as to the effect of a change in loading, weight distribution and phase angle of F2 with respect to F1. It will be seen, for example, that if F2 acts at an angle of 180 from F: the torque due to F1 will be entirely neutralized since F1 exerts a torque 15.7 inch lbs. (l5.7 1) in one direction, while F2 exerts a torque of approximately 28 inch lbs. (3.9 7-r"g) in the other direction. A resultant torque results in the direction of F2 of approximately 12.3 inch lbs. This diminishes the vertical component, the table tilt will be in the opposite direction and the direction of rotation of the screens will also be reversed.

If, by change of loading, the center of mass is lowered to the point of application of the force F1, 1as already described in conjunction with Fig. 8, the force F1 will produce no angular displacement and the vertical component will then be entirely dependent on With the screen load removed from the table, the center of mass is further lowered to a point say at C3 or between F1 and F2. For efficient screening the center of mass of the loaded system should be below the surface of the bottom screen. In order to accomplish this and assure that the center of mass will be so located under all expected loads, the counterweight 56, Fig. '7, is added to bring the center of mass below the bottom screen at maximum screen loading. The counterweight in this instance is approximately 5 lbs,

' The reason for using a secondary centrifugal force F2 to obtain the vertical component instead of simply displacing the primary force F1 from the center of mass is the practical dlfllculty of adjustment. The horizontal force must be large as compared with the vertical component, hence a small vertical displacement of F1 brings about a large change in the vertical component.

It was found by experimentation that by introducing asecondary centrifugal force below the motor in the form of a small eccentric mass the vertical component could be precisely adjusted by changing the phase-angle of the secondary force with respect to the primary. This is ac-- p The high screening efficiency of the apparatus described depends upon the high rate, small amplitude, gyrating-vibratory motion imparted to every point on the screen surface. The vibratory or vertical component of the motion increasing from zero at the center to the maximum at the outer edge of the screen combined with horizontal gyratory component produces; (1) positive distribution and circulation of material over the screening surfaces, (2) axial rotation of the screen relative to the table, and (3) the motive power for the bump and impact between the sieve and the table to prevent blinding of the sieve.

The great number of impacts developed in the rocking of the screen sieve 22 keep the screen from blinding. The pendulum principle with elastic supports used in the design balances and minimizes the inertial forces with the result that the power input is low. Moreover, my present invention provides a novel method of obtaining the impact, and the use of counterweight to adjust the position of the center of mass with an auxiliary centrifugal force to control nutatory motion provides a sieve shaker construction which is simple in construction, and to manufacture and thoroughly efiective in operation.

Fig, 9 shows a modified method of holding the screen load ll to the table and increasing the number of impacts per revolution. Instead of resting directly on the shaking table IS, the screens are held in a shallow pan 60 which is connected to the table through a central loosely fitting pivot pin tl which allows the nutating table to strike this screen at four or more places at each revolution. The adjustable arm 62 holds In some respects I am not able to account fully for the improved results which are obtained by the method and apparatus of this invention, and it should be understood that any attempt to analyze the theory which is believed to be responsible for these results is to be construed not as defining a mode of operation but merely as a possible explanation of certain physical, phenomena which have been observed.

Throughout the specification and drawings, various constants have been set forth for purposes of illustration. These constants may be varied, especially if compensatory changes are made in other parts of the several devices illustrated and I therefore am not to be limited to the precise details set forth. Consequently, all equivalentconstructions and arrangements that may be usable to accomplish the same results in substantially the same way are deemed to be included within the scope of the accompanying claims, it being apparent to those skilled in the substantially horizontal plane and a simultaneous progressive tilting motion about the center of mass of said system: an eccentric weight rigidly but rotatably supported from said table and I0- tatable in a substantially horizontal plane, said weight being spaced vertically from the center of mass of said system and being unbalanced by any other weight whereby it produces when rotated a progressive tilting motion of the system; resilient suspension means for said system, said means permitting substantially undamped movement thereof in response to forces generated by the rotation of said weight, and means for rotating said weight. I

2. In a device for imparting to a suspended system including a table a gyratory motion in a substantially horizontal plane and a simultaneous progressive tilting motion about the center of mass of said system: a prime mover rigidly supported from said table and an eccentric weight vertically spaced from the center of mass of said system and rotated in a substantially horizontal plane by said prime mover and being unbalanced by any other weight whereby it produces when rotated a progressive tilting motion of the system; and desilient suspension means for said system, said means permitting substantially undamped movement thereof in response to forces generated by the rotation of said weight.

3. A substantially dynamically isolated suspended shaking system including a substantially horizontal table and its intended load, a shaker means for producing a gyratory motion in substantially a horizontal plane and a simultaneous tilting motion about the center of mass of said system, said shaker means including a prime mover rigidly supported from said table and an eccentric weight rotated by said prime mover in a substantially horizontal plane, said weight being located below the center of mass of said system and being unbalanced by any other weight as to the tilting torque 'it produces about the center of mass; and suspension means resilient in all directions supporting said system in a plane located between the center of mass of said load and the center of mass of the unloaded system.

4. A substantially dynamically isolated suspended shaking system including a substantially horizontal table and its intended load, shaker means for producing a gyratory motion in substantially a horizontal plane and a simultaneous tilting motion about the center of mass of said system, said shaker means including a prime mover rigidly supported from said table and an eccentric weight rotated by said prime mover in a substantially horizontal plane, said weight being located below the center of mass of said system and being unbalanced by any other weight as to the tilting torque it produces about the center of mass; and suspension means resilient in all directions supporting said system in a plane struck approximately through the center of mass of the loaded system.

5. The method of producing in a suspended system a composite movement having a gyratory component in a substantially horizontal-plane and a progressive tilting component about the center of mass of said system, which comprises resiliently supporting said system substantially in dynamic isolation and applying centrifugal force to the system having its resultant plane of application vertically spaced from the center of mass whereby the compound gyratory and progressive tilting motion is obtained.'

6. The method of producing in a suspended system a composite movement having a gyratory component in a substantially horizontal plane and a progressive tilting component about the center of mass of said system, which comprises: resiliently supporting said system substantially in dynamic isolation; applying a centrifugal force to said system in a substantially horizontal plane adjacent said center of mass,

and applying a second centrifugal force to said system in a substantially horizontal plane relatively remote from said center of mass.

7. The method of producing in a suspended system a composite movement having a gyratory component in a substantially horizontal plane and a progressive tilting component about the center of mass of said system, which comprises: resiliently supporting said system substantially in dynamic isolation; applying a centrifugal force to said system in a substantially horizontal plane spaced from said center of mass; applying a second centrifugal force to said system in a substantially horizontal plane still further spaced from said center of mass, and controlling the relative amplitudes and the phase relation of said components by varying the angular relation of said centrifugal forces.

8. A device substantially as set forth in claim 1, including a screening member mounted on said table.

9. Screening apparatus comprising a suspended system including a substantially horizontal and flat sieve, and means for imparting to the system a gyratory motion in a substantially horizontal plane and a substantially simultaneous progressive tilting motion approximately about the center of gyration, of said system comprising a motor rigidly supported as a part of the system and having a substantially vertical axis and driving a substantially vertical shaft, and eccentric weight means carried by the shaft with its center of mass spaced vertically from the center of mass of said system, and resilient means suspending said system substantially as a. free body.

10. Screening apparatus comprising a suspended system including a substantially horizontal and flat sieve, and means for imparting to the system a gyratory motion in a substantially horizontal plane and a substantially simultaneous progressive tilting motion approximately about the center of gyration of said system comprising a motor rigidly supported as a part of the system and having a substantially vertical axis and driving a substantially vertical shaft, and eccentric weight means carried by the shaft with its center of mass spaced vertically from the center of mass of said system, and suspension means for said system resilient in all directions necessary to permit said simultaneous movements in response to forces generated by the rotation of said weight means; the disposition and value of the masses of the weight and the suspended system and a speed of rotation of the weight being such as to approximately balance the average centrifugal force of the gyratory motion on the contents of the sieve with the average centripetal force of the progressive tilting on said contents to maintain the contents of the sieve distributed approximately uniformly.

11. Screening apparatus comprising a suspendcd system including a substantially horizontal and flat sieve, and means for imparting to the system a gyratory motion in a substantially horizontal plane and a substantially simultaneous progressive tilting motion approximately about the center of gyration of said system comprising a motor rigidly supported as a part of the system and having a substantially vertical axis and driving a substantially vertical shaft, and eccentric weight means carried by the shaft having one eccentric weight portion near the center of mass of said system and having a lighter eccentric weight portion substantially spaced therefrom and having its center of mass angularly spaced approximately 90 ahead of the center of mass of the first-mentioned eccentric weight portion, and suspension resilient means for suspending said system as a free body with respect to the forces generated by the rotation of said weight means.

12. Screening apparatus comprising a suspended system including a sieve and means for imparting to the sieve motion having a gyratory component substantially in a horizontal plane and a substantially simultaneous progressive tilting component approximately about a point in the axis of gyration of the system comprising a motor rigidly supported as a part of the system and eccentric weight means carried by the shaft with its center of mass spaced vertically from the center of mass of'said system and resilient suspending means for said system comprising a plurality of spaced coiled springs.

13. In a device for imparting to a suspended system including a table a gyratory motion in a substantially horizontal plane and a simultaneous progressive tilting motion about the center of mass of said system; an eccentrically weighted rotor rigidly but rotatably supported from said table and rotatable in a substantially horizontal plane, the mass of the rotor being so distributed as to tend to produce a motion having a gyratory component in a substantially horizontal plane and a progressive tilting component about the center of mass of said system; resilient suspension means for said system, said means being so constructed and arranged as to permit substantially undamped movement thereof in response to forces generated by the rotation of said weight, and means for rotating said weight.

14. In a. device for imparting to a suspended system including a table a gyratory motion in a substantially horizontal plane and a simultaneous progressive tilting motion about the center of mass of said system, a substantially vertical rotatable shaft rigidly supported from said table and means for rotating said shaft, two eccentric weights rotated by said shaft in spaced substantially horizontal planes, one of said planes being more distant than the other from the center of mass of said system, and effective to produce a tilting motion progressing about a vertical axis through the center of mass of said system, and resilient suspension means for said system, said resilient means permitting substantially undamped movement thereof in all directions in response to forces generated by the rotation of said weights. p

15. In a device for imparting to a suspended system including a table a gyratory motion in a substantially horizontal plane and a simultaneous progressive tilting motion about the center of mass of said system, a substantially vertical rotatable shaft rigidly supported from said table and means for rotating said shaft, two eccentric.

weights angularly spaced from one another and rotated by said shaft in substantially horizontal planes, one of said planes'belng more distant 16. In a shaking apparatus, a substantially dynamically isolated suspended system including a table and its intended load, a prime mover rigidly supported from said table, two eccentric weights angularly spaced from one another and rotated by said prime mover in substantially horizontal planes, one of said planes being more distant than the other from the center of mass of said system and eflective to produce a tiltin motion progressing about a vertical axis through the center, of mass of said system, means for adjusting the relative angular positions of said weights around the axis of rotation thereof, and resilient suspension means supporting said system substantially in dynamic isolation.

1'7. In a device for imparting to a suspended system including a table a gyratory motion in a substantially horizontal plane and a simultaneous progressive tilting motion about the center of mass of said system, an eccentric weight rigidly but rotatabiy supportedfrom said table and rotatable in a substantially horizontal plane, said weight being spaced vertically from the center of mass of said system and effective to produce a progressive tilting motion of the system, resilient suspension means for said system, said means permitting substantially undamped movement thereof. in response to forces generated by the rotation of said weight, means for rotating said weight, a projection from said table directed upwardly, a screen resting on said projection to teeter thereon and bump against said table as said table tilts, and means for retaining said screen in position on said projection.

18. In a shaker, a load-carrying body, a rotatable shaft extending generally vertically 'in one direction only from the position of the load carried by said body and rigidly supported from said body, means for rotating said shaft, two eccentric weights rotated by said shaft in spaced substantially horizontal planes, one of said planes being more distant than the other from the center of mass of said system and effective to produce a tilting motion progressing about a vertical axis' through the center of mass of said system, and resilient suspension means for said system,- said resilient means permitting substantially undamped movement thereof in all directlons in response to forces generated by the rotation of said weights.

19. In a shaker, a load-carrying body, a rotatable shaft extending generally vertically in one direction only from the position of the load carried by said body and rigidly supported from said body, means for rotating said shaft, two eccentric weights angularly spaced from one another and rotated by said shaft in substantially horizontal planes, one of said planes being more distant than the other from the center of mass I of said system and effective to produce a tilting means for said system, said resilient means permitting substantially undamped movement thereof in all directions in response to forces generated by the rotation of said weights.

GOTTHOLD H. MEINZER.