Show deschi description pu 0 1 n of the mi mitchell vi vibrating arati ng screen scree n contributed revolutionary is the term us used ed to de uniformity over the entire surface of the scribe the mitchell electric vibrating vibration screen ca cloth oth screen invented by B A mitchell chief second there is a loss of eff effectiveness at MI A rear view of the mitchell electric vibrating screen showing the vibrator mechanical engineer of the utah copper company and recently announced as ready for distribution the term is perhaps not misapplied for the mitchell is designed on principles radically different from other screening devices mr air mitchell was assisted in perfecting the electrical vibrating unit of his machine its distinguishing feature by engineers of the general electric and westinghouse it has been in operation at the utah copper p er companas Comp anys plant at magna utah for more than two years and in its present form is the result of the practical experience gained during this period it is designed to classify any crystalline or granular material and is now being installed in several different plants for a variety of work the departure from ordinary screen design to be noted in the mitchell is in the method of vibrating the screen cloth ordinarily this is done by applying the moving force from above the screen with various rapping or pounding devices results are obtained not by the initial application of force but by the rebound of the acre screen en cloth under tension W weaknesses ea k n e s se s C corrected 0 r re ct e d admittedly there are weaknesses in this method which the mitchell has sought to cair correct ct among them are the following first the force is not applied with from the fact that the rebound and not the initial application of force affects the material a serious loss of efficiency A great deal too much material that should be eliminated is allowed to pass over the screen fourth the pounding on the screen is responsible for a rapid deterioration of the device and particularly of the screen cloth to do away with the rebound action of the screen against the material thus increasing the efficiency of each vibration and at the same time to produce much more rapid vibrations than the rebound action permits is the purpose of the mitchell screen by a strong persistent upward and rotary movement induced by electric power applied from beneath the mitchell forces the meshes directly up and into the material the screen cloth is agitated at the rate of vibrations a minute and strikes the material with an impact of from to 1000 pounds motor actuated the motor actuated vibration is applied continuously to the screening area by i means eans n of rigid arms or plates which are fastened to the ends of the vibrator casing below the vibrator the action of which is explained in detail with the accompany ing cross section view keeps these arms and consequently every wire and every mesh in the screen cloth describing a 41 Y 7 w X 4 actual installation of four mitchell Ml electric vibrating screens at the utah copper corp compa magna plant delivering a product of over tons per 24 hours th third ird lack of uniform rm application of force and the inability to produce vibrations more than mo moderately dera tely frequent cause cause minute circular path in the words if y top of the screen in other the theloa machine wibb s tand stand facing the end to your left the motion of the screen cloth is clockwise clock wise this circular movement tends to toss back the particles of material along their path the scope of the movement is small but depends for its efficiency upon the frequency of the vibrations and the force of the impact the action of the mechanism is noiseless and without racking or straining in fact when the screen is in operation with no material passing over it there is no sign of motion except the sound of the motor and a faint blur of the screen capacities in operation with one large installation of the mitchell screen the following figures on capacities have been tabulated in coarse crushing work each mitchell E E L K I 1 A C I 1 1 UN LIM J C A F L E A G vibrating mechanism of mitchell electric vibrating screen with description of operation casing the butr outer casing of the mechanism A is of tubular form the larger cross section at center encloses the driving motor B the smaller cross sections at either end enclose the ring oiling bearings C and the ball cages D the casing itself is cast in two sections which are rigidly bolted together by means of bolts E clamping securely the motor stator between the inside faces of the casing the mechanism is held firmly to the main screen frame by means of a steel strap F which encircles the middle circumference cum ference of the casting the mechanism rests at its contact with the screen frame in a ball and socket joint G the convex portion attached to the vibrator and the concave portion attached to the screen screen frame by means of this strap and joint the whole mechanism is free to move in any direction about this joint G as a center the casing forms a dust and waterproof housing for the ahe motor bearings andrall an and dball ball cages wear on the mechanism mecham sm from out side influence is thus eliminated 5 screen is delivering a screened product of more inore than 1200 tons per twenty four hours of minus vz material in fine crushing dry work each mitchell is delivering a screened product of more than tons per twenty four hours of minus 10 mesh material in fine crushing wet work each mitchell is delivering a screened product of more than tons per twenty four hours of minus 10 mesh material the mitchell is equally efficient in coarse as well as fine screening and classifies both wet and dry material it is readily adaptable to any standard type of feeding device and requires but small head room making the installation of a series ot olf screens practicable in both wet and dry work it classifies from one inch to 40 49 mesh mechanism A long shaft H runs through the center of the machine and is supported by the ring oiling bearing M C at the center of the shaft is keyed the rotor B and at each end of the shaft H is keyed the hard fibre fabre cylindrical ball cage D having radial bored round holes 1 slightly larger than the diameter of the balls J arranged as shown in section at left of drawing encircling the fibre fabre ball cage and pressed tightly into the tubular casing A are the hard steel races K in operation one or two adjacent rows of holes are filled with balls in the ball cage at each end of the shaft H any particular row of balls at one ned of the shaft is counterbalanced by a similar row of balls at the other end of the shaft but the two rows are degrees apart with reference to the circumference of the shaft when aben the ball cage is rotated by the motor centrifugal force causes the balls to travel to the inside periphery of their respective spec tive races with such force that gravity is reduced and the mechanism being free to rotate at its center at the ball and socket joint an eccentric motion is pro elimination material the action is automatic and uniform and therefore requires no supervision the cloth can readily be changed by loosening the four bolts lifting from the frame the holder with its worn out screen cloth slipping in another holder with a new screen cloth and tightening the four bolts again adjustments for change in operating conditions are made by raising or lowering the inclination racks thus raising or lowering the screen with reference to the pivot hangers the motor of the vibrator is securely encased against dirt and damage and is protected by automatic overload and low voltage release A 1 H P motor is standard for each screen but the consumption averages only about 38 H P deuced at either end of the mechanism this motion may be roughly illustrated by taking hold of a pencil at its center by the thumb and first finger of one hand and with the other hand moving one end of the pencil in a slight circular path the vibrating motion of the mechanism is positive and powerful and is at its maximum at the ends diminishing to practically zero at the center of vibration i e the ball and socket joint the operation is very quiet by varying the number of rows or b balls alls or number of balls in a row in the ball cage at each end of the shaft the strength of vibration can be accurately regulated the vibrating motion is that of a compound or on double conical pendulum acting on a horizontal axis by it the upward drive of nipsy hipsh against screening material is made possible supplemented by the backward tossing that does so much toward thoroughly segregating the material I 1 As shown in the perspective view the screen cloth is held taut between the two rigid arms of the main frame by means of a slotted pipe into inlow which aich the screen cloth bent around wrought brough taron iron bars is slipped this pipe is held firmly to the screen frame by means of bolted clamps which fit into slots in the frame the cloth can thus be adjusted to any degree of tautness by tightening or loosen this strip the screen cloth is stretched the vibrating plates are so constructed that the cloth is slightly higher where it comes in contact with these plates than where it is fastened to the main frame 0 04 YU ni z z fi ap I 1 JA front view of the mitchell electric vibrating screen ing the nuts on the bolted clamps the motion of the vibrating mechanism is transmitted to the taut screen surface by means of rigid vibrating arms or plates which fit snugly on and are firmly held to each of the tapered ends of the vibrator casing by means of wedges L on top of each plate a hardwood strip is laid and over this assures a constant contact between the screen cloth and the vibrating mechanism 1 sm the upper end of the screen frame is pivoted in brackets and the lower end is provided with elevating screws by means of which the inclination or slope of the screening surface can be easily adjusted to conform to any possible change in conditions of screening or feed |