Show ball and tube drives at rochester combined mill BY K FREITAG the ball and tube mills in the mill of the rochester combined mines company at rochester nevada which was designed and constructed by freitag ainsworth hobart building san francisco represent the latest in compact and efficient installation each unit is composed of one six foot by four foot six inch ball mill and one seven foot by twelve foot tube mill the ball mills are fed from two cylindrical bins by means of apron feeders that discharge into the center fed scoop feeders mounted on the feed of the mills the ore is crushed to one and one half inch size all mills are of the overflow type employing no screens in the discharge end but are equipped with reverse spinal liners the ball mills employ forged steel balls as a grinding medium and all new balls fed through the central feed opening are five inches in size the pebble mills employ flint pebbles as grinding medium and these are fed through the center opening in the scoop in the same way as the balls the ball mill discharges its products to a duplex dorr classifier operating in closed circuit with the pebble mill and the grinding is done so that 80 per cent will pass mesh screen one notable feature of the installation is the main drive arrangement many designs of ball and tube mill drives have been put into operation and every known method of transmission has been employed but nearly all have developed serious faults after a few months continuous operation from the early application of the long belt drives to the later short center or lenix drives many variations of belt drives have been applied of all belt drives the long center straight drive has proven the best but it requires considerable floor space and is an obstruction to installations where closed circuit grinding is employed all belted applications involving a friction clutch to take care of starting the load either to relieve the fhe belt or motor of the necessary starting torque and introduce an accessory which is reak eak mechanically as the strain on the clutch is extremely heavy when starting the mill from the rest even if rocking the mill is attempted introduction of the closed circuit with the introduction of closed circuit grinding it was necessary to develop a close center drive and the first tried was by means eans of double gear reduction using a friction diction clutch and constant speed motor later ter a chain belt was used in place of the first st gear reduction and finally single gear reduction by means of herringbone gears all al these drives mentioned had faults due ither either to the methods of application or the underrating of the excessive strain on them but the main trouble with these drives was that due consideration was not given to the end surge of all ball and tube mills no matter how well balanced ball and tube mills develop a surge endwise the bearings cannot be set close on the collars as clearance must be allowed for contraction and expansion of the mill barrels this is particularly the case with the longer tube mills in which the grinding temperature varies in shorter mills where it is possible to set a new mill with only slight clearance the wear and tear of daily use will soon be noticeable its aal S 1 ball and tube mill at rochester combined it can readily be seen that this end surge will seriously affect direct or single reduction gearing the speed of the driving shaft prevents it from following the jerky surge of the rotating mill and therefore causes excessive side wear on the driving sprockets in order to overcome the bad features embodied in the drives mentioned it was decided to eliminate them in order as they occurred oc cured that this was accomplished has been demonstrated in the continued successful operation of the mills during the past year general design Is identical in the lined drawing fig 1 it will be no noted ted that th the e general design is identical for ball and tube mills in order to secure similarity of apparius ap and consequent interchangeability ter changeability of parts it was decided decide d to build both mills as nearly alike as possible although the diameters were e different all and bearings were in made ade diameter by twenty twenty four inches in four inches long giving an extra large bearing area and also providing for large 1 openings the lin liners ers were made interchangeable as well as the feeders and discharge spouts this idea of interchangeability ter changeability of parts was carried through to motors and the entire drive arrangement ran gement the motors were built on the same frame thereby making all bearing shells interchangeable switchboard and control apparatus were made identical with the exception of course of the grid resistance selection of motors the selection of proper motors to take full advantage of the desirable features of the drive was carefully investigated and it was determined finally to use motors of the westinghouse CW type these are phase wound motors designed especially for good starting characteristics and high running efficiency fici ency with windings carefully braced and to resist the vibration incident to tube and ball mill drive A further advantage was secured due to the fact that the ball mills requiring 75 H P and the tube mills requiring H P could be furnished shed by the westinghouse company with motors of these two ratings built on the same frame so that all mechanical parts of the motors would be interchangeable as is the case with other parts of the installation the ball mills are driven by 75 H P RPM volt 3 phase 60 cycle type CW motors and the tube mills by H P motors of the same characteristics each motor is provided with a control panel of the westinghouse type RF equipped with su suitable bitable starting resistance on each of these panels is mounted an ammeter a primary circuit breaker equipped with overload trip and an under voltage release as well as a drum controller behind the board with han hand wheel dheel mounted on the face of the panel the am meters are of special advantage for the operator knows at all times just what the mills are doing and he is accordingly able to adjust his ball charge to get the best results the RF control panel has a special feature which has been found very valuable in that the operating mechanism of the motor starting controller is interlocked mechanically chani cally with the primary circuit breaker so thal thai if the circuit breaker has once tripped it cannot again be closed until the controller has been returned to the off position nordberg flexible coupling on each motor shaft is mounted a awen ty four inch nordberg flexible coupling connecting the motor to a short shaft which carries the driving sprocket for the chain drive this shaft is of the same diameter as the motor shaft and is mounted in two rigid pedestal bearings supplied with ring adia xa ahng bel BoJI M Mils f W 0 CW W sh r I 1 westinghouse bearing 24 4 War flexible Nordberg dberg shells in pedestal I 1 coupling J r morse choln chain 1 21 tooth arl ver neide if pitch ilch steel hardened I 1 J jj 4 0 casing 4 1 RPM n i jah LL 40 C out cut ieee steel hui pi man 14 bacs cut oear m 1 I 1 ba ball mih ral on ve itse st i center fool boo L j suit S lake ctr mini ns Review figure no 1 oiling removable bearing sleeves interchangeable with the motor bearings abl pedestals were also furnished by the westinghouse in company A heavy cast iron sole plate forms the base for the above bearings as well as for the main countershaft bearings and as all bearings are provided with means for lateral adjustment it forms a rigid base on which it is possible to maintain perfect alignment for the chain belts the driving sprockets and chains for the ball mills are as follows iol follows lows 75 H P 48 inch centers reduction to 1385 RPM driver 4 sprocket 21 tooth S inch face 1002 inch diameter steel hardened driven sprocket 77 tooth O inch face inch diameter chain morse type feet long and nine inches wide 15 inch pitch the driving sprockets and chains for the tube mills are as follows H P 50 inch centers reduction to 1385 RPM driver sprocket 21 tooth 1214 inch face 1002 inch diameter steel hardened driven sprocket 89 tooth 1214 inch face inch diameter chain morse type 1575 feet long and twelve inches wide 15 inch pitch both of the above drivers are encased in a galvanized iron case provided with and oil drip rings the main countershafts Counter shafts the main countershafts counter shafts for the ball and tube mills are five and one sixteenth inches in diameter mounted in heavy mill type ring oiling bearings the tow bearings next to the main pinion are mounted on a heavy cast iron which is tied to the main bearing at the discharge end of the mills insuring perfect alignment of the main drive the other end of the shaft on which is mounted the driven sprocket is carried in heavy ring oiling bearings similar to those on th emain drive end and mounted on the adia x 12 long ru beMI s CW siny hoase ben bening bearing ing shells same as motor fleo r I 1 abol morse se chum chal 12 lip tab 21 footh c v steel a I 1 4 J L 0 C cut steel pinion 01 14 fact 4 pitch 4 f pebble mill RH drive LH salt I oke mi na nino ia q R we w 1 i c 1 figure N no 0 2 sole plate which also supports the bearings carrying the motor shaft extension exien sion the main drives the main drives for the ball mills are as follows driving pinion cast steel cut teeth 15 teeth S inch pitch 14 inch face driven gear close grained semi steel cut teeth teeth 4 inch pitch 14 inch face main drives for tube mills are as follows driving pinion cast steel cut teeth 4 inch pitch 14 inch face 16 teeth driven gear close grained semi steel cut teeth teeth 4 inch pitch 14 inch face both main driving gears are made in halves and bolted to the end flanges of the mills in such a way that they can be easily removed or turned around so that both faces of the teeth can be used the speed of the six foot ball mills is 24 RPM and of the seven foot tube mills 22 RPM the drive arrangement this drive arrangement with all bearings mounted on interconnected sole plates insures perfect alignment of all shafts the employment of straight cut spur gears chain drives flexible couplings and slip ring motors is considered by the designers to be superior to any other drive arrangement for ball and tube mills the chains are relieved of all excessive strains and run perfectly without the customary jerk so familiar in the old type drives any undue strain or surge of the mills which is not absorbed by the straight cut main gears and is transmitted to the chain sprockets due to slightly worn collars on the main shaft does not affect the running of the chains as this motion is not counteracted by the motor but is absorbed by the flexible nordberg coupling connecting the motor with the floating motor extension shaft |