Tumbling mills are basically described by the type of grinding media used, whether they grind the feed wet or dry, by the type of discharge arrangement, and the shape of the cylindrical part of the mill. All tumbling mills have common features:
■ Drives
■ Support bearings in which they rotate
■ Cylinders
■ Feed and discharge ends
■ Wearing parts to protect the surfaces of the cylinder and the ends that are subject to wear
■ Discharge arrangements
All of the tumbling mill manufacturers continue to have the same basic design challenges. Drives start with the energy source and end with the delivery of the energy to the cylinder. For tumbling mills the first energy source was the steam engine, followed by the electric motor. In some cases, internal combustion engines are used to drive tumbling mills.
The early mills and the smaller mills were driven by flat belts connecting the motor and the mill pinion shaft. The limitation on this drive was the ability of the belts to transmit the power. This limitation was solved by using V-belt drives, which were suitable to about 224 kW, but for more power the drives became so wide that they were difficult to install. This led to the use of direct-coupled synchronous motors, which opened the door to much larger mills.
Gears were a very important factor in mill design. The original gears were spur gears, which allowed single-tooth contact only and limited the power that could be transmitted. Higher power transmission was obtained by using herringbone gears, which allowed more than one-tooth contact. The gear teeth for each side of a herringbone gear are cut from the outside of the gear face; one side is cut, then the gear is turned over and the other side is cut. Manufacturing limitations caused the machining of the apex at the center of the herringbone gears to waver from being a true circle. The wandering apex, which was at the center of the gear, and the pinion required that herringbone pinions be free to move laterally in either direction. With the introduction of spherical roller pinion shaft bearings, the necessary clearance in the bearings was lacking for the free movement of herringbone pinions. Single helical pinions need to be held in a fixed position, so they did not need the same freedom as herringbone pinions. Consequently, single helical gears and pinion drives were used in grinding mill drives, instead of herringbone gears and pinions, where spherical roller pinion shaft bearings had been used. For mills drawing in the range of 5,000-9,000 kW (7,000-12,000 hp), single helical drives using two or more pinions to drive one large-diameter gear were developed, and the gear manufacturers built new larger-diameter gear-cutting facilities. Large-speed reducers were often installed between the motors and the pinions, with one motor and drive shaft for each pinion.
In the 1950s, the grinding mill manufacturers were building mills that required single helical gears with a diameter that approached the limit of what could be supplied by gear manufacturers. The availability of large diameter gears was more critical in Europe than in the United States. The first solution was the development of large-speed reducers (gear boxes) that could be coupled to the discharge trunnion of the grinding mill. The output speed of the reducer was the same as the mill speed. The speed reducers had either double or triple reduction of input speed. F. L. Smidth and Allis-Chalmers designed and built these drives, which were principally used in the cement industry. The sale of trunnion drives for tumbling mills ended when the gear manufacturers installed larger — diameter gear-cutting facilities.
Motors A second solution to the problem of ever-larger, single helical gears was developed in Germany, and it became useful when the problem occurred again in the 1970s. In the 1960s, Brown-Bovari and Siemens in Germany and General Electric in the United States developed the wraparound motor drives for long-length, two-compartment ball mills that would draw more than 2,240 kW (3,000 hp). These were for dry mills for grinding cement clinker in Germany. The design of these motors was the base from which the wraparound drives for SAG mills and single — or two-compartment large ball mills were developed.
The motor was an alternating current synchronous motor, and the sequence in the electrical starting and control circuit was to rectify the alternating current to direct current then back to a low-frequency alternating current to give the motor the same speed selected for the operation of the mill. The selection of the operating frequency for the low-frequency alternating current is a control variable that can be used to adjust the mill speed for changing grinding conditions. The design of large-diameter, high-horsepower SAG mills required drives that could deliver up to 22,500 kW, and wraparound motors with the motor rotors mounted on the shells of the mills were built for mill drives exceeding 9,000 kW.
Trunnion Bearings Manual grease lubrication was used on trunnion bearings. This put a limitation on the size of trunnion bearing with the maximum size being 0.87 m in diameter. Grease lubrication also lowered the allowable loading of trunnion bearings. With the development of self-lubricating oil bearings and then oil lubricating systems, much larger trunnion bearings were available as grinding mills became larger in size.
Primary autogenous mills built in South Africa and by Aerofall Mills were mounted on pad bearings around the mill shell. The South Africans needed this design for their high-speed mills with peripheral discharges. Aerofall’s engineers wanted large-diameter trunnion bearings for its air-swept mills.