Grinding mills are used for crushing and pulverizing raw material required for thermal power and cement plants and for ore processing units of mining industry. Other heavily loaded rotating equipment in industry, like kilns, rotating furnaces, rubber mills etc. , are all powered through a girth gear, mounted externally on the cylindrical tumbling barrel, required for processing the material. This heavy duty Girth gear drive, is the most economical drive alternative, when high load carrying capacity, and long service life under severe shock loading conditions exist, as in the above cases.Depending upon the type of raw material, rotary drum size, its arrangement and output expected, girth gear size is selected. Different lubrication and gear housing arrangements exist, with its own distinct advantages. In general, intermittent / continuous spray systems, have been the leading method of lubrication for the girth gear drives, where residual compounds of diluent type, are sprayed on to the load carrying tooth flanks of the mating gears, the solvent evaporating after the spray, leaving a protective coating of heavy oil , on the tooth surface. These open gear compounds are bitumen-base, and are toxic in nature. Of late, strict environmental regulations have been introduced, with reference to the use and disposal of such lubricants, which are being classified as hazardous waste. This makes their continued use increasingly more costly, and in this day of environmental awareness, less acceptable.Need has been felt to evolve an alternate, effective, reliable, environment friendly lubrication system, for girth gear drives, so that the lubrication cost is cut down, at the same time enhancing the life of the mating gears. The changed circumstances warrant the conversion of equipment fitted with open girth gear drives, to suit the alternate lubrication scheme. Although the most economical time to install such an alternate lubrication scheme for open girth gear drives, is at the time of initial installation of equipment, reliable systems with suitable design and appropriate technology, can also be retrofitted to the existing drives, necessitating minimum changes, thus enabling smoother transition to better lubrication and performance, of the girth gear drives.
Typical girth gear drive arrangement is given in Fig. A The required rotational speed for the tumbling barrel, is obtained through the two-stage speed reducer. Different types of girth gear lubrication schemes in vogue, is given in a to e, of Fig. B. Combining the salient features of immersion, pressure, and spray lubrication principles, novel oil lubrication scheme for the girth gear drives is evolved, as a retrofit, and is described here in detail. Immersion lubrication system is designed, primarily to provide a continuous flow of lubricant to the meshing gears, as part of the pinion or the gear, is always submerged in the oil bath, and is in direct contact with the lubricant reservoir, as illustrated in arrangements a to d, of Fig B. This arrangement requires that the lubricant reservoir, always remains adequately filled, and the gear guard cover, is sealed properly to avoid lubricant losses. To ensure immersion/sump/splash lubrication systems reliability, it is important to regularly compensate for the lubricant losses, which may be due to leakage from the sump, or spillage through the drum rim seals. Inadequate sealing between the gear housing and the rotating drum, will result in dust, sand, clinker, water etc., penetrating in to the immersion bath, thus contaminating the lubricant source. Hence effective sealing, go hand in hand with circulation lubrication. The rotating girth gear, while dipping in to the oil-bath, introduces small vortices, leading to air entrapment, and foam build-up in oil. The vortex as well as the foam build-up in oil, has to be contained with-in limits, for reliable gear performance. As majority of the girth gear housings are designed for spray lubrication, they cannot be readily used, for circulation lubrication, without suitable modifications, with reference to sealing, and for holding the lubricant. Because of continuous oil circulation, heat transfer is effected from the high pressure contacting areas of the meshing gears, and this influences the sump / reservoir oil temperature. Reservoir oil temperature has to be effectively controlled through coolers in the lubrication circuit, to maintain cooler feed oil temperature with in limits, which ensures adequate oil film thickness along the contacting, highly loaded, tooth flanks of the meshing gears, thus preventing aspirates contacts, during meshing.
Lubricant is drained from reservoir (1) of Fig. C, and controlled through shut-off valve (2) of Fig. C, to the air treatment plant (3) of Fig. C, the details of which is separately given.Air bubble / foam-free oil, is then fed to the lubricant feed pump (4).The lubricant output pressure is regulated through shut-off valve (6), and pressure relief valve (5).The lubricant is further processed through a powerful magnetic filter (7.1), employing permanent magnets, which filters-off the metallic debris from the lubricant, while micro-pore strain filter cartridges (7.2) and (7.3), completes the filtration loop, thus ensuring air/ impurities free lubricant, to the supply line. Details of the filtration unit and circuit, is separately given in Fig. F. Lubricant is drained from reservoir (1) of Fig. C, and controlled through shut-off valve (2) of Fig. C, to the air treatment plant (3) of Fig. C, the details of which is separately given in Fig. E.