For more than four years two 36-in. hydraulic dredges chewed away at overburden. Here are details of the heavy-duty clutch on the job.
Moving a lake and its millions of tons of silt and rock overburden a distance of six miles, to permit open-pit mining operations in the cleared area calls for plenty of power and specialized equipment. It also presents some special problems. Application of a Canadian designed automatic clutch to the power drive on the hydraulic dredges solved one of the toughest problems faced in this particular job. This article tells how it was done.
Late in 1953 the job of removing approximately 165,000,000 cubic yards of overburden from a lake overlying an iron ore deposit was initiated. This deposit, located near Atikokan, Ont., was owned by Caland Ore Company Limited of Atikokan. The job was contracted to Construction Aggregates Corporation of Chicago, Ill. The nature of the material to be moved dictated a hydraulic removal method.
By May of 1955 plant capable of accomplishing this job had been designed, obtained and erected by Construction Aggregates Corporation. Then two 36-in. hydraulic dredges and their allied equipment began working to capacity on the job.
The hydraulic dredges loosed the worthless overburden of clay and rocks, and fed the sludge (slurry) to a pipeline with a 36-in. diameter at the dredge. A 10,000-hp electric motor-driven pump forced this slurry through the floating pipe to shore, where a second 10,000-hp pump moved the slurry through a 42-in. pipe, up and over the hill, for a total lift of some 600 feet. A third 10,000-hp pump took over here and delivered the fast moving slurry through the pipelines (the two run parallel through the bushland) a distance of 4-1/2 miles, where the effluent was dumped into a valley area.
The immensity of the job can be gleaned when one realizes that a total of 65,000 Igpm of slurry poured through each of the two pipelines, and this went on for four years. In addition, each line disgorged approximately 3,600 cubic yards per hour of solids which were carried in suspension in the slurry.
Hydraulic Dredge Arrangement
An inherent part of a hydraulic dredge such as is used on this type of job is a cutter. This cutter revolves constantly while the dredge is working, stirring up the silt, clay, sand or gravel being dug so that its mixture with water will form slurry that can be pumped and transported by pipeline. Since the cutter always works under water it normally is powered by means of a drive-shaft coupled to a driving unit which is above water level.
This is not always the case, as dredges have been designed with a submersible electric motor driving the cutter. The standard design calls for a prime mover to be mounted on the upper end of the dredge ladder. This prime mover transmits power to the cutter through a reduction gear and a driveshaft.
If the character of the material to be dredged is suitable, that is, uniform and consistent, a direct drive between the cutter and its power source would be fine. However, on this job the character of the material varies from silt, to boulders that are too large for the dredge to handle. Also the lake bottom was cleaned to bedrock. If the cutter suddenly engage bedrock or an enormous imbedded boulder then something must give. Usually the cutter driveshaft or the cutter break.
Initially, the cutter drives on these dredges were directly connected through a standard flexible coupling. It was found that the cutter shafts would not stand up with this arrangement and the shaft broke. When such occurred an expensive shutdown and repair job resulted. For this reason, a relief arrangement between the cutter and its power source was deemed necessary, and a heavy-duty automatic clutch appeared to be the logical answer. The drive was therefore remolded to utilize a BLM clutch. This clutch is manufactured by the BLM Automatic Clutch Limited.
The cutter drives of the two 36-in. dredges were set up now are powered by a-c electric motors of 1,000 hp. The motors were 2,300 v, 234 amps, 60 cycle, 3 phase with a speed of 600-300 rpm. Power was transmitted to a reduction gear through a type L size 30 BLM automatic clutch. The clutch was set to slip at a horse power slightly above that of the motor output and well below that of the breaking point of the cutter or cutter driveshaft. The high speed side of the reduction gear is that of the motor and at full motor speed of 600 rpm the low speed of the reduction gear is 19 rpm. From the low speed of the reduction gear, power is transmitted through a 14-in. diameter flanged and jaw coupled shaft to the cutter which is 72 feet from the reduction gear. The BLM Clutch The BLM is an automatic centrifugal-type clutch. It is unique for its simplicity. The drive (inner) portion is fixed to the drive motor shaft. A number of driving blocks, all of identical weight are assembled so that in rotation, all of the blocks move uniformly outward. The centrifugal force developed by this outward movement of the blocks is transmitted through friction linings riding against the driven rim. This rim, in turn, is fastened to the gear reducer input shaft. In this particular application in the dredges, the unit is normally started unloaded and the clutch serves only to limit overloading of the drive motor. However, where a unit must be started with a part or full load, the clutch permits the motor to reach a higher rpm before loading. This makes available the more favourable torque at higher speeds characteristic of the motor, for acceleration purposes. We have found that normal servicing of the BLM clutch is quite feasible in the field, usually without disturbing the shaft alignment. Removal of the driven rim accomplished simply by removing the retainer bolts, provides access to the mechanism and linings for service or replacement, as required. The linings, not being bonded or riveted to any metal parts can be removed and replaced without further disassembly, Good preventive procedures normally schedule this as a routine operation after a set number of working hours, depending upon the particular application. In addition to hydraulic dredging operations such as described, this type of automatic clutch has found application in the pulp and paper industry, in mines, construction, railroads and chemical industries. It is ideally suited as a means of applying power from a prime mover to its load in such a way that the prime mover is not overloaded beyond its capacity either during the starting period or the actual running time.