Plants removed from the lake shore must, of course, receive their ore by rail, and the enormous quantity to be handled in the relatively brief open season has made desirable in some cases a greater speed in dumping than can be secured even with the modern drop-bottom type of ore car.

Fig. 15. Car dumper.

To meet this condition an apparatus called a car dumper has been applied at some plants. This apparatus has been built in two styles, stationary and moveable. The moveable type is greatly to be preferred, because when a stationary dumper is used some style of transfer car is needed to carry the ore from the car dumper to the stocking bridge, which must stand in the location at which it is desired to stock the ore. With the traveling type of car dumper this is not necessary.

The car dumper is moved along on its own tracks, just in front of those carrying the rear end of the bridge, and the car when dumped delivers the ore to the exact spot where it is most easy for the bridge to pick it up.

The general appearance of the car dumper, just back of the retaining wall of the ore yard, with a car standing in it, is well shown by the photograph, Fig. 15.

This apparatus can easily dump one hundred and fifty cars per day of ten hours, and most of the cars used in this service carry about fifty-five gross tons. It will be seen that the amount of material which can be handled through it in a given time is enormous.

In spite of the enormous capacity and great efficiency of these machines for the purpose intended, it may be doubted whether very many plants require the dumping of so vast a quantity of ore in a given time as to compensate for the heavy interest charges on the investment involved in one of these machines, especially in those cases where the ore yard is provided with elevated tracks from which dumping may readily be done.

In such a case the amount of labor involved in dumping the desired number of cars of ore from modern drop-bottom cars is not great, and its cost in all probability does not offset the interest and amortization charges on a car dumper.

At those plants in which the location of the ore yard is relatively high, and it is impractical to provide an elevated dumping trestle, the car dumper undoubtedly furnishes the best solution of the problem when considerable tonnages are to be handled.

Power Used

Various kinds of power have been used from time to time for operating the various types of ore-handling machinery, described and illustrated above, but it is safe to say that at the present time no power except electricity is seriously considered for this service under any ordinary conditions. This is supplied either from trolley wires or from sliding contacts, like a third rail, parallel to the track, as the circumstances may require.

Whether alternating or direct current be preferable is a matter of some discussion among experts in this field, but in the majority of cases direct current at 220 volts is used, because of the superiority of control which the direct possesses over the alternating, and because with this kind of current dynamic braking can be used in addition to mechanical brakes, for retardation of the moving parts. This can not so easily be done with alternating current.

Each plant presents ore-handling problems of its own, and in each case the problem must be worked out on the basis of the conditions to be met. Enough has been said to give a substantial idea of the general outline of the operation.