Steam shovels will excavate any kind of material except solid rock, and they will load rock if it has been broken up by explosives into pieces of not more than 3-4 cu. yd. in size. The materials excavated by them are mostly sand, loose gravel, all kinds of clay, cemented gravel, hardpan, clays mixed with bowlders and other small stones, ore, phosphate rock, loose rock and thin seams of slate, shale or sandstone.
These machines are used for excavating material, loading it on cars or wagons for ballasting tracks; for filling trestles, streets, roads, dams, lots and new city additions; for widening embankments for double track, side tracks, yards, shops and station grounds; for cutting down street, road and railway grades; grading lots and new city additions, railway yards, shop and station grounds; widening cuts, removing land slides, stripping coal fields, ore beds and stone quarries; digging canals and drainage ditches, loading clays for brick yards, etc.
Construction of Steam Shovels.—The general plan of construction of the machines, shown in Figs. [1] to [9], is essentially the same in all, and consists of a strong frame, mounted on wheels, forming the base to which all working parts are attached. The boiler and machinery are placed near the rear end of the frame, and the mast, or post, and crane at the front end. The crane is made in two pieces connected only at the top or point, and at the foot of the mast. Between these pieces, serving as guides, is the dipper handle, carrying at its farther end the dipper or scoop. To the top of the post (or to the foot in some machines) the swinging circle is secured.
FIG. 3.—BARNHART STEAM SHOVEL; Marion Steam Shovel Co., Marion, O.
The most used, and hence the most important part of the machinery of the steam shovel is the gearing imparting motion to the hoisting drum, actuating the chains by which the dipper is raised and lowered. It is in almost constant use, and is often subjected to severe shocks in hard digging. Of all parts of the machinery it is the most likely to break or rapidly wear out. Naturally it has received the most attention of any part of the steam shovel in all efforts to improve the design, strength and durability of the machine. There are a number of different gears in use, and essentially they are either friction clutches or positive gearing. The use of the former subjects the machinery and crane to less severe shocks, and can be thrown in and out of gear more rapidly, but it wears out quicker, often causes delay by heating, and requires frequent repairs. Positive gearing exposes the machinery and crane to more severe shocks in hard digging, and must be started slower, especially in hard material, but while these machines are a little slower than those operated with friction clutches, they are less subject to the expense of repairs and delay due to the disarrangement of the hoisting gear, so that their total output of material about equals, and sometimes exceeds, the quicker moving friction gear machine.
The mechanism for thrusting the dipper into the bank is attached to the crane, and the forms most generally used are as follows:
1. A chain, one end of which is attached to the rear end of the dipper handle, and the other end wound around a drum receiving its motion by an endless chain passing over a sprocket wheel connected to the axle of the sprocket wheel at the top of the mast, over which the hoisting chain passes, thereby revolving both wheels. This drum is thrown into gear by a friction clutch, and its motion regulated by the cranesman's lever and footbrake.
2. A rack on the dipper handle operated by a pinion attached to a shaft revolved and regulated as above described.