The supporting base is made up of a frame of steel beams strongly braced and riveted together. This frame rests on two steel axles, one pivoted and the other held in position and geared for power traction; and carries a heavy cast-steel annular gear, the track for the rollers, and the main journal which supports the revolving frame. The pivoted axle allows the directtional movement of the machine to be changed. Small diameter steel wheels of wide tread are generally employed, though some manufacturers are now supplying a caterpillar traction which is an advantage if the machine is to be moved about much and the soil is weak.

The upper or revolving frame consists of the power plant, A-frame, boom, and shipper arm, to which is attached the dipper, or bucket.

The power plant consists of motors or engines for hoisting, thrusting, and swinging; and where steam is used. a vertical boiler. The thrusting engines are generally mounted on the upper side of the boom and work through rack and pinion. Hoisting is accomplished through steel cables; and swinging through gears.

The A-frame is made up of structural steel posts. The feet of the posts are fastened to the revolving platform on each side, forward and back. The A-frame carries a pivoted cast-steel head block at the top, to which the rods are attached which support the upper end of the boom. The A-frame is slightly inclined and is shorter than the boom.

The boom is made in two parts and of structural steel, space being left between for the passage of the shipper arm. The boom is pin connected at the base and is held in position at its outer end by bars connected to the A frame. It is wider at the center than the ends due to the added stress imposed where the shipper arm is pivoted The sheaves for hoisting are at the outer end of the boom.

The shipper arm is of oak 8 to 12 in. square, reinforced with steel channels, and from 14 to 24 ft. in length A toothed rack fastened on the underside engaging with a pinion controls the movement of the arm. The digge is attached to the shipper arm.

Typical data on representative types of shovels are given on pp. 835 and 836;

It is of all steel construction with a cast bottom

The dipper or shovel resembles the ordinary scoop bucket. which is hinged to the back of the bucket and held closed by a spring latch. By a slight jerk on a light line which leads from the back to the cabin, the operator may open the latch causing the bottom to drop and empty the dipper. The cutting edge of the dipper depends on the character of the material to be excavated. This cutting edge is of high carbon or alloy steel supplemented under ordinary conditions by teeth made from forgings with tool steel points and fastened to the outside of the bucket leaving the inside free from obstructions. The dipper is fastened to the shipper arm by means of heavy forged arms and braces.

The hoisting line is connected to the dipper by a hinged bail through a sheave. The amount of excavation for a given period of time will depend on the character of soil, size of dipper, depth of cut, height of unloading, and the rate at which empties can be brought in place for loading. Using a 1-yd. dipper, in average ground, and rating dippers per minute to allow for ordinary delays, 40 to 60 cu. yd. should be handled per hour, or 300 to 500 cu. yd. per day for 8 hr. Steam shovels cost from $8000 for the small 1⁄2-cu. yd. to $20,000 for the 12-cu. yd. capacity.

The weight will necessarily increase with the size of the shovel.

16. Locomotive Cranes.-The locomotive crane is similar in general construction to the steam shovel. It also consists of two main parts-the supporting base and the upper frame. The general construction of the supporting base is the same as that of the shovel. The upper frame carries the power equipment and the boom. The power equipment consists of a 2 or 3 drum-hoist driven by double cylinder engines supplied with steam from a vertical boiler, or driven by gasoline engines in some recent types, or electric motors in favorable situations.

The boom of a locomotive crane is longer than that used with a power shovel and is made either in the form of an A (with legs connected to the rotating platform, the apex to a drum by

sheaves and boom lines) or of box cross section, greatest at the center and tapering to the ends. The boom (ranging in length form 25 to 70 ft.) is made of latticed structural steel shapes and can be raised or lowered.

These machines are used for construction purposes where loose material is to be handled, as in loading and unloading cars of sand, crushed stone, and gravel, or for hoisting materials, placing steel, pile driving, etc. They may also be used for excavating, but have no mechanical means other than the action of the bucket itself for cutting into materials. Three types of buckets are commonly used with cranes-the clamshell, orange peel, and handling buckets-on each of which detailed information is given later.

Locomotive cranes will vary in weight from 45 tons in the smaller or 10-ton capacity, to 90 tons for the larger or 30 to 40-ton capacity. The price will vary from about $12,000 for the former to $30,000 for the latter.

MAXIMUM CLEARANCE UNDER HOOK FOR VARIOUS RADII-OHIO LOCOMOTIVE CRANE

MAXIMUM CLEARANCE UNDER BUCKET FOR VARIOUS RADII-OHIO LOCOMOTIVE CRANE

FIG. 4.-Drag scraper.

1c. Scrapers.-Scrapers as used in ex

cavating for buildings are of two general types-drags or slips, and wheeled scrapers. These are practically identical in action except that one slides on the ground and the other is mounted on wheels.

Drag Scrapers.-The drag or scoop scraper (Fig. 4) is made of steel plates, sometimes provided with an extra bottom known as "double bottom" (Fig. 5), or with runners, with a heavy iron bail hinged to the sides as a means of attaching a team of horses.