For soft, tenacious material, likely to adhere to the inner sides of the dipper, and not drop out promptly when the bottom door is opened for unloading, the dipper is shaped as shown in Fig. 10, with a larger bottom than mouth. In hard, or dry soft material the section shows parallel sides, as in Fig. 11. For general use the bottom of the dipper should be slightly larger than the mouth, as most materials contain more or less moisture which is likely to produce a partial clogging of the dipper by material sticking to the inner sides, especially between the teeth, necessitating frequent cleaning out whenever the machine is stopped while preparing to move forward, and sometimes oftener. For ordinary clay, cemented gravel, and hard dry materials, a dipper with a wide and shallow mouth, as shown in plan in Fig. 12, is preferred to the one shown in Fig. 13, which latter is better adapted for loose gravel, sand and other soft dry materials where a deep cut can easily be made. For hardpan, shale, loose rock and similar materials, ample strength of teeth and dipper is of greater importance than its shape.
[Illustration: FIG. 7.--GIANT STEAM SHOVEL; Vulcan Iron Works Co., Toledo, O.]
[Illustration: FIG. 8.--LITTLE GIANT STEAM SHOVEL; Vulcan Iron Works Co., Toledo, O.]
To prevent tenacious material from sticking to the inner sides of the dipper, and to allow it to drop out freely when the bottom door is opened, it is often good economy to place a barrel of water near the head of the machine from which a bucketful can be taken and thrown into the dipper just before each cut. The water acts as a lubricant and causes the material to drop out more readily. For cleaning the dipper, the tool shown in Fig. 14 is used.
Figs. 10 to 13.--Buckets for Steam Shovel.]
The chains have links of three-quarters-inch to one-inch diameter, and are made of iron, sometimes of steel. Their constant use necessarily subjects them to great wear, and as they are also often exposed to severe shocks (especially the hoisting chain) they must be made of the very best material and in the most careful manner. At present iron chains are preferred to those made of steel: they are more durable, and less likely to break under severe shocks. Steel chains have suffered in reputation through rapid wear and frequent breakages occurring within the last few years, but with increased experience in their manufacture and use they will undoubtedly be improved, and eventually take the lead over iron chains.