[Fig. 37] shows a frame of similar form, but in which the landing platform is placed at a height above the surface, so that the cars hoisted can be run off on a trestle and thus be delivered at the top of a car, breaker, tipple, or ore house. Its principal dimensions are as follows: height to sheave center 75 feet; base 40 feet 11¾ inches by 21 feet 8½ inches. The leg a is made of two angles. The bracing leg b is built of two angles. The diagonal braces c are single angles. The horizontal braces are angles or channels of various sizes depending on the stresses.
37. The head-sheave is supported directly on top of the main frame, as shown in Figs. 32, 34, 36, and 37, or a small superstructure a is built on top of the main frame, as shown in [Fig. 38], so that the base of the sheave journals is perpendicular to the resultant pull on the frame, that is, to the theoretical direction of the inclined leg of the frame if one is used.
38. Timber frames are usually built by the mining company from its own designs. Steel frames are generally built by the structural steel companies from detailed plans and designs furnished by the mining company, or from a skeleton diagram furnished by the mining company, giving the loads on the rope and the general conditions about the shaft to which the frame must conform, the frame being then designed and erected in detail by the steel company.
39. Enclosing Head-Frames.—Head-frames are sometimes wholly or partially enclosed to protect them and the men from the weather. A covering of boards is warmest. All woodwork should be painted with fireproof paint and ample means for extinguishing fire should be provided. A covering of corrugated sheet iron well painted on both sides to prevent rusting is often used instead of wood and lessens the danger of fire, but is not as warm a covering as wood.
Fig. 38
40. In many states, it is required by law that the top of the shaft be protected by a fence or by gates to prevent persons falling down the shaft. This protection is secured at the sides of head-frames by extra timbers or beams forming part of the frame, or by means of a fence placed near the sides of the frame. The ends of the shaft are protected by a bar placed across uprights, by gates that swing like an ordinary door, or more generally by vertical sliding gates that are raised by the cage when it comes to the surface and drop into place when the cage descends. Similar gates, doors, or bars should be used at all landings below the surface.
HEAD-FRAME SPECIFICATIONS
41. The following is a sample set of specifications for a steel head-frame to be built from detailed plans furnished by the mining company.
This head-frame to be made from drawings to be furnished by the—— Coal Company, and placed on foundations furnished by said company.