---

October, 1936

---

Headgears may he constructed of timber, steel or reinforced concrete. Originally, the former material was universally employed, but it is now only used for small temporary structures. The material most commonly used today is structural steel, and high tensile steel members are increasingly introduced to save weight. The possibilities of reinforced concrete have received serious consideration, but this material is very seldom employed.

Timber headgears have often given satisfaction for periods of thirty to fifty years, and since this material is much cheaper than steel, it is never likely to be entirely discarded. Pitch pine is the most suitable wood for these structures. The principle objections to the use of timber are its combustibility and its liability to decay. The lack of uniformity in quality, the low compressive values across the grain, and the difficulty in obtaining suitable lengths for framing large headgears are other reasons why the use of timber has declined.

Steel headgears can be designed with great exactitude, because of the reliable character and uniform quality of the material. Consequently, a lower factor of safety can be used with steel than with the other materials under consideration, and a lighter and more economical structure can be obtained. The variety of sectional shapes available in practically all desired lengths, and the satisfactory joints that can be made are further features in favour of this material. Although not combustible, steel is liable to distortion under the influence of intense heat, and because of its elastic nature, headgears constructed of this material are subject to vibration.

Reinforced concrete headgears can withstand the effects of fire and vibration to a much greater extent than can structures made of the foregoing materials. Concrete, however, is not homogeneous, and therefore it is not absolutely uniform in quality. On account of this, and also the fact that it is brittle to a certain extent, concrete work cannot be designed with perfect definition. A large factor of safety is consequently necessary, and this results in a massive structure which requires very good foundations. Because of its mass, a reinforced concrete headgear is exceptionally rigid, and may be made narrower at the base than a steel headgear and still possess the same stability. This is of importance where ground area is limited, but care must be taken that the shaft collar is sufficiently strong to withstand the close proximity of heavy loads.

Elements of Design

The design of mine headgears is governed by a large number of factors which vary according to local conditions. The most important features are the depth and inclination of the shaft, the quantity of ore to be raised per hour and its subsequent handling, the desirability of using skips or cages or both, the hoisting speed, and the material to be used in the structure.

The amount of ore raised per wind, the size of the skip or cage, the number of hoisting compartments, the area and shape of the shaft, the dimensions of the winding rope, sheaves and engine, and the type of structure are mainly determined by the depth and inclination of the shaft, the quantity of ore to be raised and the hoisting speed. The skip tipping or cage banking level, which depends upon the height required for the storing or other handling of the ore, the height of the tipped skip and its bridle or the cage and its chains, and the provision for overwinding govern the height of the headgear, and this in turn fixes the position of the winding engine. In this article, structural steel headgears for vertical shafts will be considered.

The headgear serves the double purpose of supporting the hoisting sheaves and guiding the skips or cages to the tipping or banking level. It may be designed so that the sheave framing is independent of the shaft framing, or so that the two frames form one structure and yet retain their individuality to a great extent, or again so that both frames are more intimately combined. The merits of the three designs, which are shown in Figures 2, 3 and 4 respectively, are in the order mentioned.

In Fig. 2 the shaft framing consists of a vertical tower, which is simply supported at the top by connection with the sheave framing. The shaft framing fits closely round the skip or cage ways, and is designed to take all the loads due to overwinding and skip tipping or cage landing. The sheave framing is made as narrow as possible at the top, and the sides are inclined at from 1 in 8 to 1 in 10 so as to give increased width at the foundations. This framing is designed to take all the loads in connection with the hoisting operations. Thus each structure serves its own purpose, and as there is no confusion of stresses, the design is quite simple.

In Fig. 3, the inner shaft framing legs are dispensed with, and horizontal members between the main legs of the sheave framing are arranged to carry the horizontal girders from the vertical front legs of the shaft framing. Horizontal ties are also provided at intervals between the vertical front and the inclined main legs. Otherwise, the shaft and sheave framings are made as above, and therefore the main legs in this type serve the same purpose as both the inner shaft framing legs and the main legs of the previous design.

In Fig. 4, the headgear is a six-legged structure, which is made narrow at the top and wide at the base, all legs being battered so that the side framing is in the same plane. In this case, the vertical shaft framing is incorporated in the front and main legs of the headgear.

The hoisting rate varies from 10 to 60 ft. per second according to the depth of the shaft, which may be anything from 1,000 ft. or less to 6,000 ft. or thereabouts for a single vertical wind, so that the rate may be approximated at 10 ft. per second for every 1,000 ft. of shaft depth. The quantity of ore to be raised per hour may modify the hoisting rate, and if the tonnage is large, four or six hoisting compartments may be necessary instead of the usual two. These considerations determine the amount of ore raised per wind, the size of the skip or cage, and the shaft dimensions. Cages are used when the tonnage is small or when the shaft is to be used for handling men. Skips are always used for large tonnages, but sometimes cages for carrying either men or ore cars are used in two compartments of a multi-compartment shaft.

Rectangular shafts are almost invariably used in ore mines, and the compartments are arranged in line so that the sheaves can be placed parallel Sometimes an additional compartment is included for the handling of pumps, pipes, etc , and a smaller diameter sheave is usually provided for this purpose This odd sheave may be located on the top platform, but it is generally carried on staging about half way up the headgear In either case, the introduction of this extra compartment makes the loading on the structure unsymmetrical, because the stresses on one side are greater than on the other.

In order to determine the height of the headgear, the position of the skip tipping or the cage banking level must first be fixed This depends upon the surface arrangements, and may vary from 30 to 90 ft. in the case of skips, or from shaft collar level to 30 ft. in that of cages. From the tipping or banking level to the safety catches, a distance equal to the overall height of the tipped skip with its bridle and safety hook, or that of the cage with its sling chains and hook, plus from 10 to 25 ft. for overwind should be allowed. In the case of rapid hoisting, this dimension should equal at least one revolution of the winding sheave. The height from the safety catches to the centre line of the hoisting sheaves should be sufficient to allow the skip or cage to be released without the rope sockets coming in contact with the sheave; usually 10 ft. is enough. Although prospecting shaft headgears may be only 30 to 40 ft. high, mine shaft headgears usually range from 60 to 160 ft. in height.

The hoisting engine is placed at some distance from the shaft, a good position being such that the mean angle of the hoisting rope between the drum and the sheave is from 400 to 450 from the horizontal. When so placed, the constantly varying angle which the rope makes with the sheave in winding off or on the drum is kept small, and the friction due to this cause is correspondingly reduced.

The location of the back legs of the headgear is determined in relation to the position of the winding engine. The resultant of the stresses in the hoisting rope bisects the angle between the vertical and the inclined portions, and passes through the centre of the head sheave. If the back legs coincide with this resultant, they transmit all the true hoisting loads to the foundations by the most direct path. When the base of the back legs lies within the resultant, the hoisting stresses produce tension or up-lift in the main legs, and this is an undesirable condition which must be guarded against. On the other hand, when the base of the back legs lies without the resultant, both the main and the back legs are in compression, and this is the most satisfactory condition because it ensures a stable frame.

Some designers prefer to arrange the back legs parallel to the inclined portion of the winding rope, so as to divide the hoisting stresses equally between the main and the back legs, but this system generally necessitates intermediate posts to brace effectively the back legs against bending. Another method is to construct a parallelogram between the inclined and vertical portions of the winding rope, such that the length of the inclined side is twice that of the vertical, and then produce the major diagonal to the ground in order to obtain the position of the back leg base. With the hoisting engine situated as previously described, the back legs can be placed at an angle of 300 from the vertical and fulfil the desired conditions with economy of material. These three systems are clearly shown in Figs. 5, 6 and 7 respectively. The important feature to bear in mind is that the axes of the main and back legs must pass through the centre of the hoisting sheave to produce a statically determined structure.

Drawings and Photographs accompanying the article