Hoisting Appliances

by International Correspondence Schools

HEAD-FRAMES

29. A head-frame of wood, iron, or steel is built over a shaft or slope mouth to carry the sheaves over which the hoisting ropes are conducted from the mine to the drum of the hoisting engine; it also usually carries the upper portion of the cage guides or, in the case of a slope, the tracks for cars.

Fig. 27

A head-frame must be strong enough to bear the strain brought on it due to the total load hoisted and the pull of the engine in hoisting this load; it must also be rigid in construction to withstand the severe vibration and shock to which it is subjected on account of the rapid hoisting and the jar due to the landing of the cages.

Fig. 28

The amount and direction of stresses that a head-frame must resist are usually determined by applying the parallelogram of forces as follows: [Fig. 28] is a simple head-frame at a slope; a is the drum of the hoisting engine with the rope coming from its upper side and running over the head-sheave b down to the slope cage c. Assuming that the angles e, f made by the two portions of the rope with the horizontal are equal, and that the pull on each part of the rope is 20,000 pounds, to determine the amount and direction of the resultant of the two rope pulls, proceed as follows: Extend the rope lines to the point of intersection g and from there lay off the two lines g h and g k, to some definite scale, representing the pull of the rope. If a scale of 2,000 pounds to ⅒ inch is taken (⅒ inch = 2,000 pounds), g h and g k will each be 1 inch long. Complete the parallelogram by drawing h l parallel to g k and k l parallel to g h. The diagonal g l represents the direction and amount of the force acting on the head-frame due to the pull of the two portions of the rope. The diagonal, by measurement, is 1½ inches or ¹⁵/₁₀ inches long, and since each tenth inch equals 2,000 pounds, the stress on the head-frame in the line of the diagonal g l is 2,000 × 15 = 30,000 pounds. The figure also shows that the direction of this force is vertical, hence there is no tendency for the frame to be pulled over to either side and, theoretically, side bracing is not needed.

Fig. 29

30. Consider now the case of a vertical shaft, [Fig. 29], in which, as before, a is the drum, b the head-sheave, c the cage, and d the head-frame, and assume the same pull of 20,000 pounds on each part of the rope. As before, extend the lines of the rope, which are the lines of force along which the pulls due to the engine and the load act, until they intersect at g. From this point lay off on these lines distances representing the stresses in the rope to any scale. Using the same scale as before, ⅒ inch = 2,000 pounds, the lines g h and g k representing the two forces will be each 1 inch long. Completing the parallelogram by drawing h l parallel to g k, and k l parallel to g h, and drawing the diagonal g l through g, the resultant, g l = ¹⁹/₁₀ inches, represents a stress of 38,000 pounds. The direction of the resultant is also determined, being in the line of the diagonal g l. If the head-frame shown in [Fig. 28] were used for this case, it would be overturned by this resultant force, unless the leg on the opposite side of the shaft from the engine were securely anchored, so an inclined brace m is added to resist this overturning action. The resultant of all forces acting on the head-frame should generally fall within the structure if the greatest stability is to be secured, but when this cannot be done it is necessary to resist the overturning pull by anchoring the head-frame to its foundations much more securely than is the case where the resultant falls within the structure.

The direction of the resultant force may be obtained by drawing a line through the intersection of the lines of action of the forces at g and the center of the head-sheave b, as may be seen in Figs. [28] and [29].

Fig. 30

31. In Figs. [28] and [29], the pull of one hoisting rope running from the top of the drum was considered, but in most cases it is necessary to consider the pull from two hoisting ropes, one running from the top and one from the bottom of the drum f, as shown in [Fig. 30]. a b and a′ b′ represent the directions of action of the two forces acting on the hoisting ropes, while the two vertical forces a c and a′ c acting down the shaft are approximately equal to the two forces acting toward the drum. There are, therefore, two resultants a d and a′ d′, the directions of which are determined by lines from a and a′ through the center of the sheave e. The amounts of these resultant forces can be determined by the parallelogram of forces as shown in Figs. 28 and 29. A resultant that is a mean between a d and a′ d′, both in position and amount, is sometimes taken, or the greater value as determined from a d or a′ d′ and the greatest inclination as given by a′ d′ may be used, as being the worst theoretical conditions to which the frame may be subjected. A head-frame usually has a vertical post approximately parallel to the vertical pull of the rope in the shaft, and an inclined member g h approximately parallel to the resultant determined by the parallelogram of forces. If g h, [Fig. 30], is parallel to the resultant, the vertical leg h i is under no strain and merely supports the end of g h. If the resultant falls between g h and h i, both of these legs will be under compression. If the resultant falls outside of g h, the leg g h will be under compression and h i will be under tension. The head frame will be most stable when the resultant falls between g h and h i, but this cannot always be accomplished in building the frame on account of the conditions at the head of the shaft; nor is it always advisable to do so from structural considerations.

32. Since wood is much better adapted to withstand compressive than tensile stresses and since steel is adapted to withstand either tensile or compressive stresses, it is much more important that the members of timber frame conform as closely as possible to the theoretical line worked out in Figs. 28, 29, and 30 than in the case of a steel frame. Take, for instance, the case shown in [Fig. 31], where for some local reason it is impossible to put an inclined strut in or near the line of the resultant stress to withstand the pull that tends to overturn the head-frame. In a steel structure, a can very easily be made a tension member by anchoring its lower end to a heavy foundation. This resists the tendency to overturn and makes a very stable structure. In practice, braces can generally be located parallel to the line of resultant strain, [Fig. 29], or outside this line, as shown in [Fig. 30], so that the strain due to the pull of the rope will come mainly on the inclined brace and not on the upright. To distribute the stress on the foot of the different parts of the frame, an inclined brace is usually set farther from the shaft than the parallelogram of forces locates it, and so placed that about two-thirds of the strain due to the pull of the rope comes on the brace and one-third on the upright parts of the frame. In order to give the frame a more stable base and because the base must be larger than the top of the frame to bring the foundations back from the shaft mouth, usually the members h i are also slightly inclined.

Fig. 31

Wherever permanency of head-frames is required, if steel is obtainable at a price at all comparable with wood, steel structures are being used, as timber frames rot.

TYPES OF HEAD-FRAMES

33. There are three types of head-frame construction—the A type, the square type without an inclined brace, and the square type with an inclined brace.

34. A Type of Head-Frame.—[Fig. 32] shows the construction of a triangular, or A-shaped, head-frame of which (a) is a side elevation and (b) an end view. This particular frame is largely used at anthracite mines, but the type is one quite commonly used for timber frames, though the details of construction vary in different localities. The height of the frame is from 30 to 50 feet, and with direct-acting engines this height should be sufficient to allow a play of at least two-thirds of a revolution between the cage landing and the overwinding point. The posts a are parallel to the hoisting rope b as it hangs down the shaft and the inclined brace c, which resists any thrust that would tend to rotate the head-frame, is parallel to the resultant pull of the two parts of this rope b; the inclined braces d stiffen the frame and help support the cross-timbers m that support the cage guides e. The sills f are made of three pieces of timber 8 inches by 14 inches in cross-section. The posts a rest in cast-iron shoes g that are firmly bolted to the posts and sills. The inclined braces c, d are fitted with cast-iron shoes h, i. The post a and the two braces c, d are held in place at the top of the frame by the casting j, which also supports the pillow-block k.

The posts a and the brace c are made up of two pieces of timber each 8 inches by 14 inches in cross-section. The brace d consists of one piece of timber 8 inches by 14 inches in cross-section. The transverse braces l consist of two pieces of timber 6 inches by 14 inches in cross-section, bolted through the timbers a and c. The supports m for the guides are single pieces of 8" × 8" timber. The center post, as shown in [Fig. 32 (b)], is braced by the two pieces n, o, which are supported by two timbers p, q bolted to the two outside posts. The posts a and the inclined braces c are further braced by the tie-rods r, s, t, and u, all of which are fitted with turnbuckles, as shown at v. The different posts are firmly bolted together, the bolts being fitted with cast-iron washers.

Fig. 32

Fig. 33

[Fig. 33] shows the construction of the ordinary timber gallows frame used at many ore mines.

[Fig. 34] shows a steel A frame, of which the principal dimensions are as follows: height to sheave center 48 feet; base 33 feet 10 inches by 56 feet. Legs a and b are made of laced channels, as are also the central upright posts and cross-braces. The forward inclined legs are made of I beams. The weight of the frame is 98,000 pounds without the sheaves. The advantages claimed for this type of design are that it gives a very strongly braced frame while using a minimum of material. Also, in cases of overwinding, the cage goes over the top of the frame without injury to the frame, and should men be overwound they would fall only the height of the frame instead of being crushed against the top.

35. Square Type Without Inclined Brace.—[Fig. 35] shows a steel frame in which the tendency to be overturned by the pull of the rope is resisted by a nearly vertical tension leg as explained in [Art. 32]. Each leg of the frame is built of channel bars connected by lattice bracing, as shown, and the legs are stiffened by horizontal channel cross-bars similarly braced and also by diagonal tie-rods, provided with turnbuckles.

Fig. 35

Fig. 34

Springs are sometimes placed under the journals of the head-sheaves to lessen the strain on the rope while starting the load; the 15-foot head-sheaves of the Robinson deep mine at Johannesburg have locomotive springs under the journal boxes, the actual load on each spring due to the weight of the sheave, rope, skip, and rock being equal to about 20,000 pounds; it was estimated that the sheave would thus be lowered by the load on it, about 3 inches, which would be equal to an action of a spring giving motion of 6 inches at the cage. Springs can often be used both on the rope and under the sheave in the same plant to advantage.

Fig. 36

36. Square Type With Inclined Brace.—[Fig. 36] shows a very substantial frame with square tower and inclined brace.

Fig. 37

Its principal dimensions are as follows: height to sheave center 59 feet 6 inches; base of tower 15 feet 8 inches by 14 feet; distance of bottom of inclined leg from vertical post 48 feet. Each end post a is composed of two channels, double-latticed. The horizontal members b are I beams and each inclined member c is made up of two angles. The inclined leg d is trussed as shown and built of channel and angle beams, the main member being made of two channels, the incline and base members of the truss being made up of two angles, and the short vertical member of two channels. The center post of the tower is similar to the end posts, except that the uprights are I beams instead of channels. The frame is designed for a static weight of 16,000 pounds and for a maximum strain on the cable of 32,000 pounds.

[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.

Material.—Structure to be built throughout of soft structural steel, net strength 55,000 to 62,000 pounds per square inch; elastic limit not less than 30,000 pounds per square inch; elongation, 25 per cent.; bending test, bend flat on itself without fracture.

Builder agrees to guarantee structure to withstand strains specified on drawings with factor of safety of 10, to provide for possible overwinding or sticking in shaft.

No steel shall be used less than ¼ inch thick except for lining or filling vacant places.

Workmanship.—The tower to be built in a neat and workman-like manner. The pitch of the rivets (distance between centers) shall not exceed 6 inches or sixteen times the thinnest plate, nor be less than three diameters of the rivets.

The rivets used shall generally be ½ inch, ¾ inch, and ⅞ inch in diameter.

The distance between edges of any piece and the center of rivet hole shall not be less than 1¼ inches, except for bars less than 2½ inches wide; when practicable it shall be at least two diameters of the rivet. All rivet holes shall be spaced and punched, so that when the several parts are assembled together a rivet of ¹/₁₆ inch less diameter than the hole can be entered hot into any hole, without reaming or drifting. The rivets when driven should fill the holes. The heads must be rounded; they must be full and neatly made, and be concentric to the rivet hole, and thoroughly pinch the connecting pieces together. Field riveting must be reduced to a minimum. All joints and connections shall be neatly made, the several parts to be brought together without twists, bends, or open joints.

Inspection.—All facilities for inspecting the material and workmanship shall be given by the builders during the erection of the head-frame. The company reserves the right to reject any or all parts not built in accordance with the plans or these specifications. Final inspection of work 1 month after being in actual service.

Painting.—All work, before leaving the shops, shall be thoroughly cleaned from all loose rust and scale, and be given one good coat of paint well worked into all joints and open spaces. In riveted ironwork, the surfaces coming in contact shall each be painted before being riveted together. Bottoms of bearing plates and any parts that are not accessible for painting after erection shall have two coats of paint. After the structure is erected in place, it shall be given one coat of paint. All recesses that will retain water, or through which water can enter, must be filled with thick paint or some waterproof cement before receiving the final painting. The paint shall be a lampblack paint, mixed with pure linseed oil, or of red lead mixed with raw linseed oil containing Japan dryer.

General Clauses.—The specifications and drawings are intended to cooperate and to indicate the principal dimensions and requirements necessary to the complete structure. It being understood that while some work may be shown in the plans and not described in the specifications, or vice versa, and some minor details and fastenings are omitted from both plans and specifications, the work is to be executed without extra charge therefor, the same as if the minutest details were set forth in full in both drawings and specifications. The contractor is to make good any defects of material or workmanship developing within 1 year after final acceptance.

The contractor shall furnish a location plan and also two copies of the detail shop drawings for convenience in making future alterations and repairs.

Erection.—The head-frame is to be erected complete, secured to foundations provided by the _______ Company.

Contractor shall furnish all foundation bolts and washers. Iron stairway with hand rails beside main back bracers and platform with wooden floor under sheaves, also iron stairs from platform under sheaves to back sheave pedestal for oiling. Wood furnished by the ______ Company.

Price includes all material for completion of work delivered, erected, and riveted in place and painted.

The ______ Company will furnish and place in position the sheaves, with the shafts and boxes belonging to the same, also the wooden guides.

Delivery.—The head-frame to be erected, complete, and secured to foundations in ______ weeks from date of order.