THE COLLIERY GUARDIAN. •August 9, 1918. 284 neers: Professor Hardwick and Messrs. W. H. Chambers, C. C. Ellison, J. H. W. Laverick, W. D. Lloyd, T. W. H. Mitchell, H. F. Smithson, and W. Hargreaves. The President thanked the members for the addi- tional honour they had done him in re-electing him president, expressed his regret that his health had not during the last year allowed him to devote the attention to the office that he would have liked, and hoped that during the ensuing year he would be able to serve the institute with credit and in a way that would uphold the honourable traditions of the office. Old Pumping Engines. Mr. Gerald T. Newbould (Earl Fitzwilliam’s Collieries, Rawmarsh) read a paper on “Two New- comen Atmospheric Pumping Engines” (see page 230 in our last issue). Discussion. The President, in opening the discussion, said it was most interesting to have the accounts of the work- ing of these old engines while they were still in use. So often papers referring to them were not published until after the engines were no longer in operation. It was very interesting to note that in spite of the great advances which had been made since the first engine was started 131 years ago, the actual useful work accomplished compared very favourably with what was being done to-day. It would be interesting if Mr. Newbould could give some idea of the cause of the coal consumption of the Elsecar engine being double that of the other engine. Of course it was the older engine, and fed, as he understood, by externally fired boilers as against the Lancashire boilers in the case of the Westfield engine, and if that were so it accounted for a great deal of the difference in coal consumption. But to have an engine that had been working for 95 years still giving 18 lb. per brake horse-power seemed to show that the old engineers knew their business. He would also like to know if Mr. Newbould could give some particulars as to the reliability of a plant of that description. It seemed to him quite possible that a plant of that sort, although perhaps it had not quite as high an efficiency as could be shown to-day, yet in the long run, after such a long life, would compare very favourably with modern plant as regards upkeep. He noticed Mr. Newbould referred to a spare cylinder bottom being kept. It would be interesting to know how often the bottom was knocked out of the cylinder, and also whether the false bottom in the Elsecar engine was intended to provide against any accident of that sort. It was understood that the pistons were hemp-packed, and it would be interest- ing to know whether the cylinders had got out of truth at all and had been bored out at any period. Generally, if Mr. Newbould could give any idea as to whether any expensive repairs had been done and to what extent during the life of the engines, it might add to the interest of the paper. Mr. J. H. W. Laverick (Tinsley Park) said there was an old engine at Pentrich Colliery, Ripley, near Derby; it was dated somewhere about 1800 and he was not sure whether it was not the older of the two. Mr. Newbould : The Elsecar engine is four years older. Mr. Laverick said he could remember the old atmo- spheric engines being in daily work for the raising of coal. One he had in mind was erected about 1810, but to his regret it had been smashed up, or he would have been glad to have purchased it as a curiosity, although he had no doubt in some form its parts were still doing service to-day. The working of these old atmospheric engines was a most interesting subject to mining engineers of to-day. Mr. J. R. Wilkinson said the paper recalled the difficulties under which their grandfathers worked, and he confessed to a pronounced reverence for the old machine he remembered very distinctly having been left in charge of occasionally. Steam was made in an old wagon boiler; from each end of the beam massive chains connected to the pumping rod and piston rod. The piston was packed with plaited gas- kin. Inside the piston block there was a wood ring like a cart wheel, and the piston was made compara- tively tight by this gaskin. Over the top of the cylinder was a lead pipe fitted with a tap, out of which a small stream of water came to keep the air out. The steam pressure used to be 2 lb. per sq. in., and he could recall his grandfather’s directions as to shovelling so much coal on at stated intervals, and what to do if the engine ran down, and to lift up the valve and insert a wood wedge. He could not see that to-day any more effective work was got out of their engines than was obtained from these old engines for what they were required to do in their day. The great improve- ments, to his mind, were in the boilers and the pro- duction of steam. Had the old engineers had in their days the same boiler efficiency, the probability was that they would have got extraordinary work from the fuel used. It was a subject of very fascinating interest, and they could all learn something from studying the conditions under which their grandfathers worked. Mr. S. Evans (Cresswell) also recalled early expe- riences with an old beam engine in the Black Country, where most of the old winding engines were of this type. He wondered whether much of the efficiency of these engines was not attributable to the momentum of the enormous fly wheels with which they were fitted. Mr. S. J. Rayner (Rotherham Main) asked what had been the amount of wear in the cylinders during their many years’ working; what was the original and present thickness of the cylinders? Mr. Newbould, who was accorded a hearty vote of thanks for his paper, then briefly replied to the dis- cussion. The spare cylinder bottom, he explained, was provided at a time when a slight surface crack made its appearance many years ago, but this got no worse, and the spare bottom had never been used. The cylinders had never been re-bored. With hemp pack- ing it did not, of course, matter very much if the cylinders did get slightly out of truth. The heavy coal consumption at the Elsecar engine was largely due to the externally fired boilers, and to the fact that steam was on during the whole stroke. The Pentrich engine had been described by Mr. Anderson. The flat false bottom in the Elsecar engine very probably had something to do with the high coal consumption, and it appeared to him to be a disadvantage. The engines had proved exceptionally reliable, and no expensive repairs had ever been necessary. The tupping pieces prevented any danger of the piston accidentally going too low in the cylinder and knocking the bottom out. With regard to heavy flywheels fitted to the old beam rotative winding engines, these would act in the ordinary way, keeping the speed constant during the power and steam strokes. These engines only ran at about 20 revolutions per minute, so that a large, heavy flywheel would be necessary; this, added to the effect of the constant stroke, would no doubt add to their efficiency. The Westfield cylinder had since been bored near the centre, and also below the travel of the piston, and it was found that the wear had been practically nil. This, no doubt, was due to the fact that the surface in contact with the cylinder was only soft rope. __________________________ ROOF CONDITIONS AT THE LETH- BRIDGE COLLIERY.* By J. B. De Hart. Much has been written on the numerous theories of roof pressure; some of the theories agree fairly well with observed results, but no one theory can be evolved which will show how every roof will behave. Some roofs bend a great deal before breaking, others break off short, and there are roofs which behave in all sorts of intermediate ways. The following notes on the behaviour of the roof in the Lethbridge Colliery show fairly well why, in many cases, the booms bend but do not break; they also show that this condition is not a very safe one, because when the booms are thus bent a parting of the rock above may occur at any time without warning. Entries in the Lethbridge Colliery are driven 8 ft. wide, and the booms used are 4 in. by 6 in. sawn timber. The roof is shale, and the booms are spaced approximately 3 ft. centre to centre. There are two general ways in which failure of the booms occurs. Sometimes the roof breaks from 18 in. to 2 ft. above the coal, the roof above being self-supporting. In some cases this is sufficient to cause failure of the booms; in others the booms stand up under this load until the roof breaks higher up. In this case it usually breaks in the shape shown in fig. 1. It starts to arch until it meets a hard band from 6 ft. to 8 ft. above the coal. This hard band forms a flat top from 2 ft', to 3 ft. wide. Fig. 1. Fig. 2. \ \ \ Al I \ J 3 I I /A In order to gain some idea of the load on the booms, samples of the roof were taken and their specific gravity determined. The specific gravity was 2-40, showing the weight of the rock to be 150 lb. per cu. ft. Consider a beam of length I feet supported at both ends, with a cross sectional area of A square inches and loaded with a uniform load of w lb. per lin. ft. The maximum shear is at the ends and is equal to w 1 T lb' The maximum bending moment is at the centre, and is equal to ft. lb. 8 The maximum shearing stress is lb. per sq. in. _____ The maximum bending stress is ~ lb. per o I sq. in., where y is the distance from the neutral axis to the extreme fibre and I is the moment of inertia of the beams. In the case under consideration w = 150 x 2 x 3 = 900 lb. per lin. ft., y = 2 in., 1=32 in., and A= 24 sq. in. 900 x 8 The maximum shearing stress is therefore —------ & 2 x 24 = 150 lb. per sq. in., and the maximum bending stress is 12 x 900 x 64 x 2 _ 5 4001b per sq in 8 32 The ultimate shearing strength of the timber across the grain is about 3,000 lb. per sq. in., and the ulti- mate transverse stress 4,000 lb. per sq. in., so that the bending stress is more than sufficient to cause failure. But the boom does not always fail under this load. In the first place there is some degree of fixture of the ends of the beam. If the ends were rigidly fixed horizontally, the maximum bending moment would be at the supports and would be equal to ft. lb., while the bending moment at the centre 12 would be ft. lb. The maximum stresses pro- 24 duced would be 3,600 lb. per sq. in. at the supports, and 1,800 lb. per sq. in. at the centre. Now the ends are not fixed absolutely rigidly so that the maximum * Canadian Mining Institute Bulletin. ___________________ fibre stress at the centre will have a value inter- mediate between 5,400 lb. per sq. in. and 1,800 lb. per sq. in., its exact value depending on the degree of fixture. There is another factor which affects the maximum stress produced. The elasticity of the rock is less than that of the boom, so that when the boom deflects under load the rock is left without a support at the centre, and it cracks at or near the centre. The rock then wedges itself at the points A A fig. 2. This wedging throws more of the weight on to the part of the beam directly above the supports, and thus further reduces the bending stresses. The result of these actions is that the 4 in. by 6 in. timbers will just support the 2 ft. of loose rock; but if there be a weak timber it will break, and this will throw an extra load on the adjacent timbers. They will then be overloaded, and the failure will spread along the entry in both directions. This is exactly what.happens in practice. First, as the rock loosens a number of the booms bend; then one fails at the centre, and this failure then spreads in both directions. In some cases where there is no weak boom the timbers will stand until the rock breaks higher up. When it does it usually breaks in the shape shown in fig. 1. There is little or no wedging action in this case. For the sake of simplicity, assume that the break occurs as shown in fig. 3, and that the beam is rigidly fixed at the ends. Let us determine if there is any possibility of the beam withstanding the bending stress, for this is the stress at which all the booms in this mine break. Consider a section of the beam distance x from one end, x being greater than a and less than a + b. Let Mcc = the bending moment at this section; M = what this bending moment would be if the ends were free; m = the fixing moments at the ends; i = the slope of the beam when deflected; E = the modulus of elasticity; I = the moment of interia; and w = the load per foot on the centre portion. Then M = ~ (a 4- 6) a? — (x — A — w (« — a) 2 2 3 / (x — a\ __ wax , wbx __ wax . w a2 _ w ~2~ “2 + cT 2 , „ n i wax . wbx wax . w a2 - 2ax + a2) = + —------------ + -g- wx2 , wa2 .wbx wx2 __ — + Wax- = wax + — __ — _ wa2 .................................. ~6~ ..................................1 Ma? = m — M ............................(2) and i f M) ..................(3)..... E 1 | Substituting the value of M found in equation (1) into equation (3), we get— If, wbx . w®2 . wa\ j 1 = El J (W “ WaX ~ ~2~ + ~2~ + dx 1 ✓ wax2 wbx2 . wx3 . w a2 x\ = EiV^~^r -^r + 1F + —J ,. ~ , i________ .. , and i = O when x = - since the beam is symmetrically loaded, O 2 w a2 I (4) ml wal2 wbl2 , wl3 ~2 ~S~ T6~ + ~48~ wal . wbl wl2 wa2 ..... ~T~ + ~8~ ~24 “6“ Substituting the values of M and m from equations (1) and (4) into equation (2), we get— nr wal . wbl wl2 wa2 Ma: = - - 4- -5- - = —- wax 4 8 24 6 wbx . wx2 . wa2 ............. = 2 + — + .............(6) Substituting x = 4 ft., I = 8 ft., a = 2’5 ft., and b = 3 ft. into equation (5) we get— M at the centre Z20 , 24 64 625 12 . 16 = W ( 4 + 8 - 2-4 " -6- " 10 ~ 2- + 2 + 645) ftlb. = w (16 - 16—|) ft. lb. = — ft. lb. 3 Substituting the same values of x, I, a, and b into equation (4), we get— Z20 .24 64 6 25 \ = w ( - + - - - - \ ft. lb. \ 4 8 24 6 / /120 + 72 - 64 - 25\ .. = W (—^4-------------------)ftlb- Now assuming the height of the loose rock to be 8 ft., we get— w = 150 x 8 x 3 lb. per ft. = 3,600 lb. per ft. g giving the bending moment at the centre = — x 3,600 = 9,600 ft. lb. Now M = where / is the fibre stress at a sec- y tion distance y from the neutral axis. Therefore the maximum fibre stress produced by . K 9,600 x 12 x 2 bending at the centre is given by -2-------— --------- 32 ]b. per sq. in. = 7,200 lb. per sq. in. The ultimate fibre stress due to transverse load is 4,000 lb. per sq. in. Therefore the beam is unable to support the load. What happens in practice is as follows: First of all the 18 in. to 2 ft. of rock loosens, the weight of