520 HYDROMECHANICS [HYDRAULICS. 162. Action of Water in a Water Motor. Water motors may be divided iuto water-pressure engines, water wheels, and turbines. Water-pressure engines are machines with a cylinder and piston or ram, in principle identic il with the corresponding part of a steam engine. The water is alcernately admitted to and discharged from the cylinder, causing a reciprocating action of the piston or ram. It is admitted at a high pressure and discharged at a low one, and consequently work is done ou the piston. The water in these machines never acquires a high velocity, and for the most part the kinetic energy of the water is wasted. The useful work is due to the difference of the pressure of admission and discharge, whether that pressure is due to the weight of a columh. of water of more or less considerable height, or is artificially produced in ways to be described presently. Water wheels are large vertical wheels driven by water falling from a higher to a lower level. In most water wheels, the water acts directly by its weight loading one side of the wheel and so causing rotation. But in all water wheels a portion and in some a considerable portion of the work due to gravity is first employed to generate kinetic energy in the water ; during its action on the water wheel the velocity of the water diminishes, and the wheel 13 therefore in part driven by the impulse due to the change of the water s momentum. Water wheels are therefore motors on which the water acts, partly by weight, partly by impulse. Turbines are wheels, generally of small size compared with water wheels, driven chieHy by the impulse of the water. Before entering the moving part of the turbine, the water is allowed to acquire a considerable velocity ; during its action on the turbine this velocity is diminished, and the impulse due to the change of momentum drives the turbine. Roughly speaking, the fluid acts in a water-pressure engine directly by its pressure, in a water wheel chiefly by its weight causing a pressure, but in part by its kinetic energy, and in a turbine chiefly by its kinetic energy, which again causes a pressure. Water-Pressure Engines. 163. In these water acts by simple pressure due to the height of the column in the supply pipe or the pressure in the supply reservoir. The water acts on a piston or ram which it displaces. When the height of the column exceeds 100 or 200 feet, or there is a pressure equivalent to this, water wheels are inapplicable, and turbines have the disadvantage that in such circumstances their speed is very great. Then water-pressure engines maybe very conveniently adopted. In other cases they are generally too cumbrous. When an incompressible fluid such as water is used to actuate piston engines, two special difficulties arise. One is that the waste of work in friction is very great, if the water attains considerable velocity; another is that there is great straining action on the machinery. The violent straining action due to the more or less sudden arrest of the motion of water in machinery is termed hydraulic shock. For these reasons the maximum velocity of flow of water in hydraulic machines should generally not exceed 5 to 10 feet per second. Under very high pressure, where there is less object in economizing energy, and it is very im portant to keep the dimensions of the machinery small, Mr Anderson gives 24 feet per second as the limiting velocity. In large water-pressure engines used for pump ing mines the average piston speed does not exceed to 2 feet per second. Direct-Acting Hydraulic Lift (fig. 174). This is the simplest of all kinds of hydraulic motor. A cage W is lifted directly by water pressure acting in a cylinder C, the icugui uj. vniL^ii 10 a jitLir; gicaLci iuu.li iuu int. A rtuu or plunger R of the same length e*-elof -Supply is attached to the cage. The ^==3B?5? water pressure admitted by a cock to the cylinder forces up /^~^~^-^ the ram, and when the supply ^-^.-^- _^3L_p ^ -s >Y/, valve is closed and the dis- if""^^ - 1 r! ?^-- //, charge valve opened, the ram ;r~~ i descends. In this case the i ram is 9 inches diameter, with ; It a stroke of 49 feet. It con
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sists of lengths of wrought- : ^T ^ iron pipe screwed together per i fectly water-tight, the lower end i /*" ^ i being closed by a cast-iron
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plug. The ram works in a , 1 1 cylinder 11 inches diameter, ; i of 9 feet lengths of flanged , i i o o ( ^~~^ cast-iron pipe. The ram passes t water-tight through the cylin W der cover, which is provided i with double hat leathers to R ^s prevent leakage outwards or i inwards. As the weight of <"
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the ram and cage is much : i more than sufficient to cause a i descent of the cage, part of < i the weight is balanced. A Ja l chain attached to the cage < w passes over a pulley at the i top of the lift, and carries at , , % its free end a balance weight m B, working in T iron guides. W Water is admitted to the 1 r L w cylinder from a 4-inch supply w pipe through a two-way slide, | w H worked by a rack, spindle, w and endless rope. The lift g" JJ w/ works under 73 feet of head, ._,;:[ w and lifts 1350 ft at 2 feet jg> , N My/ per second. The efficiency is ij ,. . w/ given by Mr Anderson at 75 ^>^ ; - to 80 per cent. 1 i The principal pre &Jm judicial resistance to evel of fijiif- ^ w 1 1 the motion of a ram ----== ffila r fflw ^ of this kind is the ^T^ ^H< " vtffi / friction of the cup Dvafvarge. ^J|| ^^ X; leathers, which make i -1^11 i the joint between > ? 1fflll 1 the cylinder and $$!* ram. Some experi- fyjib
ments by Mr John i$Pli- Hick give for the $8lP " friction of these ^-vMsm c
leathers the follow- ^JB j ing formula. Let %,$& F= the total fric- ^W? tion in pounds ; rf = . ff ?$j$jt diameter of ram in ty/JIm feet ; p= water ] ires- ^tyitm^ sure in pounds per / /li square foot; k a ^jll coefficient. i ^Ml ~F = kpd j- ^ffli^ " 1 fc= 0-00393 if the * jjjm leathers are new i ^^* or badly lubri- M cated ; = 0-00262 if the { .tijml LLteM 1 * leathers are in . *$$&&& r^S-jr" 7 good condition WK "L and well lubri- ; ;?-^f^ ^ - cated. Fig. 174. 1 The drawing and description of this ram are taken from Mr
Anderson s Chatham Lecture* on Hydraulic Machinery.
