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WATER MOTORS]
93
HYDRAULICS


mains very high pressures are adopted, generally 700 lb per sq. in. or 1600 ft. of head or more.

In a large number of hydraulic machines worked by water at high pressure, especially lifting machines, the motor consists of a direct, single acting ram and cylinder. In a few cases double acting pistons and cylinders are used; but they involve a water-tight packing of the piston not easily accessible. In some cases pressure engines are used to obtain rotative movement, and then two double-acting cylinders or three single-acting cylinders are used, driving a crank shaft. Some doub1e»acting cylinders have a piston rod half the area of the piston. The pressure water acts continuously on the annular area in front of the piston. During the forward stroke the pressure on the front of the piston balances half the pressure on the back. During the return stroke the pressure on the front is unopposed. The water in front of the piston is not exhausted, but returns to the supply pipe. As the frictional losses in a fluid are independent of the pressure, and the work done increases directly as the pressure, the percentage loss decreases for given velocities of flow as the pressure increases. Hence for high-pressure machines somewhat greater velocities are permitted in the passages than for low-pressure machines. In supply mains the velocity is from 3 to 6 ft. per second, in valve passages 5 to ro ft. per second, or in extreme cases zo ft. per second, where there is less object in economizing energy. As the water is incompressible, slide valves must have neither lap nor lead, and piston valves are preferable to ordinary slide valves. To prevent injurious compression from exhaust valves closing too soon in rotative engines with a fixed stroke, small self-acting relief valves are fitted to the cylinder ends, opening outwards against the pressure into the valve chest. Imprisoned water can then escape without over straining the machines.

In direct single-acting lift machines, in which the stroke is fixed, and in rotative machines at constant speed it is obvious that the cylinder must be filled at each stroke irrespective of the amount of work t.o be done. The same amount of water is used whether much or little work is done, or whether great or small weights are lifted. Hence while pressure engines are very efficient at full load, their efficiency decreases as the load decreases. Various arrangements have been adopted to diminish this defect in engines working with a variable load. In lifting machinery there is sometimes a double ram, a hollow ram enclosing a solid ram. By simple arrangements the solid ram only is used for small loads, but for large loads the hollow ram is locked to the solid ram, and the two act as a ram of larger area. In rotative engines the case is more difficult. In Hastie's and Rigg's engines the stroke is automatically varied with the load, increasing when the load is large and decreasing when it is small. But such engines are complicated and have not achieved much success. Where pressure engines are used simplicity is generally a first consideration, and economy is of less importance. § 175. Ejiciency of Pressure Engines.—It is hardly possible to form a theoretical expression for the efficiency of pressure engines, but some general considerations are useful. Consider the case of a long stroke hydraulic ram, which has a fairly constant velocity v during the stroke, and valves which are fairly wide open during most of the stroke. Let r be the ratio of area of ram to area of valve passage, a ratio which may vary in ordinary cases from 4 to 12. Then the loss in shock of the water entering the cylinder will be (r-I)”'v2/2g in ft. of head. The friction in the supply pipe is also proportional to rf. The energy carried away in exhaust will be proportional to 112. Hence the total hydraulic losses may be taken to be approximately tu?/2g ft., where § ' is a coefficient depending on the proportions of the machine. Let f be the friction of the ram packin and mechanism reckoned in Tb per sq. ft. of ram area. Then ig the supply-pipe pressure driving the machine is p lb per sq. ft., the effective working pressure will be

p-C5112/2g-f lb per sq. ft.

Let A be the area of the ram in sq. ft., v its velocity in ft. pa' sec. The useful work done will be

(p-Cya'/ag-f)At' ft. Tb per sec.,

and the efficiency of the machine will be 11 = (P 'G?1'2/22'°f)/P-This

shows that the efficiency increases with the pressure p, and diminishes with the speed v, other things being the same. If in regulating the engine for varying load the pressure is throttled, part of the available head is destroyed at the throttle valve, and p in the bracket above is reduced. without intermediate gearing, may

have an efficiency of 95 % during the working stroke. If a hydraulic jigger is used with roipes and sheaves to change Direct-acting hydraulic lifts,

4

the speed o the ram to the speed of the lift, the efficiency may be only 1 .

/

f

Level of Supply

I

50 %. E. B. Ellington has given the Efficiency t cg Jiftis witlgh hydraulic, V 3 HHCC 3 Uflfl 6 WOT 1I'l

stroke. Large pressurg engines havi a n efficiency of 85 %, but small rota- ; r / tive engines probably not more than I, i Z I 50 % and that only when fully loaded.:fi I 1 Z } 512 WE 3 /

§ 176. Direct-Acting Hydraulic, QQ: Z Lift (fig. 17r>. This is the Z f simplest of all kinds of hydraulic I Z § motor. A cage W is lifted directly I ' / 2 by water pressure acting in a Z:

cylinder C, the length of which is Z I a little greater than the lift. A ' Z ram or plunger R of the same l Z

length is attached to the cage. I The water-pressure admitted by a Z Q cock to the cylinder forces up the 'I ' Z: ram, and when the supply valve is H Z I closed and the discharge valve 'i ' Z: opened, the ram descends. In 5 » this case the ram is 9 in. diameter, 1 » Z I with a stroke of 49 ft. It consists Z § of lengths of wrought-iron pipe I ° Z } screwed together perfectly water- —% Q tight, the lower end being closed r %: by a. cast-iron plug. The ram % »

works in a cylinder II in. dia- rg % meter of 9 ft. lengths of Banged % cast-iron pipe. The ram passes, I Z % water-tight through the cylinder E, % I cover, which is provided with %

double hat leathers to prevent I: % If leakage outwards or inwards. As E, % I the weight of the ram and ca ' 5: - l% 1 ge is E %:

much more than sufficient to cause B, , N % a descent of the cage, part of the UFQ % weight is balanced. A chain at- % I tached to the cage passes over a ' 7 "Z % }“ pulley at the top of Z:

the lift, and carries 1% AW, n

at its free end a L A' 'E' ' 1

balance weight B, Disgymrge :

working in T iron i Q, 4 4* I

guides. Water is ad-:, , W :

mitted to the cylinder F it:N

from a 4-in. supply i Y, Ee ,

pipe through a two- 2. 1 2

way slide, worked by { C, Q 1

a rack, spindle and: 2.endless rope. The:

lift works under 73 Hb f r E ” 1 Q ft. of head, and lifts i 3/ Q

1350 Tb at 2 ft. per l 5* Q

second. The efh- } f iciency

is from 75 to 2 s

80%.: 2 /1 1

ll ° ' - ' :ff

jucrhciial pfeggghceprfto i, =

the motion of a ram I fi, "Iliff

of this kind is the fric- ! ...-..— / f. .'; .. 'C. tion of the cup leathers, '; wévhich makge therjoént

8 I1

anilweifhmf eSoC1iidne;€ FIG' 171° I pediments by John Hick give for the friction of these leathers

the following formula. Let F= the total friction in pounds;