Page:EB1911 - Volume 14.djvu/106

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[WATER MOTORS
HYDRAULICS

d=diameter of ram in ft.; p=water-pressure in pounds per sq. ft.; k a coefficient. F=kpd k=O~OO393 if the leathers are new or badly lubricated; =o-00262 if the leathers are in good condition and well lubricated. Since the total pressure on the ram is P==i1rd2p, the fraction of the total pressure expended in overcoming the friction of the leathers is F/P= -oo5/d to -0033/d, d being in feet. Let H be the height of the pressure column measured from the free surface of the supply reservoir to the bottom of the ram in its lowest position, Hy, the height from the discharge reservoir to the same point, h the height of the ram above its lowest point at any moment, S the length of stroke, S2 the area of the ram, W the weight of cage, R the weight of ram, B the weight of balance weight, 'w the weight of balance chain per foot run, F the friction of the cup leather and slides. Then, neglecting fluid friction, if the ram is rising the accelerating force is P, =G(H-h)SZ-R-W+B-'w(S-h) -I-wh-F, and if the ram is descending P2= -G(H;, -h)SZ+W-I-R-B+'w(S-h)-wh-F. If ~w=% GS), P1 and P2 are constant throughout the stroke; and the moving force in ascending and descending is the same, if B =W-I-R+wS-Gf2(l-I-l-Hb)/2. Using the values just found for 'w and B, P1 = P2 = § G§ Z(H-Hb) -F. Let W+R+wS+B=U, and let P be the constant accelerating force acting on the system, then the acceleration is (P/U)g The velocity at the end of the stroke is (assuming the friction to be constant) v=~J (2P€5/U): and the mean velocity of ascent is év. § 177. Armstrong? Hydraulic J igger.-This is simply a single acting hydraulic cylinder and ram, provided with sheaves so as to give motion to a wire rope or chain. It is used in various forms of lift and crane. Fig. 172 shows the arrangement. A hydraulic ram or plunger B works in a stationary cylinder A. Ram and cylinder ®< carry sets of sheaves over which passes a chain or rope, fixed at one end to the / cylinder, and at the other connected over @ ”

(°/ guide pulleys to a lift or crane. For each pair of pulleys, one on the cylinder and one on the ram, the movement of the free end of the rope is doubled compared with that of the ram. With three pairs of pulleys the free end of the rope has a movement equal to six times the stroke of the ram, the force exerted being in the inverse proportion. - § 178. Rotalive Hydraulic Engines.-Valvegear mechanism similar in principle to that of steam engines can be applied to actuate the admission and discharge valves, and the pressure engine is then converted into a continuously-acting motor. 