operates the pump as well as the advantage due to the relative sizes of the pump and the ram. FIG. 75. Example: If the ram of Fig. 75 is 3 in. in diameter and the pump is 1 in. in diameter, while the lever is 15 in. long and is connected to the pump at a distance of 1 in. from the fulcrum, what is the mechanical advantage of the entire jack? Explanation: The areas of the ram and pumps are as 9:1, hence their mechanical advantage is 9. The lever has a mechanical advantage of 10. Hence, that of the whole jack is 9X10-90, and a force applied at the end of the lever would be multiplied 90 times. This force would, however, move through a distance 90 times as great as the distance the load would be raised. The hydraulic jack has usually an efficiency of over 70% and is, therefore, a much more efficient lifting device than the jack screw. A mixture containing one-third alcohol and two-thirds water should be used in jacks. The alcohol' is added to prevent freezing. 119. Hydraulic Machinery.—In the shop, we often find water pressure used to operate presses, punches, shears, riveters, hoists, and sometimes elevators. These machines are seldom operated by hand power but have water supplied under pressure from a central pumping plant. The admission of the water and the consequent motion of the machine is controlled by hand operated valves. Most of these hydraulic machines are used where tremendous forces are required. Therefore, very high water pressures are used, occasionally as high as 3000 lb. per square inch. 1500 lb. per square inch is a common working pressure for hydraulic machines. Fig. 76 shows a press operated by hydraulic pressure. It will be noticed that the movable head is connected to two pistonsa large one for doing the work on the down stroke, and a smaller one above, used only for the idle or return stroke of the press. 120. Hydraulic Heads. Quite often we use a high tower or tank to secure a water pressure, or we make use of some natural source of water which is at some elevation. This is most often seen in the water supplies for towns and cities. Water tanks are put upon the roofs of some factories for the same purpose. The higher the tank, the greater will be the pressure which it will maintain in the system. Let Fig. 77 represent such a system. The water at the bottom has the weight of a column of water h ft. high to support and, consequently, will be under a pressure equal to the weight of this column of water. A volume of water 1 in. square and 1 ft. high weighs .434 lb., so the pressure per square inch at the base of the column in Fig. 77 will be .434×h. Notice particularly that the shape and size of the tank has no influence on the pressure, it being used merely for storage and to keep the pressure from falling too fast if the water is drawn off. The water in the tank on either side of the outlet is supported by the bottom of the tank and has no effect on the pressure in the pipe. The pressure at the bottom of the pipe would be the same if the pipe alone extended up to the height h without the tank. Also the size of the pipe has no effect on the pressure per square inch. The water in a large pipe will weigh more than in a small pipe, but the pressure will be spread over a larger area and if, the heights are the same, the pressure per square inch will be the same. In pumping water to an elevated tank or reservoir, the pressure required per square inch is also determined in the same manner and is .434 times the height to which the water is raised, plus an allowance for friction in the pipes. Thus, to pump water to a height of 100 ft. requires a pressure somewhat greater than .434 X 100 43.4 lb. per square inch. 121. Steam and Air.-Steam and air are likewise used to produce pressures in shop machinery. Being more elastic than water, they are preferred where the machines are to be operated quickly. Devices using air are called "pneumatic appliances," among the most common of which are pneumatic drills, hammers, and hoists. The air for operating these is supplied by air compressors located in the power house. These take the air from out of doors and compress it into a smaller volume. The resistance of the air to this compression causes it, in its effort to escape, to exert a pressure on the walls of the tank or pipe containing it. The more the air is compressed, the greater is the pressure exerted by it. The air pressure used in shop work is usually about 80 lb. per square inch. The air is conducted through pipes and hose to the point where it is to be used and there allowed to exert its pressure on the piston of the appliance which is to be driven. PROBLEMS 201. The specific gravity of Lignum Vitæ (a hard wood) is 1.328. Will this wood float or will it sink in water? 202. What weight on the small piston of Fig. 74 would support a weight of 30,000 lb. on the large piston if the small piston is 1 in. in diameter and the large one 12 in. in diameter? 203. If a hydraulic press works with a water pressure of 1500 lb. per square inch, what must be the diameter of the ram if a total pressure of 75,000 lb. is to be produced? 204. If the air hoist of Fig. 78 has a cylinder 10 in. in diameter inside, and the piston rod is 1 in. in diameter, and an air pressure of 80 lb. per square inch is exerted on the bottom of the piston, what weight can be lifted by the hoist? 205. If a city wishes to maintain a water pressure of 80 lb. per square inch at their hydrants, how high above the streets must be the water level in the stand pipe? 206. Fig. 79 shows the principle of one form of hydraulic elevator, the car being fastened directly to a long ram which is raised by water pressure. The weight of the ram and car are partially balanced by a counterweight. If this elevator is operated with water from the city mains at 80 lb. pressure per square inch and the ram has a diameter of 10 in., what load can be lifted allowing 30% for losses in friction, etc.? AAA FIG. 80. FIG. 81. 207. If a steam pump, such as shown in Fig. 80, has a 12 in. steam piston and an 8 in. water piston, what water pressure can be produced with a steam pressure of 90 lb. per square inch? How many gallons would be pumped per minute when the pump is running at 100 strokes per minute, the stroke of the pump being 12 in.? (1 gallon=231 cu. in.) 208. What water pressure must a pump be capable of producing in order to force the water to a reservoir at an elevation of 300 ft. above the pump? |