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FORMER GERMAN SUBMARINE TYPE XXI |
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HYDRAULIC POWER |
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SUMMARY |
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| The number of uses for hydraulic power have been greatly increased on this type of vessel, but the physical size and capacity of the system have been increased very little beyond those of the system on the type IX vessels, which were used solely for periscope hoists. Use of compressed air for snorkel hoist supersedes oil in this type of vessel. | |||||
| The system installed is unnecessarily complicated. A simple system would probably have been more reliable. Experience of American crews has confirmed the drawbacks presented in Navtechmiseu report 305-45. | |||||
| The light, intermittent duty motors and the pressure switch are of interest, although they are not in all respects desirable. Further exploitation of these features is recommended. | |||||
March, 1946 |
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PORTSMOUTH NAVAL SHIPYARD, PORTSMOUTH, N. H. |
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HYDRAULIC POWER |
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| The hydraulic system is used to operate the periscope hoist, bow and stern planes, rudder, torpedo tube muzzle doors, bow plane rigging mechanism and machine gun turret training motors. The last-named four are new types not found on other types of vessel. It is not used for snorkel hoisting. | |||||
| The system consists of two air flasks, two oil flasks, one main and one standby oil pump, together with related piping, tanks and controls. | |||||
| The system operates at pressures from 55 to 80 atmospheres (782 to 1138 psi), which is a smaller range of pressures than that accepted on earlier types of vessels. The pumps cut in at or before the lower limit is reached, and shut down when the upper limit is reached. | |||||
| The volume of each air flask is 325 liters. Total air capacity of the fully charged system is 650 liters (23.0 cu. ft.), considerably less than the capacity on the type X vessels, and only slightly greater than that on the type IX vessels. | |||||
| The volume of each oil flask is 200 liters, and the total effective oil in the flasks is 325 liters (11.5 cu. ft.), the remainder of the space being occupied by residual oil and by air entrapped at the top of the oil flask when the system is fully charged. This is greater than the capacity of the system on type IX vessels, but less than the volume of oil on type X vessels. Inside each oil flask, within a guide frame, is a float with a stopper at each end. When the system is completely charged, the upper stopper closes the air opening into the oil flask, and when the system is completely discharged, the lower stopper closes the oil outlet. This single float serves the same purpose as the two floats described in the section on type X vessels. | |||||
| Piping connections to the flasks are as follows: | |||||
| a) To air flasks - air piping from the high pressure air manifold to each flask in parallel, and between one flask and the other at the opposite end. | |||||
| b) To oil flasks - air piping in parallel with the air flasks to the tops of the oil flasks in parallel. Oil supply and discharge lines to the bottom of the flasks in parallel. Oil drain lines and test lines individually to the bottom of each flask. One of the test lines is connected to the two control valves in parallel. The supply line to one of the | |||||
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| flasks is connected to the two control valves in parallel. | |||||
| The main and standby pumps are both worm pumps (IMO type) with one drive shaft and two idlers, each operated by a motor rated at 9.6 to 29.5 kw dependent on the voltage, which can vary from 110 to 170 volts. The pumps and motors operate at 1470 to 1920 rpm, dependent on voltage and pressure head. Each is able to supply 100 liters per minute (3.5 cu. ft.) at 80 atmospheres (1138 psi). Motors are rated on an intermittent service basis (50% is the phrase used in the special specifications.) | |||||
| The hydraulic oil collecting tank has a capacity of 500 liters (17.7 cu. ft.) and the related supply tank has a capacity of 120 liters (4.25 cu. ft.). | |||||
| The system is placed in operation by first completely emptying the flasks, then charging the flasks to 53 atmospheres (753 psi) of air. Oil is then pumped into the oil flasks by means of the hand pump until a pressure of 55 atmospheres is obtained, where the oil level is supposed to have reached the level of the test line. After sampling the oil level, the power switch for the hydraulic pump motor is closed, and the pressure in the system is raised to 80 atmospheres (1138 psi) by pumping oil into the oil flasks. When the floats in the flasks reach the upper end of their travel, they close off the air connections, after which the pressure in the flasks increases rapidly. This pressure operates on the oil-air piston of the automatic control valve to open the power circuit and stop the pump motor. | |||||
| The two switches are adjusted so that, as pressure in the system decreases, one pump is started when the pressure reaches 57 atmospheres (810 psi) and the other is started only if the pressure decreases to 55 atmospheres (782 psi). When it is once started, the second pump, like the first, continues pumping until the pressure in the system reaches 1138 psi. Switching arrangements provided permit interchange of connections between the two pressure switches and the two pump motors. | |||||
| The pressure switches are of the same type found on the type XB U-234, with two cylinders each, one of which is an oil cylinder with a spring loaded piston, and the other of which is an air-oil cylinder with air pressure on one side | |||||
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| of the piston balanced against oil pressure on the other side. When the pressure in the system drops to the established point, the spring tension overcomes the oil pressure in one cylinder and moves the piston rod, thereby closing a solenoid switch and starting the related pump motor. As the pressure in the system increases, the oil pressure overcomes the spring loading and restores the piston to its compressed-spring position without, however, affecting the switch. The switch is opened when, after the system is charged and the air inlet to the oil flask has been closed by the float, the oil pressure exceeds the air pressure acting on the piston in the air-oil cylinder or the pressure switch, thereby moving the piston and shutting down the pump motor. | |||||
| The control described departs from that described in the specifications, which calls for putting the second pump on the line under the control of a float valve when the residual oil in the flask begins to be used. | |||||
| The use of two pumps and dual switches with separate settings, and the reduction in size of the flask components, together with the reduction in pump and motor sizes, indicates a desire to provide a more compact system than the one previously available. | |||||
| Materials are steel for flasks, piping and pipe connections. Copper lining for piping has not been noted. The special specifications state that the most recent materials - substitution lists are to be employed in determining materials. Further, the same specifications require a 135 atmosphere test on the flasks, but the flasks on the vessel are marked to show a flask test pressure of 120 kg/cm2 (1710psi). | |||||
| The system is, in essence, relatively simple, but the installation has been unnecessarily complicated by a multiplicity of drain lines, vent lines, and pilot lines, complete with valves, few of which appear to serve any essential purpose. | |||||
| Comments: | |||||
| Except for a correction of the system pressures, the description and comments in Navtechmiseu Report No. 305-45, "Hydraulic Systems on German Submarines", apply without qualification, i.e.; the system as installed is over-engineered and unsuited to the type of vessel on which it is installed. The pressure switch is of interest, but as has been mentioned in the type X report, it is subject to certain | |||||
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| disabilities because of its design. | |||||
| The motors, as has been noted, are rated on an intermittent service basis, and are correspondingly small and light. No exact weights are currently available, so no weight per horse power has been calculated. | |||||
| Experience with hydraulic controls on U-2513 has paralleled that on U-234. Neither vessel has considered the control switch reliable. Further, experience with U-2513 has borne out the point raised in the Navtechmiseu report that trouble could be expected from leaks through external piping and pistons, in that salt water contamination of the system has occurred. | |||||
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PLATE VI |
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FINAL DESIGN OF TYPE XXI HYDRAULIC PLANT |
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