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Airflex Clutch and Gear Assembly
Chapter 9 Pumps, Valves, and Piping

Fireman - Navy Firefighter, Fireman training manual
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Some  slippage  is  necessary  for  operation  of the  hydraulic  coupling,  since  torque  is  transmitted because   of   the   principle   of   relative   motion between the two rotors. The power loss resulting from the small amount of slippage is transformed into heat that is absorbed by the oil in the system. Compared  with  mechanical  clutches,  hydraulic clutches have a number of advantages. There is no  mechanical  connection  between  the  driving  and driven elements of the hydraulic coupling. Power is transmitted through the coupling very efficiently (97   percent)   without   transmitting   torsional vibrations or load shocks from the engine to the reduction  gears.  This  arrangement  protects  the engine,  the  gears,  and  the  shaft  from  sudden shock loads that may occur as a result of piston seizure or fouling of the propeller. The power is transmitted entirely by the circulation of a driving fluid  (oil)  between  radial  passages  in  a  pair  of rotors. In addition, the assembly of the hydraulic coupling  will  allow  for  slight  misalignment. PROPELLER The  screw-type  propeller  consists  of  a  hub  and blades all spaced at equal angles about the axis. When  the  blades  are  integral  with  the  hub,  the propeller is known as a solid propeller. When the blades are separately cast and secured to the hub with studs, the propeller is known as a built-up propeller. Some of the parts of the screw propeller are identified  in  figure  8-9.  The  face  (or  pressure  face) is  the  afterside  of  the  blade  when  the  ship  is moving ahead. The back (or suction back) is the surface  opposite  the  face.  As  the  propeller  rotates, the  face  of  the  blade  increases  pressure  on  the Figure 8-9.—Propeller blade. water to move it in a positive astern movement. The overall thrust, or reaction force ahead, comes from the increased water velocity moving astern. The tip of the blade is the most distant from the hub. The root of the blade is the area where the blade joins the hub. The leading edge is the edge  that  first  cuts  the  water  when  the  ship  is going  ahead.  The  trailing  edge  (also  called  the following  edge)  is  opposite  the  leading  edge. A  rake  angle  exists  when  the  tip  of  the propeller blade is not precisely perpendicular to the axis (hub). The angle is formed by the distance between where the tip really is (forward or aft) and  where  the  tip  would  be  if  it  were  in  a perpendicular  position. A  screw  propeller  may  be  broadly  classified as  either  fixed  pitch  or  controllable  pitch.  The pitch of a fixed-pitch propeller cannot be altered during  operation.  The  pitch  of  a  controllable-pitch propeller can be changed at any time, subject to bridge  or  engine-room  control.  The  controllable- pitch propeller can reverse the direction of a ship without  requiring  a  change  of  direction  of  the drive shaft. The blades are mounted so that each one can swivel or turn on a shaft that is mounted in  the  hub  (as  shown  in  fig.  8-10). SUMMARY This chapter has provided you with some basic information   on   several   types   of   propulsion systems used on Navy ships. You should become familiar  with  the  propulsion  system  on  your  ship. Keep  in  mind,  the  propulsion  systems  are  usually a  little  different  from  ship  to  ship. Figure  8-10.—Schematic  diagram  of  a  controllable-pitch propeller. 8-8






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