Chapter  4—REDUCTION  GEARS  AND  RELATED  EQUIPMENT from the shafting to the hub assembly. The hub is secured to the tailshaft by flange bolts. These bolts are not designed to take torque from the tail shaft.  The  hub  to  tailshaft  joint  is  sealed  by O-rings located between the hub and the tailshaft, and between the tailshaft and the tailshaft spigot. Each blade is attached to a crank pin ring by blade bolts.  The  crank  pin  ring  fits  over  and  rotates about a center post which is physically a part of the hub body. The crank pin ring is retained in the hub by the bearing ring. The area under each blade is sealed by the blade port cover and by two O-rings in the blade seal base ring. Each blade seal base ring is spring-loaded against the under- side of the blade port cover to provide a sealing surface under all loading conditions. The blade turning mechanism in the hub con- sists of a single crosshead, attached to the end of a  piston  rod.  Several  sliding  blocks  are  fitted into the machined chambers of the crosshead. An eccentric  pin  on  the  underside  of  each  crank  pin ring fits into a hole machined into each sliding block. The hub servomotor is attached to the after end of the crosshead. A piston rod carries the lines for  the  regulating  valve  pin  which  is  attached  to the end of the valve rod. This assembly forms a passage for hydraulic power oil flow and return oil flow to and from the hub. class and DD 963 class ships, is mounted to the forward end of the main reduction gear and is flange-connected to the main reduction gear shaft coupling.  The  other  type,  used  on  the  LST 1179-1198 class ships, is called the Bird-Johnson Kamewa Unit; it is manufactured of steel, is cast in  two  sections,  and  is  line-bored  for  installation over the intermediate shaft. The OD box provides a direct hydraulic oil connection to the main pro- pulsion  shaft  and  also  translates  to  the  valve  rod in response to hydraulic control oil commands. High pressure oil from the hydraulic oil power module (HOPM) is introduced through the OD box to the internal bore of the valve rod and to the hub. The oil returns from the propeller hub to the hydraulic oil sump tank by way of the an- nulus between the valve rod and the internal bore of the shafting, through the OD box. Hydraulic System Valve  Rod  Assembly The  valve  rod  assembly  is  composed  of fabricated   sections   of   seamless   steel   tubing joined by couplings to provide a mechanical link between the oil distribution (OD) box and the hub servomotor  through  the  internal  bore  of  the propeller shaft. The valve rod assembly provides a passage for high pressure hydraulic oil from the oil distribution box to the hub. Each valve sec- tion is supported at the center of the propeller shaft bore by guides. The after end of the valve rod  assembly  supports  a  regulating  valve  pin which operates in the valve pin liner of the hub servomotor. The forward end of the valve rod assembly is mechanically linked to the OD box shaft, so that the valve rod assembly turns with the shaft. The  hydraulic  system  consists  of  a  self- contained  HOPM,  a  standby  hydraulic  pump driven by the main reduction gear, the pitch con- trol  valves  manifold  block  assembly,  and  all  the associated  connecting  piping,  fittings,  and  valves. The  hydraulic  oil  is  supplied  to  the  hydraulic  oil pumps from a separate sump tank. To maintain a static head pressure when the hydraulic system is shut down, a gravity head tank is connected to the OD box. Oil Distribution Box Presently there are two types of OD boxes being used in the Navy. One type, used on FFG-7 HYDRAULIC  OIL  POWER  MODULE.— The HOPM is located adjacent to the main reduc- tion gear. It is a RESILIENT mounted, welded structural assembly, consisting of a base plate with structural  ANGLE  bar,  flat  bar,  and  mounting plates.  The  HOPM  contains  the  major  com- ponents of the hydraulic system, including (1) either the motor-driven hydraulic screw or the vane pump, coupling, and AC motor; (2) a suc- tion  strainer  for  the  motor-driven  pump;  (3)  two 40 micron duplex discharge filters; (4) the pressure control  assembly  operating  valves,  which  consist of a pressure reducing valve, an auxiliary relief valve,  a  check  valve,  an  unloading  and  check valve, and a relief and sequence valve; (5) one 10 micron duplex control oil filter; (6) a gauge panel assembly and associated instrumentation; (7) a manual bypass valve; and (8) the interconnecting piping and fittings. 4-13