Even  though  certain  oils  may  be  satisfactorily purified at operating temperatures, a greater degree of purification generally results if the oil is heated to a higher temperature. To do this, the oil is passed through a heater where the proper temperature is obtained before the oil enters the purifier bowl. Oils used in naval ships maybe heated to specified temperatures  without  adverse  effects.  However, prolonged  heating  at  higher  temperatures  is  not recommended because of the tendency of such oils to oxidize.   Oxidation   results   in   rapid   deterioration. Generally,   heat   oil   to   produce   a   viscosity   of approximately 90 seconds Saybolt universal (90 SSU). You should NEVER increase the pressure above normal to force a high-viscosity oil through the purifier. Instead,  decrease  the  viscosity  by  heating  the  oil.  Using excess pressure to force oil through the purifier results in less efficient purification. On the other hand, reducing the pressure at which the oil is forced into the purifier increases the length of time the oil is under the influence of centrifugal force and results in improved purification. To make sure that the oil discharged from a purifier is free of water, dirt, and sludge, you need to use the proper size discharge ring (ring dam). The size of the discharge ring depends on the specific gravity of the oil being purified. All discharge rings have the same outside diameter;  but,  they  have  inside  diameters  of  different sizes. The information in this TRAMAN on purifiers is general, and it applies to both types of purifiers. Before you  operate  a  specific  purifier,  refer  to  the  specific operating procedures contained in the instructions that come with the unit. ELECTROHYDRAULIC DRIVE MACHINERY Hydraulic units drive or control steering gears, windlasses,  winches,  capstans,  airplane  cranes, ammunition hoists, and distant control valves. In this part of the chapter, you will learn about some of the hydraulic  units  that  will  concern  you. The electrohydraulic type of drive operates several different kinds of machinery better than other types of drives.   Here   are   some   of   the   advantages   of electrohydraulic   machinery. l Tubing, which can readily transmit fluids around corners, conducts the liquid which transmits the force.  Tubing  requires  very  little  space. l l l The  machinery  operates  at  variable  speeds. Operating  speed  can  be  closely  controlled  from minimum to maximum limits. The controls can be shifted from no load to full load  rapidly  without  damage  to  machinery. ELECTROHYDRAULIC   SPEED GEAR An  electrohydraulic  speed  gear  is  frequently  used in electrohydraulic applications. Different variations of the  basic  design  are  used  for  specific  applications,  but the operating principles remain the same. Basically, the unit consists of an electric motor-driven hydraulic pump (A-end)  and  a  hydraulic  motor  (B-end). The B-end (fig. 10-37) is already on stroke and is rotated by the hydraulic force of the oil acting on the pistons. Movement of the pistons’ A-end is controlled by a tilt box (also called a swash plate) in which the socket ring is mounted, as shown in part A of figure 10-37. The length of piston movement, one way or the other, is controlled by movement of the tilt box and by the amount of angle at which the tilt box is placed. The length  of  the  piston  movement  controls  the  amount  of fluid flow. When the drive motor is energized, the A-end is always in motion. However, with the tilt box in a neutral or vertical position, there is no reciprocating motion of the pistons. Therefore, no oil is pumped to the B-end. Any movement of the tilt box, no matter how slight,  causes  pumping  action  to  start.  This  causes immediate   action   in   the   B-end   because   force   is transmitted by the hydraulic fluid. When you need reciprocating motion, such as in a steering gear, the B-end is replaced by a piston or ram. The force of the hydraulic fluid causes the movement of the piston or ram. The tilt box in the A-end is controlled locally  (as  on  the  anchor  windlass)  or  remotely  (as  on the steering gear). ELECTROHYDRAULIC   STEERING GEAR The steering gear transmits power from the steering engine to the rudder stock. The steering gear frequently includes  the  driving  engine  and  the  transmitting mechanism. Many different designs of steering gear are in use, and they all operate on the same principle. One type of 10-40