When the operator pulls down on the retracting lever, the force transmitted through the control cable lifts the end of the retracting lever that is attached to the return spring and tie rod. The retracting lever has a pivot point on the block mounting of the valve. As the one end of the retracting lever is lifted, the end connected to the plunger pushes down on the plunger and valve stem, allowing fluid flow through the valve from the accumulator or fluid cooler to the engine cylinder, thus forcing the ram and crosshead back to their battery position. After retraction is complete, the retracting lever is released and the return spring pulls down on the retracting lever, which in turn pulls up on the plunger and valve stem, which closes the valve. See figure 3-7. The ideal condition is that tension be kept on the purchase cable from the beginning of the retracting stroke until the ram is in its battery position. An interruption of the stroke generally disrupts this condition and creates cable backlash, which results in cable slack on the engine. If   an   emergency   arises   involving   the   safety   of personnel    or    equipment,    and    an    interruption    of full-speed retraction is necessary, the following procedures   are   recommended   to   prevent   possible damage, such as a tight kink, to the purchase cable: 1. Resume retracting very slowly at first to rid the cable system of slack. 2. Resume   full-speed   retraction   only   after   the cable slack has been eliminated and the cable has tension. 3. Inspect   sheave   damper   sheaves   for   proper seating of the purchase cable on completion of retraction. A shock absorber like the one found on automobiles   is   installed   on   the   operating   lever   to eliminate   chattering   of   the   retracting   valve   during closing. ACCUMULATOR SYSTEM LEARNING   OBJECTIVE:    Describe    the accumulator system. The    Mk    7    Mod    3    arresting    engine    has    a recirculating-type   hydraulic   system.   During   arrest- ment, the hydraulic fluid is forced from the main engine cylinder,  through  the  CRO  valve,  to  the  accumulator. An  initial  air  charge  of  400  psi  in  the  accumulator builds up to approximately 650 psi during arrestment. This increased pressure is used to force the fluid from the accumulator into the fluid cooler, thus forcing fluid from  the  previous  arrestment,  already  cooled  by  the cooler,  out  of  the  cooler,  through  the  retracting  valve, and into the main engine cylinder, returning the engine to its BATTERY position. The accumulator (fig. 3-9) is a long, steel cylinder mounted    horizontally    in    saddles    on    the    engine structure, with the fluid end toward the fixed sheaves. Inside   the   accumulator   is   a   floating   piston   that separates the air side of the accumulator from the fluid side.  The  air  end  of  the  accumulator  is  flanged  and 3-12 ROD NOZZLE SHAFT FLUID INDICATOR ROD PISTON RETAINER RING SLIPPER AND CAGE ASSEMBLY CYLINDER A STOP LEVEL INDICATOR PLATE ABEf0309 Figure 3-9.—The Mk 7 accumulator.