The Mk 7 arresting engines have an 18:1 reeve ratio, which means for every foot of ram travel there are 18 feet of purchase cable payout. The number of sheaves on the crosshead determines the reeve ratio. The types of reeves used on the Mk 7 arresting engines are 18:1 single reeve on pendant engines, and 18:1 endless reeve on barricade engines. The endless reeve barricade engines use only one purchase cable with the two bitter ends terminating on the flight deck and connected to the barricade. The single-reeved pendant engines require two purchase cables, one reeved around the 28-inch pitch diameter sheaves and one reeved around the 33-inch pitch diameter sheaves of the fixed sheave and crosshead assemblies. FAIRLEAD SYSTEM SHEAVES Three types of sheaves are used in Mk 7 arresting gear.  They  are  the  horizontally  mounted  retractable sheave,   the   vertical   through-deck   sheave,   and   the fairlead   sheave.   All   these   sheaves   are   designed   to accommodate 1 7/16-inch diameter purchase cable and are made of forged aluminum alloy. A  typical  sheave  includes  a  base  and  cover  that retains   the   sheave   assembly.   See   figure   3-27.   Two grease fittings provide access for lubricating the sheave bearing  and  spacers.  The  sheave  assembly  has  three races: an inner race, a bearing race, and an outer race. Mounted  with  each  sheave  is  a  two-piece  concentric spacer.  The  inner  spacer  is  made  of  steel  and  is  the lubricant    distributor.    The    outer    spacer,    made    of phenolic,  is  bonded  to  the  sheave,  provides  a  bearing surface, and is a lubricant retainer for the sheave. The horizontal and vertical sheaves are identical except for their mounting arrangement. The function of a retractable sheave is to provide a means  of  lowering  deck  sheaves  that  would  interfere with the passage of aircraft and deck equipment when in  the  raised  operating  position.  Figure  3-28  is  an example of a retractable sheave installation. Each  retractable  sheave  is  operated  by  an  electric motor   unit   controlled   by   a   deckedge   push-button station.  In  addition,  an  indicator  light  box  is  installed adjacent  to  the  deckedge  push-button  station  to  show the  position  of  the  sheave—a  green  light  when  the sheave is fully raised, or a red light in all positions other than fully raised. The retractable sheaves may also be operated by means of handwheels in case of emergency. The handwheel is located below decks on the   operating   unit.   To   eliminate   the   chance   of   the retractable  sheave  being  lowered  inadvertently  during landing operations, the handwheel is removed from the unit whenever it is not actually being used. The  retractable  sheave  operating  unit  is  bolted  to the  bottom  of  the  retractable  sheave  assembly.  It  is accessible for maintenance and manual operation from the  compartment  that  is  directly  below  the  retractable sheave. The retractable sheave operating unit is a self-contained unit consisting of a high-torque electric motor,  a  geared  drive  system,  and  limit  switches.  See figure 3-29. The motor is coupled to the wormshaft, which has a worm splined to the shaft. The worm engages the worm gear, which is free to rotate on the sleeve. There are two lugs on the back of the worm gear that, after some free rotation,  engage  two  lugs  on  the  back  of  the  clutch bevel  gear.  The  free  rotation  is  to  prevent  putting  an immediate load on the electric motor. The clutch bevel gear is splined to the sleeve, so that any rotation of the clutch bevel gear rotates the sleeve. 3-28 ABEf0327 COVER INSERT INNER SPACER INNER RACE OUTER SPACER (BONDED) SHEAVE INNER SPACER GREASE FITTINGS BASE BEARING RACE Figure 3-27.—Typical sheave assembly.