Fairlead System Sheaves
Four types of sheaves are used in Mk 7 arresting
gear. They are the horizontal deck sheave, the
vertical through-deck sheave, the fairlead sheave,
and the retractable 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 3-23. 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
Figure 3-23.–Typical sheave assembly.
retainer for the sheave. The horizontal and vertical
sheaves are identical except for their mounting
arrangement.
Retractable Sheave
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 3-24 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 3-25.
The motor (2) is coupled to the wormshaft (12),
which has a worm (15) splined to the shaft. The
worm (15) engages the worm gear (10), which is
free to rotate on the sleeve (4). There are two lugs
on the back of the worm gear (10) that, after some
free rotation, engage two lugs on the back of the
clutch bevel gear (8).
The free rotation is to
prevent putting an immediate load on the electric
motor (2). The clutch bevel gear (8) is splined to
the sleeve (4), so that any rotation of the clutch
bevel gear rotates the sleeve.
The sleeve is directly keyed to the lead screw of
the retractable sheave so that rotation of the sleeve
raises or lowers the sheave. The amount of sleeve
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