The bottom of the upper lever is fitted with a roller
that bears against the flat surface of the lower lever. One
end of the lower lever has a bushed hole to receive a
pivot pin. The pivot pin passes through the lever and
through two mounting holes in the stanchion.
The bottom of the lower lever is fitted with a roller
that bears on the stem screw on top of the valve sleeve.
The vertical position of the roller on the lower lever
determines the vertical distance that the valve sleeve
may move. Thus, it controls the size of the initial
opening of the control valve.
The levers are mounted in such a way that, as the
upper lever is withdrawn, the lever arm ratio of both
levers is increased by an equal amount. When the upper
lever is fully withdrawn, the ratio of each lever is 2:1,
and the ratio through the lever system (upper and lower
levers) is 4:1. In this case the initial control valve
opening is minimum. A plunger movement of 1 inch,
acting through the upper lever, would move the lower
lever 1/2 inch; the lower lever, in turn, would move the
valve sleeve and stem 1/4 inch downward.
A critical point to consider is the position of the
levers when the valve stem is seated by cam action at
the termination of each arrestment stroke. The levers
are so mounted and adjusted that the bearing surfaces of
the levers are level when the valve is seated. When the
bearing surfaces are level, the distance across the lever
system is the same regardless of the ratio setting.
Because of this, the point of closing of the valve is
independent of the aircraft weight selector. It is a
function of the cam only; therefore, it is constant.
As the engine is retracted, the upper lever rises a
distance equal to the movement of the plunger. If the
ratio is 1:1, the valve sleeve rises the same distance. In
Figure 3-4.Sectional view of Mk 7 CROV.