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TM 55-1905-223-24-9
(5) Relief Valve. This valve diverts the vane pump discharge via the shuttle valve back into the fluid reservoir
whenever system pressure exceeds 930 psi. The pressure at which this valve opens can be adjusted if necessary as
described in paragraph 3-12.
(6) Lockvalve. The outflow from each pumpset passes through a lockvalve located in the uniblockjust before
it is piped into the port and starboard cylinders located in the uniblock. When there is no rudder order, the lockvalves
establish a bidirectional fluid lock around both pistons to prevent rudder travel. The lockvalves automatically isolate the
idle pumpset so that pump changeover is accomplished without reconfiguring the system.
(7) Manifold Outlet Filters. These filter elements are cleanable, reusable 60-mesh wire screen.
e. DARBS Valve. A three-position double acting relief/bypass/shutoff (DARBS) valve protects the system from
overpressure damage in the event the rudder strikes an underwater object.  A three-position mode selector spool
operates the valve in three ways:
(1) Relief. This is the normal operating position. Relief valves are factory set to open at 1160 psi. That way,
they protect system piping from overpressurization.
(2) Bypass. In bypass, the rudders can be mechanically centered or moved as required during installation,
removal, or maintenance.
(3) Shutoff. This position isolates the relief valves from the system so that a failure of one or both does not
disable the steering gear.
f. Hydraulic Cylinders. Port/starboard tiller arms are operated by doubleacting hydraulic cylinders. This cylinder is
closed at one end and has a piston and piston rod fitted into the other end. Fluid ports near each end drive the piston
inward or outward as required for right or left rudder movement. Spherical mounted pivot pins couple the cylinder to its
mounting pad and the piston rod to the tiller arm. O-rings limit fluid losses around the piston and the piston rod.
g. Hard-Over Stops. Rudder travel is limited to 35 degrees right or left by mechanical stops that limit the stroke of
the pistons. In each cylinder, the outer limit is reached when the piston face contacts a fixed stop inside the tube. The
inward stroke of the piston reaches a limit when a threaded collar on the outer end of the piston rod makes contact with
the outer flange of the cylinder cap. Threads on this rod allow inward or outward adjustment of this stop. When both
rudders are at the right hard-over limit, one piston is stroked to its outer limit and the other to its inward limit. Since the
outer limit is fixed, the other piston must be adjusted so that its inward limit matches exactly. The same adjustment is
made to the other piston with both rudders at the left hard-over limit.
h. Tiller Arms. A forward facing tiller arm is keyed to each rudder stock so that torque applied to the arm turns the
rudder. The arm is made of upper and lower steel plates welded to a steel block. The block is machined in two halves
that bolt together around the stock and provide a keyway for positive lock. The tapered arm has inner and outer
bushings and pivots. The inner pivot couples the arm to the actuator piston rod. The outer pivot connects both tillers to
a common tie rod (jockey bar). By joining the arms, the jockey bar makes it possible to operate the rudders with only one
cylinder in use. Single cylinder steering is necessary for cylinder replacement when at sea. Also on each tiller arm is an
attachment for the arm of the Model 306 Followup Unit. Used on Hulls 2001 through 2007.
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