Page 246 - Introduction to Marine Engineering

Steering gear 223

flexibility of different arrangements in the event of component failure.
Either pump can be used with all cylinders or with either the two port or
two starboard cylinders. Various valves must be open or closed to provide
these arrangements.
The use of a control valve block incorporating rudder shock relief
valves, pump isolating valves, ram isolating and bypass valves, offers
greater flexibility with a four-ram steering gear. In normal operation
one pump can operate all cylinders. In an emergency situation the
motor or a pair of hand pumps could be used to operate two port rams,
two starboard rams, two forward rams or two after rams.
The crosshead arrangement on the four-ram type steering gear
described incorporates what is known as the 'Rapson Slide'. This
provides a mechanical advantage which increases with the angle turned
through. The crosshead arrangement may use either a forked tiller or a
round arm tiller (Figure 12.8). The round arm tiller has a centre
crosshead which is free to slide along the tiller. Each pair of rams is
joined so as to form a double bearing in which the trunnion arms of the
crosshead are mounted. The straight line movement of the rams is thus
converted into an angular tiller movement. In the forked tiller
arrangement the ram movement is transferred to the tiller through
swivel blocks.
To charge the system with fluid it is first necessary to fill each cylinder
then replace the filling plugs and close the air cocks. The cylinder bypass
valves should be opened and the replenishing tanks filled. The air vents
on the pumps should be opened until oil discharges free of air, the
pumps set to pump and then turned by hand, releasing air at the
appropriate pair of cylinders and pumping into each pair of cylinders in

Ram
Ram

Round arm Forked
tiller "tiller
N ; >
C • •10 j • ! t V ] o
.
\J I/
(a] Round arm tiller (b) Forked tiller
Figure 12.8 Crosshead arrangements