Page 232 - Introduction to Marine Engineering

212 Steering gear

be switched on and therefore a constantly running pump is required
which pumps fluid only when required. A variable delivery pump
provides this facility.

Variable delivery pumps

A number of different designs of variable delivery pump exist. Each has
a means of altering the pump stroke so that the amount of oil displaced
will vary from zero to some designed maximum value. This is achieved
by use of a floating ring, a swash plate or a slipper pad.
The radial cylinder (Hele-Shaw) pump is shown in Figure 12.1.
Within the casing a short length of shaft drives the cylinder body which
rotates around a central valve or tube arrangement and is supported at
the ends by ball bearings. The cylinder body is connected to the central
valve arrangement by ports which lead to connections at the outer casing
for the supply and delivery of oil. A number of pistons fit in the radial
cylinders and are fastened to slippers by a gudgeon pin. The slippers fit
into a track in the circular floating ring. This ring may rotate, being
supported by ball bearings, and can also move from side to side since the
bearings are mounted in guide blocks. Two spindles which pass out of
the pump casing control the movement of the ring.
The operating principle will now be described by reference to Figure
12.2. When the circular floating ring is concentric with the central valve
arrangement the pistons have no relative reciprocating motion in their
cylinders (Figure 12.2(a)). As a result no oil is pumped and the pump,
although rotating, is not delivering any fluid. If however the circular
floating ring is pulled to the right then a relative reciprocating motion of
the pistons in their cylinders does occur (Figure 12.2(b)). The lower
piston, for instance, as it moves inwards will discharge fluid out through
the lower port in the central valve arrangement. As it continues past the
horizontal position the piston moves outwards, drawing in fluid from the
upper port. Once past the horizontal position on the opposite side, it
begins to discharge the fluid. If the circular floating ring were pushed to
the left then the suction and discharge ports would be reversed (Figure
12.2(c)).
This pump arrangement therefore provides, for a constantly rotating
unit, a no-flow condition and infinitely variable delivery in either
direction. The pump is also a positive displacement unit. Where two
pumps are fitted in a system and only one is operating, reverse operation
might occur. Non-reversing locking gear is provided as part of the
flexible coupling and is automatic in operation. When a pump is stopped
the locking gear comes into action; as the pump is started the locking
gear releases.