General cargo ship mechanically operated hatch covers
The single pull hatch cover (Figure 13.12) consists of a number of transverse
panels which span the hatchway and are linked together by chains. In the
closed position, the panel sides sit firmly on a horizontal steel bar attached to
the top of the hatch coaming. Just inside the side plates is a rubber gasket
housed in a channel on the underside of the hatch cover and which rests on a
steel compression bar to form a weathertight seal (Figure 13.13). When closed
the covers are held on to the seals by a series of peripheral cleats. Rollers are
arranged on the sides of the covers to facilitate opening and closing.
To open a single pull cover the securing cleats are first freed and each panel
is raised off its compression bars by hydraulic jacks. The cover wheels, which
are arranged on eccentrics, are rotated through 180 deg and locked into position.
The jacks are then removed and the cover can be pulled backwards or forwards
as required.
The rail arrangement on both sides at the end of the hatch coaming, is
designed to turn the panels so that they are left stacked upright in the space
provided.
Fig: Single pull steel hatch cover showing fittings and opening
Figure : Detail of single-pull cover showing scaling arrangement and
jacking system (The Henri Kummerman Foundation)
Fig:A multi-panel end folding hydraulic cover for weather deck use
An alternative arrangement uses a fixed chain drive on the periphery of the
hatch, complete with its own electric or hydraulic motor.
Folding covers
Covers of the folding type, are hinged together and arranged to be opened by a
wire pull or operated hydraulically. The multi-panel, end-folding, hydraulically
operated cover shown in Figure 13.14 has a part panel at each end with hinged
arms. As the system is acted upon by the hydraulic cylinder, they fold and
travel on the wheels to stow on the end extensions of the rails.
Other folding covers may have fewer panels but work in a similar way, with
hydraulic cylinders supplying the effort. The hydraulic system for the hatch
covers may be based on a fixed displacement pump the output from which is
bypassed until required for cylinder operation. When pump output is
channelled to the cylinders, system pressure and pump motor load rise.
Protection is provided by system relief valve and motor protective devices.
Fig:The Navire Hydratorque rotary actuator and hinge (Navire Cargo Gear International AB, Sweden)
Figure 13.15 shows an interesting hydraulic arrangement termed the Navire
Hydratorque hinge. It incorporates a pair of helixes attached to two pistons.
When hydraulic pressure is applied between the two pistons it forces them and
the helixes apart thus causing rotation through the mating helixes and
operation of the hinge. Pressure applied to the outside of the pistons creates a
torque in the opposite direction.
Maintenance
Hatch cover equipment like the other deck machinery, has to exist in a very
hostile environment and the importance of regular maintenance cannot be
over-emphasized. Drive boxes and electrical enclosures should be checked
regularly for water-tightness.
Drive chains, trolleys and adjusting devices such
as peripheral and cross-joint cleats should be cleaned and greased regularly.
Seals, compression bars and coamings should be inspected and cleaned at each
port. Drain channels should be cleared regularly.
On the subject of seals and cleats it is important not to overtighten cleats.
The seal should be compressed but not beyond the elastic limit of the gasket
material. Standard rubber gaskets can be expected to last from four to five years
of normal service.
In freezing conditions special grease or commercial glycerine
should be spread over the surface of all gaskets to prevent them from sticking
to their compression bars. Quick-acting cleats are fitted with thick neoprene
washers arranged to exert the correct degree of compression. After a time these
lose their elasticity and the cleat must be adjusted or replaced.
Hydraulic systems
The most important thing about any hydraulic system is to ensure that the
hydraulic oil remains clean (regular inspection of filters). Any protective boots
fitted over rams should be periodically examined as also should flexible hoses.
Hydraulic hoses should have their date of manufacture printed on them and can
be expected to have a life of about five years.
Many types of mechanically operated hatch covers can now be found
at sea. The principal ones are listed in Table below
Fig shows Types of mechanically operated hatch covers
Summarized below some of the basic operation of deck machinery and maintenance guide :
Pump and motor systems are used for powering deck machinery such as
winches and windlasses. Pump and actuating cylinders are normally employed
for hatch covers. One or more pumps will be used to supply the volume of fluid
at the pressure required to operate one or more motors.
......
The operation of mooring a vessel has traditionally required the attendance of
a large number of deck crew fore and aft. Supervision of the moorings was also
necessary to maintain correct tension through changes due to the tides and the
loading or unloading of cargo.
......
The three essential components for a hydraulic circuit, are the hydraulic fluid
held in a reservoir tank, a pump to force the liquid through the system and a
motor or cylinder actuator to convert the energy of the moving liquid into a
working rotary or linear mechanical force. Valves to control liquid flow and
pressure are required by some systems.
......
Electric motors on vulnerable deck areas may be protected against ingress of
water by being totally enclosed in a watertight casing. Vents are provided on
some winches, which must be opened when the motor is operating in port.
......
The windlass cablelifter brakes must be able to control the running anchor
and cable when the cablelifter is disconnected from the gearing when
letting go'. Average cable speeds vary between 5 and 7 m/s during this
operation.
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A large number of ships are fitted with deck cranes. These require less time to
prepare for working cargo than derricks and have the advantage of being able
to accurately place (or spot) cargo in the hold. On container ships using ports
without special container handling facilities, cranes with special container
handling gear are essential.
......
Hatch cover equipment like the other deck machinery, has to exist in a very
hostile environment and the importance of regular maintenance cannot be
over-emphasized. Drive boxes and electrical enclosures should be checked
regularly for water-tightness.
......
The duty of a deck winch is to lift and lower a load by means of a fixed rope on
a barrel, or by means of whipping the load on the warp ends, to top or luff the
derricks, and to warp the ship.
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