Marine Auxiliary Machinery Chapter 9 Lesson 6 Deck Machinery Cargo Access By Professor Zhao Zai Li 05.2006
CARGO ACCESS (1) Although not strictly a ‘machinery’ item, the mechanical complexity of present-day cargo hatch covers-whose periodic maintenance may fall in the domain of the ship’s Engineering Department-warrants some mention in this chapter. Many types of mechanically operated hatch covers can now be found at sea. The principle ones are listed in Table 9.1.
Table 12.1 Types of mechanically operated hatch covers
CARGO ACCESS (2) The most common type of hatch cover is the ‘single pull’. The complete cover 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 under-side of the hatch cover and which rests on a steel compression bar to form a weathertight seal (Figure 9.13).
CARGO ACCESS (3) When closed the covers are held onto the seals by a series of peripheral cleats. Rollers are arranged on the sides of the covers to facilitate opening/closing. The opening arrangements are shown in Figure 9.14.
Figure 9.13 Detail of single-pull cover showing scaling arrangement and jacking system
Figure 9.14 Single pull cover showing fitting and opening arrangements
CARGO ACCESS (4) 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 180º and locked into position. The jacks are then removed and the cover can be pulled backwards or forwards as required. Instead of the opening/closing arrangements shown in Figure 9.14 the hatch may be fitted with a fixed chain drive on the periphery of the hatch, complete with its own electric or hydraulic motor.
Folding covers These may be wire operated or hydraulically operated. A multipanel end-folding hydraulic cover is shown in Figure 9.15, while Figure 9.16 shows an interesting hydraulic hinge arrangement. Known as 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 apart thus rotating the helixes. Pressure applied to the outside of the pistons creates a torque in the opposite direction.
Figure 9.15 A multi-panel folding hydraulic cover for weather deck use
Figure 9.16 The Navire Hydratorque rotary actuator and hinge
Maintenance (1) Hatch cover equipment has to exit in a very hostile environment. The Importance of regular maintenance cannot be over- emphasised. 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.
Maintenance (2) On the subject of seals and cleats it is important not to over tighten 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.
Maintenance (3) 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 axe 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 make sure that the hydraulic oil remains clean (regular inspection of filters). Any protective boots fitted over rams be periodicaly 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 ．