Page 136 - Ship Construction.DJ Eyres 6Ed

Ch13-H8070.fm Page 125 Wednesday, October 18, 2006 6:56 AM
Plate and Section Preparation and Machining 125
which was convenient for weathering purposes, but this system is not now
employed; sections are laid horizontally in convenient batches.
Where cranes are used for material handling it is important that there be
adequate coverage for the full extent of the stockyard, and the material may
be delivered to the covered shed areas. Cranage is generally of the electric
overhead gantry type, traversing the stockyard on rails and extending into
the workshop area; alternatively the plates may be placed on a roller con-
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veyor for delivery to the workshops. Lifts are made with magnetic clamps
for plates and slings for the sections, the crane capacity rarely exceeding
five tonnes. To improve the handling facilities in the stockyard a device
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known as a ‘captivator’ is now available. Plates required for the work being
processed may be pre-stacked at the captivator which can be remotely
controlled so that as each plate is required it lifts, using magnetic clamps,
the plate from the top of the pile and then transfers it to a roller conveyor.
A stockyard employing this technique can in fact become highly automated.

MANGLES Mangles is the name given to a heavy set of plate straighten-
ing rolls through which the plate is passed prior to its being worked. During
transit plates may become distorted, and for many of the modern machin-
ing processes it is important that the plates should be as flat as practicable.
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Two sets of mangles may be provided, one for heavy plate and one for light
plate, but types are now available which permit a wide range of thickness to
be straightened.

SHOT BLASTING Plates and sections are in most cases now shot blasted
to remove rust and millscale. The principles of shotblasting are dealt with in
Chapter 27.
Shot blasting plant in shipyards is generally of the impeller wheel type
where the abrasive is thrown at high velocity against the steel surface and
may be re-circulated. The plant can be so arranged that the plate passes
through vertically, but more often it is installed so that the plate may pass
through in the horizontal position. In the former case equipment must
be provided to allow the plate to be raised into the vertical position from
the roller conveyor before it enters the blasting plant, and to lower it on
removal. There is a further problem with lighter plate which may more
easily distort when stood on end. The only disadvantage of the horizontal
plant is the removal of spent abrasive from the top of the plate, which may
be relatively easily overcome. A separate shot blast plant is often installed
for sections.

PRIMING PAINT Following the shot blasting of plates and sections, the
material passes immediately through an airless spray painting plant. In one
pass the material is automatically sprayed with a priming paint of controlled
coat thickness. A number of suitable priming paints are available; the