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Conveyor belt construction & troubleshoot guide for self- unloading bulk carrier

Self unloader conveyor-belt how it works
Fig: Self unloader conveyor-belt how it works

The specification of the conveyor belt, or the type to be used depends on various factors; speed, width, length of travel, inclines, components of its carriage etc.

The belt is made up of:
Top and bottom layers or covers of the belt are built of rubber, elastometers, mixing compounds and various chemicals. This composition is to ensure durability and compatibility with service conditions required.

The main function of the top and bottom covers is to protect the structure of the belt within i.e. the carcase. It has to withstand and resist abrasion, which is caused by the cargo loading on its carrying side and running on rollers on its underside. The top cover is generally greater in thickness than the bottom cover. Between the belt covers and the carcase are special fabric layers called ‘breakers’, which are used to increase the adhesion of the covers to the carcase.

Self unloader how it works
Fig: Self unloader how it works

The belt carcase:
is the tensile structure of the belt between the top and bottom covers. These are of various types: woven fabric – multi-ply, reduced-ply, PVC impregnated solid weave, steel-chord cable etc. The functions of the belt carcase are to:

  1. Provide strength.
  2. Withstand tension.
  3. Provide lateral stability (cross rigidity).
  4. Ensure alignment on the track.
  5. Absorb the impact of cargo loading.
  6. Offer resistance to tear.
  7. Have ability to hold mechanical fasteners.
  8. Provide strength to spliced areas.

Self unloader discharging stone cargo
Fig:Self unloader discharging stone cargo
Fabric conveyor belt:
The carcase fabric is made of lengthwise yarns called ‘warp’ and crosswire yarns called ‘wefts’. The ‘warp’ yarns are tension bearing, while the ‘weft’ yarns provide cross rigidity and impact tolerance. The warp and weft yarns are interwoven in many patterns: plain weave, straight warp weave, etc. with fillers and binders.

Multi-ply carcase:
Usually has three or more plies (layers) of interwoven fabric which are bonded together by elastic-polymer compounds. The number of plies (layers) determines the belts load ability and tensile strength. These types of fabric belts are of an older design, and have been phased out.

Reduced-ply carcase:
Uses weaves of high-strength synthetic fibres, nylon and polyester, which are formed into special fabric designs forming solid weaves of higher dimension, reducing the number of plies required. It incorporates thin layers of rubber between plies, or in the special weave design of its fabric.

Solid weave belts:
are formed from a single ply layer of fabric yarns, woven into a special design creating one solid weave, which is impregnated and covered with PVC. This type of belt requires relatively thinner covers and is more abrasion resistant.

Steel-cord conveyor belts:
are belts whose carcase is made of a single layer, of numerous parallel steel cables, embedded in rubber bonding. These steel cables forming the carcase are the belt’s tensile members. The belt construction is of three types:
  1. That which uses cables and rubber.
  2. That which is reinforced by transverse synthetic fabric cords and has one or more plies (layers).
  3. That which utilises transverse reinforcement including steel chords, or steel wires interwoven. This type of belting is used where operating tensions are high, extension of the belt is unacceptable, and the belt take-up has a limited travel area.

The life of the belt depends on:
Structural stretch is the elongation of the carcase when warp fabric stretches under load, and part of this stretch remains unrecoverable, resulting in an increase in the overall length of the belt. It is therefore necessary that good practices such as maintaining the correct operating tension, and gradual start-up without sudden shock loads, are carried out to increase the belt life.

The technical information supplied by the manufacturer must be referred to for the characteristics of the belts in use, and also to make a choice when ordering the belts. The following information is normally available:

Deck conveypr belt

Deck conveyor system discharges bulk material by using deck cranes that load the cargo into hoppers located on deck. From the hoppers the cargo is fed onto longitudinal belt conveyors and transferred to a cross conveyor which discharges via a boom conveyor into the shore receiving facilities.

Depending on the number of cranes and hoppers, a peak capacity of 4,000t/h can be reached and an average discharge rate of 1,000-2,500t/h can be maintained. This rate is mainly dependent on the cranes, which may have to be upgraded to provide the best possible discharge rates.

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