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INTRODUCTION

This chapter describes steel cord belting with references to its major components of steel cord, cover rubber, inside rubber and if specifi ed a breaker. This will be followed by a description of belt cover compounds, steel cord belt manufacturing methods, and special considerations in fi eld applications.

BELT CARCASS

Purpose

The belt carcass primarily provides resistance to tension forces that build up in the conveyor system from frictional resistance and vertical changes in loading. Also it provides the strength to resist belt tearing and impact damage and still allow fl exibility for load support. Splicing is a critical part of the carcass design and will be discussed separately.

Steel Cord Carcass (see Figure 8-1.)

The steel cord carcass is made up of three major components: top cover, insulation gum, steel cord bottom cover and in some cases a breaker in one or both covers. The belt strength or rating is determined by the cord diameter and pitch. The top and bottom covers protects the steel cords, the insulation gum penetrates and adheres to the steel cords which then provide corrosion resistance.

Top Cover

Insulation Gum

Bottom Cover Steel Cord

Figure 8-1. Steel Cord Belt Components

BELT EDGE PROTECTION - MOLDED EDGES

Molded (Capped) Edge Belting

All steel cord belts have molded edges which are necessary to protect the steel cords against corrosion and damage from rubbing contact against the conveyor structure.

Cut/Slit Edge Belting

Steel cord belts are manufactured to width and unlike fabric belts are not slit to narrower widths widths. If a steel cord belt were manufactured in a wide width and slit into narrow widths, the belt would have a tracking issue with each belt section tending to run toward opposite edges of the conveyor system.

CARCASS PROTECTION

Breaker

Depending on the application and customer desire, some steel cord belts are specifi ed with a breaker in the top and/or pulley cover. The breaker aids in abuse resistance by reducing the impact, slitting and gouging of the covers and also offers some limited protection

BELT MANUFACTURING METHODS

Belt Covers

The elastomeric covering on belts is there to provide protection for the carcass, and/or provide a specifi c property. These coverings are applied by several processes, depending on the material or thickness of the covering. The belt insulation compound is calendered or extruded to the correct width and gauge and laminated onto the cover which has been previously produced in the same manner. The lamination rolls for both the top and bottom covers are then forwarded to the belt building operation.

Steel Cords

The carcasses of steel cord belts are most often composed of galvanized plated steel cords uniformily spaced across the belt width. Although other constructions are available for special applications, most manufacturers typically use a 7 x 7 or 7 x 19 construction for the best combination of strength vs fl exibility.

Belt Building

The total number of cords required for the belt width are threaded from spools containing long lengths of cord through a device that controls cord tension. The cords then travel through sizing combs to control the cord pitch. The rubber covers are then cold compacted onto the previously tensioned cords with time and pressure.

Belt Covers with Breakers

Depending on the applications, some customers may specify a breaker be placed within the top cover and/or bottom cover for added protection against the abuse of impact, cover cutting and gouging. Below details the fabric process.

Textile Fabric Treatment

Generally, most multi-fi lament textiles (nylon, polyester, etc.) require an RFL treatment to ensure adequate adhesion in service. RFL is an industry term designating a treatment mixture of resorcinol formaldehyde latex (RFL), whereby the woven textile is dipped in the emulsion and dried under specifi c temperature and tension conditions. This process is used for most rubber based belting (NR, SBR, NBR, CR, EPDM, etc.)

Release Coating

Before entering the curing press, a light coat of release agent is applied to both surfaces of the belt. This is done to prevent the unvulcanized belt from sticking to the press surface after cure and is generally applied as a dust, liquid for fi ber.

Before release coating and before curing, the cover is usually perforated with fi ne pricker needles to help release gases that may be present within the body of the belt. These holes are completely sealed during the vulcanization operation.

Curing

The belt is vulcanized in a fl at platen press (index cure) with a temperature in the range of 280-320°F (140-160°C) while under pressure. Edge irons are set at the desired belt width to retain and/or mold the rubber covered edges.

Since it is essential that a small excess of material be present to create proper pressure during cure, a small overfl ow of cover occurs at the side retaining irons. This fl ashing or rind is removed by trimming or buffi ng as the cured belt exits the press and is inspected in preparation for packaging. Samples from both ends of the belt are cut and tested in the QA laboratory prior to releasing the belt for shipment.

Branding of the belt with the manufacturer’s name, grade or type of belt, and date of manufacture is generally accomplished by placing a metal stencil on the uncured belt at regular intervals. This produces an embossed label cured onto the surface.