THE APPLICATION OF STEEL BAND CONVEYORS IN INDUSTRY
By John Dungey, Sandvik Coromant S.A. (Pty.) Ltd.
Issued by Lindsay Smithers Transvaal (Pty.) Limited – The Certificated Engineer November 1965.
A substantial part of the manufacturing costs in the industry has been due to the expense incurred by the movement of materials, but appreciable saving has been effected in recent years by the application of materials handling plant, in which the conveyor plays an important part.
We are all familiar with the endless belt type of conveyor which was evolved in the year 1868 from experiments carried out at Liverpool on the handling of grain. This originally consisted of an endless canvas band, but the modern conveyor belt is made up of plies of woven cotton, canvas or duck, fractioned with a rubber compound, covered with rubber, the whole being vulcanised together uniformly. The strength of the belt lies in the duck or fabric, the rubber covering merely acting as protection.
The beginning of this century saw further development in the handling of materials, with the introduction of the flexible steel band conveyor, by the Sandviken Steelworks of Sweden. In 1901 this company produced the widest cold-rolled strip then available, which it is believed was 300 mm or about 12 in. On being exhibited a then outstanding achievement of rolling, its possibilities as a conveying medium were seen, and an experimental conveyor was built, for conveying planks and lumber. This was a success, as it was followed by installations in the pulp and paper industry as well as in sawmills for the handling of chips and wood scrap.
During the last 40 years the steel band conveyor has been continuously developed until there are now thousands of installations throughout the world, and whilst it can handle practically all materials which are transportable on other types of conveyor, it possesses the outstanding advantage of presenting a smooth hard surface from which sticky materials can more readily be removed. This quality is of particular interest to the sugar industry, because of present developments in bulk handling of raw sugar.
Steel Band Conveyors:
Steel conveyor bands can be supplied in two qualities, namely: cold rolled hardened and tempered carbon steel, and cold hard rolled stainless steel, the former being used for general purposes and the latter for the chemical and food industries for conveying material through higher temperatures, for hygienic reasons, or to resist corrosion.
The carbon band, the carbon content is 0.60 to 0.65 per cent, and the stainless bands are classified as 18-8 quality, being J 8 per cent. chromium and 8 per cent. nickel. The analyses of both bands are given below and in Table 1. are given their mechanical and physical properties.
The bands are extremely straight and flat, which is accomplished by a special truing process, the deviation from straightness not exceeding ± 0.6 in on a length of 330 feet.
They can be supplied in single widths from 4 to 4 in, in thicknesses from 0.6 to 1.6 mm i.e., .024 to .064 in, and in lengths from 300 to 500 feet, depending on width and thickness. The ends are usually joined together by a small lap riveted joint, to form an endless length, although for special purposes they can be welded, a very recent development is an overlap joint with a thin layer of rubber between, which is vulcanised. Wider bands can be obtained by joining several single bands together with a patented longitudinal joint. It may be interesting to note that the widest steel band conveyor in the world is 12 ft 10 in wide, being made up of five single bands joined together. It has been working in England since about 1935 and, incidentally, is used as a storage conveyor for cube sugar. There are two conveyors side by side, receiving the full production when the packeting machines are shut down, and each conveyor when fully loaded holds 60 tons of cube sugar. A check-up was made on these conveyors after they had been in continuous operation for 12 years when it was found that not one single penny had been spent on replacement or repairs. Two similar conveyors have recently been installed in England for the storage of soap powder.
Details of Installation:
The construction of a steel band conveyor is very similar to the belt type since the band is made endless, passes around terminal pulleys, and can be supported on idler rollers. Unlike a rubber and canvas belt, a steel band does not stretch appreciably in operation. The extension is so small that it can be ignored so that only a very short range is necessary at the tension end for adjusting purposes.
The terminal pulleys are usually mild steel but can be lagged with rubber to increase the tractive effort required for very long or heavily loaded conveyors. They are usually narrower than the band, contrary to the usual rubber belt conveyor practice, partly because of the transverse stiffness of the steel band, to prevent material getting between the band and pulley and also to give good tracking of the band. For the discharge of material at any point along its length, the conveyor is fitted with simple "v" shaped or oblique ploughs of the fixed or travelling type, these being fitted with a steel blade which enables even sticky materials to be efficiently discharged from the smooth surface of the band. For the conveying of bulk materials, the band is usually supported on idler rollers, but in one case such as in the handling of delicate core in foundries, the band is arranged to slide on horizontal and continuous supports of steel or wood.
As the steel band is very thin and light, the power consumption is low.
Due to the transverse rigidity of the band, almost the whole width can be utilised by the load, and for the same reason supporting idler rol1ers can be spaced well apart, and the width of tread of individual rollers required to support the band can be very small. This reduces the risk of accumulation on the return idlers which support the carrying face of the band, as opposed to tubular rollers frequently used on rubber belt conveyors.
Steel bands are heat resistant within wide limits and their physical and chemical properties do not change by being kept in stock for long periods. This quality makes them very useful for ovens and other warm localities such as boiler plants, etc. Usually, the working temperature of carbon steel bands does not exceed 250°F., but in case of very even heating, such as in baking and drying ovens, a temperature up to 750°F is permissible.
Whilst flat steel bands have been common for many years, troughed steel bands can now be supplied, to give increased carrying capacity, and for handling material which might tend to run off a flat band. These can be arranged either with inclined idler rollers somewhat similar to those used for belt conveyors or with rollers arranged on a pivot attached to a spring steel cross strap so that the idlers are practically horizontal when the band is running light. When the band is loaded and is consequently troughed the idlers conform to the same curvature.
Uses:
The introduction of steel band conveyors in England about 30 years ago originated mainly in their application to the brick industry, as for reasons previously mentioned, they are ideal for handling wet. sticky, plastic clay. In this industry, one company has over 40 steel band conveyors in use, incorporating 1? miles of the band, whilst another has conveyors using 3? miles of the band, the longest conveyor exceeding 900 feet between centres.
At present over 11 conveyors with a total length of about 2 000 feet have been installed as part of the 40 000 ton handling plant constructed at the Plaistow Wharf Refinery of Messrs Tate & Lyle, Ltd., London. In placing this order they stated they did so without hesitation after having had at least 20 years' experience of satisfactory performance of steel bands when conveying raw sugar at their three refineries.
Steel band conveyors are in use in many other countries for the handling of sugar, and it is interesting to record that a sugar company in the U.S.A. who formerly used rubber belt conveyors, have recently changed these to steel band and have written that the new steel band conveyors are performing very satisfactorily and the change looks like the answer to their raw sugar handling problems.
Whilst the foregoing has emphasised the application of steel band conveyors in the sugar industry, thousands of steel band conveyors are being used for other applications such as the handling of coal, sand and gravel, limestone, foundry sand, chemicals, linoleum mix, chalk, charcoal, sacks, packages and boxes, in assemble and production lines. whilst in the food and confectionery industry, there is a wide range for canning, handling of ice cream through refrigerated chambers, conveying of margarine, suet, tea, for the production of pastries, and for packing tables etc. In the baking industry, steel bands have been supplied all over the world for the baking of biscuits, and it may be interesting to record that there are a number of these installations in Australia and South Africa. One band supplied 15 years ago is still giving good service, and there are many of these bands which have been in use for over 20 years, in other countries.
Reference has been made to the use of steel bands for conveying purposes, but in recent years, further, development has arisen, by using the bands for heat exchange problems, where heat is extracted from or put into material whilst being conveyed. So far, steel bands have been supplied for the cooling of soap powder, the cooling and solidification of gelatine - a plant for each of these applications has recently been set to work in Australia - confectionery, pitch, sulphur, grease, synthetic resin, metasilicate of soda, wax for the heating of material such as breeze, which is made into briquettes, and for evaporation of moisture such as is necessary to change liquid latex into solid rubber film, and for many other similar cases.
For putting heat into materials, or for evaporating moisture, the steel band is enclosed, and heat is applied, whilst for cooling the band, instead of being supported by idler rollers, it passes over shallow water tanks almost the full length between the terminal pulleys. The edges of the water tanks are fitted with brass strips to support the band, and additional support is obtained across the width with bent brass bars of Vee formation. The water in the tanks is either supplied from the main pipe, or is circulated using small pumps, and the pressure tends to lift the band so that the whole area of the band is in contact with the cooling water, which overflows into a collecting channel. The steel band is enclosed in an airtight chamber or ducting, usually of timber, through which cooled in a contra-wise direction to the travel of the band, which gives additional cooling on the top surface of the material being conveyed. When the liquor has to be cooled and solidified, the edges of the steel band are fitted with rubber strips to form a trough.
It may be interesting to give two examples of the benefits derived from this type of process plant. In the gelatine industry where liquor is fed on the band at about 100°F, the solidified material is automatically cut into strips about 10 in x 6 in at the delivery end of the conveyor, and fed on to wire mesh trays in about five minutes, as compared with 24 hours under the previous system of solidifying in tins, whilst in the confectionery industry, the cooling time has been reduced to one-twentieth with a considerable reduction of labour.
Conclusion:
Whilst a good deal more technical information might have been given, the purpose of this paper is to draw attention to the ever-increasing application of steel band conveyors. It is hoped that this will prove of particular interest to those engaged in the sugar industry, given the world-wide tendency to handle and store sugar in bulk rather than in bags. With this development in mind, coupled with the general expansion and modernisation in industry, an organisation has now been set up in South Africa to manufacture these conveyors. Facilities are therefore available for supplying all information to meet specific requirements.