Belt Conveyors - Design Matters

Belt conveyors should be designed to load and unload materials from one stage of processing to another in the fastest, smoothest, most judicious, safest, and most economical way with minimum spillage.

Belt conveyors can be designed for practically any desired path of travel, limited only by the strength of the belt, angle of incline or decline, or available space. Some of the profiles are more desirable than others. For example, transfers between conveyors are to be avoided where ever possible because of the additional wear on the belts at the loading points, the dust raised, and possible plugging in the transfer chutes.

Material characteristics
Belt conveyors are capable of handling an almost unlimited variety of materials. However, their successful performance depends upon a design that is based on a thorough understanding of the characteristics of the material to be handled and careful consideration of its anticipated behavior while being carried on the belt, as well as during delivery to and from it. Some of the characteristics which affect the basic design of the conveyor belt are the size of lumps, bulk density, angle of repose, abrasiveness, moisture content, dustiness, stickiness, temperature, and chemical action, etc.

The successful design of a belt conveyor must begin with an accurate appraisal of the characteristics of the material to be transported. The flowability of a material, as measured by its angle of repose and angle of surcharge, determines the cross-section of the material load which can be safely carried on a belt. It also is an index of the safe angle of the incline of the belt conveyor. The flowability is determined by different material characteristics such as size and shape of the fine particles and lumps, roughness or smoothness of the surface of the material particles, proportion of fines and lumps present, and moisture content of the material.

Conveyor Capacity
The conveyor capacity is the total weight of the material conveyed in one hour with the belt continuously carrying a uniform cross-section of material and traveling at a uniform speed. It is usually in tonne/hr. The conveyor capacity is determined by the belt speed, width, and angle of the belt.

The angle of the surcharge is one of the most important characteristics in determining the carrying capacity as it directly governs the cross-sectional area of material in the belt and hence the volume being conveyed. The surcharge angle depends on the friction between the belt and the material and how the material is loaded. The steeper the conveyor, the greater the belt capacity and the lesser the surcharge angle.
Another important factor in determining the belt capacity is the toughing angle. Belts are troughed to allow the conveyor load and transport materials. As the trough angle increases, more materials can be transported.

Belt width and speed
Belt width is a function of the largest lump size (largest product particle size), operating speed, and the desired belt capacity. As a thumb rule, belt width should be between three to five times the height of the maximum lump. 

A number of factors determine the correct conveyor belt speed. These include the material particle size, the inclination of the belt at the loading point, degradation of the material during loading and discharge, the width of the conveyor structure, belt tensions, and power consumption. Increasing belt speed increases belt capacity, which decreases belt width and tension. Moreover, this increases the importance of the design of transfer points and generally reduces the life of all conveyor components.

Idler rollers
It is important to select the best idler design for a specific condition since idlers greatly influence belt tensions, power requirements, belt life, capacity, and operational success of a conveyor. Use of the right carrying idler and proper spacing can minimize frictional resistance. Some idlers carry larger cross-sectional loads than others for appropriate materials. Others are needed for such purposes as training the belt, cushioning the belt against heavy lumps, and handling extremely abrasive or sticky materials.

The spacing or pitch of idlers has a direct bearing on the sag of the belt between the idler sets. The idlers on the carrying side of a conveyor must support both the belt and the load carried by the belt while on the return side; the idlers must only support the empty return belt. It follows therefore that the idlers on the carrying side must be positioned at smaller intervals than on the return side.

An excessive sag in the belt between idlers results in a higher absorbed power for the conveyor and therefore the pitch of the idlers in conjunction with the tension in the conveyor should ensure that the sag is limited to between 1,5% and 3%.

Belt power and tensions
Belt tensions not only determine the required strength of the belt but also influence the design of many other mechanical parts. Furthermore, the power required to drive the conveyor is calculated from the belt tensions. There are several factors that contribute to belt tensions and their relation to power requirements. The main factors are the power needed to overcome frictional resistance and lift the load, acceleration and deceleration, and drive arrangements. Good design of the conveyor belt assures lower belt tension by the proper arrangement of drive pulleys, their lagging, and degrees of belt wrap. Tensions resulting from acceleration can be limited by appropriate electrical controls.

Conveyor belt
Selection of the conveyor belt is the most important design consideration since the belt constitutes a large portion of the initial cost of a belt conveyor. Also, it is subject to the most wear and contributes substantially to the operating costs of a conveyor. Selection of the most suitable belt for the required service involves careful consideration of the construction of the belt in conjunction with the idlers and other mechanical components. There are so many belt constructions. The rubber belt manufacturing industry is making rapid strides in making belts with the use of synthetic fibers and steel-cord carcasses. Belts with these improved constructions are frequently available.

Pulleys and shafts
Since conveyor pulleys and shafts form a composite structure in operation, it is accepted engineering practice to consider their design together. There are standard tables and formulas to assist the designer in selecting the most effective pulley and shaft combination.  Some belt conveyors are designed with special-purpose pulleys, such as those with rubber lagging for better traction, herringbone-grooved lagging for a relatively 'non-skid' grip on a wet belt, and wing or slat pulleys for use in handling sticky materials.

In addition to the data required to design a basic belt conveyor, there are numerous accessory items of equipment that must be considered in light of their contribution to the economical and successful operation of the conveyor. For example, belt take ups are necessary for maintaining the proper belt tension for drive pulley traction. Cleaning devices can reduce material cleanup problems and eliminate a source of misalignment which might otherwise result in damage to the belt. Other accessory devices are available for protecting the belt and/or performing certain process functions. Such devices include belt covers and devices for weighing and/or sampling the conveyed material accurately and continuously.

The design criteria for most of the belt conveyor systems are somewhat conservative and are based on many field trials over a wide range of operating conditions for average size conveyors. Advanced technical design methods are often required for high energy, high tension conveyors having some of the following conditions namely length over 1000 meters, horizontally curved, head and tail drove, high lift or large decline with braking requirements, and undulating geometrics, etc. Dynamic analysis and computer modeling is often used in assessing the dynamic action and control of long overland and high lift, high horsepower conveyor systems.