Why a conveyor belt is required?
In any mineral processing plant the ore has to be transported and fed to different processing equipments, located at different points in a continuous rate. Conveyor belt helps in serving this purpose. Conveyor belts can be as short as few meters to as long as several kilometres. Moreover there is flexibility of lifting the load over a gradient and also turning a radius. Some variations of the conveyor belt can also serve in lifting the load vertically. The conveyor belt also serves as an environment friendly way of transporting ore over long uneven terrain due to its minimum impact on ground. Thus today it is the most widely used method of handling loose bulk materials
What are the components of a belt conveying system and what are their functions?
The belt conveying system consists of a drive pulley, a head pulley, some carrier rollers, some return rollers, a suitable belt tensioning arrangement system and a continuous loop of belt running over all these items. All these items are mounted on a suitable structure of steel.
Drive pulley – It gives motion to the conveyor belt. The drive pulley can be at the head end or at the tail end, depending on the layout of the conveyor, space available and loading conditions.
Head pulley – It is the head end where generally the belt discharges the carried material and return for reloading.
Carrier rollers – These are the set of rollers on the top side of the conveyor path and are the main load bearing units of the conveying system. There are three or five rollers in a set of carrier rollers. The outer rollers in the set are inclined upwards from the centre so as to raise the edges and give it a trough-like profile. Moreover the carrying capacity of the belt is also increased by giving the trough shape rather than keeping it flat. This trough shape helps in carrying the material effectively without any spillage. Spacing along the belt is at the maximum interval which avoids excessive sag in between the rollers.
Return rollers – As the name suggests these rollers are placed in the return side of the conveying system to carry the empty belt towards the drive pulley. The return rollers are horizontal straight rollers which overlap the belt by a few inches at each side. The spacing of return rollers are more compared to carrier rollers as it has to guide only the empty belt.
Conveyor belt – This is the heart of the system and the costliest individual item in the system. The standard rubber conveyor belt has a foundation of sufficient strength to withstand the driving tension and loading strains. The foundation can be of cotton, nylon, or steel cord, which is bound together with a rubber matrix and completely covered with a layer of vulcanised rubber.
Belt tensioning arrangement – Any conveying system needs to have a proper belt tensioning arrangement since the drive from electric motor is transferred to the conveyor belt through the drive pulley by means of frictional grip. Thus if proper grip is not there may be belt slippage. The tensioning device has to adjust the belt for stretch and shrinkage of the belt during operation of the system.
How the conveyor belt gets drive?
The conveyor belt is driven by an electric motor through a gear box and drive pulley. There is also some fluid coupling in between the motor and gear box so as to enable smooth start and to avoid slippage due to heavy inertial load in a static belt system.
To induce motion without slipping requires good contact between the belt and drive pulley. This may not be possible with a single 180 degree turn over a pulley and some form of “snubbed pulley” drive or “tandem” drive arrangement may be more effective. This will depend on the type of load, length of the conveying system and gradient over which the conveying system has to carry the load.
How belt tensioning arrangement works?
In most of the conveying systems where there is no space constraint, gravity-operated arrangements are used which adjust the tension continuously. The belt can be made to pass through some idler pulleys and take up pulleys on the return side. The take up pulley can be directly connected to some dead weight or some rope and pulley attachment can be provided in case there is shortage of vertical space. Hydraulics have also been used extensively, and when more refined belt-tension control is required, especially in starting and stopping long conveyors, load-cell-controlled electrical tensioning devices are used. In case of short length of conveyors screw type tensioning arrangement can also be provided.
What are safety requirement of the belt conveying system?
A series of belts should incorporate an interlock system such that failure of any particular belt will automatically stop preceding belts. Interlock with devices being fed by the belt is also important for the same reasons. It should not be possible to shut down any machine in the system without arresting the feed to the machine at the same time. Similarly, motor failure should also lead to the automatic tripping of all preceding belts and machines. Sophisticated electrical, pneumatic and hydraulic circuits have been widely employed to replace all but a few manual operations. The reliability of belt systems has been enhanced by advances in control technology, making possible a high degree of fail-safe automation. Nowadays most of the modern plants use PLC system for controlling the operation of all plant equipments.
Precautionary measures are to be taken while loading the conveyor belt. Several methods can be used to minimise loading shock on the belt. A typical arrangement is screening the fines on to the belt first to provide a cushion for the larger pieces of rock.
Proper sensing devices and guiding devices should be installed to check the run out of belt.
At the loading point the carrier rollers may be rubber coated so as to reduce wear and damage from impact loading and the gap between the rollers is also less than the remain length of the conveying system. This helps in reducing the sock loading of the belt.
What are other variations of conveying system?
Shuttle belts are reversible self-contained conveyor units mounted on carriages, which permit them to be moved lengthwise to discharge to either side of the feed point. The range of distribution is approximately twice the length of the conveyor. They are often preferred to trippers for permanent storage systems because they require less head room and, being without reverse bends, are much easier on the belt.
Where space limitation does not permit the installation of a belt conveyor, gravity bucket elevators can be used. These provide only low handling rates with both horizontal conveying and elevating of the material. The elevator consists of a continuous line of buckets attached by pins to two endless roller chains running on tracks and driven by sprockets. The buckets are pivoted so that they always remain in an uptight position and are dumped by means of a ramp placed to engage a shoe on the bucket, thus turning it into the dumping position.
Sandwich conveyor systems can be used to transport solids at steep inclines from 30 to 90 degree. The material being transported is “sandwiched” between two belts which hold the material in position and prevent it from sliding back down the conveyor even after the conveyor has stopped or tripped. As pressure is applied to material to hold it in place, it is important the material has a reasonable internal friction angle. The advantage of sandwich belt conveyors is that they can transport material at steep angles at similar speeds to conventional belt conveyors.
Screw conveyors are another means of transporting dry or damp particles or solids. The material is pushed along a trough by the rotation of a helix, which is mounted on a central shaft. The action of the screw conveyor allows for virtually any degree of mixing of different materials and allows for the transportation of material on any incline from the horizontal to vertical. The main limitation of screw conveyors is their capacity, which has a maximum rate of about 300 m3/ h.