1.0 VARIOUS MATERIAL HANDLING SYSTEMS
Bulk Material Handling is an engineering field that is centred around the design of equipment used for the transportation of materials such as ores and cereals in loose bulk form. It can also relate to the handling of mixed wastes.
The purpose of a bulk material handling facility is generally to transport material from one of several locations (i.e. a source) to an ultimate destination. Providing storage and inventory control and possibly material blending is usually part of a bulk material handling system.
Bulk material handling systems can be found on mine sites, ports (for loading or unloading of cereals, ores and minerals) and processing facilities (such as iron and steel, coal fired power stations refineries).
In ports handling large quantities of bulk materials continuous ship unloaders are replacing gantry cranes.
Methods of Material Handling:
Several factors have to be considered in choosing suitable material handling method for particular manufacturing operations:
Shapes, weights and characteristics of parts
Types of distances of movement and the positions, orientations of parts during movement and at their final destination
Conditions of the path along which parts are to be transported
Degree of automation
Amount of control desired and integrations with other equioments
Operators skill required
For small batch manufacturing operations, raw materials and parts can be handled and transported by hand. But this method is generally costly. More over, because it involves human beings this practice is unpredictable, unreliable and can be unsafe to the operators depending upon the weight and shape of the parts to be moved and the environmental factors, such as heat and smoke in foundries and forging plants. In automated manufacturing plants, computer controlled materials and parts flow is being rapidly implemented. These changes have improved repeatability and lowered labor cost.
Automated Guided Vehicle Systems [AGVS].
Vehicles that interface with workstations for automatic or manual loading and unloading. These systems have automatic guidance equipments and follow a prescribed path. Flexible manufacturing systems guided vehicles generally operates under computer control. Automated Guided Vehicle
Automated Guided Vehicles (AGV), popularly known as battery-powered driverless vehicles. AGVs are becoming an integral part of automated manufacturing systems. They are with programming capabilities for destination, path location, and positioning. The AGVS belong to a class of highly flexible, intel1igent versatile material-handling systems used for materials loading and unloading throughout the facility.
Automated Guided Vehicle (AGV) is a advanced material handling applications. AGV play an important part in automating the manufacturing unit because they not only connect all faces of the factory by the horizontal movement of materials, but allow management to control and direct manufacturing processes.
The AGV can be defined as a vehicle equipped with automatic guidance equipment, either electromagnetic or optical. Such a vehicle is capable of following prescribed guidepaths and may be equipped for vehicle programming and stop selection, blocking, and any other special functions required by the system.
One of the interesting feature of AGVs is the collision avoidance capability. That is, the vehicle comes to a dead stop before any damage is done to personnel, materials, or structures.
The Components of an AGV
There are four main components of an automated guided vehicle system:
1. The vehicle: It is used to move the material within the system without a human operator.
2. The guide path: It guides the vehicle to move along the path.
3. The control unit: It monitors and directs system operations including feedback on moves, inventory, and vehicle status.
4. The computer interface: It interfaces with other computers and systems such as the mainframe host computer, the automated storage and retrieval system (AS/RS), and the flexible manufacturing system (FMS).
AGVs Control Systems
Three types of AGVS control systems are available:
In this system, all the transactions and AGVS vehicle movements are controlled and monitored by the system controller. The guide path controller controls the guide path of the AGVS and transfers the information to the AGVS process controller. The AGVS process con troller directs the movement of the AGVS vehicles.
Remote dispatch control system:
In this system a human operator is required to issue instructions to the vehicle through a remote control station. The control system sends destination instructions directly to the vehicle. Therefore, the human operator does not have any direct control over the AGVS vehicle.
Manual control system:
In the manually controlled system, the operator gives commands to AGVS. The operator gives instructions like loading the vehicle, entering a destination and unloading. A manually controlled system is simple and the least expensive of all control systems. The efficiency of the system depends on the skill and performance of the operator.
Applications of AGVS
AGVS have numerous applications and have already been applied by many manufacturing plants and companies. New applications are being developed as technology improves and as experience is gained. Some of the most common applications of the AGVS are:
Raw material storage
Finished goods storage
Flexible manufacturing systems
Technical and Economic Benefits of AGVs
Economic Justification: AGV systems are proving to be the most economical method of moving material.
Interface with Other Systems: AGV systems are designed to interface with other material-handling systems including conveyors, automatic storage/retrieval systems, production lines, and other devices.
System Accountability: Computer control means planned delivery, transaction audit records, on-line interface to production and inventory control systems, and management information on vehicle and workstation production.
Reduced Labor/increased Productivity: In cases where driverless vehicles are used, substantial savings are realized due to labor reduction.
Guide path Easily Expanded: As material movement needs change or plant size in creases, AGV systems can be expanded or modified quickly and at low cost.
Expandable System Capacity: As material movement needs increase, load movement capacity in the AGV system is easily accomplished by adding one or more vehicles.
Unobstructed Aisles: Control wires for AGV systems are installed in the floor and, therefore, leave no above-floor obstructions.
Destinations Unlimited: AGV systems can be designed with an unlimited number of pick-up and delivery points.
Less Equipment Damage: There is less product and equipment damage when AGV systems are used to move material because the vehicles travel on a predetermined route.
Reliable System Control: All automatic guided vehicles are equipped to allow manual override for special material or vehicle movement situations.
Energy Conservation: AGV systems require very little energy to operate.
Ease of Installation: AGV systems can be installed in less time than most other material handling systems. -
Installation in Existing Buildings: AGV systems can be installed in existing buildings with minimum interference to ongoing operations.
Various types of equipments can be used to move materials, such as conveyors, rollers, self powered mono rails, carts, forklift trucks, various mechanical, electrical, magnetic pneumatic and hydraulic devices and manipulators. Manipulators are designed to be controlled directly by the operators or they are automated for repeated operations such as loading and unloading parts from machine tools , presses, and furnaces. Manipulators are capable of gripping and moving heavy parts and orienting them as required between manufacturing and assembly operations. Machines are often used in sequence where work pieces are transferred directly from machine to machine. Machineries with the capability to convey parts without the use of additional material handling schemes are called integral transferred devices.
Material handling systems are typically comprised of moveable items of machinery such as conveyor belts, stackers, re-claimers, bucket elevators, ship loaders, unloaders and various shuttles, hoppers and diverters combined with storage facilities such as stockyards, storage silos or stockpiles.
A belt conveyor consists of two or more pulleys, with a continuous loop of material - the conveyor belt - that rotates about them. One or both of the pulleys are powered, moving the belt and the material on the belt forward. The powered pulley is called the drive pulley while the unpowered pulley is called the idler. There are two main industrial classes of belt conveyors; Those in general material handling such as those moving boxes along inside a factory and bulk material handling such as those used to transport industrial and agricultural materials, such as grain, coal, ores, etc. generally in outdoor locations. Generally companies providing general material handling type belt conveyors do not provide the conveyors for bulk material handling. In addition there are a number of commercial applications of belt conveyors such as those in grocery stores.
The belt consists of one or more layers of material. Many belts in general material handling have two layers. An under layer of material to provide linear strength and shape called a carcass and an over layer called the cover. The carcass is often a cotton or plastic web or mesh. The cover is often various rubber or plastic compounds specified by use of the belt. Covers can be made from more exotic materials for unusual applications such as silicone for heat or gum rubber when traction is essential.
A stacker is large a machine used in bulk material handling applications. A stacker's function is to stack bulk material such as ores and cereals onto a stockpile. A reclaimer can be used to recover the material.
Stackers are nominally rated in tph (tonnes per hour) for capacity and normally travel on a rail between stockpiles in the stockyard. A stacker can usually move in at least two directions typically: horizontally along the rail and vertically by luffing its boom. Luffing of the boom minimises dust by reducing the height that the coal needs to fall to the top of the stockpile. The boom is luffed upwards as the stockpile height grows.
Some stackers are able to rotate by slewing the boom. This allows a single stacker to form two stockpiles, one on either side of the conveyor.
Stackers are used to stack into different patterns, such as cone stacking and chevron stacking. Stacking in a single cone tends to cause size segregation, with coarser material moving out towards the base. Raw cone ply stacking is when additional cones are added next to the first cone. Chevron stacking is when the stacker travels along the length of the stockpile adding layer upon layer of material.
A reclaimer is a large machine used in bulk material handling applications. A reclaimer's function is to recover bulk material such as ores and cereals from a stockpile. A stacker is used to stack the material.
Reclaimers are volumetric machines and are rated in m3/h (cubic meters per hour) for capacity, which is often converted to t/h (tonnes per hour) based on the average bulk density of the material being reclaimed. Reclaimers normally travel on a rail between stockpiles in the stockyard. A bucket wheel reclaimer can typically move in three directions: horizontally along the rail; vertically by "luffing" its boom and rotationally by slewing its boom. Reclaimers are generally electrically powered by means of a trailing cable.
A bucket elevator, also called a grain leg, is a mechanism for hauling flowable bulk materials (most often grain or fertilizer) vertically.
It consists of: 1. Buckets to contain the material; 2. A belt to carry the buckets and transmit the pull; 3. Means to drive the belt; 4. Accessories for loading the buckets or picking up the material, for receiving the discharged material, for maintaining the belt tension and for enclosing and protecting the elevator.
A centrifugal discharge elevator may be vertical or inclined. Verical elevators depend entirely on the action of centrifugal force to get the material into the discharge chute and must be run at speeds relatively high. Inclined elevators with buckets spaced apart or set close together may have the discharge chute set partly under the head pulley. Since they don't depend entirely on the centifugal force to put the material into the chute, the speed may be relatively lower. Nearly all centrifugal discharge elevators have spaced buckets with rounded bottoms. They pick up their load from a boot, a pit, or a pile of material at the foot pulley.
The buckets can be also triangular in cross section and set close to on the belt with little or no clearance between them. This is a continuous bucket elevator. Its main use is to carry difficult materials at slow speed.
Early bucket elevators used a flat chain with small, steel buckets attached every few inches. Current construction uses a rubber belt with plastic buckets. Pulleys several feet in diameter are used at the top and bottom. The top pulley is driven by an electric motor.
The bucket elevator is the enabling technology that permitted the construction of grain elevators. A diverter at the top of the elevator allows the grain to be sent to the chosen bin.