Conveyors are used when material must be moved in relatively large quantities between specific locations over affixed path. The fixed path is implemented by a track system, which may be in-the-floor, above-the-floor, or overhead. Conveyors divide into two basic categories: (1) powered and (2) non-powered. In powered conveyors, the power mechanism is contained in the fixed path, using chains, belts, rotating rolls, or other devices to propel loads along the path. Powered conveyors are commonly used in automated material transport systems in manufacturing plants, warehouses, and distribution centers. In non-powered conveyors, materials are moved either manually by human workers who push the loads along the fixed path or by gravity from one elevation to a lower elevation.
Types of Conveyors
A variety of conveyor equipment is commercially available. In the following paragraphs, we describe the major types of powered conveyors, organized according to the type of mechanical power provided in the fixed path.
Roller and Skate Wheel Conveyors. These conveyors have rolls or wheels on which the loads ride. Loads must possess a flat bottom surface of sufficient area to span several adjacent rollers. Pallets, tote pans, or cartons serve this purpose well. The two main entries in this category are roller conveyors and skate wheel conveyors, pictured in Figure 10.6.
In roller conveyors, the pathway consists of a series of tubes (rollers) that are perpendicular to the direction of travel, as in Figure 10.6(a). The rollers are contained in a fixed frame that elevates the pathway above floor level from several inches to several feet. Flat pallets or tote pans carrying unit loads are moved forward as the rollers rotate. Roller conveyors can either be powered or non-powered. Powered roller conveyor are driven be belts or chains. Non-powered roller conveyors are often driven by gravity so than the pathway has a downward slope sufficient to overcome rolling friction. Roller conveyors are used in a wide variety of applications, including manufacturing, assembly, packaging, sortation and distribution.
Skate-wheel conveyors are similar in operation to roller conveyors. Instead of rollers, they use skate wheels rotating on shafts connected to a frame to roll pallets or tote pans or other containers along the pathway, as in Figure 10.6(b). This provides the skate wheel conveyor with a lighter weight construction than the roller conveyor. Applications of skate-wheel conveyors are similar to those of roller conveyors, except that the loads must generally be lighter since the contacts between the loads and the conveyor are must more concentrated. Because of their light weight, skate wheel conveyors are sometimes built as portable equipment that can be used for loading and unloading truck trailers at shipping and receiving docks at factories and warehouses.
Belt Conveyors. Belt conveyors consist of a continuous loop: Half its length is used for delivering materials, and the other half is the return run, as in Figure 10.7. The belt is made of reinforced elastomer (rubber), so that it possesses high flexibility but low extensibility. At one end of the conveyor is a drive roll that powers the belt. The flexible belt is supported by a frame that has rollers or support sliders along its forward loop. Belt conveyors are available in two common forms: (1) flat belts for pallets, individual parts, or even certain types of bulk materials; and (2) troughed belts for bulk materials. Materials placed on the belt surface travel along the moving pathway. In the case of troughed belt conveyors, the rollers and supports give the flexible belt a V-shape on the forward (delivery) loop to contain bulk materials such as coal, gravel, grain, or similar particulate materials.
Conveyors Driven by Chains and Cables. The conveyors in this group are driven by a powered chain or cable that forms an endless loop. In some cases, the loop forms a straight line, with a pulley at each end. This is usually in an over-and-under configuration. In other conveyors, the loop has a more-complex path, with more than two pulleys needed to define the shape of the path. We discuss the following conveyors in this category: (1) chain, (2) slat, (3) in –floor towline, (4) overhead trolley, and (5)power-and-free over-head trolley.
Chain conveyors consist of chain loops in an over-and-under configuration around powered sprockets at the ends of the pathway. One or more chains operating in parallel may be used to form the conveyor. The chains travel along channels in the floor that provide support for the flexible chain sections. Either the chains slide along the channel or they ride on rollers in the channel. The loads are generally dragged along the pathway using bars that project up from the moving chain.
The slat conveyor sues individual platforms, called slats, connected to a continuously moving chain. Although the drive mechanism is a powered chain, it operates much like a belt conveyor. Loads are placed on the slats and are transported along with them. Straight line flows are common in slat conveyors systems. However, because of the chain drive and the capability to alter the chain direction using sprockets, the conveyor pathway can have turns in its continuous loop.
Another variation of the chain conveyor is the in-floor towline conveyor. These conveyors make use of four-wheel carts powered by moving chains or cables located in trenches in the floor, as in Figure 10.8. The chain or cable is called a towline; hence, the name of the conveyor. Pathways for the conveyor system are defined by the trench and cable, and the cable is driven as a powered pulley system. Switching between powered pathways is possible in a towline system to achieve flexibility in routing. The carts use steel pins that project below floor level into the trench to engage the chain for towing. (Gripper devices are substituted for pins when cable is used as the pulley system, similar to the San Francisco trolley.) The pin can be pulled out of the chain (or the gripper releases the cable ) to disengage the cart for loading, unloading, switching, accumulation of parts, and manually pushing a cart the main pathway. Towline conveyors systems are used in manufacturing plants and warehouses.
All of the preceding chain and cable drive conveyors operate at floor level or slightly above. Chain-driven conveyors can also be designed to operate overhead, suspended from the ceiling of the facility so as not to consume floorspace. The most common types are overhead trolley conveyors. These are available either as constant speed (synchronous) or as power-and-free (asynchronous) systems.
A trolley in material handling is a wheeled carriage running on an overhead rail from which loads can be suspended. An overhead trolley conveyor, Figure 10.9, consists of multiple trolleys, usually equally spaced along a fixed track. The trolleys are connected together and moved along the track by means of a chain or cable that forms a complete loop. Suspended from the trolleys are hooks, baskets, or other receptacles to carry loads. The chain (or cable) is attached to a drive wheel that supplies power to move the chain at a constant velocity. The conveyor path is determined by the configuration of the track system, which has turns and possible changes in elevation. Overhead trolley conveyors are often used in factories to move parts and assemblies between major production departments. They can be used for both delivery and storage.
A power-and-free overhead trolley conveyor is similar to the overhead trolley conveyor, except that the trolleys are capable of being disconnected from the drive chain, providing this conveyor with an asynchronous capability. This is usually accomplished by using two tracks, one just above the other. The upper track contains the continuously moving endless chain, and the trolleys that carry loads ride on the lower track. Each trolley includes a mechanism by which it can be connected to the drive chain and disconnected from it. When connected, the trolley is pulled along its track by the moving chain in the upper track. When disconnected, the trolley is idle.
Other Conveyor Types. Other powered conveyors include cart-on-track, screw, vibration-based systems, and vertical lift conveyors. Cart-on-track conveyors consist of individual carts riding on a track a few feet above floor level. The carts are driven by means of a rotating shaft, as illustrated in Figure 10.10. A drive wheel, attached to the bottom of the cart and set at an angle to the rotating tube, rests against it and drives the cart forward. The cart speed is controlled by regulating the angle of contact between the drive wheel and the spinning tube. When the axis of the drive wheel is 45°, the cart is propelled forward. When the axis of the drive wheel is parallel to the tube, the cart does not move. Thus, control of the drive wheel angle on the cart allows power-and-free operation of the conveyor. One of the advantages of cart-on-track systems relative to many other conveyors is that the carts can be positioned with high accuracy. This permits their use for positioning work during production. Applications of cart-on-track systems include robotic spot welding lines in automobile body plants and mechanical assembly systems.
Screw conveyors are based on the Archimedes screw, the water-raising device devised in ancient times (circa 236 B.C.), consisting of a large screw inside a cylinder, turned by hand to pump water up-hill for irrigation purposes. Vibration-based conveyors use a flat track connected to an electromagnet that imparts an angular vibratory motion to the track to propel items in the desired direction. This same principle is used in vibratory bowl feeders to deliver components in automated assembly systems (Section 19.1.2). Vertical lift conveyors include a variety of mechanical elevators designed to provide vertical motion, such as between floors or to link floor-based conveyors with overhead conveyors. Other conveyor types include non powered chutes, ramps, and tubes, which are driven by gravity.
Conveyor Operations and Features
As indicated by our preceding discussion, conveyor equipment covers a wide variety of operations and features. Let us restrict our discussion here to powered conveyors, excluding non powered types. Conveyor systems divide into two basic types in terms of the characteristic motion of the materials moved by the system: (1) continuous and (2) asynchronous. Continuous motion conveyors move at a constant velocity vc along the path. They include belt, roller, skate-wheel, overhead trolley, and slat conveyors.
Asynchronous conveyors operate with a stop-and-go motion in which loads, usually contained in carriers (e.g., hooks, baskets, carts), move between stations and then stop and remain at the station until released. Asynchronous handling allows independent movement of each carrier in the system. Examples of this type include overhead power-and-free trolley, in-floor towline, and cart-on-track conveyors. Some roller and skate-wheel conveyors can also be operated asynchronously. Reasons for using asynchronous conveyors include: (1) to accumulate loads, (2) temporary storage, (3) to allow for differences in production rates between adjacent processing areas, (4)to smooth production when cycle times vary at stations along the conveyor, and (5) to accommodate different conveyor speeds along the pathway.
Conveyors can also be classified as: (1) single direction, (2) continuous loop, and (3) recirculating. In the following paragraphs, we describe the operating features of these categories. In Section 10.6.3, we present equations and techniques with which to analyze these conveyor systems. Single direction conveyors are used to transport loads one way from origination point to destination point, as depicted in Figure 10.11(a). These systems are appropriate when there is no need to move loads in both directions or to return containers or carriers from the unloading stations back to the loading stations. Single direction powered conveyors include roller, skate wheel, belt, and chain-in-floor types. In addition, all gravity conveyors operate in one direction.
Continuous loop conveyors form a complete circuit, as in Figure 10.11(b). An overhead trolley conveyor is an example of this conveyor type. However, any conveyor type can be configured as a loop, even those previously defined as single direction conveyors, simply by connecting several single direction conveyor sections into a closed loop. A continuous loop system slows materials to be moved between any two stations along the pathway. Continuous loop conveyors are used when loads are moved in carriers (e.g., hooks, baskets) between load and unload stations and the carriers are affixed to the conveyor loop. In this design, the empty carriers are automatically returned from the unload station back to the load station.
The preceding description of a continuous loop conveyor assumes that items loaded at the load station are unloaded at the unload station. There are no loads in the return loop; the purpose of the return loop is simply to send the empty carriers back for reloading. This method of operation overlooks an important opportunity offered by a closed loop conveyor: to store as well as deliver parts. Conveyor systems that allow parts to remain on the return loop for one or more revolutions are called recirculating conveyors. In providing a storage function, the conveyor system can be used to accumulate parts to smooth out effects of loading and unloading variations at stations in the conveyor. There are two problems that can plague the operation of a recirculating conveyor system. One is that there may be times during the operation of the conveyor that no empty carriers are immediately available at the loading station when needed. The other problem is that no loaded carriers are immediately available at the unloading station when needed.
It is possible to construct branching and merging points into a conveyor track to permit different routings for different loads moving in the system. In nearly all conveyor systems, it is possible to build switches, shuttles, or other mechanisms to achieve these alternate routings. In some systems, a push-pull mechanism or lift-and-carry device is required to actively move load from the current pathway onto the new pathway.
4/17/2007
|
