MATERIAL-HANDLING SYSTEMS:CONVEYORS

CONVEYORS

Conveyors are perhaps the oldest means of handling material. As is the case with industrial trucks, there is a wide range of conveyors available for different applications. They may take the form of a simple gravity chute (where a load placed at point A simply slides down to point B with gravity) or a sophisticated high-speed sortation conveyor that can sort hundreds of thousands of items per hour. Virtually any type of conveyor, in any type of industry / application, can be designed to handle vir- tually any type of material, including bulk materials (coal, sugar, etc.), loose items (nuts, bolts, screws, etc.), packaged goods (such as a men’s shirt or lady’s blouse), individual products (such as a light bulb or an engine block in a fixture), and palletized or containerized loads.

Due to limited space and extensive variations in design, only a basic set of conveyors are presented in this chapter. Such a list would include: (1) chute conveyor (Figure 8), (2) belt conveyor (Figure 9), (3) roller conveyor (Figure 10), (4) wheel conveyor (Figure 11), (5) slat conveyor (Figure 12),

(6) Chain conveyor (Figure 13), (7) tow-line conveyor (Figure 14), (8) trolley conveyor (Figure 15), and (9) power-and-free conveyor (Figure 16).

Belt conveyors come in a wide variety of forms. The simplest form is a flat belt conveyor, which is often roller supported (i.e., there is a series of rollers placed underneath the belt). Belt conveyors are typically driven by a drive pulley / roller connected to an electric motor. For conveying items that may not be containerized, however, belt conveyors can also be of the slider-bed-supported type, where a flat metal surface is placed underneath the belt. An everyday example of the slider-bed supported belt conveyor can be found at the check-out lanes of most grocery stores.

An important characteristic of belt conveyors is that items or containers placed on such conveyors retain their relative position as they are conveyed, unless, of course, one of the items is intentionally

stopped or removed. Belt conveyors can also be used for moving loads between floors; this may be done with a belt incline or a spiral belt conveyor, which consists of sections of powered belt conveyors spiraling up or down. Depending on the application, there is a large variety of plastic belts one may use for horizontal conveying and belt inclines.

Telescoping belt conveyors have been used successfully at shipping / receiving docks to load / unload trailers. (The belt conveyor telescopes into the trailer to minimize walking distances during manual load / unload operations.) In addition, (troughed) belt conveyors are used extensively in han- dling bulk materials horizontally as well as up / down an incline.

Roller conveyors also come in a wide variety of forms and are used in wide ranging applications. Typically, a roller conveyor is driven either by a belt (placed underneath the rollers), a drive shaft (that runs along the conveyor underneath the rollers, the rollers are connected to the drive shaft with industrial-grade rubber bands), a chain-and-sprocket mechanism (a sprocket is attached to the end of each roller, the chain runs along one end of the conveyor), or an electric motor placed inside the rollers themselves.

Roller conveyors are most useful when the load to be conveyed has a smooth conveying surface. (Loads with irregular conveying surfaces would be better suited for belt or slat conveyors.) As a rule of thumb, for load stability, the roller spacing on the conveyor should be such that the smallest load conveyed is supported by at least three rollers at all times. Otherwise, the load may rock back-and- forth as it travels on the conveyor. In addition to conveying containerized loads, roller conveyors can be used for accumulation, which is a critical function required in some conveyor systems. Zero- pressure-accumulation is a type of technique often used with roller conveyors to accumulate many loads without buckling (i.e., without loads falling off the conveyor as more loads arrive at the ac- cumulation point).

Wheel conveyors (or skate wheel conveyors) are relatively inexpensive because they are not powered (i.e., gravity or a human’s push is often used to move the loads). They are quite cost effective in moving light to medium-weight loads over short distances. A collapsing variety, which can collapse and expand and has legs with adjustable height, has been used at shipping / receiving docks to load /

unload trailers. The conveyor is stretched into the trailer to minimize walking distances during manual load / unload operations.

A slat conveyor consists of individual slats attached to a chain (or other driving mechanism) that runs underneath the conveyor. Often the slats are made of wood or steel. Slat conveyors, similar to belt conveyors, provide a smooth conveying surface to move heavy and / or irregularly shaped loads that may or may not be containerized. Most conveyors used in the baggage claim areas of airports can be classified as slat conveyors.

Chain conveyors are often used for moving medium-weight to heavy loads over short distances. They are also used for performing 90° transfers, where a set of chain conveyors pops up (or moves up) from underneath the load to lift it an inch or so, moves it laterally off of one conveyor and onto another one, and then moves back down to release the load.

Tow-line conveyors consist of a drive mechanism (often a chain) that may be buried in the floor (in-floor tow line conveyor) or supported by an overhead track (overhead tow-line conveyor). Hooks or dogs attached to the chain move the load forward when the chain is powered. Such motion can also be described as synchronized motion because all the loads attached to the conveyor move at the same time and at the same speed.

Tow-line conveyors generally offer a cost-effective means of moving loads (often placed on carts with nonpowered wheels) over long distances with no human operators. Each cart being pulled by the conveyor can be preprogrammed manually to be diverted off the conveyor automatically at specific points (marked on the floor, often by magnets). Hence, a cart manually hooked onto the conveyor at point A can be diverted off the conveyor at point B or point C with no human intervention. Tow- line conveyors, especially the in-floor variety, are also used in automotive manufacturing, where the automobile bodies are moved through the paint and assembly areas. Such tow-line conveyors are also known as ‘‘drag chain’’ conveyors.

Trolley conveyors, which are often used for moving loads overhead, are very similar to tow-line conveyors. A chain is placed inside an overhead track, which may be I-shaped or U-shaped. The carriers are attached to the chain via trolleys. When the chain is powered, the carriers move forward. Based on the product / part being handled, there is a wide range of designs used for the carrier, or

hook attached to the carrier. Again, the type of motion provided is synchronized motion. Trolley conveyors have been used extensively in manufacturing, and similar applications to move parts from, say, the paint department, to workstations and assembly stations where parts are needed. They often travel overhead but can dip to deliver the parts to human operators.

Power-and-free conveyors have also been used in manufacturing and similar applications. Often they are installed overhead, although floor-supported versions (known as inverted power-and-free) are also available. A power-and-free conveyor consists of two tracks: a power track, which is similar to a trolley conveyor in design, and a free track. The carriers or hooks are placed on the free track via trolleys. When the dog on the power track catches (or engages) the trolley on the free track, the load moves forward. To stop motion (or to divert the load), the free track moves away from the power track and the dog disengages. Hence, merge and divert points can be created by using a power-and- free conveyor. Also, loads can be accumulated by disengaging the dog. The power track may run directly above the free track, or the two tracks may run side by side. Note that a power-and-free conveyor provides synchronized motion as long as the dog is engaged with the carrier in the free track.

Conveyors that provide asycnchronized motion are more advanced in design. One example is the cart-on-track conveyor and its variations, which allow the carts to be stopped or moved at different speeds on the conveyor at the same time. (The ‘‘cart’’ is often a fixture to hold the part.) Monorails, which strictly speaking are not classified as conveyors, also provide asynchronized motion since each carrier on a monorail is individually powered through an electric motor attached to the carrier. Power for the electric motor is received from the track, which also serves as a conduit to serve information to / from each carrier on the track. By moving sections of track vertically or horizontally, monorails can move individual carriers between floors, or they can route individual carriers in different directions (i.e., a carrier arriving at an ‘‘intersection’’ may continue straight through, or it may be diverted off to another segment of the track).

There is also a family of special-purpose conveyors known as sortation conveyors. (Interestingly, the word ‘‘sortation’’ is not in the English dictionary, but it has been used in the material-handling literature for a long time.) As the name implies, the primary function of a sortation conveyor is to sort incoming (mixed) items into specific lanes based on specific characteristics of each item. For example, if incoming items represent shipments to various retail stores, then a sortation conveyor can be used to sort all the items so that items destined to store A are diverted into lane A, items

destined to store B are diverted into lane B, and so on. Naturally, sortation conveyors play a significant role in distribution centers and similar facilities where a large number of items must be sorted by customer, or by destination, or some other user-specified criteria.

With sortation conveyors, the items to be sorted are moved in a fashion similar to those conveyors described above, but their main distinguishing characteristic is the fact that loads can be automatically diverted off at specific points and at high speeds. (As described earlier, loads can also be diverted off at specific points with an in-floor tow-line conveyor, for example, but the tow-line conveyor would be exceedingly slow for most sortation purposes.)

Items to be sorted enter the sortation conveyor at a point known as the ‘‘induction point.’’ (There may be multiple induction points.) In high-volume systems, the induction point is typically fully automated; the item is automatically identified (often via bar coding the item and using laser scanners) and it is automatically ‘‘inserted’’ into the sortation conveyor. The insertion mechanism depends on the type of sortation conveyor used. For example, most sortation conveyors (except for tilt tray and cross-belt sorters, described below) require a minimum amount of clearance (or physical distance) between the items to be sorted; this clearance is automatically inserted at the induction point, which releases the items with the proper clearance between them.

There is a wide variety of sortation conveyors available. Naturally, the sortation rate (number of items sorted per minute) depends on the type of conveyor used. Generally speaking, although it is often not up to the user, it is easier and less costly to achieve high sort rates if all the items are uniform in size and weight and they are packaged / containerized in a consistent fashion. The sortation function slows down or becomes more sophisticated as more variance in item weight / size (or pack- aging) is encountered.

Basic types of sortation conveyors, their method of operation and basic characteristics (including sort rates), are presented in Table 2 (Kulwiec 1999). As can be seen in Table 2, sort rates may be as small as 20 items / min (or 3 sec per item) and as large as 670 items / min, which is less than 0.1 sec per item! Among the conveyors described in Table 2, the sliding shoe conveyor and the cross-belt sorter are sortation conveyors that have been introduced relatively recently.

In addition to those conveyors listed in Table 2, special-purpose sortation systems have been developed for overnight delivery companies and the United States Postal Service (USPS) for mail / parcel handling. Although a very small fraction of parcels / mail may be lost, delayed, or misdirected, such systems have been used successfully for many years to sort mail and parcels that show consid- erable variation in size / weight. Given the increasing volume of domestic and international air travel, sophisticated sortation systems have also been developed for some of the world’s largest airports, where thousands of pieces of luggage must be moved and sorted among a large number of arriving and departing flights every day.