Conveyor sortation systems

The conveyor industry has entered the "blur stage" as cartons are sorted so fast that it is impossible for a person to visually count them.

By Mark Vaughan September 1, 1999

The conveyor industry has entered the “blur stage” as cartons are sorted so fast that it is impossible for a person to visually count them. Today’s high-speed single sortation systems have mechanical throughput rates exceeding 200 cartons/ min, or 96,000/8-hr day.

A conveyor sortation system is divided into six main subsystems: order selection, premerge accumulation, merges, induction, sorters, and takeaways. This final article in a two-part series discusses induction, sorters, and takeaways; the first installment ( PE , June 1999, p 66, File 4510/4585) looked at the other three subsystems.


The induction subsystem (Fig. 1) has two primary functions: Generates the required spacing between cartons to ensure the maximum sort rate, and identifies the destination of each carton prior to sortation. Spacing is accomplished with different types and configurations of belt units, depending on the rate.

Induction subsystems are normally categorized by low, medium, and high rates.

Low-rate induction is in the range of 10-40 cartons/min. Gapping of cartons is not as critical as a high-rate operation; therefore, the induction method is fairly simple.

Product is gapped by two or three increasing speed belt units. Gaps between cartons vary in length. Carton destination is determined by manual encoding or bar code scanning. (The manual method is discussed here).

Typically, cartons are staged on an accumulation conveyor, waiting for release onto the first belt unit of the induction subsystem. The two or three belts that make up the induction unit run at fixed, but progressively faster, speeds in order to pull a gap between cartons.

For example, the first belt may run at 100 fpm, the second at 180 fpm, and if there is a third, it runs at the same speed as the sorter. Cartons entering the induction stop on the second belt unit when a package-present photoeye is blocked. The operator visually identifies the destination code and enters it into the encode panel.

As soon as the operator enters the number, the carton is discharged onto the sorter, and the next piece advances to the package-present photoeye. Depending on the dexterity of the operator and number of digits entered, an encode operator can induct as many as 50 cases/min, but the average is closer to 30.

If a company needs to capture a unique product identification or accountability number, automatic scanning equipment would probably be applied.

Medium-rate induction is in the range of 40-80 cartons/min. Equipment applications include two manual encoding or manual laser scanning stations, or automatic laser scanning.

Manual laser scanning is similar to manual encoding. The only difference is the manual encoding station is replaced with a gun-type, handheld laser scanner.

Rather than keying in a destination code, the operator simply points the laser gun at the bar code label and fires. The scanner reads the destination code and instantaneously discharges the carton onto the sorter. As in manual encoding, dual induction enhances the rate of a gun scanning operation since two operators can scan more product than one. However, there is a labor impact which must be evaluated against automatic laser scanning.

The next level of sophistication is a fixed position, automatic laser scanner. New design considerations must now be taken into account such as field of vision and fixed gapping.

To maintain flow at the high end of the medium-rate spectrum, the induction subsystem provides a fixed gap between cartons, which is accomplished with clutch controlled brake belt units. Three belt induction units work together to keep uniform spacing between cartons.

The accumulation conveyor feeding product onto the first induction belt runs faster than the receiving belt. Cartons are slugged together to eliminate gaps. The second belt, which runs faster than the first, pulls a gap between the cartons.

The final fixed gap is achieved when the cartons pass from the second belt onto the third, which is running at the same speed as the sorter. The fixed position laser scanner reads the bar code label as it travels on the third induction belt. The divert destination is transmitted from the scanner to the sorter controller, and the appropriate diverter activates to sort the carton.

In a dual induction configuration, cartons are scanned on the pre-sorter belt merge, which performs the same function as the third belt in a single line induction configuration. The scanner is positioned at the discharge or funnel end of the merge, after all of the cartons are properly aligned for scanning. The dual induction feature allows the accumulation units and induction belts to run at slower speeds, since each side is essentially providing 50% of the total throughput rate.

Induction units for a tilt tray sorter fall into the medium rate category since their single unit induction rates rarely exceed 60 cartons/min. The tilt tray sorter achieves its throughput by adding more induction stations along the sort loop.

High-rate induction is in the range of 80-200 cartons/min. There are basically three types of equipment: single line, multiple line, and combination servo-meter belts with single line induction.

The device used to identify the carton’s destination at these high rates is an omnidirectional laser scanner. The scanner is capable of reading the bar code label placed on top of the carton, regardless of the label orientation.

Logic, controls, and mechanical equipment used to set the gap between cartons are basically the same. The induction subsystem provides variable, optimal gaps based on carton length.

The induction subsystem optimizes gaps by presenting product to the sorter in a uniform fashion. The induction subsystem controller accomplishes this goal through the use of servo-driven brake belt units and photo-eye controls.

Servo-driven or electronic belt units accelerate or decelerate cartons momentarily to adjust the size of the gap between them. Photoeye controls measure the length of the cartons as they enter the induction units. This information is passed to the subsystem controller, which varies the speed of the indexing belts to achieve the minimum gap required to properly sort cartons.

The last belt in the induction unit runs at the same speed as the sorter. Overhead is the omnidirectional laser scanner.

These induction types use similar componentry and provide the same functionality. However, their operations are vastly different because of the speed of the upstream conveyors that feed the induction units.

Single-line induction

Single-line induction requires that all infeed conveyors, including the accumulator prior to induction and those ahead of the main merge, discharge product at the full sorter rate.

Accumulators prior to the merge are commonly equipped with a “slug discharge.” In simple terms, slug discharge allows an entire train of accumulated product to release as a group, without singulation or gapping between cartons. This slug of cartons travels through the main merge and onto an accumulator conveyor prior to the induction units.

The accumulator prior to induction is normally designed with “dynamic accumulation.” Its function is to keep the cartons together in a train, but not allow excessive line pressure to build.

Induction units accelerate or decelerate the servo-driven belt units to achieve the optimum gap between various carton lengths. When cartons travel onto the last induction belt, they accelerate to sorter speed and are read by the omnidirectional laser scanner.

Multiple-line induction

Multiple-line induction provides two high-speed inputs to the sorter, each capable of delivering cartons at less than the sorter rate. Upstream feed conveyors can now run at slower speeds. Product travels from an accumulator onto indexing belts, then discharges onto a merge unit where scanning occurs prior to sortation.

Release of product from multiple induction belts onto the belt merge is regulated by the subsystem controller. It adjusts the release by monitoring package-present photoeyes and discharges on a first come/first serve basis.

Cartons are properly gapped by servo-driven indexing belts, then discharged onto the merge unit in a synchronized release fashion. Cartons maintain their optimum gap because the last belts of the induction units and the merge are running at the same speed as the sorter.

Combined induction

Attributes of both servo-controlled meter belts and single-line induction methods can be applied to achieve high throughput rates. As many as four infeed accumulation units are regulated by the induction subsystem controller for release into a merge unit.

Since the rate through this merge is the same as the sorter, a combining slat/shoe type merge is typically used. Delivery of product from the accumulation lines is controlled by servo-driven indexing belt units.

Cartons are released from the indexing belt units onto the merge on a first come/first serve basis. If all the induction units have product waiting for release, the subsystem controller cycles to each line in sequence, allowing one carton to discharge before switching to the next line. Cartons are properly gapped by the indexing belt units, and the gap is maintained as the cartons travel on the combining merge unit.

As many as four units can provide the product necessary to maintain the sorter rate. This factor allows the conveyors feeding the combining merge unit to run at a much slower speed. Each of the four infeed conveyors is capable of delivering product at a rate somewhat greater than 35% of the sorter capacity. The sorter can still run at full capacity when only three of the feed lines are delivering product.

The combining merge discharges cartons onto a single skewed live roller conveyor. This unit aligns the product to one side of the conveyor before the product enters the single-line induction unit.

This second induction belt unit is the “gap optimizer,” which ensures the proper spacing between cartons prior to entering the sorter. The induction belt units of the gap optimizer are servo-driven and accelerate or decelerate to create proper spacing between cartons.

The third belt of the gap optimizer runs at the same speed as the sorter, and product is scanned by the overhead laser prior to entering the sorter.


This subsystem (Fig. 2) creates whatever shipment integrity is required by the user by sorting mixed orders or batches to planned destinations. The sortation subsystem controller is the brains of the operation and executes the plan.

The sortation process involves identifying the carton’s destination, tracking it along the conveyor path, and physically diverting it to the proper destination. Equipment includes an identification device, such as a scanner or keypad, timer, and mechanical diverter (sorter).

The identification device reads the carton’s identity and transmits the information to the controller.

The timing device monitors the speed and position of the sorter conveying surface, usually a belt or chain, and transmits the information to the sortation subsystem controller. The controller then links a specific carton with a position on the sorter conveying surface. The controller calculates the amount of time necessary for that carton to travel to its destination, and then activates the appropriate diverter when the carton arrives there.

Update photoeyes validate the data given by the timing device. The photoeyes are polled by the subsystem controller to ensure the tracked carton reaches the diverting position. If the carton is not in its appropriate window, the photoeye reports back to the controller and necessary corrections are made.

Cartons out of their assigned window bypass their sort destination and divert to a “jackpot” line or continue onto a recirculation conveyor. Product from recirculation re-enters the system, normally at the main merge.

Sorters are available in several types. The best choice depends on rate and product characteristics, such as dimensions, weight, condition of conveying surface, and fragility.

Sorters are classified into three rate categories: low rate (0-40 cartons/min), medium rate (40-80 cartons/min) and high rate (80-200 cartons/min).

Low-rate sorters include 90-deg transfers, barrier-type diverters, and pusher sorters. Medium-rate sorters are pop-up wheel and roller sorter. High-rate sorters are the sliding shoe, tilt tray, and cross belt.

90-deg transfers

There are two types of 90-deg transfers: pop-up chain and pop-up roller. Both versions are used where the sort rate is 15 transfers/min or less. (A transfer is a device that moves product onto or off of a conveyor line, changing the product’s direction and axis.)

Barrier diverters

These products, also known as blade diverters, plow arms, or vertabelts, are as simple as a steel channel or as sophisticated as a powered belt. Vertical powered belts are frequently used for systems with a wide product mix or oversized cartons. The belt helps positively direct the cartons on the takeaway conveyor.

Pusher sorters

One of the first mechanized products is the right angle, pusher-type diverter. A version similar to the pusher is a puller diverter.

When the pusher diverter is activated, the carton moves sideways rather than forward. Pusher diverters are a fixed 90-deg type that use a back and forth motion, or a rotating parallelogram where the pusher surface has some forward motion in addition to its side motion. The latter provides a gentler diverting action.

The puller arm reaches out from the divert side of the conveyor, squares and edge aligns the carton against an aluminum plate, and pulls the carton off the conveyor.

Pop-up skewed wheel sorters

Skewed wheel sorters consist of a high-speed belt transportation conveyor with two rows of skewed wheels built into the belt unit at each divert point. When a carton reaches the divert destination, wheels raise above the conveying surface of the belt and steer the product off the sort line.

Skewed wheels are normally driven by elastomer bands and run at a speed slightly faster than the sorter to provide additional driving force when diverting. The pop-up skewed wheel sorter is a reliable method of high speed diverting.

Skewed-roller sorters

Skewed-roller sorters have many of the same features and attributes as the skewed wheel. The unit consists of a high-speed belt transportation conveyor with a single row of polyurethane rollers built into the belt unit at each divert point. Unlike the skewed wheel, rollers of this sorter are mounted flush with the conveying surface of the belt and are normally in the direction of flow position.

The main advantage of the skewed roller over the skewed wheel is that it can sort to both sides. When a carton approaches the appropriate divert destination, rollers raise above the conveying surface of the belt and pivot to the left or right to divert the carton. Polyurethane rollers run at a speed slightly faster than the sorter.

Sliding shoe sorters

The sliding shoe is a high-speed device than handles a variety of products. The carrying surface consists of conveyor flights or slats driven on the outer edges by a chain. Sliding shoes, which are supported by the conveyor slats, push the product off the flight conveyor at the appropriate sort destination. The number of shoes used to divert a carton is determined at induction, depending on the length of the load.

In a single side diverting application, sliding shoes are positioned on the side opposite the divert side. As the carton reaches the appropriate sort destination, a divert gate automatically changes the path of the sliding shoes, diagonally sweeping the carton off the conveyor. Shoes remain on the divert side of the sort conveyor until the return trip on the underside of the unit, where a fixed gate brings all the shoes back to one side.

In a two-sided diverting application, sliding shoes are presorted to the left or right before coming to the top of the sorter. The sortation controller takes the divert destination data from the scanner, and marries that with the carton length information from induction. The controller then preassigns the appropriate number of sliding shoes to the right or left of the sorter, depending on the destination.

Tilt-tray sorters

The tilt-tray sorter is a fast, bidirectional device capable of handling cartons as well as soft goods and products difficult to convey. The basic design consists of a carriage with a tray device mounted on top that is pulled through an enclosed track. Carriages are capable of traveling at speeds in excess of 400 fpm. Belt units are typically used to induct product onto the trays from several locations along the sort loop.

Once the product is discharged from the induction belt onto a tray, it passes under an omnidirectional laser scanner where the item is identified and a divert destination assigned. The item is now married to its carrying tray by the sortation controller, and is tracked to the appropriate divert destination, where the tray is automatically tilted to the left or right, for correct sortation. Inducting product from many points along the sort loop allows the tilt tray to attain high throughput rates.

Cross belt

The cross-belt sorter offers performance features beyond the operating characteristics of the tilt-tray system. This improvement is due to the powered belt conveyor on each carrier that moves product on and off in a very predictable manner. As a result, positive sorting is achieved, faster line speed increases the sort rate, narrower off loading chutes can be installed, and more off load chutes can be concentrated in a given amount of space. The cross belt sorter is capable of handling up to 17,000 items/hr.

Powered product movement also means items can be off-loaded predictably regardless of shape, size, or surface characteristics. Item information may be manually or automatically input into the sortation control system to specify the item destination.

Position and size of items are automatically measured on the induct lines. This feature allows the proper positioning of the packages for transfer to the sortation line. Long items are placed across two carriers. When an item reaches its divert point, the carrier conveyor is activated and the item is gently diverted to its destination.

Takeaway conveyors

Sorter takeaway conveyors (Fig. 3) are an overlooked feature of the system, but play an important role in the overall achievement of the desired throughput rate. Their function is to accept product diverted from the sorter conveyor so the discharge rate of the sorter is not impaired.

The type of takeaway used depends on many factors, including rate, product mix, product weight, and fragility of the product. Three types of conveyors are available: powered takeaways, gravity takeaways, and gravity chutes.

Powered takeaways

Powered takeaway conveyors are typically used to facilitate diverting on the less positive high speed sorters. The actual equipment application is live roller or belt conveyor.

Live roller wedge sections at each divert point are often used on sorters. The wedge sections are slave driven off the sorter.

Belt conveyor takeaways are applied for their ability to grip product and pull it from the sorter. This feature is important when a sorter reacts differently to mixed size and weight products.

Belt units are typically perpendicular to the sorter and run at faster speed, so the product is pulled away. Live roller wedge takeaway conveyors are normally positioned at the same angle as the diverter.

The live roller wedge typically runs faster than the sorter to aid in discharging the product. To facilitate the diverting process, some live roller takeaways are equipped with plastisol or urethane coated rollers. Coating on the rollers grips the cartons more positively than plain steel and can improve the takeaway feature.

A challenge with these conveyors is the transition from the takeaway which runs faster than the sorter to the downstream conveyors which run slower. If the downstream conveyor is slowed down too soon or too much, product backs up onto the takeaway and impairs diverting. Careful consideration must be given to the speeds of downstream conveyors and the distribution of cartons, especially in a high rate sortation environment.

Gravity takeaways

Gravity takeaway conveyors are commonly used with high speed positive sorters where the product is assured of being cleanly diverted, or slower speed sorters where the spacing between cartons is not as critical. With a positive diverting sorter, such as the sliding shoe, gravity conveyors make an effective takeaway when properly engineered.

Gravity wheel conveyors are more effective than gravity rollers because the wheels create less coefficient of friction on cartons. The gravity takeaway conveyor is normally pitched away from the sorter to assist the flow of diverted cartons.

The gravity wheel conveyor also acts as a speed transition unit, allowing the diverted product to gradually decelerate from the sorter speed before discharging onto the downstream conveyor. This fact allows downstream conveyors to run at slower speeds than the sorter. Gravity takeaway conveyors are also an economical alternative to consider when using a slower speed sorter with positive diverting action.

Chute takeaways

Chutes are most commonly used as a takeaway device when product orientation and fragility are not a consideration. Tilt tray sorters often divert products into chutes. Products difficult to convey, such as soft goods, bundled material, or envelope type packages, can be handled very effectively in a chute. This approach is also used with positive-type conventional sorter equipment when product orientation is not a factor.

Chutes provide a large target for the diverter to hit, and the pitch of the chute facilitates the takeaway. The chute must be at a steep enough angle to move the product away from the sorter quickly.


Subsystems are critically linked to each other. The output of one subsystem is the input to another. They must work in harmony to achieve maximum throughput. This goal is only achieved by a thorough understanding of the user’s requirements and good design team.

One other item critical to achieving desired throughput is the working staff assigned to the sortation system. Not only must harmony exist between equipment, but also with the human and computer element.

— Edited by Ron Holzhauer, Managing Editor,, 630-320-7139

Key concepts

The induction subsystem generates the required spacing between cartons and identifies the destination of each prior to sortation.

The sorter identifies the carton’s destination, tracks it along theconveyor path, and diverts it to the proper destination.

Takeaway conveyors accept product diverted from the sorter conveyor so the discharge rate of the sorter is not impaired.

Types of subsystems

1. Order selection

2. Premerge accumulation


Indexing belt

Live roller

3. Merge

Live roller


Sliding shoe



4. Induction

Single line

Multiple line


5. Sortation

90-deg transfer

Barrier diverter

Pusher sorter

Pop-up skewed wheel/roller

Sliding shoe

Tilt tray

Cross belt

6. Takeaway