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.
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 first article in a two-part series looks at the first three elements; part two examines induction, sorters, and takeaways.
Most conveyor sortation systems have order selection modules (Fig. 1). The number of modules and their configurations depend on the amount of conveyable stockkeeping units (SKUs), how SKUs are picked, and throughput requirements.
There are several similarities in conventional modules:
- Product is picked by batch or discrete order
- Repack is normally picked from carton flow track modules and casepack is gathered from pallet flow track modules
- Each pick face has reserve positions immediately behind it
- Product is picked to a takeaway conveyor.
A conventional, three-level-high module can have one, two, or three exit conveyors depending on throughput requirements, product separation needs, and cost. Less than three exit conveyors results in a serpentine conveyor that links multiple levels.
An accumulation conveyor (Fig. 2) is required because of the need to temporarily stop, hold, and release product. There are three primary types of accumulation conveyors to convey product from selection to the merge: wheel, indexing belt, and live roller. The type applied is based on product conveyability criteria and equipment cost.
Wheel accumulation is an economical way to transport and gather product. Items are moved forward on a narrow drive belt and supported on both sides by wheels. When stopped, the drive belt lowers, removing drive pressure from the product.
Indexing belt is a different concept. Slugs of product, usually 100 ft in length, are formed and indexed forward as a "train" from one belt to the next until arriving at the merge.
Live roller is very prominent regardless of throughput volume. Polymer pads on a drive chain activate the rollers. When a carton stops, an air-powered mechanism drops the metal chain from the rollers in that conveyor section to stop flow. Padded chain conveyors are cost effective, quiet, and reduce mechanical problems.
Presortation (or main) merge is the primary collection point for all inputs to the sorter. The main merge has two important responsibilities.
- It allows upstream conveyors to run without impairment. This fact means the merge must react to imbalances to prevent stoppages on upstream conveyors.
- It provides a constant, metered flow of product to the sorter. The sorter induction unit must not be starved or overfed in order to maintain system throughput.
Since imbalances occur, the merge must prioritize the release sequence of the infeed conveyors. This action is accomplished by the merge logic.
Regardless of whether there are 2 or 20 premerge accumulation lines, the merge subsystem must be able to identify the amount of accumulation on each line and prioritize the release sequence. Photo sensors are placed at intervals along the premerge accumulation conveyors, usually at the 50%, 75%, and 100% full marks, to determine the status of each line.
The merge subsystem controller (PLC, PC, or microprocessor) constantly monitors the status of each line. The merge can run in a manual mode where the operator chooses which line to release, or in automatic mode.
If operating in automatic and the premerge accumulation lines are less than 75% full, the merge subsystem cycles between the premerge accumulation lines, allowing each to discharge for a predetermined period before switching to the next line. This approach provides a constant flow of product to the induction subsystem and keeps the premerge accumulation lines at about the same level.
If a line reaches the 75% level, the merge subsystem controller automatically prioritizes that line as the next to release. The 75% full line runs until the condition no longer exists and then resumes its normal release sequence.
There are many types of merges in use today -- some very simplistic and slow and others more elaborate and fast. Each type performs basically the same function.
Equipment application is based on carton size, throughput rates, and number of infeed conveyors. Basically, there are two merge designs: parallel and perpendicular.
In a parallel merge , the premerge accumulation lines are parallel or inline with the discharge conveyor and feed directly into the end of the merge unit. In a perpendicular merge , the premerge accumulation lines are at 90, 45, or 30-deg angles to the discharge conveyor and feed into the side of the merge unit.
In general, all merge subsystems consist of two basic components: infeed flow control devices and merge beds (including the funneling device).
Infeed flow control devices regulate the movement of product onto the merge bed. Depending on the desired rate, they can be a simple mechanical traffic cop or carton stop, or a sophisticated servo-driven indexing belt unit.
Merge beds are the conveyors that transport the product from the flow control device to the output conveyor. They are live roller, belt, or live tube or slat.
The merge bed also includes a funneling device which directs product from the wide area of the merge to the narrow output conveyor. This action is accomplished using conventional, fixed guard rails, live guard rails, moveable blades, skewed live rollers, or moving slats or shoes.
Live roller merge
The live roller merge is used in parallel configurations and accepts 2-6 premerge accumulation conveyors, depending on the width of the units. It is considered a low-to-medium volume merge capable of 10-80 cartons/min.
In its simplest form, the live roller merge collects product at the charge end from two premerge accumulation lines and feeds product at the discharge end to a single conveyor. The infeed flow control device is a traffic cop for low rate applications, or an indexing belt for high rate applications. The merge bed consists of a live roller conveyor or belt and fixed guard rails which funnel product into a single output conveyor.
Live roller and herringbone equipment are commonly applied to satellite merges or those located away from the main merge.
The herringbone is similar to the live roller since the merge bed itself is a live roller unit. Both types are considered low-to-medium rate in the range of 10-80 cartons/min. The difference is in the manner in which they funnel the product.
The herringbone merge is designed with skewed rollers. The driving action of the skewed rollers causes the product to travel towards the centerline of the conveyor. The centerline of the single output conveyor is aligned with the centerline of the herringbone merge. Since varying weights and sizes of product react differently to the skewed rollers, fixed guard rails are used to aid in funneling.
Sliding shoe merge
As system throughput increases to more than 80 cartons/min, the need for faster, more sophisticated merging devices becomes essential. The sliding shoe handles these jobs.
The merge bed consists of a live slat or tube-type unit with sliding shoes to push and align the product to the center of the merge. Sliding shoes are positioned by a fixed guide to form a merge pattern. Shoes on the left and right side of the pattern push product towards the center. Product is combined at the center of the merge unit and discharged onto the center of the output conveyor.
The sliding shoe is unique in the way product is metered onto the merge by the infeed conveyors. As many as four infeed conveyors discharge product onto the merge in a metered fashion through the use of servo-driven indexing belt units.
The merge subsystem controller constantly monitors the availability of product on the indexing belts for discharge onto the merge. A package-present photoeye located near the discharge end of the indexing belt alerts the controller to the status of that line. The controller discharges product from each line identified as having a carton ready for release.
Product metering onto the merge serves two purposes: Keeps the accumulation lines feeding the merge at approximately the same accumulation level, and indexing belts provide a suitable gap prior to entering the induction subsystem.
The sawtooth is a perpendicular type merge. Instead of feeding into the charge end of the merge like the parallel configurations, conveyors feed into the side of the sawtooth merge, usually at 45 or 30-deg angles. This side-feed feature allows a sawtooth merge to support as many as 12 infeed conveyors, and is capable of rates in excess of 80 cartons/min.
Infeed flow control devices are typically brake belt or indexing belt units, depending on the desired throughput rates. The merge bed is basically a live roller bed, with one important difference: There is a powered wedge section provided at each infeed conveyor induction point.
Rollers on the main body of the sawtooth merge are skewed slightly so the product that enters from the wedge section is driven to one side and edge-aligned. The combination of the skewed rollers and fixed guard rails acts as the funneling device to discharge product onto the output conveyor.
The sawtooth merge is controlled manually by an operator, or automatically by a subsystem controller. In the manual mode, the operator is responsible for the timely release of each infeed conveyor. Typically, a control console is provided with start and stop pushbuttons for each infeed conveyor.
In higher rate applications, indexing belts are the infeed flow control device to ensure gaps are created as cartons enter the sawtooth merge. In addition, the merge bed runs at a slightly faster speed than the infeed conveyor to ensure a gap and minimize the potential for jams.
This version is perpendicular and resembles the sawtooth. The flexible merge (Fig. 3) can receive product from any two infeed conveyors at the same time, which gives the overall system a great deal of operational flexibility.
The flexible merge provides twice the throughput of a conventional sawtooth. It is designed for applications in the range of 80 cartons/min or greater. The two output conveyors can feed two individual sorters, each at the sorter rate, or feed a single sorter at rates lower than the sorter rate. -- Edited by Ron Holzhauer, Managing Editor, 630-320-7139, firstname.lastname@example.org
Sortation conveyors include six subsystems: order selection, premerge accumulation, merge, induction, sorter, and takeaway.
An accumulation conveyor temporarily stops, holds, and releases product.
The merge is the primary collection point for all inputs to the sorter.
Types of subsystems
1. Order selection
2. Premerge accumulation
Pop-up skewed wheel/roller
Designing conveyor sortation systems
There are several key factors to consider when designing a conveyor sortation system:
- User requirements
- System concept
- Equipment application
- System installation and startup.
User requirements form the basis of design. Important criteria to establish are:
- Product characteristics such as length, width, height, and weight of cartons to convey
- Throughput requirements expressed in cartons/min
- Operating parameters such as shift schedule, number of orders, and whether or not those orders must be palletized for delivery.
The best system concept is selected from evaluating several operating procedures. Some criteria used for selecting the best concept are equipment costs, physical capacities, staffing levels, operational flexibility, system manageability, and system response.
At this point in the design, the project team made up of the user, conveyor manufacturer, and/or consultant have decided the basic operating plan and conveyor sortation system layout.
Equipment application involves the conveyor manufacturer's engineering group applying the best products to do the job. Requirements, functionality, and costs are important ingredients in this step. A firm price is prepared and contract executed.
System installation and startup cover the detailed engineering, manufacture, installation, and commissioning of the system. Project management is included to ensure successful completion of the job. Contractual throughput rates are met and the system is turned over to the user.
An important consideration for conveyor sortation system design is the throughput rate. System throughput refers to the amount of product selected, conveyed, sorted, and shipped in a given period of time. It is usually expressed in cartons per minute, per shift, or per day.
A user of a conveyor sortation system views system throughput in more specific terms: System capacity compared to operational capacity. System capacity is often referred to as the mechanized equipment rate. It is defined as the maximum rate at which the sorter can effectively divert product. This rate is based on carton size, weight, and spacing.
When a sorter is tested for the sort rate, average size cartons, usually 14-24 in. in length, are used. Cartons are spaced and aligned properly to provide the right conditions for sorting. In the real world of mechanized distribution, these conditions rarely exist for a sustained period. The equipment rate, however, is still important because it:
- Identifies the rate required in other subsystems such as the premerge and takeaway conveyors
- Serves as a contractual point the conveyor manufacturer must meet before turning the system over to the user.
Operational capacity is the average, sustained rate the staff and mechanized equipment can produce. This figure is less than the mechanized equipment rate.
Operational capacity divided by the system capacity results in operational effectiveness expressed as a percent. The higher the percent, the more effective the staff and equipment. This performance measure is based on a number of factors:
- Workload and staffing balance
- Individual and team productivity
- System downtime.
These rates are critical in the sorter subsystem. A conveyor system is like a funnel. Inputs from various locations meet at a single point -- the sorter. If the sorter can't take product away fast enough, it backs up to the order selection modules, shutting down the picking operation. Therefore, the sorter subsystem is the cornerstone to the mechanized operation when it comes to determining system throughput.
For an early look at the second part of this series, check the Plant Engineering web site: www.plantengineering.com.
See the "Material handling" channel on www.plantengineering.com for more articles related to this topic.
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