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Sortation Systems - Selecting a Sortation System - Automated Conveyor Sortation

Considerations When Selecting a Sortation System

“A place for everything and everything in its place” is a philosophy nowhere more applicable than when you’re overseeing a multi-million dollar distribution center (DC). A DC lives and dies on knowing when and where everything is received, stored, and shipped out.

Tracking the “history” of a carton sitting in front of you is difficult enough. Keeping track of it at speeds of up to 600-feet per minute is a whole other challenge. That’s precisely when a company needs to determine if  sortation conveyor systems are justified and if so, which sortation system is right for its operation.

A sortation system (sorter) is an integrated material handling conveyor system that automatically diverts product to a conveyor or chute for delivery to other areas of the DC including put-away, consolidation, replenishment, picking, audit, and outbound shipments. A sorter usually is chosen when the required speed and accuracy of an operation are too great to compensate for with manual labor.

An example would be a distribution center that receives truckloads of unsorted boxes of running shoes and sorts them onto pallets by style, size, width, and color. Unfortunately, every size comes in a shoe box that looks exactly alike except for the bar-code label. Needless to say, this would be a nightmare scenario to manually sort and palletize several thousand boxes of shoes. Whereas a sortation system can identify and sort based on the desired characteristic to a specific location so they can eventually be consolidated onto a pallet for further processing or storage. Obviously, this solution would require that the DC receives and ships a large enough quantity of shoes to justify a sufficient payback timeline or return on investment (ROI).  When calculating ROI one must consider several factors such as the cost of land, labor, inventory, taxes, etc. in order to justify an automated sortation solution. So where does one begin?

The first question to start with is how many cartons will be shipped out in a day and across how many work-hours or shifts? If a DC ships more than 10,000 packages per day in one eight hour shift, then there is reason for further evaluation. Whereas a DC handling less than 10,000 cartons is typically not a likely candidate due to the longer time period required for ROI. So if the numbers justify further evaluation, how does one choose the proper type of sorter?

Selecting a sortation systems will depend on several factors. It is recommended one invests a significant amount of time to understand the entire process and to interview all work cells and levels of operation. The following questions, however, will typically help narrow the options for a shipping sorter:

  1. What are the goals? Specify the needs among speed, accuracy, reducing human touches, freeing up floor space, process changes, building expansion, etc.
  2. What are the product characteristics (dimensions, weights and types)? Cartons smaller than 6” wide x 9” long x 1” tall in any dimension will affect the choice of conveyor and sorter technology. While plastic bags or different pallet types will also severely limit technology selection.
  3. Is the product trucked by pallet load, individual cartons or in another fashion?
  4. Will cartons be shrink-wrapped or have other reflective properties? This adds complexity to the system’s equipment selection such as scanners and photo eye.
  5. Does each individual carton have a readable (via inline scanner) bar code and where is the bar code located on the product?
  6. What is the average weight and the maximum weight of the product? This will affect equipment selection and/or drive size requirements.
  7. How many cartons need to be conveyed and sorted across a specific amount of time? Simple math provides the average throughput rate required for the system.
  8. Is this the peak rate the system will see? In most cases, an operation will have busier times of day than others and so a system will need to, whenever feasible, be designed for the required peak rate.
  9. If it is a shipping sortation system, how many dock doors will the sorter feed?  Can this quantity of doors accept the volume of product being delivered by the sorter?
  10. What is the anticipated growth of operations? Typically a sorter should be designed to accommodate at least three years of future business growth. Additionally, one should also have a strategy for sortation system modifications to support long-term (5-10 years) growth.

These key questions will narrow the applicable sortation options so one can begin understanding the rough cost. However, there are many other questions which should be answered before making a final selection. In fact, if the solution provider is not spending as much or more time up-front understanding the operational requirements needed to design the system, then chances are you will not be happy with the end result (garbage in = garbage out). With so many considerations to comply with, how does one choose the proper type of sorter?

Once you have determined your goals, needs and system requirements, it is time to analyze what type of sortation system will best satisfy your company’s needs. There are a number of commonly utilized sortation systems available today, depending on what you wish to accomplish. They can typically be broken down into three categories of high, medium and low throughput rate. Before proceeding it should be noted that with the global push towards “green” solutions, the rising cost of electricity and the anticipated passage of a “cap-and-trade” bill; one should strongly consider a low-voltage, 24 volt DC (VDC) sortation system. 24VDC systems are where the marketplace is heading due to energy, ergonomics, safety, maintainability, modularity, floor space utilization and noise concerns. Plus they are rapidly becoming very cost competitive with standard AC voltage technology. Their throughput rate typically falls within the medium rate category.


High Throughput Sorters – Greater Than 60 Product Sorts per Minute (PPM)

Sliding Shoe Sorter

Rate range: 70-200 PPM targeted range

The high initial investment and additional sound dampening necessary may deter some operations from the sliding shoe sorter; but this sorter works well when handling high volume and a variety of product sizes due to the sorter being a push type versus a pull type. The product is conveyed on a series of closely linked “slats” that are typically constructed of hollow-extruded, 3” to 4” wide aluminum tubes. The flat surface and closely linked slats have very little open gap between them which makes them excellent for handling a variety of different sized products. Upon each slat is a “shoe” that wraps around the slat so it easily slides and always remains attached to the same slat. When the moving product is near its intended divert destination, multiple shoes slide and push the product down a chute or a conveyor section known as a spur. Due to the high speed of the product (400-600+ feet per minute), the spur is usually a gravity-sensitive design to minimize product rotating and causing jams in the throat of the spur. This also provides a tertiary advantage of minimal maintenance and no electrical power required for the spurs.  Minimal maintenance does not come without preventative inspections to the sorter. 

Cross-Belt or Tilt-Tray Sorter

Rate range: 60+ PPM. Maximum rate will vary depending on number of induction locations.

These sorters are typically chosen for retail and postal distribution centers because they can sort a wide variety of products from CDs and Ziploc bagged products to large/heavy postal bags. These systems also have the highest initial costs which usually limits them to large applications with specifications that are impossible to accomplish by means of a conventional sorter. Operations calling for a cross-belt or tilt-tray sorter benefit from the ability of using multiple induction points.  Cross-belts and tilt-trays circulate in a horizontal loop by running on a single rail system. The difference is that a cross-belt has short individual conveyor sections oriented 90° to the direction of travel. When the cross-belt arrives at the desired divert location, the conveyor runs either left or right to discharge the item. The width of the cross-belt will vary based upon maximum product size, but they are typically less than 20” wide which means they are usually specified for smaller items that are difficult to handle. A tilt-tray is very similar to a cross-belt except product is inducted onto a concave or box-shaped tray. When the tray arrives at the desired divert location, the tray tilts either left or right to discharge the item. The width of the tilt-tray will also vary based upon maximum product size; but these sorters are typically specified for larger/heavier items since it is quicker, easier and less costly to pneumatically/electrically tilt a tray than to start a conveyor from a dead stop under a heavy load. The sorter typically discharges to chutes due to cost savings when used with a high number of divert locations.

30° Narrow Belt Sorter

Rate range: 60-110 PPM targeted range

There is one patented narrow-belt sorter design that can achieve rates above 100 PPM due to a unique divert mechanism and patented controls logic. This design raises and lowers each row of divert wheels independently in a wave action. This allows a significant reduction in the amount of gap required between products, which in turn substantially increases the throughput rate.


Medium Throughput Sorters – Between 20 and 60 PPM

24 Volt Direct Current (24 VDC) Sorter

Rate range: 0-40 PPM

The 24 VDC boasts significantly reduces power consumption and does not require expensive induction equipment.   Different from standard AC voltage systems, product accumulation is also possible between divert points as well as on inclines or declines which becomes very helpful in shipping sorter applications.  With very low noise, fewer safety considerations, reduced air compressor requirements, low spare parts inventory, lower maintenance costs, and easy to maintain equipment; 24 VDC systems are an excellent choice to consider whenever throughput rates are below 40 PPM.  The disadvantage is that the initial cost may be higher but varies depending upon manufacturer.  The divert mechanism is typically either pivoting or pop-up style wheels similar to those described below. Or at a throughput rate of less than 20 PPM one can also use a 90° transfer divert (described further below). Take-away methods are 24VDC powered spurs, gravity conveyors or gravity chutes.

Pivot-Wheel Sorters

Rate range: 40-70 PPM targeted range

With pivot-wheel sorters, the product is conveyed on top of a belt that is the full conveyor width and at each divert location is a divert section that has several smaller divert wheels that spin about 40% faster than the speed of the belt. As the product nears the divert spur, the sorter uses cams or pistons to rotate/pivot these wheels to divert product at a 30˚ angle. The rotated wheels cause the product to divert off onto a powered or gravity spur, or a chute. Often times a slave-driven, powered spur is the preferred choice for a few reasons. First, it uses very little electricity since it is slave driven. Second, since there is no motor the maintenance requirements are significantly reduced versus a motor-driven powered spur. Third, the slave-driven power will help divert the product more reliably and with fewer jams than the gravity spur or chute. This pivot style sorter also tends to divert product more reliably than the pop-up wheel sorter (with one exception).  Although bi-directional sorting is possible, additional equipment is required, which may contribute to frequent jams when a variety of sizes, weights and products are sorted.

Pop-up Wheel Sorters

Rate range: 40-70 PPM targeted range. See exceptions below.

This sorter is operates very similar to the previously mentioned pivot-wheel sorter. Instead of rotating/pivoting, however, the wheels are preset to a specific angle and then are very quickly raised and lowered using cams or pistons to divert the product. In my opinion, the only other difference is that a pivot-wheel sorter tends to render slightly fewer nuisance jams. This seems to become more evident when conveying product without a firm, flat bottom surface such as lower grade corrugate or dimpled totes.  Included within the family of pop-wheel sorters is the narrow-belt sorter. This sorter operates nearly identical to the pop-up wheel sorter except that instead of the conveying belt being full width, it is comprised of several 1”-2” strip belts spaced 2”-3” apart.


Low Throughput Sorters – Between 0 and 20 PPM

90° Transfer Sorters

Rate range: 0-20+ PPM targeted range

The two main types of 90° transfer sorters are lineshaft and narrow belt. Lineshaft sorters continue to lose market share due to their limitations versus other types of sorters as well a reduction of their previous cost advantage. However, they are still frequently used in split-case (pick and pass) applications due to good space utilization, low-pressure accumulation capabilities and reduced power requirements. Within a small footprint, a tote (or cartons) dedicated to a single order requiring multiple items (eaches) can be sorted to both sides of the lineshaft conveyor every 50’-100’. This  allows an operator to pick individual items from carton flow rack located in their area (zone), place those items into the tote and then put the tote back on the conveyor so it can be sorted directly to the next picking zone. This setup significantly reduces order fulfillment time and labor costs since an individual picker  only has to concentrate on their smaller zone. They become very familiar with where each item is specifically located in the rack and they do not need to walk great distances. Once the tote is automatically sorted to the next zone, another picker is waiting to add more items to the tote. This continues until the order is complete and the tote is conveyed out of the carton flow area for packaging or shipment. Another type of 90° transfer sorter is called a narrow-belt sorter. This sorter can provide higher throughput rates due to a more positive divert action. This is not typically used for the split-case picking operation since it requires a larger floor space. Also, one will frequently see the 90° narrow-belt sorter used to sort small products such as music CDs which is something only cross-belts and tilt-trays can offer, but at a much higher price.  Both the lineshaft and narrow-belt styles can, without sacrificing additional floor space, also be configured to sort products bi-directionally (left and right).

Pusher and Swing-Arm Sorters

Rate range: 0-20 Product Sorts per Minute (PPM) targeted range

Pusher and Swing-Arm sorters have a low initial cost but are quickly becoming obsolete technology or only used in unique situations due the potential for product damage, high noise, low throughput rates and large required footprints or gaps between products. Note: All sorter rates specified are a conservative rule-of-thumb that can be achieved for nearly all applications. Although higher rates may be obtainable; the specific application, product sizes, product type, type of induction system and scanner technology will dictate the maximum rate. So please practice due diligence and realize that sales and marketing literature will typically publish maximum rates which are only achievable under a perfect scenario.

Advances in sortation, such as the arrival of radio frequency (RFID) bar-coding, have significantly changed the sorting landscape. Distribution centers can now, with 99.5% accuracy, establish the number of products in the warehouse, their specific location, where they are to be moved, and where they are to be shipped. Bar-codes interface with a Warehouse Management System (WMS) to act like a traffic cop to track the movement of all items. Manually tracking inventory of the products is no longer necessary, eliminating not only human error but the need for “human touch” to move product from Point A to Point B. Along with giving more accurate counts, the RFID can help increase speed and, ultimately, productivity.

However, the additional cost to implement an RFID system—a label with a radio chip is used instead of a traditional bar-code—many DC operations are still in the “discussion” phase.  As the cost-vs.-benefits gap continues to slowly narrow, an RFID system that can track an item from inception through manufacturing will become more of a necessity. The medical device and prescription drug industry will most likely be among the first to implement RFID systems, where both speed and accuracy are required.

Choosing the proper sortation systems can differ by industry, especially as companies start to investigate more accurate and lower cost ways to handle, store and transport their product. Or as flexible work cells and seasonality requires more flexible, modular and adaptable systems to meet the growing needs of a distribution center.

As a facility manager once pointed out, “In the time it takes a human being to read a label on a single carton and determine where it needs to go, a high-speed sorter can read and act upon upwards of hundreds of cartons.” And when a distribution center is handling over a million units per week, that increase in throughput translates into a boost in productivity that’s hard to ignore. Evaluating your company’s situation regarding your storage requirements, shipping specifications, and most importantly your future goals, provides a helpful analysis to determine if an automated sortation system is right for your operation. 

TriFactor, LLC, a material handling systems integrator based in Lakeland, Fla.