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 single carton sitting in its storage location is difficult enough. Keeping track of it at speeds of up to 600 feet per minute is a totally different challenge. That’s precisely when a company needs to determine if a sortation system is 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 DC 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 shoeboxes that look exactly like every other shoebox, except for the bar code label. Needless to say, it would be a nightmare scenario to manually sort and palletize several thousand boxes of shoes. However, 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 requires that the DC receives and ships a large enough quantity of shoes to justify a sufficient payback timeline or ROI. When calculating ROI, one must consider such factors as the cost of land, labor, inventory and taxes to justify an automated sortation solution. So where does one begin?

NARROWING DOWN THE OPTIONS

Choosing the right sorter depends on several factors. To assure accuracy, a significant amount of time should be invested in understanding the entire process and interviewing all work cells and levels of operation. These questions 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 inches wide by 9 inches long by 1 inch tall in any dimension will affect the choice of conveyor and sorter technology. Plastic bags or unusual pallet types will also limit technology selection.

3. Is the product trucked by pallet load, individual cartons or some other way?

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 eyes.

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 will be conveyed and sorted in a specific amount of time? Simple math provides the average throughput rate required for the system.

8. What is the system’s expected peak rate? 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 number 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 (five to 10 years) growth.

These key questions can help narrow the applicable sortation options to begin understanding the rough cost. However, there are many other questions that should be answered before making a final selection. In fact, if the solution provider does not spend as much or more time up-front understanding the operational requirements needed to design the system, then there’s a very good chance the system will not perform as expected. With so many considerations, how does one choose the proper type of sorter?

Once you have determined your goals, needs and system requirements, it’s time to analyze what type of sortation system will best satisfy your company’s needs. There are a number of sortation systems available, depending on what you wish to accomplish and they are based on high, medium and low throughput rate.

At this point, it should be noted that with the global push towards “green” solutions, the rising cost of electricity and the anticipated carbon reduction legislation, it is prudent to consider a low-voltage, 24 volt DC (VDC) sortation system. This is 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 category.

HIGH-THROUGHPUT SORTERS—GREATER THAN 60 PPM

1. Sliding Shoe Sorter (rate range: 70 to 200 Product Sorts Per Minute (PPM) targeted range)*

The high initial investment and additional required sound dampening may deter some operations from selecting a sliding shoe sorter. However, it works well when handling high volume and a variety of product sizes since it is a push type rather than a pull type. The product is conveyed on a series of closely linked “slats” that are generally constructed of hollow-extruded, 3- to 4-inch-wide aluminum tubes.

The flat surface and closely linked slats have very little open gap between them, which make them excellent for handling a variety of different sized products. Upon each slat is a “shoe” that wraps around the slat so it slides easily 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.

Because of the product’s high speed (400 to 600 feet per minute or more), the spur is usually a gravity-sensitive design to minimize product rotating, which causes 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.

2. Cross-Belt Or Tilt-Tray Sorter (rate range: 60-plus PPM; maximum rate varies depending on number of induction locations)

These sorters are usually chosen for retail and postal distribution centers because they can handle a wide variety of products from CDs and Ziploc bagged products to large/heavy postal bags. Since these systems also have the highest initial costs, this usually limits them to large applications with specifications that are impossible to accomplish with a conventional sorter.

Operations calling for a cross-belt or tilt-tray sorter benefit from the availability of 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 degrees 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 inches wide, meaning 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, it tilts either left or right to discharge the item. The width of the tilt-tray will also vary based on maximum product size. However, these sorters are generally 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.

3. 30 Degree Narrow Belt Sorter (rate range: 60 to 110 PPM targeted range)

There’s 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. This, in turn, substantially increases the throughput rate.

Medium-Throughput Sorters—Between 20 And 60 PPM

1. 24 Volt Direct Current (VDC) Sorter (Rate range: 0 to 40 PPM)

The 24 VDC boasts significantly reduced 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. This is 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, although it 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 degree transfer divert. Take-away methods are 24 VDC powered spurs, gravity conveyors or gravity chutes.

2. Pivot-Wheel Sorters (rate range: 40 to 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 with several smaller divert wheels that spin about 40 percent 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 degree 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. Second, since there is no motor, the maintenance requirements are significantly lower than for 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; See below). 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.

3. Pop-Up Wheel Sorters (rate range: 40 to 70 PPM targeted range)

This sorter is quite similar to the 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. The only other difference is that a pivot-wheel sorter tends to have slightly fewer nuisance jams. This seems to become more evident when conveying product without a firm, flat bottom surface, such as lower grade corrugated or dimpled totes.

LOW-THROUGHPUT SORTERS—BETWEEN 0 AND 20 PPM

1. 90 Degree Transfer Sorters (rate range: 0 to 20-plus PPM targeted range)

The two main types of 90 degree transfer sorters are lineshaft and narrow belt. Lineshaft sorters continue to lose market share due to their limitations versus other types of sorters and a reduction in 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 to 100 degrees. This allows an operator to pick individual items from carton flow racks located in their area, 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 pickers only need to concentrate on their smaller zone. The pickers become very familiar with where each item is specifically located in the rack and they don’t need to walk long 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.

2. Pusher And Swing-Arm Sorters (rate range: 0 to 20 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.

Sortation And The Role Of RFID

Advances in sortation, such as the arrival of RFID barcoding, have significantly changed the sorting landscape. Distribution centers can now, with 99.5 percent 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 WMS to act like a traffic cop to track the movement of all items. The manual inventory tracking 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 productivity.

However, the additional cost to implement an RFID system—a label with a radio chip is used instead of a traditional barcode—many DC operations are still in the “discussion” phase. As the cost-verses-benefits gap continues to narrow slowly, 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 the first to implement RFID systems, where both speed and accuracy are required.

The value of a sortation system was best expressed by a facility manager who 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 DC is handling over a million units per week, that increase in throughput translates into a boost in productivity that’s difficult 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 you.

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.

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