Conveyor selection: It's all about throughput

Speed alone kills; maximum throughput for your line must acknowledge buffers and accumulators.

By Mike Pehanich, Plant Operations Editor

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When you’re planning conveyors, accumulators and buffers for your process or packaging system, focusing on throughput can carry you to the right solution.

Forget equipment that can run at the speed of light and conveyors that spiral and bend and pass through contraptions as if they were designed by Rube Goldberg. The key to manufacturing is getting more good product out the door while holding onto your pennies.

Higher throughput is everything. Nothing matters if you can’t make more product for less money.

Within that dictum is the key to selecting conveyors for processing or packaging lines. For the conveyors you choose are never more important than the product they carry or the equipment they connect.

Speed kills

Ask plant managers and manufacturing personnel about the most important equipment issues facing them today and you are likely to hear comments like faster changeover, more automation or greater line speed.

"People think that faster is better. But speed kills," says Patrick Helm, a former General Mills packaging engineer, now managing partner of The Manufacturing Systems Group Inc. (phelm@cinci.rr.com), Cincinnati, and adjunct professor at the University of Cincinnati’s school of applied science. Linking conveyor selection to throughput optimization is one of his major themes with students and packaging engineers. "The reality is that, without throughput, efficiency, line speed, changeover and automation are not important."

Nor is more automation necessarily the best solution.

"Eliminating bottlenecks is certainly key in improving productivity. But one of the biggest bottlenecks of all is lack of flexibility or adaptability on a line," says Bob Woelfling, vice president of manufacturing for Hershey Foods Corp., Hershey, Pa. "Adaptability is becoming a key factor in every part of our (manufacturing) system, especially packaging areas.

"Ten years ago, we were driving productivity at Hershey with automated case-packing and realized tremendous productivity in that fashion," Woelfling continues. "Then came display-ready cases, and most had a different configuration. We couldn’t get the hard automation we had installed to adapt.

"We have adapted since, but that was a five- to seven-year issue with us," he continues. "That was an early indicator. You automate something and think you have found a packaging configuration that is going to solve your situation. Then the marketplace changes everything!"

"Automation is expensive, and it requires higher-level skill sets for operations and maintenance personnel as well as increased planned downtime. It also reduces the flexibility of a system," echoes Helm. "With more automation, you have less opportunity to deviate. When you are designing a packaging system, you design it with a product family in mind. You have to know what those islands of automation will do. When designing a line today, you’re better off reducing its complexity."

Though the terms are often used interchangeably, Hershey’s Woelfling likes to differentiate between flexibility and adaptability.

"Flexibility for me means changing your line over on the fly [to run another product or size]," he says. "Adaptability is adjusting to a marketplace change, adapting your lines to a different product or package than you have run before. Conveyors play no small part in the adaptability/flexibility issue.

"In the overall picture, it’s how quickly you can follow the market trends that counts," he sums. "I’m looking for the kind of flexibility that can adapt in fractions of a shift, not fractions of a year."

Food manufacturers expect more from processing and packaging systems than ever before. Customers are demanding customized products and package sizes. That translates into large numbers of SKUs coming off production lines.

"You have to think what makes the system effective, and for most food manufacturers flexibility is an important requirement," says Helm. That means more planned downtime between product runs.

The first thing to consider when aiming to optimize the throughput of any production system is recognizing that it is a production system to begin with, and not just a concatenation of equipment.

"You have to look at the whole picture," says Mark Garvey, president of the Garvey Corp. (www.garvey.com), Blue Anchor, N.J., a manufacturer of conveyors, accumulators and special food systems. "People generally manage a very specific area of a process. They may spend time and money improving ‘productivity’ in that area, but what they have done may have had no real effect on overall productivity."

Conveyors: the great supporting cast

Think of the production system as a line of movement through islands of transformation – where breading is applied, product is cooked and the product changes by entering a container; where the container is sealed, the product is checked for contaminants, safety or integrity and where it is packaged or multi-packed or palletized.

These machines are Helm's islands of automation. To bridge these islands, you use conveyors.

"Conveyors are transports or bridges, and they are as important as the islands of automation themselves," says Helm. "I call them the bridges of automation and compare the production line to the Florida Keys. You have all these islands connected by Highway 1. Think of the Keys as islands of automation and the causeways between them as the conveyors."

Effectiveness is process reliability, Helm explains, and only running time improves it. "It’s calculated as the difference between the time a line makes product and the unplanned downtime. But most lines have from 20- to 30-percent planned downtime built into their schedule." Cutting that planned downtime – not faster lines -- is the key to greater throughput, he says.

A half-dozen changeovers in a day at 10 minutes per changeover translates into an hour of lost production time. In a climate of frequent changeover, reducing this planned downtime can boost production significantly.

Keep your lines well-buffed

Helm identifies three functions for conveyors: transport, buffering and accumulation. Transport conveyors move product from point A to point B. Effective buffering and accumulation can cut down or eliminate the "stops and goes" that gimp production and boost waste.

Buffering conveyors are meant to hold more product than can be used by the machine they precede. Accumulation conveyors are meant to accumulate product after a machine while a downstream machine is not running. Both are critical to dealing with constraints.

A constraint is a bottleneck or hang-up in the system. Its cause can be equipment breakdown, a slow or hampered machine or inadequate conveyors. Identifying constraints may be the most important step in process or packaging system improvement. Such analysis also provides critical insight into the selection and optimization of conveyors, buffers and accumulators.

Knowing the location of constraints will enable you to determine strategic buffer locations and the capacity of those buffers, and confirm needed rate capacity of "non-constraints," those line areas that run unencumbered, says Garvey. Understanding these constraints enables one to predict throughput and determine efficiency thresholds for each machine.

"As everyone pursues high speed in their lines, the thing that makes their system successful is buffering," says Hershey’s Woelfling. He also notes that food manufacturers have used simulation and modeling extensively to fine-tune buffering and accumulation requirements.

"In a close-coupled system, when a non-constraint [free-running machine or line portion] shuts down, it shuts down the constraint because the whole line is an interconnected system," says Garvey. "But you can protect the constraint by adding buffers in key locations. These buffers allow the constraint to run even when you have a malfunction of a non-constraint. If you do this right, you will increase throughput."

Buffers upstream of a constraint will be full, he explains. They can even run faster than the overall line and produce excess product to fill the buffer. During a malfunction of a non-constraint machine occurs upstream, the constraint can keep running with the product in the buffer. This provides time for the operator to fix the upstream problem and start again before the product in the buffer runs out.

Accumulators located downstream of the constraint will be empty most of the time by design. The downstream machines can even run faster to keep excess product from building up in the accumulator. Then when a downstream machine malfunctions, the constraint can still run. The excess product simply gets stored in the buffer.

In either case, the result is greater throughput.

Virtually all conveyance, buffering and accumulation needs can be accurately figured by determining throughput requirement and the instantaneous operating speed of the line, says Helm.

Hand in hand with proper equipment and conveyor selection is the communication between machines. Field devices must enable machines to communicate so that each machine is aware of the activity of the machines before and after it. The machines and/or their operators need to know when machines are running fast or slow or if they are running at all.

"The minimum signals required from one machine to another are: I am running; I have an emergency stop; I have a cycle stop," says Helm. This communication is absolutely critical to sustaining production through the inevitable constraints on a production line.

Where do most processors err when putting together packaging or processing lines?

"The two classic mistakes that people make are buying a machine that runs too slow or not buying a long-enough conveyor," says Helm. "You can make a conveyor a lot longer to protect your line against constraints, but this costs you more in equipment and also increases planned downtime because you have more bottles to move out when the line shuts down."

Though the principles of selecting conveyors for greater throughput are most easily followed in a packaging context, the model works equally well applied to a processing system.

CAPTIONS:

[spantech-print]

Three transport conveyors end-to-end: Chocolate chip cookies, hot out of the oven, are transferred from one conveyor to another using SpanTech’s MicroSpan transfer conveyor as an alternative to roller transfers.

[For one of the PPT slides:]

In this illustration of a packaging line, product flows from the process line (product delivery) to unit load handling, passing through primary, secondary and tertiary packaging stations and a unit load handling system before being stacked on pallets.

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