by Jim Butschli, Technical Editor; Packaging World 10/97

Conveyor systems are often overlooked, as are the components that maximize conveyor efficiency. But at the United States Postal Service's (USPS) Carol Stream, IL, Processing and Distribution Center, a $4.5 million conveyor system retrofit commands center stage. The system carries more than 4.6 million pieces of mail through the center every day.

Commenced in mid-'96, the retrofit is expected to be completed within the next month. For the retrofit, USPS kept the framework of the original conveyor system that was installed for the 750,000 sq' facility's May 1992 opening. That saved time and money. The original conventional rollers, on the other hand, were removed using simple hand tools.

In their place are individually powered "intelligent" rollers that are linked by a computer to control modules and photoelectric sensors through a Smart Distributed System (SDS) intelligent bus network and SmartControl system from Honeywell's Micro Switch Div. (Freeport, IL). The galvanized steel Intelliveyor® rollers are supplied by Interroll (Wilmington, NC). Serving as systems integrator for the project was Key Handling Systems (Moonachie, NJ). KHS oversaw several subcontractors involved in setting up the electrical and software systems involved with the retrofit.

Mail is conveyed in plastic or corrugated trays or tubs. Typically, each 24" long x 11" wide tray, for example, holds about 300 pieces. Trays and tubs are transported through the center along some 11,000' of conveyor that includes about 115 curves or lane merges. Scanners are used to read bar-coded labels on trays and tubs to help sort and divert them to their intended destinations within a 450,000 sq' work area. Like the conveyor control system, scanners too are linked to a central computer, though they don't communicate directly with the Honeywell components.

Zone controls

The new conveyor control system is divided into 27"-long zones at the Carol Stream facility. Each zone includes a photoelectric sensor and nine rollers positioned on 3" centers. One is a motorized drive roller while the other eight are considered slave rollers.

In most instances, four zones are controlled by Honeywell's Powered Roller Interface Module (PRIM). Each plastic-housed PRIM is equipped with several connector interfaces. One is used to connect to the electrical power source. Four accommodate motor drive circuitry that permits communication between the zone's drive rollers and sensors. Each PRIM is also connected to the central computer through a data bus. A data bus is a path for transmitting data between devices. The central computer permits communication between PRIMs over the SDS network.

As a tray enters a particular zone, a sensor (part of the Honeywell system) signals that zone's PRIM module, which in turn actuates the drive roller. The drive roller initiates movement of slave rollers through the assistance of "O-rings," also from Interroll. Each roller has two polyurethane, belt-like O-rings attached at one end, positioned about 1" apart from each other. Throughout the system, one O-ring connects a roller to the previous roller, while the second O-ring connects from the roller to the following one. That way, when one roller moves, the O-rings begin to rotate the next roller.

'Intelligent' system

A PRIM communicates to the next zone to let it know a tray is approaching. If the next zone is empty, the PRIM activates the drive roller of the current zone to move the tray to the next zone. However, if there's already a tray within that next zone, the PRIM signals the current drive roller to stop, thereby preventing trays from colliding.

In the past, if a tray with fewer pieces was sandwiched between two trays with a heavier load, impact between the continuously moving trays tended to push the lighter-weight tray off center, requiring postal workers to constantly monitor tray progress and reposition them when necessary.

"These are zero pressure accumulation conveyors, and they prevent bumping or jackknifing of trays in transport. They also improve our conveyor control," says George Cavell, a postal consultant who oversaw the retrofit. While not on the USPS payroll, Cavell maintains an office at the Carol Stream facility, serving as an intermediary between the project's contractors and the USPS.

When a sensor detects a tray entering the final zone controlled by a PRIM, it sends a message across the SDS network to the host computer. The computer then sends a message to the next PRIM to be ready for a tray.

The system's communications "intelligence" is superior to the facility's former conventional conveyor system, notes Cavell. "Before we added the new conveyors, our conveyors ran continuously. Now they only run when a tray is in the zone, so run time has been cut way down, even during peak periods."

Multiple benefits

As a result of the conveyor system, operational efficiency at the Carol Stream center is up while costs for electrical consumption and maintenance are down. Cavell explains that a USPS decision analysis report projects that the retrofit will save nearly $280,000/ yr through reduced routine preventive and corrective maintenance, and lowered electrical utility bills.

The reduction in power consumption is easily explained: When a zone is empty, the conveyor system automatically shuts off electrical power to that zone. Power is restored immediately when a sensor detects a tray. "Depending on the specific area within the system, power requirements are down anywhere from 50 to 85 percent," says Cavell. The USPS report forecasts electrical savings of nearly $60,000/yr.

Another important advantage of the retrofitted conveyor system is that it requires little maintenance. "With this system, there's no regular preventive maintenance other than cleaning and removing paper dust from the sensors," says Cavell. "Basically, we don't do anything unless a component fails. If that happens, to a motor for instance, we just replace it.

"On the old system," he continues, "there was a rigorous regular preventive maintenance schedule, as well as repair and breakdown maintenance. Often that involved changing oil and grease, which are considered hazardous wastes. We no longer have to deal with that."

USPS projections forecast savings of $153,000/yr in routine annual preventive maintenance costs, and another $64,000/yr in annual corrective or repair savings.

Declining decibel levels

While those savings are impressive, the driving factor behind the retrofit was combatting plant noise. Increasing volume raised USPS's concern for the health of its employees, while bringing about the possibility that the facility would have to adhere to costly Occupational Safety and Health Administration (OSHA) noise-reduction programs. Compliance with such programs, according to USPS's report, would have cost upwards of $115,000 annually in adminstrative expenses, settlements and the purchase of protective hearing devices.

"Decibel levels were creeping up around 81 or 82 decibels," notes Cavel. "At about 85 db, we would be at the point where we'd have had to implement OSHA's Hearing Conservation Cost Avoidance Program. We wanted to avoid those potential costs and maintain a healthy working environment."

The original conveyor system contributed significantly to noise levels. "Until the retrofit, the Carol Stream facility used a traditional roller conveyor system with a chain that used a clutch mechanism to drive the rollers. That chain made quite a bit of noise," recalls Cavell.

The retrofitted conveyor system, he contends, "is inherently quieter. It doesn't use this chain, and the O-rings put more tension on the rollers to hold them more tightly in place on the conveyor than did the previous chain. That helps reduce 'chattering' of the rollers within the frame."

Besides the quieter conveyor system, USPS invested in hundreds of "mats" that are suspended from the ceiling. These help baffle sound within the facility. "Since making the changes, decibel levels at the plant now range between 67 and 72," Cavell estimates. "We've lowered our decibel level between 10 and 13 db in general." Because of the way sound is measured, notes Cavell, that represents a significant reduction.

Ready for the future

So what's the payback for the retrofit project? "Normally we would shoot for a minimum return of investment of 12 to 20 percent per year, for five to seven years," says Cavell. "But because of intangibles such as health claims and cost-avoidance hearing programs, ROI is more difficult to pin down. As a result, our ROI is likely to be longer than normal."

Nevertheless, Cavell feels USPS's investment is prudent in large part because the retrofit has improved efficiency. "There's more uptime now, so the facility is able to handle more volume, more reliably, in less time. That's difficult to measure, but it's important," he says.

Uptime becomes more significant in a rapidly growing community like Carol Stream, where the zip codes handled by the USPS facility continue to generate increasing numbers of parcels from business and residential customers.

Carol Stream's sister facility in nearby Palatine, IL, completed a similar conveyor system retrofit at roughly the same time. Cavell says that while the Palatine Processing and Distribution Center handles more mail, its conveyor system is less extensive than Carol Stream's. There, the round-the-clock operation is poised to transport more pieces of mail in less time, and at less cost, thanks to the retrofitted conveyor system.

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