Design engineers like yourself use clutches and brakes to solve a myriad
of real-world problems. Today's clutches and brakes, often supplemented by
control electronics, offer performance unavailable to engineers only a decade
No more drift. Working with application engineers from Warner Electric Corp., engineers Gary Marsh and Mike Buttrill of BAE Inc., Dallas, TX, put together a closed-loop control system that automatically compensates for clutch/brake drift in the baggage-handling conveyor system at Dallas-Fort Worth International Airport (DFW).
The conveyors that form the backbone of airport baggage-handling operations must meet demands for non-stop operation, precision, repeatability, and accuracy. Furthermore, system throughput requirements typically change as air traffic volume at an airport changes. At DFW's Terminal 2E, a belt conveyor system carries output-bound passengers' bags to piers to await loading onto planes. The bags pass through a scanning station, and six scanner heads sweep each bag, looking for a bar code. Upon receiving the baggage identification data, the baggage-handling control system identifies the correct divert point.
Downstream from the scanning station, the outbound conveyor system consists of an upper and a lower line. On each line, twenty pusher arms divert bags down chutes to other conveyors that lead to loading piers. Originally, the pusher arm system used limit switches and cams to control each pusher arm's arc of movement. Unfortunately, this approach led to some problems. "Using a purely mechanical approach to the design of the pushers resulted in drift problems, " explains Les Sanders, supervisor of facility maintenance at American Airlines. "During periods of high-frequency cycling, the clutch/brake units would heat up, and their accuracy would begin to suffer. Sometimes a pusher arm would return to its proper zero position without a hitch, at other times it would drift out of position."
The same problem occurred when maintenance personnel installed new friction facings in a clutch/brake. During the seating-in period, the unit's performance could vary. "As the throughput demands of our operation here increased," says Sanders, "so did the frequency of drift." He points out that drift/cycle can prove nearly immeasurable, but cumulative drift can become critical. After drift reached a certain point in the old system, a pusher would cycle back to an incorrect zero position. The pusher could wind up so far out of alignment that bags struck it as the conveyor moved them along. Result: an unscheduled interruption in operations until workers set matters right.
Anticipating the need for higher performance from the system in the near future, Sanders and his colleagues at American Airlines turned to BAE Inc. in search of a design solution. Gary Marsh, Mike Buttrill, and the other members of the BAE team set out to find the best means of automatically compensating the drift experienced by each of DFW's 40 pushers. "What we needed was a clutch/brake that could recognize those occasions when it was drifting, then correct itself without requiring operator input," explains Buttrill.
Warner electric application engineers helped the BAE team select a control system that employs a fully programmable closed-loop position control, an electromagnetic clutch/brake, a clutch/brake control, and a shaft encoder. The positioning control, a Warner Electric CBC-1500AH, continuously calculates a brake actuation point to minimize error from cycle to cycle. Encoder feedback closes the position loop by supplying the controller with pulses proportional to load motion. This feedback enables the CBC-1500AH to determine the optimum brake actuation point.
A marker input from the encoder provides data needed by the controller's autohome feature to find the user-defined zero position when powering-up. Users can program the new control system for use in either incremental or absolute modes of operation; onboard potentiometers provide dual-channel torque adjustment for soft starts and stops.
"Even when we're handling bags at the rate of 80 bags per minute per pusher," reports Sanders, "the positioning controls get the pusher arms back to the correct zero position at the end of every cycle. And we've essentially cured the problem of accidental contact between the pusher arms and the bags on the conveyors."
Off the ropes. A few years ago, intense competition from servo- and step-motor systems forced makers of electromagnetic friction clutches used in business machines like copiers, printers, and fax machines to look to their knitting. With the introduction of low-inertia servomotors, most clutch makers feared a significant decrease in demand for their products. Also, the popularity of steppers with low-cost controls did little to warm the hearts of electromagnetic clutch builders.
As Alabama's legendary football coach Bear Bryant said long ago, "When the going gets tough, the tough get going." Manufacturers of electromagnetic clutches here in the U.S., faced with a hard-nosed challenge to their technology, sized up the competition, dug in, and got going.
Barry Bistis, president of API Deltran, Amherst, NY, says changes in office machines drove his company's redesign of small clutches. Markets wanted smaller, lighter, faster office machines. "The way systems are designed now, there's no longer just one big motor drive and heavy clutches running different parts of the machine. There are steppers and different types of motors within the machine that power subsystems."
These changes created a situation in which clutches in business machines typically see low inertial loads. Thus the clutches used in new office equipment these days usually are 26- and 36-mm OD sizes. Deltran makes its models CS-08 (26 mm) and CS-11 (36 mm) for such equipment.
Consolidation of clutch functions into the two most common sizes made manufacture in the U.S. practical, says Bistis. In machines manufactured in Japan, "many times you'll find a half-dozen or more clutches on a machine," he explains. "For the most part those clutches aren't necessarily the same. As a matter of fact, it's very rare that they would be the same."
At API Deltran, success depended upon consolidating the abilities of these disparate clutches into a few common designs. This approach enabled the company to raise the volumes of particular components and therefore reduce their unit cost.
Because of the lower inertial loads on them, today's office-machine clutches must dissipate less energy than units formerly employed in such equipment. By designing small clutches so that they comfortably handle the energy dissipation required of them, and by using new materials in the clutches, engineers at API Deltran extend unit life rather dramatically. In the past, low-cost clutches typically didn't survive one million operational cycles. These days it's not unusual for clutches to provide more than 10 million maintenance-free cycles.
Changes in materials used in new low-cost clutches significantly improved their performance. "Particularly some of the bearing materials, and the integration of bearings into substrates on parts, for example TeflonTM coatings on parts," says Bistis. "The bearings now used are made of high performance plastics that provide much longer life." Even in thrust bearings, long made from MylarTM, high-performance plastics reduce wear and extend life.
Coil redesign in small clutches, which might appear routine to outsiders, also has advanced. In many of API Deltran's small clutches, engineers integrate connectors into the coils. Forming terminations into the bobbin assembly makes it possible to wind coils on high-speed machines, and permits automatic assembly rather than hand-assembly. This design also makes it possible to eliminate time-consuming hand soldering.
What reward has API Deltran achieved after putting forth all this effort on low-cost clutches? "We became not only competitive but, in this country, the leader in the area," Bistis proudly claims.
Pushin' pasta. An electric clutch protects a variable-speed ac motor on a conveyor that carries finished pasta products, and also smoothes operating cycles. A. Zerega's Sons, Inc., in Fairlawn, NJ, uses a Totally Enclosed, Non-Ventilated (TENV) Stearns(R) Super-Mod(R) clutch, made by the Stearns Div. of Rexnord Corp. and especially suited to food industry applications.
America's first pastamaker, Antoine Zerega, founded A. Zerega's Sons and, in the 1890s, introduced semolina milled from durum wheat to the U.S. Today, the company produces pasta for private brands and the firm's own Columbia brand.
On A. Zerega's Sons' production line, after the extrusion and forming of short products like bow-tie, spirals, or shells, pasta travels through long drying ovens on belt conveyors. When it leaves the oven, another belt conveyor transports the pasta to a bucket elevator that loads a packaging machine's hopper. Because the bucket elevator stops and starts as necessary to maintain the product level in that hopper, the feed conveyor must also stop and start, to prevent spillage.
During this process, one of the conveyors, equipped with a motor and a variable-speed ac drive, switched on and off at least 45 to 50 times every hour to control the feed. Larry Filato, maintenance manager at A. Zerega's Sons, knew this process represented an inefficient use of the ac drive. And he believed that the screwy duty cycle could cause premature drive wear or even failure.
To deal with the problem, Filato placed a clutch between the motor and drive. Unfortunately, it proved excessively noisy and failed after a few months. He then upgraded to the Super-Mod TENV SM 50-1040 clutch, which has operated quietly and reliably for more than a year. "Even if the clutch does cycle a lot," says Filato, "it's going to prevent wear and tear on the ac drive, and that's the most expensive part of the line."
Rated at 16 lb-ft static torque, the C-face unit is designed for superior thermal capacity. Its cast-aluminum housing meets IP54 requirements and prevents dirt and moisture from affecting clutch operation. An integrally cast magnetic body and endbell function as a large heat sink, while an unusual fan creates bi-directional air movement within the clutch to stabilize its internal temperature.
"We put the clutch on over a year ago. When the line runs, it cycles about once a minute. All we've had to do is blow the dust off it with an air hose now and then," says Filato. "The plant is open 24 hours a day, and we only shut down on Christmas and New Year's Day, so it takes a lot of wear and tear."
Compact vehicle brake. Tow tractors, the vehicles that haul material to and from aircraft, require components that can fit into confined areas. A disc brake designed for mounting on steering and drive axles enables PDI Ground Support Systems, Inc., Cleveland, OH, to offer builders of tow tractors the first front steering axle system equipped with compact disc brakes. Manufactured by Northwestern Motor Co., Eau Claire, WI, the tow tractors come in two versions: military and commercial.
Made by Tol-O-Matic, Inc., Hamel, MN, the QC-220 hydraulic disc brake used on both versions of the tractor delivers high braking torque in a small package. It's a member of a family of calipers that range in weight from 3 to 4.5 lbs, and have replaceable pads and two pistons instead of one to conform to the unit's low profile. The 220 Series brakes come rated to 1,500 psi and use standard EPR seals (Buna-N seals are optional). Tol-O-Matic offers the standard 220 Series brake with round pads that have a wearable volume of 1.66 cu-inches. To improve life, the QC 220 contains 3.612 cu-inches of wearable volume.
"Most tow-tractor manufacturers have a very tight envelope in which the front steering axles, wheels, and tires operate," says Irwin Haber, president of PDI Ground Support. "When you put disc brakes into the product, you're forced in many cases into using a larger wheel and tire." Unfortunately, the larger wheel and tire reduce the vehicle's turning angle to less than 55 degrees. That's a significant disadvantage when the tractor must maneuver around aircraft and other equipment out on a flight line.
Until the QC-220 came onto the market, compact disc brakes were not available for use on PDI Ground Support's low-profile steerable axles. The new brake allows PDI to put disc brakes on an axle capable of maintaining a 55-degree turning angle for use in tight areas.
"This is the first tractor front axle with compact disc brakes," says Haber. The quick-change disc pads require minimal disassembly during maintenance, saving time and money for the vehicle's user. More conventional designs employ drum brakes, and servicing drum brakes is more expensive than working on the new Tol-O-Matic disc brakes.
Baggage conveyors/sorters, pasta production machinery, office machines, a tow tractor for airlines (and the military)--all very different applications. But in each of them, clutches and brakes play an important role, helping these very different types of equipment work more efficiently and survive for longer periods.
Les Gould contributed to the preparation of this article.
Moving baggage, Texas style
Controlled by clutch/brake units, baggage pushers divert bags to the appropriate loading pier at Dallas-Fort Worth International Airport. By automatically correcting for clutch/brake drift, the Warner Electric control system prevents downtime and ensures efficient baggage sorting.
During operation, as a pusher arm swings through its arc and returns, the CBC-1500AH reads encoder data. It enables the control system to accurately determine the position of the pusher arm at the end of every cycle. If the pusher arm has not returned to its proper zero position, the control system calculates the arc of movement necessary to correct the arm's position. During the next operating cycle, the control system corrects the drift, and the arm returns to the specified zero position.
To reduce any jerkiness that might result from comparatively large corrections, users can program the control system to base its calculations on an average of several cycles. If large corrections don't present problems, the control can be programmed to base corrections solely on the previous cycle.