A candy maker has streamlined its material handling capabilities by replacing its rod-less-cylinder actuators with rigid-chain units.

Y&S Candies, a division of Hershey Foods Corp., Lancaster, PA was experiencing frequent failures with the multiple rodless-cylinder systems in the plant, imposing a heavy burden of downtime and repair costs. The systems are used to transfer and position a railed load of approximately 7.5 tons over a total motion of about 9 ft.

The long-stroke rodless (pneumatic) cylinder systems originally selected by Y&S for the load transfer/positioning application were easily adapted to the job. In each setup, two cylinders are installed in a pit beneath the floor and attached to a drawbar that pulls the load along railed track.

From the beginning, however, problems with the rodless cylinders have cost Y&S hundreds of man hours in repair work and thousands of dollars in replacement parts. According to Y&S maintenance group leader Bob Devolder, the biggest problem has been the pneumatic systems' inability to withstand shock loads that sometimes occur in the load transfer application.

"Some shock loads occurred when an operator was in too much of a hurry during loading, and the load slammed or jarred the drawbar," says Devolder.

Shock loads also occurred because the air cylinder system did not permit tandem synchronization of the paired cylinders. The railed load would sometimes stick on one side. One cylinder might then pull ahead in an attempt to push the load, while the other, encountering excess resistance, would first bind up - then become unstuck and suddenly lurch ahead into the load-bearing drawbar.

"In both cases," Devolder explains, "the shock load was transmitted to the cylinder body and piston, since the air cylinders were mechanically attached to the drawbar. We believe the shock loads were a principal cause of the principal cause of the frequent air leaks we experienced, as well as of mechanical damage to the cylinders themselves."

The shock loads may also have been behind another recurring problem.

"We continually had to replace cylinder supports bolts that were simply shearing off." Says Devolder, explaining that the bolts are used to hold the linear actuators in a vertical position for the activating the drawbar.

Beyond the continual repairs for system failures, the rodless cylinder actuators were also subject to extensive downtime for maintenance and setup. One regular service requirement, for example, was to clean and replace clogged air lines and filters.

Frequent maintenance was also required to adjust the flow controls on the incoming and exhaust air lines. "That's the way we tried to synchronize the two cylinders, since the rate of air flow directly affects cylinder speed," says Devolder. He points out, however, that a number of variables also affect cylinder speed, making the synchronization effort both painstaking and a matter of trial-and-error, which meant Devolder's crew had to adjust the flows controls at least every other night.

Another tedious task was adjusting the actuator "prox switches" - the devices that signal the control system that cylinders have reached a 100% forward or back position. Unless the cylinders have reached the 100% forward position, the prox switch will not signal the control system to retract the cylinders.

In cases where the switches vibrated out of adjustment, or for whatever reason failed to signal, a mechanic had to lift the pit cover plate and index the drawbar until it was in the correct position, then readjust the prox switch. According to Devolder, such adjustments were being made at a rate of at least once or twice a week.

In the face of mounting downtime and costs to repair and maintain its rodless cylinders, Y&S looked into alternative load transfer/positioning systems. One system investigated was a ball-screw type linear actuated system offered by Serapid U.S.A, Inc. Troy, MI.

"I learned about the Serapid system through a magazine ad," says Devolder. "The description of the system's design intrigued me. I called Serapid to get more details, and was quickly convinced it was probably the solution we needed."

The Serapid rigid-chain actuator eliminates the "out-of-sync" conditions that had been a major problem with the rod-less cylinders at Y&S. The system uses a single motor to drive the paired pushing mechanisms in a standard power transmission arrangement. This ensures continuous synchronization, even with unequal loads on the two sides.

"The rigid-chain assembly is virtually maintenance-free; you never even have to lubricated it," says Devolder of his decision to go with the Serapid system. "It's also tough enough to absorb the shock loads in our application, which we think was a big cause of our problems with the rodless cylinders."

On this basis, Y&S purchased a single Serapid rigid-chain actuator as a prototype replacement for one of its rodless cylinder systems. The new system was easily retrofitted to both the existing underfloor pit and drawbar. Since then, additional rigid-chain units have been purchased, and a program is now in place to retrofit all of the company's rodless cylinders used in the load transfer/positioning application.

The Serapid rigid-chain system is a self-contained linear actuator consisting of a rigid chain assembly and a gear drive. A unique characteristic of the system is the ability of its power chain not only to pull, but also to push a load with the rigidity of steel bar.

This thrust capability is designed into the chain links, which have special shoulders that rest against each other as the links emerge from the drive housing. Regardless of load weight, the system provides long travel strokes and high positioning accuracy. When the chain is retracted, it relaxes for coiling in a highly compact storage pattern.

The Serapid actuator system can replace any of the conventional devices now commonly used for load transfer positioning, including rodless cylinders and ball screw and belt drive actuators.

The rigid chain assembly needs no tensioning devices such as those required with belt-drive systems. The rigid chain is very tight and precise, and does not stretch as temperature or use increases. In addition, no elements of the rigid chain assembly are subject to wear.

Y&S Candies has benefited from many of these advantages in its load transfer/positioning application. "In light of our experience with our Original system, we especially appreciate the high reliability and low maintenance of the Serapid system," says Devolder.

"But we're also impressed by its performance," he continues. "Serapid set the speed of the system to meet our needs exactly. In each installation, we've found both the speed and accuracy to be superb. The movement of the drawbar is always accurate and in time."

As a retrofit replacement of the rodless cylinder system, the Serapid system has been adapted to fit the existing the underfloor pit and the frame around it. As the chain retracts, it simply accumulates inside the assembly's lower guide.

In many applications however, it's the high maneuverability and unmatched compactness of the Serapid system that make it the most practical solution available.