Case Packer Renewal

Using control retrofits to address production bottlenecks can boost manufacturing efficiency. For a major tissue manufacturer, replacing the pneumatic cylinders on old case packers and moving that process into a tightly-controlled motion system increased case packer speed by 160 percent.

"Twenty years ago, pneumatic cylinders controlled virtually every axis of a case packer," says Peter Ratcliffe, president of RPT Motion based in Quebec, Canada. "New case packers today are often all servo-controlled. We are taking old case packers and adding two-axis pushers and servo-controlled elevators to provide significantly more speed."

Opportunity Knocks

Retrofits are, by their nature, opportunistic projects. In this situation, the company upgraded part of the production line upstream and it now had more product than it could case pack. Another plant in the corporation replaced an existing case packer of the same model. That plant gave the old case packer to this plant for nothing. But this plant needed a system to distribute product to both case packers.

At the heart of the new design was the replacement of a two-position lane changer, which was powered by a pneumatic cylinder. The cylinder couldn't deliver four positions or keep up with the higher speeds required by the two machines. To serve all four lanes, the design needed to stroke over 40 inches while indexing the lane changer every 10 seconds or less and maximizing time available for product flow.

"We couldn't fit the ball screw and nut into our existing design with a fast enough pitch to suit the higher speeds," says Ratcliffe. "It would hit its limits and not allow us to reach the high linear speeds we needed. We could have found a ball screw with a fast lead, but it would not have fit into the current assembly, forcing us to redesign the entire slide system for a bigger, heavier screw."

To solve this design problem, RPT worked with Kerk Motion (www.kerkmotion.com) on a system to feed the required four lanes (two lanes in each of two case packers). Engineers chose a VHD Series lead screw which comes in a variety of different leads in the same size physical package. To meet the high speeds required by the lane changer, RPT selected a 2.400 inch travel per revolution screw of ¾ inch nominal diameter which had an efficiency about equal to that of a ball nut.

Design Challenge

One design challenge in implementing the lead screw was its slightly lower efficiency and limits on top speed. But by matching the linear speed the application required to the available rotary speed of a longer screw, engineers were able to move to a product with a faster lead. If they were designing the system today, the engineers could create a system that's 40 percent faster because a lead screw with a four inch lead is now available.

"The Kerk VHD lead screw is a convenient solution for linear motion applications," says Ratcliffe. "We looked at its limitations in terms of efficiency and top speed, and optimized its application to meet the requirements. By managing peak motor torques and acceleration rates, we were able to get a system that performed over the long term in an outstanding and predictable fashion with virtually zero maintenance."

The lane changer operated without failure or noticeable backlash for three years — unheard of in most linear motion applications, according to Ratcliffe. The use of the self-lubricating Kerk VHD Series also held up well in the plant's tough environment.

Almost everything in the cartoning and case packing area wears a layer of tissue dust. Grease captures the dust and creates an abrasive paste. But the VHD screws do not require grease to operate. Further, no greasy ball circuits are used which might be damaged by dust-laden paste clinging to a ballscrew.

Ratcliffe says, even in that environment, the lead screw lasted four or five times longer than they thought it would. The plant actually replaced it long before it showed any signs of wearing out because it is not an expensive component, whereas stopping the line is a costly undertaking.

"It is a tough, smooth-running mechanical component," he adds. "We benefit from it in our business because, in the same frame size, we can go from the smallest pitch that is incredibly accurate and goes 1/16^th of an inch per revolution all the way up to almost four inches per revolution. So within the same linear motion subsystem envelope, we can design a tremendous range of speed, force and accuracy without changing a single component in our design."

According to Ratcliffe, backlash in a motion control system like this one is also a major problem. "We were basically hanging one end of a 600-lb pivoting conveyor on the end of a linear slide, and had very rapid moves anywhere from 10 to 40 inches," he says. The system must keep all four lanes filled with product in response to downstream demand. The zero backlash feature of the lead screw helped create a system that is solid and accurate which requires no maintaining.

"Any wear that does eventually occur in the screw or nut is compensated by the nut which adjusts automatically to maintain a zero backlash over a very long service life," Ratcliffe says.

Motion System Overview

RPT Motion delivered a total of 20 axes of servo motion control for the application — three case packers with six axes each, plus two lane changers powered by servos. Two of the axes on the case packers and the lane changers use lead screws. The other drive mechanisms are powered through timing belts because peak forces in the 900-lb range exceed the capacity of lead screws in that size, which is about a 350-lb design load.

	For the high speeds required for the lane changer, RPT chose a 2.400 inch travel per revolution screw of ¾ inch nominal diameter (top) with approximately the same efficiency as a ball-nut configuration.

On each case packer, two axes control what had been pneumatic elevators to form the case load. Complete servo control of the acceleration, deceleration and speed of each lift significantly increases the speed of this section of the case packer. The other four axes on the case packers are two-axis pushers.

According to Ratcliffe, case packers designed 25 years ago typically form a case load and a pneumatic pusher pushes the caseload into the formed case. But the catch is that the next caseload can't begin forming until that pusher retracts all the way past the two lanes of elevators. Instead, the retrofitted servo machines use high-speed, timing-belt-powered systems to push the case loads in. Then, each pusher backs out of the case and moves vertically above it and back over the two lanes that are already forming.

"By adding the vertical axis to the pusher system, we gain back a tremendous amount of case load forming time," says Ratcliffe. "That's one of the major benefits of the retrofit and a way to retrofit older style case packers for higher production and provide lane changers to intelligently divide up the flow of upstream production in a logical way."

The big accomplishment of the retrofit program is adding case-packing capacity, and using servo controls to enable the case packers to run at 160 percent of their original speed. With the two lines, they are able to more easily coordinate product packaging changes, while continuing to pack product. A third case packer is used for special products and only runs 25 percent of the time.

The new lane changers provide the mechanism to properly divide up production and distribute it logically. This permits the plant's main Allen-Bradley PLC to switch on the third case packer and coordinate production by feeding one, two or three case packers at will.

	A unique lead screw design helped case packers achieve high linear speeds and size restrictions of machine retrofit.