World's First Servo-Driven Lumber Stickering Stacker Saves Energy

MoCo Engineering and Fabricating builds lumber stacking and other machinery for various sawmill applications. However, changing market needs are putting the OEM's reputation for innovation to the test. For MoCo, the challenge was clear: Find a way to increase machine productivity, energy savings and safety on its stacker equipment.

MoCo's flagship 10-axis "stickering stacker" is the company's most sophisticated machine. Used on sawmill lines, the stickering stacker inserts wooden or aluminum spacers (called stickers) on 1- or 2-ft centers between each course of cut-to-length lumber, while simultaneously stacking the courses. Typically, the motion on a stickering stacker was driven either by hydraulics, variable speed ac motors or a combination of both. For some time, MoCo had been considering an all-electric, servo-driven stacker designed to improve performance and reduce energy use. An experiment conducted by MoCo a few years ago proved that servos could operate well under sawmill conditions, but the company was unsure whether the market would support such a machine.

However, increased energy costs, decreased lumber prices and rising environmental concerns are impacting lumber mill profitability. To preserve profits, mills need greater productivity, less downtime, improved safety features, more energy efficiency and less potential for environmental cleanup issues. MoCo decided it was time to move forward with an all-electric, servo-driven machine that could provide lumber mills with an impressive return on investment. The company converted an existing 10-axis hydraulic stacker system to the world's first line-regenerative, electric servo-driven synchronized stickering stacker — using a drive and control system from Rexroth. As Allan Hahn, MoCo control specialist put it, "The servo-electric concept helps us offer greater efficiency and long-term cost savings. The ability to reduce both energy use and environmental impact makes this stacker well-suited for today's challenging market."

To obtain the ideal components that would make the servo-electric concept a reality, MoCo teamed up with local Rexroth automation distributor Northwest Motion, a supplier to MoCo since 2000. Based on its experience with servo designs, Northwest Motion recommended a Rexroth IndraDrive drive system, IndraDyn servo motors and a common dc bus with regenerative capabilities so excess power could be diverted from one axis to another, or onto a mill's main power grid.

MoCo's documented tests indicate substantial energy savings. Energy consumption drops from the 100–180 kW range for a previous hydraulic machine to 58 kW or less for the new servo-driven machine. The stacker draws about 35–45A of power when running at 10 cycles per minute, compared with 270A for a hydraulic machine.

"Unlike a hydraulic system, the stacker uses power only when cycling," says Hahn. "At the end of the day this machine cycles only 40 percent of the time compared to a hydraulic machine, which typically runs a 75 hp electric motor all the time powering the hydraulic pump even when it isn't needed." Energy reductions of 40 to 75 percent, with savings of up to $45,000 per year have been documented. The line-regenerative power supply puts approximately 5 kW of power lasting several seconds back onto the ac power grid during deceleration of the main hoist axis alone. The

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