By Mike Bacidore, managing editor
Pneumatic technology has had a successful role in lumber-processing machinery for years. But, just like other motion alternatives, there’s always a need to make pneumatic components perform better.
Manufacturers of equipment used in harsh sawmill environments traditionally used pneumatic cylinders made of steel. Designers considered steel to be the only material capable of tolerating the debris, temperature fluctuations, vibration, and shock generated in tough applications such as trim-saw lines.
Sawmill equipment manufacturer USNR, headquartered in Woodland, Wash., is one machine builder that recently sought improved pneumatic cylinder performance alternatives for its new Accu-Trim trim-saw machine.
The Accu-Trim planermill trim saw has 11 14-in.-diameter saws on 1-ft centers, which are used to trim planed boards to length prior to sorting. The machine processes 4-6-in. wide boards that are 2-4 in. thick and 6-10 ft long. Boards are fed into lugs just before the trim saw, and they pass laterally via chain conveyor through the machine. The saws are independently actuated in a quick down-and-up motion to trim the boards to specified lengths as they pass through the trimmer. The extension and retraction of each saw is driven by 80-mm bore pneumatic cylinders (Figure 1). The cylinders are controlled by a master PLC, and require 80 psi air to complete the down-and-up stroke.
Figure 1: The infeed side of the open trimmer shows how the extension and retraction of each saw is driven by pneumatic cylinders operating at 80 psi and controlled by a master PLC.
The PLC also monitors and controls the complete trimmer/sorter system for the planer mill. Rather than use position sensors in this tough environment, the saws are actuated up and down based on timing the system’s encoder counts.
The PLC code is written by USNR PLC programmers, and is customized for each mill application.
Need for Speed
“To meet increasing customer demands for speed, we needed saw-actuation cylinders that could extend 90 mm in 50 msec, and retract in 80 msec,” says Tommy Green, USNR’s chief engineer for lumber handling equipment. “Faster cylinder-actuation speeds mean an increase in the number of boards processed per minute. The trim saw rate had been 140-160 boards per minute, but our goal was 200 boards per minute—a significant production increase.”
Green adds that, in the worst case, the saw has to cycle down, cut the board, then cycle up for the next board. This all has to be done in about 300 msec.
USNR’s engineers researched using aluminum pneumatic cylinders coupled with a concept known as ideal cushioning. They discovered a real potential for cost and performance gains.
“We initially had doubts about the durability and performance of aluminum cylinders in such a harsh, demanding environment,” says Green (Figure 3). “Our concern was how the cylinders would withstand a bad block cut, which can send a chunk of wood spinning out of control, and really batter the trimmer. We also were skeptical whether the aluminum cylinders could perform at the speeds desired without quick deterioration from the impact shock in each cylinder.”
Figure 2: The trimmer is about 8 ft wide, 20 ft across, and 8 ft. tall with multiple 14-in. diameter saws.
Test Results Don’t Lie
USNR tested components from several different suppliers to determine what type of pneumatic cylinder was best suited to meet Accu-Trim’s speed and durability requirements. “We built a test stand in our shop to cycle the saw ladder just as it would happen in the trimmer,” says Green. “We ultimately chose a Bosch Rexroth ISO/VDMA 523 ‘ideally cushioned’ aluminum cylinder because it performed best. The heavy-duty aluminum, wall thickness, and forged housing of the cylinder held up well during testing. The cushioning method successfully dampened the shock, and prevented the rod ends of the cylinders from breaking. All of these benefits, including an overall weight reduction afforded by the aluminum, were remarkable given the unprecedented speed we demanded of the cylinder.” Green adds the superior cushioning in these new aluminum cylinders allows faster motion with less force, as well as faster and more-efficient stroke cycles.
After seeing how 523 dampens shock in an aluminum cylinder, and ultimately enabled Accu-Trim to cut 200 boards/min at 40-60 msec/stroke, USNR specified these components for its Accu-Trim Trimmer high-speed trim saws.
“We saw that aluminum cylinders enable faster cycle rates and greater productivity at a lower overall component cost,” summarizes Green. “The speed and productivity benefits from aluminum’s lighter weight, combined with eliminating end-of-stroke bounce, coupled with the lower cost of aluminum compared to steel, make aluminum cylinders increasingly competitive for us.”