Lubricated and Trouble-free Pneumatic Cylinders

Tom Kreher, Applied Pneumatic Controls

Peter Russell, purchasing agent for Roseburg Forest Products, asked the question, “Isn’t there a better way to lubricate our dry (veneer) stackers than to buy 128 (aerosol) lubricators?”

Hanging 128 inline aerosol lubricators throughout the system like decorations on a Christmas tree did not seem to be the best answer. The Christmas tree effect is similar to the air systems from the 1940s – the kind that Rosie the Riveter might have used during WWII. This method of lubricant application is cumbersome and failed to deliver the appropriate amount of lubricant.

Peter felt there must be a better way, and he was right. Applied Pneumatic Controls has dabbled with a centralized lubrication system that will enable pneumatic devices to be lubricated effectively.

Veneer Stackers

Veneer is optically graded and sorted in motion by a conveyor. Each sheet is assigned an address or bin number that travels with it via an electronic shift register until it reaches the designated bin.

Veneer is transported on the underside of the conveyor vacuum plenum. Each bin has two pair of (knock-down) air cylinders. One pair is on either side of the conveyor vacuum plenum. These cylinders, approximately 2 x 9-inch stroke, thrust two bars of ultrahigh molecular weight polyethylene (UHMW) downward against the sheet of veneer to force it away from the vacuum conveyor and down into the bin below.

The cylinders must extend and retract immediately to clear the path for the next sheet of veneer zipping along at 400 feet per minute. There are other cylinders on the stacker that center the veneer to keep the ends even, but the knock-down cylinders have the higher maintenance challenge.

On some stackers, cylinders are replaced and repaired every few weeks. On other stackers, the corresponding cylinders last a year or more without service. Obviously, the cylinders on bins that see the greatest use have the most wear.

Compare the cost of replacing cylinders for just one bin every four weeks to cylinders that run for a year. The cost of a single high-maintenance bin is about $7,000 per year. If a bin runs without difficulty all year and if the cylinders undergo regular preventive maintenance (PM), the cost is about 1/12th of the high-maintenance bin, approximately $600 per bin.

The average cost per bin is $3,500, about half of the extreme repair cost. For an average 10-bin stacker, the yearly maintenance cost might be $35,000. For a well-lubricated and well-maintained 10-bin stacker requiring only PM work, the cost may be reduced by 90 percent. Mechanical wear, alignment and other factors must be considered, but good lubrication may be the most important factor in cylinder uptime.

Lubrication Application Issues

One 10-bin stacker may have 20 inline lubricators for the knock-down cylinders and 10 more for the even-ender cylinders. The time consumed repeatedly filling seven-ounce lubricator bowls without stopping the machine is often eliminated by using a remote fill that has float-operated shut-off valves in the lubricators, a central bulk tank and a timer that force fills the lubricators periodically.

This is a sound practice, but it adds approximately $100 per lubricator. The remote fill devices for a system with 30 lubricators would add $3,000 to the initial cost of lubricators. Over time, the labor saved justifies the cost of this refill system.

A significant problem with multiple individual aerosol lubricators is adjustment. It is impossible to set multiple lubricators identically. Change in temperature, pressure, viscosity, flow, age and contamination all amplify the existing differences. While discussing a stacker, the maintenance supervisor said, “Some of our cylinders starve for lubrication, some get enough and some flood.”

This happens at random without warning and is subject to change as bin assignments and grades change. It is almost impossible to correct. There is a long list of applications in many different industries that are plagued with the limitations of conventional aerosol lubrication for similar pneumatic valves, cylinders, tools and other devices.

The following is a list of what a lubricant system should provide:

Centralized - Now is the time for centralized lubrication for service and monitoring.

Distribution - The amount of lubrication should be adequate and even at varying flow rates.

Distance - The lubrication system should work well over a 100-foot distance and 50-foot elevation.

Installation - The installation should be straightforward for the maintenance staff.

Cost - The cost should not be significantly more than the traditional (Christmas tree) practice.

Labor - The lubrication system should be durable and cut man-hours needed to adjust, refill and monitor lubrication. It will reduce cylinder repair and replacement by suppyling the correct amount of oil.

The Solution

A lubrication system was built for stackers. Mike Nordyke, lead millwright at Roseburg Forest Products, was approached with the design. Mike considered the idea and technique and agreed to try it. He had been fighting lubrication on green veneer stackers for a long time. Mike installed the pilot system on one of his stackers without difficulty or questions.

Weeks of trouble-free maintenance became months, When frequently asked about the performance of the lubrication system, Mike replied “I’ve stopped checking on it.” The system has become durable enough to reduce the number of routine inspections needed. Not long after that, Mike’s department wanted to install the smoke system on another stacker. In less than a year, he ordered another system similar to the first one, and installed it without any difficulty.

Duane Hopkins, maintenance superintendent who worked with Mike, recognized the potential and suggested oil smoke lubrication to associates at Roseburg Forest Products and with others who struggle with lubrication of their stackers. His support was encouraging and added credibility to the new concept of oil smoke.

Don Kapitanovitch, field service engineer, saw the oil smoke systems at Roseburg Forest Products and discussed them with Mike and Duane. Next he came to Applied Pneumatic Controls to learn more. Don told his customers about oil smoke as he serviced their stackers. Major companies from British Columbia (Canada), Washington State, Oregon, Montana, Idaho and Northern California are using oil smoke systems on retrofitted machines and have requested that original equipment manufactures put oil smoke on their new machines.

Possible Side Effects

Oil smoke systems favor a chimney that helps larger particles separate and fall back to the reservoir.

Large manifolds are mandatory to keep the velocity of the lubricant-loaded air below 1,200 FPM. The compressed air can suspend a limited amount of oil smoke which helps self-limit the lubricant (maximum lubricity). Turbulence in a small manifold causes wet out, replenishment and flooding.

Boundary layer turbulence will wet out a small amount of oil even at low velocity. This should be drained periodically before it becomes a nuisance and floods cylinders. When possible, slope the manifold pipes.

The pee trap effect of a low area can hold accumulated liquid that may increase the restriction and turbulence. Eventually the liquid condensate and oil may block air flow.

Applied Pneumatics Controls learned that yes, there is a better way to lubricate veneer stackers. It became evident that veneer stackers and similar applications commonly suffer unsatisfactory lubrication, which limits profits and production. When asked how his staff liked the lubrication system on their year-old stacker Rod exclaimed, “It’s maintenance–free”.

All of these photos were taken at Roseburg Forest Products, Plywood No. 4 in Riddle, Oregon.

The logs are peeled to make the veneer on the “green” end of the mill where the veneer has a high moisture content. These stackers are like those that stack the veneer sheets after they come out of the ovens; hence the name dry stackers.

The veneer is conveyed four feet wide ±500 feet per minute. Veneer sheets are sorted and thrust into the designated bins at approximately 125 sheets per minute. It takes a minimum of six cylinders; four to knock down and two to square ends per bin. The combined cylinders may complete 750 or more cycles per minute. These cylinders need proper lubrication.

Figure 2. Dry stacker, end view. This is an 18-month old 12-bin dry stacker at Roseburg Forest Products, Plywood No. 4, taken from the in-feed end. This machine was ordered from the manufacturer with oil smoke lubrication specified. The two central lubrication systems are on floor level under the in-feed ramp.

Figure 3. Nordyke retrofit 10-foot green stacker. This older “green veneer” stacker was retrofit with oil smoke lubrication. The photo is at bin level where the operator works.

Figure 4. New 12-bin dry stacker with the oil smoke system located in the middle (two gray boxes) under the cat walk. This unit was not in production when photo was taken.

Figure 5. The 12-bin stacker. Note the high location of multiple valve manifolds, 20 feet or more above the lubrication units that are under the in-feed conveyor. Contains 128 valves, 20 feet high, along 80 feet of both sides.

Figure 6. Side view from bin level of the 12-bin dry stacker. The valves are above the catwalk. The hydraulic scissors jack lowers the stack of veneer as it builds to keep the level even with the end-squaring cylinders until the bin is full.

Figure 7. Side view of the 12-bin stacker showing veneer bin at the bottom (slightly cluttered). This stacker is in normal production when this photo was taken. Note the absence of haze from exhausted lubricant. The pneumatic devices on this stacker are clean and well-lubricated from a convenient central source.

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