by Jack Byrne
As equipment gets heavier and threatens the integrity of access flooring, the cleanroom industry considers a 35-year old technology that moves equipment on a floating cushion of air.
If it's not a corollary to Moore's Law, it should be: Larger wafers mean heavier processing equipment. A lithography machine that five years ago weighed three tons today tops the scale at more than eight tons. Stepper machines have gone from less than one thousand pounds (450 kg) to 7,000 pounds (3,150 kg) in recent years. And it's not just semiconductor processing equipment that's getting bigger and heavier. The pharmaceutical and medical device industries are experiencing a similar trend.
Installing and rearranging such massive equipment poses a considerable threat to cleanroom access floors. Unlike solid foundation floors, access floors were never designed to support heavy processing equipment. The very perforations that allow airflow to exhaust through a raised floor render an aluminum access floor panel structurally unable to withstand the weight of heavy equipment. And access panels are very susceptible to damage if weight is concentrated on a small area by wheels or other narrow supports (see Figure 1).
That's why equipment manufacturers and cleanroom personnel hold their breath when moving equipment in the cleanroom. Overloading a floor can damage floor panels and pedestals and the equipment and cables underneath the floor. The damage can be devastating, leading to contamination breaches, cleanrooms shut down for floor repairs and delayed start-up.
Further complicating the problem is the fact that typical load-limit specifications supplied by floor manufacturers don't always provide sufficient guidance for avoiding overload. Many users don't even know these limits exist; others find them difficult to understand. Questions about load limits are usually resolved with a call to the manufacturer for help on a specific floor, or by arranging special panel tests.
Overload can occur both from equipment at rest and under transport. To avoid the former, cleanroom operators position heavy equipment directly on the sub-floor frame or support pedestals, effectively bypassing the access floor and eliminating risk. Conventional moving methods, however, have failed to circumvent the potential for overload and floor damage.
The problem of moving heavy equipment without damaging floors was solved 35 years ago with a few molecules of air. Faced with the difficulty of moving heavier aircraft sections during the manufacturing process then precisely positioning larger jet engines for mounting under an airplane's wing, Boeing Commercial Airplane Co. developed a concept to move equipment with the power of compressed air.
The air-film technology, called Aero-Caster, floats heavy loads on a thin film of air, allowing a single individual to move several tons with just one hand. Realizing the widespread applicability of the air-film concept, in 1967 Boeing spun off a company called AeroGo (Seattle, WA) to further refine and market it.
Today, AeroGo builds Aero-Caster systems for a variety of industries worldwide, including the most stringent Sub-ISO Class 3 (Class 1) cleanroom applications. The technology commonly moves hundreds of tons as well as industrial loads of more than 5,000 tons.
Cleanroom personnel have likely seen the systems at work during installation of new equipment. Over the past decade, many cleanroom equipment manufacturers embraced the concept to avoid damaging their customers' floors during move-in and installation. They also employed it on their own assembly lines to move heavy equipment.
In addition, access floor manufacturers are beginning to urge cleanroom operators to protect their floors by adopting the technology. As it garners more attention, cleanroom users are seeking a better understanding of floor-loading problems and how the Aero-Caster technology works.
Conventional equipment moves
Traditionally, there have been two methods to move loads in the cleanroom: via wheels alone and wheels with heavy floor overlays. Wheels alone are used with lightweight machines. The wheels, also called rollers or casters, are placed under the machine in multiple locations, and the load is pushed where needed. Although inexpensive to purchase, this technique is suitable for only the lightest machines. Yet even with light machines, using wheels alone often results in unintentional overload. The concentrated point load at narrow wheels is the worst way to stress a floor panel.
Tests performed to determine actual floor loads of heavy equipment versus manufacturers' specification limits show how easily access floors can be overloaded by wheel moves (see the Table on page 34). For example, a 6,000-pound (2,700-kg) machine moved on four wheels, puts a concentrated load of 1,880 pounds (850 kg) on a floor panel. This point load at each wheel far exceeds the 1,000-pound (450-kg) concentrated load limit recommended by manufacturers and severely stresses the floor panel, leading to extreme panel deflection and probable failureplus all the costly problems and interruptions that result. Many operators have witnessed this type of failure, and there are other disadvantages to wheel systems.
Even if secure at rest, the application of force to propel the wheel forward can punch through a panel or deform and fatigue it over time (hence the creation of lower “rolling load” specifications as depicted in Figure 2). In addition, users tend to select hard narrow wheels to reduce pushing force, which further concentrates the stress. Wheel use is also hampered by the extra time, trouble and special care that may be required when trying to turn, rotate or finely position the load. Due to machine variations and uneven loading, the machine weight will not be equally distributed on each wheel, so the stress will be higher at some panels.
Since the advent of heavier machines, users have implemented heavy plate overlays to help with the problems of wheel moves. After first overlaying heavy metal reinforcement plates on top of the raised floor, mechanical jacks are used to position the machine load onto a hand truck or other conveyance with wheels or rollers.
Overlays essentially transfer or spread the machine load over a larger area than just the point of a wheel. To effectively reduce the concentrated load and stress on the floor panel, the overlays must be sufficiently stiff and thick to spread the weight over a broad area. Arriving at the correct overlay dimensions requires careful deflection calculations and possible testing to assure risk avoidance. Further, the overlay must cover the full expected path of the move plus a sufficiently wider load-transfer area. In some new installations, entire floor sections end up covered with overlay plates.
If used properly, overlays can distribute the load effectively and satisfy manufacturers' uniform load specifications in most cases. But there are other problems with the approach. The plates are heavy and difficult to handle and may even require equipment to move. Laying them is labor intensive, puts personnel safety and the floors integrity at risk and has the potential to create contamination.
They are also costly and a challenge to store. One new facility even suffered severe floor damage and start-up delays when overlays stacked on a carrying cart pushed the cart's wheels through the new floor. In addition, overlays do not eliminate or reduce the difficulties of wheel moves previously mentioned. Moving the bulky load precisely with an electrically powered hand truck requires experience and skill.
Still, for equipment weighing a few thousand pounds, the overlay procedure worked in the past and continues to suffice for some applications today. But for moving megaton-sized equipment, the Aero-Caster technology better protects access flooring, human safety, the cleanliness of the room and the equipment itself.
Floating equipment with air-film technology
While wheels concentrate a load's force onto just a tiny percent of a typical floor panel, the technology spreads the load over a larger area. The approach is similar to the metal-on-metal air bearings used in some lithography machines. In these applications, a thin film of air separates two metal sections that are gliding without friction. The Aero-Caster systems accomplish the same effect with an inflatable torus bag attached to a lightweight aluminum-frame structure. Instead of a narrow wheel, the wider Aero-Caster is typically 21 or 27 inches (500-700 mm) in diameter depending on the weight. This wide profile spreads the weightthe concentrated load is less than 15 pounds on a one-inch square (7 kg, or 1 kg/sq. cm). Thus, no structural overlay plate is needed to prevent damage.
Figure 3. The Aero-Caster principle.
With profiles less than three inches (76-mm) high, the systems slide under frames and between equipment support legs. Figure 2 illustrates how four Aero-Casters are placed under a machine for a floating move. The concept includes an integral jacking system to raise the load and move it anywhere in the cleanroom. Personnel use the cleanroom's standard compressed air supplyno other power is necessaryto inflate the torus bag and raise the load (see Figure 3 for a more detailed operating description). The air pressure is lowless than half the pressure in an automobile tire. Nitrogen has been used in a few cases.
Air is applied until the pressure exceeds the load's weight, then the air slowly and evenly escapes to create a thin film of air 0.003- to 0.005-inches (.08- to .1-mm) thick. The load is virtually floating over the floor's surface with a friction factor of 0.04, which means a 40-pound force (18-kg force) is all that's required to move each 1000 pounds (450 kg) of load. A few people can move a megaton load safely and efficiently in any direction, navigate tight spaces and precisely position equipment in the target location. The technology also reduces or eliminates the need for realignment and calibration of equipment since there is no vibration and less lifting during the move.
The technology effectively eliminates the risks of overloading access floor panels. For example, a 6,000-pound (2,700-kg) machine transported via air puts a uniform load of 450 pounds (200 kg) on a floor panel, an amount safely below the manufacturers' uniform load limit of 600 pounds (270 kg) (see the Table on page 34). When a 15,000-pound (6,750-kg) machine is transported via air, it achieves a uniform load of only 1,120 pounds (510 kg), compared to the load limit of 1,200 pounds (540 kg). Larger systems are also available to move even heavier sizes.
The approach requires no heavy floor overlay plates to protect the floor panels from damage. For perforated or grated floors, a thin polystyrene sheet is used to create the air seal that powers the technology. The lightweight sheet can be rolled up for storage between uses.
Aero-Caster systems have virtually no moving parts or lubricants, making them inherently suitable for cleanroom use. They are available in powder-coated, anodized or painted models depending on the cleanroom specification level.
The airflow created during transport of equipment is minimal, just enough to create a slight flow several molecules thick. Properly operated, the air-film systems stir up no more air than people walking. The technology has been tested by major cleanroom operators and found to generate no measurable contamination above background.
In addition to protecting cleanroom floors from heavy loads, the approach also reduces personnel time and labor costs associated with moving, rearranging and positioning equipment in the cleanroom.
The configuration most suited to cleanroom use is the Load Module system, which features a single Aero-Caster per module. Used in groups of four or more, the modules lift and move loads of any size, shape or weight. Standard Load Module systems are priced at several thousand dollars and up depending on the size and load capacity of the system. Aero-Casters are available in sizes for any weight, including the heaviest next-several generations of equipment. Many custom-engineered systems have also been produced to meet specific customer applications.
When selecting and using the systems for moving heavy loads, users should consider floor type and specifications, machine weight, equipment frame and legs and path-placement locations. Oversize landing pads have also been developed for using air-film technology at rest underneath super-heavy machines, which eliminates the risk of concentrated floor overloading at rest (when uninflated during start-up or pauses in the move).
Access floor manufacturers are working to increase the structural strength of cleanroom flooring, moving from the typical current 2,000 pound (900 kg) concentrated load rating specification to 3,000-pound (1,350-kg) models at standard deflection limits.
Even with a more robust floor, ever-larger equipment will continue to pose a threat to a floor's integrity, says Peter White, a cleanroom flooring expert and former executive with Tate Access Floors. White now heads H.K. Techfloor (Toledo, OH), a manufacturer of access flooring aluminum panels and subsidiary of Hae Kwang Co. Ltd.
“There's a great deal of concern among cleanroom users that raised floors are not able to support the move-in of new equipment. No one wants a few million dollars worth of equipment to go through the floor,” says White. “Users are considering the size of the wheels that are transporting the equipment and wondering whether to put down sheets of steel to distribute the load over a larger area. But the heavy plates don't distribute the load to the same degree as the Aero-Caster technology, which to me is the safest way to move equipment across the floor.”
Several Japanese manufacturers of large semiconductor equipment use air-film technology to move units through production and also for transit to customer locations and installation. “We started to install 21-inch (500-mm) Aero-Casters in these applications, but have since moved up to supplying 27- to 36-inch (700- to 900-mm) models as cleanroom equipment has grown continually heavier,” says Jun Daigo of OS Engineering in Tokyo.
Jack Byrne is presideFnt of AeroGo, Inc., a manufacturer of heavy load moving equipment and flexible lean-flow manufacturing systems based on air-film technology. He served as founding president of a high-tech materials startup and as a professor of operations strategy for the University of Oregon MBA program. He can be reached at (206) 575-3344 or at [email protected].
Moving heavy loads by air
Key uses of Aero-Caster technology:
- Delivery of new equipment into cleanrooms (used or specified by equipment manufacturers).
- Rearranging or moving equipment in cleanrooms.
- Used by manufacturers in their assembly production flow processes (as a substitute for conveyors or other movers).
Benefits for cleanroom applications:
- Prevents floor damage by eliminating the concentrated point loading of wheeled systems.
- Cleanuses only compressed air.
- Low frictioneasily and safely handles heavy loads.
- Allows precise positioning.
- Lowers the cost of moves because it requires fewer people and less preparation.