If designing an automated assembly system for an ordinary environment can be challenging, designing a system for a clean room is even more so.
Since clean space is expensive, the footprint of the system becomes a priority. The system must be as compact as possible without sacrificing access or performance. Because the ceiling of a clean room is lower than the rest of the plant, the height of the assembly system can be an issue, as well.
Machine components should be built from materials that are easy to clean and won’t generate particulates. Hard, anodized aluminum and 304 stainless steel are common. Other metal parts are usually plated rather than painted, to avoid putting paint flecks into the air. Wire ways and pneumatic lines run below the system, so tabletops can be cleaned easily.
Depending on the application, assembly systems may require special actuators and sealed, permanently lubricated bearings. Engineers cannot specify just any actuator for a clean room. In general, ballscrew-driven linear actuators are preferred over belt-driven actuators, since the belt will generate more particles than a ballscrew.
Bosch Rexroth’s new PSK-gen 2 precision linear actuators are a good example. For most applications, actuators are designed to ensure that outside contaminants do not penetrate the mechanics of the actuator. For production in clean rooms and other controlled areas, actuators must also be designed to ensure that contaminants do not escape from the mechanics of the actuator. The revised PSK actuators have been designed to ensure just that. The ballscrew nut and linear carriage assembly are sealed from both sides, which significantly reduces the risk of lubricants escaping outside.
At the same time, Bosch Rexroth has expanded its lubrication concept to include four lubrication versions. The standard version comes with conventional industrial lubricant with initial greasing. Engineers can select between two different lubricants: LSS standard and LSC clean room. Another option is to connect the actuator to a central lubrication system. Automated re-lubrication with liquid grease increases operational reliability. There’s less risk of a technician forgetting to lubricate the mechanism. The fourth lubrication version is a simple preservation of the runner blocks and ballscrew assemblies so that approved greases can be used.
Made in the United States, PSK actuators come in three sizes and are mounted on a smooth, space-saving and rigid precision steel profile frame with integrated guide tracks. Engineers can select which side to have a smoothed reference edge. This speeds up assembly and makes it easier to align the axis. The frame is more torsion-resistant compared to the previous models and offers higher planar moments of inertia. Optional magnetic field sensors can be adjusted over the entire travel distance.
It’s important to note that an actuator’s cleanliness rating is usually tied to a specific speed, and exceeding that speed will make the device less clean. The faster you run a module, the more particulates it is likely to generate. As long as the actuator is run at or below the rated speed, it will provide the specified level of cleanliness.
Some assembly processes require special attention in clean room operations. Ultrasonic welding could produce particulates, so additional exhausting and flushing with ionized air may be necessary. Adhesives and solvents also may require additional venting outside the clean room.
Exhaust air from pneumatic devices should be directed to manifold and expelled from the clean room. In some cases, the assembly machine might need to be completely shrouded in a Lexan enclosure, which is pressurized with filtered air, creating a clean room within a clean room.
In addition, stations should be designed to minimize the amount of moving parts above product assembly areas. Holes and slots that could collect dust or stray parts should be covered, and traps can be built into feeder bowls to collect particulates generated by plastic parts vibrating against each other.