Designing for cleanrooms
There's a lot to consider when designing a robot for a cleanroom application. You're bringing together two genres of priorities, that of the robot; to be precise, smooth and long-lasting, and that of the environment; to prevent contamination in accordance with industry standards. Here, Chris Johnson, managing director of precision bearing expert, SMB Bearings, explains why you need to consider both these genres when choosing cleanroom bearings.
Most processes requiring high cleanliness standards are now performed in a clean room. Clean rooms are used in the biotechnology, pharmaceutical and telecommunications industry to name just a few. The cleanliness of a room is classified from ISO 1 (strictest) to ISO 9 (room air), based on the ISO 14644-1 criteria.
A huge industrial sector that makes use of clean rooms is semiconductor manufacturing. This advanced industry uses thin film technology to form microprocessors, sensors and flash memory. It must take place in a room of class ISO 4, or cleaner. A single dust particle is all it takes to ruin a central processing unit (CPU) that might sell for £400, so companies are eager to minimise the risk of this happening.
Computer processors start out as silicon wafers, which are subjected to intense vacuums, high-energy plasmas and ultraviolet light. It's these steps that turn them into CPUs, memory chips and graphic processors. Of course, these processes aren't human friendly, which is why we rely on robot arms to move the wafers from one processing station to another.
The demand for cleanroom automation equipment is increasing at a rapid rate with the growth of both the medical and consumer electronics industries. FANUC UK launched its new M20iB/25C robot for cleanroom environments to help meet this demand at the end of 2017. This has been designed specifically to ISO Class 4 standards — with the added bonus of high payload and excellent reach.
It's interesting that FANUC isn't promoting this new robot as a system solely for cleanrooms, but rather as a well-rounded robot for food-applications, pharmaceutical areas and normal handling of work pieces. This shows the capabilities of the latest cleanroom robots — they aren't limited by their cleanroom label, but rather enabled by excellent workmanship and ingress protection (IP).
Part of this workmanship will come down to making expert decisions on what goes into the robot. Every component that goes into the robot, such as the bearings, will need to be carefully considered in terms of how they will enable the robot functionality, and what effect they will have on the cleanroom environment.
Using any old bearing and lubrication combination, without consideration, will bring about concerns over outgassing. This is when the grease vaporises to an unacceptable level, contaminating the environment and failing to lubricate the bearing sufficiently. Instead, special low volatility greases with very low outgassing rates that are designed to perform well under these conditions should be used.
Our cleanroom grease has very low dust generation and uses no ingredients containing lithium, molybdenum, calcium, aluminium, barium, zinc or sodium or other contaminants. These are found in many standard types of grease, and would cause issues of contamination if used. Evaporation levels are also extremely low with this cleanroom grease.
Dry lubricants such as molybdenum disulphide or tungsten disulphide can also be a useful alternative in such applications. This lubricant is technically a solid, which eliminates the problems of outgassing, keeping the environment clean and the bearings sufficiently lubricated.
Moving delicate objects, such as a semiconductor wafer, requires robots to be precise, smooth and predictable in every movement they make. Ultimately, the robot needs to move an object, exactly where it is expected to. This is where stainless steel, precision thin bearings, manufactured to very low tolerances, come in.
The tolerance level dictates how far away from the nominal dimension, the actual measurement can be as part of the manufacturing of the bearing. The lower the tolerance, the closer you are to getting a set of bearings, of the exact same, shape and size. It sounds basic, but we are talking micrometres.
Because of this, thin section bearings have high accuracy of rotation. These bearings are often preloaded (fitted with a small constant thrust load) for increased rigidity and even greater running accuracy.
Precision thin section bearings also take up a lot less space, weigh much less and have significantly lower friction levels than heavy duty bearings. They require much less lubricant due to shallower raceways, so they have less turning resistance on start-up and during running. Of course, less lubrication also brings a reduced likelihood of contamination to the environment.
Bringing it all together
We've established you're better off using stainless steel thin section bearings with cleanroom grease in cleanroom areas that are sensitive to metallic elements and other contaminants. But there is once more factor to consider - shielding or seals?
Both shielding and seals serve to keep dirt out of the bearing, which is not really a problem in a cleanroom. However, these additions have the very important job of keeping lubrication in the bearing, something that is definitely a priority in a cleanroom.
However, you may experience a small amount of outgassing from rubber seals as they are made from nitrile rubber. As this may not be acceptable in many cleanroom environments, stainless steel shields are recommended as a cleaner option.
Unlike standard bearings, stainless steel bearings do not require protective oil on external surfaces to protect against corrosion. This oil would contaminate a cleanroom environment so not needing it with the stainless steel material is very helpful. This drastically reduces outgassing.
Cleanroom robots certainly bring their fair share of challenges to facility managers, suppliers and employees. Between concerns of the robot’s general capabilities in terms of precision and accuracy, and the constant threat of contamination, there is a plethora of issues that need to be handled at once, for the facilities to run efficiently. Suddenly, the simplicity of opting for a stainless steel bearing with cleanroom grease seems like a very simple, effective contribution to a very big task.