Bearings: the inside matters

The demand for cleanroom automation equipment increases at a rapid rate with the growth of the medical and consumer electronics industries. Chris Johnson, managing director of SMB Bearings, examines outgassing and how to prevent contamination

Robots for cleanroom applications bring together two genres of priorities. From the machineís perspective, the robot needs to be precise, smooth and long-lasting. From the environment point of view, the androidís operation and maintenance should prevent contamination in accordance with industry standards. Combining these is a challenging task; companies need careful consideration when choosing cleanroom bearings.

Manufacturing processes in the biotechnology, pharmaceutical and telecommunications industries, for example, require compliance to cleanliness standards. The cleanliness of a room is determined by ISO classifications rated according to how much particulate of specific sizes exist per cubic metre. The "cleanest" cleanroom is a Class 1 and the "dirtiest" a Class 9; based on the ISO 14644-1 criteria.

A huge industrial sector that makes use of cleanrooms is semiconductor manufacturing. This advanced industry uses thin film technology to form microprocessors, sensors and flash memory. It must take place in an ISO 4 cleanroom 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 is these steps that turn them into CPUs, memory chips and graphic processors. Of course, these processes are not human friendly, which is why companies 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, the factory automation specialist, launched its new M20iB/25C robot for cleanroom environments at the end of 2017 to help meet this demand. The unit has been designed specifically to ISO Class 4 standards with the added bonus of high payload and excellent reach.

It is interesting that FANUC is not 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 robotís functionality and what effect they will have on the cleanroom environment.

The risk of outgassing

Using any old bearing and lubricant combination without thought will bring concerns about 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 grease with very low outgassing rates that are designed to perform well under these conditions should be used.

The advice is for choosing cleanroom grease with very low dust generation and whose manufacturing process uses no ingredients containing lithium, molybdenum, calcium, aluminium, barium, zinc or sodium or other contaminants. These, however, are found in many standard types of grease, and would cause contamination issues 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.

High accuracy

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 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.

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 robotís functionality and what effect they will have on the cleanroom environment

Bringing it all together

Companies are 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 one 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, undoubtedly, a priority in a cleanroom.

Companies 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.

Unlike standard bearings, stainless steel bearings do not require protective oil on external surfaces to shield against corrosion. This oil would contaminate a cleanroom environment so not needing it with the stainless steel material is very helpful and drastically reduces outgassing.

Cleanroom robots certainly bring their fair share of challenges to facility managers, suppliers and employees. Between concerns of the robotís 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.

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