How to choose linear motion components for vacuum applications？

Vacuum applications are common in the manufacture of semiconductors,liquid crystal and plasma displays,fiber optics,and aerospace components,where linear motion systems are used for the positioning,handling,and alignment of critical equipment.While vacuum applications are often likened to clean room applications,there are key differences between the two in the "special environment" category.The standards that define clean rooms refer to the presence and release of solid particles into the environment,and the vacuum environment is affected by the release of gases and vapors into the environment,which may hinder the ability to generate or maintain the desired degree of vacuum.Technically,any environment with a pressure below atmospheric pressure(101,325 Pa or 760 Torr)is considered a vacuum.

 

Outgassing can be reduced by a process called baking,in which materials or devices used in a vacuum are heated to high temperatures - usually 200°C,although higher temperatures are sometimes required - ranging from a few hours to a few days.The baking process forces the release of trapped gases and vapors,ensuring that little or no gas is released when the product is introduced to the vacuum environment.Baking the product before use in the vacuum chamber is standard practice for most vacuum levels,but for ultra-high vacuum(UHV)environments,baking is mandatory.

When selecting linear moving components for vacuum applications,there are two main goals:to use materials with minimal outgassing,and ensure these materials can withstand the required baking process.Fortunately,stainless steel is a standard material for many components(or a widely used choice)and is one of the preferred materials for vacuum environments due to its low outgassing properties,wide temperature range, and corrosion resistance.

Most types of recirculating bearing linear guides and screws,including CAM roller guides,can be made entirely of stainless steel.However,for components using recirculating balls,replacing standard bearing balls with stainless steel versions can result in a 30 to 40 percent reduction in load capacity.For vacuum applications requiring high load capacity,load-bearing balls can be made of ceramic that is non-porous,non-corrosive,lightweight,and can withstand extreme temperatures,including temperature fluctuations.Even the rack and pinion assembly can be made of stainless steel rack and bronze pinion for vacuum environments.

For components or systems where stainless steel is not an option,electroless nickel plating(which can be applied to steel or aluminum)is an acceptable finish in vacuum environments.(Anodizing is often used in aluminum and is not recommended for vacuum applications because it causes the surface to retain water vapor and is an important source of outgassing.

 

Plain linear guides and lead screws are also ideal for vacuum applications,as plain bearings and lead screw nuts can be made from a variety of materials.For example,although most polymers are outgassing sources,certain materials(such as PEEK,PPS,and PTFE)are relatively low hazard and suitable for most vacuum environments.(However,if the component will be baked,be sure to check the allowable temperature range of the material.

 

 

Lubrication is also an important source of outgassing of linear moving parts.Ideally,of course,no lubrication would exist in a vacuum,but completely lubricated systems are rarely possible.(Remember that screws use radial bearings as end supports,and other components in the vacuum chamber(e.g. motor bearings)also need lubrication.

One solution to the lubrication problem is to use plain bearings or lead screw nuts made of self-lubricating,vacuum-compatible polymers or composites,such as PTFE-infused PEEK.For recirculating linear guides and ball screws,ceramic balls do not need lubrication.

In some vacuum environments,solid lubricants such as molybdenum disulfide or tungsten disulfide are recommended,but these materials are not suitable for semiconductor and LCD manufacturing applications due to particle production.Another solid lubricant option is coated with a thin layer of silver(especially recommended by some ball screw manufacturers),which provides lubrication properties and is suitable for semiconductor manufacturing and UHV environments.When the use of grease cannot be avoided,there are a number of vacuum compatible options available for rolling bearing linear guides and ball screws.

 

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