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Technical Literature

Precision Motion in Vacuum

Generating and controlling motion in a vacuum environment requires careful consideration to materials, stage configuration, and operating temperatures. As a vacuum chamber pumps down to a negative atmosphere, outgassing takes place, effecting the ability to control motion. As well, the lack of air dictates that other means of cooling be used, should heat (generated from motor operation) effect the process.

A vacuum is any pressure that is less than atmospheric pressure. Typically measured in terms of "Torr", a Torr is 1/760 of atmospheric pressure at 0 0 C and sea level. The various vacuum levels are defined as follows:

Rough Vacuum 1 to .760 Torr
Medium Vacuum 10 -3 to 1 Torr
High Vacuum 10 -8 to 10 -4 Torr
Ultra High Vacuum 10 -11 to 10 -9 Torr
Extremely High Vacuum <10 -11 Torr

Materials & Stage Configuration
While each vacuum application may vary, with respect to allowable materials that are used in vacuum, there are some basic guidelines that should be followed with the construction of motion systems. Most linear and rotary stages today utilize a steel or stainless steel bearing construction. Stage housings are typically aluminum or cast iron, and the drive mechanisms are almost always steel or stainless steel with a bronze nut or steel ball nut.

In defining a motion system, all aspects of the stage's mechanical construction and material selection are critical. While most stages utilize a low friction ball or roller bearing configuration for vacuum, the bearing structure should be based primarily on the specific motion requirements. Generally, however it is difficult to utilize recirculating ball bearing configurations, such as gothic arch type, as there is a substantial content of plastic in the bearing puck. Though some vacuum environments allow Teflon, most other plastics are usually forbidden in vacuum. To generate smooth, precise motion, precision crossed roller bearings, of steel or stainless steel are ideal for vacuum chambers. Crossed rollers provide the lowest friction, which will eliminate heat and require the least amount of maintenance over other mechanical bearings. Additionally, when performance is a concern, crossed rollers provide the highest level of stiffness and accuracy over other mechanical bearings.

Vacuum environments ranging up to high vacuum, but limited to the 10 -6 Torr range, typically allow for a wider selection of materials. Teflon, phenolic, Nylon and other nonmetallic materials may be suitable. Above 10-7, and ranging through the Ultra High vacuum level, materials should be limited to: Steel, Stainless Steel (300 series when possible), Aluminum, Copper*, Nickel*, Ceramics, and Titanium.

*Copper and Nickel are not good, should vacuum require baking.

As a vacuum pumps to a negative atmosphere, the process of outgassing acts upon materials that are porous. This not only impacts that choice of base material but also the types of finishes that are used. Stage construction in aluminum or cast iron are typically acceptable, however cast aluminum and anodized aluminum should not be avoided. Cast aluminum is porous and will hamper the efforts to pump down. Anodize will outgas and contaminate the vacuum. Preferably, stage constructions should be bare aluminum or cast iron (or with a nickel plating, if allowed).

The drive mechanism, either ball screw or lead screw should also be given proper consideration. Many ball screws use plastic inside the ball nut for deflectors or wipers. Again, Teflon wipers are acceptable but other plastic wipers should be eliminated. Steel or stainless steel lead screws typically uses bronze (Aluminum, Phosphor or Oilite) or Turcite nuts. In most vacuum applications, this becomes an issue of application specifics as some vacuum environments may allow bronze or Turcite.

Limit switches and electronics are of great concern in vacuum; particularly position feed back devices such as encoders. While all wiring must be Teflon coated, switches and encoders often have other materials that are not vacuum compatible. Micro Slides will provide magnetic read switches (manufactured by Hamlin) that have proven to be acceptable in vacuum. Linear encoders can be made vacuum compatible by removing the rubber seals and re-wiring with Teflon coated wires. Most encoders (linear and rotary) have electronics inside that are mounted to phenolic boards. Depending on vacuum levels, this may be a concern. Micro Slides has provided linear (RSF) encoders in vacuums up to 10 -7 Torr.

Motors in vacuum present simlar concerns as other electronic devices. Step motors are more common, as they can be operated open loop (without position feedback) but it is just as easy to prepare a dc servo motor for vacuum. Motor preparation includes re-potting the motor with a vacuum compatible material and rewiring the motor leads with Teflon coated wires. The potting process is done in a vacuum chamber and heated to provide a bake out. When applying step motors in a vacuum, cosideration must be given the heat that is generated by the motor and how it will be transfered to the stage. It is often more practical to oversize the motor and operate it at lower currents to reduce the generation of heat.

Cleanliness and Outgassing
In maintaining the cleanliness and quality of the vacuum environment, there are several sources of gas that effect the system. Each of these sources should be addressed carefully to avoid impact to the process within the vacuum chamber. They are:
As grease and oil will not only outgas, but can potentially contaminate the chamber. All parts should be properly degreased. Parts requiring lubrication should be lubricated with a vacuum rated lubrication that will not damage the environment. There are four fundamental steps that should be followed for proper chemical cleaning of vacuum materials. These steps may vary based on individual requirements and regulations:
General (non-chemical) cleaning can be achieved through brushing or wiping however where possible the tools should be harder than the material being cleaned, to avoid deposits. No abrasive particles, such as grinding wheels, should be used. Glass bead or alumina power blasting (or electropolishing) can be used to remove paints or markings, but a water rinse should be used afterward to wash away loose material. These cleaning processes are designed to smooth out the surface and lower outgassing rates.

Temperature
In vacuums that have fluctuating temperature or operate at a significantly different temperature from normal room temperature, cast iron would be a better choice for the stage housing. As temperature varies, a cast iron stage would act more similarly to a steel bearing structure than aluminum. Thermal expansion between the bearing structure and the stage housing will effect the bearing preload and stiffness, which may impact the desired performance. In addition, methods of removing heat that is generated should be evaluated. Without air flow, heat conduction may be a concern depending on what the heat source is mounted to.

Conclusion
In addition to the selection of stage materials and configurations, it is important that good basic practice be used on designing or preparing a stage for vacuum. Most vacuum stages are 'sized' with a pump that is based on the volume of the chamber. Efforts should be made to eliminate aspects of the motion system that would cause the pump to work harder than normal. For example, blind holes will create a slow draw of air that is trapped under a fastener. All holes should be fully vented or hardware should be vented, to prevent the trapping of any air. In general, stage and bearing designs should be configured where there are no pockets or crevises of air that can be trapped, causing additional work for the vacuum pump.

The concept of anti-friction bearings and low friction ball screws is quite appealing to vacuum environments as the lubrication requirements are minimal. Many times it is acceptable to utilize a permanent dry lubricant that can be applied to bearings or screw that is vacuum compatible and eliminate the need for future maintenance. Micro Slides provides its stages with an option for TD-2; a Tungsten disulfide permanent dry lubricant. TD-2 is a metallic film lubricant that is applied without heat, binders or adhesives. It bonds instantly to metal surfaces without adhering to itself, leaving a minimal build up of .5 microns. TD-2 is suitable in temperature ranges from -188 oC to 500 oC and is vacuum compatible to 10-14 Torr. Stages that are manufactured by Micro Slides for vacuum will be thoroughly degreased and relubricated with a vacuum rated lubricant (either TD-2 or Fomblin).
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