This invention relates to an improvement in linear bearing assemblies and is particularly directed to a means of restraining rolling element cages and rolling elements to prevent their migration away from the center position.
In linear bearing assemblies with roller elements (rollers or ball bearings), the rolling elements tend to migrate. Migration is caused primarily by imperfections in the components, misalignment, and/or variations in the load on the rolling elements and cage.
This may not be a problem in leadscrew or other high force drive systems as stops at the ends of the bearings will relocate the rolling elements and their cage. When it is not permissable to hit stops at the end of travel or, if the drive forces are not strong enough, to keep the rolling elements and cages in place, rack and pinions, pulleys and filaments have been used. These mechanisms are bulky and have rubbing surfaces requiring additional forces to be applied.
When the available driving force is limited, however, another means is necessary to keep the rolling elements from migrating very far or not at all and this invention is directed toward this end. This is particularly important where movement of the apparatus is controlled to submicron distances, step-by-step, as in an X-Y table in an inspection microscope or a particle beam machine for making semiconductor wafers and masks.
It is therefore an object of this invention to improve linear bearing assemblies by providing a means for reducing or preventing migration of the rolling elements.
Another object of this invention is to provide a means in a linear bearing assembly which reduces or prevents migration of the rolling elements without providing bulk and without rubbing surfaces which require additional forces to be applied to overcome the friction of the rubbing surfaces.
Another object of this invention is to improve an X-Y table for a particle beam machine for precise positioning of the top surface of the table which is critical to the proper operation of the particle beam machine.