Linear Bearings and Guideways Primer

Linear bearings and guideways are primarily used in positioning table (positioning stage) applications.  The main function of a positioning table’s linear or rotary bearing system is to carry the user mounted load while the table is in motion.  The bearings are also a key element in determining the overall positioning table accuracy, repeatability, straightness and flatness.  Each bearing design provides advantages and disadvantages in load capacity, size, cost, stiffness, and friction.  Selecting the right linear or rotary bearing system for a positioning table application is critical.

The four primary linear bearings used in positioning table applications are cross roller, round rail, square rail, and air bearing.

Cross Roller Linear Bearings feature cylindrical rollers with ground “V” ways.  The larger surface contact between the rollers & “V” ways typically increases the table load capacity by up to 3 times more over a comparable ball & rod type system.  These table designs also produce better flatness and straightness specifications over traditional ball & rod type systems.  Thus cross roller linear bearings are typically used in higher accuracy type of applications.  However, the disadvantages of this type of bearing system are short travel lengths, minimal moment load capacity, large horizontal envelope area, and no possibility of using protective cover plates or waycovers.

Cross Roller Linear Bearing

Cross Roller Linear Bearing

Round Rail Linear Bearings use four bushings with recirculating ball bearings which are mounted within either two, or four pillow blocks (aluminum or steel housings).  The pillow blocks are then mounted to the underside of the carriage, which rides on two round hardened & ground shafts (which are mounted to the base).  Travel lengths are only limited by the available shaft and base length.  The contact point between the recirculating balls in the bushing and the round shaft produces a very low friction positioning table.  The greater number of balls contacting the ground shaft provides for a larger load capacity system.  This table design provides long travel lengths, good load capacities, large moment load capacities, and can accommodate protective cover plates & waycovers.

Round Rail Linear Bearing

Round Rail Linear Bearing

Square Rail (linear guide) Bearing Systems are very similar in operation as the round rail systems.  The round shaft has been replaced with a rectangular (square or T-shaped) rail, while the round rail bushing has been replaced with a rectangular bearing block.  The recirculating ball bearings in the bearing block contact more surface area on the curved ball race on the square rail which provides a table that has increased load capacity, increased moment load capacity, and higher system rigidity over the round rail positioning table.  Because of the precision ground ball races on the rails, these linear bearings will have better flatness & straightness specifications than a round rail system.  Table travels are only limited by the available rail, and base length.  This table design is also able to handle shock & vibration forces better than a round rail system due to its robust bearing design, and can accommodate protective cover plates & waycovers.

Square Rail Linear Bearing

Square Rail Linear Bearing

Air Bearing Linear Tables create a small air cushion between the table carriage and the table base (guide rail).  This provides a non-contact linear bearing system that is rigid, friction free, and cog-free.  Using a very accurate guide rail (rectangular or square) can produce excellent flatness & straightness specifications, especially if the air bearing linear table is mounted to a granite base.  Typical drive mechanisms for these tables include high accuracy acme screws and linear motors.  Incorporating a high accuracy non-contact linear motor drive system with a high accuracy non-contact linear encoder for position feedback creates a very accurate sub-micron or nano-precision positioning table, one that could virtually last forever.

Air Bearing Linear Table

Air Bearing Linear Table

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Tags:  Cross Roller Linear Bearings, Round Rail Linear Bearings, Square Rail Bearings, Linear Guides, Air Bearing Linear Tables, Round Shafting, Shaft Supports, Linear Rails, Roller Recirculating Units, Linear Guideways, Linear Splines, Splines, Linear Slides, Slides, Linear Tables, Linear Stages

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