The RPS4014 linear drive system from Nexen (Vadnais Heights, MN) adds two rollers for a total of 14, and a wider rack to double thrust capacity of 14000 Nm. It is available in a standard grade with positional accuracy of +/-50 micrometers and a machine oil finish; a premium grade achieves positional accuracy of +/-30 micrometers and has a Raydent surface treatment to prevent corrosion.
Both versions of the drive have zero backlash and are unaffected by thermal expansion. In applications with speeds under 30 meters/min., the premium-grade drive can run lubrication-free with only a small sacrifice in life.
For more info, contact http://info.hotims.com/22908-319.