Profiled rails have high load-carrying capacity for their size. But when smooth, precise motion is needed and the load being moved is not very high, they can be overkill for the application. That’s why most profiled linear guide manufacturers now make miniature versions, with many of the same performance benefits and features of standard sized profiled rails, but in a smaller package.
There’s no official designation for “miniature” profiled rails, but most manufacturers classify rails size 15 and smaller as miniature, with sizes as small as 1 mm rail width!
Miniature vs standard
Besides their smaller size, the most significant distinction between miniature and standard profiled rails is the number of rows of balls in the bearing. Most standard sized bearings have four rows of balls (two on each side of the bearing), which gives them high load and moment capacity. Miniature versions, because of their compact size, typically have just two rows of balls (one on each side). This reduces both their load and moment capacities, but “long” versions provide increased pitch and yaw moment capacity, while “wide” versions provide higher roll moment capacity.
Another difference between standard and miniature profiled rail bearings is the retention of the balls in the bearing housing. This is important because virtually all standard sizes have balls that are retained in the housing, meaning they won’t fall out during general handling and installation. Because of their limited real estate, miniature versions generally don’t incorporate ball retention mechanisms, or if they do, they are only effective with extremely careful handling and installation. Because of the difficulties with ball retention, miniature profiled rail bearings are typically mounted on an arbor (essentially a plastic, “dummy” rail) during the manufacturer’s assembly and packaging. The arbor not only helps to retain the balls in the housing, but also aides with mounting the bearing on the rail.
Because of their small size, miniature rails cannot be heavily preloaded, but light preload can be applied—typically in the range of 1 to 2%. Accuracy grades for miniature rails follow those of standard profiled rails, ranging from what most manufacturers term “normal accuracy” to “super precision.”
Miniature profiled rails are often used in the semiconductor, electronic, and medical industries, where contamination is rare, and more emphasis is placed on preventing contamination from being released into the environment by moving components. The use of non-contact or low-friction seals helps miniature rails meet this requirement. Conversely, many manufacturers offer secondary seals for cases where the bearing needs to be protected against contamination. And for applications with corrosive environments, miniature rails are commonly supplied with metal parts that are made from stainless or corrosion-resistant steel.
To further support their use in clean environments, some manufacturers have submitted their miniature profiled rails for cleanroom compatibility testing. With low-friction seals and cleanroom grease, many components can meet ISO 14644-1 class 4 or 5 (US Fed Std 209E class 10 or 100) requirements for particle generation.
Miniature rails are more than just “smaller” versions of standard profiled rails. They offer a space-saving solution for linear motion applications that require smooth, precise motion with lower load and moment-handling requirements and minimized particle generation.
Feature image credit: NSK Americas