Traditional profile monorails deliver the highest accuracy for linear motion. This comes from an expensive grinding process. Accuracy is presented in terms of grinding tolerances on the rail and divides into accuracy grades. The same is true for some types of steel U-shaped rails, which offer grades from honed to ground. But this data on grinding tolerances is usually not the same as accuracy in operation.
Most products have values for “running parallelism.” This tells where the carriage will be while it is running through its stroke. It’s the most critical piece of information for engineers. Linear guides typically have tolerances for parallelism to keep the top of the carriage true to the bottom of the rail, and the outside of the carriage true to the side of the rail. This tolerance scheme ensures proper alignment over the length of the rail.
Running-parallelism tolerances, however, are usually established when the rail is in a bolted-down condition on an extremely smooth test surface. The tests do not take into account variances in mounting surfaces, preload adjustment, deflection due to loading, or temperature changes. This could lead to problems when choosing a rail based on test data for running parallelism.
Redi-rail linear guides simplify accuracy problems for machine designers. The guides hold tolerances that rival those of profile monorails but the manufacturing process costs less. Modern standards of geometric dimensions and tolerances hold the top of the carriage parallel to the bottom of an extruded aluminum rail. Ground and polished steel shafting is pressed and bonded into the aluminum rail. This produces a consistent running surface for the bearings and holds the carriage parallel to the side surface.
Redi-rail linear guides hold tolerances of ±0.001 in. over rail lengths of up to 19 ft. The devices are used in linear applications such as pick-and-place robots, packaging machinery, material handling, and assembly equipment.
This information supplied by Pacific Bearing Co., Rockford, Ill.
For more information, Circle 627.