Excessive radial clearance in my linear motion axis, necessary because of the tolerances I could achieve using conventional woodworking techniques, was the major cause of inaccuracies and non-repeatability in my system. We learned recently in class that appropriate preload is a feasible approach to eliminate radial clearance and improve geometric repeatability without compromising overall bearing stiffness.
There is a large variety of possible ways to build preload into a sliding contact linear bearing. I decided to explore the use of an elastomeric compliant layer in the slider since it seemed like a concept I could eventually incorporate into my desk. Few suitable compliant materials have the wear and frictional properties to be good bearing materials, so this compliant layer will be sandwiched between the original wooden slider and a new bearing plate made of a wear-resistant material.
All of the repeatability and stiffness benefits of preloaded bearings only hold if the preload is never completely relieved under normal operation. I estimated that the primary source of aberration in the boxway is due to local imperfections in the side rails due to the poor surface finish achievable in the oriented strand board I used to build them. To the first order, I estimated that these imperfections have a characteristic dimension of approximately 0.1 mm. In order to ensure some preload is retained through the entire travel, I invoked St. Venant’s Principle and designed for a preload displacement of 10 mm.
Using these values, I built a simple spreadsheet that predicts the marginal amount of actuation force due to the preload and specifies the geometry of individual parts to help support fabrication. For a hypothetical material with a modulus of 0.001 GPa (based off of linoleum), the appropriate displacement would result in an additional 140 N of actuation force required. Since my toy linear motion axis does not drive anything other than its internal resistance, I figured my stepper motor would be able to move it without too much trouble.