One of the factors to consider when selecting shafts for linear bearings is surface roughness, which describes the microscopic asperities, or peaks, and valleys present on a material’s surface. But surface roughness is an important specification for all types of linear guides and screws — whether plain or recirculating, round shaft or profiled rail, ball screw or lead screw.
The ASME B46.1-2019 standard, Surface Texture (Roughness, Waviness, and Lay) defines surface roughness as, “the finer spaced irregularities of the surface texture,” and states that these irregularities often result from, “the inherent action of the production process or material condition.”
In other words, the combination of the material’s properties — such as particle size and distribution of elements in metal alloys — and the machining process determine the surface roughness of a part.
Surface roughness is most commonly expressed as an average value, denoted Ra. The Ra value is the arithmetic average of the absolute deviations of the profile from the mean line, recorded over an evaluation length. (Note that some references use an RMS value of roughness, termed Rq, although it has mostly been replaced by the Ra specification.) Another specification, useful for determining how much material should be removed during machining or finishing, is Rz, which gives the average distance between the five highest and five lowest points along the evaluation length.
Roughness, texture, or finish?
Three terms — surface roughness, surface finish, and surface texture — are often used interchangeably, but they actually refer to different quantities or characteristics. Surface roughness, as explained here, is a quantitative measure of surface asperities.
Surface texture is defined by the ASME B46.1-2019 standard as being made up of three parameters: lay, waviness, and roughness, of which waviness and roughness are quantifiable values.
Surface finish is a qualitative classification which, technically, applies to stainless steel materials. Guidelines for describing surface finishes are given in the ASME SA-480/SA-480M document, Specification for General Requirements for Flat-Rolled Stainless and Heat-Resisting Steel Plate, Sheet, and Strip.
Surface roughness is important for linear guides, ball screws, and lead screws for several reasons, but probably the most important is its contribution to friction. A higher (poorer) surface finish on rolling or sliding elements means more friction between those elements, and in turn, greater wear and reduced life. In fact, dynamic (kinetic) friction between two moving components is directly related to the components’ surface finish. This is one of the key reasons that rolled ball screws undergo an additional polishing step after the rolling process — because the rolling process produces an oxidation layer on the screw raceways, and polishing removes this oxidation layer and improves the surface roughness to match that of a ground screw.
A poor surface roughness can also damage plastic or rubber parts that ride on metal shafts or guides. For example, both the rubber seals used on ball nuts and the plastic nuts often used on lead screws can be damaged if the surface roughness of the ball or lead screw shaft is too high.
In recirculating linear bearings, the surface roughness of the recirculation guides can create pulsations that show up as planar errors during travel. And a maximum surface roughness is sometimes recommended for the mounting and reference surfaces used for profiled linear guides, to avoid plastic deformation of these surfaces caused by stresses from the guide mounting screws.
For plain bearings that are lubricated through the transfer of material from the bearing to the shaft (or guide), some amount of surface roughness is necessary, since the “peaks” on the surface of the guide serve to scrape off small amounts of material from the inside of the bearing. These small bits of material settle in the “valleys” on the surface of the shaft (or guide) and provide lubrication as the bearing travels back and forth. Similarly, when external lubrication is applied to plain bronze bearings, the valleys in the surface of the shaft capture and hold the lubrication to reduce friction and wear.