Pros and cons of different guide ways

Three basic types of guide ways can support linear motion between machine tool components: rolling-element, hydrostatic and plain sliding ways.

In rolling-element ways, moving elements like the table are supported on the flat, straight guide ways that are mounted on stationary elements like the bed by rolling elements, such as cylindrical rollers. The friction in rolling-element guide ways is relatively low, so they provide little resistance to moving along the direction of the guide way. However, the contact patch is small—essentially a line on the side of each cylindrical roller. Rolling-element guide ways, therefore, have lower stiffness and less damping than other kinds of ways. In addition, these guide ways do not require any power to support the load.

Courtesy of S. Smith

Figure 1. A scraped surface with blue marking compound. The blue spots mark small depressions that will contain oil.

Hydrostatic ways are formed by combining a hydrostatic bearing with a flat, straight way. The hydrostatic bearing consists of a pocket supplied with pressurized oil. The gap between the edges of the pocket and the guide way form a restriction, the flow of pressurized oil through the restriction controls the pressure in the pocket, and the pressurized oil supports the load. Hydrostatic ways have little friction, but they are active, requiring a pocket, a restriction, a pump and a flow of oil through the restriction even when the bearing is not moving.

Plain sliding guide ways do not require an external power source. The sliding element and stationary element are typically cast iron, and their surfaces are closely matched geometrically, but they are not perfectly smooth. A thin film of oil separates the two surfaces. It is retained in small, randomly dispersed depressions on the surfaces that act as oil reservoirs.

The reservoirs are replenished by occasionally squirting a little oil on the way in front of the sliding element. The oil is pulled by the motion of the sliding element from the reservoirs into the space between the contacting points of the two surfaces. This effect forms a hydrodynamic bearing, and the oil film is created and maintained by the motion. The critical features in the bearing are the close fit of the geometric form between the stationary and moving surfaces, and the surface texture. Unlike the other guide ways discussed above, the features on plain guide ways are obtained by hand scraping.