In the small world of precision spindles, there exist two types of bearings; rolling element bearings and non-contact type bearings. Rolling element bearings, as the term indicates, rotate due to integral components that roll such as balls and rollers. Non-contact type bearings would be defined as those that support rotation, but have no contact between the rotating and non-rotating components. Air bearings, magnetic bearings and fluid fed bearings are included. Fluid fed bearings, either hydrostatic or hydrodynamic, are integrated in high-precision spindles.
Throughout this discussion we will focus on the spindles designed to incorporate the oil fed hydrostatic or hydrodynamic bearings. Each style relies on a fluid, usually oil, to create a clearance between the moving and stationary elements. However, that is where the similarity ends. Hydrostatic bearings support a load by means of high pressure oil. Hydrodynamic bearings support a load by a lubricant film.
Journal bearings support the radial loads and thrust bearings support the axial loads. The fluid fed bearings are usually radial or axial in design. Some designs have been developed to incorporate both radial and axial functions in an opposing pair of angular or spherical bearings. Basically, the bearings float the load on a renewing film of lubricant. The hydrostatic and hydrodynamic bearing designs are substantially different from each other and are almost always application dependent.
Hydrostatic bearings require an external high pressure oil source along with a restrictor integrated into the design to provide a controlled oil film and generate load support. Generally, the oil is distributed to multiple cavities through metering devices called restrictors. Spindles built with hydrostatic bearings characteristically have high stiffness, extreme accuracy, and excellent vibration dampening features. The extreme accuracy is a result of the averaging of form errors of the surfaces that make up the bearing elements. Because a hydrostatic bearing is not dependent upon rotation to generate the oil film, it can accommodate heavy loads at zero speed. The low random motion error and vibration dampening makes the hydrostatic bearing ideal for ultra-precision machining such as those required for precision optics including mirrors.
Stay tuned for next month when we dive into Hydrodynamic spindles!