A centerless grinder has three main components:

Because the entire workpiece is supported by the regulating wheel and the blade, centerless grinding provides a more rigid system, enabling more efficient and accurate grinding compared to grinding between centers.

If you’re trying to grind a small-diameter bar between centers, it’s very difficult because the bar wants to move and flex.  

In centerless grinding, there’s no flexing because the workpiece is supported on three points.

The workpiece is rotating but not moving its position.

 

In the 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.

The hydrodynamic bearing is dependent on rotation and a low pressure oil supply to build up a wedge of oil between two surfaces – the shaft and bearing – to create the oil film producing the load support.  The geometry of the bearing surfaces aid in forming the wedge.  A low pressure oil source is required.  Spindles built with hydrodynamic bearings feature extreme accuracy, and very good vibration dampening.  High loads at start-up should be avoided.

The advantages of the fluid fed bearings, as outlined, outweigh the increased costs.  Spindles built with fluid fed bearings are more expensive to build.  This is mainly due to low volume, the unique geometry of the bearings and the additional processing necessary to ensure the precision.

Either style of fluid bearing dictates the need for an auxiliary temperature controlled oil supply system.  High pressure oil supply systems are needed for the hydrostatic bearing while low pressure oil systems are used for hydrodynamic bearings.

Because both types of fluid fed bearings are utilized in precise applications, temperature control for the oil supply system is a necessity.  Oil filtration, generally in the range of 1-micron is also required.  An accumulator in the oil supply system is recommended so that oil pressure can be maintained during shutdown in the event of a power failure.

When maintained as directed, spindles with fluid fed bearings can provide infinite (in theory) bearing life since there is no metal to metal contact.  Failures are usually due to human error such as poor maintenance of the oil filtration, use of improper fluid viscosity, temperature control system failures, and machine crashes.