Hydrostatic bearing technology using water to suspend ball bearings has the potential to transform high-speed milling by smoothing the machining process and adding years to milling equipment.

Spindle suppliers to the metalworking industry have indicated that a change is about to occur for spindle bearings used in milling. New to the milling market is the introduction of hydrostatic bearings, so-named because they utilize water as their floating medium. The water is used, not so much as a lubricant, but rather, as a means of suspending the bearings completely. In this way they do not come in contact with any other surface.

Fluid bearings have been used before in other applications, as has the use of water as the suspension medium. What’s new is that hydrostatic bearings have not traditionally been used in the milling spindle market. That is, up until now. Recently a demand has grown due to the fact that the reigning favorite, hybrid bearing technology using ceramic ball bearings set in steel races, has reached its theoretical limit as to how much speed and power it can provide the milling spindle without significantly shortening the life of the equipment. Because of their capacity to accept greater loads, fluid bearings surpass the limitations of hybrid bearings. Furthermore, the inherent smoothness of motion they provide, along with their tendency to absorb vibration, makes them ideal for use in high speed/high power machining. It also suggests that spindles with hydrostatic bearings will last longer in a field that is typically hard on shock-sensitive equipment.

One industry, in particular, that could realize significant improvement in milling operations, is the aerospace industry. This field often couples high spindle speeds with greater cut depths for the purpose of hogging out aluminum aircraft components. Hard steel mold and die milling also stands to benefit from the spindle’s potential to extend the life of expensive milling machinery. Both industries have the know-how, the process components, as well as the desire, to leverage higher spindle speed and power. They are basically waiting for the technology that can speak to these needs. Hydrostatic bearings may find their greatest use in the metal shops of these fields.

Air and electromagnetism are two alternates to waterborne bearings. Both technologies can be utilized to suspend the bearing apart from other surfaces. But both have serious shortcomings curtailing their use. Air bearings generally do not possess the degree of stiffness that is necessary in most milling operations while magnetic bearings are simply too expensive to be a viable option at present.

Since fluid bearings are suspended free of contact, this enables the hydrostatic bearings to endure heavier loads. The bearings do not touch other parts of the spindle so there’s a greater area of fluid to absorb the pressure. If the bearings were not suspended, the force would be concentrated in those places where they touch other components and this would create undue stress on these parts.

Fluid bearings can use oil or other fluids, but its use is better confined to lubrication purposes in which the ball is not completely isolated by the oil medium. Oil’s relative viscosity in comparison to water is what keeps it from seeing more widespread use as a suspension agent in spindle bearings. Its viscosity slows the machinery in much the same way friction does. A motor must exert more energy to overcome the viscosity’s resistance and this jacks up costs.

For all its advantages, the switch to completely hydrostatic technology will not occur overnight. Water’s chief shortcoming is that it does not provide the non-reactive and non-conductive properties machinists have come to expect of oil. To use water as a suspension agent, spindle manufacturers must take a number of additional steps during manufacturing. These include coating internal components, sealing the motor and using de-ionized water, all of which can add onto the spindle’s production costs. It is hoped, however, that as hydrostatic spindles grow in popularity, the price of manufacturing them will gradually come down. The incentive for customers to make the initial investment is durability and projected lifetime. In other words, hydrostatic spindles stand to save their users money in the long run. The fact is that hydrostatic technology is not exclusive to any one company. Its combination of properties practically ensures that it will gain an increased acceptance as the primary foundation technology on which metalworking spindles are built.

Source: The Fluid Transition
Peter Zelinski
Modern Machine Shop, April 2001
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