Roller bearing spindles are infrequently used in the machining of optical components or high-precision parts including small features. This is largely a result of insufficient reliability at higher speeds, changing properties and frequent errors in shaft motion. Whilst it is true that spindles with oil-hydrostatic bearings demonstrate the sufficiently increased robustness and low errors in shaft motion necessary to allow precision-precision machining, they also result in large power losses at higher spindle speeds, and complex sealing technology, all of which contributes to greater up-front and subsequent running costs. This combination ultimately curtails the use of this spindle type in these sorts of applications.
Figure 1. Shear-losses of a journal bearing with speed and rela. shaft eccentricity using aerostatic bearing technology (left) and oil-hydrostatic bearing technologies (right) for same bearing dimensions (oil-hydrostatic bearing gap 7 x larger)
Levicron’s Patented Bearing Technology
The use of an optimized bearing orifice design, combined with aerostatic bearings, enables near proportional increases in load capacity with bearing supply pressure, thus enabling load capacities established in oil-hydrostatic spindles at the same supply pressure. This can ultimately culminate in an increase of allowed side load at the tool due to the reduced distance from the tool tip to the front radial bearing of the spindle, as well as the sealing technology.
Given that shear losses in fluid and gas bearings under shaft rotation come about largely due to the viscosity of the fluid or gas used, and not because of alterations in pressure, bearing losses themselves are largely invariant to supply pressure.
An aerostatic bearing that comprises state of the art bearing orifices will not only see an increase in supply pressure, but also a proportional increase in air consumption. This can potentially lead to excessive air consumption reaching as high as 1.000 NI/mind at 60 bar supply pressure. To combat this issue Levicron’s patented bearing technology avoids the use of orifices enabling economical manufacturing of small capillaries under 30 microns in diameter, of which 4-8 times more are used as in standard bearings.
This not only allowed reductions in air consumption of up to 70%, but load capacity and stiffness increased at large shaft eccentricities in situations where larger orifices would be forced to operate in choked conditions.
Figure 2. Stiffness of an aerostatic journal bearing with orifice diameter and relative shaft eccentricity
Using this knowledge not only have Levicron developed high-pressure aerostatic spindle technology which enables customers to achieve load capacities reached using oil-hydrostatic bearings, but this technology also enables speeds, power consumption and error-motion levels of spindles with aerostatic bearing systems. Our new spindle developments UASD-H40, featuring a brand new and patented automatic and spring-less HSK-E40 tool interface, and our UASD-H25/A, as a derivation of the well-known ASD-H25/A, feature the new high-pressure aerostatic spindle technology to combine robustness and industry standards with ultra-precision properties and high-speed capability.
Figure 3. Capillary bearing design
UASD-H25/A- High-Pressure Aerostatic Spindle Technology
UASD-H25/A – as reliable, fast and accurate as expected, but featuring up to three time the load capacity
Featuring alongside other spindle models at this year’s EMO exhibition will be Levicron’s new UASD-H25/A. This model, a derivative of the existing ASDH25/A, not only features the new high-pressure aerostatic bearing technology, but also allows up to 200% increases in load capacity whilst ensuring the speed, error-motion and stability already achieved with standard pressure models. Machining steel under moderate chip load with larger tools and highspeed machining of copper material in a new dimension with aerostatic tool spindles becomes reality.
Figure 4. 200 % increase in radial load capacity at spindle nose (top) and 150 % in axial direction at only 50 % increase in air consumption
Table 1. Direct comparison spindle type ASD-H25/A and the new UASD-H25
||0 - 60
||0 – 60
|Ult. load capacity at spindle nose, radial
||900 (275 %)
|Ult. Load capacity , axial
||1400 (255 %)
|Static stiffness, radial
||83 (202 %)
|Static stiffness, axial
||120 (185 %)
|Static air consumtion
||90 (180 %)
|Dynamic run-out at tool
|Error in shaft rotation
||< 23 nm
||< 28 nm
Furthermore, it is simple to turn the existing ASD-H25/A into the new UASD-H25/A, requiring only the introduction of the new axial bearing design and a new connector for the air supply, and all of this with only a 50% increase in air consumption. The spindle performance, as well as the static load capacities have been verified in machining trials.
This information has been sourced, reviewed and adapted from materials provided by Levicron GmbH.
For more information on this source, please visit Levicron GmbH.