Hybrid bearings enable faster spindle speeds

A novel design effectively addresses the major limiting factors to achieving higher speeds with aerostatic bearings

Driven by the demands of the machine tool industry, and by the requirements for micromachining in the electronics and optical industries, ultra-high-speed aerostatic bearings are now achieving speeds in excess of 3 x 10e6 DN - equivalent to shaft surface speeds of more than 150m/s.

However, with bearing clearances in the range of 5-20um, air films are subject to very high shear rates at these speeds; the resulting viscous forces having a detrimental affect on both the thermal and dynamic characteristics of the bearing.

Loadpoint has overcome these problems with a new design of grooved hybrid aerostatic bearings.

The benefits of this design are improvements in high-speed performance that may be exploited in terms of increased limiting speed, higher stiffness or reduced running temperature.

Loadpoint's new design effectively addresses the major limiting factors to achieving higher speeds with aerostatic bearings: high friction losses due to shearing of the air within the bearing gap; and half speed whirl, a destructive instability that adversely affects the stiffness of the bearings; in some cases reaching a level that can lead to seizure.

Combining aerostatic and aerodynamic principles, Loadpoint's grooved hybrid bearings - journal types with helical grooves and thrust bearings with spiral grooves - are designed to pump air into the centre of the bearing when they rotate.

At high speeds, the pumping action of the grooves significantly changes the pressure distribution within the bearing, generally improving load carrying capacity and stiffness.

The grooves also change the velocity gradients within the bearing's air film; that, in turn, reduces the bearing's viscous friction loss and improves its whirl response.

Whirl response is generally more complex than for a conventional aerostatic bearing; it is highly dependent on groove design, where, in general, the higher the groove volume, the more the response departs from that of the conventional bearing.

The response takes a similar form with maximum loss of aerodynamic lift occurring at a particular speed.

However, unlike conventional air bearings, this speed does not always occur at half shaft rotational speed; and the maximum loss isn't always the total aerodynamic component, which would otherwise result in total loss of load capacity.

As viscous friction loss in aerostatic bearings is proportional to area and inversely proportional to gap, the grooved hybrid bearings also produce lower levels of friction than conventional bearings operating with the same minimum clearance.

The benefits become more pronounced at high speeds, as the ratio of groove clearance to ridge clearance in the hybrid bearing increases.

A typical comparison between grooved hybrid and conventional aerostatic bearing performance illustrates this point.

Each curve shows the trade off between bearing stiffness, taking into account whirl response and viscous friction loss, as shaft speed increases from 0 to 160,000rev/min.

As speed is increased, the bearings minimum clearance reduces from 28 to 16um due to shaft growth.

At low and moderate speeds the results are similar to the conventional bearing exhibiting a marginally better trade off between stiffness and power loss.

However, at speeds above 120,000rev/min, the grooved bearing shows clear benefits and has the potential to reach significantly higher speeds.

Comparing the effects of the grooved hybrid design across annular thrust bearings and journal bearings shows that the benefits are more pronounced in the former types.

Taking the example of a machining spindle arrangement, the thrust bearings do not experience whirl, so the tradeoff is usually stiffness against power loss.

This is an important consideration as thrust bearings are invariably larger in diameter than journal bearings, and surface speeds are higher.

Importantly, grooved hybrid thrust bearings can be designed to give the same static stiffness as a conventional thrust bearing; however, because of the grooving, they will exhibit significantly lower power consumption throughout the speed range.

Furthermore, their stiffness increases with speed due to aerodynamic effects, whereas conventional aerostatic thrust bearings do not generate any aerodynamic lift at all.

An early application illustrating the benefits of Loadpoint's grooved hybrid technology is the rotor of a bore- grinding spindle.

This uses four short aerostatic journal bearings to achieve high stiffness and to support a high powered overhung motor.

In this application grooved hybrid bearings have been compared with conventional aerostatic bearings and found to increase whirl speed by approximately 25%.

Power consumption, measured in terms of drive current was also reduced by 10%.

A further application concerns a high-speed micromachining spindle designed to take milling tools down to 50um in diameter.

Here, high speeds need to be accompanied by submicron running accuracy and low thermal growth.

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