Grooved Hybrid Air Bearings

Grooved Hybrid Air Bearings
1
P. Stanev
F. P. Wardle
2
1
J. Corbett
1) Cranfield University, School of Industrial and Manufacturing Science, Bedford, UK
2) Loadpoint Ltd, Cricklade, Swindon, UK
Experiments
Grooved hybrid air bearings combine aerostatic and
aerodynamic design principles to optimise ultra high speed
performance. Aerostatic lift is generated by feeding the bearing
with pressurised air through orifice restrictors as in
conventional bearing designs. Aerodynamic lift is controlled by
the addition of helical grooves machined into the shaft or
bearing journal. Partially grooved bearings (shown above) can
be designed to improve high speed stiffness whereas fully
grooved bearings may be designed to improve high speed
stability.
At high speeds the grooves pumping action significantly changes the
pressure distribution within the bearing and can improve load carrying
capacity and stiffness. The grooves also change the air velocity gradients in
the bearing affecting the basic mechanism of whirl instability inherent in
fluid film journal bearings - usually improving the threshold at which
instability occurs. Grooves may extend the full length of the bearing or part
way across the bearing depending upon required operating characteristics.
In general, fully grooved bearings give better stability whereas partially
grooved bearings may give better stiffness and load carrying capacity. The
improvements being confined to high rotational speeds.
Ultra high Speed Machining Applications
Parametric Studies
●
●
●
●
Grooved hybrid bearings have a large number of design parameters and
optimum designs are most easily identified with the aid of a computer
model that simulates air flow through the bearing. Such a model has been
developed and used to study the effect of groove parameters on bearing
stiffness and stability. Figs 4 to 7 give some examples of the results. In
these figures bearing speed is represented by the non dimensional
parameter Compressibility Number defined as :-
Bore Grinding
Milling
Micromachining
Printed circuit board drilling
In these applications machining spindles are run at speeds well in excess of
100,000 rpm. The spindles use aerostatic bearings because of :● Improved life at ultra high speeds
● Low friction losses
● Low motion errors
Bearing diameters are usually more than 25 mm and surface speeds often
exceed 3.0 x 106 DN (bearing bore in mm x shaft speed in rpm).
Ultra High Speed Aerostatic Bearing Characteristics
All aerostatic journal bearings develop an aerodynamic component of lift
and at surface speeds as low as 0.5 x 106 DN this can exceed the
aerostatic lift, fig 1 and hence dominate the bearing’s high speed
performance.
Aerodynamic lift increases bearing load capacity and stiffness but the
benefits are at the expense of reduced stability. With aerodynamic lift being
dominant at high speeds and with air spindles needing to use large
diameter (heavy) shafts to maximise bearing stiffness the aerostatic
bearing becomes prone to whirl. As for aerodynamic bearings the whirl
frequency is close to one half of the shaft’s rotational speed and is usually
excited when the rotational speed approaches twice the frequency of a
critical speed of the spindle. This type of whirl is often referred to as ‘half
speed whirl’ and is destructive in the sense that any attempt to run through
it will lead to wear and ultimately seizure of the bearing. Half speed whirl
therefore limits spindle speed to about twice the first critical speed of the
spindle. Two whirl modes are common on ultra high speed aerostatic
spindles, fig 2 and are known as ‘translational whirl’ or ‘conical whirl’.
Whichever mode is predominant simply depends on the dynamic
characteristics of the bearing - shaft system. Conical whirl is likely to occur
in arrangements where the shaft overhang from the bearings is large and
conversely translational whirl is more likely when overhangs are low.
Experimental work was aimed at validating the computer model. A
machining spindle with integral high frequency motor and two partially
grooved hybrid bearings was manufactured, fig 8. Pressure sensors were
built into one of the bearings to measure the pumping effect of the grooves
and a slave bearing and capacitance gauge were mounted on the front of
the spindle to enable stiffness to be measured.
Λ= 6µω (R/C)2/Pa
where µ is the viscosity of air, ω is bearing rotational speed, R is shaft
radius, C is clearance measured over ridges and Pa is atmospheric
pressure.
Stiffness measurements were limited in speed to less than 40 000 rpm due
to the characteristics of the slave bearing. However the latter could be
removed and pressure measurements taken to the spindle’s maximum
speed of 120 000 rpm. This speed limit being a function of the drive motor.
Fig 4 illustrates that in the case of a partially grooved hybrid bearing there is
an optimum groove depth which gives maximum stiffness at high speeds.
The improvement over a conventional aerostatic bearing though is at the
expense of low speed stiffness.
Experimental measurements of spindle stiffness showed it to increase with
speed. Factoring spindle stiffness to account for the overhang of the slave
bearing and capacitance gauge enabled the bearing stiffness to be
determined. Comparing bearing stiffness measured in this way with
theoretical predictions from the computer model showed reasonable
agreement over the range of speeds for which measurements could be
taken.
The partially grooved hybrid bearing giving the maximum stiffness in fig 4
also exhibits high stability, fig 5. The improvement over a conventional
aerostatic bearing is only significant at high speeds, however this is an
important advantage as for aerostatic bearings stability is seldom an issue
at low speeds.
Pressure in the bearing journal measured midway between orifices in one
row also increased with speed. For these measurements the spindle was
not subjected to any load so the increase in pressure was predominantly
due to the pumping effect of the grooves. Comparing experimental
measurements of pressure with theoretical predictions from the computer
model again showed reasonable agreement.
For the case of a fully grooved hybrid bearing the improvement in high
speed stability over a plain hydrostatic bearing can be substantial, fig 6.
Conclusions
● A computer model of a grooved hybrid bearing has been developed and
demonstrated to give reasonable agreement with experimental
measurements performed on a high speed spindle.
● Grooves can be added to otherwise conventional aerostatic bearing
designs to modify their high speed performance.
● More specifically groove designs may be optimised to enhance bearing
stiffness or stability at high speeds.
● Improvements in high speed stiffness or stability are however offset by a
reduction in stiffness or stability at low speeds.
Grooved Hybrid Bearing
The grooved hybrid bearing combines aerostatic and aerodynamic design
principles. It contains helical grooves, either on the shaft or in the journal
bore, located outboard of the air feed jets and arranged to pump air into the
centre of the bearing when the shaft rotates, fig 3.
● Partially grooved bearing designs give maximum high speed stiffness
with some improvement in high speed stability.
However the additional stability is at the expense of stiffness which is lower
than that of the conventional bearing throughout the speed range, fig 7.
● Fully grooved bearing designs give maximum high speed stability but at
the expense of bearing stiffness.