26-11-2012, 12:48 PM
DYNAMIC PROPERTIES OF AEROSTATIC JOURNAL BEARINGS
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Abstract:
Aerostatic bearings are in specific cases used for high speed rotors,
where most importance have problems of stability. The knowledge of
gas film stiffness and damping is necessary for calculation of the rotor
stability limit. Algorithms for calculation of aerostatic journal bearings
stiffness and damping already exist, but up to now they were not
experimentally verified. The stand for demonstration of rotor dynamic
phenomena at CTU in Prague was supplemented with super-structure
enabling identification of stiffness and damping matrices coefficients.
The stiffness and damping coefficients are determined from the
response to harmonic excitation, invoked in two different directions by
means of piezo-actuator.
INTRODUCTION
Aerostatic bearings are characterized by high precision of operation and they are
aimed first of all for applications with low rotational speeds. However, in specific cases,
e.g. in polluted environment or with high requirements to bearing stiffness, they are used
also for high-speed applications. As an example of successful high speed applications of
aerostatic bearings one can mention dental drills with maximum speed of 750.000 rpm or
drilling and grinding spindles with operating speeds above 100.000 rpm. The most
important problem of high-speed rotors is stability. The stability limit must be determined
with sufficient precision in advance, because the rotor instability in reality ends
immediately by heavy failure. The knowledge of gas film stiffness and damping is
therefore essential condition for calculation of the rotor stability limit.
Existing programs for calculation of aerostatic journal bearings stiffness and damping
could not be up to now verified due to the lack of suitable experimental equipment. The
stand for demonstration of rotor dynamic phenomena – Rotor Kit Bently Nevada (RKBN)
- installed at CTU in Prague has modular design, which makes it suitable for different
modifications. RBKN was therefore supplemented with super-structure enabling
identification of stiffness and damping matrices coefficients.
IDENTIFICATION OF BEARING STIFFNES AND DAMPING
Plain journal bearings with full fluid film (see Fig.1) have some special features.
Journal movement in one direction results in generation of fluid in arbitrary direction. The bearing force can be therefore resolved into radial component, which returns the journal
to its original position, and tangential component, which is the cause of bearing
instability. For securing the stability of high-speed rotors it is necessary to use bearings
with more complex geometry of sliding surface – lemon, offset halves (Fig 1) or tilting
pad type.
TEST BEARINGS
Four variants of aerostatic journal bearings were proposed for experimental
verification of dynamic characteristics. Bearing diameter is 30 mm, their length to
diameter ratio l/D is equal either to 1,5 (Fig. 5) or to 1,0 (Fig. 6).
The advantage of greater l/D ratio is higher value of angular stiffness, which may
eliminate problems with tilting of the test bearing when it is excited by dynamic force.
The shorter bearing, on the other hand, is closer to bearing geometry used in reality.
Both bearing types have either two rows of feeding orifices distant one quarter of bearing
length from the boundary, or one row of orifices in centre plane of the bearing. There are
8 orifices around the bearing periphery of 0,2 mm (l/D=1,5) or 0,3 mm (l/D=1,0) in
diameter. The bearing diametral clearance varies from 0,03 mm to 0,045 mm, which
corresponds to relative clearance of from 1.10-3 to 1,5.10-3. By increasing the feeding
hole diameter or bearing clearance one can get another bearing geometry to broaden the
measured dynamic characteristic data range.