Bearing structures

A bearing structure, preferably for supporting a rotary table (4) in metrological apparatus, includes both a journal bearing (34,36,38) and a thrust bearing (56,66). The journal bearing is made up of three dry pads (34,36,38), one (38) of which is radially adjustable, and the thrust bearing (56,66) comprises first and second air bearings. The axis of rotation of the bearing is vertical, and the lower (66) of the air bearings is less stiff than the upper one (56).

This invention relates to bearing structures and is particularly but not 
exclusively applicable to bearing structures for rotary workpiece support 
tables in metrological apparatus, such as rate tables, angular measuring 
systems and apparatus for the measurement of profile errors including 
roundness, straightness and surface texture. 
There is a need for a precision bearing, for example for the rotary 
workpiece support table in metrological apparatus, which is capable of 
supporting a relatively high load, is stiff in the sense of substantially 
preventing radial, axial and angular displacements of the member supported 
by the bearing, provides low friction so that the member is freely 
rotatable and provides vibration damping. Various forms of bearings have 
been proposed in metrological apparatus in the past. For example in one 
bearing structure which has been in commercial production, the vertically 
disposed shaft of a workpiece support table has been supported at its 
bottom end by a hemispherical dry thrust bearing and near its upper end by 
a journal comprising dry pads engaging the cylindrical surface of the 
shaft. This dry bearing structure suffers from the problem that it has 
only a low load bearing capacity and that the shaft has to be longer than 
is desirable to provide the required accuracy. 
Oil hydrostatic bearings have also been proposed. In one example of such a 
bearing, oil is continuously pumped into the space between conical 
surfaces provided on the shaft and a surrounding stationary support 
member. Although this type of bearing is capable of supporting high loads, 
it is extremely expensive as it requires a considerable amount of 
ancilliary equipment such as a pump, a reservoir for the oil and 
associated piping. 
Also, air bearings have been used but these are unstable so that in 
metrological apparatus high coning errors would be introduced particularly 
if the load is offset relative to the axis of rotation of the workpiece 
support table. 
In one preferred aspect of the present invention, there is provided a 
composite bearing in which first and second members are supported for 
relative rotation, a bearing of a first type being provided to constrain 
said members against axial movement and a bearing of a second type being 
provided to constrain said members against radial movement. In a preferred 
embodiment, the bearing of the first type is an air bearing and the 
bearing of the second type is a dry friction bearing. 
In the preferred embodiment of the invention, there is provided a bearing 
structure comprising air bearing means constraining first and second 
relatively rotatable members against axial movement and a bearing of a 
different type constraining said members against radial movement. 
In a further alternative aspect of the present invention, there is provided 
a composite bearing comprising first and second relatively rotatable 
members having radially extending bearing surfaces and cylindrical bearing 
surfaces with dry friction bearing means acting between the cylindrical 
surfaces and air bearing means acting between the axial surfaces. 
With the preferred embodiment of the invention, a bearing structure having 
low friction, high stiffness, a high level of stability and a high level 
of vibration damping may be provided at relatively low cost.

With reference to FIG. 1, the metrological apparatus, which is particularly 
for measuring roundness, comprises a bench 2 within which is a chassis 
structure (not shown) supporting a turntable 4 by means of a bearing to be 
described in detail with reference to FIGS. 2 to 5. The turntable 4 is for 
supporting a workpiece whose surface is to be measured. A column 6 is also 
supported on the chassis structure adjacent the turntable 4. A carriage 8 
is mounted on the column 6 for vertical movement and supports a transducer 
(not shown), which may for example be inductive, to which a stylus 10 is 
connected. In operation of the apparatus shown in FIG. 1, a workpiece is 
mounted on the turntable 4 so that it may be rotated, by rotation of the 
turntable 4, relative to the stylus 10 which is positioned in contact with 
the workpiece surface to be measured. Alternatively, a non-contact 
transducer may be employed. The apparatus also includes signal processing 
means (not shown) for processing the signal output by the transducer in 
order to provide information as to the characteristics, such as the 
roundness, of the workpiece. 
The bearing structure upon which the turntable 4 is mounted is shown in 
section in FIG. 2. It comprises a rotor 12 supported for rotation about a 
vertical axis by a stator 14 which is fixed relative to the chassis of the 
apparatus indicated diagrammatically at 16. The rotor 12 and stator 14 are 
respectively the rotational and stationary parts of the bearing. The rotor 
12 comprises s spindle 18 and upper and lower discs 20,22 secured to the 
spindle 18 by bolts 24 and 26 (only one each of which is shown in FIG. 2). 
The stator comprises a member 28, which is also shown in FIGS. 3 and 4, 
having a cylindrical bore 30 in which the spindle 18 is located. An 
annular recess 32 is formed in the bore 30 near to the upper end and 
contains three dry friction bearing elements 34, 36 and 38 which are made 
of a low friction plastics material, such as an acetal copolymer with a 
PTFE filler and which engage an annular shoulder 40 formed on the spindle 
18, to constrain the spindle against radial movement. The elements 34, 36 
and 38 are disposed at equiangularly spaced positions and are secured in 
place by bolts 42. The element 38 includes two thin portions 38a which 
permit the central portion 38b to move radially. An adjusting screw 44 and 
compression spring 46 are arranged in the member 28 for adjusting the 
force with which the centre portion 38b of the element 38 is pressed 
against the spindle 18. The elements 34, 36 and 38 accurately define the 
radial position of the spindle 18 whilst permitting the spindle 18 to 
rotate relative to the member 28. 
The stator 14 includes a further member 48 which is secured to and 
surrounds the lower portion of the member 28. An annular air supply 
channel 50 is defined between the members 28 and 48 and is connected to an 
air supply line 52 through a radial bore 54 formed in the member 48. The 
channel 50 is connected to the space 56 between the upper, radially 
extending surface of the member 28 and the lower, radially extending 
surface of the disc 20 through blind radial bores 58 and upwardly directed 
bores 60 formed in the member 28 to form an air bearing in the radially 
extending space between the upper surface of the member 28 and the lower 
surface of the disc 20. Flow restrictors 62 are provided in the bores 60. 
Downwardly directed bores 64 communicating with the blind bores 58 are 
also formed in the member 28 to provide an air bearing in the radially 
extending space 66 between the lower radial surface of the member 28 and 
the upper surface of the disc 22. The bores 64 contain flow restrictors 68 
and are positioned slightly radially inwardly relative to the bore 60 so 
that the stiffness of the air bearing in the space 66 is less than that in 
the space 56. The width of the spaces 56 and 66 is preferably in the range 
10-15 microns, for example 12 microns, so that the air bearings 
effectively and stably constrain the rotor 12 against axial movement 
relative to the member 28 of stator 14. The lower bearing prevents 
vertical vibration of the rotor. 
The air in the spaces 56 and 66 may move both radially inwardly and 
radially outwardly. The air which moves radially inwardly is exhausted 
through passages 70 which extend radially through the spindle 18 to an 
annular space 72 formed between the inside surface of the spindle 18 and 
an exhaust block 74 mounted inside the spindle 18. The space 72 is 
connected to an exhaust 76, which includes a silencer (not shown), by a 
passage 78 in the block 74. 
An inflatable clamp 80 is mounted on a structure 82 which is secured to the 
members 28 and 48 inflatable clamp 80 surrounds the disc 20 and may be 
inflated by supplying fluid, such as air, to it through passages 84 to 
clamp the rotor 12 relative to the stator 14. The purpose of clamp 80 is 
to selectively hold the turntable stationary, e.g., when mounting a 
workpiece thereon. During operation of the apparatus, clamp 80 is released 
so that the turntable may be rotated. 
A housing 86 (FIGS. 2 and 5) secured to the bottom of the stator 14 
contains a single DC electric motor. The motor comprises an annular magnet 
88 fixed in the housing 86 and comprising alternately arranged north and 
south poles, a shaft 90 which is secured by bolts 92 to the disc 22 of the 
rotor 12, and an insulating disc 94 having conductors 96 formed thereon in 
the manner of a printed circuit, which conductors 96 constitute the 
windings of the motor. The shape of the conductors 96 can be seen in FIG. 
6. The disc 94 is clamped tightly between insulating rings 98 by means of 
a spigot 100 which is secured to the shaft 90 by bolts 102 so that the 
disc 94 is fixed relative to the shaft 90. Power may be supplied to the 
winding 96 by brushes 104 fixed inside the housing 86. Thus, the motor 
shaft 90 forms an extension of the spindle 18 which may therefore be 
directly driven by the motor. Preferably, the motor is arranged to rotate 
at between 0.03 and 10 rpm. 
Air seals 106 are provided between various parts of the rotor and stator. A 
centring ring 108 is provided between the disc 20 and the spindle 18 and a 
further centring ring 110 is provided between the shaft 90 and disc 22. A 
multicore electric cable 112 extends through the centre of the rotor 12 
and the motor shaft 90 for supplying power and control signals to the 
turntable 4 for centring and levelling the turntable by means (not shown). 
Power and signals are supplied to the cable 112 via a slip ring 
arrangement which cooperates with the lower end of the motor shaft 90. 
The structure described with reference to FIGS. 2 to 6 of the drawings is 
relatively inexpensive, is capable of supporting a high load, is stable 
and well damped and the direct motor drive ensures that unwanted 
vibrations, such as may arise when a belt drive is used, are avoided. 
Various modifications are possible within the scope of the invention. For 
example, although in the preferred embodiment, the bearing is mounted to 
provide for rotation about a vertical axis, it could be arranged at a 
different attitude, such as to provide rotation about a horizontal axis. 
Although the dry friction bearings have been provided so that they act 
between an inner surface of the stator and an outer surface of the rotor, 
the bearing could be redesigned within the scope of the invention so that 
the dry bearings act between an outer surface of the stator and an inner 
surface of the rotor. Further, although in the embodiment illustrated the 
air bearings have been formed between radially extending plane surfaces, 
it would be possible within the scope of the invention to form the air 
bearings between conical surfaces. However, the arrangements shown in the 
drawings in which the air bearing is formed between plane surfaces is 
preferred since it is simpler and each bearing performs only the single 
function of constraining in either the radial or the axial direction. 
Although in the embodiment the air bearings act as thrust bearings and the 
dry friction bearing is a journal, the embodiment being for an application 
in which high thrust forces but low radial forces will be encountered, the 
invention may be applied to an arrangement in which low thrust forces but 
high radial forces may be encountered, in which case the air bearing is 
preferably formed as a journal and the dry friction bearing as a thrust 
bearing. A particular advantage of the arrangement shown in the drawings 
is that the turntable may be brought to rest without loss of its 
geometrical position. This is difficult to achieve with prior art 
bearings.