Rotary bearing and drive mechanism

Abstract: A rotary bearing has a pair (32,40) of bearing members, one of which has convex (34) and planar (36) surface portions, and the other of which has concave (42) and planar (44) surface portions. The convex and concave surface portions and the planar surface portions of the two bearing members bear against each other via an interstitial bearing medium of air, with the axial forces between the convex and concave surfaces counteracting the axial forces between the planar surface portions. When incorporated into a drive mechanism, one of the bearing members is connected to a drive shaft via a first torsionally stiff flexible diaphragm (130), and the other bearing member is mounted to a motor (50) (which rotates the drive shaft) via a second torsionally stiff flexible diaphragm. During operation, flexing of the diaphragms (110,130) accommodates eccentricity in the mounting of the motor (50) without transmitting unwanted loads to the bearing.

Description: The present invention relates to a rotary bearing, and to a mechanism which may be used to provide drive for a rotary bearing. A rotary bearing and/or a drive mechanism may be used in an articulating probe head for a coordinate positioning machine, such as a coordinate measuring machine. Such a probe head may be mounted to the movable arm of e.g. a Cartesian coordinate measuring machine to enable orientation of an inspection probe relative to a part under inspection with two rotational degrees of freedom. An articulating probe head employing a rotary bearing and drive mechanism is known from, for example, EP 317397. EP 288999, on which is based the preamble of claim 1, shows a torsionally stiff but otherwise flexible coupling between a motor drive and a bearing. The present invention provides a drive mechanism for rotating first and second relatively rotatable bearing members about an axis, the drive mechanism including first and second bearing members, a motor having an elongate drive shaft, the elongate drive shaft being connected to the first bearing member and the motor being connected to the second bearing member, the motor being adapted to rotate the shaft and first member relative to the motor, and a torsionally stiff flexible diaphragm which connects the drive shaft to the first bearing member, characterised by a further torsionally stiff flexible diaphragm which connects the motor to the second bearing member. Preferably the first and second members respectively have concave and convex spherical surface portions which circumscribe the axis and bear against each other via an interstitial bearing medium, the first and second members further being provided with planar surface portions which circumscribe the axis and bear against each other via an interstitial bearing medium, wherein axial bearing forces on the members at the spherical surface portions are counteracted by axial bearing forces on the members at the planar surface portions. Preferably, the interstitial bearing medium will be provided by fluid, and more preferably air. Embodiments of the invention will now be described, by way of example, and with reference to the accompanying drawings, in which: Fig 1 is a perspective view of an articulating probe head in which bearings and drive mechanisms according to the present invention may be employed; Fig 2 is a section through part of the probe head of Fig 1; Fig 3 is a perspective of a detail of Fig 2; Fig 4 is a section through a detail of Fig 2; Figs 5a and b are perspective view of possible defects in the bearing of Figs 2 and 3; and Fig 6 is a perspective view of the bearing and motor mounting in Figs 1 and 2. Various aspects of the present invention will be illustrated and described within the context of their use in an articulating probe head for a coordinate measuring machine, although they have more general applicability, and may be applied independently of each other. Referring to Fig 1 an articulating probe head 10 includes a support or fixed structure provided by a cylindrical housing 12, by means of which the probe head 10 may be mounted to the movable arm of a coordinate measuring machine (not shown). First and second rotary axis members 14,16 are serially mounted to the housing 12 in a manner enabling their rotation about first and second rotary axes D,E respectively. A touch probe 18 is connected to the second rotary axis member 16 via an extension bar 20, and is therefore rotatable relative to the movable arm of the machine with two degrees of freedom; the probe 18 may thus be used to measure the position of variously oriented surfaces of a component under inspection. Referring now to Figs 2 to 4, rotation of the first and second rotary axis members 14,16 relative to the housing 12 and first rotary axis member 14 respectively is provided by means of rotary air bearings 30A,B and motors 50A,B. Each air bearing 30A,B includes a first bearing member 32 having a substantially spherical convex surface 34 and a planar annular surface 36, extending around the axis of rotation and interconnected via an intermediate member 38. Convex spherical surface 34 and planar annular surface 36 bear, via an interstitial bearing medium which in this example is air, against concave spherical surface 42 and planar annular surface 44 respectively, provided on second bearing member 40. The air is supplied to the bearing faces via conduits 46, provided within the body of second bearing member 40. A further conduit 48 conducts air at supply pressure to an annular plenum chamber 60 defined by the surface of intermediate member 38 and adjacent surface 62 of second bearing member 40. Air bleeding from conduits 46 to supply the interstitial bearing medium exits the bearing at manifolds M, which provide the requisite pressure drop, given that air in plenum chamber 60 is at supply pressure. Air from plenum chamber 60 may be taken via a further conduit 64, provided in the body of first bearing member 32, to supply a further second bearing member of a further rotary axis. In this way, air at supply pressure may be transferred between relatively rotatable parts without the need for trailing leads, which would otherwise restrict the relatively free rotation provided by the air bearing. The spherical surfaces 34 and 42, in conjunction with a force urging the said surfaces into engagement, define the location of the first bearing member 32 relative to the second bearing member 40 in a direction radial and parallel to the axis of relative rotation; alternatively, it may be said that the surfaces 34 and 42, together with the engaging force, co-operate to eliminate (within the stiffness limits of the bearing) relative translation of the first and second bearings members 32,40. The annular surfaces 36,44 co-operate to provide the force urging spherical surfaces 34,42 into engagement, so that, in effect, the forces on the first and second bearing members 32,40 at the spherical surfaces 34,42 and planar surfaces 36,44 counteract each other. Planar surfaces 36,44 also eliminate, to within the stiffness limits of the bearing, all relative rotation of the first and second bearing members 32,40, other than relative rotation about a single axis; this being the axis of relative rotation D,E. Because surfaces 34,42 are spherical, rotation of first bearing member 32 relative to second bearing member 40 will always occur about a point passing through the centre of a notional sphere defined by spherical surfaces 34,42. As a result, imperfections in the manufacture of, e.g. first bearing member 32, do not hinder satisfactory operation of the bearing. Referring now to Figs 5a and 5b, it can be seen from Fig 5a that, as a result of the manufacture of intermediate member 38, annular surface 36 of first bearing member 32 does not lie parallel to the rear face 80. However, since the axis of relative rotation will always pass through the centre of the notional sphere, the surfaces 34,42 and 36,44 will co-operate to provide swash-free rotation of the first bearing member 32 relative to the second bearing member 40. As can be seen from Fig 5, annular surface 36 will rotate eccentrically with respect to the axis D,E of relative rotation, but this will not be a hindrance to the operation of the bearing provided that first bearing member 32 does not foul on second bearing member 40 during the course of rotation. In practice, it is desirable that the first bearing member 32 is constructed such that planar annular surface 36 and rear surface 80 are machined parallel to each other, so that an axis member mounted to first bearing member 32 will rotate in a plane orthogonal to the rotary axis of the first bearing member 32. Additionally, it is preferable that upper surface 90 of first bearing member 32 is machined parallel to annular surface 36, in order that a code wheel 200 for a rotary encoder (not shown) may be mounted thereon, to provide an accurate measure of the magnitude of relative rotation between first and second bearing members 32,40. Referring additionally to Fig 6 rotation between two rotary axis members, e.g. 14 and 16, is achieved by means of a motor 50. Motor 50 has a casing 100 mounted via a flexible diaphragm 110 to the first rotary axis member 14, on which second bearing member 40 is provided. The output shaft of the motor is connected via a gearbox 120 to a flexible diaphragm 130, which is in turn connected to the first bearing member 32, provided on second rotary axis member 16. This mounting of the motor ensures that, in the event of eccentric rotation of annular surface 36, as illustrated for example in Fig 5b, deformation of diaphragms 110 and 130 will accommodate the resultant unwanted movement of the motor 50 and gearbox, preventing the transmission of forces to the bearing. The relative torsional stiffness of diaphragms 110,130 enable the control of motor 50 to provide accurate rotational positioning of the first and second rotary axis members, on the basis of outputs from the rotary encoders. Flexible diaphragms 110,130 thus allow the rotary bearings to define the axis and manner of rotation without interference due to transmission links with the motors performing the said rotations. This mounting for the drive mechanism has general applicability, and does not require the bearing configuration shown in Figs 2 to 5 but may instead be beneficially exploited with a standard bearing configuration. To enable automatic exchange of one-touch probe 18 for another, typically for the purpose of performing a variety of measuring operations without the need for recalibrating the machine after each exchange operation, the second rotary axis member 16 is provided in two parts 216A,B. Parts 216A,B are repeatably engageable by means of a kinematic support provided by three pairs of adjacently positioned balls on part 216A, and three balls appropriately spaced on part 216B, as is well known in the art. Clamping and release may be provided by an electromagnetic system as described e.g. in our co-pending UK Application 9610936.8 and US Patent Application 5,041,806. The bearings of the present application have been described with reference to air as the interstitial bearing medium. Other interstitial bearing mediums such as other fluids, or ball races may be provided. Additionally, planar annular surface 36 is illustrated as lying outside the sphere defined by spherical surface 34. In an alternative embodiment rear face 80 of the first bearing member 32 may be machined to provide the planar surface. The embodiments of the present invention illustrated herein all include bearings in which one bearing body or bearing member of a bearing assembly bears directly via an interstitial bearing medium against the other body or member of an assembly. However the present invention applies equally to bearings and drive mechanisms which include a pair of relatively rotatable bearing bodies, both of which also rotate relative to a mechanically earthed member, such as are used in inertially balanced mechanisms (see e.g. WO90/07097).

Claims: A drive mechanism for rotating first and second relatively rotatable bearing members (32;40) about an axis, the drive mechanism including first and second bearing members, a motor having an elongate drive shaft, the elongate drive shaft being connected to the first bearing member (32) and the motor (50) being connected to the second bearing member (40), the motor being adapted to rotate the shaft and first member (32) relative to the motor, and a torsionally stiff flexible diaphragm (130) which connects the drive shaft to the first bearing member, characterised by a further torsionally stiff flexible diaphragm (110) which connects the motor to the second bearing member. A mechanism according to claim 1 wherein the first and second bearing members are part of a bearing assembly, and bear against each other via an interstitial bearing medium. A mechanism according to claim 2 wherein the bearing medium is air. A mechanism according to claims 2 or 3 wherein the first and second (32;40) bearing members respectively have concave (42) and convex (34) spherical surface portions which circumscribe the axis and bear against each other via the interstitial bearing medium, the first and second members (32;40) further being provided with planar surface portions (44;36) which circumscribe the axis and bear against each other via the interstitial bearing medium, wherein axial bearing forces on the members at the spherical surface portions are counteracted by axial bearing forces on the members at the planar surface portions. An articulating probe head for orienting a probe (18) relative to an arm of a coordinate positioning machine, the probe head including: first and second rotary axis members (14,16) which are mounted to a base (12) and the first rotary axis member (14) respectively; and first and second drive mechanisms each according to any one of the preceding claims which provide relative rotation of the base and first rotary axis member, and the first and second rotary axis members respectively.

Inventors: HENNING BRIAN CECIL ROBERT, MCMURTRY DAVID ROBERTS, WELLS PETER JOHN, HENNING, BRIAN CECIL ROBERT, MCMURTRY, DAVID ROBERTS, WELLS, PETER JOHN
Applicants: RENISHAW PLC
Application year: 1997
Main classification: B23Q1
IPC codes: B23Q1, F16C23, F16C11, F16C27