Linear bearing assembly

Abstract: An air pad (19) comprising a body (22) with two opposite faces (23, 24), and having a number of supporting orifices (31) on one face (23) and at least one compensating orifice (32) on the other face (24); each compensating orifice (32) being located on a micropad (33) comprising a plate (34) integral with a shank (36) sliding in a cylindrical chamber (38) communicating with the supply conduits (26-29) of the orifices (31, 32); and the plates (34) cooperating with a surface (42) opposite the surface of the slideway (21) and located on a bar (43) fitted to the slideway (21).

Description: The present invention relates to a linear bearing assembly including an air pad comprising a body having two opposite faces. More specifically, the present invention relates to a linear bearing assembly for supporting the work table of a measuring machine or machine tool, e.g. a printed circuit drilling machine. A linear bearing assembly according to the preamble of claim 1 is known from US-A-3 855 901. Known bearing assemblies of the aforementioned type are widely employed on the slideways of mutually mobile members, by virtue of the low friction coefficient and the straightforward design of the bearing pads. The precision of the slideway depends on the size of the gap between the respective pad and slideway surfaces, which gap decreases in size alongside an increase in the load on the pad. In the presence of light loads and low air supply pressure, even small variations in load result in a considerable variation in the size of the gap, thus impairing the precision of the slideway. For applications in which the pads are used for supporting the work table of machine tools or measuring machines, which normally feature an air pressure of 2-3 bar, attempts have been made to increase the specific load by constructing extremely heavy tables, which are invariably expensive by virtue of requiring extremely powerful drive means. Even in the case of massive work tables, however, when accelerating or decelerating, the load on the pads varies enormously and differently from one pad to another. The load on the temporarily relieved pads therefore falls drastically, thus varying the gap and resulting in vibration of the table. Further vibration may also be caused by the work holder and/or tool engaging the work. One design proposal for eliminating such vibration is to connect each air pad to a vibration damper operated solely when accelerating or decelerating. Such pads, however, fail to provide for compensating for variations in load caused by other factors, such as the action of the tool on the work. Yet another proposal is for a work table which is moved along two axes by means of an intermediate plate made of granite and in turn moving in relation to a fixed slideway also made of granite. An extensive portion of each face of the intermediate plate presents a number of supporting orifices cooperating with a number of vacuum elements for creating a certain preload on the slideway surfaces. Vacuum preloading as described above presents several drawbacks. Firstly, the preload achievable is relatively low, thus providing for only very limited compensation of the variation in load. Secondly, the number of vacuum elements is necessarily limited, by virtue of requiring a large-section chamber and being located on the same face as the supporting orifices. Finally, the vacuum elements require a number of conduits connected to a vacuum source and distinct from those of the supporting orifices. As such, the supporting system is not only ineffective, but also highly complex and expensive. It is an object of the present invention to provide a perfected air pad of straightforward design, which provides for a high degree of reliability and for eliminating the aforementioned drawbacks typically associated with known supporting systems. According to the present invention, there is provided a linear bearing assembly comprising the features of claim 1. A preferred non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: Fig.1 shows a schematic side view of the work table of a printed circuit drilling machine incorporating linear bearing assemblies in accordance with the present invention; Fig.2 shows a front view of the Fig.l work table; Fig.3 shows an enlarged, partially sectioned view of a detail in Fig.2; Fig.4 shows a view of the pad along line IV-IV in Fig.3; Fig.5 shows a partially sectioned view of the pad along line V-V in Fig.3; Fig.6 shows an enlarged section along line VI-VI in Fig.5; Fig.7 shows a performance graph of the pad. Number 10 in Fig.s 1 and 2 indicates the bed of a machine tool, e.g. a drilling machine for printed circuit boards 9. Bed 10 is normally made of granite and constitutes the fixed slideway of a work table 11 fitted in known manner with the usual fixture 12 for fastening boards 9. Table 11 is moved along a given axis, e.g. longitudinal axis Y, by a screw 13 fitted to bed 10 and rotated selectively by a servomotor. Table 11 (Fig.3) advantageously presents a hollow structure, and is covered by a finish sheet. Underneath, table 11 presents two longitudinal bars 14 (Fig.2) between which screw 13 is fitted, and to which is fitted the usual nut screw 16 engaging screw 13. Each bar 14 presents a pair of air pads 17 by which it is guided in sliding manner along the surface of a respective vertical wall 18 of bed 10. By virtue of operating in opposite directions, the action of pads 17 of both bars 14 is compensated automatically. Each of the four corners of table 11 presents an air pad 19 traveling on an accurately ground guide surface 21 on bed 10. Normally, the weight of table 11 and fixture 12 produces a thrust on each pad 19 equal to a quarter of said weight. When table 11 is subjected to a given amount of acceleration or deceleration, a corresponding force F in the corresponding direction is produced at the connection point M of nut screw 16. Due to the lever arm between the center of gravity C of table 11 and point M, force F produces a thrust which is transmitted to pads 19, and which is subtracted from the weight on pads 19 on the side towards which force F is directed (the left side in Fig.1), and is added to the weight on pads 19 on the opposite side. Moreover, when boards 9 are engaged by the work holder or tool, an additional instantaneous force is produced on pads 19, which force is distributed variably on the four pads 19 as a function of the position of table 11 in relation to the machining head, so that each pad 19 is subjected to a different, variable thrust in the course of the machining cycle. Each pad 19 consists of a cylindrical or prismatic, more specifically, parallelepipedic body 22 (Fig.3), and presents two opposite parallel faces 23, 24 constituting the two bases of the parallelepipedon. Bottom face 23 presents four shallow grooves 25 (Fig.4) parallel to its sides and so substantially forming a rectangle, the length of grooves 25, however, being such as to fall short of the corners of the rectangle. Between faces 23 and 24, body 22 presents a number of conduits comprising a longitudinal conduit 26 (Fig.5) and three transverse conduits 27, 28 and 29. One of said conduits, e.g. conduit 28, is open outwards and connectable, via a pipe not shown, to a compressed air source; and conduits 27 and 29 are located along the two shorter sides of the rectangle formed by grooves 25. Bottom face 23 of body 22 presents four supporting orifices 31 (Fig.4), each having its axis perpendicular to face 23 and a diameter preferably of about 0.25 mm, and each preferably formed in a cylindrical insert 30 housed in a respective seat on body 22. Each insert 30 is located at the mid point of a respective side of the rectangle formed by grooves 25, which thus assist in forming the gap between face 23 of pad 19 and guide surface 21. Internally, two orifices 31 communicate with transverse conduit 28 (Fig.5), and the other two orifices 31 with longitudinal conduit 26, more specifically, at the intersection with conduits 27 and 29. According to the present invention, top face 24 of body 22 presents at least one compensating orifice 32 (Fig.5). Advantageously, provision is made for four orifices 32 having their axis perpendicular to face 24 and of substantially the same diameter as orifices 31 (Fig.4). Orifices 32 are interspaced with orifices 31, and, more specifically, are located substantially at the four corners of the rectangle formed by grooves 25. Each compensating orifice 32 is located on a micropad 33 (Fig.6) comprising a plate 34 for compensating or preloading pad 19. Plate 34 is circular with a perfectly flat active surface 35, and forms one piece with a cylindrical shank 36 having a conduit 37 coaxial with orifice 32. Conduit 37 connects orifice 32 to a cylindrical chamber 38 in body 22, in which shank 36 slides, and which communicates at the bottom with conduit 27 or 29. Shank 36 also presents an annular groove 39 housing a seal 41, e.g. of the type known as an O ring, for sealing the cylindrical surface of chamber 38. The two pairs of micropads 33 cooperate with two ground contrast surfaces 42 (Fig.3) on two straight parallel metal bars 43 fitted longitudinally to bed 10. More specifically, for each bar 43, bed 10 is fitted with a number of substantially equally-spaced spacer ribs 44 (Fig.1) to which the respective bar 43 is fitted. The thickness of ribs 44 (Fig.3) is slightly greater than the total thickness of body 22 and plate 34, so as to permit a very small amount of axial travel of plate 34. The section of shank 36 is so sized that, when pad 19 is supplied with compressed air, the thrust produced by the air in chamber 38 does not exceed the lift of micropad 33, i.e. the thrust produced by the air from orifice 32 in the gap between surface 35 of micropad 33 and surface 42 of bar 43. In turn, surface 35 of plate 34 is so sized that the four micropads produce a thrust ranging from 1/3 to 2/3 of the static weight supported by pad 19. Top face 24 of body 22 also presents a threaded hole 46 possibly extending as far as conduits 26 and 28, and in which is screwed the threaded end of an element 48 for connection to table 11. Element 48 presents a head 49 having a threaded hole in which is screwed the threaded end 51 of a stud 52 having a larger-diameter socket head 53. Table 11 in turn presents, substantially at each of its four corners, an internally threaded sleeve 54 in which is fitted a counterelement, consisting of a hollow cylinder 56, for connection to pad 19. Cylinder 56 is threaded externally, and presents a socket head 57 at the top for screwing it inside sleeve 54. Cylinder 56 is designed to house pin 52, and presents a bottom wall 58 having a central hole enabling passage of the threaded end 51 of pin 52. Between wall 58 of cylinder 56 and head 53 of pin 52, there is provided a Belleville washer compression spring 59 so sized as to withstand at all times the weight of table 11 and fixture 12, and the additional thrust produced under normal operating conditions. Finally, head 49 of element 48 presents a semispherical upper surface engaging a conical surface formed coaxially in bottom wall 58 of cylinder 56. Between pad 19 and table 11, there is thus formed a spherical articulated joint enabling self-adaption of the connection. Compensated air pad 19 operates as follows. When idle, surface 23 of body 22 of each pad 19 is held contacting guide surface 21 of bed 10 by table 11 via joint 48-56; and micropads 33, for lack of compressed air to support them, are set to the lowered position with a gap between plates 34 and bars 43. Conversely, when pad 19 is supplied with compressed air, the air inside chambers 38 (Fig.6) acts on shanks 36 so as to immediately move plates 34 towards bars 43 (Fig.3); and the compressed air issuing from orifices 31 now creates a supporting gap between bottom surface 23 of body 22 and guide surface 21 of bed 10, so that pads 19 hover over guide surfaces 21. Moreover, compressed air also issues through conduits 37 (Fig.s 3, 6) from orifices 32 of micropads 33, for preventing surface 35 of plates 34 from contacting the bottom surface 42 of respective bars 43. A compensating gap is thus formed between surfaces 35 and 42, which gap produces a given thrust which is added to the resultant of the weight and thrust exerted by table 11 on pad 19, which thus behaves as though it were preloaded, i.e. subjected to a greater load. As we know, the thrust of an air pad depends on the air pressure of the orifices and the size of the active surface of the pad, and varies in inverse proportion to the pressure of the compressed air supply to the pad. The size of the gap varies alongside the load on the pad. Curve 61 in the Fig.7 graph shows in µm, and as a function of load, the supporting gap size of a pad similar to pad 19, supplied with compressed air at 4 bar pressure, but having no compensating micropads 33. As can be seen, the variation in the size of the gap is minimum for loads of over 10 kg, whereas, for loads of less than 10 kg, it moves rapidly from a few tens of a mm to values of the order of a tenth of a mm. Curve 62 shows the supporting gap size of pad 19 compensated by means of four micropads 33 with a preload thrust of roughly 10 kg. As can be seen, the variation in the size of the gap is minimum even in the case of loads approaching zero, as when accelerating or decelerating sharply, so that table 11 is subjected to no vibration, and remains perfectly stable under all operating and load conditions. Fig.7 also shows two curves illustrating the rigidity of the pad as a function of load, rigidity here being intended to mean the load to gap size ratio expressed in kg/µm on the scale to the right. Curve 63 shows the rigidity of the noncompensated pad, and curve 64 the rigidity of the pad when compensated. As can be seen, to achieve the same rigidity as the compensated pad, the noncompensated pad must be loaded to at least approximately 20 kg, whereas the same degree of rigidity is achieved on the compensated pad with a load of barely 14 kg. The advantages of air pad 19 according to the present invention will be clear from the foregoing description. In particular, as compared with noncompensated pads, pad 19 provides for reducing the weight of the work table, and consequently for proportionally reducing the size of the table motor and so increasing the pressure of the compressed air supply to the pads. Moreover, it provides for more or less eliminating any vibration caused by variations in the size of the supporting gap when accelerating or decelerating or due to engagement of the work by the work holder or tool. Finally, even as compared with vacuum preloading systems, the pad according to the present invention provides for more effective compensation, as well as for supplying supporting and compensating orifices 31 and 32 from the same operating fluid source. To those skilled in the art it will be clear that changes may be made to the air pad as described and illustrated herein without, however, departing from the scope of the present invention. For example, changes may be made to the number of supporting and/or compensating orifices; pads 19 may advantageously be derived from noncompensated pads, such as pads 17 (Fig.2); micropads 33 may be other than circular in design; and pads 19 may be applied to the tables of other precision machines, e.g. measuring machines, or to other members traveling along respective slideways.

Claims: A linear bearing assembly including an air pad (19) for slidably supporting a first member in relation to a second member, said pad (19) comprising a body (22) having two opposite parallel faces (23, 24), said body (22) being provided with a number of supporting orifices (31) located on one of said parallel faces (23, 24), and with at least one compensating orifice (32) associated with the other one of said parallel faces (23, 24), said orifices (31, 32) being supplied with compressed air, said body (22) being removably connected to said first member (11), said second member (10) comprising two contrast surfaces (21, 42) facing said parallel faces (23, 24), characterized in that said body (22) is connected to said first member (11) by an articulated joint (46-56), and said compensating orifice (32) is provided on a micropad (33) movable in relation of said body (22) in a direction parallel to the axis of said compensating orifice (32), said contrast surfaces (21, 42) constantly covering the orifices (31, 32) of said parailel faces (23, 24), said micropad (33) being urged by compressed air toward the relevant contrast surface (42), and having an active surface (35) so sized as to cause said compensating orifice (32) to generate a preload for compensating any variation of the load on said pad (19) occurring during the sliding of said first member (11). A bearing assembly as claimed in Claim 1, characterized in that said micropad (33) comprises a plate (34) integral with a cylindrical shank (36) sliding in a seat (38) formed in said body (22) and supplied with compressed air, said shank (36) having an annular groove (39) housing a seal (41) for sealing said seat (38), said compensating orifice (32) communicating with the seat (38) via a conduit (37) in said shank (36). A bearing assembly as claimed in Claim 2, characterized in that said supporting orifices (31) and said seat (38) are supplied with compressed air by a common set of conduits (26-29) located in said body (22), said preload ranging from 1/3 to 2/3 of the load of said pad (19). A bearing assembly as claimed in Claim 2 or 3, characterized in that the cross section of said shank (36) is so sized as to enable the compressed air to move said micropad (33) toward said relevant contrast surface (42). A bearing assembly as claimed in one of the foregoing Claims, characterized in that said body (22) is cylindrical or prismatic, the bases of which form said parallel faces (23, 24), said supporting orifices (31) having their axes perpendicular to the relevant parallel face (23), a shallow groove (25) being provided in correspondence of said supporting orifices (31) on said relevant parallel face (23). A bearing assembly as claimed in Claim 5, characterized in that said compensating orifices (32) are individually provided on corresponding micropads (33), said compensating orifices (32) presenting substantially the same diameter as said supporting orifices (31). A bearing assembly as claimed in Claim 6, characterized in that said body (22) is parallelepipedic and comprises four supporting orifices (31) interspaced with the same number of compensating orifices (32), said grooves (25) being rectilinear and being located along the corresponding sides of said relevant parallel face (23) to form a corresponding rectangle. A bearing assembly as claimed in Claim 7, characterized in that each one of said groove (25) extends as to fall short of the corners of said rectangle, said supporting orifices (31) being located each one at the mid point of the relevant groove (25), said compensating orifices (32) being located at each corner of the parallel face (24) opposite to said grooves (25). A bearing assembly as claimed in one of the foregoing Claims, characterized in that said articulated joint (48, 56) comprises an element (48) fitted to said body (22) and having a semispherical head (49), and a counterelement (56) fitted to said first member (11) and having a conical surface engaging said said head (49), and a compression spring (59) for pressing said counterelement (56) on to said element (48). A bearing assembly as claimed in Claim 9, characterized in that said compression spring is a Belleville washer (59) located between a cylindrical cavity of said counterelement (56) and a stud (52) housed in said cylindrical cavity, said Belleville washer (59) being so sized as to withstand the weight of said first element (11) and the additional thrust produced thereon under normal operating conditions. A bearing assembly as claimed one of the foregoing Claims, wherein said first member comprises a work table (11) of a machine tool, and said second member (10) comprises a fixed straight slideway, characterized in that one of said surfaces (21, 42) is formed of the upper surface (21) of a granite block (10), the other one (42) of said contrast surfaces (21, 42) being formed on a pair of metal bars (43) each one fitted to said block (10) by means of a number of spacing ribs (44) fitted a predetermined distance apart to said block (10) and the relevant metal bar (43).

Inventors: OTTONE RENATO, PIOVANO LUIGI, OTTONE, RENATO, PIOVANO, LUIGI
Applicants: PLURITEC ITALIA, PLURITEC ITALIA S.P.A.
Application year: 1992
Main classification: F16C29
IPC codes: F16C32, F16F15, H05K3, G12B9, B23Q1, F16C29