WO2005036004A1

WO2005036004A1 - Ball-screw transmission for linear axis

Info

Publication number: WO2005036004A1
Application number: PCT/IT2003/000634
Authority: WO
Inventors: Roberto Colombo
Original assignee: Technai Team S.R.L.
Priority date: 2003-10-16
Filing date: 2003-10-16
Publication date: 2005-04-21
Also published as: AU2003279558A1

Abstract

A transmission kinematism for linear axis of a machine or apparatus comprises a ball-screw device for transforming rotative motion in a motion along the linear axis of a member mechanically connected to the translating recirculating balls nut along the screw of the device and means for supporting and coupling one end of the screw to the shaft of a driving servomotor (7). The driving servomotor has a rotor shaft with at least a tubular end (11) protruding externally from a radial-axial bearing (3) between the tubular end portion of the shaft and the stator casing of the motor connected to a support of the servomotor that may be fixed on a mounting plane of the machine or apparatus. The screw (4) has an end portion the size and shape of which are adapted to fit inside the tubular end of the rotor shaft of the servomotor and an axial threaded hole (21) in which a mounting screw (18) is tightened for establishing a pre-tensioned axial connection between the screw and the shaft. Means are provided on said tubular shaft for establishing a radial and torsional constraint between the tubular end portion of the shaft and the end portion of the screw received therein and axially connected to the shaft.

Description

"BALL-SCREW TRANSMISSION FOR LINEAR AXIS"

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to transmissions for linear axis of machine tools and more in particular transmissions using a rotating servomotor and a mechanical conversion of rotating motion into a linear motion of a saddle by a ball-screw device.

BACKGROUND OF THE INVENTION

The state of the art of transmissions for linear axis of machine tools contemplates two alternative approaches: - the first approach is based on the use of a rotating servomotor and mechanical conversion into a linear motion of the saddle by an appropriate transmission; the other approach is based on the use of special linear motors wherein electric power is transformed to directly produce a linear motion of the part of the motor to which the saddle is connected.

Using actuators in the form of a linear motor, even if it allows outstanding performances in terms of precision of regulation and dynamical response of the system, remains a relatively expensive solution and nowadays is used in a limited number of applications. To a large measure, the market requests the use of a classic solution with mechanical transformation of the rotating motion impressed by a common servomotor in a linear motion.

In these transmissions, stiffness of the kinematic chain of transformation of the rotating motion impressed by the driving servomotor in the desired displacement along the linear axis of the saddle moving together with the nut of the ball-screw device is an essential condition for ensuring a satisfactory precision of positioning of the saddle and a reliable dynamical response of the transmission device. Modern production criteria and reduction of manufacturing costs in the field of machine tools, devices for positioning and/or scanning and other mechanical precision devices, impose the use of functional parts and components that are available in the market at relatively low prices because they are manufactured in specifically organized production lines for exploiting the advantages of large scale production. The various components and devices having precise specifications and ensuring certain performances may thus be bought and assembled with other components and functional parts by the manufacturer of the machine tool.

Figure 1 shows a transmission for linear axis according to prior art.

The axial stiffness of the screw 4 is ensured by two bearings and relative supporting members firmly anchored on the structure 1. The recirculating ball nut 9 to which is firmly connected the saddle 10 to be positioned and moved along the linear axis with extreme precision, advances along the screw 4. The servomotor indicated as a whole with the number 7 is commonly a completely assembled component, the drive shaft of which 11, adequately supported by two internal bearings of the servomotor, protrudes for a certain length outside the body 5 of the motor.

A mechanical joint 6 connects the end portion of the drive shaft 11 to the end portion of the screw 4 or more precisely to a stem 12 projecting for a certain length from the bearing 3, which is installed in a dedicated terminal support 2 connected to the structure 1 of the machine. The servomotor 7 is fixed to the terminal flange of the sleeve 8 which on its turn is rigidly fixed to the support 2. The joint 6 is capable of compensating within a certain tolerance misalignments of the "iperstatic" assembly that is realized between the support-bearing 2-3 and the two internal bearings (not shown in the figure) of the shaft 11, of the servomotor 7.

The critical aspects of a classic ball-screw linear transmission such as that of Figure 1 are the axial stiffness (of the screw stem and of the terminal supports) and the torsional stiffness of the coupling joint and of the screw itself because of the many interconnections among the members constituting the whole transmission.

In order to improve precision by limiting the elastic deformation under the stresses that occur in operation (for example for reducing the occurrence of finishing defects due to vibrations in case of a cutting machine tool) and for enhancing the fastness of dynamical positioning for the same mobile masses of the system, or in other words to ensure a larger regulation pass band with reduction of the tracking error and consequent reduction of the time needed for stopping exactly at the desired location, an integration of the screw and of the control servomotor has been proposed. This is accomplished by mounting the rotor structure of the servomotor, the encoders of the angular position etc. on an extended stem of the screw and by successively completing the assembly of the stator structure and casing of the motor, obviously sustained by bearings mounted on the screw stem.

This solution, notwithstanding its conceptual validity and reduced encumbrance has been scarcely accepted by manufacturers because of complications that such a solution implies in assembling the machine. Indeed, intervention of skilled persons is required for assembling "in place" the servomotor, that is necessarily bought in "kit", on the projecting stem portion of the screw already installed on the machine. Even the carrying out of texts for verifying respect of certain declared performances specifications of the servomotor are more difficult or impossible.

These attendant drawbacks have not favored the use of this solution instead of the use of a classic transmission that is typically realized with completely pre- assembled functional parts, singularly tested and qualified.

OBJECT AND SUMMARY OF THE INVENTION

Thus, there is still a demand for the largest number of applications, of transmissions for linear axis with enhanced the kinematic and dynamic performances and at the same time requiring a reduced number of components and/or devices for constituting the kinematic chain for transforming/transmitting the motion, as well as its overall encumbrance. Of course, these objectives should be achieved while preserving singularity of each functional component of the transmission and in particular of the control servomotor that, being an electromechanical component of the transmission, is fabricated and assembled with specifically developed techniques.

To these needs this invention relating to a ball-screw transmission for linear axis in which the control servomotor embodies the support of the end stem portion of the screw, provides an outstandingly effective solution.

From one side, the servomotor is a completely assembled element, offering a choice between two configurations of the mounting support; one appropriate for being fixed on a machine tool with coupling flange on a plane orthogonal to the axis of the screw and the other on a plane parallel to the axis of the screw.

Problems arising from an imperfect coaxiality between the axis of the screw and the axis of the shaft of the servomotor, which in prior art transmissions imposed the use of a special mechanical joint between the two rotating members, according to the configuration of Figure 1, are practically prevented.

According to an essential aspect of the transmission of this invention, the external end portion of the shaft of the servomotor is tubular and receives therein the end portion of the screw establishing a perfectly co-axial engagement of the two parts with a very small clearance, which may be pre-tensioned by tightening an axial mounting screw.

The radial and torsional coupling between the stem end of the screw and the drive shaft may be ensured in different ways. For instance, the end of the screw and the shaft may be engaged by pins or conic keys, though this may hinder disassembling. According to a particularly preferred embodiment, a conical collar installed around the tubular end of the drive shaft of the servomotor inside of which the stem end of the screw is received is tightened by means of a plurality of screws disposed at regular intervals along the circumference of the collar, in order to elastically force the end portion of the tubular portion of the drive shaft to grip onto the stem of screw thus establishing a strong radial and torsional mechanical coupling.

In practice, compared to the known kinematic chain of Figure 1 , the device of the invention eliminates two bearings (that is the internal bearings of the servomotor that sustain the assembly shaft-rotor) by integrating in the servomotor assembly a special radial-axial bearing capable of supporting the end portion of the screw and which at the same time constitutes also the bearing of the rotor-shaft of the servomotor.

Even the need of a compensation joint between the screw and the drive shaft is eliminated as well as the relative sleeve for mounting the servomotor. The support block to be fastened on the structure of the machine is a component of the servomotor, realizable in two alternative configurations adapted for fixing either on an orthogonal or on a parallel plane to the axis of the screw.

Stiffness characteristics of a transmission kinematism realized according to this invention are sensibly enhanced. In practice, kinematic and dynamic characteristics are comparable to those of a motor integrated on the end stem portion of the screw though maintaining the advantages of allowing the marketing of a crucial functional object such as the servomotor in a completely pre- assembled form to be readily tested and mounted on the machine.

BRIEF DESCRIPTION OF THE DRAWINGS

The different aspects and advantages of the invention will become even more evident through the following description of several embodiments referring to the attached drawings, in which:

Figure 1 shows a transmission for linear axis according to the prior art;

Figure 2 shows a transmission for linear axis realized according to this invention; Figure 3 is a sectional detail view of the control servomotor integrating a special unified bearing for supporting the shaft of the servomotor and the end portion of the screw;

Figure 4 is a magnified detail view of the axial, radial and torsional coupling elements of the shaft of the motor and the end portion of the screw, according to the preferred embodiment shown Figures 2 and 3;

Figure 5 shows an alternative embodiment of the coupling elements and in particular of the radial and torsional coupling elements of the screw and the drive shaft; Figure 6 shows another possible alternative embodiment of the coupling elements and in particular of the radial and torsional coupling elements of the screw and the drive shaft;

Figures 7 and 8 show two alternative forms of the support of the servomotor and of the end portion of the screw, respectively for mounting on a parallel plane to the axis of the screw and on a plane orthogonal to the axis of the screw of the structure of the machine.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

An embodiment of this invention is shown in Figure 2, that depicts a kinematism functionally comparable to that realized according to the prior art of Figure 1, and in the detail views of Figures 3 and 4, wherein the same numerals have been used for indicating the same functional parts.

As may be easily noticed in the magnified views of Figures 3 and 4, the servomotor indicted as a whole with the numeral 7, has a shaft 11 provided with a shoulder or flange 13 against which the inner ring 14 of the radial-axial bearing 3 abuts. The external ring of the radial-axial bearing 3 is firmly connected to the flange 23 of the stator case 5. The support black 2 that is firmly connected to the flange 23 of the stator case of the motor is alternatively configured for being fixed on a plane of the structure 1 orthogonal to the axis of the screw (as in the depicted example) or on a plane of the structure 1 parallel to the axis of the screw. The end portion of the drive shaft 11 on which the radial-axial bearing 3 is mounted, that projects for a certain distance out of the bearing 3, is tubular, preferably with a circular section.

The size and shape of the end portion of the stem 12 of the screw 4, are such for the stem to fit inside the tubular end of the drive shaft 11 with the minimum clearance allowed by the grade of finishing of the two matching parts.

The end portion 12 of the screw 4 has an axial threaded hole 21 for receiving a mounting screw 18 therein.

After having fixed in perfect alignment the support block 2 of the servomotor on the dedicated mounting plane (or flange) of the structure 1 of the machine, which in the case of Figures 2, 3 and 4 is a plane perfectly orthogonal to the axis of the screw, the mounting screw 18 is tightened in the axial hole of the end portion 12 of the screw 4. In doing so, the two parts are connected in a precise axial alignment, and a pre-tensioning of the axial coupling of the two members is established by tightening the mounting screw 18. To this end cooperate the shoulder 19 of the ring nut 16 and the shoulder 20 of the drive shaft 11, that bind the bearing 3.

After having established a pre-tensioned axial connection between the screw 4 and the drive shaft 11, a radial and torsional coupling between the two rotating elements of the kinematism must be realized.

According to the preferred embodiment illustrated in Figures 2, 3 and 4, the ring nut 16, screwed on a threaded external part of the tubular shaft 11 for tightening against the outer face of the inner ring of the bearing 3, mounted on the drive shaft, is provided with a male conical tubular tail 17 extending towards the rim of the tubular end portion of the shaft 11.

A female conical collar 15, having a plurality of tightening screws 24 disposed at regular intervals around its circumference that screw in respective threaded holes of the ring nut 16 screwed on the tubular shaft 11, cooperates with the male conical tubular tail 17 of the ring nut 16 for establishing a radial and torsional coupling of the shaft and the screw.

After having fixed the support 2 of the servomotor on the structure 1 of the machine and after having pre-tensioned the axial connection of the screw with the drive shaft, the screws 16 are tightened causing a reciprocal compenetration of the two conical elements: the conical collar 15 and the conical tail 17 of the ring nut 16. This determines the transmission of a radial force towards the axis such to cause by elastic deformation a forced constraining of the tubular shaft 11 on the stem 12 of the screw 4 fitted inside it, completing the assembly of the kinematism of the ball screw transmission for linear axis of this invention.

As schematically depicted in the figures and more easily observable in Figure 3, the rotor 8 of the servomotor is mounted on the shaft 11 of the servomotor, while the stator 22 is mounted inside the stator case 5 of the servomotor.

An encoder of angular position 25, which may commonly be an optical encoder, is mounted on the shaft 11, according to common practice.

The other parts and details that have not been cited are all of commonly used kind and do not pertain directly to the illustration of the essential features of this invention.

Figure 4 is a detail view of the coupling that is established between the screw 4 and the drive shaft 11 of the servomotor supported by the support block 2 fixed to the structure of the machine and incorporating the unified support bearing 3 of the end portion of the screw 4 and of the drive shaft 11 of the servomotor 7.

Optionally, the radial and torsional coupling between the end portion 12 of the screw and the tubular shaft 11 of the servomotor may be established even by other mechanical devices appropriate to ensure a firm torsional coupling.

Figure 5 shows the use of an alternative device to that described in the previous figures.

In this case the ring nut 16 that blocks the bearing 3 is a normal ring nut without any conical appendix.

The radial and torsional constraint between the end portion 12 of the screw and the tubular end portion of the drive shaft 11 is established by a double-cone ring 26 permanently mounted by dimensional interference on the tubular end portion of the drive shaft 11. The ring cooperates with the inner conical surfaces of two collars 27 and 28 that may be tightened one against the other by tightening the screws 29 regularly distributed around their circumference.

Also in this case, by tightening the screws 29, a radial force towards the axis is generated by the reciprocal compenetration of the conical surfaces of the double cone ring 26 and of the collars 27 and 28, such to cause by elastic deformation of the tubular shaft 11 a forced constraining on the end portion 12 of the screw 4.

Figure 6 shows a third example of how a forced constraint of the tubular shaft 11 on the end portion 12 of the screw can be assured.

According to this alternative embodiment, end of the tubular shaft 11 of the motor a front toothing is realized, for example a Hirth or Curvic toothing with trapezoidal teeth.

A similar front toothing 31 is realized a bushing 30 that is permanently interference fitted on the end portion 12 of the screw (the dimensional interference is calculated to ensure an adequate compression taking into account the torsion torque to which the transmission will be subjected in operation) and which eventually may even be blocked by a radial safety pin or by structural adhesives, according to common assembling practices.

According to this alternative embodiment, the axial coupling is established by tightening the mounting screw 18 takes place on the sides of the teeth of the two toothings 31-32, an axial reference abutment is established absolutely free of any clearance because the assembly is pre-tensioned by the mounting screw 18. Moreover, the coupling established on the sides of the teeth disposed as circular crowns ensures also a coaxial centering reference and thus the coupling of the end portion 12 inside the cavity of the tubular shaft 11 may tolerate a relatively large clearance.

Of course, even other functionally equivalent devices of radial and torsional constraint may be used according to design preferences.

Figure 7 is a schematic partially sectional view of the servomotor 7, of the relative support 2 and of the unified radial-axial bearing 3, according to the first embodiment depicted in Figures 2, 3 and 4.

Figure 8 is a schematic partially sectional view that shows an altemative embodiment of the support 2, appropriate for mounting on a flange or plane of the structure of the machine orthogonal to the axis of the screw.

Claims

C L A I M S

1. A transmission kinematism for linear axis of a machine or apparatus comprising a ball-screw device for transforming a rotative motion in a motion along said linear axis of a member mechanically connected to the translating recirculating balls nut along the screw, means for supporting and coupling one end of said screw to the shaft of a driving servomotor, characterized in that said servomotor has a rotor shaft provided with at least a tubular end protruding externally from a radial-axial bearing between said tubular end portion of the shaft and the stator casing of the motor integral with a support of the servomotor that may be fixed on a mounting plane of said machine or apparatus and said screw has and end portion the size and shape of which are adapted to fit inside said tubular end of the rotor shaft of the servomotor and an axial threaded hole in which a mounting screw is tightened for establishing a pre-tensioned axial connection between said end portion of the screw and said shaft; and in that it comprises means for establishing a radial and torsional constraint between said tubular end portion of the shaft and said end portion of the screw received into said tubular end of the shaft and axially connected together.

2. The transmission kinematism of claim 1, characterized in that said means are composed of a ring nut screwed on a threaded portion of said tubular end of the shaft against the inner ring, connected to said shaft, of said radial-axial bearing, said ring nut having a tubular male conical tail extending towards the rim of the tubular shaft; a collar provided with a plurality of tightening screws in holes realized at regular intervals around the circumference of the collar that screw in respective threaded holes of said ring nut and having a female conical surface surmounting said tubular male conical tail of said ring nut; the forced reciprocal compenetration of the conical portions of said ring nut and of said collar generating a radial force towards the axis establishing by elastic deformation said radial and torsional constraint between the tubular shaft and the end portion of the screw.

3. The kinematism according to claim 1, characterized in that said means consists in a double cone ring permanently interference fixed over the tubular end portion of the drive shaft; two collars having inner female conical surfaces surmounting respective male conical surfaces of said double cone ring are tightenable one toward the other by tightening screws distributed at regular intervals around their circumference for compressing radially the end portions of said double cone ring and the tubular extremity of the drive shaft to constrain the end portion of screw.

4. The kinematism according to claim 1, characterized in that said means consists in a first front toothing on the rim of the tubular shaft of the motor; an analogous front toothing on the side facing toward the motor of a bush interference fixed on said end portion of the screw; said mounting screw establishing a pre-tensioned axial connection upon a reciprocal penetration of the teeth of said two front toothings that establishes also a torsional constraint between said end portion of the screw and said tubular drive shaft.

5. A driving servomotor for a transmission for linear axis of a machine or apparatus comprising a ball-screw device for transforming rotative motion imported by the servomotor, characterized in that said servomotor has a rotor shaft provided with at least a tubular end protruding externally from a radial-axial bearing between said tubular end of the shaft and the stator casing of the motor integral with a support of the servomotor that may be fixed on a mounting plane of said machine or apparatus, said tubular end receiving therein an end portion of the screw of said device the size and shape of which conform to the size and shape of the cavity of the tubular end of the motor shaft, and having an axial threaded hole in which a mounting screw is tightenable for establishing a pre-tensioned axial connection between said end portion of the screw and the tubular shaft; and in that said tubular end of the shaft comprises means for establishing a radial and torsional constraint between said tubular end of the shaft and said end portion of the screw.

6. The servomotor of claim 5, characterized in that said means consists in a ring nut screwed on a threaded portion of said tubular end of the shaft against the inner ring, connected to said shaft, of said radial-axial bearing, said ring nut having a tubular male conical tail extending towards the rim of the tubular shaft; a collar provided with a plurality of tightening screws in holes realized at regular intervals around the circumference of the collar that screw in respective threaded holes of said ring nut and having a female conical surface surmounting said tubular male conical tail of said ring nut; the forced reciprocal compenetration of the conical portions of said ring nut and of said collar generating a radial force towards the axis establishing by elastic deformation said radial and torsional constraint between the tubular shaft and the end portion of the screw.

7. The servomotor of claim 5, characterized in that said means consists in means consists in a double cone ring permanently interference fixed over the tubular end portion of the drive shaft; two collars having inner female conical surfaces surmounting respective male conical surfaces of said double cone ring are tightenable one toward the other by tightening screws distributed at regular intervals around their circumference for compressing radially the end portions of said double cone ring and the tubular extremity of the drive shaft to constrain the end portion of screw.

8. The servomotor of claim 5, characterized in that said means consists in a first front toothing on the rim of the tubular shaft of the motor; an analogous front toothing on the side facing toward the motor of a bush interference fixed on said end portion of the screw; said mounting screw establishing a pre-tensioned axial connection upon a reciprocal penetration of the teeth of said two front toothings that establishes also a torsional constraint between said end portion of the screw and said tubular drive shaft.