"APPARATUS FOR FOR THE DYNAMICAL BALANCING OF A ROTATING

BODY"

Technical Field

The present invention refers to an apparatus for dynamically balancing a rotating body, with a supporting frame, an assembly fixed to and rotating with said body, comprising masses the position of which is adjustable, and at least one electrical motor connected to the masses and adapted to adjust said position, power supply and control means adapted to feed and control said at least one electrical motor, and spaced apart from the rotating body, and electrical connection means between the power supply and control means and said at least one electrical motor, with mutually slipping contacts comprising first contact elements and second contact elements that are connected to one and to the other of the rotating body and the supporting frame.

In particular, the invention refers to a balancing apparatus for dynamically balancing the grinding wheel of a grinding machine.

Background Art

Unwanted vibrations are generally present in a grinding machine, and are generated by out of balance conditions of the grinding wheel due to various possible reasons such as: shape and/or constitution defects (parallelism errors between the faces of the wheel, concentricity errors between the external abrading surface and the internal centering hole, inhomogeneity of the material, porosities, etc.), inaccurate assembling to the rotating spindle (causing the wheel center of gravity being spaced apart from the rotation axis) , and, in general, deteriorations

due to wear and/or splinter during the machining operations of the workpieces. The vibrations may cause inaccuracies in the machined workpieces such as roundness errors (ovality, lobing) , and introduce loads and stresses that may damage the machine tool.

Known balancing apparatuses are coupled to the grinding wheel and comprise movable masses, that are driven by electrical motors, during the rotation of the wheel, along radial or angular paths, in order to compensate the out of balance.

The driving motors are part of the apparatus, rotate along with it and along with the wheel, and are fed by a fixed external power source, through an electrical coupling including, for example, slip rings and brushes. The vibration caused by the out of balance is picked up by a sensor, and some parameters of the vibration (freguency, amplitude, and/or other ones) are valued, and then are displayed and/or processed in a unit that also comprises the above mentioned power source. The unit may then send proper "balancing" signals to the motors, in order to properly drive the movable masses.

A balancing apparatus comprising the above mentioned features is shown and described in patent US-A-3698263. According to such patent, the driving motors are controlled by an operator on the basis of the vibration parameters that are displayed in an instrument.

A problem of the known apparatuses is represented by the wear of the brushes that remain in contact with the relevant slip rings in the course of the grinding wheel rotation, and by the consequent need of relatively frequent maintenance and replacement operations.

Disclosure of Invention

Object of the present invention is to provide a balancing apparatus that overcomes such problems of the known apparatuses, and allows to improve the reliability, and to

reduce the costs and the down time of the machine tool. This object is achieved, and other advantages are allowed, according to the present invention, by an apparatus in which the electrical connection means comprise a movable support, connected to one of said supporting frame and rotating assembly, that carries said second contact elements and is adapted to assume a first position, in which said first and second contact elements are mutually spaced apart, and a second position, in which said first and second contact elements touch each other for providing electrical connection between the power supply and control means and said at least one electrical motor.

Brief Description of the Drawings

The invention is described hereinafter with reference to the annexed drawings, given as non limiting example only, wherein:

Fig. 1 is a schematic partially cutaway side view of an apparatus according to the present invention;

Fig. 2 is an enlarged cutaway side view, of a detail of the apparatus of figure 1, and

Fig. 3 is a cross-section, along line III-III, of a part of the apparatus of figure 2.

Best Mode for Carrying Out the Invention

According to figure 1, a grinding wheel 2 of a grinding machine is connected to a rotating body, or spindle, 4 and rotates together with it about an axis A with reference to a relatively fixed supporting frame, or tool slide, 6, under the control of a control unit, or programmable logic controller, 8, and of other actuating means, not represented. A cavity 10 in the spindle 4 houses a balancing device 12, for the dynamic balancing of the wheel 2, and a vibration sensor 14 is fixed to the frame 6 of the grinding machine.

Power supply and control means comprise a survey and processing unit 16 that is electrically connected to the vibration sensor 14, the latter sending to the former electrical signals responsive to the vibrations of the rotating parts (grinding wheel 2 and spindle 4) .

The survey and processing unit 16 is electrically connected, on its turn, to the programmable logic controller 8 and, through electrical connection means with a distribution terminal 18, to the balancing device 12. The device 12, that is better visible in figure 2, substantially comprises a framework 19, fixed, in a way not shown, to the spindle 4 (i.e. rotating with it), and defining two cylindrical surfaces 21, 23 aligned about axis A, and a rotating assembly with two tubular elements 20, 22, respectivelly coupled to the cylindrical surfaces 21, 23 by means of rolling bearings 21* , 23\', each tubular element 20, 22 carrying a compensation mass 24, 26, respectively, having the shape of a hollow half-cylinder. The tubular elements 20, 22 comprise also relevant circular flanges 25, 27 that define toothed inner surfaces 25*, 27\'. Two electrical motors 28, 30, with rotating stems 32, 34, are fixed to the framework 19 so that the stems 32, 34 lie parallel to the axis A and are coupled, by means of gears 36, 38 to the toothed surfaces 25\', 27* of tubular elements 20, 22 to make them rotate about axis A. In such a way it is possible to adjust the angular position of masses 24, 26 with respect to the framework 19, in order to perform balancing operations in a way that is known per se. The distribution terminal 18 - the main functional details of which are shown in figure 3 - provides the electrical connection between the survey and processing unit 16 and the balancing device 12. Unit 16 comprises a power source 40. The terminal 18 includes slipping contacts and substantally comprises two parts: a non-rotating part 42 fixed to the frame 6, and a rotating part 44 steadily connected to the spindle 4.

The rotating part 44 comprises a substantially cylindrical end 46 that carries first electrical contact elements, which include three slip rings 48, and are connected to motors 28, 30 by means of cables 50. The non-rotating part 42 comprises a housing 51 and a movable support 52 that carries second electrical contact elements including three brushes 54 electrically connected to the unit 16, the brushes 54 and the slip rings 48 providing the electrical connection between the power source 40 and the motors 28, 30.

Fulcrum or pivoting means 56 define a rotation axis B, which is substantially parallel and offset with respect to axis A, and allows pivoting movements between the support 52 and the housing 51. A retraction device comprises a spring 58 arranged between the housing 51 and an extension plate 53 of the movable support 52 to pull the support 52 to a first position, defined by stop means 55, in which the brushes 54 are spaced apart from the rings 48, as shown with dashed lines in figure 3.

Operative means comprise electromagnets 60, connected to and controlled by the survey and processing unit 16, adapted to attract the extension plate 53 and consequently move the support 52 to a second position (shown in figure 3 with continuous lines) in which the brushes 54 touch the rings 48 and electrically connect the unit 16 to the balancing device 12.

In the course of normal operation of the machine tool, and of the mutual rotation between parts 44 and 42, the brushes 54 retract under the action of spring 58, and do not slide on the corresponding rings 48. Whenever the performance of a balancing cycle is needed, the operative means 60 are energized, the brushes 54 are moved to contact the slip rings 48, and through the elecrical contact so provided in the terminal 18, the motors 28, 30 are properly controlled to drive the balancing masses 24, 26. A balancing cycle is generally needed as a consequence of

the replacement or of the dressing of the grinding wheel 2, and when the survey unit 16 picks up, through the sensor 14, values of the vibration parameters that exceed determined limits. The balancing cycle ends when the values of the surveyed vibration parameters lie within a predetermined tolerance range: the time required for a fully automatic cycle does not generally extend beyond one or two minutes, whereas, in case that a partially manual operation is needed, a few minutes more are required.

From the above, it is apparent that the time in which the electrical connection between unit 16 and balancing device 12 is needed, is extremely reduced with reference to the overall working time of the machine tool (a very rough valuation of the ratio between the two above mentioned times might be of one to some hundreds) .

The possibility of spacing the brushes 54 apart from the slip rings 48 allows to reduce the mutual slidings to the strictly necessary ones, and, as a consequence, to render the wear of the electrical contacts substantially negligible.

It is pointed out that the electromagnets 60 may be replaced with any different device that performs the same operation of properly moving the support 52 to the second position, providing the required electrical connection.

Other different embodiments of an apparatus according to the present invention are possible, including differences with respect to what is shown in the drawings and above described. As an example, the balancing device may be connected externally to the grinding wheel spindle, in a per se known way.