Revolving joint

A revolving joint comprising at least one anti-friction bearing and an electromotive drive unit. The anti-friction bearing is in contact with at least one row of rolling bodies arranged in a row rotating the joint around the rotational axis of the anti-friction bearing. The rolling bodies are in contact with at least one race that can be driven to rotate by the drive unit.

FIELD OF THE INVENTION

The invention concerns a revolving joint with at least one rolling contact bearing and with an electromotive drive unit, wherein the rolling contact bearing is provided with at least one row of rolling bodies which are arranged around the rotational axis of the rolling contact bearing and are in contact with at least one running track which can be driven by the drive unit.

BACKGROUND OF THE INVENTION

A revolving joint of this type is described in DE 10210071 A 1. Integrated and low-noise revolving joint systems can be used in a versatile manner and generally serve to drive, to connect and support and to detect the positions of rotating masses and to couple them to static components.

Cataloged revolving joints with rolling contact bearings are used nowadays in many industrial applications. In most cases, these are rolling contact bearings with an outer race and inner race, and are optionally provided with axial fastening bores and/or fastening threads to attach and to fasten them to the static component and the rotating mass.

In the sphere of medical technology, revolving joints with rolling contact bearings are used in computer tomographs. Furthermore, the revolving joint with rolling contact bearings is used in similar radiographic equipment to investigate items of luggage in the security sphere, for example at airports. Known revolving joints with rolling contact bearings are unsuitable to meet the constant rise in rotational speeds of these applications with the simultaneous requirement for reduced noise, low starting torque, small construction space, low weight and high running accuracy, and no longer meet the demands of using them in this way.

For computer tomographs to record and reconstruct images, the precise angular position and position of the rotating components has to be known. There are various measuring devices which operate according to various measuring principles (inductive, optical, capacitive, etc.) for this.

SUMMARY OF THE INVENTION

It is the object to provide a revolving joint which meets the previously mentioned requirements.

This object may be achieved according to the disclosure hereof by a measurable running noise at the electromotively driven revolving joint, said running noise, measured as air-borne sound, having cumulative sound pressure levels with values of at most 70 dB(A), preferably 67 dB(A). The measurement of noise is undertaken at 1 m horizontal distance from the bearing plane on the theoretical continued line of the rotational axis.

The rolling contact bearing has at least one inner running track and one outer running track. A plurality of running tracks are also conceivable in the rolling contact bearing. The rolling bodies are in contact, and during operation of the rolling contact bearing are in rolling contact, with at least one inner running track and outer running track, and, alternatively, also with running tracks oriented axially.

The rolling bodies revolving in the rolling contact bearing cause noises in the form of solid-borne sound. In general, the higher the operating speed, the louder is the noise generated. A solid-borne-sound insulating means of the rolling contact bearing prevents the solid-borne sound from propagating into the surrounding parts and therefore reduces the production of noise. In this case, the surface area and/or lateral area of at least one of the rolling contact bearing races is insulated from the surrounding parts in terms of solid-borne sound. The material used here for the solid-borne-sound insulating means has an impedance ratio p of at least 3 in comparison to the material of the rolling contact bearing race. The impedance ratio p is defined as follows:

p=E1×ρ1E2×ρ2p Impedance ratioE1 Modulus of elasticity of the rolling contact bearing raceρ1 Density of the rolling contact bearing raceE2 Modulus of elasticity of the insulating materialρ2 Density of the insulating material

A further feature of the low-noise rolling contact bearing is the surface topography of the running tracks, which is determined by a defined waviness and/or surface roughness. The roughness of the running track is set to a value of not greater than Ra 0.25. The Ra detail is a known and internationally standardized value which is defined as an arithmetic mean value according to DIN EN 4287: the following features are based on the waviness concept, which is also known per se:at least the surface of the running track is described by any desired number of surface lines adjacent parallel to one another with profiles which are each wave-shaped in the circumferential direction and therefore deviate from the imaginary ideal profile which encircles the rotational axis and is designed entirely in the form of a circular curve,the profiles are each described by waves which follow one another in an alternating manner in the circumferential direction (about the rotational axis) and, in the process, repeatedly intersect the particular ideal profile,average values of sums of all of the waviness amplitudes between wave crest tip and wave trough bottom of any desired number of periods of the waves of a measuring range on one surface line in each case correspond at maximum to a quotient of the constant 0.33 in mm/min and of the rotational speed in rpm, andthe measuring range is defined by the curve length along the respective running track between the respective contacts with the running track of two rolling bodies of a row that follow each other in the circumferential direction.

According to this definition, for example for an operating speed n=120 min−1and a distance between rolling bodies=30 mm (nominal size, without taking any dimensional deviations within permissible tolerance limits into consideration), a max. permissible waviness amplitude of the running tracks of 0.0028 mm arises.

A revolving joint system which is of low-noise design according to the invention and comprises a rolling contact bearing and drive unit is compact and takes up little construction space. It reliably absorbs high radial, axial and torque loads. The direct drive is preferably an integral part of the revolving joint system and therefore does not take up any additional construction space. The rotating revolving joint system causes a particularly low level of noise in all rotational speed ranges, but in particular at rotational speeds above 100 rpm (1/min), preferably above 160 rpm because of the integration of a rolling contact bearing of particularly low-noise design, optionally combined with the solid-borne-sound insulating means. The revolving joint can be produced cost-effectively owing to the small number of components used and its compactness.

The direct drive ensures that the driving forces of the direct drive are transmitted to the revolving joint with rolling contact bearings without the interconnection of further drive components (such as, for example, belts or gearwheels, etc.), brake generators or deflecting and clamping rollers.

The electromotive drive unit which is preferably integrated into the revolving joint as a direct drive is designed as a torque motor of annular or segmental construction, with a static component of the revolving joint being connected to at least one stator comprising iron cores and electrical windings. One of the rotating components of the revolving joint is equipped with permanent magnets.

It is furthermore advantageous that, with the aid of the torque motor of segmental construction or designed as an annular motor, different motor powers can be provided by putting together one or more individual segments up to a completely closed ring for the drive. The total driving power thereof is dependent on the number and size of the stator segments used. If only low rotational speeds and small torques are required, then wide gaps can be left between the individual stator segments. The torque motor is even operational if there is only one stator segment, if a relatively low driving power is required. If a high power is required for active rotation, acceleration or braking of the revolving joint system, then the annular motor construction is to be selected.

In contrast to the annular motor, a torque motor of segmental construction has to be operated in a controlled manner so that the magnetic forces of each individual segment are synchronized with one another and therefore the best possible efficiency and the lowest noise level are achieved. According to one refinement of the invention, the frequency-converter input signal required for this, in order to activate the segments, is provided by the integrated sensor system for detecting position. If very high torques are required (for example during a short starting phase of the rotating components), then efficient operation of the annular motor likewise by means of the controlled operation is possible. By means of the controlled operation of the drive unit, high torques can be realized at a simultaneously relatively low power stage of the frequency converter.

In order to detect the position of the rotating parts, for example those for recording and reconstructing images in computer tomographs, a position measuring system can additionally be integrated with the sensor system into the revolving joint system. The sensor system has at least one sensor and a signal transmitter. The sensor serves to detect signals of the signal transmitter (encoding means) or else has a plurality of the previously mentioned components in any desired embodiment. For example, the encoding means is formed by an elastomer belt which is charged with magnetized particles with alternating polarity (alternately north and south poles). The sensor system may also have further electronic components, for example transducers.

Rolling contact bearings of any desired number and any desired constructional shape are inserted into the revolving joint. The rolling contact bearings are realized in a single row or in a number of rows. The rolling bodies are balls or rollers which can be held in cages.

The material of the rolling bodies and of the running tracks is preferably steel or else all other conceivable materials, such as materials with a density which is less than 5 grams per cubic millimeter. Materials of this type are, for example, ceramic materials.

Low-noise thin-ring four-point ball bearings are preferably used. According to one refinement of the invention, for bearings of this type, the ratio of the diameter of the reference circle to the diameter of each one of the rolling bodies of a row is greater than 30:1, preferably 40:1, with the reference circle being the imaginary circle which is arranged concentrically with respect to the rotational axis and which, irrespective of possible changes in position of the rolling bodies due to clearances in the rolling contact bearing, intersects the center axes of the rolling bodies, which center axes are oriented parallel to the rotational axis. In the case of rollers, the center axes are the rotational axes or axes of symmetry and, in the case of balls, are imaginary axes running through the center of the ball and parallel to the rotational axis of the revolving joint.

The thin-ring four-point ball bearing is the simplest and most robust constructional form of the bearing. It is configured in such a manner that average axial, radial and torque loads can be reliably absorbed. The thin-ring bearing with its low inherent and dimensional stability is supported by being fitted into the surrounding parts.

Furthermore, wire-race bearings, angular ball bearings and roller bearings are used.

Furthermore, the integrated and low-noise revolving joint system affords the possibility of integrating the running tracks of the rolling contact bearings into the surrounding parts, the running tracks of the rolling contact bearings then being directly incorporated into the corresponding surrounding part. A surrounding part is, for example, a housing into which the rolling contact bearing, or at least one of the bearing races or at least one of the running tracks is integrated, and/or a rotor or a shaft on which the rolling contact bearing, or at least one of the bearing races or at least one of the running tracks is arranged.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1shows a revolving joint1with at least one rolling contact bearing2and with an electromotive drive unit3in a partial section along the rotational axis2aand not illustrated to scale. The rolling contact bearing2has a row of rolling bodies4arranged around the rotational axis2aof the rolling contact bearing2. The rolling bodies4are in contact with a running track5, which can be driven by the drive unit, on an inner race6and in contact with a running track7on an outer race8. The outer race8is fixed in a rotationally fixed surrounding part in the form of a housing9.

Between the outer race8and the housing9there is a solid-borne-sound insulating means10in the form of an insulating layer11which is made from an elastomer and is vulcanized onto the outer race8. Alternatively, the insulating layer11is an insert.

A torque motor of the drive unit3has permanent magnets12which sit directly on a rotor13of the revolving joint1. The rotor13of the torque motor is coupled in a rotationally fixed manner to the driven running track5and is separated from electrical windings (not illustrated in detail) of a stator15of the torque motor by an air gap14encircling the rotational axis2a.

The ratio of diameter D of the reference circle to the diameter K of each one of the rolling bodies4of the row is greater than 30:1, with the reference circle being the imaginary circle which is arranged concentrically with respect to the rotational axis2aand which intersects the center axes16of the rolling bodies4, which center axes are oriented parallel to the rotational axis2aand butt perpendicularly into the plane of the illustration. The rolling bodies4and bearing races6and8are optionally made of steel or of ceramic, with combinations of rolling bodies and components of the rolling contact bearing made of steel with components and rolling bodies made of ceramic being conceivable.

In the revolving joint there is a sensor system18comprising a sensor17and an encoding means20with which at least relative positions in the circumferential direction between the rotor13and the stator15can be detected. The sensor17is fixed on the stator15and the encoding means20on the rotor13. A connecting cable21branches off from the sensor and can also optionally be connected to the control unit22(indicated by dashed lines). From the control unit22, a connecting cable19leads to the drive unit3, and therefore the control unit22can convert output signals of the sensor system18into input signals in order to control the torque motor.