GB2224085A

GB2224085A - Controlling the clearance or preloading of a rolling bearing

Info

Publication number: GB2224085A
Application number: GB8918154A
Authority: GB
Inventors: Erich Giebner
Original assignee: SKF GmbH
Current assignee: SKF GmbH
Priority date: 1988-08-09
Filing date: 1989-08-09
Publication date: 1990-04-25
Anticipated expiration: 2009-08-09
Also published as: DE3826945A1; FR2635286B1; GB8918154D0; GB2224085B; FR2635286A1

Description

2 2 4 0 8 5 CONTROLLING THE CLEARANCE OR PRELOADING OF A ROLLING BEARING

is The invention relates to controlling the clearance or preloading of a rolling bearing.

In the course of operation, the rolling bearings of work spindles, in particular machine tool spindles, are subject to heating which is caused by bearing friction and which is dependent on the speed of rotation of the spindle and on the forces acting on the bearing. The normal temperature behaviour in uncooled bearing causes a reduction in bearing clearance at high spindle speeds, so that the admissible upper speed limit is limited or it is necessary to use a bearing having a large bearing clearance at low speeds of rotation. Therefore, the effects of normal temperature behaviour are in contradiction to requirements, since at precisely the moment when high operating forces occur, for example during a machining operation in which a high degree of bearing rigidity is required, low speeds of rotation are used, whereas at high speeds the requirements in respect of bearing rigidity are lower. However, if bearings are used which for reasons of bearing rigidity are already provided with preloading at low speeds of rotation, the maximum admissible speed of rotation is reduced even further.

To overcome this circumstance it is normal to cool the bearings and/or their structural components, such as shafts, housing, cooling bushes and so forth, so as to counteract to a large extent any heat-induced variation in bearing clearance. For example, it is known from US-PS 27 47 945 to measure continuously the temperature of the bearing inner ring by means of a temperature sensor and, according to the measuring signals, to supply coolant individually to different regions of the rolling bearing. For this purpose, the measuring signals are fed to a circuit arrangement whose output signals control solenoids in the coolant conduits. If, for example, the measuring signals of the temperature sensor indicate a temperature of the bearing inner ring below a lower limit value, a small portion of the coolant delivered by a pump and simultaneously serving as lubricant is fed through bores in the bearing outer ring to the rolling bodies, whereas the coolant flow proper by passes the bearing and is fed back into a sump. However, if the temperature of the inner ring exceeds an upper limit value, the full coolant flow is fed to the bearing on both end sides.

However, a disadvantage of using temperature sensors is that heating up of the bearing can only be detected when it has already occured. Accordingly, the reduction in bearing clearance and/or increase in bearing preloading, and thus the increased load, has thus already occured before cooling measures are taken. Moreover, in many applications, particularly in the case of machine tool spindles, the requirements for a mounting having as little clearance as possible cannot be satisfied just by eliminating variations in bearing temperature; it is also necessary to match the bearing clearance and/or preloading to different operating conditions (such as machining force and spindle speed).

The invention is based on the object of devising a mounting for work spindles, which is suitable for a high speed range but, therefore, does not exhibit too little rigidity at low speeds of rotation and under high bearing forces.

is The invention provides a method of controlling the clearance or preloading of a rolling bearing supporting a work spindle to take account of different operating conditions by the use of a fluid, the amount of fluid delivered being controlled by valves which are controlled by measuring signals, characterised in that the forces acting on the bearing in the course of operation are measured and the force-measuring signals are used to generate control signals for the valves.

The advantages of the invention lie particularly in that bearings having optimum bearing clearance (or bearing preloading depending on the type of bearing and particular application) can be used for mounting work spindles, in such a way that even at low speeds of rotation the work spindle is mounted with a sufficient degree of rigidity. The requirement for a mounting which remajIns stable when subject to vibrational forces is taken into account by the fact that relatively low speeds of rotation are used under high machining forces and with heavy workpieces. However, if the machining forces are slight and high work spindle speeds are thus used, speed-dependent substantial heating up of the bearing and often inadmissibly high bearing preloading is promptly counteracted, that is before the heating-up occurs.

According to one advantageous development, it is also possible to feed fluid of different temperatures to the bearing and/or bearing structural components so that, if necessary, controlled heating of the bearing ring can also be effected. In a development of this type the bearing clearance of the unstressed bearing could be designed in optimum manner for an average speed of rotation and control of the valves (and also of an optionally provided heating device) could be effected in such a way that, at low speeds of rotation and under corresponding high machining forces, the bearing outer ring is cooled and/or the bearing inner ring is heated so as to reduce the bearing clearance or increase the pre-load, whereas under low machining forces and high speeds the bearing inner ring is cooled and/or the bearing outer ring is heated so as to prevent excessive preloading of the bearing in the upper speed range.

In another advantageous development of the invention, a force-measuring bearing, for example comprising integrated wire strain gauges, is provided for mounting the work spindle. The use of such standard rolling bearings enables complicated and expensive additional measures to be obviated.

If the mounting according to the invention is used with a work spindle driven directly by an electric motor (so-called motor spindle), the stator of which is cooled by a fluid, in a further advantageous development the fluid heated by the cooling of the stator i) can be intermixed, according to corresponding force-measuring signals, with the fluid serving to cool the bearing, ii) can be used for the heating thereof in a heat exchanger or else iii) can be used directly for heating too cold bearing rings.

For the sake of completeness it should also be mentioned that a correspondingly designed control circuit, which comprises a microprocessor or signal processor and which uses the measured signals to 1 1 1 generate control signals, enables the fluid flow to be so controlled, as a function of the magnitude of the force-measuring signal, that upon exceeding a given limit force (above which heating up of the bearing would occur as a result of increased bearing friction) any reduction in bearing clearance is likewise counteracted.

It is also pointed out that both liquid and gaseous fluids and combined cooling with fluids of both types are suitable for putting the invention into effect.

In the accompanying drawings Figures 1 and 2 are diagrammatic illustrations of two types of apparatus for controlling the clearance or preloading of a rolling bearing.

Figure 1 shows an apparatus for controlling the bearing clearance of a rolling bearing 10 of a work spindle mounting (for example the work spindle of a machine tool) to take account of different operating conditions, the bearing having an outer ring 11 and inner ring 12. The rolling bearing 1 is a force-measuring bearing, in the outer ring 11 of which there are integrated sensors 13, for example wire strain gauges. The rolling bodies rolling over the measuring point in the load zone induce d.c. voltage signals in the sensors 13, the amplitude of which is directly proportional to the bearing load.

The bearing outer ring 11 and inner ring 12 are mounted in a known manner, which is therefore not illustrated in detail and with the provision of cooling surfaces or cooling bodies, respectively in a bearing housing and on the spindle. Depending on the particular application and development, for example cooling bushes, recessed cooling passages or other structural bearing components (housing, shaft) can be used for this purpose.

is The signal output of the sensors 13 is fed to a control circuit 14, by which the incoming force-measuring signals are used to generate, in accordance with their respective amplitude, control signals for mixing valves 15 and 16 and a throttle-type valve 17. The control circuit 14 may comprise a signal processor with an A/D converter connected upstream thereof and an amplifier circuit (including a D/A converter) for the output signals, and the valves 15, 16 and 17 may be provided with magnetic or electromotive actuating means, such as an electric motor.

A blower 18 delivers an air current, serving as the cooling medium, along a cold air duct 19 to respective first inlets to the two mixing valves 15 and 16 and, along another cold air duct 20 through a heating device 21 both to the throttle valve 17 and, along a hot air duct 22, to respective second inlets of the mixing valves 15 and 16.

If substantial bearing forces occur during operation, for instance because a thick shaving is being removed in the course of machining a workpiece, force-measuring signals of high amplitude are delivered by the sensors 13 to the control circuit 14. The control circuit 14 then emits control signals to the actuating means of the mixing valve 15 associated with the bearing outer ring 11, by which the cold air duct 19 is opened to a greater extent and the hot air duct 22 is throttled. The bearing z, 1 is outer ring 11 is thus cooled to a greater extent. At the same time, however, the control circuit 14 also emits control signals to the actuating means of the mixing valve 16 associated with the bearing inner ring 12, by which the cold air duct 19 is throttled and the hot air duct 22 is opened to a greater extent. The bearing inner ring 12 is thus heated. Another control signal emitted by the control circuit 14 causes the throttle valve 17 to remain substantially closed so that the air heated by the heating device 21 is fed to the mixing valves 15 and 16. The air, fed to the bearing rings 11 and 12 and/or to their structural components is then discharged along a common outflow duct 23.

Cooling of the bearing outer ring 11 and heating of the bearing inner ring 12 cause a reduction in bearing clearance or an increase in bearing preloading if the bearing was installed already with preloading in the inoperative condition, thereby providing the degree of rigidity of the mounting accommodating the high bearing force.

However, if low bearing forces occur (at a high speed of rotation when removing a thin shaving), it is necessary to counteract any speeddependent substantial heating up of the bearing. The low-amplitude forcemeasuring signals used by the control circuit 14 to generate control signals then cause the cold air duct 19 to be throttled by the mixing valve 15 associated with the bearing outer ring 11 and the hot air duct 22 to be opened further, whereas the hot air duct 22 is throttled by the mixing valve 16 associated with the bearing inner ring 12 and the cold air duct 19 is opened further. In this case, the bearing outer ring 11 is heated and k the bearing inner ring 12 is cooled, thereby counteracting any excessive reduction in bearing clearance caused by heating of the bearing and/or any increase in bearing preloading. Here too, the control circuit 14 ensures that the throttle valve 17 remains substantially closed.

Therefore, cold air is respectively mixed with heated air in accordance with the amplitude of the force-measuring signals, wherein by controlling the throttle valve 17 it is additionally possible to regulate the amount of heated air fed to the mixing valves 15 and 16.

The heating device 21 may comprise an electric heating coil, through which current constantly flows during operation and which emits heat'. In a more expensive development, it is also possible for the heating device 21 to be switched on and off when required by the control circuit 14 (by way of the control line 24 indicated in dashed line). If the work spindle is a spindle driven directly by an electric motor (so-called motor spindle), wherein the stator is cooled by air, the stator may serve as the heating device 21, in which case the air heated by cooling the stator is fed to the two mixing valves 15 and 16. Here the throttle valve 17 is of particular importance, because an adequate supply of air to the stator has to be assured even if the outflow through the hot air duct 22 and the two mixing valves 15 and 16 is only slight. Of course, it is also possible for a heating device to be additionally provided here.

However, a simplified apparatus can also be provided, for example, by that illustrated in Figure 2. Here the blower 18 delivers air through two cold 1 is air ducts 25 and 26 to two throttle valves 27 and 28 which are associated respectively with the bearing outer ring 11 and the bearing inner ring 12. The control signals coming from the control circuit 14 and derived from the force-measuring signals of the sensors 13 cause a greater or lesser flow of cold air to be fed to the bearing rings and/or their structural components, so as to achieve thereby a temperature regulation and adjustment of bearing clearance or bearing preloading.

1

Claims

CLAIMS is A method of controlling the clearance or preloading of a rolling

bearing supporting a work spindle to take account of different operating conditions by the use of a fluid, the amount of fluid delivered being controlled by valves which are controlled by measuring signals, characterised in that the forces acting on the bearing in the course of operation are measured and the force-measuring signals are used to generate control signals for the valves.
2.. A method according to Claim 1, characterised in that fluid of different temperatures are supplied through the valves, according to force-measuring signals.
3. Apparatus for carrying out the method according to claim 1 or 2, characterised in that force-measuring means (13) are disposed on the bearing (10) supporting the work spindle, the force-measuring signal outputs of which means are connected to a control circuit (14) to generate control signals, and in that the control signal outputs of the control circuit (14) are connected to controllable valves (15, 16; 27, 28) through which fluid can be fed in a controlled manner to the bearing (10) and/or its structural components.
4. Apparatus according to claim 3, characterised in that wire strain gauges are disposed in the load-bearing region of the bearing for measuring the forces acting on the bearing.

Q c 1
5. Apparatus according to Claim 3, characterised in that a forcemeasuring bearing (10) is provided for mounting the work spindle.
6. Apparatus according to any one of Claims 3 to 5, characterised in that a heating device (21) is provided for heating at least part of the fluid.
7. Apparatus according to Claim 6, characterised in that the heating device (21) is connected to the control signal output of the control circuit (14) and is controllable according to the force-measuring signals.
8. Apparatus according to any one of Claims 3 to 7 for a work spindle driven by an electric motor having a stator cooled by fluid, characterised in that the fluid which has been heated by the stator can be fed to the bearing (10) and/or its structural components, after being mixed with cooler fluid, according to the force-measuring signals.
9. A method of controlling the pre-load or clearance of a rolling bearing in use comprising determining the condition of the bearing, generating a signal according to the condition and utilising the signal to control the delivery of a fluid to adjust the condition of the bearing, wherein the forces acting on the bearing are used to generate the signal.
10. Apparatus for carrying out a method as claimed in claim 9, comprising a sensor connected to the bearing, a control circuit connected to receive signals from the sensor, a ducting system and valves in the ducting system which are controlled by the control circuit according to signals received from 1 12 - the sensor.

1 NblIshed 1990 atThePatentOMee, State House, 6671 High Holborn, LondonWCIR4TP. Further copies maybe obtainedfrom, The Patent OfficeSales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Can. 1187 f