DE19946383A1 - Control unit for keeping constant axial force on bearing comprises component which generates large amount of force, e.g. hydraulic piston highly pretensioned spring between which outer ring of bearing is mounted - Google Patents

Abstract

The control unit for keeping constant the axial force on a bearing comprises a component which generates a large amount of force, e.g. a hydraulic piston (1) and a highly pretensioned spring (6), e.g. a sheet spring. The outer ring of the bearing (2, 4) is mounted between these. The axial force on the bearing depends only on the force generated by the piston and not on the friction between the bearing and its housing (3) or the piston and housing.

Description

Angular contact ball bearings have largely established themselves for the storage of high-speed machine tool main spindles. They are characterized by their low friction, the stiffness that can be set by the axial preload force and the high speeds that can be achieved. Angular contact ball bearings can absorb axial and radial loads at the same time via the contact angle set in production ( Fig. 1).

To generate the axial preload and to absorb higher axial loads in both load directions (tension / compression), the bearings are usually arranged in axially clamped sets [1]. A distinction is made between rigidly and elastically arranged sentences ( Fig. 2).

The angular contact ball bearings now come into operation and especially at high speeds changes in the bearing kinematics, d. H. the contact angle, the contact forces and the Shifting behavior. These changes in kinematics define one of the application limits for angular contact ball bearings.

The changes in the kinematics are triggered by three effects, the effects of which overlap and intensify [1-6]:

• - The centrifugal forces on the balls of the bearing change the balance of forces on the Ball and thereby cause changes in the contact angle on the bearing rings. The centrifugal forces acting on the inner ring widen this compared to Outer ring on. Even with axially preloaded and therefore free of play This reduces angular contact ball bearings, which are always present Bearing rings approach each other.
• - The temperatures that occur during operation at the bearings have the same effect, since the spindle is less able to dissipate the heat coupled into it than the housing. Therefore, a higher temperature develops on the inner ring of the bearing than on the outer ring. Together with the thermal expansion of the rolling elements, this effect also helps to reduce the radial internal clearance ( FIG. 3 shows these changes in a simplified manner).

Since the radial and the axial relative displacements of the bearing rings are linked to one another via the pressure angle as in a wedge gear, the radial approach of the bearing rings must trigger an axial compensating movement. This movement is also shown in Fig. 3. Fig. 4 shows a comparison of calculated values for this shift with measured values.

If this axial compensating movement is prevented, the deformations in the rolling contacts - including the preload of the bearing - must increase. Especially in axially rigidly clamped bearing assemblies, there must be an increase in the preload during operation, since the axial displacements of the bearing rings shown in FIG. 3 are prevented by the bearings arranged in opposite directions. Fig. 5 shows a comparison of calculated values for this preload increase with measured values: Even if the bearing package accelerates rapidly to a speed of 19000 l / min - i.e. without significant temperatures developing in the bearing package - the bearing preload increases by the installation value from 500 N to 9000 N. In practical use, this would lead to a sharp increase in the frictional torque and the temperature of the bearing and greatly reduce the service life.

If the application does not require a fixed bearing package (lathe spindles, for example, must have a fixed bearing), fast-rotating angular contact ball bearings in milling spindles in particular are usually set elastically for this reason. This happens e.g. B. by placing springs with a flat characteristic between the outer bearing rings, or by using pneumatic or hydraulic pistons to generate the preload force. The elastic adjustment of the bearings has the disadvantage that the axial rigidity of the bearing package decreases because the bias springs are in the power flow (see FIG. 6 - spring characteristics of a rigid and an elastically adjusted bearing package). However, since maintaining an approximately constant preload is the prerequisite for achieving high speeds, this disadvantage is often accepted when designing spindle bearings.

The shifts required to maintain the preload can then take place by approaching the outer rings of the bearings against the spring forces. Such and similar solutions are e.g. B. in the documents DE 41 26 317 A1, G90 04 901.2, DE 43 33 196 C2, US 5,803,619, US 5,564,840 and US 4,022,645.

The disadvantage of such and similar solutions is that the friction between the outer rings and the housing often impedes the axial movements required to maintain the preload. This is underlined by the measurement results in FIG. 8. Here, the displacements between the outer rings of a hydraulically operated bearing package according to FIG. 7 were measured during operation and the calculated values compared. The fact that the measured values remain behind compared to the calculated values indicates an increase in the preload in the bearing package, which is also quantified in FIG. 8 for the different temperature states.

From this the requirement can be derived that a device for keeping constant or The preload can only be set reliably if the Friction is excluded, or the influence of friction on the movements will be annulled.

The former can be done by smoothly realizing the movements that the bearings have to carry out in order to keep the preload constant or to adjust the preload. The most widespread solution according to the state of the art for avoiding the friction effects and their highly undesirable effects on the bearing preload is an axially movable radial roller bearing - a so-called ball bushing. Fig. 9 shows a spindle bearing, in which the load-bearing non-locating bearing is axially movably received in such a ball bushing.

However, the ball bushing in turn brings a number of disadvantages with respect to required high manufacturing accuracy, the heat dissipation from the floating bearings out and the behavior towards dynamic loads.  

The state of the art can therefore be summarized as follows:

• - In axially clamped bearing assemblies, the effects of centrifugal forces and thermal expansion of the housing and shaft parts can lead to strong increases in the preload forces acting axially between the bearings [3, 5, 6]. This increase in preload can lead to the destruction of the bearings and limits the performance and service life of the spindle bearing.
  The increase in preload can only be prevented if the bearing rings can perform axial compensatory movements in the order of a few µm per mm bearing bore [1-6]. For a bearing with a 100 mm bore, relatively large compensating movements of the order of a few tenths of a millimeter are required to maintain the preload, which, on the other hand, must be carried out to the µm.
• - To avoid the increase in the preload and to allow the compensating movements, there are a variety of z. T. patented solutions, in which the use of force-setting elements, such as. B. springs, hydraulic pistons etc. between the axially clamped bearings, the preload of the bearings is kept constant. These solutions are characterized by the fact that the force-setting actuator exerts a force of the magnitude of the desired preload when the shaft rotates between the outer rings (the statements presented here relate to the more common case that the shaft with the bearing inner rings performs the rotation and thus the outer ring stands) initiates the axially clamped bearing.
  Appropriate solutions are such. B. in the documents US 4,033,645, US 5,803619, US 5,388,917, O 169 034 B1, DE 43 33 196 C2, DE 41 26 317 A1, RU 2015426 and in the utility model G90 04 901.2.
• - On the other hand, it is known that the friction z. B. between the outer rings of the bearings and the housing components or between the moving components of the force-setting elements in the order of magnitude of the desired bearing preload. The axial compensation movements required to maintain the preload can thus be prevented by frictional forces. The force controller is then ineffective due to the influence of the friction (this is expressly pointed out in EP 0 169 034 B1, for example).
  In other words, the static frictional force severely limits the effectiveness of the purely force-providing elements for keeping the preload constant and can also cancel it entirely.
  This principle-related disadvantage therefore affects all solutions that work with exclusively force-setting elements such as springs and hydraulic pistons. This also applies to the solutions proposed in US 4,033,645, US 5,803619, EP 0 169 034 B1, DE 43 33 196 C2, DE 41 26 317 A1, and in utility model G90 04 901.2.
  In these solutions, only force-setting elements are presented that only release forces of the size of the preload to the outside and consequently by forces of this size - that is, e.g. B. the frictional forces can also be blocked.
• - Finally, solutions are known which are intended to rule out the influence of the friction on maintaining or adjusting the preload. Such solutions are known from the documents US 2,556,368, EP 0 169 034 B1, DE 27 47 976 A1 and DE 198 18 634 A1. An elastic suspension of the bearings in springs is proposed here in order to completely rule out the influence of the friction on the preload.
  In particular, the solution presented in EP 0 169 034 B1, however, has the disadvantage that, for. B. in the radially exempted part of the "sleeve 80" received bearing 18 in Fig. 3 is supported radially only indirectly. Angular contact ball bearings, in particular, tend to react to radial loads by tilting the outer ring [1, 3, 6] if this is not prevented by proper guiding in the housing. In principle, such guidance cannot be guaranteed with a solution according to EP 0 169 034 B1.
• - In other known solutions an attempt is made to use the different thermal expansion coefficients of different materials or material combinations in order to generate the forces required to maintain or adjust the bearing preload. Such solutions are e.g. B. in the document DE 32 39 305 A1, or the article "Game compensation in precision spindle bearings" from construction 25-08 and the writings cited there.
  The temperature condition of the bearings themselves is used here as a manipulated variable for the generation of force or displacement for keeping constant or adjusting the bearing preload. The solutions described thus have the disadvantage of a delayed response to changes in the speed or load state of the spindle. Especially with spindles that cover a large speed range, such solutions are not suitable for the destructive increase in the preload, the z. B. results from an increase in speed (see FIG. 5) to avoid. Instead, they gradually reduce the increased preload as the bearing temperature increases. In addition, it is known that especially in the flat tapered seats of cylindrical roller bearings, as shown in the publication of the construction, the highest frictional forces can arise, which would in turn be directed against the desired actuating movement. Furthermore, it is considered extremely unfavorable in bearing technology [1, 3, 6] to move the inner ring of the bearings on the rotating shaft, because fretting corrosion then forms between the bearing seat and the spindle. Such a displacement of the inner ring on the shaft is required in the solution presented in DE 32 39 305 A1.
• - In order to avoid these disadvantages, there are considerations to use displacement elements to adjust the preload. Moving elements are characterized by the fact that, depending on an actuating signal, they assume a certain position which is not influenced by the resistance forces against the movement - including the friction. This is achieved by the position adjusters being able to generate very high actuating forces that are significantly greater than the resistance forces that may occur.
  If the dependency between the set path - i.e. the distance between the bearing rings - and the resulting bearing preload is known exactly, the bearing preload can be set independently of the friction.
  The position control signal must then be constantly adjusted by a control to the operating state of the camp.
  Classic pioneering elements are e.g. B. Piezo quartz or ceramics. Corresponding applications for regulating the bearing preload are e.g. B. proposed in the documents DE 39 00 121 A1, US 5,564,840. However, piezo actuators have the disadvantage that they only allow a short actuating movement of approximately one μm per mm of overall length for a given overall length. In order to be able to generate the necessary travel ranges in the range of a few tenths of a millimeter, a large number of piezo elements would have to be stacked up, or very long piezo elements would have to be used - an expensive solution in view of the high cost of the elements.
  This disadvantage is even expressly pointed out in US 5,564,8490 and a complex and complex arrangement is presented, by means of the combination and mutual actuation of the actuators, nevertheless being able to provide paths of the required magnitude.
  In both documents - DE 39 00 121 A1, US 5,564,840 - the outer rings of the bearings are also accommodated directly in the housing, the position adjuster does not engage around the outer ring, so that in one direction of movement - namely when the bearings are relieved - the clamps The bearing according to Fig. 8 is likely.

An improvement beyond the prior art can be achieved by the following presented solution can be achieved because they are easier to manufacture and with less Effort is to be done and the disadvantages that other solutions such. T. have (reduced rigidity, jamming of the bearing rings in the housing etc.) avoided. Other features of the invention will be apparent from the following statements can be seen on the following drawings:

Fig. 10 sleeve for positioning the outer rings with the aim of keeping constant or adjusting the axial bearing preload.

Fig. 11 pressure control in the piston chamber depending on the speed and temperature.

Fig. 12 Positioning of the bearing rings via the hydraulic pressure regulated as a function of speed and temperature.

Fig. 13 Use of the positioning unit as an overload protection.

Fig. 14 examples of the use of the preload controller in different bearing arrangements.

Figure 15 is an alternative arrangement of the outer rings of the bearing 2..

Fig. 16 controller for keeping constant the axial bearing preload by the forced positioning of the outer races of bearings axially braced by means of a linear actuator.

Fig. 17 controller for keeping constant the axial bearing preload by the forced positioning of the outer races of bearings axially braced by means of a linear actuator consisting of a hydraulic piston and a spring.

Fig. 18 Schematic diagram of the control for keeping the axial bearing preload constant by the forced positioning of the outer rings of axially clamped bearings.

Fig. 19 Setting the installation or standstill preload.

First embodiment

The outer ring of the bearing 2 , which generates the axial preload of the bearing 4 , is received in a sleeve 1 , which is slidably movable in the housing 3 or a further sleeve 5 . The sleeve 1 is designed so that it represents the piston of a hydraulic cylinder, the wall of which is formed by the housing 3 or a further sleeve 5 . The piston is axially loaded by a spring 6 with a steep characteristic curve which is already strongly preloaded in the mounting situation of the bearing shown in FIG. 10. The power flow between hydraulic medium 7 and spring 6 is closed by the housing 3 or sleeve 5 .

The sleeve 1 is then in an equilibrium position, which is due to the balance of the force of the hydraulic piston 1 and the preload of the bearing or bearings 2 on one side and the counterforce of the spring 6 and the friction between the piston 1 and the housing 3 or sleeve 5 is defined on the other side.

If the forces acting on the piston 1 through the hydraulic medium 7 and the spring 6 are large compared to the bearing preload or the friction in the sliding joint 8 between the housing 2 or sleeve 5 and the piston 1 , then the equilibrium position of the Do not depend very much on the piston preload and especially on the friction. Instead, the position in which the piston 1 is located is determined almost exclusively by the pressure on the piston surface.

This dependency is used to position the outer rings of the bearings depending on the speed and temperature at a distance that corresponds to a constant preload of the bearings. For this purpose, the speed of the spindle and the temperature of the spindle bearing are measured and used as an input signal for a servo valve 9 , which causes the pressure on the piston 1 to drop in a suitable context with increasing speed ( FIG. 11). In this case, the distance between the outer rings of the bearings 4 and 2 is shortened, so that the preload between the bearings is reduced compared to the value that they would assume without this regulation.

Second embodiment

In the second exemplary embodiment, the temperature of the bearings is also included as a manipulated variable for the pressure in the piston 1 and thus the axial distance between the bearing rings and thus the axial bearing preload. In this case, the axial distance between the spindle bearings may have to be taken into account when determining the setpoint for the hydraulic pressure on the piston 1 . If the bearings are arranged at a small axial distance - ie the axial distance is less than approximately twice the value of the bearing bore - the pressure and the speed must also decrease with the temperature. With increasing axial distances, the reduction in pressure, which occurs as a function of the temperature, must be reduced.

The setpoint for the pressure reduction as a function of speed and temperature must therefore be determined as a function of these two manipulated variables, taking into account the bearing and spindle geometry and the installation conditions. Powerful computer programs are now available for this purpose, the results of which, as shown in FIGS. 4 and 5, agree well with measured values.

The information about the spindle speed can advantageously from the converter of the Drive or the machine control can be taken while the signal for the temperature when the temperature is included as a manipulated variable in the control loop should be recorded by an appropriate sensor. In In this context, it should be noted that many high-performance engine  Spindles for monitoring the bearings already have such a temperature sensor feature.

By decreasing the pressure in the piston chamber 7 , the spring z. T. relax, reducing the piston space. This change in the piston position reduces the distance between the outer rings of bearings 2 and 4 . The direction of this movement corresponds to the displacements described above, which are required to increase the speed and temperature in order to maintain the preload in the bearing ( FIG. 12, cf. FIGS. 3 and 4). The arrows drawn in FIG. 12 are intended to symbolize the various forces acting on the piston. Their size should correspond approximately to the size of the forces in relation to one another.

If the pressures are now set such that the movement of the piston and thus the displacement of the outer rings towards one another exactly corresponds to the displacements quantified in FIG. 4, the preload in the bearing package remains constant.

Another advantage of the system described is that the actuating forces are transmitted between the hydraulic medium and the spring to move the piston, are large compared to the preloads. This will not only influence the Friction on preload significantly reduced. In addition, one is preloaded bearing package also "rigid" in the sense of the above statements, d. H. the The axial stiffness is the same in the pulling and pushing directions and about twice as high like the rigidity of a storage package made in the classic way Size and design. Only when the external load is the force of the hydraulic piston exceeds, there is a greatly increased displacement. But since the hydraulic and spring forces large compared to the preloads and thus also against the expected operating loads, the bearing package corresponds in its Stiffness behavior of a rigidly set bearing.

Third embodiment

If the bearing 2 is arranged on the load side, the clamping of the bearing can also take over the function of an overload protection if the servo valve which regulates the piston pressure is equipped with an overpressure function ( FIG. 13).

To simplify matters, the mode of operation of the invention was always illustrated using the example of an O- Package explained, which was formed from two angular contact ball bearings. At this point expressly pointed out that the preload is also different or arranged storage packages - e.g. B. from the so-called tandem O packages from three or four bearings, which can also be adjusted axially over the entire length of the spindle - can be kept constant in the manner described.

Fourth embodiment

Another sensible variant consists, in contrast to the previously described arrangement of the bearings within a piston 1 that can be displaced in the housing, to receive the outer ring or bearings of the bearings 2 directly in the housing ( FIG. 15).

swell

[1] NN, the work spindle and its bearing - the heart of powerful machine tools, FAG publication, publ. No. 02113 DA
[2] Oswald Bayer, correctly dimensioning spindle bearings, workshop and operation, volume 130 (1997), issue 4, soap 222-226
[3] TA Harns, Rolling bearing analysis, Third edition, John Wiley & Sons, ISBN 0-471-51349-0
[4] M. Weck, U. Tüllmann, angular contact ball bearing - a machine element for the storage of fast rotating spindles, contribution to the seminar "Design of spindle bearing systems for high-speed machining", WZL of RWTH Aachen University, 1997, lecture 4
[5] NN, WZL, VDW research report "Development, high-speed, roller-bearing main spindles for machine tools", report on the research activities of the WZL in the context of the research project of the same name in the years 94-98
[6] Udo Tüllmann, "The behavior of axially braced, high-speed angular contact ball bearings", dissertation RWTH Aachen, volume 25/99, ISBN 3-8265-6682-3

Claims (3)

1. Controller for keeping the axial bearing preload constant by positively positioning the outer rings of axially clamped bearings with the aid of a linear actuator, characterized in that the outer ring (s) of the bearing or bearings 2 of an axially clamped bearing package, which generate the axial preload against the other bearings 4 the package are arranged between an actuator 9 , which can generate high forces and a strongly biased spring 6 with a steep characteristic curve, whereby the position of the outer rings of the bearings 4 between the actuator 9 and spring 6 and thus the axial distance of the outer rings of the bearings 2 and 4 and thus the axial preload of the bearings 2 and 4 then essentially depends on the amount of force which the actuator emits and only slightly on the amount of the axial preload itself and above all also the friction between the outer rings of the bearings 2 and the housing 3 or is influenced by the friction between piston 1 and housing 5 ( Fig. 16) - depending on whether the outer ring of the bearing 2 is directly in the housing 3 , or whether it is accommodated in the piston 1-.
It should be noted that the actuating forces that the actuator can develop are far higher than the axial preload of the bearings. The arrangement of the components ensures, however, that when the pressure, ie position value, is correctly adjusted to the speed and possibly the temperature, these high actuating forces are not passed on to the bearings, but are canceled out between the actuator 9 and the spring 6 . The preload of the bearings does not correspond to the actuating forces, but results from the axial distance between the outer rings of bearings 2 and 4 , which is inevitably determined by the force of the actuator.
The solution described can be carried out particularly advantageously by using a hydraulic piston 1 as the actuator, while a plate spring is used as the spring ( FIG. 17). 2. Regulator for keeping the axial bearing preload constant by means of forced positioning <of the outer rings of axially clamped bearings with the aid of a linear actuator according to claim 1, characterized in that the force of the actuator is then set as a function of the speed of the bearings and possibly also their temperature, whereby the axial position of the outer rings of the bearings 2 between actuator 1 and spring 6 and thus the distance between the outer rings of the axially clamped bearings and thus the axial preload of the bearings exactly and almost without influence of the friction between the relatively moving parts in a suitable dependency of the speed and temperature can be set ( Fig. 18). 3. Regulator for keeping the axial bearing preload constant by positively positioning the outer rings of axially clamped bearings with the aid of a linear actuator according to claim 1, characterized in that the axial standstill or installation preload between the bearings 2 and 4 is set in that the controller without spring 6 is mounted and the actuator is controlled so that it emits a greatly reduced force which corresponds to the desired axial preload of the bearings and that the axial distance L ( FIG. 19) is then measured and the width of the ring 11 is ground in this way that the axial position of the outer rings of the bearings 2 between the built-in spring 6 and the actuator 9 working with the normal operating load corresponds exactly to the axial position which the outer rings of the bearings 2 have already assumed during the procedure described above.