The present invention relates to a method of setting a desired axial preload in a bearing arrangement by selecting an appropriate shim.
Bearing preload is a force acting between the rolling elements and bearing rings that is not caused by external load. Preload can also be regarded as negative internal clearance, and may be applied to increase stiffness, enhance running accuracy, reduce noise level and improve service life. In bearing types which are designed for transmitting both axial and radial loads, such as taper roller bearings, an axial preload is applied. Sufficient preload is important for ensuring that the loads are evenly distributed amongst the rollers. On the other hand, excessive preload will cause friction and wear, which shortens bearing life.
A common method of setting axial preload in e.g. a double-row taper roller bearing arrangement is to use spacing elements or shims. One or more shims of specific thickness are used to set the desired preload, but due to manufacturing tolerances of the bearing, the shaft and the housing, a shim of the same thickness does not always generate the same preload in the same bearing arrangement. The shim sets a specific gap over which one of the bearing rings is axially displaced relative to the other, but the magnitude of this gap is not known with precision. Therefore, preload is often measured after the bearing arrangement has been assembled and, if necessary, a shim of different thickness is used.
In U.S. Pat. No. 5,115,558, an apparatus is disclosed for determining shim thicknesses used to position bearings on shafts located in recesses formed in a casing. An apparatus plate of predetermined thickness is located between the flanges of two casing halves. An axial force measuring cell is fitted on the bearing and the two casing halves are clamped together with a predetermined load. Axial forces produced by the clamping action of the casing on the shafts are measured during operation and correlated with a predetermined, desired axial force by a computer. The optimum thickness of the shims for the individual shafts is calculated from data supplied to the computer.
There is still room for improvement.
The present invention resides in a method of setting a desired axial preload FP in a bearing arrangement having an inner ring and an outer ring, whereby one of the bearing rings is axially displaceable relative to the other of the bearing rings.
Specifically, the method of the invention is based on selecting a shim that will generate the desired preload at a predetermined axial force applied by a clamping element, such as a locknut. The method comprises steps of:
The analytical model can therefore be used to identify the reference shim as the shim from the plurality of shims that generates the measured preload Factual at the predetermined axial force FA. Consequently, an initial axial gap δf generated by the reference shim is known. The analytical model is further used to identify a target shim from the plurality of shims that would generate the desired preload FP at the predetermined axial force FA. An initial axial gap generated by the target shim δtarget is therefore also known.
The desired axial preload FP is set by replacing the reference shim with a shim that has a thickness of t+(δtarget−δref) and applying the predetermined axial force FA on the bearing arrangement.
Any known method of measuring bearing preload may be used to determine Factual. In a preferred embodiment of the invention, the step of measuring preload comprises measuring a value σactual of a parameter that is representative of the stiffness of the bearing arrangement. As known to the skilled person, a direct relationship exists between bearing preload and bearing stiffness. Suitably, the analytical model then defines a correlation between the measured parameter and bearing preload and further defines a correlation between the measured parameter and applied axial force for the plurality of shims. The analytical model is thus used to determine the actual bearing preload Factual from the measured parameter value σactual, and is also used to determine a target value σtarget of the measured parameter corresponding to the desired preload FP. The target value for the initial axial gap δtarget is then obtained by identifying the target shim as the shim from the plurality of shims that generates the target parameter value σtarget at the predetermined axial force FA.
The analytical model may be constructed using finite element analysis of the bearing arrangement, which takes into account the geometry of the components of the bearing arrangement and preferably also interference between the components.
In one example, the bearing arrangement comprise a double-row taper roller bearing or a double row angular contact bearing, in which first and second bearing inner rings are mounted on a shaft and first and second bearing outer rings are mounted in a housing. The displaceable bearing ring may be a first inner ring or a first outer ring of the bearing arrangement. The arrangement further comprises a fixed abutment comprising an axial side face against which the shim is mounted in pressing contact.
The fixed abutment may be formed on the part to which the displaceable bearing ring is mounted. In one embodiment, the displaceable inner ring is the first inner ring of a double-row taper roller bearing. The fixed abutment may be formed by an axial side face on the shaft to which the bearing inner rings are mounted. Alternatively, the fixed abutment may be formed by an axial side face of the second inner ring.
When the first outer bearing ring is the displaceable bearing ring, the fixed abutment may be formed by an axial side face of the second outer ring or by an axial side face on the housing to which the bearing outer rings are mounted.
In another embodiment, the bearing arrangement comprises a double-row taper roller bearing and further comprises a flange element mounted between the locknut and the first inner ring. For each shim in the plurality of shims, the initial axial gap generated by each shim and the associated clamping force at which the shim makes contact with the displaceable inner ring is calculated. The calculation suitably takes into account an initial interference between the nut and the flange. Preferably, press-fit data is also taken into account.
In one example, the stiffness of the bearing arrangement is measured by applying an impulse in an axial direction to e.g. the shaft, which causes the bearing arrangement to vibrate. At an opposite end of the shaft, an accelerometer may be mounted that is connected to a processing unit that analyses the accelerometer signal and determines an axial mode eigenfrequency. The analytical model in this example is then configured to correlate measured eigenfrequency and bearing preload and applied axial force.
It is also possible to measure a bending mode eigenfrequency associated with the delivered impulse.
In a still further example, actual bearing preload is measured by measuring an axial displacement of an axial surface of the bearing arrangement relative to a fixed reference.
The method of the invention thus allows a variety of measurement techniques to be employed and enables a straightforward selection of a shim that will generate the desired preload in a particular bearing arrangement. These and other advantages of the present invention will become apparent from the following detailed description and accompanying drawings.
In the following, the invention is described with reference to the accompanying drawings, in which:
a shows a partial cross-sectional view of a bearing arrangement which is being preloaded by applying an axial force and using a shim that sets a maximum amount of relative axial displacement between an inner and outer ring of the bearing arrangement;
b shows a partial cross-sectional view of the same bearing arrangement when the maximum relative axial displacement has occurred;
A bearing arrangement suitable for supporting a pinion shaft in a truck transmission is shown in
Prior to preloading, an initial axial gap δ exists between the shim 35 and the first inner ring 13. The magnitude of the initial axial gap δ defines the maximum amount of relative axial displacement between the inner and outer bearing rings, which is predominantly responsible for setting bearing preload.
The locknut 40 is torqued to apply a predetermined axial force FA. While the first inner ring 13 is not in contact with the shim 35 and the shaft abutment 27, the force applied by the locknut 40 follows a force circuit that flows from the first inner ring 13 to the shaft 25 via the first set of rollers, the first outer ring 10, the housing 20, the second outer ring 11, the second set of rollers and the second inner ring 14. This force circuit will be referred to as the preload force circuit, and is shown by the line indicated by reference numeral 45 in
As the axial force applied by the locknut increases, the first inner ring 13 is axially displaced towards the shim 35 until at a certain clamping force, the axial gap becomes zero, as shown in
For the depicted bearing arrangement, it has been found that approximately 10% of the excess axial force flows through the preload circuit, meaning that the magnitude of the predetermined axial nut force FA influences bearing preload, even when a shim is used. In order to set a desired preload, it is therefore necessary to select a shim that will generate the desired preload at the predetermined axial nut force FA.
The present invention defines a method of setting desired preload, based on selecting an appropriate shim.
The method of the invention makes use of an analytical model that correlates applied axial nut force and resulting bearing preload for a plurality of shims. Each shim is defined in terms of the clamping force FC that “closes” the clamping circuit. Furthermore, for each clamping force, the associated initial axial gap δ between the shim and the bearing ring is calculated. Suitably, a table of values is generated, such as shown in Table 1:
In the depicted arrangement, an initial interference between the locknut 40 and the flange element 30 is one of the factors that is taken into account in calculating the clamping force FC and initial axial gap δ, along with bearing geometry and, preferably, press-fit data. The analytical model correlates bearing preload and applied axial force for each shim, enabling a library of bearing preload curves for different shims to be generated, such as shown in the graph of
Bearing preload (y axis) is plotted against the axial force applied by the locknut 40 (x axis). The linear curve 200 shows the relationship between bearing preload and axial force when no shim is present, i.e. when all of the axial force flows through the preload circuit 45. The curves 201-210 respectively show the relationship between bearing preload and axial force for a first shim, a second shim, a third shim, a fourth shim, a fifth shim, a sixth shim, a seventh shim, an eighth shim a ninth shim and a 10th shim. Only some of the curves have been numbered so as not to obscure the drawing.
Let assume that the desired preload to be set in the depicted bearing arrangement is FP and that the locknut 40 applies a predetermined axial force FA. It can be seen from curve 205 for the fifth shim that this shim generates the desired preload FP at the predetermined nut force FA. The fifth shim clamps at a clamping force Fc5. Let us assume that the initial axial gap δ5 that corresponds to the clamping force Fc5 is 34 microns. Thus, a shim that generates an initial axial gap of 34 microns needs to be selected in order to set the desired preload FP. It is not possible, however, to know which value of initial axial gap a real shim generates.
The method of the invention therefore comprises a step of mounting a reference shim having a thickness t. Returning to
Therefore, the difference between the actual initial axial gap and the desired initial axial gap is 28−34 microns=−6 microns. Consequently, the desired bearing preload can be set by replacing the reference shim 35 of thickness t with a shim that has a thickness of t−6 microns.
In a preferred embodiment of the method, the step of measuring bearing preload comprises measuring a parameter that is representative of the stiffness of the bearing arrangement. An example of a suitable measurement apparatus is shown in
The bearing arrangement of
Next, an impulse I that causes the bearing arrangement to vibrate is applied. In the depicted apparatus, an impact device 60 delivers an impulse I in axial direction to the shaft. The device is sensorized and the magnitude of the impulse is recorded.
The bearing arrangement is a system comprising bodies of different stiffness whose values determine the eigenfrequencies and mode shapes of the system as whole. The magnitude of the axial force applied to the system mainly affects the stiffness of the bearings through where the preload force is transmitted. It is therefore possible to determine a correlation between applied axial force, the preload force and the measured eigenfrequencies.
In the depicted apparatus, the axial mode eigenfrequencies are measured by an accelerometer 65 mounted at an opposite axial end of the shaft 25 from where the impulse is applied. It is also possible to measure the bending mode eigenfrequencies using an accelerometer mounted on the shaft circumference.
The measurement apparatus further comprises an analysis unit 70 which receives a frequency signal from the accelerometer 65 and an impulse signal from the sensor (not shown) on the impact device 60. The analysis unit 70 analyses these signals, to determine the eigenfrequencies, and is programmed with an analytical model that correlates eigenfrequency and bearing preload. The analytical model further comprises a correlation between eigenfrequency and applied axial force for a plurality of different shims.
An example of an analytical model is represented by graph of
Let us assume that the accelerometer measures an axial eigenfrequency of σactual Hz. The actual bearing preload is obtained from the preload force curve 400 and corresponds to a magnitude Factual. The measured eigenfrequency corresponds to a unique value of bearing preload, regardless of which shim has been used. For setting a desired preload, however, it is necessary to identify which shim from the library of shims has, in fact, been used. As explained previously, the known axial force applied by the locknut FA, in combination with the actual measured quantity, is used for the identification. From
The method for selecting an appropriate shim for setting a desired preload FP is then identical to the method described above.
The reference shim is a shim which closes the clamping circuit at a clamping force Fc, ref. Let us assume that this clamping force is associated with an initial axial gap δf. At the desired preload FP, the associated eigenfrequency is σtarget, which is the eigenfrequency generated when a target shim defined by the fifth curve 405 is subjected to the predetermined axial nut force FA. The target shim closes the clamping circuit at a clamping force Fc, target. Let us assume that this clamping force is associated with an initial axial gap δtarget. The desired bearing preload can therefore be set by replacing the reference shim 35 with a shim that has a thickness of t+(δactual−δtarget).
A number of aspects/embodiments of the invention have been described. It is to be understood that each aspect/embodiment may be combined with any other aspect/embodiment. Moreover the invention is not restricted to the described embodiments, but may be varied within the scope of the accompanying patent claims.
This is a United States National Stage application claiming the benefit of International Application Number PCT/EP2013/054475 filed on 6 Mar. 2013 (Jun. 3, 2013), which is incorporated herein by reference in its entirety.”
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/EP2013/054475 | 3/6/2013 | WO | 00 |