DE3437379A1 - Device for measuring the rotary or bending force exerted on a shaft - Google Patents

Abstract

In the invention, use is made of air gap eddy current probes (3, 4, 9 and 11) in order to be able to measure the torsion and the bending stress, the expansion or the compression which are caused by an alignment error etc. of a shaft (1) by determining the change in permeability of a highly magnetostrictive surface (2) on the shaft (1). This surface (2) can consist, e.g., of a sleeve from an amorphous metal. The change in permeability is proportional to the torsion or stressing of the shaft. There is no physical connection between the surface (2) to be investigated and the sensor or the probe (3, 4, 9 and 11). <IMAGE>

Description

DEVICE FOR MEASURING ON A SHAFT

ROTATING OR BENDING FORCE EXECUTED The invention relates to a device for measuring a torsional or bending force resulting from a shaft misalignment that acts on the shaft from a distant point by observing the Change in permeability of a highly magnetostrictive surface. The invention relates to especially a technique where there is no physical connection between the area to be examined and the sensor.

There has been a need for a method of measuring for many years the torsional load on a rotating shaft, which is contactless, with low power and works intrinsically safe and no significant modifications to the shaft, none Electronics to be attached to the shaft, no wearing parts (e.g. slip rings) and does not require substantial weight addition to the shaft.

Possible solutions to this are found in US Pat. Nos. 3,340,729 and 4,135,391 and 4,364,278. The torsion measurement is based on this prior art on the application of the transformer effect, the shaft, the cladding or Sleeves that are under torsional load act as a transformer core then the primary and secondary windings of a transformer suitably around the shaft on, the change in permeability of a magnetostrictive wave, cladding or Sleeve are detected. The one caused in the shaft as a result of the torsional load Change in permeability creates an asymmetrical magnetic field, creating a magnetic one Flux is caused in the secondary winding, which in the windings an electrical Induced current. This electrical current is then related to the torsional load the wave. The state of the art discussed and explained how that magnetostrictive phenomenon applies.

A major disadvantage of the above solution is that a Considerable power is required to produce a measurable signal on the secondary winding to obtain. This fact makes it difficult to secure the measuring device in a dangerous environment to install. In addition, the induced field, that interacts with the wave, imposes speed restrictions. By using a sleeve or sleeve made of a highly magnetostrictive material, such as an amorphous metal, the power consumption can be reduced will; however, the speed limit remains along with the spatial one There are restrictions near the shaft.

It is therefore the object of the invention to provide an improved device for Measure the torsional or bending force exerted on a shaft to propose the is consistent with the above criteria.

This object is achieved by the invention described in claim 1 solved.

Advantageous further developments and refinements thereof are the subject matter of claims 2 to 22. This object is further achieved by the in claim 23 described invention solved.

According to the invention a device for measuring the on a rotating or stationary shaft proposed torsional or bending force, wherein the shaft has a highly magnetostrictive surface, the permeability of which as a function of the loading due to the load exerted on such a shaft Torsion or their Changes in bending, stretching or compression. Further is a device for measuring this change in permeability by a change in coil impedance provided, which includes a probe device, the coil in the predetermined Distance is arranged close to the surface of the shaft.

Ultimately, with the help of an electrical circuit, the change is made the coil impedance measured and a readable output signal obtained, the one Represents a function of the applied torsion or the applied bending force.

The invention is described in more detail below with reference to the drawing. It shows: Fig. 1 a shaft and four probes embodying the invention, in perspective view; 2A shows an alternative embodiment of FIG.

1 in perspective view; FIG. 2B is a cross-sectional view of FIG Fig. 2A; 3A shows a further alternative exemplary embodiment of FIG. 1 in perspective Opinion; Figure 3B is a cross-sectional view of Figure 3A; Figure 4A shows another alternative Embodiment of FIG. 1 in a perspective view; 4B is a cross-sectional view of Fig. 4A; FIG. 4C shows a further alternative exemplary embodiment of FIG. 1 in perspective view; 4D is a cross-sectional view of FIG Figure 4C; 5 is a circuit diagram showing the processing of the various Electrical signals originating from the probes of FIGS. 1 to 4D illustrate in the invention; 6 is a circuit diagram of an alternative embodiment; Fig. 7 is another Circuit diagram of an alternative embodiment; 8 is a graphical representation the probe output signals of the invention; Figure 9A is a graphical representation of Output signals at which the shaft surface has been biased and FIG. 9B shows a graphical representation of the difference between the two curves of FIG. 9A.

Fig. 1 illustrates a shaft 1 with an indicated center line for which the shaft torsion and the deflection should be measured. Located on shaft 1 a cylindrical collar 2 made of an amorphous metal such as Metglas (trademark). This collar 2 forms a highly magnetostrictive surface on the shaft 1, their permeability turns out to be a function of the load due to the shaft 1 applied torsion or the bending, elongation or elongation of the shaft changes.

About the change in permeability of the magnetostrictive shaft surface measure up four probes 3, 9, 4 and 11 are provided. The two probes 9 and 11 are, as can be seen from the drawing, in a horizontal, the center line the plane intersecting the shaft 1 and the two probes 3 and 4 in a vertical, arranged in the orthogonal plane.

Thus, the probes are arranged and located opposite one another in quadrature. Each probe has a coil (e.g.

3a), which at a predetermined distance close to the sleeve or the Collar 2 is arranged. As explained below, they are electrical circuits provided in the probe itself or assigned to the change in coil impedance to measure and generate a readable output signal that is a function of the exercised Torsion or the exerted bending force.

The surface or the collar 2 also has an axially extending one Gap 5, which is used to measure the rotational speed, as this Gap 5 creates a discontinuity when sliding past the probe. However, this one is Gap 5 for the measurement of torsion or bending irrelevant.

The highly magnetostrictive surface (collar 2) can have several different Types are running. In the preferred embodiment, one is made of amorphous Metal existing sleeve glued onto the shaft 1. As mentioned above, will a typical amorphous metal sold under the trademark Metglas. Other types of attachment of the sleeve can consist of electroplating, welding, etc.

In addition, the surface of the shaft can be modified with the help of chemical conversion techniques treated to create a magnetostrictive surface. This tech niken can for example consist of an ion implantation or an ion bombardment. Ultimately the shaft can be provided with a coating or a lining, and by painting, flame spraying, plasma spraying, laser melting or vapor deposition.

The four probes 3, 9, 4 and 11 are all at a predetermined distance 14 arranged from the shaft and represent eddy current probes. As eddy current probe, which is suitable for the present purpose can be a non-contact Find probe use made by the Sently Nevada Corporation of Minden1 ,, Nevada, is manufactured. Such a probe is a transducer, the one Distance converted into a voltage and with normal use, just as it is sold today becomes, the distance (namely an air gap) to any conductive material, such as e.g. a motor shaft. This probe is therefore used to measure vibrations. The actual transducer in the probe consists of a coil of wire. This coil can e.g. have a diameter of 5 to 25 mm and is powered by an oscillator controlled, which is either part of the probe or a separate unit. In this way eddy currents are induced, in this particular case in the amorphous metal sleeve or the magnetostrictive surface.

Such currents are, of course, induced in metals when these are always brought into an alternating magnetic field. These eddy currents generate a magnetic field that is opposite to the inducing magnetic field. That resulting field, which causes a change in coil impedance, is determined with the aid of the Coil of the eddy current probe measured.

Such a change in coil impedance is shown in FIG size and the phase relationships of the eddy currents and their inducing magnetic field currents at. This relationship is dependent on many factors including electrical conductivity the wave, the magnetic permeability and inhomogeneity as well as the oscillator frequency and of course depending on the air gap. In the invention, as explained below, by isolating the sensitivity to permeability the Rotational force or torque exerted on a rotating or stationary shaft

Torsion can be measured. A single probe, e.g. probe 3, can can only be used to measure such torsion. However, it will be two Probes, e.g. probes 3 and 4, the output signal doubles, while gap changes and the oscillation are averaged out. Four probes 3, 4, 9 and 11 have the further advantage that the output voltage is doubled again while the probe response to torsion is about four points averaged on the shaft as well as a compensation of the gap changes and the Vibration is made.

Accordingly, Fig. 1 represents a basic building block of which other embodiments of the invention along with changes in FIG Surface preparation and the display electronics can be derived.

If another system structure uses this basic unit as a building block, in which diametrically opposed probes are used, vibration errors can occur be excluded. In addition, by using a torsionally preloaded Surface increases the sensitivity to low torsional loads will. Become two pre-stressed surfaces, i.e. two or more separately arranged Sleeves or surfaces such as shown in Figure 2A, and four or If more probes are used, the sensitivity to non-torsion may be Loads, i.e. the sensitivity to bending, stretching and compression be reduced. Furthermore, by different combinations of the probe output signals the sensitivity to loads on the shaft that are not due to torsion are increased, i.e. e.g. the bending of the shaft can be measured, while the torsional sensitivity is reduced.

In general, the use of an eddy current probe results from the use of a single coil operating at a high frequency of more than 100 kHz is controlled and thus as a source of excitation for the induction of eddy currents acts in the magnetostrictive surface. This same coil also detects that of the induced eddy currents caused by the magnetic field and thus their changes in impedance. Such a change in impedance is measured and provides relevant data that concern the torsion, bending and other deformations of the shaft 1. A coil 3a is shown schematically in the probe 3 of FIG. As shown and as with The coil of the commercially available probe is circular. However can these can also be oval or rectangular. Such training also changes that Shape of the electromagnetic field to increase the torsional load sensitivity and increase the sensitivity to loads that are not due to torsion are to decrease.

Finally, additional probes can be provided, each make an angle of 900 to one another in order to improve the signal-to-noise ratio and the effects mechanical or electrical degeneration to reduce.

FIGS. 2 to 4 illustrate various modifications of the one shown in FIG. 1 Fig. 2A and 2B is an additional magnetostrictive Surface 8 designed as a sleeve on shaft 1. That extra surface 8 is arranged axially offset next to the original surface. In the surface 8 and the surface or the collar 2 is each a gap 5 is provided, which only used to measure the speed of rotation of the shaft. Two pairs of probes, consisting of the probes 3 and 4 or 6 and 7, are arranged diametrically opposite one another, wherein The shaft oscillation and the gap changes are compensated for in each probe pair. This is achieved by algebraically summing the probe output signals. On the other hand, from the output signals of the in the same radial, by the Center line of the wave extending plane provided pairs of probes, namely the neighboring probes, e.g. probes 4 and 7 or 3 and 6, the difference is formed, so this action generates a signal that is proportional to the torsion exerted, and eliminates common mode errors such as temperature effects and stresses that are not due to torsional forces and a bend, a stretch as well cause compression of the shaft. These loads are indicated by arrows in FIG. 1 indicated, marked with T (elongation) and C (compression).

To achieve a linear signal, the two can be magnetostrictive Surfaces 2 and 8 are subject to different loads; so e.g. the surface 2 can be stressed in a negative direction of torsional stress, while the surface 8 is in a positive Direction of torsional stress is biased.

Such a preload is achieved by initially placing the shaft Twisted positively or negatively by a certain amount, then the magnetostrictive Applies surface, either by gluing or by a coating etc., as explained above, and then the torsion or twisting of the Wave eliminated.

The results with and without bias are shown in FIGS 9B clarifies. 8 shows the output signals of a probe on the basis of a curve 35, whose surface is not under any load. This curve 35 has one on the Zero-torsion point-centered tip with minimum output size on that with a applied torsion coincides with zero. The disadvantage of this curve shape is there in that an insensitive or ineffective area around the torsion value zero present around. The area insensitive to torsion can, as shown, can be moved below the work area by clicking on the surface exerts a slight negative torsional prestress, which results in the output characteristic curve 36.

It can thus be seen that the relationship between torsion and tension at least from zero torsion value to almost the middle of the positive torsion range represents a linear function.

The curves of Figure 9A illustrate the output signals from the probes of Figure 2A, both positive and negative bias curves being provided will. In this case, both curve peaks are outside of the marked "Workspace". 9B then shows the difference between the two curves. The resulting curve is obviously linear over the work area. It can thus be seen how a preload difference between enables an output signal on both adjacent magnetostrictive surfaces, which is a linear function with the torsion exerted over a predetermined Work area changes. The working range can also be above the torsion value zero lie, in which case the two surfaces have a different one, however should have positive bias.

If the bias difference is not optimal, it sets the output is not a linear function of torsional load, so is an electronic linearizer required as explained below.

FIG. 2B shows a cross-sectional view of FIG. 2A, with the surface 2 is attached to the shaft 1 with the aid of a thin, even layer of adhesive 13 (the gluing is carried out e.g. with epoxy glue). There is also the gap 5 for measuring the speed (rpm) as well as the air gap 14 between the end of the respective probe and the magnetostrictive surface 2 is shown. A complex system using eight probes is shown in FIG. 3A. This The structure is characterized by redundancy in order to increase reliability. This structure also enables a bending moment to be determined in two planes, as shown in connection with the associated circuitry.

The cross-sectional view according to FIG. 3B illustrates the orthogonal one Relationship of the probes with respect to the center line of the shaft.

Figures 4A and 4B show an alternative method of attachment for the magnetostrictive surfaces, which are in the form of film strips 15 and 16, e.g. B. made of amorphous metal are shown. These film strips 15, 16 are in one Angle of 450 to the shaft axis along the Main stretch and main compression lines attached to the shaft experiencing a torsion. The shaft is preloaded and the Foil strips 15 glued with their ends, with the one lying under the probes Middle part remains unattached and is also pulled taut as a result of the bias will. The film strips 16 are attached in a similar manner, but with the exception that the bias is in the opposite direction, thus making this Strips are held just as close to the shaft.

Another coil structure is shown in FIGS. 4C and 4D, wherein the measuring coil 17 surrounds the magnetostrictive surface 2.

Fig. 5 shows a block diagram of a display device. This display device gives the speed (rpm), the power (J / sec), the mean torsion, the torsional vibration (or the torsional vibration amplitude) as well as the dynamic torsion again. That The display device shown can be used in conjunction with one, two or four probes find, the latter case being shown. In detail, these are the probes 3, 9, 4 and 11, as illustrated in Fig. 1, for example. Each probe stands with one Oscillator-demodulator unit 18 in connection, which, as explained above, For example, a high-frequency signal of more than 100 is sent to the coil 3a of the probe 3 kHz and with the help of the demodulator the voltage and thus the probe impedance is measured as soon as it equates with the monitored surface permeability changes.

All output signals of the oscillator-demodulator units 18 are added with the aid of a summing amplifier 19.

One through the one provided in the magnetostrictive surface 2 Air gap 5 generated voltage pulse passes through a high-pass filter 20 and is in converted to a speed signal (/ min), which is displayed by the unit 25.

The low-pass components of the summed probe signals are calculated using the low-pass filters 21 and 22 are filtered out.

This creates almost a DC component that is contained in the unit 23 linearized and scaled and with the help of the display unit 27 for the middle or average torsion is reproduced. A multiplication unit 24 enables by multiplying the torsion and the speed a display of the power the unit 26.

The output signal of the low-pass filter 22 provides, as shown, a dynamic torsion signal. With the help of a peak value rectifier 41 can a torsional vibration amplitude can be displayed on the unit 28.

The linearizer discussed in connection with the graphs of Figures 8 and 9B 23 may vary depending on the desired work area and application Preloads may or may not be required.

The schematic circuit according to FIG. 6 illustrates an alternative Embodiment of a torsion display unit, the probe pairs 3 and 4 respectively 6 and 7 changes in two separate but adjacent magnetostrictive ones Determine surfaces as shown in Figure 2A. The demodulator signal outputs of the Adjacent probes 3 and 6 or 4 and 7 are calculated by means of the units 29 of a difference and then added with the aid of the summing amplifier 19. Through the Difference between the signals of the neighboring surfaces increases the sensitivity with respect to common mode errors, such as with respect to temperature as well as Loads that do not result from torsion and bending, elongation and the compression of the shaft lock in. The algebraic summation or addition of the probe output signals that belong to the same surface, allow a weakened sensitivity to inhomogeneities in the magnetostrictive surface and against vibrations and thus against the Distance between one opposite a given magnetostrictive surface Pair of probes and its rotating shaft.

In Fig. 6, the summing unit 30 sums the output signals of all of them Oscillator-demodulator units 18. The output signal of the unit 30 is with Using the electronics 31 for the bending stress processed and scaled and then controls an alarm device 32. This alarm device 32 has a Setting value from which an alarm is displayed. This enables a Overload as a result of loads that are not related to torsion, such as bending, stretching and / or compression. Looking again at Figs. 2A and 2B, the X plane represents the common plane in which the probes lie and which extends through the centerline or axis of the shaft. The foregoing is also shown in Fig. 7, but an additional four are perpendicular to each other standing probes 9, 10, 11 and 12 (see. Fig. 3A) provided, the sum of the Probe signals via the bending stress unit 31 controls an alarm device 32, which - as shown - is intended for the bending stress in the Y-plane; i.e., the plane in which these probes lie, as best seen in Figure 3B.

The vibr ce circuit part of Fig. 7 clarifies in:, cyuq aüf Fig. 6 a dual display or Uber \ .; chircsgc ät, with additional tuning circuits 33 and 34 are provided, which are the independent torsional output variables compare the values of each probe set consisting of four probes, i.e. the Probes 3, 6, 4 and 7 or 9, 10, 11 and 12 can be generated. Agree the independent Output variables that are to be compared with one another within specified Limits do not match, an "out of bounds" is made by the voting circuitry Tolerance "indicator. In other respects, the same parameters are measured.

As explained above, the use of opposite, Biases defining a working area have a linear output variable. However An alternative technique can be provided that is in addition to the first two surfaces makes use of a third magnetostrictive surface on the shaft, wherein the third surface is not biased. And then a surveillance or display device provide a linear, extensive work area without that there are ambiguities around the bias points.

The invention thus has the following advantages: 1. It is simple compared to the known transformer technologies.

2. It works without contact.

3. It has a high inherent sensitivity with a high degree of freedom from noise and low power requirements.

4. It has a high intrinsic linearity if a suitable bias is used.

5. It has a high level of reliability as there are no wear and tear Parts or "flying" electronics are present.

6. It can be installed and adjusted at the location where the shaft is used.

7. It has little effect on weight and balance the wave.

8. It does not affect the structural integrity of the shaft.

9. It can be run intrinsically safe in a hazardous environment will.

10. It is insensitive to gap changes as a result of a Vibration, misalignment and temperature and has decreased Sensitivity to bending, stretching and / or compression (shaft misalignment) on.

11. It can control the mean torsion, dynamic torsion, shaft speed, the performance as well as an alarm for stresses that cannot be attributed to torsion are, determine.

12. It requires a low cost.

13. It is not affected by dirt, oil or soot.

In addition to the above advantages, when compared with the known Transformer technology a significant advantage of the invention insofar as the invention has a high inherent sensitivity, partly from the use of several Probes and results from the high freedom from noise, which in turn is a consequence of the Possibility of algebraic summation and subtraction of different ones Probe signals is. This also has an influence on the reduced sensitivity versus bending, stretching, etc. Ultimately, there is the other major difference between the known transformer techniques and the invention in the simple Structure, whereby the invention can be rrrbar and ,, writable on the insertor shaft mGnt 1 is. In addition, little space is required because the probes are small cylindrical units carctelles, which can be easily established near the expansions.

Claims (23)

DEVICE FOR MEASURING THE ROTATING OR BENDING FORCE EXECUTED ON A SHAFT Claims: 1. Device for measuring the torsion or bending force on a rotating or stationary shaft is exerted e t through - a highly magnetostrictive surface (2, 8), the permeability of which manifests itself as a function of the stress caused by the the shaft (1) exerted torsion or the bending, stretching or compression of the shaft (1) changes, - a device (3, 4, 6, 7, 9, 10, 11, 12; 18) for determining the Change in permeability of the magnetostrictive surface (2, 8) via a change in coil impedance, the device being a vortex current probe device whose Coil (3a) arranged at a predetermined distance in the vicinity of the surface (2, 8) is, and an oscillator (18) with an operating frequency of about 100 kHz or higher for controlling the coil (3a) and for inducing the eddy currents in the Has surface (2, 8), and - an electrical circuit for measuring the Change in impedance of the coil (3a) and to generate a readable output signal, which is a function of the applied torsion or the applied bending force. 2. Apparatus according to claim 1, characterized in that g e k e n n -z e i c h n e t that a demodulator (18) is provided to determine the change in the coil impedance is. 3. Apparatus according to claim 1, characterized in that g e k e n n -z e i c hn e t that the probe device has a coil (17 ') which surrounds the shaft (1). 4. Apparatus according to claim 1, characterized in that g e k e n n -z e i c h n e t that the highly magnetostrictive surface (2, 8) attached to a shaft (1) Sleeve is made of an amorphous metal. 5. Apparatus according to claim 4, characterized in that g e k e n n -z e i c h n e t that the sleeve is attached by gluing, plating or welding. 6. The device according to claim 1, characterized in that g e k e n n -z e i c h n e t that the highly magnetostrictive surface (2, 8) consists of one applied to the shaft (1) Coating consists. 7. Apparatus according to claim 6, characterized in that g e k e n n -z e i c h n e t that the coating by painting, flame spraying, plasma spraying, laser melting or vacuum evaporation is applied. 8. The device according to claim 1, characterized in that g e k e n n -z e i c h n e t that the highly magnetostrictive surface (2, 8) produced by a chemical technique converted surface of the shaft (1) is. 9. Apparatus according to claim 8, characterized in that g e k e n n -z e i c h n e t that the chemical technique includes ion implantation or ion bombardment. 10. Device according to one of claims 1, 4, 5, 6, 7, 8 or 9, in that the surface (2, 8) is pretensioned provided or otherwise attached under a bias. 11. The device according to claim 1, characterized in that g e k e n n -z e i c h n e t that two or four probe devices are provided, which are diametrically opposite or are arranged in quadrature and that circuits for algebraic summing of the probe output signals for vibration compensation, for increased sensitivity to the effects of torsional stress and to reduced sensitivity provided against inhomogeneities in the magnetostrictive surface (2, 8) are. 12. The device according to claim 11, characterized in that g e k e n n -z e i c h n e t that the oscillation compensation changes the distance between a probe device pairing (3, 4; 9, 11) and a rotating shaft (1). 13. The device according to claim 1, characterized in that g e k e n n -z e i c h n e t that on the shaft (1) another magnetostrictive surface (8) as well as in A probe device (6, 7) is provided near the surface (8), the both surfaces (2, 8) are provided with a bias difference that corresponds to the corresponds to the prescribed dynamic working range on the shaft. 14. The device according to claim 13, characterized in that g e k e n n -z e i c h n e t that a circuit for algebraic subtraction of the output signals two adjacent probes (3, 6; 4, 7) is provided to the sensitivity to Common mode errors such as the temperature and stresses that cannot be attributed to torsion and which include the bending, stretching and compression of the shaft (1) (misalignment) to decrease. 15. Apparatus according to claim 11 or 14, characterized in that it is -k e n n z e i c h n e t that on the shaft (1) two magnetostrictive surfaces (2, 8) as well several probe devices (3, 4; 6, 7) are provided for each surface to be monitored are, in each case the number of probes per surface is the same, and that one electrical circuit is provided that the adjacent surface signals subjecting the same surface signals to an addition. 16. The device according to claim 15, characterized in that g e k e n n - z E i c h n e t that an alarm module (30, 31, 32) for informing an operator from excessive loads that cannot be attributed to torsion, is provided, the module adding up all probe output signals and adding this value compares with a specified maximum value. 17. The device according to claim 15, characterized in that g e k e n n -z e i c h n e t that two opposing probes for measuring the not per radial plane torsional stress in an axial plane with respect to the shaft axis are provided. 18. The device according to claim 1, characterized in that g e k e n n -z e i c h n e t that the geometry of the coil (3a) is circular, oval or rectangular, whereby the generated electromagnetic field is formed so that the sensitivity increased to torsional stress and the sensitivity to Stresses that are not due to torsion is reduced. 19. The device according to claim 1, characterized in that g e k e n n -z e i c h n e t that the metal sleeve made of amorphous metal consists of a film (15, 16). 20. The device according to claim 1, characterized in that g e k e n n -z e i c h n e t that the magnetostrictive surface (2, 8) has a gap (5) and that a device for scanning the gap (5) is provided to determine the shaft speed to display. 21. The device according to claim 4, characterized in that it is n -z e i c h n e t that the metal sleeves are made of amorphous Metal in the form of strips (15) are formed with a bias at an angle of 450 with respect to the shaft axis are attached, partially enclose the shaft (1) and partially are glued to the shaft (1) by means of their ends. 22. The device according to claim 21, characterized in that g e k e n n -z e i c h n e t that strip (16) with respect to the first strip (15) at an angle of 900 are attached and with the opposite prestress along the main torsional stress lines is oriented in the wave. 23. Device for measuring stresses, g e -k e n n z e i c h n e t through - a magnetostrictive surface (2, 8), the permeability of which changes changes as a function of the load, - a device (3, 4, 6, 7, 9, 10, 11, 12; 18) to determine the change in permeability of the magnetostrictive surface (2, 8) via a coil impedance change, the device being eddy current probe devices, whose coil (3a) is arranged at a predetermined distance in the vicinity of the surface (2, 8) is, and an oscillator (18) with an operating frequency of about 100 kHz or more about controlling the coil (3a) and inducing the eddy currents in the Has surface (2, 8), and - an electrical circuit for measuring the Change in impedance of the coil (3a) and to generate a readable output signal, which is a function of the stress exerted.