GB2052078A

GB2052078A - Load Cell

Info

Publication number: GB2052078A
Application number: GB8014791A
Authority: GB
Original assignee: ASEA AB
Current assignee: ABB Norden Holding AB
Priority date: 1979-05-04
Filing date: 1980-05-02
Publication date: 1981-01-21
Also published as: SE7903900L; DE3015535A1; FR2455734A1; JPS56629A; GB2052078B

Abstract

A load cell, which is particularly suitable for measuring the forces in a rolling mill, has a measuring body 17 in the form of a hollow cylinder for measuring a load acting on areas of the end surfaces 7, 8 of the measuring body and tending to compress the measuring body in its axial direction. The measuring body is provided at a plurality of locations along the circumference of its arcuate wall with measuring devices 18-20, for example magnetoelastic transducers or strain gauges, for measuring the stress or strain in the measuring body caused by the load. The wall of the measuring body is provided with substantially axially disposed through- going slots 21-24 which screen the measuring devices from circumferential tensile or compressive stress which can arise in the measuring body depending on the distribution of the load on the end surfaces of the measuring body. <IMAGE>

Description

SPECIFICATION Load Cesl Technical Field This invention relates to a load cell of the kind comprising a measuring body in the form of at least a sector of a hollow cylinder for measuring a load acting on areas of the end surfaces of the measuring body and tending to compress the measuring body substantially in its axial direction, said measuring body being provided at a plurality of locations along the circumference of its arcuate wall with measuring devices for measuring the stress or strain in the measuring body caused by said load.

Annular load cells of this kind are frequently employed for measuring the large rolling forces in rolling mills, the measuring body of the load cell being located, as disclosed in U.S. Patent No.

3,577,108, around the mill screw which presses the upper and lower bearing housings together, the load cell being arranged so that it is subjected to the force with which the rolls are pressed together. For this purpose, the measuring body of the load cell is suitably located around the mill screw between the mill screw nut and the mill stand.

The load conditions arising when annular load cells are located in this way in a rolling mill are, however, unfavourable. One reason for this is that it is difficult to machine the contact surface in the mill stand with sufficient accuracy, but mainly because the elastic deformation, and subsequently, to a certain extent, also the plastic deformation, 6f the surfaces cause them to become conical. Plastic deformation occurs primarily at the surface of the mill stand, since the material from which this is made is usually of a much lower quality than the material from which the mill screw nut is made. As a result, the two load surfaces constituted by the end surfaces of the measuring body of the load cell are subjected to an uneven load in the radial direction, with the maximum load occurring at the outer periphery of the load surfaces.The originally hollow-cylindrical measuring body of the annular load cell then becomes elastically deformed, with a drawn-in "waist", which results in a high circumferential compressive stress arising in the central parts of the cross-section of the measuring body, which is substantially constant in the radial direction. This normally gives rise to great deviations from the measurement values obtained from a load cell which has been calibrated between plane-parallel surfaces. For example, in the case of an annular load cell resting on a plane surface with the axis of its measuring body vertical, when a load is applied to the radially inner half of the upper end surface, a 45 per cent higher signal has been measured compared with the signal obtained in the case when an evenly distributed load is applied to the upper end surface of the load cell.

The present invention aims to provide a load cell of the kind referred to in which the abovementioned problem does not arise.

Disclosure of the Invention According to the invention, a load cell comprises a measuring body in the form of at least a sector of a hollow cylinder for measuring a load acting on areas of the end surfaces of the measuring body and tending to compress the measuring body substantially in its axial direction, said measuring body being provided at a plurality of locations along the circumference of its arcuate wall with measuring devices for measuring the stress or strain in the measuring body caused by said load, and said wall being provided with substantially axially disposed through-going slots arranged on both sides of the measuring devices, which slots are arranged to screen the measuring devices from circumferential tensile or compressive stress which can arise in the measuring body depending on the position of said areas of said end surfaces to which said load is applied.

The load cell in accordance with the invention is substantially insensitive to the load distribution on its end surfaces, and a highly accurate measurement of the axially applied load is obtained.

The load cell in accordance with the invention must not be confused with the load cell disclosed in U.S. Patent No. 3,093,999, in which several magnetoelastic transducer elements are located adjacent to each other in one plane, the transducer elements being mutually spaced apart by slots. As is clear from this U.S. Patent, the purpose of the slots is to isolate the transducer elements mechanically from each other so that the lateral strain does not cause sensitivityreducing transverse forces. Such transverse forces would occur between the inner transducer elements but not in the outer elements of the load cell, so that a different sensitivity would be obtained for different transducer elements and thus a minor error would occur in the integration of a force distributed unevenly over the load surfaces.The present invention, on the other hand, aims at eliminating the very great measurement errors which are caused by the great circumferential stresses arising as a consequence of uneven loading of the measuring body in the radial direction.

Brief Description of Drawing The invention will now be described, by way of example, with reference to the accompanying drawing, in which Figure 1 a and Figure 1 b show two extreme load cases which shall be handled by the load cell in accordance with the invention, Figure 2 is a schematic perspective view of a load cell in accordance with the invention, Figure 3 is a perspective view illustrating the deformation of, and the circumferential stress in, an unevenly loaded sector-shaped load cell, and Figure 4 is a schematic perspective view illustrating the location of a load cell in a rolling mill.

Description of Preferred Embodiments Figure 4 shows a mill stand 1 with a mill screw 4 and a mill screw nut 2, a load cell 3 in accordance with the invention being positioned around the screw 4 between the nut 2 and the mill stand 1.

The problem in connection with the load cell 3 is illustrated by Figures 1 a and 1 b. If the load on a hollow-cylindrical measuring body 6 is concentrated at the outer periphery of the load surfaces 7, 8 (said outer periphery being shown by arrows 9, 10, 11, 12 in Figure 1 a), this results in a bending moment on the measuring body which endeavours to bend the wall of the body inwardly intermediate the load surfaces 7, 8. The result is a high circumferential compressive stress in the central part of the body (see the horizontal arrows in Figure 1 a), which stress is substantially constant in the radial direction.

If, instead, the load in concentrated at the inner periphery of the load surfaces 7, 8, as shown in Figure 1 b (see the arrows 13, 14, 15, 16), a high circumferential tensile stress is obtained in a corresponding manner in the central part of the measuring body (see the horizontal arrows in Figure 1 b), which stress is again substantailly constant in the radial direction.

Figure 2 shows a load cell comprising an annular measuring body 1 7 with a number of stress transducers 18, 19, 20, etc., around the circumference, each transducer being either of the magnetoelastic type, for example of the kind disclosed in U.K. Specification No. 767,791, or a strain gauge. Each of these transducers is flanked on both sides by a substantially axially disposed slot 21-24, etc., each slot being radially disposed and passing right through the wall of the measuring body 17. The height of these slots is smaller than the height of the body 17 and the slots do not extend to the load surfaces 7, 8 at any point. These slots screen the measuring elements from the above-mentioned circumferential compressive or tensile stresses, which are caused by a load distribution which is uneven in the radial direction.The slots should be located close enough to the transducers to prevent disturbing circumferential stress fields from coming inside the two slots associated with the same transducer, but not so close that, in the case of magnetoelastic transducers, the magnetic flux is cut off, or, in the case of strain gauges, the measurement of the gauges is disturbed.

The load cell described above may be employed for more difficult load cases too, for example in cases where the measuring body tends to become twisted.

In this connection it should be pointed out that circumferential stresses may occur also if, instead of a measuring body in the form of a closed ring, a sector-shaped measuring body 31 is used, the arc length of which relative to its height is not negligibie. The measuring body 31 of such a load cell is shown schematically in Figure 3. Although, not shown in this Figure, the body 31 would be provided with a plurality of stress transducers, each flanked with substantially axially disposed slots passing right through the wall of the measuring body, as in the case of the load cell shown in Figure 2. Clearly, said slots have to be positioned sufficiently close to the associated transducer that the sector which is formed between the two slots is relatively narrow in relation to the height of the measuring body.

The invention may be applied to measuring elements of other types too, for example of oscillating string type.

Claims

1. A load cell comprising a measuring body in the form of at least a sector of a hollow cylinder for measuring a load acting on areas of the end surfaces of the measuring body and tending to compress the measuring body substantially in its axial direction, said measuring body being provided at a plurality of locations along the circumference of its arcuate wall with measuring devices for measuring the stress or strain in the measuring body caused by said load, and said wall being provided with substantially axially disposed through-going slots arranged on both sides of the measuring devices, which slots are arranged to screen the measuring devices from circumferential tensile or compressive stress which can arise in the measuring body depending on the position of said areas of said end surfaces to which said load is applied.

2. A load cell according to claim 1, in which the measuring devices are selected from the group consisting of magnetoelastic transducers, strain gauges and transducers of the oscillating string type.

3. A load cell according to claim 1 or 2, in which said slots pass substantially radially through said wall.

4. An annular load cell comprising a hollow cylindrical measuring body for measuring a compressive force acting on the annular end surfaces of said measuring body in the axial direction of the latter, said measuring body being provided at a plurality of locations around its circumference with measuring devices for measuring the stress or strain in the measuring body caused by said load, and said measuring body being further provided with substantially axially and radially disposed through-going slots arranged on both sides of the measuring devices, said slots being arranged to screen the measuring devices from circumferential tensile or compressive stress which can arise in the measuring body in the event of uneven loading in the radial direction of one or both of said annular end surfaces.

5. A load cell constructed and arranged substantially as herein described with reference to, and as illustrated in, Figure 2 or Figure 3 of the accompanying drawing.