GB2126357A - Load-measuring devices - Google Patents

Abstract

A ring-shaped load-measuring device distorts under axial load so that it remains substantially circular in plan but takes up a circumferentially- undulating shape, the ring (1) being circular in cross section. Strain gauges (6,7,8,9) on the ring give an indication of the load. The load-transmitting members that transmit the distorting forces to the ring include balls or the like that make substantially point contact with the surfaces that they bear against. The balls may be located within cradles fixed to the casing within which the ring is mounted, so that the balls bear directly upon the ring: the reverse arrangement is possible in which the ball-locating cradles (2,3) are located on the ring and the exposed faces of the balls (5) bear against the casing. <IMAGE>

Description

SPECIFICATION Improvements in or relating to load.measuring devices This invention relates to load-measuring devices, and in particular to the known type of ring-shaped device for measuring the loads set up in engineering equipment such as a rolling mill. The ring is interposed between two adjacent components in the suspension of the rolls, coaxiallywith the axis of the load which is to be measured. A set of small load-transmitting members, spaced at regular intervals around one of the axially-facing surfaces of the ring, separates the ring from one of the components.

A second set comprising the same number of like members, spaced at the same intervals but angularly staggered relative to the first set, separates the opposite axially-facing surface of the ring from the second component. When the two components tend to converge under load the two sets of loadtransmitting members therefore tend to converge also, so distorting the ring so that it remains substantially circular when viewed in plan, but presents a continuously undulating appearance to an observer who passes around the outside of the ring and looks radially inwards. From readings of strain gauges, bonded to the surface of the ring, the axial load between the two components may be determined.

US Patents Nos. 3636760 and 4089216 show examples of rings of this kind. In all of these examples the ring is of substantially rectangular cross-section, and the load-transmitting members that make contact between the ring and the two components to either side of it are also substantially rectangular in shape, when viewed parallel to the axis of the ring, and are integral with or bonded to the ring itself. The rings shown in published UK Application No.GB 2036344A are of rectangular cross-section also, but the load-transmitting members are of truncated-sector shape when viewed in a direction parallel to the axis of the ring: this specification also discloses the option that the two sets of load-transmitting members, instead of being fixed to the ring, may be mounted in two different parts of a casing within which the ring is mounted, so that as the two components converge those two parts move similarly and the members mounted on them bear against a plain rectangular-section ring to which strain gauges are bonded, distorting it so that the gauges give a reading from which the applied load may be calculated.

While it is not difficult to visualize the general shape to which such a rectangular-section ring is distorted under load, the precise shape is harder two define. Each element of the ring may be considered as being subject to both a force and a moment related to each of three axes passing through that element: one axis lying radial relative to it, a second tangential, and the third axis lying at right angles to the other two. Under this complex of forces, what were initially the flat surfaces of the axially-facing faces of the ring acquire a very complicated contour.

This complex twisting of the ring under load means that whereas initially each rectangular or sectorshaped load transmitting member made face-to-face contact with whatever was bearing against it (i.e.

with the ring if the member is mounted in part of the ring casing, or with one of the converging components if mounted on the ring), nevertheless after the ring has twisted that member will be canted about a radius. In consequence the contact will become edge-to-edge instead of face-to-face, and the application of the load to the member will inevitably change as the load changes. The transition from one such mode of contact to the other will be accompanied by changes in the transmission of load and moments by that particular member and these changes cannot be predicted or observed with accuracy. Further, the load-transmitting members will distort in response to the distortion of the ring as the load and moments change, and will thus transfer part of their stiffness to that of the ring.Consequentlythere will always be non-linearity and some inaccuracy in whatever calibration is made to match applied load to the readings ofthe gauges.

The present invention seeks to provide firstly a ring type gauge which distorts in a less complex fashion when subjected to load. Secondly, and preferably, such a gauge in which the mode of contact between the load-transmitting members and those surfaces that they bear against is less subject to the kinds of unpredictable changes just described.

According to the invention in its broadest aspect, a load-measuring device includes a ring adapted in use to distort to the undulating shape already described and the ring is of circular cross-section, so that the ring is basically a torus, preferably solid.

According to the second aspect of the invention, the load-transmitting members associated with such a ring make substantially point contact with the surfaces that they bear against. The members may include parts which are separate both from the ring and from the component from which they are transmitting force to the ring. The parts may be in the form of balls which may for instance rest in part-spherical cavities formed in cradles. The cradles may be fixed to the ring, in which case the exposed surfaces of the balls will bear against the components that are converging upon the ring. Alternatively the cradles may be mounted upon parts of a casing within which the ring is mounted, so that the exposed surfaces of the balls bear against the ring.

The invention is also defined by the claims, the disclosure of which is to be read as part of the disclosure of this specification, and will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which: Figure lisa plan view of a ring; Figure 2 is a section on the line ll-ll in Figure 1, omitting all but a single example of cradle and each type of strain gauge, and Figure 3 is an exploded perspective view of an alternative device.

Figure 1 shows a solid toroidal ring 1. Eight cradles 2 are welded to the upper surface of the ring in regular angular spacing, and another eight similar cradles 3 are welded to the bottom surface of the ring, also in regular angular spacing but staggered relative to the locations of cradles 2. A hemispherical cavity 4 is formed by end-milling in each of cradles 2, 3to receive a ball 5 (Figure 2): cavity and ball are nominally of equal radius so that the practical clearance between them is slight. Strain gauges 6 and 7, of electrical resistance type, are bonded to the upper and lower surfaces of ring 1 as shown, and similar strain gauges 8, 9 are bonded to the inner and outer surfaces of the ring.When the ring is now interposed between two loaded components, so that one component bears upon the balls held in the cavities of cradles 2 while the other bears in the opposite direction upon the balls carried by cradles 3, the ring will tend to distort to undulating shape and if strain gauges 6 - 9 are appropriately connected in known manner (for instance as shown in Figure 3 of UKApplication No. GB 2036344A) a measure of the applied load may be derived from the resulting output signal of the circuit. As explained in Application No. GB 2036344A, the contribution of gauges 6 and 7 to the output signal will tend to indicate the full degree of distortion of the ring, while the output of gauges 8 and 9 will compensate forthat part of the total distortion that is due to factors - for instance temperature variation around the perimeter of the ring - that are not due to the applied load.

In one such ring, already tested, the diameter of the torus centre circle was 6.5 inches (165 mm) and the diameter of the circular section of the torus was 1.146 inches (29.11 mm). Each of cradles 2,3 was of rectangular outline when viewed in a direction parallel to the axis of the ring, the radial dimension being 0.625 inches (15.87 mm) and the dimension normal to the radius being 0.5 inches (12.7 mm). The cavities 4 were 0.375 inches (9.53 mm) in diameter, and the balls 5 fitted within them were nominally of the same diameter Electric resistance strain gauges 6 - 9 were of normal foil type and the ring 1 was made of a high-grade steel of composition 0.4%C, 0.3%Ni, 0.6%Mn, 1.0%Mo, 0.2%V, and 3.25%Cr, balance Fe, suitably heat-treated to the required hardness and toughness.The ring was calibrated to measure applied loads up to 50kN.

Not only does the circular section of such a ring offer the advantage, over conventional rectangularsection rings, of a simpler pattern of distortion under applied load and thus simpler calibration when used as a load cell; in addition the pattern of contact between balls 5 and cradles 2,3 and the opposed surfaces of the loaded components will aiways be the same. Substantially point contact will exist at all times, and the fact that the distortion of the ring will cause the location of the various points to change slightly is of little consequence.

in the alternative apparatus shown in Figure 3 the ring 10 is a plain ring mounted within a casing in two parts 11,12. A key 13 on part 11 engages with a slot l4on part 12, and another key 15 on part 11 engages with a slot 16 in ring 10, so that the two parts of the casing and the ring are all free to slide relative to each other in a direction parallel to their common axis 17. Strain gauges (not shown) are bonded to ring 10 in a manner similar to that in which gauges 6 - 9 were bonded to ring 1. However, instead of the cradles being bonded to the ring as before, sectorshaped cradles 2a formed with cavities 4 are now mounted on part 11 and similar cradles 3a on part 12 of the casing. When the casing is now interposed between two components urged together by an axial load, the load is transmitted to the casing so that parts 11,12 tend to move axially together. The balls carried by cradles 2a and 3a therefore bear against the plain ring 10 in opposite directions and distort it to undulating shape.

Claims (7)

1. A ring-shaped load-measuring device, adapted in use to distort under an axial load so that it remains substantially circular in plan but undulates circumferentially, and in which strain gauges associated with the ring distort with it so that their output may be used to give an indication of the load, and in which the ring is of substantially circular outline when viewed in radial section.

2. A load measuring device according to Claim 1 in which the ring is of solid radial section.

3. A load measuring device according to Claim 1 including load-transmitting members associated with the ring to transmit to it the forces that distort it in use, and in which those members are adapted to make substantially point contact with surfaces that they bear against.

4. A load measuring device according to Claim 3 in which the load-transmitting members are in the form of balls located in part-spherical cavities formed in cradles.

5. A load measuring device according to Claim 4 in which the cradles are fixed to the ring.

6. A load measuring device according to Claim 4 in which the cradles are mounted upon parts of a casing within which the ring is mounted, so that the exposed surfaces of the balls bear against the ring.

7. A load measuring device according to Claim 1, substantially as described with reference to the accompanying drawings.