WO1993022632A1

WO1993022632A1 - Load measuring devices

Info

Publication number: WO1993022632A1
Application number: PCT/AU1993/000190
Authority: WO
Inventors: Arthur Kellenbach
Original assignee: Arthur Kellenbach
Priority date: 1992-04-30
Filing date: 1993-04-30
Publication date: 1993-11-11

Abstract

A strain gauge load measurement device consists of a beam in the form of a metal bar (40). A fixed load application point may comprise a rib (42), and longitudinally movable load application points may comprise rollers (47, 48) in grooves (44, 45, 46) of diameter larger than that of the rollers. A strain gauge (21) is located at the mid-point of the bar, and the device is embedded in elastomer (49). A three-point version is also described, and a load measuring pin.

Description

LOAD MEASURING DEVICES

FIELD OF THE INVENTION

This invention relates to bending beam load measuring devices. In such devices, the strain in a beam resulting from simple bending in response to an applied load is measured to determine the load. The load and the support forces necessary for the equilibrium of the beam are preferably applied at pairs of locations on the beam, and a strain gauge transducer is provided centrally of these locations. In an embodiment of the invention, a "three point" beam device is also disclosed.

BACKGROUND ART

As the load on a simple bending beam is increased, the relative distances between the load points on the beam will change slightly, since the bending of the beam changes distances along its original longitudinal axis. Where load points are in the form of knife edges or are otherwise fixed longitudinally, these offer considerable mechanical resistance to these changes of distance, and hence to the bending of the beam. This can lead to inaccurate measurements and to hysteresis effects in repeated weighings. One way of reducing those problems, by the use of elastomeric load transfer devices, is described in Australian Patent No. 590520. The present invention presents alternative approaches to the solution of this problem, in devices of great simplicity and robustness, using inexpensive materials and requiring little machining or other finishing.

A further source of inaccuracy in simple bending beam devices of the kind described above is the introduction of spurious strain from twisting of the beam as a result of off-centre or other asymmetrical loading. This problem is addressed in preferred embodiments of the invention.

SUMMARY OF THE INVENTION

In one aspect, the present invention resides in a bending beam load measuring device including a beam, strain gauge means located on the beam equidistantly between a pair of load receiving means and between a pair of support force receiving means, characterised in that at least one of said force receiving means is capable of movement in the longitudinal direction of the beam.

In another aspect, the invention resides in a bending beam load measuring device including a beam, strain gauge means located on the beam between a pair of support force receiving means, and load force receiving means located between said support force receiving means, characterised in that the load line of said load force passes through the beam substantially equidistantly between said support force receiving means.

In a further aspect, the invention resides in a load measuring pin comprising a substantially cylindrical pin body, a first pair of peripheral ribs on the pin body, strain gauge means located substantially midway between said ribs and orientated to respond to strain arising from bending of said body by forces normal to the plane of said flat, and means for the application of force midway between said ribs.

The invention will now be described by way of example, with reference to the accompanying drawings, in which several alternative embodiments are described.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a side elevation of a first embodiment of the invention; 5 Fig. 2 is a cross-section taken on the line 2-2 in Fig. 1; Fig. 3 is a cross-section taken on the line 3-3 in

Fig. 1; Fig. 4 is a fragmentary side elevation of a further 10 embodiment of the invention;

Fig. 5 is a cross-section taken on the line 5-5 in

Fig. 4; Fig. 6 is a fragmentary side elevation of a further embodiment of the invention; 15 Fig. 7 is a cross-section taken on the line 7-7 in Fig. 6; Fig. 8 is a fragmentary side elevation of a further embodiment of the invention; Fig. 9 is a cross-section taken on the line 9-9 in 20 Fig. 8;

Fig. 10 is a fragmentary side elevation of a further embodiment of the invention; Fig. 11 is a cross-section taken on the line 11-11 in Fig. 10; 25. Fig. 12 is a fragmentary side elevation of a further embodiment of the invention; Fig. 13 is a side elevation of a further embodiment of the invention; Fig. 14 is a force diagram illustrating principles of 30 the invention in a second aspect;

Figs. 15 and 16 are graphs showing the relationship between load line displacement and change in transducer output; Fig. 17 is a side elevation of an embodiment of the 35 invention in its second aspect; Fig. 18 is an end elevation of the embodiment of Fig.

17; Fig. 19 is a side elevation of a further embodiment of the invention; Fig. 20 is an end elevation of the device.illustrated in Fig. 19; Figs. 21 to 31 are fragmentary views showing alternative techniques for force application; Fig. 32 is a side elevation of a further embodiment of the invention; and

Figs. 33 and 34 are fragmentary views showing an example of the application of devices incorporating the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the embodiment of the invention illustrated in Fig. 1, a weighing device using a simple bending beam consists of a beam in the form of a steel bar 60 of rectangular cross-section. The bar 140 is provided with a strain gauge transducer 41, located in this example on the underneath surface of the bar and centrally of its length, to produce an electrical signal representative of the strain in the bar 40 resulting from bending of the bar out of its plane.

The bar 40 is also provided with a lateral load-receiving rib 42, which in this example is of semi-cylindrical cross-section. Spaced from the rib 42 so that it and the rib 42 are equidistant from the longitudinal centre of the beam shown by the broken line 43, is a laterally directed semi-cylindrical groove 44.

Also equidistant from the centreline 42 are laterally directed cylindrical grooves 45 and 46 in the lower surface of the bar 40. Resting in the groove 44 is a pin 47, and in the grooves 45 and 46 are pins 48, all the pins in this embodiment being of cylindrical shape. The radius of the pins 47 is less than the radius of the grooves in which they are located, so each of the pins is capable of limited rolling movement within its groove.

The device thus far described is in this form encapsulated in an elastomeric material 49 such as rubber, the thickness of the encapsulation being such that the outer extremity of the surface of the pins 47 and 48 and the rib 42 are substantially at the surface of the encapsulating material.

In use, the illustrated device is placed on a supporting surface, or within a supporting and enclosing region, and the load to be weighed is rested on the rib 42, which provides a fixed point of load application, and the pin 47. As the load is applied and the bar 40 bends slightly, the pins 47 and 48 will roll relative to the surface of the grooves 44, 45 and 46, while the rib 42 will provide a fixed point of support for the load. In this way, the small changes in length referred to above will be accommodated without the production of spurious strain in the bar and without the degree of hysteresis which would otherwise be encountered. The elastomeric encapsulation 49 retains the pins in their grooves and provides a light centring force upon removal of the load.

Illustrated in Fig. 4 and the cross-sectional view in Fig. 5 (taken on the line 5-5 in Fig. 4) is an alternative approach to the provision of the fixed load application point, represented by the rib 42 in Fig. 1. In Figs. 4 and 5, this is replaced by a cylindrical pin 51, operating in a V-shaped groove 50.

Figs. 6 and 7 show a cylindrical pin 52 operating within a groove 53 which is convex in the transverse direction. Such an arrangement, substituted for the grooves 44, 45 and 46 of Fig. 1, provides a degree of lateral accommodation which reduces the effects of off-centre loads referred to above. Where such an approach is used, it is preferred that one of the load receiving means, conveniently the rib 42, has no convexity, to provide a full load-bearing line of support.

Figs. 8 and 9 illustrate an arrangement which is an alternative to that shown in Figs. 6 and 7, where a barrel-shaped pin 54 co-operates with a cylindrical groove 55, to provide similar lateral accommodation.

It will be clear that other permutations of the elements in question are possible. For example, the full load receiving line which is preferred could be provided by the pin 47 of Fig. 1, while the rib which forms the other load-receiving element may have a curved upper surface, as shown in Figs. 10 and 11, where such a rib 56 is shown.

While it is believed to be preferable that one of the load application points be fixed in the sense of not comprising a rolling device, this is not believed to be essential.

Other variations in the construction of weighing devices utilising the design principle described herein may be made. For example, where it is desirable to accommodate greater relative movement between the bar and a pin than is freely provided by a groove, the pin may be located on a flat surface region of the bar as illustrated in the case of pin 57a in Fig. 12.

While the devices illustrated so far have four points of force application, this is not essential. An example of an alternative configuration is shown in Fig. 13, where a three-point device comprises a longitudinally fixed load receiving pin 57 mounted in a v-shaped groove 58, and the support force receiving points comprise barrel-shaped pins in part-cylindrical grooves 59 of greater radius.

Such a three point device for bending beam strain-based load measurement will now be examined in more detail. The device can be viewed generally as load measuring apparatus for the measurement of a single load force, applied to a bending beam between its points of support.

Fig. 14 illustrates the basic configuration with which the aspect of the invention is concerned. In this figure, a load force L is applied to a beam which is simply supported adjacent each end. It can readily be shown that the bending moment at the centre of the beam, measured by means of a strain gauge assembly (not shown in Fig. 14) is proportional to the product of the distances A and B between the load line of the force L and the points of support of the beam. The maximum bending moment at the centre of the beam, or in other words the output of the strain gauge there located, will occur, for a given load, when the load is located at the point midway between the points of support. Should, due to manufacturing tolerances or errors or to misalignment of the apparatus or of the applied load, the line of application of the load move to one side or the other of the centre point, an error will be introduced into the load measurement. The aspect of the invention presently under consideration proceeds from the observation that in a configuration such as that shown in Fig. 14, provided the point of application of the load is close to the centre point, apparently significant errors in its location will not give rise to intolerable errors in the measurement of the load.

Since the strain at the centre of the beam is proportional to the product of the distances between the load line and the respective supports, the relationship between the error in the strain gauge output and the displacement of the load line will be parabolic. Figs. 15 and 16 show this relationship on two scales, for the case of a bending beam the nominal length of which is 200mm. In these Figures, the change in transducer output for a given load is plotted against displacement of the load line from the point midway between the points of support of the beam.

It will be observed that for displacements of the load line up to 5mm, only a very small change will occur in the transducer output, in this case 0.25% for a 5mm (5%) displacement.

In accordance with this aspect of the present invention therefore, a three-point bending beam load measuring apparatus is characterised in that the load to be measured is applied to a bending beam in the region of the midpoint between the points of support of the beam and the load is measured by the strain produced in the beam by the bending thereof due to the load.

The accuracy with which the load application point is located, and the accuracy with which the load line of the applied force is controlled, will of course depend on the design parameters of the equipment. Apparatus of this aspect of the present invention is characterised, however, by the fact that the load application point is substantially at and at least within a range from the midpoint between the support locations which provides the degree of accuracy required for the application in question.

Since there are many applications for weighing equipment where measurement of load within one or two percent is adequate, the recognition that this can be achieved by a simple bending beam device opens the way for the production of low cost and robust devices. Cost reduction can be achieved by eliminating the need for machining, heat treating and other expensive processes.

The manner in which the load force to be measured, and the supporting reaction forces, are applied to the beam may take any suitable form, and will vary widely from one application to another. While the techniques illustrated in Figs. 1 to 12 in connection with elastomer-encapsulated four-point devices will be preferred, other approaches may be used, and elastomer encapsulation is not essential. By way of example, the Figures 17 to 30 illustrate some suitable approaches, by no means exhaustively.

Figs. 17 and 18 show, respectively in side and end elevation, a bar 20 provided with a lateral load force receiving rib 60 and a pair of support ribs 61 located equidistant from the rib 60.

In a modified form of this device illustrated in Figs. 19 and 20, the ribs 61 are laterally convex to provide

an arcuate footing, to minimise the effect of off-centre loading or uneven surfaces. Any suitable mix of straight or arcuate ribs of this kind may be provided.

In Fig. 21, the load force to be measured is applied to the beam 20 by means of an elastomer load transfer element 71.

In Fig. 22, the load is applied to the beam 20 by means of a pin 72 which passes through the beam.

In Fig. 23, an elastomeric sleeve 73 is interposed between the beam 20 and the pin 72 in a construction otherwise similar to that shown in Fig. 22.

In Fig. 24 the force is applied to a cylindrical force receiving pin 74.

In Fig. 25 the load force is applied to a pin 75 which is embedded in an elastomer body 76.

In Fig. 26 the force is applied to a cylindrical pin or roller 74 which rests within a part-cylindrical groove in a surface of the beam 20. Preferably, the diameter of the groove is slightly greater than that of the pin. This configuration has been found to provide a particularly hysteresis-free device, as the pin can roll relatively to the beam as the distance between the grooves changes slightly with bending.

In a manner analogous to the ribs of Figs. 19 and 20, one or more of the rollers 74 in a device according this embodiment, may be provided with a diameter which increases from each end toward its centre. Alternatively, the depth of one or more of the grooves may be laterally convex. In Fig. 27 the force is applied to a ball 77 which rests in a part-spherical depression in the beam 20.

In Fig. 28 the load is applied to a rigid, force receiving element 78 fixed to the beam.

In Fig. 29 the force is applied to the beam 20 by means of a housing 79 separated from the beam by an elastomer body 30.

In Fig. 30 the force is applied to a pin 81 extending from one side of the beam 20, the pin 81 being rigidly fixed to or received by a structure which is not shown.

In Fig. 31 the force is applied to a pair of pins, which are sufficiently close that the effect of the spread of the force application is small, enabling weighing to be achieved within the desired accuracy. It will be appreciated that devices other than pins may be used in such a configuration, and that three or even more load application points may be used in this way.

It is to be understood that the supporting reaction forces may be applied to the beam by any of the means described herein in relation to the applied load, and that various combinations of types of support may be employed to suit the particular application of the equipment.

In the device illustrated in Fig. 32, the load application points 82 and the points of support 83 are formed by laterally extending rounded ribs protruding from the surface of the bar 20. The device illustrated in Fig. 32 differs importantly from the previous embodiments, in consisting of a series of integral three-point load measuring devices. Consideration of Fig. 32 will show that between each successive support rib 83 there is a device of the kind described above, so that by the placement of strain gauges on each of the sections of the member 20, the total load applied can be measured as the sum of the loads on the individual sections. While the device of Fig. 32 consists of three such sections, it will be appreciated that any number of sections may be provided.

Figs. 33 and 34 illustrate a simple application of devices of the present invention, to weighing frames. In this embodiment, a weighing frame consists of a load carrying frame 85 constructed from welded steel angle to provide a horizontal web 86 and a vertical flange 87, and a floor support frame 88 similarly comprising a lower floor contacting web 89 and a vertical flange 90.

At each corner of the lower frame 88 a pocket is formed by a wall member 91 and a three-point load measuring device of the kind illustrated in Fig. 13 is placed in each of these pockets, as shown in Fig. 33. The pocket is closed by an end wall 92 downwardly depending from the web 86 of the load carrying frame 85, and the web 86 rests upon the load receiving roller 82 of each weighing device.

The upper and lower frames are loosely fixed together by pins 93 which are passed through aligned apertures in the flanges 87 and 90. The diameter of the pin and the diameter of the flanges is chosen to provide sufficient freedom of relative vertical movement between the frames 85 and 88 to accommodate deflection of the member 20. The new approaches to bending beam devices explained above can be applied to weighing devices in the form of load-bearing pins. Such devices in the prior art consist of a hollow pin with a number of strain gauges distributed on its inner surface, and load-receiving areas on its outer surface, separated by peripheral grooves. The present invention provides an alternative approach.

Devices constructed in accordance with this aspect of the invention will find application in areas where an accuracy in the region of 98% is acceptable in a robust device which can be incorporated into the structure of transport equipment including trucks and trailers, materials handling equipment, and other apparatus subjected to shock loading and hostile environments.

The cross-sectional view in Fig. 35 shows a load weighing pin incorporating the invention mounted in a load receiving and support structure. The pin 110 comprises a cylindrical body of suitable steel or other material, and is provided with an inner pair of peripheral ribs 111 and an outer pair of peripheral ribs 112. The ribs 111 and 112 are disposed symmetrically about the mid-point of the device, and a e radiused in cross-section to present a rounded line contact with the structure described below.

A flat 113 is machined on the lower surface of the pin in the central region thereof, for the reception of a strain gauge assembly 114, which is arranged to respond to strain arising from bending of the pin by a load applied as described below. A drilled passage 115 allows for electrical access to the strain gauge 114 from the end of the pin. The outer ribs 112 are located in apertures 116 in a support structure 117, while the inner ribs 111 are located in passage 118 in a load-bearing structure 119.

To ensure that the flat 113 remains horizontal, so that the strain measured is properly representative of the load on the ribs 111, orientation means are preferably provided, such as a key 120 fitting a keyway 121 in one end of the pin 110 and engaged with the structure 117. Alternative arrangements, for example a protruding key on an end of the pin 110 cooperating with a pair of locking plates attached to the supporting structure. Such a key should be vertically orientated to avoid exerting vertical forces on the pin which may give false readings.

While the use of simple radiused ribs 111 and 112 as the load transfer points for a bending beam weighing device would normally be shunned as leading to unacceptable inaccuracy and hysteresis arising from friction between the opposed surfaces, it has been found that in a device subjected to dynamic loads involving shock loading, such hysteresis effects are not a problem, within the level of accuracy required, apparently because the continual load changes prevent the "sticking" of the load application points.

The invention is capable of embodiment in an even simpler form, employing the above teachings of three-point bending beam load measuring devices. Such an embodiment is illustrated in Fig. 36, where a bolt 121 is provided with three radiused peripheral ribs, comprising a single centre rib Ilia and a pair of outer ribs 112. The centre rib Ilia is partially relieved at the location of the central gauging flat 122, and as in the case of the Fig. 35 embodiment, means such as a key (not shown) are provided to ensure the correct orientation of the pin.

Such a three-point device may be used in situations where the accuracy obtainable with such a device, as explained above, is acceptable.

It will be appreciated that the embodiments described above are given by way of example, and the invention is capable of embodiment in other forms. For example, the load application points need not be in the form of continuous peripheral ribs, but may comprise formations located only at the region of load transfer.

The materials used in the devices described will be chosen for the suitability of their properties. The bars and the pin may suitably be of steel of suitable hardness and tensile strength.

Claims

CLAIMS :

1. A bending beam load measuring device including a beam, strain gauge means located on the beam equidistantly between a pair of load receiving means and between a pair of support force receiving means, characterised in that at least one of said force receiving means is capable of movement in the longitudinal direction of the beam.

2. A device according to claim 1 further characterised in that at least one of said force receiving means is incapable of movement in the longitudinal direction of the beam.

3. A device according to claim 2 further characterised in that only one of said force receiving means is incapable of movement in the longitudinal direction of the beam.

4. A device according to any preceding claim further characterised in that the force receiving surface of at least one of said force receiving means is laterally convex, and of at least one has a constant height in the lateral direction of the beam.

5. A device according to claim 4 further characterised in that the force receiving surface of at all but one of said force receiving means is laterally convex.

6. A device according to any preceding claim further characterised in that at least one of said force receiving means capable of movement consists of a substantially cylindrical roller located within a substantially cylindrical lateral groove in a surface of the beam, the diameter of said groove being greater than the diameter of the roller.

7. A device according to claim 6 further characterised in that the or at least one said roller is barrel-shaped.

8. A device according to claim 6 further characterised in that the wall of the or at least one of the substantially part-cylindrical grooves is laterally convex.

9. A device according to any preceding claim further characterised in that the beam and said force receiving means are embedded in an elastomeric body.

10. A device according to any one of claims 6 to 8 further characterised in that the beam and said force receiving means are embedded in an elastomeric body, said body retaining the or each said roller within its groove.

11. A device according to claim 9 or claim 10 further characterised in that a surface of said body substantially coincides with the outer surface of said force receiving means.

12. A device according to any one of claims 2 to 11 further characterised in that the or each load receiving means incapable of movement consists of a lateral rib on the beam.

13. A device according to claim 12 further characterised in that the load bearing surface of the or at least one rib has a constant height in the lateral direction of the beam.

14. A device according to claim 12 further characterised in that the load bearing surface of the or at least one rib is laterally convex.

15. A device according to any one of claims 2 to 11 further characterised in that the or each.load receiving means incapable of movement consists of a substantially cylindrical roller located within a V-shaped lateral groove in the beam.

15. A bending beam load measuring device including a beam, strain gauge means located on the beam between a pair of support force receiving means, and load force receiving means located between said support force receiving means, characterised in that the load line of said load force passes through the beam substantially equidistantly between said support force receiving means.

17. A device according to claim 16 wherein said beam comprises a metal bar, at least one of said support force receiving means consisting of a lateral ridge on the bar.

18. A device according to claim 17 further characterised in that said ridge is laterally convex.

19. A device according to claim 18 wherein said load force receiving means consists of a lateral ridge on the bar.

20. A load measuring pin comprising a substantially cylindrical pin body, a first pair of peripheral ribs on the pin body, strain gauge means located substantially midway between said ribs and orientated to respond to strain arising from bending of said body by forces normal to the plane of said flat, and means for the application of force midway between said ribs. 21. A pin according to claim 20 wherein said means for the application of force comprises a second pair of peripheral ribs on the body and located equidistantly between the first pair of ribs.

22. A pin according to claim 20 wherein said means for the application of force comprises a single peripheral rib on the body and located equidistantly between the first pair of ribs.

23. A pin according to any one of claims 20 to 22 wherein said ribs are of radiused cross-section.

24. A pin according to any one of claims 20 to 22 wherein said strain gauge is mounted on a flat formed in said body .

25. A pin according to claim 24 further comprising means for engagement with a supporting structure to maintain the orientation of the plane of said flat normal to the direction of the applied load.

26. A pin according to claim 25 wherein said means is a keyway in an end of said pin.

27. A load measuring device comprising a pin according to any one of claims 20 to 26, a support structure apertured to receive said pin and engage said first pair of peripheral ribs, and a load bearing structure apertured to receive said pin and engage said second pair of ribs or said single rib.

28. Load measuring apparatus substantially as described herein with reference to the accompanying drawings.