AU661746B2 - Elastomeric load transfer element - Google Patents

Description

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AUSTRALIA

Patents Act 1990

ORIGINAL

1 COMPLETE SPECIFICATION STANDARD PATENT Invention Title: "Elastomeric Load Transfer Element" The following statement is a full description of this invention, including the best method of performing it known to me: St St CC C

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S CC CCI 4 I 'SCott S C i 3/4 etc ccC FIELD OF THE INVENTION This invention relates to means for the transfer of forces to a beam in load measurement devices of the single bending beam type. In a single bending beam load measurement device, load and reaction forces are applied to a beam in equilibrium by means of force transmission means spaced from one another in the longitudinal direction of the beam. At a selected region of the beam the strain therein is measured, which arises from the bending of the beam by said forces, this strain being proportional to the distances between the location of the strain measurement region and the force transmission means and between the force transmission means themselves.

15 In such devices the beam is in use subjected to bending out of its rest plane, such that the alignment of the beam with the original line of application of the forces changes, as does the distance between fixed points on the beam. Such behaviour is to be with that of the beams which are employed in other devices such as, for example, shear beam load cells and parallelogram beam devices where distortion of the beam does not involve bending along the length thereof. Examples of parallelogram beam devices are described in Laimins U.S. Patent 3,512,595 and Yamato Scale Company Australian Patent Specification No.

535,837.

Single bending beam devices are also subject to other modes of deformation as a result of imperfect loading conditions, and undergo, for example, Itwisting due to uneven load application and/or uneven support.

BACKGROUND ART The problem of transferring forces to such single bending beam structure is addressed in Australian 7174 4/44 -4- -3- Patent Specification 44956/85, and in that specification elastomeric elements are used in two different ways to achieve force transfer to the beam.

In one embodiment described in 44956/85, forces are applied to pins which are mounted in elastomeric sleeves within the beam, while in another embodiment, forces are applied to cylindrical elastomeric elements mounted above and below the beam.

SUMMARY OF THE INVENTION The present invention seeks to provide arrangements which yield improved performance in such bending beam weighing devices, at lower cost. In particular, an object of the invention is to provide means for force transfer which will enable the bending beam to be of 15 simple construction and of cheap material such as spring steel, which will require a minimum of preparation and machining of the beam itself and which will be tolerant of misalignment and the sort of abuse to which weighing devices are often subject in the field, without intolerable loss of performance.

This invention also relates to a weighing system which is particularly although not exclusively, suitable for use in the weighing of small animals, and a further object of the present invention is to provide a weighing system in which a load placed on a load receiving platform or otherwise over an extended area may be weighed with the use of a single strain gauge only, instead of requiring at least two strain gauges as in the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS I Fig. 1 shows schematically in side elevation, a single bending beam weighing device; Fig. 2 illustrates a load transfer element incorporating the present invention; 'Ii I z.z. z -4- Fig. 3 illustrates the results of a creep test; Fig. 4 illustrates the results of a dynamic test; Fig. 5 shows in plan view a bending beam weighing device incorporating load transfer elements of the present invention; Fig. 6 shows an end elevation of the device of Fig.

Fig. 7 is a sectional elevation taken on the line 7-7 of Fig. 5; and Fig. 8 is a fragmentary sectional elevation taken on the line 8-8 of Fig. A single bending beam weighing device is illustrated schematically in side elevation in Fig. 1. In this i arrangement, the bending beam 10 is coupled to 15 relatively rigid members 11 and 12 by means of force transfer devices 13, the arrangement being such as to allow the beam 10 to bend relative to the members 11 and 12, with the application of a load to one or the other of these members. In the case illustrated, load forces are applied to the member 11, and reaction zforces are applied to the member 12 to maintain the c system as a whole in equilibrium.

rcc, t. The force transfer means 13 must have the capacity to transmit to the beam 10, the forces which cause its bending, while generating a minimum of resistance to the bending of the beam 10 and a minimum of hysteresis and creep upon distortion, so that the strain in the beam which results from its bending, may accurately be I related to the forces applied to it. A typical application of such an arrangement is in weighing devices, where a load or part of a load to be weighed is applied to the member 11.

As load is applied, to the member 11, it will stay largely parallel/to the surface on which the structure

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is mounted, while the beam bends so that the point of attachment of the load transfer means to the beam moves in an arc. The devices 13 must therefore be capable of undergoing complex distortions without influencing the bending of the beam 10, and must be capable of accommodating compression, bending, torsion and shear, and must not exhibit non-linearity, creep or hysteresis to an extent which will destroy the usefulness of the device.

Rubber mountings have been used for a number of years in the application of loads to electronic loadcells, for the purpose of protecting the loadcell from shock loading which is otherwise a major cause of loadcell failure, and to absorb vibrations which which can otherwise lead to fatigue problems.

Such rubber mountings have been used with single point *loadcells employing parallelogram beams or shear beams, where the load receiving part of the cell i remains parallel to the fixed part of the cell as the load varies. Single bending beam loadcells, on the other hand, are normally subjected to a larger deflections in the direction of loading and therefore to greater changes in the distance between the point of receptionof the load and the original line of application of the load, (ii) a significant change in the angle between the beam at the region of force application and the load line, as that part of the beam moves in an arc rather than parallel to a direction at right angles 3to its rest plane.

The use of such rubber mountings with bending beam Sloqadcells therefore leads to levels of inaccuracy an I reainsparlle to he ixe par ofthecellas he -6which are quite unacceptable.

The invention stems from the realisation that the load transfer means in single bending beam devices must be regarded as an integral part of the the bending beam loadcell, in that the spring characteristics of the transfer means become part of the spring characteristics of the system, and combine to determine the effective characteristics of the system in terms of hysteresis, linearity and creep.

A load transfer element incorporating the present invention is illustrated in Fig. 2, and will there be seen to comprise a body of elastomeric material such as natural rubber, having the shape of a rectangular t prism with waisted sides 15. Bonded to each of the upper and lower surfaces of the body is a rectangular HI steel plate 16, to which is attached by, for example, I spot welding, a threaded stud 17. To provide a means for fine adjustment of the device in a manner to be described below, the studs 17 are offset from the true 20 centreline of the body, the distance A shown in Fig.

.2 being slightly greater than the distance B. To show the orientation of the body in view of this subtle Sasymmetry, notches 18 are formed in the plate 16 on tcc c the side to which the studs are closest.

f 25 Elements of this design are suitable for use in i bending beam devices in which the beam comprises a flat strip of spring steel. Typically, the beam will j be 40 mm wide and 10 mm thick, and of a spring steel commonly used for such applications as automotive leaf springs and therefore cheap and readily available.

I: For such a beam, transfer elements of natural rubber having a hardness of 60 Duro-A, 60 mm long, 20 mm high and 30 mm wide, with the waisted region 20 mm thick, have. been found to give excellent results. We have P 1 4 .r i -7experimented with elements of various hardnesses (for example by increasing the hardness to 65 Duro-A in order to increase the load capacity) and found that inferior creep and dynamic performance results.

To illustrate these effects, the graphs of Figures 3 and 4 show the creep and dynamic performance of three test elements. Element A was a standard commercially available vibration-isolating mounting, element B was an element having the form described in relation to Fig. 2 and of a modified natural rubber elastomer Duro-A, while element C was of the same shape and dimensions as B but made from natural rubber of hardness 60 Duro-A.

Fig. 3 shows the results of a creep test, which was 'C 15 carried out by installing the mountings on a test Sframe incorporating a beam of the kind described above, then exercising and stabilising the device.

The readings obtained are shown on the vertical axis as counts in 5000 for full scale.

9, 20 The test load was applied, and the device tared off after 5 seconds. Readings were then taken at the te indicated time intervals to 30 minutes, when the load was removed. After a further 5 seconds, the device was tared off again, and readings taken at the same 25 intervals. During relaxation, each of the materials a reversed the creep exhibited under load. It will be apparent from Fig. 3 that the beam using element C performed in a very superior fashion, with an error of only .12 after 1.5 min. The elements B exhibited more creep, and the commercial element was quite unsatisfactory, with a an error due to creep after minutes, of greater than 1 SIn the dynamic test of which the results are shown in I -8- Fig. 4, the same three mountings were placed in the test frame, exercised and stabilised. The test load was applied, and after 5 seconds the device was tared off. Readings were then taken up to 20 seconds, and the test frame was subjected to violent shaking for seconds. The reading at the end of this period of dynamics was taken at 30 seconds after the original application of the test load. The results demonstrate the marked superiority of the elements B and C over the known mounting A, and in turn the superiority under dynamic conditions, of the element C.

A consequence of the performance of load transfer elements of the present invention in conjunction with inexpensive steel bars is the ability to make a bending beam weighing machine with a capacity of 1000 kg and an accuracy better than 0.5 of reading, at a maximum manufactured cost of transducers which is less S" than one quarter of that of the transducers normally used for such applications.

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*r 20 Important to the success of these elements is the t choice of elastomer such that the spring characteristics of the element closely approximate j those of an ideal spring in terms of freedom from e creep, hysteresis and non-linearity. The material found to give the best approximation is natural a organic rubber with carbon black reinforcement.

S Also contributing to the success of such elements is the use of waisting of the sides, which firstly enables a reduction in the resistance of the element to bending while maintaining sufficient load-carrying Si capacity and secondly reduces the extent to which bending of the element produces tearing forces within the elastomer. This shape also facilitates the maintenance of the positign of the load line through -9the element during bending and is therefore a preferred feature of embodiments of the invention.

Bending beam weighing devices employing transfer elements of the present invention enable elegant design solutions in weighing equipment, as is exemplified in the weighing platform illustrated in Figs. 5 7. In this device, a pair of bending beams and 21 consisting of steel bars drilled or punched for the reception of studs 17, are provided with force transfer elements 13 constructed in accordance with Fig. 2. The device further consists of upper frame 23 comprising a horizontal web 24 and a vertical flange 25, and a lower frame 26 comprising a S' horizontal web 27 and a vertical flange 28, the 15 relative dimensions of the frames being such that the upper flange 25 exteriorly overlaps the lower flange 28 by a distance which is sufficient to enable normal operation, but providing lateral overload protection for the device by limiting lateral movement.

20 Each of the bars 20 and 21 is of course provided with a single strain gauge (not shown). Such a weighing frame can be manufactured with great economy.

Because of the offsetting of the studs 17 of the transfer elements 13 described above, rotation of the elements through 180o, provides a fine adjustment of the location of the points of application of force to the beam. This provides a means of calibration or optimisation of the performance of the system, so that the bars 20 and 21 may be manufactured without such fine tolerances as wi/1l destroy the economy of the design.

It will be appreciated from the above that the transfer devices of the present invention are not All, limited to the use of particular materials.

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Claims (2)

1. 2. Apparatus according to claim 1 in which both said load and reaction forces are so applied. or. #4
2. 3. Force transmission means according to claim 1 or claim 2 wherein said elastomer is natural rubber. DATED this llth day of January 1993 ARTHUR KELLENBACH Patent Attorneys for the Applicant: HALFORD CO. i is AB S T RACT For use in weighing devices of the kind employing strain measurement in a bending beam, a force transmission element comprises a rectangular prismatic body of elastomeric material such as natural rubber having waisted sides Fixing studs (17) are attached by means of plates (16) bonded to the upper and lower surfaces of the body. The material of the body is chosen to exhibit characteristics of creep, hysteresis and linearity approximating those of an ideal spring. A weighing device in which a distributed load is measured with the use of a single bending beam is also described. I c r I i i i:i