The present invention relates to weighing machines.

BACKGROUND OF THE INVENTION

A conventional weighing machine is known comprising a base and a weighing platform separated therefrom and carried by elastic beams which deflect under load.

It is known that for such beams with two loading points that there is a definite point somewhere along their lengths which will always deflect by the same amount regardless of how a given total load is apportioned between the said loading points. This is known as the invariant point and, for a beam of uniform cross-section having symmetry about its centre in respect of the support and loading points, this invariant point lies at the beam centre, and, from its motion, the magnitude of the total applied load may be determined.

Platform-type machines are known in which two parallel spring beams of the type described are situated adjacent the parallel sides of the structure and particularly, in the inventor's co-pending application number PCT/GB89/ 01281 entitled "Weighing Machine" there is disclosed the use of a stiff bar spanning the invariant points of the springs, the motion of these points being averaged at the centre of said bar to a value from which the total load applied may be inferred.

SUMMARY OF THE INVENTION

According to the present invention there is provided a weighing machine comprising a base, spring beam supports

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arranged at selected positions on the base, at least two elastic spring beams arranged on the supports in crossed formation, a weighing platform resting on the spring beams such that the invariant deflection points of the beams due to the imposition of a load applied to any point of the platform, are in co-incidence at the position of cross-over of the beams, and means for measuring said deflection whereby to determine the magnituαe of an imposed load on the platform.

Preferably there are two separate spring beams in crossed formation each in the form of an elongate spring strip with the longitudinal axes of each strip at right angles to one another.

Advantageously, the two crossed beam spring may be held together at their invariant deflection points.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference to the accompanying drawings wherein;

Fig 1(a) is a perspective view of two elongate spring strips arranged in crossed formation, as an illustration of the theory behind the invention;

Fig 1(b) is an end view along one of the spring strips of Fig la to illustrate the deflection characteristics at the coincident invariant points;

Fig 2(a) illustrates means for uniting the two spring strips of Fig 1(a) at the coincident invariant points as would occur in practice;

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Fig 2(b) is a part sectional perspective view of further means for uniting the two spring strips of Fig 1(a) at their coincident invariant points;

Fig 3 is a plan view of a platform weighing scale incorporati the invention; and

Fig 4 is a cross-sectional view along the line A-A of Fig 3.

PREFERRED EMBODIMENTS OF THE INVENTION

The theory behind the present invention may be understood with reference to Fig 1(a) , which depicts two spring beams 1 and 2 arranged in crossed formation at right angles with their invariant points I vertically coincident and being supported at ends 3 and loaded at defined loading points 4.

The beams 1 and 2 are elastically similar in that equal loadings at the loading points 4 result in equal deflections, and equal pressures at the invariant points produce equal deflections thereat. In practice the spring beams 1 and 2 will most usually be geometrically identical, but it is possible to devise beams which are geometrically different but still possess the required elastic similarity.

The beams 1 and 2 are shown separated, see Fig 1(b) by a short distance as would occur if the lowermost beam 2 were the more heavily loaded. If equal and opposite forces are applied to the invariant points to cause their mutual approach till they touch, there will be superimposed deflections which are equal and opposite as a consequence of the stipulated elastic similarity.

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Since the invariant points have been moved by equal amounts, they are now in a position which is the average of the two original load-induced deflections and this average deflection provides a measure of the total of all the loads on the supported platform and may be converted to a visually sensible indication by any known means. The same result may be obtained with initially separated unloaded beams provided that the fastening means for uniting the beams maintains the initial separation distance at all times.

It is desirable to arrange for the various support and loading points to be simultaneously engaged or nearly so when the machine furnished with this type of spring system, for example a platform weighing scale, is not loaded. Inevitably there will be slight discrepancies owing to manufacturing inaccuracies, but this may be rectified by endowing the platform with sufficient dead weight or arranging for the base and platform to be permanently drawn together by additional spring means, this method being commonly used on personal weighing machines. The force required to secure the required total engagement is usually quite small in relation to the load measuring capability of the machine.

The angle at which the spring beams 1 and 2 are crossed may be chosen to suit convenience in other respects. It is also possible to use more than two spring if general design considerations indicate this to be desirable: the united invariant points always move to provide a measure of the total platform loading by averaging the otherwise unrestrained motion of all the springs.

To cater for situation where the uppermost spring beam 1 is not the more heavily loaded, the invariant points may be fastened together. With thin springs it will often be sufficient to use a spot weld.

In Fig 2 two further methods of connecting the spring beams 1 and 2 are shown. The first method illustrated in Fig 2(a) involves the provision of holes in the spring beams 1 and 2 at their invariant deflection points, to accept a connecting pin 5. The pin 5 is furnished with a domed end E for the purposes of engagement with an indicating device in the weighing machine in which the spring beams 1 and 2 may be mounted.

With spring beams of appreciable width and robustness it is also desirable to incorporate a small-diameter washer b to separate the spring beams 1 and 2 and eliminate effects which may arise from longitudinal twisting of the springs. The method has the advantage that it is not necessary to provide the ends of the spring beams 1 and 2 with locating features: it is sufficient to provide the base and platform of the weighing machine in which they are used with abutments closely to engage the sides of the spring beams at the support and loading points. The relative position of all the parts mentioned is then exactly determined.

The spring beams 1 and 2 may also be connected by a clip 7 of metal or a plastics material as shown in Fig 2(a).

A platform weighing machine incorporating the crossed spring beam arrangement shown in Fig 1(a) is illustrated in Figs 3 and 4.

The platform weighing machine comprises a circular base 8 provided with four upstanding projections 9 arranged at the ends of two diameters of the base 8 at 90° to one another.

The projections 9 form support points for crossed spring beams 1 and 2 as shown.

The crossed springs 1 and 2 are joined together at their coincident invariant point by means, for example, of the spring clip 7, shown in Fig 2, the domed head 10 formed on one limb of the U-shaped projection 7 constituting, the take-off point of the weighing scale.

A circular weighing platform 11 is mounted on the crossed spring beams 1 and 2 as shown, the weighing platform 11 having a peripheral depending side wall 12 joined by a flat plate 13 from which project four loading points 14 each positioned to sit adjacent the ends of the crossed spring beams 1 and 2, such that support points 9 and loading points 14, lie on respective circles of lesser and greater diameter, with the centre of the circles lying at the co-incident invariant point.

The top of the weighing platform 11 has an inclined peripheral wall portion 15 with respect to the vertical side wall 12 joined by a horizontal platform surface 16.

A dial glass 17 is formed in a section of the inclined wall portion 11 extending substantially half-way around its length.

A dial carrier 18 is attached to the peripheral side wall 12 and extends beneath the dial glass 17 and parallel to the inclined attitude of the dial glass 17.

A dial pointer 19 extends over the dial carrier 18 and is attached to a pinion 20 lying on the central axis of the weighing scale. A spring loaded rack 21 cooperates with the pinion 20 to cause rotation of the dial pointer

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- 1 - 19 in accordance with deflection of the coincident invariant point of the crossed spring beams 1 and 2 transmitted therethrough via a primary lever 22 pivoted at 23 on the plate 13 and in contact at one end with the takeoff point 10 and at the other end through a pivotally mounted bell-crank 24 attached to the rack 21, the point of pivot of the bell-crank being at 25.

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