GB2118312A - Load cell - Google Patents

Abstract

A load cell 2 to detect when the weight of an article, for example a fire extinguisher, is less than a predetermined value. The load cell has a chamber 6 filled with hydraulic fluid.The ceiling of the chamber is a rubber diaphragm 10 supporting a flat plate 20 on which a steel ball 22 rests continually on the same point on the plate. The ball is slidably guided vertically by the wall of an upright cylindrical tube 16 making mere line contact around the ball. The top end of the ball emerging from the tube carries a platform 34 on which the article is placed. Thus the weight of the article is transmitted through the ball to the fluid in the chamber so the fluid pressure is a function of the weight. The fluid pressure is observed by an instrument connected by a conduit 32 to the chamber. <IMAGE>

Description

SPECIFICATION Load cell This invention relates to a load cell ofthetype in which hydraulic pressure can be developed as a measure of an applied force, comprising a chamber of variable volume filled with hydraulicfluid, and said cham ber having a wa 11 partwhich is displaceable by theappliedforcetovarythechambervolume.

According to the invention a load cell of the type referred to, comprises a rigid substantially spherical element whereby said applied force is transmissible to said wall part, and the spherical element being guided at sides thereof for substantially rectilinear displacement of said spherical element.

The spherical element can be metal, for example, steel, ball which may be solid.

The chamber may be closed at one end by an elastic, for example rubber, diaphragm. The wall part comprises said diaphragm to which there may be applied a rigid plate externally of the chamberwhich is displaceable into the chamber with the diaphragm in response to applied force.

The spherical element may merely rest on the plate and be guided for substantially rectilinear displacement, substantially normal to the plate, by guide means spaced from the center ofthe sphere by a distance slightly largerthan the radius ofthe sphere.

For example the guide means may be a cylindrical passage of a radius slightly larger than that ofthe sphere. Thus only at circumferential portions of one diametral plane at any one time will the sphere contact the guide means, and then at most the contact will be line contact. Accordingly frictional resistance to displacement ofthe spherical element along the guide means is small.

The plate which may be flat, may extend across the displacement path ofthe spherical element which because it is guided makes contact at one end with the plate at substantiallythe same position on the plate no matter at what angle the force being measured (and directed towards the plate) is applied to the element.

The chamber may be part of a hydraulic system comprising a conduit connecting the chamberwith fluid pressure observing or detecting means adapted to provide a visual and/or other signal or indication as a function ofthe hydraulic pressure which is a function of the magnitude of the force applied to the sphere displacing the diaphragm. The fluid in the hydraulic system may be under pressure before any force is applied to the sphere. This pre-pressurising of the system may be of relatively low value, for example in the range of 140 gm/cm2to 700 gm/cm2 (substantially 2 Ibs/sq.inch to 10 Ibs/sq.inch).

The invention will now be further described, by way of example, with reference to the accompanying drawings in which: Fig. lisa vertical cross-section of an embodiment of a load cell formed according to the invention; Fig. 2 shows partly in section part of a hydraulic system which can be connected to the load cell in Fig.

1; Fig. 3 is afragmentary plan view showing a capto retain the spherical element in the load cell in Fig. 1, and Fig. 4 is a diagrammatic side elevation of the load cell in Fig. supporting afire extinguisher.

With reference to Fig. lea load cell 2 is shown having a rigid base plate 4 on which chamber 6, of circular cross-section is provided defined in a cyiindricai element 8 which is a one-piece machining with a side wall 8a and a base 8b welded to plate 4. Thetop ofthe chamber is sealed by a flexible rubber diaphragm 10 clamped to wall 8a by an annularflange 12which is secured to the wall by bolts 14 and provided on a lower end of a vertical cylindrical sleeve 16.An annular recess 18 in the underside oftheflange 12 accommodates a rigid, flat pressure plate or disc 20 which in the drawing merely rests on the diaphragm, though it may be secured thereto. The sleeve 16 has an internal diameter slightly largerthan the diameter of a solid steel ball 22 which is thus a sliding fitinthe sleeve and capable of moving vertically along the sleeve axis X.

The ball sits on the plate 20 making contact therewith at substantially the plate's centre Y and merely makes line contact (around the ball at the circumference of a diametral plane Z) with an inner surface 24 of the sleeve 16, which inner surface may be coated with lowfriction material. This low friction material may be plastics, for example nylon.

A ring-shaped cap 26 retainsthe ball in sleeve 16 whilst allowing an upper end E ofthe ball to project through the cap. The cap 26 can be formed by two semi-circular rings 26a and 26b (Fig. 3) secured together by horizontal screws 28 clamping the cap to an annularwaist30 in the sleeve 16.

Aconduit32 opening to chamber6 connects the latter with hydraulic fluid pressure detection and/or indication means The conduit 32 and chamber 6 are filled with hydraulicfluid at a desired pre-determined pressure.

This pressure exerts an upward force on the diaphragm and ball. When an external force having a downwards vertical component in excess of the upward force is applied to end E of the ball, the diaphragm is displaced downwards into chamber 6, by an amount which is a function of the applied force and displaces hydraulicfluid along conduit 32 to act on the pressure observing or detection means arranged to give a visual and/or audible and/or other signal.Thatsignal is a function of the applied external force, which may be the weight of an article. Accordinglythe load cell may be used to obtain a weight measurement.

Fire extinguishers comprising cylinders filled with fire-fighting orsmoothering gas, for example, HALON (Trade Mark), under pressure are deemed to have fallen below the required standard if, prior to use, the gas has leaked from the cylinderto an extent that the weightoftheextinguisherfallstoorbelowa pre-determined value.

To determine whetheror not such an extinguisher F (Fig. 4) is upto standard, it can be weighed on the load cell 2 by loading the extinguisher onto the end E ofthe ball 22. This may be done, for example, by permanently standing the upright extinguisher on a platform 34 welded or otherwise secured to end E ofthe ball. The platform which can rock a little since the ball can pivot about its centre, is restrained from excessive rotation, about axis X, by a depending peg 36 in an upstanding tube 38 secured to the base 4.

The conduit 32 is connected to a pressure gauge 40 (Fig. 2) through a block 42 containing a one-way valve 44, connected to the conduit and housed in a recess closed by a removable cover 46. To raise the hydraulic pressure in the conduit 32 and chamber 6 to the desired pre-determined value a fluid pump is connected to the valve 44 and operated to attemptto pump fluid into the system until the desired pressure value is attained.

The area of the diaphragm 10 exposed to chamber 6, the pre-pressure in the hydraulic system (before any weight is applied to the ball 22) and the calibration of the gauge40 are chosen sothatthegauge reading accurately shows when the weight of the extinguisher Ffalls below the pre-determined weight value.

The gauge may have a scale over which a needle moves to indicate detected weight. Alternatively as shown in Fig. 2, the gauge may have a needle carrying a flag of plate arrangement with at leastfirst and second coloured sections observable in turn through window 48. When the weight ofthe extinguisher F is above the desired minimum one colour is on view through window48. If the weight eventuallyfalls below the desired value the other colour comes into viewto indicate this. At the same time a pressure switch connected to the conduit 32 or actuated by a moving part (for examplethe needle offlag arrangement) of the guage can be operated to provide an electrical signal to operate othervisual and/oraudible warning means at, for example, some distant location.

The initial pressure in the hydraulic system and the size of the diaphragm are aiso chosen so that even when the weight of the extinguisher is belowthe desired minimum, the pressure plate 20 is still displaced below the ceiling of recess 18 so that the weight of the extinguisher is borne by the floating diaphragm. Then if the temperature to which the cell is exposed rises up to a pre-determined maximum for which the cell is designed, the resultant expansion of hydraulic fluid is accommodated for by raising the diaphragm and pressure plate in the clearance in the recess, and not (in the main) bytransmission ofthe fluid volume increase to the gauge to give a faulty overweight reading.

To further limit the increase in fluid volume due to thermal expansion, the volume of hydraulic fluid used in the system should be small. This can be achieved by using a narrow bore conduit 32 and a chamber 6 of small volume. In the case of the chamber, the volume can be kept small by forming the chamber base 8b with a central protrusion or plateau 8e occu pying a considerable part of the volume surrounded by the side 8a.

It will be appreciated that under all circumstances the ball never makes more than line contact with the surface 24. Therefore friction opposing ball displacement along axis E is kept to a minimum. Furthermore, the ball always transmits force to the pressure plate 20 at the centre Y of the latter. This reduces the risk of skewing the pressure plate 20 which otherwise might cause the plate to rub on the side ofthe recess and reduce the accuracy of the weight measurement. Also skewed pressure/plate 20 may so reduce the volume of one part of chamber 6 that there is an excessively high localised increase in pressure in that one part giving an inaccurately high weight measurement.

A plurality offire extinguishers may each stand on a respective load cell; each extinguisher being retained in a holder arrangement exemplified, for example, at H (Fig. 4) having surfaces, which touch sides of the cylinder of the extinguisher, formed of lowfriction material.

Ifthe load cell isto be used at lowtemperature, it may be preferred to use a silicone oil as hydraulic fluid.

If desired an openable and closable adaptor valve arrangement (not shown) may be provided leading through the wall 8a to the chamber 6. Thisvalve arrangement may be connected to a vacuum pump to evacuate airfrom the hydraulic system when the latter is being filled with hydraulic fluid, and may also be used as an alternative to valve 44 to introduce hydraulicfluid into the system.

Claims (12)

1. A load cell inwhich hydraulic pressure can be developed as a measure of an applied force, said load cell comprising a chamber of variable volume filled with hydraulic fluid, said chamber having a wall part which is displaceable by the applied force to vary the chambervolume, a rigid substantially spherical element whereby said applied force is transmissibleto said wall part, and the spherical element being guided at sides thereof for substantially rectilinear displacement of said spherical element.

2. A load cell as claimed in claim 1, in which said wall part comprises an elastic diaphragm in combination with a plate, and the element having a spherical surface portion contacting a face of the plate.

3. A load cell as claimed in claim 1 or claim 2, in which said element is based on a sphere having a great circle surrounded by guiding means whereby the element is guided substantially rectilinearly.

4. A load cell as claimed in claim 3, in which the guiding means is a cylindrical passage having a radius slightly greater than that of said great circle.

5. A load cell as claimed in any one preceding claim, in which said element carries platform means on a side of the element remote from said wall part.

6. A load cell as claimed in claim 5, provided with obstruction means to restrict swinging ofthe platform means.

7. A load cell as claimed in any one preceding claim, in which the spherical element is a metal ball.

8. A load cell as claimed in any one preceding claim in combination with means responsive to fluid pressure in the chamberto provide an indication correlated to said applied force.

9. The combination claimed in claim 8, in which the applied force is the weight of an article on the load cell.

10. The combination claimed in claim 9, in which the article is a fire extinguisher.

11. A load cell substantially as hereinbefore de scribedwith reference to Figs. 1 and 3 of the accompanying drawings.

12. Apparatus to measure the weight of a fire extinguisher, substantially as hereinbefore described with reference to the accompanying drawings.