GB2110392A

GB2110392A - Overload protection device

Info

Publication number: GB2110392A
Application number: GB08135437A
Authority: GB
Inventors: Jan Ferdinand Oortgijsen
Original assignee: Maatschappij Van Berkels Patent BV
Current assignee: Maatschappij Van Berkels Patent BV
Priority date: 1981-11-24
Filing date: 1981-11-24
Publication date: 1983-06-15

Abstract

A load cell comprising a guided- beam transducer 11 having a fixed member from which a pair of deformable beams 13A, 13B extend to support a deflectable structural member 14 is provided with an overload protection device. This comprises U-shaped member 17 secured to the weight-receiving member 15 and roller-bearing arrangements 18A, 18B so that member 17 is slidable with respect to member 14. Device 16 further comprises spring 22 effectively located between members 14 and 15 to bias these members apart and pin 20 secured to member 14 and protruding through slot 19 in member 17 in order to limit relative movement of members 14 and 17. Device 16 is assembled so as to be rigid with respect to loads within the tolerances of the cell but resiliently collapsible with respect to greater loads. Bearing arrangements 18A, 18B render protective operation of device 16 substantially independent of the position of the load on the weight- receiving member 15. <IMAGE>

Description

SPECIFICATION Overload protection device The invention relates to an overload protection device for a load cell.

Load cells have been used in the electronic scale industry for the weighing ofgoods and usually comprise a cantilevered structural member which deforms or deflects when loaded with a weight, the magnitude of deflection or deformation being measured by a sensor to provide a measure of the weight of goods. The satisfactory operation of such a load cell is dependent upon the unknown weight imposing a deformation or deflection within the tolerances of the cantilevered structural member and the sensor and since this cannot be guaranteed it has been proposed to provide an overload protection device for the load cell.

One known form of overload protection device simply consists of a stop located in the path of movement of the structural member whilst another known form of overload protection device, described in U.S. Patent Specification No.

4254841, provides a spring-loaded bracket assembly between the load cell and the member on which the unknown weight is placed, the assembly being rigid with respect to loads within the tolerances of the load cell and sensor but resiliently collapsible with respect to greater loads.

In both these known forms of overload protection device successfui protective operation is dependent upon the unknown weight being imposed on the weight-receiving member centric with respect to the structural member of the load cell. However, in practice the weight-receiving member is usually in the form of a platter very much greater in surface area than the structural member of the load cell -- typically by a factor of 50 to 1 - and eccentric loading is therefore a probable occurrence. We have also ascertained that eccentric overloads give rise to additional longitudinal and torsional deformations which are not restricted by the operation of either of the known forms of overload protection devices discussed above.

It is an object of the present invention to provide an overload protection device for a load cell which is effective to provide protection against both centric and eccentric overloads.

According to the present invention there is provided a load cell having a deformable or deflectable structural member in combination with a weight-receiving member and an overload protection device effectively located between said members, wherein said device comprises a springloaded assembly which is rigid with respect to loads less than a predetermined value and resiliently collapsible with respect to loads equal to or greater than said predetermined value, a roller bearing arrangement being provided in said device to render the resiliently collapsible movement thereof substantially independent of the position of the load on the weight-receiving member.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a side elevation view of a load-cell according to the present invention; Fig. 2 is an underneath plan view of Fig. 1; Fig. 3 is a part sectional view taken on A-A in Fig. 2; Fig. 4 is a part sectional view taken on B-B in Fig. 2; and Figs. 5A, 5B and 5C illustrate alternative constructions of a detail.

In the drawings, the load-cell 10 comprises a guided-beam transducer 11 having a fixed member 12 from which a pair of deformable beams 1 3A, 1 3B extend to support a deflectable structural member 14, sensors (not shown) being mounted on the beams 1 3A, 1 3B in a manner known per se effectively to measure the extent of deflection of member 14 when subjected to a weight or force F in the direction illustrated.

Weight or force F is imposed on a weightreceiving member 1 5 which transmits the weight or force to member 1 4 via an overload protection device 16.

The device 1 6 as can best be seen from Fig. 2 comprises a U-shaped component 1 7 secured to member 1 5 and having legs 1 7A, 1 7B which embrace the lateral surfaces 1 4A, 1 4B of the member 14, respective roller bearing arrangements 1 by, 1 SB are located between leg 17A and surface 14A and between leg 17Band surface 1 4B so that component 1 7 is slidable with respect to member 14 in the direction X-X (Fig. 1) which is perpendicular to the weightreceiving member 1 5. Device 1 6 further comprises a slot 19 in component 17 extending in the direction X-X and a protruding pin 20 extending through slot 1 9 and secured to member 14 whereby relative movement between member 14 and component 1 7 is limited. Also, member 14 comprises a blind hole 21 containing a tensionadjustable spring 22 which bears against the member 1 5 so that in normal operation of the device 1 6 pin 20 bears against the bottom edge of slot 19 as shown in Fig. 3 and renders device 16 rigid.When an overload occurs, the weight or force F equals or exceeds the tension setting of spring 22 causing device 1 6 to collapse resiliently so that component 1 7 slides over member 14 via the roller bearing arrangements 1 by, 1 SB until pin 20 abuts the top edge of slot 19 or until member 15 abuts fixed stops 23 (Fig. 1). Clearly stops 23 can be adjustable in position and need not be set accurately since the longitudinal extent of slot 19 prevents the overload being transferred into excessive deflection of the member 1 4.

As is illustrated in Figs. 3 and 4 adjustment of the tension of spring 22 is effected by a threaded spindle 24 carrying a spring seat 25 and extending through the member 14 from the base of the hole 21 so that the position of the seat 25 and hence the spring tension can be adjusted from outside the member 14.

As has been explained the roller bearing arrangements 1 8A, 1 8B constrain the sliding movement of component 1 7 relative to member 14 to take place in the X-X direction which is perpendicular to weight-receiving member 15.

Accordingly even when weight or force F is located eccentrically with respect to the member 14 - that is to say when weight or force F is not at position C in Fig. 3 - the roller bearing arrangements still render the device 1 6 operative against overloads. This arises because the arrangements 1 8A, 1 8B are each elongate in the direction X-X and are laterally disposed on opposite sides of member 14 in the plane B-B.

As illustrated in Figs. 1-4 the arrangements 1 8A, 1 8B are each in the form of a pair of spaced spherical rolling elements or balls 30 located in grooves 31 but of course the number of such balls 30 may be increased as desired to improve the effectiveness of the arrangement as a roller bearing and the materials from which the balls 30 and grooves 31 are constructed may be of seiected hardness to give the required eccentric load-handling capabilities. Although not shown balls 30 are mainted in grooves 31 by a suitable retainer of conventional construction.

Figs. 5A, 5B and 5C illustrate alternative possible constructions for arrangements 1 8A, 1 8B. Fig. 5A shows the grooves 31 formed by inset metal plates 33; Fig. 5B by metal pins 35; and Fig. 5C by inset blocks 37.

Claims

1. A load cell having a deformable or deflectable structural member in combination with a weight-receiving member and an overload protection device effectively located between said members, wherein said device comprises a spring-loaded assembly which is rigid with respect to loads less than a predetermined value and resiliently collapsible with respect to loads equal to or greater than said predetermined value, a roller bearing arrangement being provided in said device to render the resiliently collapsible movement thereof substantially independent of the position of the load on the weight-receiving member.

2. A load cell as claimed in claim 1, wherein said spring-loaded assembly comprises a tensionadjustable spring whereby said predetermined value is alterable.

3. A load cell as claimed in claim 1 or claim 2, wherein said roller bearing arrangement comprises spherical rolling elements.

4. A load cell as claimed in claim 3, wherein said roller bearing arrangement is disposed in a plane perpendicular to said weight-receiving member.

5. A load cell as claimed in claim 4, wherein said roller bearing arrangement comprises a pair of roller bearings each located in said plane and each extending perpendicularly to said weightreceiving member.

6. A load cell as claimed in claim 1, and substantially as hereinbefore described with reference to Figs. 1 4 or as modified by any one of Figs. 5A, 5B or 5C.