Weighing scales – Self-positioning – Spring
Patent
1998-12-23
2000-11-07
Gibson, Randy W.
Weighing scales
Self-positioning
Spring
73862471, 73862639, G01G 308, G01L 104
Patent
active
061439932
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a load cell of the shear beam type including a beam member for measuring the shear force imposed on the beam member by the load to be measured and one or more strain gauges mounted on the beam member so that they are affected by the deformation in the beam material.
BACKGROUND OF THE INVENTION
It is previously known to use beam-shaped load cells or force transducers based on resistive strain gauges in electronic weighing equipment of various types. The reason for this is that this type of load cell has proved to be very reliable and they have also the ability to maintain calibration and reproduction data for a long time and under severe weighing conditions. The beam-shaped load cells are also comparatively small in size, which makes them suitable for built-in and low-profile weighing applications. The load cells are usually cylindrical and when they are installed they can therefore often replace an existing shaft or any other machine element in order to sense a strain or measure a load without requiring any modification of the equipment in which they are installed.
A significant quality of shear beam type load cells is the ability to withstand side forces, that is, forces acting in directions other than in the measuring direction of the load cell. The load cells do not require any side supporting means, they can withstand side forces up to 100-200% of the nominal load value.
However, a load cell can also be affected by other undesired forces and as is usual in measuring applications it is the temperature and specifically temperature variations that cause measuring errors. Of course the strain gauges which are used in the load cell have themselves a certain temperature dependence caused by the fact that the measuring element and the resistive wire have different thermal expansion coefficients. If, however, at least four strain gauges are used and if they are of the same kind and glued on the same material, then the changes in resistance caused by the temperature variations will be the same for each of the strain gauges. As the strain gauges are arranged in a bridge circuit, the balance of the bridge will not be changed.
Undesired load forces affecting the measuring accuracy are also caused by, for instance, temperature expansions in the weighing container or platform. For that reason the load cell is usually provided with a bearing or other type of embodiment for eliminating these types of disturbing forces, such as side forces or twisting moments of torsion.
Swedish patent 82 01365-7 discloses a cylinder-shaped load cell with such a built-in bearing for permitting a side movement of the measuring body on a plane base support member. The load cell can be positioned directly on a foundation or other base support member in such a manner that it is permitted to move in a side direction. By means of such a built-in bearing the load cell can be loaded via a loading bracket without any additional bearings required.
In many applications, however, these types of movements on a plane foundation or base support member are not permitted by the actual design of the weighing equipment. Furthermore, the bearing built into the cylinder-shaped measuring body makes this type of load-cell comparatively expensive. The built-in bearing, in the form of, for instance, slide bearing or a roll bearing, must be designed with a high degree of accuracy and withstand the often very heavy load forces which are applied to this type of load cell.
Another type of prior art load cell is disclosed in U.S. Pat. No. 3,960,228. This load cell has a free deflectable beam secured to a foundation or other mounting means in such a manner that the free deflectable weigh end of the beam is suitably supported for receiving a vertical load force to be measured. The strain gauges are positioned on opposite sides of the beam and orientated substantially at a 45.degree. angle to the longitudinal neutral axis of the beam. A recess is formed in the free end of the deflectable beam in the load-meas
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Gibson Randy W.
Nobel Elekronik AB
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