Strain gage load cell

Theload cell herein disclosed is designed for precise measurement of forces along or parallel to a single axis, while being substantially immune to forces applied along the other mutually perpendicular axes as well as moments applied about any of the three mutually perpendicular axes. Rigid force input and force output members are resiliently interconnected by a pair of parallel flexure beams which establish with such members a parallelogram type of linkage which is relatively yieldable to forces parallel to the selected measurement axis while being relatively stiff and unyieldable to forces parallel to the other two axes and to moments about any of the three axes. A strain gage-equipped sensing beam is mounted within the plane of the parallelogram linkage and between the two flexure beams. One end of the sensing beam is rigidly connected to the fixed force output member, while the other end is free to flex in cantilevered fashion responsive to displacement of the force input member. The sensing beam thereby provides part of the resilient interconnection between the force input and force output members, and is dimensioned so that its stiffness relative to that of the two flexure beams causes it to absorb 80 to 90% of the applied loads. Adjustable overload protection means are provided, which cause excessive forces to be safely by-passed around the resilient members and the strain gages.

BACKGROUND OF THE INVENTION 
Strain gage type load cells capable of measuring forces along a single axis 
while being relatively immune to extraneous forces and moments are known 
in the art. Examples of such load cells are shown in U.S. Pat. Nos. 
3,994,161; 4,022,288; 4,103,545; 4,181,011; and 4,196,784. These patents 
show the use of a parallelogram linkage comprising two parallel relatively 
massive and unyielding structures interconnected by two flexure beams 
which are dimensioned to be relatively compliant or yieldable to forces 
parallel to the single axis but being otherwise substantially unyielding 
to extraneous forces and moments. Also shown is the placement of a sensing 
beam between such flexure beams, either anchored at both ends to the 
unyielding structures or cantilevered from only one of such structures. 
It is a principal object of the present invention to provide an improved 
strain gage type load cell having a high degree of accuracy in response to 
loads parallel to a predetermined axis, while having enhanced immunity to 
the effects of extraneous loads and moments. It is a further object of 
this invention to provide such a load cell which can be accurately 
fabricated with a minimum of machining operations, and which has improved 
protection of the resilient members and strain gages from accidental 
overloading in any direction. 
SUMMARY OF THE INVENTION 
The load cell of the present invention comprises a cantilever type sensing 
beam provided with strain gages, and mounted parallel to and between two 
flexure beams which interconnect rigid force input and force output 
members. One end of the cantilever sensing beam is anchored to the 
stationary force output member, while the load to be measured is applied 
to the other end of the sensing beam by a force-transmitting flexure beam 
which itself aids in preventing extraneous loads from being transmitted to 
the sensing beam. The arrangement of the rigid and resilient members 
reduces the amount of machining required and permits one basic structure 
to be readily modified by minimal dimensional changes during machining to 
establish a variety of useful capacity ranges. The configuration also 
permits the use of a simple adjustable threaded member to vary the 
overload at which the resilient members will be by-passed.

DETAILED DESCRIPTION OF THE DISCLOSURE 
Referring to FIG. 1 of the drawings and its schematic equivalent in FIG. 5, 
load cell 10 of the present invention is shown in an application such as a 
highly accurate scale for precise measurement of the quantity of food 
being packaged in a container. Load cell 10 is shown mounted between an 
upper weighing platform 12 and a lower stationary base or support 14, both 
shown fragmentarily in phantom. The force or load to be measured is 
applied along the illustrated Z axis and would most commonly be applied 
downwardly through platform 12 into load cell 10. 
The load cell generally comprises an inverted L-shaped rigid force input 
structure 16 having a horizontal upper leg 18 which may be bolted to 
platform 12 and a vertical leg 20 descending from the right end of leg 18. 
The load cell further comprises an oppositely arranged rigid L-shaped 
force output structure 22 having a horizontal lower leg 24 secured to base 
14 and a vertical leg 26 along the left edge of the structure. These two 
structures 16 and 22, which are sufficiently rigid to be essentially 
non-yielding within the contemplated range of loads, are resiliently 
interconnected by three elements. The first two of these elements are 
upper and lower flexure beams 28, 30, respectively, and the third element 
includes cantilevered sensing beam 32, to be further described below. 
Flexure beams 28, 30 are provided with end portions 34 of reduced section 
depth in the Z axis direction, to function as hinge points to permit the 
structure defined by vertical legs 20 and 26 and beams 28 and 30 to 
function as a flexible parallelogram in response to loads applied parallel 
to the Z axis. 
The third interconnecting element comprises, in addition to cantilevered 
sensing beam 32, a force transmitting or directing beam 36 arranged 
parallel to the Z axis and connected at one end to the free end of beam 32 
and at its other end to vertical leg 20 of force input structure 16. Force 
transmitting beam 36 is provided with end portions 38 of reduced section 
depth in the X axis direction to function as hinge points so that such 
beam will be flexible so as not to transmit to sensing beam 32 forces 
parallel to the X axis. 
Clearances or gaps 40 are provided between the adjacent portions of the 
force input and output structures 16 and 22 and flexure beams 30, 32, to 
permit a controlled amount of deflection of the vertically movable force 
input structure relative to the stationary force output structure. The 
effective size of the clearance is further controlled by an overload 
protection feature, to be described below. 
The front and rear faces of load cell 10 are provided with generally 
circular cavities or depressions 42, which reduce the thickness of certain 
of the elements and make them more compliant or responsive, to a 
controlled degree, to certain of the applied loads. Sensing beam 32 is 
relatively massive and rigid at its left end, but is provided with a neck 
44 of reduced depth in the Z axis direction to concentrate the stresses at 
a point where the strain gages 46 may be applied to its upper and lower 
surfaces, as shown in FIG. 4. These strain gages are symmetrically 
arranged about the minimum depth point at the central vertical plane 
defined by axes Z-X. The strain gages are bonded to the surface of the 
beam in the conventional manner, as will be understood by those skilled in 
the art. Access hole 48 is provided in vertical leg 26 to permit passage 
of the necessary electrical cable. The strain gage tabs (not illustrated) 
are preferably mounted away from neck 44, in lower stress areas, such as 
at the extreme left end of sensing beam 32 or along the cylindrical edge 
wall of cavity 42. 
The curved shape of sensing beam 32 facilitates placement of the strain 
gages at the point of maximum stress, as is preferred, but also provides 
maximum beam stiffness for a given level of sensitivity, which is also 
desirable. The attachment of the strain gage tabs at closely adjacent yet 
substantially stiffer parts of the structure is also advantageous, to 
eliminate beam-reinforcing effects of the connecting wires and solder 
joints, which could otherwise affect the accuracy of the load cell. 
An overload protection system is provided to prevent excessive deflection 
of the sensing beam which could damage the strain gages. This system is 
shown in detail in FIG. 6, and comprises a first overload screw 50 which 
is threaded into internally threaded hole 52 in horizontal leg 18 and 
extends downward into the clearance gap 40. The lower tip of screw 50 is 
normally spaced a small predetermined distance above a hardened pad 54, 
which screw 50 abuts whenever a downward force in excess of a 
predetermined safe level is applied to the load cell. The vertical 
position of screw 50 within hole 52 is therefore adjusted as desired to 
establish the level of downward loading which be permitted before the load 
cell will bottom out and cease to transmit additional forces to the 
sensing beam. Once screw 50 bottoms out on pad 54, such additional forces 
are safely by-passed from leg 18 directly through vertical leg 26 and 
horizontal leg 22 to base 14. 
Excessive loading from upward forces is prevented by a second overload 
screw 56 which freely passes through a shouldered clearance hole 58 in leg 
18 and is threaded into a threaded hole in leg 26. The vertical gap 
between the shoulder of hole 58 and the underside of the head of screw 56 
will control the amount of upward force can be applied to leg 18 before 
the load cell bottoms out to rigidly connect leg 18 to leg 26. In this 
manner, excessive upward as well as downward forces applied to the load 
cell can be readily by-passed around the sensing beam and other delicate 
components. Alternatively, screws 50 and 56 may be spaced transversely 
from each other, rather than longitudinally. 
It is preferred that a similar overload screw arrangement will be provided 
in the lower right corner of the structure of FIG. 1, that is, between the 
right end of horizontal leg 24 and the bottom of vertical leg 20. In this 
fashion, any overload will be absorbed by the two devices. 
The operation of the load cell in response to various applied forces and 
moments will now be described. Visualization of the operation of the load 
cell will be aided by reference to FIG. 5, which, due to its schematic 
presentation, eliminates much of the extraneous structural details 
illustrated in FIG. 1. As will be understood by those skilled in the art, 
the placement of sensing beam 32 midway between upper and lower flexure 
beams 28, 30 results in the isolation of the sensing beam from certain of 
the extraneous forces and moments. Other residual or extraneous forces 
will be cancelled out by the Wheatstone bridge arrangement (not 
illustrated) in which the four strain gages 46 may be conventionally 
wired. 
When a force to be measured is applied along or parallel to the Z axis, 
force input structure 16 will resiliently deflect downwardly parallel to 
the Z axis as a result of the parallelogram linkage described above. 
Because the hinge point or flexural pivot points 34 are readily compliant 
or flexible in response to forces along such axis, a substantial portion 
of the force will be absorbed by sensing beam 32, with maximum strain 
occurring at neck 44. Of course, as explained above, the downward 
deflection of force input structure 16 will be transferred into the free 
end of sensing beam 32 by the force directing or transmitting beam 36. 
Beam 36 is relatively stiff in response to tensile loads applied along its 
vertical axis, so it will experience substantially no axial elongation, 
with the result that virtually all of the downward deflection of force 
input structure will be communicated to the right end of sensing beam 32. 
The relative stiffnesses of flexural pivots 34 and neck 44 are selected so 
that sensing beam 32 will preferably absorb at least 80 percent of a load 
along the Z axis, for maximum accuracy of response. 
As long as the magnitude of the force along the Z axis is below a 
predetermined limit, so as not to cause horizontal leg 18 to deflect more 
than is permitted by overload screws 50 and 56, the output of the strain 
gages will accurately measure the magnitude of the applied load. 
The load cell is designed so as to be relatively immune from the specific 
point of application of the force in the direction of the Z axis. If such 
force is applied at a point toward the extreme right or left of the 
structure, a moment about the Y axis will be produced, which moment will 
be absorbed by the widely spaced upper and lower flexure beams 28, 30, and 
these beams will absorb such moment by axial compressive forces in one of 
the beams and axial tensile forces in the other of the beams. Such beams 
are dimensioned to be relatively stiff in response to forces directed 
along their longitudinal axes. The placement of sensing beam mid-way 
between these two beams, plus the weakness of flexural pivots 38 to forces 
in the X direction, minimizes the likelihood of any undesired axial 
loading of sensing beam 32. 
Similarly, moments about the Y axis will also tend to be resiliently 
absorbed by flexure beams 28, 30, thereby isolating sensing beam 32 from 
such forces. 
Extraneous forces applied parallel to the X axis will have the same effect 
and will be absorbed in the same fashion as moments about the Y axis, as 
described above. 
Moments about the X axis will be absorbed primarily by shear forces created 
in upper and lower flexural beams 28, 30. Again, the relatively wide 
vertical spacing between such beams helps to reduce the magnitude of the 
forces they will experience in response to such moments. To a lesser 
extent, shear forces will also be created in force transmitting beam 36 
and flexural pivots 38. 
Forces along the Y axis will produce the same effects as the 
above-described moments about the X axis. 
Finally, moments about the Z axis will also be resisted by shear forces, 
primarily in beams 28, 30 and to a lesser extent by shear forces in beam 
36. 
FIGS. 7 and 8 illustrate in front and side fragmentary views a modified 
embodiment wherein force transmitting beam 36 is more compliant or 
resilient to forces along the Y axis or moments about the X axis, thereby 
helping to isolate sensing beam 32 from such extraneous forces. This 
result is accomplished by providing additional hinge points 60' of reduced 
cross-section in the Y direction which function to increase the compliance 
of such beam in response to these forces and moments. 
Other modifications which may prove to be desirable, while retaining 
essentially the same configuration, involve relocating the neck portion 44 
of sensing beam 32 toward the right to a point nearer the vertical center 
line of symmetry of the load cell. Such a placement may aid in eliminating 
the effect of bending moments induced by the lack of perfect alignment of 
the point of load application with the strain gage location. 
Alternatively, such bending moments may be more effectively isolated by 
shifting the location of force transmitting beam 36 toward the left until 
it is aligned with the vertical center line of symmetry of the load cell. 
Applicants have not as yet constructed load cells having these latter two 
configurations, so no experimental data is yet available comparing the 
performance of these three slightly different configurations. However, the 
basic coaction of the various elements would be common to all three 
configurations, with only differences in magnitude of extraneous forces 
possibly varying from one configuration to the other. 
The seemingly complicated and elaborate shape of the structure of FIG. 1, 
while being functionally identical to the simplified schematic of FIG. 5, 
results from machining considerations. The illustrated load cell can be 
cut from plate stock of the desired height and thickness. Thereafter, 
opposed cavities 42 can be readily machined in the two faces, to establish 
the desired reduction of cross-section of the resilient members along the 
Y axis. The circular openings and slots can be readily machined by simple 
drilling or milling operations in order to achieve the desired thickness 
of the various hinge points. The critical dimensional points are all 
arcuate, to accommodate the simplest drilling or milling steps. The 
arcuate shape of neck 44 is particularly advantageous as a way to maximize 
the stress at the narrow section, while retaining stiffness and lower 
stresses in the remainder of beam 32. 
The disclosed shape and machining techniques are also advantageous in that 
they permit the same load cell blank and design to be used for several 
load capacity ranges, simply by using a larger or smaller diameter drill 
or milling tool or by changing spacing of these holes to thereby narrow or 
widen the thickness of the flexural pivot points. 
The use of the two rigid L-shaped structures 16, 22 not only permits a 
simple and adjustable overload protection device, as described above, but 
also eliminates the need for supplementary force input and reaction 
brackets; the necessary bolted connections for which would introduce 
potential slippage and inaccuracy of load cell output. 
This invention may be further developed within the scope of the following 
claims. Accordingly, the above specification is to be interpreted as 
illustrative of only a few operative embodiments of the present invention, 
rather than in a strictly limited sense.