CA1107310A - Leverless scale sensor - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

This weighing scale has a platform, a load cell supporting the platform and a base supporting the load cell. The load cell utilizes electrical strain gage elements, which are connected in an electrical circuit controlling an indicator. The scale is constructed to be unaffected by the eccentricity of the load on the platform. The scale is particularly suited for use in retail or other establishments, where loads are placed on the platform casually and rapidly, so that eccentric loading is common.
The load cell comprises an integral block of material divided by milled apertures and recesses into two vertical members and three horizontal members connecting the vertical members. The platform is supported on one of the vertical members. The base is connected to the other vertical member.
The upper and lower horizontal members carry most of the bending moments due to eccentric loads from one vertical member to the other. The middle horizontal member is constructed so that it carries most of the vertical component of the load. The strain gage elements are on this middle horizontal member and thus measure only the vertical load.

Description

~ ON
The invention is particularly concerned with the load cell structure, by which -the scale is made insensitive to eccentric loads on the weighing platform. This load cell structure is of general utility in other load cell applications where insensitivity to eccentric loads is desirable.
The load cell comprises an integral block having two apertures milled horizontally and dividing the block into two vertical members and three horizontal members connecting the vertical members.
The upper and lower horizontal members are constructed to transmit most of the bending moments due to eccen-tric loads from one vertical member to the other. The middle horizontal member is constructed to transmit mos-t of the compressive loads.
Each of the three horizontal members has two regions of minimum cross-section spaced horizontally. In one embodiment, the middle member includes two vertically narrow neck sections adjacent its ends and a vertically wider middle section. A
second set of four holes is bored transversely through the middle section. A vertical slot extends downwardly from the upper horizontal slot and connects one vertical pair of holes.
; Another vertlcal slot extends upwardly from~the lower horizontal slot thro~ugh the other vertical pair of these holes. There are thus created two flexible beam elements extending nwardly from the two vertical members. The inner ends of those elements are vertically aligned and are connected by a flexure element having narrow neck portions adjacent its upper and lower~ends. The fIexure element bends easily in response to horizontal forces applied at its ends~;and does not transmit suoh forces.~ It does transmit~vertlcal foroes from one beam ;-{~

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element to the other. The flexible elements are thus stressed only by the vertical forces applied to the load cell and are not stressed by moments due to eccentric loads of the platform.
The strain gages are placed on the flexible elements and measure only the weight of the load, being unaffected by the eccentricity of its posi-tion.
In another emodiment, the upper and lower members have two regions of minimum cross-section spaced horizontally by a distance at least about twice the spacing of the two regi-ons on the middle member. The upper and lower members have spring rates of about one-tenth that of the middle member. Thus, the middle member has greater stiffness to vertical loads.
The terms "horizontal" and "vertical", as used herein, accurately reflect the orientation of a load cell used in a weighing scale. When the load cell is used for other purposes, i.e., for measuring forces other than those directed vertically downward, it may be used in other orientations than that shown, and the directions presently called horizontal and vertical will be correspondingly changed.
DRAWINGS
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Fig. 1 is an elevational view of a weighing scale, ;
embodying the invention, with certain parts broken away and others shown in section ; Fig. lA is a right-hand elevation of the scale of Fig. 1, with some parts broken away and others shown in section on the line lA-lA of Fig. 1.
Fig. 2 is a plan view of the weighing scale of Fig. 1 ~with most of the platform broken away.
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Fig. 3 is an elevational view, on an enlarged scale, of an inteyral block employed in the load cell of Fig~ l, with most other parts removed.
Fig. 4 is a sectional view taken on the line 4 4 of Fig. 3.
Fig. 5 is a sectional view taken on the line 5~5 of Fig. 3.
Fig. 6 is a wiring diagram showing strain gage elements of the scale and the circuit controlled thereby.
Fig. 7 is a view of the load cell block of Fig. 3, with dotted lines added to show the distortion of the block under load.
Fig. 8 is an elevational view similar to Fig. 3, showing a modification.
Fig. 9 is a sectional view taken on the line 9-9 of Fig. 8.
Fig. 10 is a perspective view showing a modified form of load cell block.
Fig. ll is a sectional view tak n on the line 1l-ll of Fig. 10.
Fig. 12 is a perspective view similar to Fig. 10, showing another modification.
Fig. 13 is an elevational view of a weighing scale embodying the invention, with certain parts broken away.
Fig. 14 is a cross-sectional view taken on the line~
14-14 of Fig. 13.
Flg. 15 is a fragmentary view taken on the line 15-15 of Fig. 13.
:: : : ~ . : , ~ Fig. 16 is a view similar to Fig. 13, showing a :
modification.

Fig. l7~is a view taken on the line 17-17 of Fig.~16.
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DETAILED DE.SCRIPTION

FIGS. 1-6 These figures illustrate a scale including a platform 1 supported on a load cell 2, which is in turn supported on a fixed base 3. The load cell 2 comprises an integral block 4 of elastic material, usually metal, having a rec-tangular contour as viewed in front elevation, and shown in Fig. 3 separated from other parts of the scale. A set of four holes 5a, 5b, 5c and 5d is bored through the block from front to back, as viewed in Figs. 1 and 3. The holes 5a and 5b constitute a firs-t vertical pair, and the holes 5c and 5d constitute a second vertical pair. Each vertical pair of holes has i-ts axes aligned in a vertical plane. An upper slot 6a (see Fig. 3) is cut through the block 4 and connects the upper holes 5a and 5c. A
lower slot 6b is cut through the block and connects the lower holes 5b and 5d. The holes 5a, 5b, 5c, 5d and the slots 6a and 6b divide the block into two relatively rigid vertical members 4a and 4b and three relatively flexible horizontal members 4c, 4d and 4e.
The middle flexible horizontal member 4d comprises a flexible beam element 7, a flexure element 8, which is strained as a column, and a flexible beam element 9. The flexible member 4d is divided into the flexible beam elements 7 and 9 and the flexure element 8 by a set of four holes lla, llb, llc .
and lld. A slot 12 extends upwardly from the slot 6b through the hole lld and opens into the hole llc. Another slo-t 13 extends rom the s:Lot 6a downwardly through the hole lla and ~opens into the hole llb.

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The fron-t an~ rear faces of the flexure 8 are re-cessed, as shown at 14 in Fig. 4, so that the flexure 8 has the cross-sectional conEiguration of an I-beam.
The Elexible element 7 comprises a narrow neck portion 7a between the holes 5a and 5bl which is integral at its left end, as viewed in Fig. 3, with -the vertical member 4a and ex--tends to the right from the narrow neck portion 7a to an exten-sion 7b integral with the top of -the flexure 8. Similarly, the flexible beam element 9 comprises a narrow neck portion 9a integral a-t its right-hand end with -the middle of the vertical member 4b and extends from the neck 9a toward the left to an extension 9b integral with the bottom of the flexure 8.
The vertical members 4a and 4b and the upper and lower horizontal members 4c and 4e may be described as a frame en-closing the middle horizontal member 4d, which constitutes a fifth member of the load cell block.
A bracket 15 tFig. 1) has a pair of clevis arms 15a, 15b, attached to the front and back of the vertical member 4a by means of bolts 16. The clevis arms extend above the load cell block 4 and are connected to a horizontal arm 15c which is attached by means of screws 18 to a beam 20 having an inverted channel-shaped cross-section. A spider 21 includes a pair of side plates 22 and a pair of end plates 23. Each side plate is attached at its middle portion, as by welding, to one flange of the beam 20 and its end portions diverge from that beam. The ends of the side p:Lates 22 are connected, as by welding, to couplings 24, which are also attached to the ends of the end plates 23. Screws 25 extend through the platform 1 and through the couplings 24. Wing nuts 26 cooperate with screws 25 to hold the platform :in place on the spider 21.

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The load cell 2 is suppor-ted on -the base 3 by a gener-ally similar suppor-ting struc-ture, no-t shown in detail, in-cluding a bracket 17, a channel-shaped beam 19, side plates 27, couplings 2~, end plates 29, and screws 30. The screws 30 are threaded into -the base 3.
A block 31 is fastened in channel-shaped beam 19. A
screw 33 having a slotted end 33a at its lower end and a hexa-gonal head 33b at its upper end is threaded through the block 31. It may be adjusted vertically and locked in place at any position in block 31 by means of a jam nut 34. The block 31 is provided with a recess 31a in i-ts upper surface, which recess may receive par-t of the head 33b. The screw 33 serves as an overload stop to limit the downward movement of the vertical member 4a of the load cell. A similar overload stop is provided in the channel-shaped beam 20. That stop includes a block 35 and a screw 36 having a projecting hexagonal head 36b, and serves to limit the downward movement of the platform with re-spect to vertical member 4b. Strain gage elements 41 and 42 are placed on the upper and lower sides, respectiuely, of the narrow neck section 7a. Similar strain gage elements 43 and 44 are placed on the upper and lower sides of the narrow neck section 9a. When a load is placed on the scale, the strain gage ele-ments 41 and 44 are subjected to compressive strains and the elements 42 and 43 are subjected to tensile strains. These strain gage elements are connected in a bridge circuit 45 (Fig. 6) connected to a power supply 46 and having output terminals con-nected to an indicator 47, which may alternatively be a recorder.
Considering first the condition where the center line of the load is aligned vertically with the center of the verti-cal member 4a, the frame 4a, 4b, 4c, 4e deflects in a fashion similar to a paral:Lelogram~linkage, so that the members 4a and 4b ' 31(~

remain vertical, bu-t the member 4a moves lateraly toward the member 4b. Member 4c is strained in tension and member 4e in compression. Any moment due to the load is resisted by the flexible members 4c and 4e. The moments actiny at the opposite ends of the member 8 are substantially equal and opposite and therefore balanced. Any moment transmitted through the middle flexible member 4d is too small to be significant. Under ideal conditions, the member 8 remains vertical. The member 8 is so flexible at its narrow neck portions between the holes lla and llc and between the holes lld and llb that it bends easily in lateral directions, and no moment is transmitted through it.
Hence, the only strain in the member 4d is due to the weight of the load, and the strain gage elements 41, 42, 43 and 44 accu-rately measure that load, which measurement appears at the indicator 47.
Under such loading conditions, both the slots 12 and 13 are slightly narrowed when the load is applied.
Consider now the situation where the load is not aligned with the vertical member 4a. Take, for example, the condition where the center of the load is at the middle of the left-hand edge of the platform l, as viewed in Fig. 2. The load then applies a counterclockwise moment to the top of vertical member 4a, as viewed in Fig. 3, which tends to strain the flex-ible member 4c in tension and to strain the flexible member 4e in compression. If the center of the load is moved to the right in Fig. 2 along the horizontal center line of the platform 1, then as it passess over the vertical member 4a,~the direction of the moment changes from counterclockwise to clockwise. As the load moves farther to the right, the moment increasingly strains the flexible member 4c in compresslon and the member 4e in tenslon.
Thus, under some load conditions, the directions of the stralns :: : :::
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in the members 4c and 4e may be opposite Erom the strains under some other load conditions.
Depending upon the eccentrici-ty of the load, the slots 1~ and 13 may be both narrowed or both widened or one might be narrowed and the other widened. The flexure 8 always remains substantially vertical, although it typically departs slightly from the ver-tical under most load conditions.
If the load on the platform 1 is eccentric, there will be transmitted to the vertical member 4a both a downward force, which is a measure of the load, and a moment whose direction depends upon the displacement of the center of gra~ity of the load with respect to the center of the platform. The moment tends to twist the block 4 by moving the upper end of the verti-cal member 4a laterally with respect to its lower end. This moment may act either perpendicular to the plane of the paper or parallel to the plane of the paper. Components acting in both of those directions may be, and usually will be, present in the moment in any particular eccentric load situation.
If there is a moment perpendicular to the plane of the paper, the upper flexible member 4c and the lower flexible mem-ber 4e both resist that moment, and are strained by it. However, in the middle flexible member 4d, the flexure 8 is easily bent by that moment, without being greatly strained. This ease of bending in this dlrection is facilitated by the recesses 14 (Fig. 4), which allow the flexure 8 to bend more easily than the wider extensions 7b, 9b with which it is integrally con-~nected. Before the flexure 8 bends far enough to be signifi-cantly strained, the members 4c and 4e develop suffici.ent stress to resist the moment. Substantially all the moment is carried by the members 4c and 4e and none by the fifth member 4d.

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Similarly, as to moments parallel to the plane of the paper~ the middle member 4d does not resis-t compressive or ten-sile forces applied through the narrow neck sections 7a and 9a.
Instead, the flexure 8 simply bends at its narrow neck sections as required to accommodate the forces applied at the ends of the flexible member 4d, without transmitting substantial strain through that member.
As to any moment or combination of moment components, the strain gage elements 41, 42, 43 and 44, being located on the flexible member 4d, are not strained by moments, but only by the vertical forces applied to the platform 1. Hence, the unbalance of the bridge circuit 45 depends only on the weight of the load placed on the platform 1, and that weight is re-flected in the indicator 47, without dis-tortion due to any moment caused by eccentric loading.

FIGS. 8-9 These figures illustrate a modified form of integral block 51 for use in a load cell such as employed in Fig. 1.
Four holes 52a, 52b, 52c and 52d are made through the block 51.
The holes 52a and 52c are connected by an upper slot 53a. The holes 52b and 52d are connected by a lower slot 53b. The holes 52a, 52b, 52c and 52d and the slots 53a and 53b~divide the block 51 into two relatively rigid vertical members 51a and 51b and three relatively flexlble horizontal members 51c, 51d and 51e. A slot 54 extends downwardly from the slot 53a, extending almost all the way through the flexible member 51d. A similar slot 55 extends upwardly from the slot 53b almost all the way through the member 51d. The slots 54 and 55 separate the flex-ible member 51d into two flexible beam elements 56 and 57 separated by a verl:ical Elexure element 58. The flexure 58 is :
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notched in its upper and lower ends, as shown at 5~a in Fig. 8.
The notches 58a increase the flexibility of the flexure 8 in response -to moments parallel to the plane of the paper, as viewed in Fig. 8. The flexure 58 is also notched on its front and back faces, as shown at 58b (Fig. 9). The faces of the flexure 58 are recessed as shown at 58c. The recessess 58c and notches 58b increase the flexibility of the flexure 58 in re-sponse to moments perpendicular to the plane of the paper, as viewed in Fig. 8.
Notches such as those shown at 58b may be employed, if desired, on any of the other modifications illustrated.
The operation of the load cell block 51 is similar to that of the block 4 as described in connection with Figs. 1 to 7.

FIGS. 10-ll These figures illustrate a modified form of load cell block which may be used in the weighing scale of Figs. 1 to 6 in substitution for the block 4.
The block 61 has a configuration which appears rec-~~tangular in elevation. Four holes 62a, 62b, 62c and 62d are bored through the block. The holes 62a and 62c are connected by a slot 63. The slots 62b and 62d are connected by a slot 64. The holes 62a, 62b, 62c, 62d and the slots 63 and 64 divide~
the block 6I into two relatively rigid vertical members 61a and 61b and three relatively flexible horizontal members 61c, 61d ~ -and 61e. Member 61d is provided with four holes 65. The two left-hand holes 65 are~connected by a slot 66 which extends up-wardly from the slot 64. ~The~two r1ght-hand~ho1es 65~are con-nected by a slot 67 which extends downwardly from~slot 63. The holes 65 and the~s~ots 66 and 67 divide ~the~m1ddle member 61b - . . . , ~, .

~?73~3 into a pair of flexible beam elements 68 ancl 69 and a verti-cal flexure 70. The flexible beam element 68 extends from -the vertical rigid member 61a through a narrow neck section 72 to an integral connection with the top of the flexure 70. The flexible beam member 69 extends from vertical member 61b through a narrow neck section 73 to an integral connection with the bottom of the flexure 70. The narrow neck sections 72 and 73 are considerably wider in the vertical direc-tion than the narrow neck sections 7a and 9a of Fig. 3. The narrow neck section 72 is provided on its opposite faces with a pair of recesses 72a, 72b (see Fig. 11), leaving a thin web 74 across the middle of the narrow neck section 72. A pair of holes 75 are bored through a thin web 74, leaving only a narrow bridg-ing portion extending horizontally across the middle of the web 74. Strain gage elements 76, 77, 78 and 79 are affixed to the opposite faces of the bridging portion of the web 74. The upper and lower strain gage elements 76 and 77 are slanted at 45 to the horizontal and in mutually perpendicular directions.
The other two strain gage elements 78 and 79 are similarly oriented, except that gage element 78 is oriented perpendicu-larly to its immediately opposite strain gage 76. These strain gage elements 76, 77, 78 and 79 measure shear strains rather than compression and tension strains.
;~; The front and reax faces of the block 71 between the vertical members 61a and 61b are recessed, as shown at 61f, so that the middle part of the block is thinner than the vertical ; members 61a and 61b. This configuration makes the recessed parts of the block more highly stressed in response to a given load, and hence the strain gage elements 76, 77, 78 and 79 are re sensitive.

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FIG. 12 This figure illustra-tes a further modified form of load cell block, generally indicated at 81. The block is pro-vided with four holes 81a, 81b, 81c and 81d. The holes 81a and 81c are connected by a slot 82. The holes 81b and 81d are connected by a slot 83. The holes 81 and the slots 82 and 83 divide the block into two vertical members and three horixontal members, as in the case of the other species. The holes 81c and 81d have considerably larger vertical dimensions than the holes 81a and 81b, so that a narrow neck section 84 between the holes 81c and 81d is substantially narrower than the neck section 85 between the holes 81a and 81b. Strain gage elements 86 and 87 are placed only on the narrow neck section 84 which is more highly strained than the thicker neck section 85 and hence more sensitive. This arrangement of the wide and narrow neck sections makes the cell block as a whole stiffer (because of the wider neck section 85) without loss of sensitivity (because the s-train gage elements are placed on the narrow neck section 84).
The other parts of the structure of the load cell block 81 correspond to those of the block 4, and need not be further described. Note that the left-hand pair of inner holes is cut through by a vertical slot branching from the upper horizontal slot ln Figs. 1-7 and 12, and by a vertical slot branching from the~lower horizontal slot in Figs. 8 and 9. On : .
the other hand, the right-hand pair of inner holes is cut through by a vertical slot branchlng from the lower horizontal slot in Pigs. 1-7 and 12 and by a vertical slot branching from the upper horizontal slot in Figs. 8 and 9. In the structures shown in FLgs. 1-7~

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and 12, the central flexure 8 or 88 is stressed in tension by a load on -the platform 1, while in the structures shown in Figs. 8 and 9, the central flexures 58 and 70 are strained in compression. For use in a weighing scale, the modifications where the central flexure is strained in tension are preferred.
However, all the load cells shown are universal in -the sense that they wlll accept and measure either compression loads or tension loads.
Figs. 13-15 These figures illustrate a scale including a platform 101 supported on a load cell generally indicated at 102 which is in turn supported on a fixed base 103. The load cell 102 comprises an integral block 104 of elastic material, usually metal, and four strain gage electrical resistance elements 105a, 105b, 105c and 105d.
The block 104 comprises two horizontally spaced, vertically extending members 104a and 104b and three vertically spaced, horizontally extending members 104c, 104d and 104e.
The vertically extending member 104a is provided with an integral laterally projecting wing 106 extending outward from its lower end. The wing 106 is fastened to the base by means of screws 107. A shim 108 separates the bottom of the wing 106 from ~ the base 103.
; ~ The vertically extending~member 104b is provided with a similar integral wing 111 at its upper end. The wlng 111 is attached by screws 112 to the platform 101. A shim 113 is provided between the wlng 111 and the platform 101. The upper horizontally extending member 104c has two horizontally spaced ~regions of reduced cross-section shown~at 114, created by recesses of arcuate cross-section in the upper surface of ~ - 13 -::: ~ : :

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the block 10~ and simllar aligned recesses of arcuate cross-section in the lower surface of the member 104c. The regions of minimum cross-section 114 are spaced horizontally by a distance Ll. The block 104 has two transverse bores 115 and 116 or irregular contour, which separate the horizontally extending member 104d from the upper and lower members 104c and 104e~ The upper surface of the middle horizontally extending member 104d is flat, and the four strain gage elements 105a, 105b, 105c and 105d are mounted on that f:Lat surface. The middle member 104d has two horizontally spaced regions 117 of reduced cross-section, defined by two recesses of arcuate cross-section, in its under surface. The regions 117 of minimum cross-section in the middle member 104d are separated horizontally by a distanCe L2-The lower member 104e is also provided with two regions of minimum cross-section, shown at 121, which are constructed similarly to those in the upper member:104c and are spaced horizontally by the same distance Ll.
The distance Ll is made at least about twice the ~20 distance L2 and may be made as much as six times L2.
Making Ll greater than L2 increases the moment of inertia of the outer paralleIogram comprising the vertical members 104a and 104b and the horizontal members 104c and 104e.
The vertical dimensions of the reduced regions 114 and 1~1 are made smaller~ than ~he vertical dimensions of the reduced regions 117 in the middle member 104d. The spring rate of the ~ , :
upper and~lower members 104c and 104e, taken together is about 10% of the spring rate of the middle member 104d. Hence, the mlddle member 104d carries most of the ve~rtical load whlch lS
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centrally applied to the platform 101~ but the upper and lower : ~:

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members 104c and lO~e resist most of the torque due to off-center loads, i.e., loads spaced horizontally from center line 122 of the platform 1.
The cross-section areas at 114 and 121 are required to be increased when the dimension Ll is made greater than :the dimension L2, but this increase in cross-sectional areas provides substantially greater resistance to off-center loads.
The structure illustrated is stiffer and has a higher natural frequency than would be the case if Ll were equal to L2.
Such a higher natural frequency results in quîcker response of the scale to a rapidly applied load. It also allows the use of a higher limiting frequency in a high pass filter in the output of the circuit containing the resistance elements 105a, 105b, 105c and 105d. The higher the noise frequencies which are cut off, the less low frequency noise is received in the electronic circuits to which the resistance elements are connected.
The flat surface of the middle member 104d makes it easier to apply the strain gage elements 105, and to get a good bond betwéen the gage and the underlying surface. There are : four strain gage elements, the elements 105a and 105c being vertically aligned with one reduced section 117 and the elements 105b and 105d being vertically aligned with the other reduced .
section 117.
It is preferred to form all four e~:em~nts:105 on a single sheet of plastic material, as shown in Fig. 15, so that most of the bridge clrcuit includ:ing those-elements ~ :
:~ is mounted on a single plastic sheet 123. Thus, only a single part,~name].y sheet 123, has to be carefully located with respect to the:reduced sections 117. ~If the resistance :: elements are affixed separately, each of the four has to be carefully located. As shown in Fig. 15, five terminals ~: : : : :

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are brought ou-t from the plastic sheet 123 supporting the elements 105 -to facilitate the insertion of calibratiny resistance elements in the circuit.
The block 104 may be made from run-of-the mill bar stock whose dimensions are not carefully controlled. The various holes and recesses in the bar stock may be made by a numerically controlled milling rnachine. By using recesses 114 in the outer surfaces of the upper and lower members 104a, 104b, it is assured that all of the dimensions which critically determine the performance of the load cell are between surfaces which are established by the operation oE the milling machine, and not by any surface of -the original bar stock. Thus, the effective height H of the block 104 is between the horizontal center line of the reduced regions 114 and the horizontal center line of the reduced regions 121. The upper flat surface of the member 104d, on which the strain gage elements ~, 105 are mounted is located at a distance H from each of those horizontal center lines. Thus the flat surface on the member 104d contains the neutral axis of the load cell 102. All torques due to off-center loads have minimal effect at the neutral axis of the load cell. Thus, the effects of those off-center loads do not appear in the output of the strain gage circuit.
The thickness of the shim 108 determines the deflection of the vertical member 104b at which the base 103 serves as an overload stop for the bottom end of the member ~ ~ ~ 104b.
;~ The s~ide surfaces of the member 104d are cut away to make that member substantlally thinner than the upper and ' 30 ~ lower members,104c and;lO4e,, as shown in Fig. 14. The thinness '~
of the member 104d makes it less resistive to off center loads : : : :

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so that most of -those loads are carried by the upper and lower members 104c and 104d~
Figs. 16=17 These figures illustrate a modification of the invention in which the load cell 102 of Figs. 13 and 14 is replaced by a load cell 126 comprising a block 127 of resilient material and four strain gage elements 128a, 128b, 128c and ; 128d. The structure of the load cell 126 is generally the same as that of the load cell 102, except that the mlddle member 104d on the load cell 102 is replaced by two parallel members 131 and 132. The reduced regions in the members 131 and 132 are defined by two intersecting bores 133,134, which also may be described as separating the two middle members. The upper member 131 has its upper surface flat and separated from the neutral axis of the load cell by a distance X. The lower surface of the member 132 is also flat and is separated from the neutral axis of the load cell 126 by the same distance X.
The members 131 and 132 carry a greater pro~ortio~ of the torques due to off-center loads than does the member 104d of Figs. 13 and 14. Nevertheless, the strains due to those loads have equaI and oppos1te effects on the bridge circuit including the strain gages 128, and thus those effects cancel.
The operation of the apparatus in Figs. 16 and 17 is otherwise generally slmilar~to the operation of the apparatus shown in Figs. 13 and 14. ~-In the structure of Figs.~16 and 17 any thermal stresses resulting from heating oE the members 131 and 132, ~ -~either by the electric current flowing through the gage elements 30~ ~ or from other sources,~are self-canceling, so that the reading of the scale is not affected by such thermal stresses.

In the structure of Figs. 13 and 1~, the -thermal stresses on the gages 105 should also be self-correcting.
Under particular operating conditions, where the t~mperatures at the gage elements 105 are not equal, it is conceivable that a thermal stress may be encountered which is not self-canceling. In the event, the structural arrangement shown in Figs. 16 and 17 may be used.
Although a preference is expressed above for arranging all four gage elements 105 on a single sh~eet of plastic material, separate gage elements may be used, or pairs of gage elements may be arranged on each of two sheets.
Where eccentric loading is referred to herein, the eccentricity is with reference to the geometrical center of the load cell, i.e. the intersection of centerline 122 in Fig. 1~ with the upper surface of the load cell.
The wings, 111 and 106, allow the load cell to be attached to the platform 101 and the base 103 by means of bolts made to either British or metric dimensions. The bolts are not threaded to the wings, 111 and 106, but pass through with slight clearance. The bolts 112 are threaded only into the nuts under the wing 111. The bolts 107 are threaded only into the base.

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The usual dimensional tolerances of milling machines are not close enough to give the performance required withln the assigned ~limits of error. After the gages 105 are mounted, it is necessary to calibrate the load cell by filing or otherwise removing small amounts of material selectively from one or more of the reduced sections, 114, 117, and 121. In removing such material, it is taken away from the least sensitive side ::
f the load cell. If the gage elements are thereafter removed or replaced, another calibration by selective removal of :
material is required.

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Ideally, it would be desirable to have the upper and lower members, 104c and 104e, carry only the torques due to eccentric loads and to have the middle member 104d carry only the vertical loads. Necessarily, this ideal cannot be attained. However, by proper design and calibration of the members 104c, 104d and 104e, as described above, the performance can be made to approach that ideal within any assigned limits of error.

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Claims (21)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A load cell, comprising:
a. an integral frame including two horizontally spaced, elongated, vertically extending members and three vertically spaced, elongated, horizontally extending members connected at their ends to the vertically extending members;
b. each horizontally extending member having two horizontally spaced regions of reduced cross-section, said members and their regions of reduced cross-section having surfaces defined by bores extending horizontally through said frame at right angles to said members;
c. strain gage resistance elements attached to at least one of the reduced cross-sectional regions of the middle one of the three horizontally extending members;
d. means for applying a force to be measured to the upper end of only one of the vertically extending members;
e. means for applying a reactive force to the lower end of only the other vertically extending member; and f. means including said resistance elements for measuring said force to be measured.

2. A load cell as in claim 1, in which the middle horizontally extending member has at least one flat and horizontal side at the neutral axis of the load cell and the strain gage elements are affixed to the flat side.

3. A load cell as in claim 2, in which:
a. both top and bottom sides of the middle horizontally extending member are flat and equally spaced from the neutral axis of the load cell; and b. the strain gage elements are attached to both flat sides of said middle member.

4. A load cell as in claim 1 or 2, in which the reduced cross-sectional regions in the upper and lower horizontally extending members are defined in part by recesses of arcuate cross-section in the upper surface of the upper member and the lower surface of the lower member and vertically aligned with said bores.

5. A load cell as in claim 1, in which the horizontal spacing between the reduced cross-sectional regions of the upper and lower horizontally extending members is at least twice the horizontal spacing between the reduced cross-sectional regions of the middle horizontally extending member.

6. A load cell as in claim 5, in which the first-mentioned horizontal spacing is no greater than about six times the second-mentioned horizontal spacing.

7. A load cell as in claim 1, including:
a. an integral wing projecting from the upper end of one of said vertically extending members;
b. a platform for receiving a load;
c. means mounting said platform on said wing;
d. a base;
e. a second wing projecting from the lower end of the other vertically extending member;
and f. means mounting said second wing on said base.

8. A load cell as in claim 7, including:
a. a shim between said second wing and said base;
b. said base extending beyond said shim and under said first vertical member and serving as an overload stop, when the load on the platform is sufficient to deflect said first vertical member through a distance equal to the vertical dimension of said shim.

9. A load cell as in claim 1, in which:
a. the middle horizontally extending member comprises two flexible elements respectively fixed at one end to respective ones of the two vertically extending members and having their opposite ends extending therefrom toward the other of the two vertically extending members, said opposite ends being vertically spaced and aligned; and b. a vertically extending flexure element connected between said opposite ends of the two flexible elements, said flexure element being stressed vertically by a force acting on said one vertically extending member and being laterally flexible in response to horizontal forces, so that substantially only vertical forces are transmitted through said connected flexible elements and substantially all moments due to eccentric loading are transmitted from one vertically extending member to the other through the upper and lower horizontally extending members.

10. A load cell as in claim 1, in which:
a. said means for applying a force to be measured comprises a platform with horizontal dimensions in each of two mutually perpendicular directions which are greater than the corresponding horizontal dimensions of the load cell, so that the platform may receive loads eccentrically located with respect to the load cell.

11. A load cell as in claim 9, in which all said members are portions of an integral block of elastic material having:
a. a set of four horizontal cylindrical holes having parallel axes and extending through said block in two pairs, the axes of each said pair being aligned in a vertical plane; and b. two slots, each slot connecting one hole of each pair with one hole of the other pair, said slots being perpendicular to said force directions, said slots and holes cooperating to define said members, said middle horizontally extending member being located between said slots.

12. A load cell as in claim 11, in which said middle horizontally extending member has:
a. a second set of four horizontal cylindrical holes having parallel axes and extending therethrough in two pairs, the axes of each pair being aligned in a vertical plane;
b. a third slot extending from one of said two slots into both of the holes of one pair of said second set; and c. a fourth slot extending from the other of said two slots into both of the holes of the other pair of said second set, said second set of holes and said third and fourth slots defining said flexure elements and cooperating with said first-mentioned set of holes to define said flexible elements.

13. A load cell as in claim 11, in which:
a. the reduced cross-sectional regions of said middle horizontally extending member are located in said flexible elements;
b. said strain gage elements are located on both reduced cross-sectional regions of said middle horizontally extending member.

14. A load cell as in claim 11, in which the two holes of one pair have a greater dimension in the direction of said force than the other two, so that one of said flexible elements is narrower in the locality between the two holes of said one pair than the other flexible element in the locality between the other two.

15. A load cell as in claim 12, in which the opposite faces of the flexure element are recessed from the adjacent faces of the block, so that the cross-section of the flexure element, taken along its center line and perpendicular to the recessed faces, is substantially that of an I-beam.

16. A load cell as in claim 12, including transversely extending semicyclindrical notches on the opposite recessed faces of the flexure element at the middle thereof.

17. A load cell as in claim 12, in which said flexure element has semicylindrical notches on both of its side faces adjacent each end thereof.

18. A load cell as in claim 12, in which:
a. the reduced cross-sectional regions of said middle horizontally extending member are located in said flexible elements;
b. one of said reduced cross-sectional regions has a pair of aligned recesses extending parallel to said holes, leaving a thin web of material between said aligned recesses;
c. a pair of holes extending through said web and having their centers aligned along a line parallel to the force being measured, said holes being separated by a narrow bridging portion of said thin web; and d. at least some of said strain gage resistance elements are located on said narrow bridging portion of the web.

19. A load cell as in claim 11, in which said holes determine the location of four narrow neck sections of equal dimensions in the upper and lower horizontally extending members, said narrow neck sections being located adjacent the opposite ends of each member.

20. A load cell as in claim 11, in which said pairs of holes define said horizontally spaced regions of reduced cross-section, said regions being of equal dimensions and located adjacent the opposite ends of the middle horizontally extending member.

21. A load cell as in claim 14, in which said strain gage resistance elements are located only on said narrower flexible element.