This document explores the characteristics and performance of medium to high capacity load cells. It discusses factors like linearity, hysteresis, output symmetry, and rejection of extraneous loads. Different load cell types are represented, including low profile, flange mount, single column, and hollow column cells. Guidance is provided on optimizing strain gage performance and selecting the appropriate load cell style for different applications.

1. 1
Optimizing Strain GageOptimizing Strain Gage
Load CellLoad Cell
PerformancePerformance
by LaVar Cleggby LaVar Clegg
Interface, Inc.Interface, Inc.
Western Regional Strain Gage CommitteeWestern Regional Strain Gage Committee
Conference September 13, 2011Conference September 13, 2011

2. 2
SummarySummary
 This paper explores the fundamentalThis paper explores the fundamental
characteristics of medium to high capacitycharacteristics of medium to high capacity
load cells and how they are affected byload cells and how they are affected by
the types and implementation of strainthe types and implementation of strain
gagesgages
 Guidance for successful application ofGuidance for successful application of
load cellsload cells

3. 3
Load Cell Types RepresentedLoad Cell Types Represented
Low Profile, center thread Low Profile, flange mount
Single Column Hollow column

4. 4
Performance FeaturesPerformance Features
 1. Linearity over measuring range1. Linearity over measuring range
 2. Hysteresis (descending loads)2. Hysteresis (descending loads)
 3. SEB (why is it useful ?)3. SEB (why is it useful ?)
 4. Output symmetry4. Output symmetry
 5. Rejection of extraneous loads5. Rejection of extraneous loads
 6. Sensitivity to mounting6. Sensitivity to mounting

5. 5
1. Linearity1. Linearity
 A.A. Important because load cell non-Important because load cell non-
linearity represents system error when thelinearity represents system error when the
instrumentation is linear, as it typically isinstrumentation is linear, as it typically is
 B.B. Only smooth calibration curves can beOnly smooth calibration curves can be
corrected by compensation in thecorrected by compensation in the
instrumentationinstrumentation

6. 6
Low Profile Linearity ExampleLow Profile Linearity Example
 We use a 10,000 lbf low profile type loadWe use a 10,000 lbf low profile type load
cell to examine excellent linearitycell to examine excellent linearity
behavior over a wide measuring rangebehavior over a wide measuring range

7. 7
Low Profile ConstructionLow Profile Construction
 Shear beam gage shownShear beam gage shown
Load Surface
Shear beam
Gage
Base

8. 8
Compression CalibrationCompression Calibration

9. 9
10K Compression (cont’d)10K Compression (cont’d)
 a. Nonlinearity is relatively low 0.02%FSa. Nonlinearity is relatively low 0.02%FS
 b. Calibration points fit a polynomial curveb. Calibration points fit a polynomial curve
very closelyvery closely
 c. Curve is smooth clear down to zero loadc. Curve is smooth clear down to zero load
(tested over 10 to 10,000 lbf range)(tested over 10 to 10,000 lbf range)

10. 10
Expanded Scale (10X magnified)Expanded Scale (10X magnified)

11. 11
Comparable behavior in tensionComparable behavior in tension

12. 12
Expanded Scale (10X magnified)Expanded Scale (10X magnified)

13. 13
Column Cell LinearityColumn Cell Linearity
 a.a. Nonlinearity relativelyNonlinearity relatively
large due to the expansionlarge due to the expansion
or contraction of theor contraction of the
column diameter with loadcolumn diameter with load
 b.b. But well-behaved smoothBut well-behaved smooth
calibration curves normallycalibration curves normally
fitting a 2fitting a 2ndnd
degreedegree
polynomialpolynomial
 c.c. Tension and compressionTension and compression
opposite polarity of non-opposite polarity of non-
linearitylinearity

14. 14
2160-1000 kN (225 Klbf) Example2160-1000 kN (225 Klbf) Example
Tension NL =
- 0.073%FS
Compression NL =
+0.053%FS

15. 15
Strain Gage InfluenceStrain Gage Influence
 The preceding example load cell wasThe preceding example load cell was
made with modified-Karma alloy gagesmade with modified-Karma alloy gages
 Constantan alloy gages tend to produceConstantan alloy gages tend to produce
higher nonlinearity, about 0.10%FS, in ourhigher nonlinearity, about 0.10%FS, in our
experienceexperience
 However, a generalization should not beHowever, a generalization should not be
made without considering all effects ofmade without considering all effects of
geometry and transverse gage factorgeometry and transverse gage factor

16. 16
Flange Mount Low Profile LinearityFlange Mount Low Profile Linearity
 a.a. Nonlinearity relatively low, benefiting from the solidNonlinearity relatively low, benefiting from the solid
hubhub
 b.b. But well-behaved smooth calibration curves, fit 2But well-behaved smooth calibration curves, fit 2ndnd
or 3or 3rdrd
degree polynomialdegree polynomial
 c.c. Tension – compression symmetry is excellentTension – compression symmetry is excellent

17. 17
1238 - 250 kN Flange Mount Example1238 - 250 kN Flange Mount Example
Tension NL =
- 0.008%FS
Compression NL =
- 0.010%FS

18. 18
Hollow Column LinearityHollow Column Linearity
 a.a. Nonlinearity slightly better than single columnNonlinearity slightly better than single column
 b.b. But well-behaved smooth calibration curves, fit 3But well-behaved smooth calibration curves, fit 3rdrd
degree polynomialdegree polynomial
 c.c. Tension – compression symmetry is goodTension – compression symmetry is good

19. 19
2350 - 2000 kN Hollow Column2350 - 2000 kN Hollow Column
ExampleExample
Tension NL =
- 0.070%FS
Tension NL =
- 0.032%FS

20. 20
2. Hysteresis2. Hysteresis
 A.A. Often misunderstoodOften misunderstood
 B.B. Descending calibration points are validDescending calibration points are valid
only for the particular FS value of a testonly for the particular FS value of a test
 C.C. Nevertheless, the measure ofNevertheless, the measure of
hysteresis has value as an indicator ofhysteresis has value as an indicator of
the range of error to expect from loadthe range of error to expect from load
points that do not necessarily ascendpoints that do not necessarily ascend
from zerofrom zero

21. 21
Example of good hysteresis behaviorExample of good hysteresis behavior
 Same 10,000 lbf low profile type load cellSame 10,000 lbf low profile type load cell
we examined for linearitywe examined for linearity
 Descending curve as well behaved asDescending curve as well behaved as
the ascending curvethe ascending curve

22. 22
Smooth descending curveSmooth descending curve
 H = +0.03%FS and descending curve closes atH = +0.03%FS and descending curve closes at
zero load. Closure requires well behavedzero load. Closure requires well behaved
hysteresis and very low creephysteresis and very low creep

23. 23
Expanded Scale (10X magnified)Expanded Scale (10X magnified)

24. 24
Many levels of performanceMany levels of performance
 In calibrating load cells from manyIn calibrating load cells from many
manufacturers around the world, it is seenmanufacturers around the world, it is seen
that not all are as well-behaved as thethat not all are as well-behaved as the
preceding examples of Interface cellspreceding examples of Interface cells
 The differences are in the subtleties ofThe differences are in the subtleties of
design and quality controldesign and quality control

25. 25
interface
Example of lower quality load cellExample of lower quality load cell
(not an Interface load cell)(not an Interface load cell)
A. Nonlinear near zero load
B. High hysteresis
C.Non-closure of zero return indicates high creep
D.Takes a 4th
degree polynomial to fit a curve

26. 26
3. SEB3. SEB
 Static Error Band (SEB) is often misunderstood. The SEBStatic Error Band (SEB) is often misunderstood. The SEB
output line provides a single slope calibration constant thatoutput line provides a single slope calibration constant that
minimizes error on average over a force range.minimizes error on average over a force range.
Demonstration of SEB vs Terminal Output
-0.060
-0.050
-0.040
-0.030
-0.020
-0.010
0.000
0.010
0.020
0 20 40 60 80 100
Load (%FS)
Errorfromstraightline(%FS)
Data points Terminal Output Line SEB Output Line
Ascending
Descending

27. 27
4. Output Symmetry4. Output Symmetry
 Important when both tension and compressionImportant when both tension and compression
loadings use the same instrumentation gainloadings use the same instrumentation gain
 Generally, low profile shear cells better thanGenerally, low profile shear cells better than
column cellscolumn cells
 Symmetry Error of our example cells:Symmetry Error of our example cells:
Low Profile 10 Klbf 0.01%Low Profile 10 Klbf 0.01%
LP Flange 200 kN 0.03%LP Flange 200 kN 0.03%
Hollow column 2000 kN 0.05%Hollow column 2000 kN 0.05%
Single Column 1000 kN 0.25%Single Column 1000 kN 0.25%

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5. Rejection of extraneous loads5. Rejection of extraneous loads
 It is desired to measure FzIt is desired to measure Fz
 Fx, Fy, Mx, My, Mz are extraneousFx, Fy, Mx, My, Mz are extraneous
Fz
Fx
Fy
Mz
Mx
My

29. 29
Axial Load vs. Eccentric LoadsAxial Load vs. Eccentric Loads
Axial EccentricAxial Eccentric

30. 30
Method of testing eccentric load sensitivityMethod of testing eccentric load sensitivity
 Sensitivity of lessSensitivity of less
than 0.1% / inchthan 0.1% / inch
is achieved onis achieved on
shear low profileshear low profile
type cellstype cells
A force is applied on
a moment arm while
monitoring load cell
output

31. 31
Eccentric adjustment exampleEccentric adjustment example
1020-25K Eccentric Load Plot
-0.40
-0.30
-0.20
-0.10
0.00
0.10
0.20
0.30
0.40
0 30 60 90 120 150 180 210 240 270 300 330 360
Position
Error(%/inch)
Initial Adjusted

32. 32
6. Sensitivity to mounting6. Sensitivity to mounting
 Degrees of reducing installation influenceDegrees of reducing installation influence
Basic cell Factory-
installed base
Factory-
installed stud
Integral
machined stud
D e c r e a s i n g m o u n t i n g s e n s i t i v i t y

33. 33
Preloading large threadsPreloading large threads
 Challenging, as in this 10MN wire rope testChallenging, as in this 10MN wire rope test
10 MN (2.2 Million Pound) load cell

34. 34
Advantage of flange load cellsAdvantage of flange load cells
 Screws can be installed with conventionalScrews can be installed with conventional
torque wrenches or hydraulic torquetorque wrenches or hydraulic torque
wrencheswrenches

35. 35
Example of mounting sensitive load cellExample of mounting sensitive load cell
(not an Interface cell)(not an Interface cell)
A. Nonlinear near zero load
B. High hysteresis
C.Requires hard bearing plates
D.No polynomial fit
100 Klbf

36. 36
Finite Element Analysis of MountingFinite Element Analysis of Mounting
 Performance is always dependent upon fixation of the load cell to itsPerformance is always dependent upon fixation of the load cell to its
live end and dead end structures. Here in this cutaway view thelive end and dead end structures. Here in this cutaway view the
screw clamping force is being analyzed.screw clamping force is being analyzed.

37. 37
Strain Gage AlignmentStrain Gage Alignment
Alignment of gages is critical to rejection ofAlignment of gages is critical to rejection of
extraneous loads. Here the gage can be seenextraneous loads. Here the gage can be seen
well-aligned with the precisely applied scribe linewell-aligned with the precisely applied scribe line

38. 38
Tension LinkTension Link
 Another type of high capacity load cellAnother type of high capacity load cell
 Load is measured between clevis pins inLoad is measured between clevis pins in
the ends of the cellthe ends of the cell

39. 39
Tension Link GagingTension Link Gaging
 Strain gages are strategically placed toStrain gages are strategically placed to
sense tensile stress in the web sectionsense tensile stress in the web section

40. 40
ConclusionConclusion
 Medium to high capacity load cells areMedium to high capacity load cells are
successfully made in a variety of stylessuccessfully made in a variety of styles
 The appropriate style is determined by theThe appropriate style is determined by the
demands of the applicationdemands of the application
 Measurement performance andMeasurement performance and
environmental needs can be met throughenvironmental needs can be met through
good engineering design andgood engineering design and
manufacturing of load cellsmanufacturing of load cells