Load Cells
Erwin Kohler
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Definition
• A load cell is a force transducer that converts force or weight into an electrical signal
• The strain gage is the heart of a load cell
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Factors Affecting Load Cells
• Manufacturer:– The precise positioning of the gage– Mounting procedure – Materials used
• User:– Impact and hysteresis– Test modes: tension/compression– Off-axis loads
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Classification and ApplicationSingle point
Platform scales1-1000 Kg Compression
Silos, tanks and weighbridges5,000-60,000 Kg
Bending beam
Low profile: load and pallet scales
5-500 Kg
Shear beam
Low profile: load and pallet scales500-5000 Kg
S-type (tension-compression)
Hanging scales 50 – 5000 Kg
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Compression Load Cells
Preferred by testing engineers– Higher output– Better fatigue life– Better resistance to extraneous loads– Greater stiffness– Overload protection
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Load Cells in Engineering Testing
Force versus deflection. Fatigue tests Adhesive and paints peel force
Falling Weight Deflectometer (pavement testing)
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Load Cells Errors
• Impacts and hysteresis• Metal retains a history of its previous loadings• When the loading is properly done, then the curve will return to
zero
• If, by mistake, 80% of load is applied by impact and then returned to 60%, then the load cell would be operating on a minor hysteresis loop
• DON’T IMPACT LOAD CELLS
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Load Cells Errors• Test modes and calibration
• Load cells are routinely conditioned in one mode (either tension or compression) and then calibrated
• Determine test mode needed
• Some cells can be operated in both modes without reconditioning, but result in non-return to zero after looping through both modes
• Special fatigue load cells: • Sharp corners and edges are polished to avoid crack propagation
• Longer service life including fully reversed, full capacity loading cycles
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Load Cells Errors• On-axis loading
• All on-axis loading generate some level of off-axis extraneous components because of load carrying members causes friction that transmits side load
• Avoid flat-to-flat load, because it is impossible to maintain two surfaces parallel enough to guarantee that the force will end up being centered
• Compression cells have load buttons
Load button:
- 4” for 10,000 lbf - 6” for 50,000 lbf -12” for 200,000 lbf
3/8” off center with 5 degrees misalignment causes large moment
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Load Cells Errors• Nonrepeatibility
• Function of the resolution and stability of the signal conditioning electronics
• Load cells typically have nonrepeatibility which is better than the load frames, fixtures and electronics used to measure it.
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Reference Prices
LOAD SENSORS.Low Cost Compact Compression Load Cells $399
.Subminiature Compression Load Cells - High Stability $395
.Miniature Load Beams $599
.Miniature & Compact Tension/Compression Load Cells - High Output $565
.Industrial Flat Disc Tension/Compression Load Cells - Stiff Shear Design $899
ACCESSORIES
.Compression Load Button Adapters for Load Cells $60
.Tension Pull Plates for Load Cells $150
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Manufacturers
• Measurement specialists www.measurespec.com
• Entran sensors and electronics www.entran.com
• Tedea Huntleighwww.aboutloadcells.com
• Interface www.interfaceforce.com