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