Single-Ended Beam Load Cells

Take the beam as a cantilever and cut it anywhere between the bolted end and the point where the load arrives. Two internal actions cross that cut. The shear force equals the applied load, full stop. The bending moment equals the applied load multiplied by the distance from the cut to the load. Distance is in one expression and not the other: gauges reading bending see a signal that changes as the load moves along the beam, gauges reading shear see the same number wherever it sits.

That buys tolerance, not indifference: the loading hole is still where the load is meant to arrive. But a load that lands off its centreline, or through a foot that has rocked out of position, reads correctly anyway, which is why shear beams end up under decks and vessels where the load path is built on site.

A bending beam gauges the top and bottom surfaces, where bending strain is largest. Its output is proportional to moment, so the distance from the mount to the load point is part of the calibration and the loading point has to stay where the design put it. Bending beams are consequently lighter-duty, tightly fixtured parts. Our BSH runs 50 to 300 kg and is exactly that.

A shear beam gives up some of the strain magnitude a surface-gauged beam enjoys, and gets back positional tolerance and a much stiffer section. That is the right trade above roughly a thousand pounds per corner, where nothing is going to hold the load point to a millimetre.

NTEP is the National Type Evaluation Program run by the National Conference on Weights and Measures. It evaluates a weighing component as a type (not each unit) against NIST Handbook 44, and issues a Certificate of Conformance describing exactly what was tested. Where a scale determines a price or a quantity in commerce, every element in the chain needs one.

The SBS is covered by NTEP CoC 95-146, and that certificate applies to the 1,000 through 10,000 lb capacities, and the store's own option labels agree, since exactly those ratings are marked NTEP. Be blunt about the edges. The 250 and 500 lb below the band, and the 15,000 and 20,000 lb above it, are the same part built the same way to the same ±0.03% specification, but they sit outside the evaluated type, so a scale built on them cannot be sealed for trade however well it performs. Nor does the certificate cover the finished scale: the indicator and the system have their own path, and one uncertified element breaks the chain.

The SBS is alloy steel, buyable at 250, 500, 1,000, 2,500, 4,000, 5,000, 10,000, 15,000 and 20,000 lb, compensated over -10°C to +50°C and safe to +70°C. The SBSB is stainless, 1,000 to 10,000 lb, compensated over the narrower -10°C to +40°C. Across the overlap the SBS is the cheaper part and the certified one; outside it, the only one. Both sheets carry capacities the store does not stock (750, 2,000 and 3,000 lb on the SBS, metric ratings to 5,000 kg on the SBSB), built to order.

Output is 3.0 mV/V ±0.1% on the SBS; the SBSB is specified as 2.0 or 3.0 mV/V ±0.1%, so state which, and never mix sensitivities on one deck. Both take 10 V excitation and are 350 ohm output impedance, so check the indicator can drive four in parallel. Insulation resistance is ≥5,000 MΩ at 50 V DC. It is the first thing to measure on a drifting scale, because a load cell taking on moisture shows there before the reading does.

As the beam deflects, the loading point does not travel straight down. It swings through a small arc about the fixed end, so the top of the load path moves horizontally on every cycle, and a rigid post between deck and loading hole turns that motion into a side force fighting the measurement. A long steel deck is the larger version of the same problem, growing and shrinking with temperature against four load cells anchored to the foundation.

The fix is a loading interface that lets the deck move while still transmitting vertical force honestly: a rocker or self-aligning foot screwed into the loading hole, or a link that hangs the load. Check rods and bumpers then locate the deck against sideways shove, set with clearance so they carry nothing. Anything snug is carrying load.

Four load cells wire into a summing junction box, which parallels the bridges and returns one signal to the indicator. It is not a terminal strip: it carries the trim resistors that scale each load cell's contribution, and using them is a required commissioning step.

No two load cells have identical sensitivity and no deck distributes load evenly across four supports, so the same test weight over each corner reads four different numbers. Trimming pulls the high corners down until all four agree, and only then do you span-calibrate. The other order bakes in whichever corner you happened to load. If the corners will not converge, look at the structure: a deck sitting on three of its four feet is an error no trim resistor can reach.

No, and this is the detail most often got wrong. CoC 95-146 covers the SBS from 1,000 through 10,000 lb. The 250 and 500 lb ratings, and the 15,000 and 20,000 lb ratings, are built and specified identically but sit outside the evaluated type, so they cannot be used in a scale sealed for commerce. The SBSB carries no NTEP certificate at any capacity.

No. Both families are specified for compression, with load arriving downward through the threaded hole. Hanging a load from that thread loads the element backwards and the calibration does not apply. For tension use an S-beam.

Often. The datasheet lists the SBS as equivalent to the Vishay Sensortronics 65023, Vishay Revere Transducers 5123, Rice Lake RL30000, Vishay Tedea-Huntleigh 3411 and Artech 30310. Confirm the part coming out is also 3.0 mV/V, and replace all four at once where you can.

Not without recalibrating. The connection is four-wire (red and black for excitation, green and white for signal) with no sense lines, so cable resistance is part of what was calibrated at the factory. The SBS ships with 20 ft (6.0 m), the SBSB with 6 m. Coil the excess and strain-relieve it to the frame, never to the moving deck.