S-Beam Load Cells

The web is the only part of an S-beam where strain is a clean function of axial load. It sits on the neutral axis of the body, which means bending moments arriving from off-axis load produce roughly equal and opposite strain above and below it, and those cancel in the bridge.

The curved arms do the opposite job. They are compliant by design, so a moment applied perpendicular to the load axis is absorbed as elastic deflection in the arms rather than being transmitted into the web. That is the mechanism behind an S-beam's tolerance of imperfect mounting, and it is why gauging the arms would destroy the very property that makes the geometry useful.

It also sets the limit. The arms can only absorb so much before the web starts seeing the moment as apparent load, which is why an S-beam is more forgiving than a bending beam but less forgiving than a shear beam at the same capacity. Past that point the error is indistinguishable from real force, because to the bridge it is real strain.

Four foil gauges are bonded to the web and wired as a Wheatstone bridge, arranged so that under load two of them stretch and two of them compress. Opposite arms of the bridge therefore move in opposite directions, and their contributions add rather than cancel.

Reverse the load and every gauge reverses with it. The bridge output changes sign, so the same load cell reads pull as positive and push as negative. On the BSS, rated output is 3.05 mV/V, which is 30.5 mV at the rated 10 V excitation. That is why one part number covers pull and push with no change of hardware.

The same arrangement is what makes the load cell insensitive to temperature. A uniform temperature change affects all four gauges equally, and because they sit in opposing arms the effect cancels at the output rather than showing up as drift.

Height. The load path runs through the long axis of the body, so an S-beam needs vertical room between its two connection points. Where clearance is the constraint, a pancake or a load button does the same job in a fraction of the height.

Capacity headroom. Our S-beams top out at 20,000 lb. Beyond that, a pancake or column load cell carries the load.

Lateral tolerance. An S-beam handles a few degrees of misalignment through its arms, but it is still an axial sensor. If side load cannot be designed out, a shear beam tolerates it better and will hold its accuracy where an S-beam will not.

Rated output differs by family. The BSS is 3.05 mV/V, so 30.5 mV at full load on the rated 10 V excitation. The BAB-MT2 is 2 mV/V, or 20 mV at 10 V. The BMF runs 1.0 to 1.3 mV/V depending on the model.

Two things follow. Check that the instrument's input range suits the signal, because a smaller signal uses less of it. And never mix sensitivities on one summing junction box: its trim resistors are there for the small differences between load cells of one family, not a gap that size.

A rod end is a threaded shank carrying a spherical bearing. Screwed into the load cell's threaded connection, it lets the attached member swivel through several degrees while the force stays on the load cell's measuring axis. It is the default answer wherever the two mounting points cannot be guaranteed collinear, which in a fabricated frame is almost always.

Specify the bearing's static radial rating above the load cell's safe overload, not above its rated capacity. A load cell that survives a 150% excursion is of no use if the rod end fails first. Match the thread to the capacity band rather than to a general rule, because it changes across the range.

A clevis is a U-shaped bracket with a pin through both ears and through the load cell's connection. Unlike a rod end, it allows rotation about one axis only, which is exactly what you want on a crane or hanging installation where the load swings in a predictable plane.

Pin shear strength has to exceed the load cell's safe overload rating with margin, and the pin should be a close fit in the bore. A loose pin lets the assembly shift under alternating load, which shows up as hysteresis in the reading long before it shows up as a mechanical problem.

Where a threaded stud goes straight into the load cell with no bearing, a matched pair of spherical washers between the load cell and the frame takes up angular error. The convex and concave faces slide against each other during tightening and then lock, so the joint ends up square to the load path even when the surfaces it bolts to are not.

Direct stud mounting with no compensation is acceptable only when the load path is verified axial. Any angular error left in the joint becomes a permanent offset that no calibration will remove, because it varies with load.

Thread rods into the load cell by at least one full thread diameter. Less than that and the joint carries load on the first few threads only, which concentrates stress exactly where the section is smallest. On a 1/2 inch connection that means half an inch of engagement as a minimum.

Do not use the load cell body to react tightening torque. Hold the flats or the wrench feature on the fitting, never the load cell, and never apply torque through the element. A load cell that has had a rod tightened against its own body may read correctly at zero and still be out of specification under load, because the element has been pre-stressed.

Exact thread sizes for every capacity are in the dimensions table on each product page.