A filling machine’s load cell kept drifting. The weight reading at zero would shift by 20 g after a week, then 50 g after a month. The operator re-zeroed it every morning. The customer thought the load cell was failing. It wasn’t failing — it was being overloaded every cycle.
The overload that doesn’t look like overload
The load cell was rated for 50 kg. The product being weighed was a 25 kg bag. The cell was at 50% of rating, well inside the safe range. But the filling process dropped the product from 300 mm above the cell. The impact load at the moment of the drop — before the cell’s damping settled — was about 3x the static weight. That’s 75 kg. The cell was designed to tolerate 150% of rated load (75 kg) for a few cycles. The machine ran 20 cycles a minute. The cell was hit with 75 kg, 20 times a minute, all day. The strain gauges fatigued. The zero shifted. The cell drifted.
The datasheet’s overload rating (usually 150%) assumes occasional overloads, not continuous impact. Continuous impact loading on a load cell is a different failure mode: the gauges deform permanently, the zero point moves, and the calibration curve becomes non-linear.
The fix that didn’t need a bigger cell
We sized a bigger cell — 100 kg instead of 50 kg. The impact load of 75 kg is now 75% of rating instead of 150%. The cell runs in its linear range. But the bigger cell has less resolution at 25 kg (the output is half). The resolution went from 5 g to 10 g. For this application, 10 g resolution was fine — the product tolerance was ±50 g.
The real fix was mechanical. A rubber buffer pad was installed under the weigh hopper to absorb the impact. The product now lands on the pad and the hopper, not directly on the cell. The impact load dropped from 3x static to 1.2x static. The cell now sees 30 kg max. It runs in its sweet spot. Drift gone.
The third fix was the filling control. The PLC was opening the fill valve fully until the target weight, then slamming it shut. That caused the product column in the hopper to bounce. The fix was a two-stage fill: fast fill to 90%, slow dribble fill for the last 10%. The impact dropped, the overshoot dropped, and the cell’s settling time dropped. The machine fills faster now because the cell settles in half the time.
The load cell protection checklist
| Protection | What it does |
|---|---|
| Rubber buffer / dampers | Absorb impact before it reaches the cell |
| Two-stage filling | Slow the last 10% to reduce impact |
| Overload stops (mechanical) | Hard stops limit cell travel at 110% of rating |
| Proper cell sizing | Run at 30-70% of rating for linear response |
| Flexible mountings | Isolate thermal expansion and vibration |
| Environmental seal | Keep dust and moisture off the strain gauges |
The zero-drift diagnosis
When a load cell drifts, check the mechanical system first, the electronics second, the cell third. Ninety percent of drift problems are mechanical: a product buildup on the hopper, a binding flexure, a rubbing cable, a cooling fan blowing on the cell. The cable rubbing on the machine frame is a classic — the insulation wears, the signal leaks, the zero drifts. On this machine, the cable had worn through its sheath where it passed a sharp edge. The zero drifted more in humid weather. A $3 cable gland and a cable route with no sharp edges fixed it.
Load cells fail from impact, not from weight. A 50 kg cell hit with 75 kg impact, 20 times a minute, will drift no matter how good it is. Add a buffer pad, two-stage fill, and check the mechanical path before you blame the cell. The drift was the machine telling you it was being hit too hard.
