Every undersized palletizer I have been called to look at failed on arithmetic, not on mechanics. The axes were fine. The numbers someone fed into the sizing spreadsheet were not. After a few of these autopsies, my pre-catalogue routine has shrunk to three checks.
1. The payload is never the box weight
A 25 kg case is not a 25 kg payload. You lift the case plus the gripper — a vacuum head with frame, ejectors and cabling easily adds 15–20 kg — and then you multiply by the dynamic factor your acceleration profile implies. At 1.5 m/s² of vertical acceleration, the Z axis briefly "sees" roughly 15% more than the static mass. I size for case + gripper + dynamics, then add a plain 25% margin, because the day production swaps in a heavier case format — and they will — nobody reopens the sizing sheet.
2. Cycle time is a sum of small betrayals
The catalogue number assumes an ideal trapezoidal move. A real cycle adds: gripper approach, vacuum build-up (0.3–0.8 s depending on cup volume and case porosity), lift, travel, place, release, retract. On a recent line the travel was 40% of the cycle; the gripper dwells were another 35%. If you need 10 cases a minute and your honest sum says 6.4 seconds per case, no servo tuning will save you — you shorten the path, overlap the vacuum build-up with the approach, or split the work across two axes zones. This is the same discipline we apply when we write down our sizing notes on palletizer work envelope, loads and cycle times — unglamorous, and it is most of the job.
3. Long spans sag, and sag eats repeatability
A single-drive X axis of 10–12 m will wind up like a torsion spring under acceleration, and the beam itself deflects under load at mid-span. The maths of a linear actuator is clean; a 12-metre gantry is not a linear actuator, it is a structure. Past roughly 8 m I start costing a dual-drive X with electronic synchronisation, and I check mid-span deflection against the placement tolerance of the case, not against the encoder resolution the brochure advertises.
None of this needs exotic software. It needs the discipline to write the three numbers down — real payload, honest cycle sum, span behaviour — before the catalogue opens. The catalogue is where projects go to become expensive.
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