Sizing the axes of a Cartesian palletizer: the three numbers I check before opening any catalogue

Every undersized palletizer I have been called to evaluate failed due to mathematical errors, not mechanical issues. The critical issue lies in the numbers inputted into the sizing spreadsheet. My pre-catalogue routine now consists of three checks.

Firstly, the payload is not simply the weight of the box. A 25 kg case carries a payload of more than just the box weight. The vacuum gripper, its frame, ejectors, and cabling add an additional 15-20 kg. Furthermore, when accounting for the dynamic factor resulting from 1.5 m/s² of vertical acceleration, the Z axis experiences roughly 15% more load than the static mass.

I size for the combined weight of the case, gripper, and dynamics, then add an additional plain 25% margin to account for potential changes in case format that may occur in the future.

Secondly, cycle time is more than just the catalogue's ideal trapezoidal move. It includes additional factors such as gripper approach, vacuum build-up (which can range from 0.3 to 0.8 seconds depending on the cup volume and case porosity), lift, travel, placement, release, and retract. In a recent case, travel time accounted for 40% of the cycle, while gripper dwell time added another 35%.

To meet a production rate of 10 cases per minute, the honest cycle time sum was calculated at 6.4 seconds per case. This highlights the importance of accurate calculations, as mere adjustments to the servo tuning will not suffice. To improve efficiency, one must consider shortening the path, overlapping vacuum build-up with the approach, or dividing the work across two axis zones.

Lastly, long spans, particularly those exceeding 8 metres, are subject to sagging. Under acceleration, the single-drive X axis behaves like a torsion spring, and the beam itself deflects under load at its midpoint. Although the mathematics for a linear actuator may seem straightforward, the actual structure behaves differently. I therefore start considering a dual-drive X configuration with electronic synchronisation past roughly 8 metres.

Furthermore, I ensure that mid-span deflection is checked against the placement tolerance of the case, rather than relying solely on the encoder resolution advertised in the brochure.

These three points - accurate payload, realistic cycle time, and consideration of span behaviour - represent the core of my pre-catalogue sizing process. These are the numbers I must verify before opening any catalogue. The catalogue, despite its importance, often leads to increased expenses if these critical factors are not carefully considered.