Sizing and specification
Calculating fleet size
Time per task = travel out empty + pick + travel back loaded + drop + an allowance for queueing and passing
Fleet size = tasks per hour × time per task ÷ 3,600 ÷ availability per machine
Parameter
Value
Average travel speed
60–70% of rated speed
Jacking pick and drop
10–15 s each
Underride fork pick and drop
20–30 s each
Forklift, low level
30–45 s each
Forklift, high level
60–120 s each; the upper end for high positions
Tote robot
20–40 s each, by shelf height
Queueing allowance
15–25%; the upper end for narrow aisles and larger fleets
Availability
0.85, after charging, faults and servicing
Average speed is taken at two-thirds of rated because of turning, slowing and passing — the number most often overestimated. Round the result up and add one spare machine. High-level work should be banded rather than averaged: in a high-bay warehouse the top and bottom levels can differ by more than a factor of two, so weight by where the locations actually are.