Walk along a well-designed assembly line and you notice the operators rarely turn around. Everything they need arrives at the end of their arm, at the moment they need it. That is not a coincidence; it is the result of four decisions made before the line was built.
Decision one: the reach envelope
The comfortable working area for a standing operator is roughly 500 mm deep and 900 mm wide, centred on the workstation. Anything outside that envelope costs a step or a twist, and a twist repeated 600 times a shift becomes a back injury and a quality defect.
Primary parts go inside 400 mm. Secondary parts and tools go between 400 and 600 mm. Anything beyond 600 mm should be replenished by a tugger or a water spider, not reached by the operator.
Decision two: container size and weight
A container should hold between 30 minutes and two hours of consumption. Less than 30 minutes and the operator spends the shift swapping boxes; more than two hours and the container is too big for the reach envelope and too heavy to lift safely.
For a part used 40 times an hour, a 60-minute container holds 40 pieces. For a part used twice an hour, the same container holds two pieces, which is not worth a dedicated location. Low-consumption parts belong in a shared rack with a kanban card, not at the station.
Keep the full container under 12 kg. Above that, the lift becomes a manual handling risk and the container will be dragged rather than lifted, which damages the rack and spills parts.
Decision three: presentation angle
Gravity flow racks present parts at 15 to 20 degrees from horizontal. Steeper and the parts jam forward; shallower and they do not slide. The angle depends on the container base: cardboard needs more, plastic totes with a smooth base need less.
At the pick face, the container should sit at 900 to 1100 mm above the floor for a standing operator, and at 700 to 900 mm for a seated one. The pick opening should face the operator, not the aisle.
Decision four: the replenishment trigger
Two-tray kanban is the simplest system that works. Each part has two containers: one at the line, one in the supermarket. When the line container empties, the operator places the empty in the return lane and takes the full one. A water spider collects empties on a fixed route and refills the supermarket.
The trigger must be visible from the aisle, not from inside the rack. Colour-coded empty containers, an andon light or a simple card in a holder all work; a note on a clipboard does not.
What to avoid
- Shelving behind the operator. It looks tidy and forces a 180-degree turn every cycle.
- Parts in bags inside boxes inside racks. Every layer adds four seconds.
- Mixed parts in one container. Sorting at the station is a non-value operation that also creates defects.
- Racks that block the aisle. A 1.2 m aisle becomes 0.8 m as soon as a rack is added, and the tugger route breaks.
Cost and payback
A gravity flow rack costs 200 to 600 US dollars per location, depending on width and roller type. The payback comes from reduced reaching time and reduced replenishment travel. On a line with 12 stations and a 30-second takt, saving four seconds per cycle per operator is worth roughly 13 percent of direct labour, which usually pays for the racks within a year.
The calculation is worth doing before the purchase, because it also tells you which stations deserve the expensive racks and which can keep a simple shelf.
