Replenishing pick locations: when and how much

A picker walks into the aisle with a list, reaches the shelf and finds it empty. Now he runs to the reserve area, asks where the goods are and waits for a forklift. Those minutes appear in no report, yet by the end of the day orders leave late, people are tired and the customer is waiting. In many warehouses slow picking is not the pickers' fault; the pick location is simply not being refilled in time.

Two locations: pick and reserve

In many warehouses part of the stock sits at the pick location (the lowest level, near the aisle) and the rest in the reserve area, on upper levels or on a separate rack. The pick location is built for fast picking: small and convenient. The reserve holds a lot but picking from it is slow. The pick location usually gets a day or two of demand; the rest stays in reserve. Replenishment is the timely move of goods from reserve to pick location. If it is late, the picker stops at an empty shelf; if it is overdone, the goods do not fit and block the aisle.

Working out the limits

Each item needs two numbers: a minimum quantity (min) and a maximum (max). Min is enough stock to keep the pick location from running dry while replenishment is under way. Max is the capacity of the location.

A conditional example. An item is picked at 96 units a day on average, with an eight-hour shift, so 12 units an hour. Replenishment (take from reserve, bring, put away, record in the system) takes about an hour and a half, during which 18 units go. For the unexpected we add half as a buffer: 9 units. That is 27, rounded to a min of 30. On a busy day demand doubles, so for those days min = 60. If the shelf holds 120 units, max = 120. When stock falls to min a signal is raised, and at replenishment the location is topped up to max.

The figures are conditional. Measure last month's picking volume and your own replenishment time.

When to replenish: three ways

• By limit: replenish when stock falls to min. Accurate, but stock has to be watched.
• By schedule: at the start of the shift and after lunch a responsible person walks round all pick locations. The easiest route for a small warehouse.
• On request: the picker sees the gap and reports it. The weakest way, since the stop has already happened.

In practice a mix works well: by schedule in quiet hours, by limit for fast movers.

Who does it and when

Assign replenishment to one named person, otherwise "everyone does it" turns into "nobody does it". The best time is before the busy hours: in the morning at the start of the shift, or at the end of the evening shift as preparation for tomorrow. At peak time the aisles are full of pickers and a forklift gets in their way. If the move needs a forklift or trolley, book its time in advance: without equipment, replenishment runs late.

Replenishment is also a movement: record what, from where, to where and how much. Otherwise the system balance and the shelf balance drift apart. New stock goes behind or below the old, not on top of it, so first in is first out.

How to keep control

Three simple measures show whether the process works:

• how many times a day a picker met an empty location: the aim is to approach zero;
• the time from the signal to the goods being on the shelf: it should not exceed the calculated replenishment time;
• the gap between system balance and shelf balance after replenishment: once a week compare five random locations.

The person in charge spends fifteen minutes a week reviewing these figures. If an empty location keeps recurring for one item, recalculate its limits or the size of its pick location.

Common mistakes

• One and the same limit for every item, although demand differs.
• A pick location too small for demand, forcing replenishment three times a day. Enlarging it or adding a second location is cheaper.
• Replenishment is not recorded and the balance goes astray.
• Replenishment falls in the busiest hours.

Conclusion

Replenishment is easy to overlook, but it noticeably affects picking speed. During a warehouse audit, SkladKontrol looks into the causes of stoppages in the picking area, while cycle counting helps to check pick-location balances regularly without closing the warehouse.