Successful VNA
Introduction
Very Narrow Aisles (VNA) is an attractive solution for increasing storage capacity, particularly in a tall building. It is not uncommon for a new VNA system to become a nightmare, imposing a desperate limitation on warehouse throughput. Initial analysis may have demonstrated a capacity of 25 – 30 pallet moves per hour, whereas only 12 moves per hour are actually achieved.
The resulting crisis is not only expensive to put right, but can have a detrimental effect on customer service. Rather than fixing the cause, it is very common then to see radical changes made, including changing a proportion to wide aisles and moving the fast movers to a different warehouse. Alteration to the procedures or small design changes can usually achieve the design throughputs.
This article looks at what makes a successful VNA system by looking at a number of recent applications.
What is VNA
VNA is not defined by measuring the aisle width, but by the type of equipment used. There are two distinct truck types used in VNA applications. The first is the VNA pallet placer, which has a mechanism to slide the pallet sideways into the location. This type of truck is frequently called a combi, lateral stacker or turret truck.
The second type of truck is a high level order picker, where the operator has to handball the items in and out of the location. Most of these trucks have forks but they cannot be used to place a pallet in the narrow aisle since they are only forward facing. The actual width of the narrow aisle depends mainly on load size and weight but can be anything between 1.2m and 2.2m
Overcoming the limited resource
One of the most common problems of a new VNA system stems from the fact that it becomes a limited resource within the warehouse. Procedures that used to be acceptable in a more flexible warehouse aren’t adapted to cope with this limited resource. In one operation I saw recently, the VNA trucks were seen as a tool that was picked up when required. Over a shift only 12 moves per hour were achieved. Timing individual moves showed that most moves took 3 minutes, which should have given an overall rate of 20. This was caused by the operator being given a picking assignment that included wide aisle and narrow aisle work. When a pallet was required from the narrow aisle he looked for the nearest VNA truck, collected his pallet, swapped to a pallet truck and moved the pallet to despatch.
Even if the truck was used all the time, then there were other reasons why the predicted performance was not being achieved:
Picking individual orders required an aisle change between each move
The transfer aisle was furthest away from the doors
The putaway was done by a different operator so that dual cycling was
never achieved
The solution was to assign a VNA driver to a group of aisles. He was responsible
for all moves in this group and was able to some dual cycling and move
an average of 10 pallets between aisle moves. Most moves were then achieved
in less than 2 minutes.
Use Of P&Ds
A P&D is a "Pick up and Deposit Point". These are the interface between the VNA trucks and the other trucks. The are two basic designs: cantilever and castelated.
Of all the VNA operations that fail, most do not install or use P&D stations. The floor at the front of the aisles becomes congested and constrains throughput. The VNA operators tend to manage the whole process rather than concentrating on the VNA moves. In general the more P&Ds the better, as it de-couples the VNA from other operations, allowing the limited resource to work more effectively.
The usual reason given as to why the P&Ds aren’t used is that it is slower. This is true for the feeder counterbalance trucks, however they are not the limited resource. The VNA trucks should not be any slower, in fact the third level of the P&D is usually the optimum position to minimise VNA travel time. There are some VNA operations that do not use the P&Ds, these have very responsive feeder trucks, that work extremely closely with the VNA drivers.
Pallet Overhang
Installations where there is a varying pallet overhang require particular attention to the P&D design. It is important that the pallet and not the load is centred between the beams. To achieve this the feeder trucks need to place the pallet accurately. The position of the load is irrelevant as long as it is within the design constraints. Problem installations are apparent by the following observations:
The operators take 2 or 3 attempts to place each pallet
Looking down the aisle the wooden bases are not in a neat line
When carrying a load the driver cannot go at full speed for fear of his
load colliding with a load in the rack.
By designing the P&Ds with simple guides the operation can move upto
twice as many pallets per hour.
Are RDTs necessary
It is often believed that to achieve good throughputs from a VNA installation then Radio Data Terminals (RDTs) are essential. This is not the case. Simple pallet labels can achieve most of the same efficiencies. The main benefit of RDTs is to prioritise work, balancing the customer requirements with operational efficiency.
Mixed operations
When high level picking is too slow, it is often thought that pedestrian picking from the ground floor locations would overcome the problem. This is usually a good solution with the following provisos:
A good safety interface is installed to prevent a truck entering an aisle
with a picker in
As soon as there are more pickers than aisles, congestion becomes apparent,
as it is not possible to overtake.
The use of powered pallet trucks which can lift the picker to 1m should
be considered to increase the number of available pickfaces
Zoning
A VNA system can be improved by zoning the products. It is usually much better to keep the fast movers at low level rather than at the front of the aisle. It is not always understood that a truck goes slower when the mast is raised. Also frequent lifting massively reduces battery life.
Not all Operations are suitable for VNA
Not all operations can be converted to VNA. The usual reason is a potential shortage of aisles. Say 300 picks per hour are required at peak and 60 per hour are achievable. At first sight 5 trucks and 5 aisles are sufficient. However it is quite usual for demand of slow movers to be erratic and in one particular hour 50% of the demand maybe in a single aisle.
Having a large number of short aisles may also cause problems, as aisle changing in narrow aisle trucks is time consuming. Before a VNA system is sanctioned detailed analysis of actual orders and receipts is essential.