Crane takes the strain
October 1st 2006

When an unreliable and inflexible automated stacker crane needed replacing at the Oxford University Press distribution hub, a twin manual stacker crane solution from Jungheinrich proved to give better space utilisation, greater flexibility, faster throughput and lower costs than alternative VNA lift truck options

Automation may bring many benefits, but flexibility and longterm reliability aren't always guaranteed, as specialist academic publisher Oxford University Press has discovered over recent years at its Corby central distribution centre.

"Our previous crane was over 15 years old and we were getting a lot of problems with it through wear and tear," explains OUP's engineering manager. "As for the racking we had, it was so old that it wasn't compliant with modern SEMA recommendations…"

When the decision was made to replace both crane and racking, the firm invited a number of materials handling equipment suppliers to tender, challenging each one to come up with the maximum number of storage locations within the fixed space of the previous bulk store.

"We approached four manufacturers for their suggestions and looked at how many pallet locations each was providing. Three of them wanted us to have VNA trucks of one kind or another but Jungheinrich just came in, looked at the store and gave us more locations than any of the other options," says the engineering manager.

The end result was the installation of two manual stacker cranes and 6200 pallet locations of fresh racking, all supplied and installed by Jungheinrich. The two railguided man-up stacker cranes are used across two eight-hour shifts a day for both full pallet storage and picking operations within the 20m high racking, working in six aisles just 1500mm wide (pallet to pallet) and almost 50m long.

Compared to a VNA lift truck solution, the new cranes have brought a wealth of benefits, says OUP. To begin with, the mains power means there's no daily charging to be carried out. The cranes are also rated at 100 Amps, rather than the 250-300 Amps typical of most lift trucks, so the running costs are significantly lower.

They're also much faster than VNA trucks in terms of acceleration and maximum travel speed, as well as lifting and lowering speeds. And on top of all that, they only need servicing every 700 hours, rather than the 250 hour service intervals typical of most VNA trucks.

Problems covered Compared to the single automated pallet crane used before, meanwhile, the presence of two cranes rather than one means the store can still function even if there's a problem with one. Being manual units, they also have the advantage of allowing both full pallet and picking operations, to which end a special mechanism lowers the operator cab partially to bring drivers down to the level of the lateral forks as required.

Onboard fluorescent lighting provides sufficient illumination for drivers to read location information off individual pallet positions, doing away with the need for the previously unlit store to be given any fresh lighting. Integrated seats allow the drivers to take the weight off their legs when not directly engaged in picking.

Transfer from aisle to aisle is achieved without the normal complications of a special transfer car. Instead, the guidance rails just curve around at aisle ends into a transverse transfer aisle at one end of the racking. This allows drivers to simply turn into and out of individual aisles as they wish and also allows for a further run of racking set across the aisle ends around the two conveyorised P&D stations.

Simple mechanism Turns in and out of the aisles are via a mechanised system on the lower guidance rail which allows a driver-adjustable metal wedge on the front of each crane to simply flip the spring loaded points as needed.

With no moving parts it's a very simple mechanism with almost no points of failure – a boon compared to the complicated combination of hydraulics, mechanics and electrics often found in other systems.

Installation of the racking was also much simpler than it might have been with VNA lift trucks, as the racking uprights were all set on adjustable studs, doing away with the need for a superflat floor.

An exhaustive electronic protection system has also been installed for health and safety purposes. This ensures no power is provided to the cranes unless two doors leading into the store are bolted shut, preventing pedestrians from accessing the zone while the cranes are working.

Individual aisles are also protected by light barriers and a traffic light system that shows clearly when aisles are occupied by a crane and cuts off power to the aisle if a second crane attempts to access it. The two cranes also have sensors that will cut off power if they come within two metres of each other, and individual aisles are also alarmed to ensure pedestrians can only access them once they've been switched to maintenance mode.

The whole system only went live this year but already, it's obvious how reliable it is, says OUP's engineering manager. "I'm very happy with the performance of the cranes. My expectation from past experience was that we'd only get about 50-60% availability but nothing's gone wrong at all so far, thanks to the quality of the machines."

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