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General Motors has saved energy, improved productivity and optimised line-side delivery operations at an Astra plant by switching to electric tow tractors powered by Hawker batteries from Enersys.

The introduction of these tractors to replace gas-powered units also reduced vehicle movements in production and storage areas.

‘We can use the new tractors right through the shift to deliver parts to the production line without any disruption to the operations,’ said Richie Mellor, senior engineer at the Worldwide Facilities Group, GMM Ellesmere Port Plant.

‘Enersys and the tractor manufacturer have supplied us with a complete package that met our specification, delivered operational benefits and a number of savings,’ he added.

The Ellesmere Port plant manufactures Astra cars and vans and employs more than 2,000 people on an 130-acre site.

This operation incorporates a number of processes that require just-in-time delivery of components and assemblies direct to the line side.

Parts are handled on special trolleys and carts, which until 2008 were towed by LPG powered tractors.

A major drawback was that these tractors had to unhitch from the trolleys and leave the production building and visit outdoor gas bottle stores whenever they needed refuelling.

This generally occurred once every shift, which was time consuming and could disrupt the smooth operation of the production line.

The tractors were noisy and had begun to suffer from reliability problems.

As part of its ongoing investment programme, the site identified benefits and savings it could achieve by switching to electric-powered tractors operating continuously throughout the shift.

It approached a number of potential suppliers and, after testing equipment on site for a year, decided on a combination of 25 Linde P602 tow tractors and Hawker batteries with Hawker Lifespeed high-speed chargers.

The choice of battery and charger was crucial to the whole project because these allow the trucks to top-up charged for short periods during scheduled work breaks to maintain enough power to operate right through the shift.

This is supplemented by overnight charging at the weekend, when the factory is shut down, to ensure the batteries are always full for the start of the next week’s first shift.

‘With the old tractors it took around 10 minutes to leave the working area to replace the gas bottle, but with the new electric tractors we don’t need to do this,’ said Mellor.

With 25 tractors in the fleet this activity added up to 250 working minutes lost across each shift, time that is now available for productive work.

It also eliminates the need for the tractors to go outside, which means they are not exposed to rain and dirt and reduces the need for cleaning.

The new tractors are reliable and only need to be serviced twice a year, whereas the old units were serviced every two months.

‘With no need to leave the building we save 50 truck movements each shift and avoid opening the doors as often,’ said Mellor.

Although recognising the potential benefits in switching to electric power the factory still needed to present a solid business case.

Enersys is a global motive power partner for General Motors and, from operations in the US, the two companies knew that batteries on lift trucks in production facilities last an average of five and a half years.

Enersys has used information on charging and utilisation patterns downloaded from the chargers since 2008 to predict a seven-year lifetime for the batteries on the tow tractors.

One of the very few disadvantages in switching from gas to electric power was the new wiring needed to supply the chargers.

There was a cost involved but the site estimates it is saving around 50 per cent on fuel and this will offset the investment well within the projected lifetime of the tractors and batteries even without taking into account the savings in maintenance, cleaning and heating.

Energy saving was a big driver for the project.

Ellesmere Port had reduced the energy required to produce each Astra by 50 per cent since 2003.

The tractors are used for two operations in the plant.

The first is a kanban operation that ensures delivery of small parts in boxes to the line side.

Production staff call for more components whenever needed and these are delivered to the point of fit by the tractor.

Full boxes are placed on gravity-fed racks and empty boxes removed for return to the store.

The second application is known as ‘dolly delivery’ and involves the transfer of larger components and assemblies to the line side.

These items are in a tilting steel container placed on a presentation rack.

A special trailer holds up to four of these racks ready for transfer to the appropriate location in the production area.

The trailer has side gates that allow the rack to be rolled out and presented at the line side.

This is a new application in the factory that was previously serviced by lift trucks handling two stillages at a time.

It has therefore reduced the number of lift truck movements by removing the handling requirement and improved overall line side efficiency.

‘Our objective is to ensure all parts are delivered in the right sequence for the line,’ said Mellor.

‘Removing the lift trucks was a safety consideration.

‘It’s much quieter now and the racks are taken to line side in a more organised way,’ he added.

The plant specified the largest capacity batteries that would fit into the chassis.

The units are conventional lead/acid but designed for rapid charging applications.

For example, cell interconnectors have been configured for the higher currents delivered by the 10kW Lifespeed chargers.

The battery and charger communicate with each other to ensure the correct amount of energy is supplied as quickly as possible but without overcharging.

The batteries are designed for low water consumption with low gassing emissions for minimal maintenance and extended lifetimes.

This allowed the chargers to be positioned without restrictions.

One of the key decisions was where to install the chargers.

The kanban operation relies on tractors working in dedicated stores areas to feed specific parts of the line and the most efficient position was adjacent to the storage racking.

For dolly delivery, where the trucks are wider roaming, the more efficient option was to group charging points close to where the drivers take their breaks.

This maximises the amount of time available for charging, 30 minutes during lunch or 10 minutes during tea breaks.

The plant is currently running two shifts but expects to move to three during the first quarter of 2011 as demand for cars and vans continues to increase.

In practice, the short-term charging is barely needed to keep the batteries topped up for two shifts but this will change in the future.

‘Enersys downloads information from the chargers regularly and this shows that the system can cope with three shifts,’ said Mellor.

‘We’ve tested the tractors to full capacity and have the evidence that proves it works,’ he added.

The tractor drivers were given training on the equipment and undertake basic checks on their own equipment each day.

Routine service and maintenance of the batteries is carried out by Enersys engineers.

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