General Motors has saved energy, improved productivity and optimised line side delivery operations at its Astra car plant in Ellesmere Port by switching to electric tow tractors from gas-powered units.
The introduction of the new tractors, powered by Hawker batteries from EnerSys, also reduced vehicle movements in production and storage areas.
Richie Mellor, senior engineer at the Worldwide Facilities Group, GMM Ellesmere Port, said: “We can use the new tractors right through the shift to deliver parts to the production line without any disruption to the operations. EnerSys and the tractor manufacturer have supplied us with a complete package that met our specification, delivered operational benefits and a number of savings.”
The Ellesmere Port plant manufactures Astra cars and vans and employs more than 2,000 people on the 130 acre site. The 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 system allows the trucks to be 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.
“This saves a lot of time each day. 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,” says Mellor.
With 25 tractors in the fleet this activity added up to 250 working minutes lost across each shift, time which 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 which 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,” says Richie Mellor. “Think about the amount of lost heat that saves. It also extends the life of the doors and means no dirt or dust is brought back inside.”
Although it recognised 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 USA the two companies knew that batteries on lift trucks in production facilities last an average 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.
“EnerSys is one of our global partners but we also looked at potential solutions from other suppliers,” says Richie Mellor.
One of the 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 and has won eight prestigious Green Apple Awards since 2000.
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 which allow the rack to be rolled out and presented at the line side. This is a new application in the factory which 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.”
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 inter-connectors 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.