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Elastomers replacing rubbers in springs

16 July 2007

Thermoplastic elastomers are replacing conventional rubbers and steel in high performance springs used in railway rolling-stock buffers, draw gears and draft gears. German company Durel is in the vanguard of this trend, manufacturing spring pads from rods and tubes supplied by Quadrant Engineering Plastic Products (Quadrant EPP) in Arnitel thermoplastic copolyester elastomer.

Draft gears and buffers serve to smooth out train movement, providing comfort for passengers and protection against damage to freight goods, by separating the movements of individual railcars during acceleration and braking.

Copolyesters combine the strength and processing characteristics of engineering plastics with the performance of thermoset elastomers, providing benefits in processing and productivity. Arnitel has exceptional flexibility and outperforms conventional rubber and steel springs.

Durel's production process requires it to use spring pads formed from very high precision extrusions. It has found that Quadrant EPP is one of the very few companies that can meet its very precise and demanding quality requirements.

Quadrant EPP uses two extrusion processes to make the extrusions. Tube extruders equipped with vacuum calibrators ensure that the outside dimensions meet very tight tolerances. Equal accuracy can be obtained by using cold die extrusion, which the company uses for rods and a limited number of tubes, but this requires the line to run at a lower speed.

“The quality of the extrusion is critical to the application,” says Quadrant EPP sales engineer Françoise Donovan. “The surface of the rods and tubes must be absolutely smooth in order to prevent the spring from cracking or even exploding during the forming and machining operations that occur before assembly. Even a small fissure invisible to the naked eye would not meet Durel’s specifications.” She says that when Durel worked with other suppliers, it had to machine the stock shapes again in a lathe in order to achieve this perfection. “Working with Quadrant EPP saves them time and money.”

Quadrant EPP dedicates one extrusion line on a full time basis to production of stock shapes for Durel, and can allocate a second line when necessary. Total output amounts to between 20 and 30 tonnes per month. “Durel used to work with another extrusion company, but it was not able to meet those demands,” says Donovan.

“Quadrant EPP is our biggest elastomer supplier, it is capable of supplying high quality product, and it is flexible in production quantities,” says Durel general manager Christian Georg. “They are highly responsive to our special needs.”

In the springs, a series of doughnut-like pads in Arnitel are sandwiched between annular metal plates along a metal rod. Durel uses a patented process to make the springs, which entails pre-compressing the pads to improve their elastic response. “Pads that don’t undergo our forming process have a relatively low level of spring behaviour,” says Georg. “After we have powered them up, their properties improve by a factor of around three.”

Quadrant EPP says Arnitel-based springs should last for several decades, providing the most efficient energy absorption systems for a given railcar weight. The springs deliver high energy absorption, freedom from maintenance, and also toughness when exposed to extreme climatic conditions (temperature range from -60 to +50°C). Their predictable absorption and release of energy helps trains to start, accelerate, decelerate and stop smoothly. The pads do not rust, fracture or lose their springiness.

Durel is not the only company to make composite metal/elastomer springs, but it is the first to use Arnitel. According to Georg, the TPE is one of very few materials in the world that have the energy density to make the springs work.

Durel has been making the springs since 2000, and the business is now growing rapidly. Georg says the company expects to sell more than 30,000 springs this year, compared with some 10,000 in 2006. It sells them to railway companies in Europe, Russia, Africa and India, and has just entered the Chinese market.

Quadrant EPP says the technology used for making the springs could be extended to numerous other heavy duty shock or vibration absorbing and noise reduction applications. For the moment, however, Durel is concentrating on the railway business. “We are working at full capacity,” says Georg.

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