Toothed belts push the limits

Tom Shelley reports on state of the art for industrial and automotive toothed belts.

Toothed belts can now be made that are as strong as steel chain. These have the advantage of not needing lubrication or adjustment to compensate for wear, and run happily for years at high temperature.

For automotive applications, a further breakthrough has been made by using sprockets that are slightly oval. This reduces vibration and loading on the belt, which helps to enhance belt life.

Gates, a fluid power and power transmission company, has separate industrial and automotive divisions and although the underlying technologies for the products are the same, the capabilities offered to both sectors are slightly different.

Wolfgang Körfer, business unit director for Europe, is responsible for the industrial products. He says: "Our range of rubber products run at up to 150ºC, plus a bit higher for special applications. For standard polyurethane they run at up to 85ºC and for high temperature polyurethane based belts up to 120ºC."

Most products use glass fibre cord for strength. However, polyurethane belts branded Poly Chain use carbon fibre, giving them immense strength. As the brand name implies, they are being promoted as a direct replacement for chains and require neither re-tensioning nor lubrication.

Various applications have already been found for them ranging from industrial machines to motorcycles. Industrial belts of various kinds have been supplied down to 3mm width for servo drives and up to 340mm width for pump applications. As for all belts, it is essential they are tensioned correctly for maximum life.

Andrew Vaux, responsible for technical support and training for the automotive aftermarket in the UK says the new and correct method of tensioning, "is to use an special instrument which uses a non contact sensor to measure the natural frequency of vibration when the belt is tapped."

Nigel Blunsden, customer support manager for the automotive products says: "Provided they are correctly designed in [to engines], automotive timing belts will run for up to 240,000km in a vehicle. And a belt is being proposed for an existing customer's engine that will push this to 300,000km, substantially more than the life of most engines."

The achievement of very high mileages can be assisted by using sprockets that are slightly oval on crank shafts, cam shafts, or fuel injection pumps with radii typically varying by 1.5 to 3.5mm.

In response to new emissions regulations, particularly for diesel engines, these Eco Sprockets have been developed to reduce belt loads and angular vibrations while improving dynamic belt drive performance. The design of these sprockets is supported by a specially written dynamics software tool, which calculates optimum design from input phasing, torque and radii. The bottom line is that these belts can be made narrower, cheaper higher and allow for higher vehicle mileages.

Running temperatures can be from -40 to 180ºC, although full life tests on engines turned over by electric motors are undertaken in hot boxes maintained at up to 200ºC. The individual glass fibre cords are designed to withstand up to 1000N load each, or in some case 1500N. In order to ensure reliability the belts are designed to withstand peak loads of up to 30,000N and a maximum speed while normal engine running conditions only imposed loads of around 3000N at maximum engine speeds of up to 6,000rpm.

Tom Shelley

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