Material success for timing belts

Paul Boughton

The driving force for innovations in timing belts has been the automotive industry, with the increasing demands on service life, noise characteristics, temperature resistance and reliability. David Clark reports.

In recent years, the development of timing belts has been heavily focused, on the one hand, on the optimisation of timing belt geometry to increase the power density and to improve noise characteristics and, on the other hand, on the development of new, more powerful and more resistant materials.

As a result of new engine technologies and improved sealing of engines, the ambient temperatures and dynamic forces have increased significantly, while there have also been significantly higher service life expectations. The dynamic loads on some engine types, eg direct-injection diesel engines, have almost tripled in the past 20 years, with ambient temperatures rising above 130 degC. The result of this was that traditional timing belt materials could no longer be used.

With new, constantly improving materials, it has been possible to fulfil these requirements without ignoring the compactness of the drives. In addition to the traditional automotive application, the camshaft drive, the new material developments are increasingly finding their way into industrial applications.

The fabrics are still based on nylon, but today they are designed using special coatings, so that the wear behaviour has been improved by a factor of three.

In addition to the traditional chloroprene material, temperature-resistant rubber compounds such as HNBR have become much more prevalent and are used as standard in the automotive industry. They are permanently resistant up to 135 degC and at the top end of the scale up to 150 degC.

In order to increase the tooth strength, ie to prevent teeth shearing off during transmission of high torques, increasingly high-strength rubber materials are being used, and these materials are further reinforced by the inclusion of fibres within them. This means that 30-50 per cent higher power values are possible, depending on the timing belt technologies. Glass fibre is still primarily used as a traction mechanism.

Due to its favourable bending properties, this material is an excellent choice as a traction mechanism for dynamic drives, especially in conjunction with small pulley diameters. Here too, there are further developments with regard to flexibility and traction characteristics. High-strength glass-fibre tensile members demonstrate 30-40 per cent higher tensile strength and tensile members made from aramid, an equally high-strength and high-modulus material, are available as an alternative.

The high-strength glass-fibre tensile members were selected for the latest Gates innovation. These afford the PowerGrip GTX high-performance synchronous belt, one of the strongest rubber synchronous belts on the market, increased strength and lower strain, low-noise operation and an extremely high resistance against shock load. It is suitable for the widest range of applications, for which long service life and thorough operational reliability are mandatory, and serves the heaviest drives in the most diverse industries and markets.

Following the development of high-performance belt drives for the automotive industry, dragsters, motorcycles (including Harley Davidson), snowmobiles and thousands of other industrial applications, Gates has now developed a rubber-carbon belt, the Carbon Drive CDN belt, for a fold-up city bicycle in cooperation with the designers' engineering team. The bicycle, which included the cutting-edge carbon belt system, won an award for its innovative folding kinematics.

The rubber carbon belt system targets the mid-level priced bicycle market, building on the platform created by the high performance polyurethane Gates Carbon Drive belt already well established amongst mountain bike fans. Carbon fibre was the enabling technology, resulting in a belt with the highest modulus and strength. Unlike chain, carbon fibre is length-stable and will not stretch.

The 11mm pitch belt profile is designed specifically for optimal belt drive bicycle performance and to meet the demands of human power. The rubber carbon belt provides the smooth ride cyclists want, without some of the downsides of the traditional bike chain. It is quieter and requires less maintenance, and is also grease and oil free.

When talking about material developments, polyurethane timing belts should not be forgotten. They also offer long-lasting and energy-efficient solutions for different applications. Gates produces linear, wide and endless flexible timing belts, which can all be modified with various coatings, profiles and special machining operations. In order to achieve variations in the coefficient of friction, abrasion resistance or belt thickness, various materials such as polyurethane, elastomer, foam, PVC and special coatings may be applied.

The addition of profile shapes makes these belts perfect for packaging machines, transport applications and other automation equipment. The cast profiles, like the timing belts, are made of heavy-duty polyurethane. By welding the profile to the belt, a homogeneous connection is created between it and the belt. The profiles can be made in almost any design.

Gates' wide belts are used in applications where the precise positioning of the product must be guaranteed. Thanks to their easy cleaning and the positive positioning characteristics, these belts are used in the food, hygiene, tyre and confectionary industries, in which belt slip must be avoided. GMT3 is the latest addition to the 'Wide Timing Belts' product line. The special GMT3 tooth profile was designed as an alternative to existing flat belts.

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David Clark is a Gates Senior Engineer & Technical Expert, Dumfries, Scotland.