Tension member requirements for timing belts, and a comparison of their properties

Author: Dr. Jürgen Vollbarth

Use of polyurethane timing belts and their quality features

Polyurethane timing belts of the BRECO® and BRECOFLEX® brand employed in all fields of industry, from medical engineering through to the manufacture of aircraft very successfully fulfil their required tasks. A considerable part of these tasks is accomplished within the general function areas 'driving' and 'transporting'. Moreover, higher requirements, such as extremely high precision in movement transmission and difficult ambient conditions for the drive, play a decisive role when selecting the drive element and its execution. Difficult ambient conditions or the use of Polyurethane timing belts are mostly created by the choice of various cleaning and disinfection agents or high performance lubricants containing special additives. Applications in a deep freeze environment also place increased requirements on the timing belt as a drive element.

The applications require, amongst others, high performance, longevity, a large variety of design variants and conformity with the different international directives, e.g. the FDA directive in the USA, to mention just one. These requirements can only be met when the right choice of raw materials is made, when the production technology is of a high standard and, last but not least, when an excellent quality of all materials used and processes involved is ensured. These are the aspects under which the tension members for timing belts are scrutinised in this article.

Performance and quality features of tension members

Figure 1: 0.9 mm tension member in standard and highly flexible execution The use of tensile rigid and flexible tension members combined with highly abrasion resistant thermoplastic polyurethane forms the basis for high-performance products with a long operating life. Especially the security of the running and function properties of the timing belts substantially depend on the performance and quality features of the tension members. The properties of the source material used with its alloy components and their changes encountered during the manufacturing process of wires and cords are as decisive as the configuration and design of the tension member. Figure 1 shows two different designs with the same cord external diameter.

Parameters such as the diameter of the individual wires and their number, the relation to the overall cord diameter, as well as the lay stranding factor are of decisive significance for the choice of the design and the attendant functional properties. Once the design features are defined, emphasis has to be placed on the quality of manufacture and workmanship of the individual cords and of the entire tension member, as well as their stability. To this effect, corrosion protection, cleanliness of the tension member surface, number of welding points over a defined length, and the lay stranding quality play a major role.

Given the above, cleanliness of the tension member surface and a possibly applied bonding agent, if any, affect the bond between the polyurethane base body of the belt and the tension member itself. This bond and its long-term stability affect the power transmission from the belt tooth into the tension member and back, and, as a result, the performance of the belt with regard to power transmission. The material properties of the individual wires and the quality of the lay stranding of the cord have, amongst others, a significant effect on the stretching ability of the tension member, and therefore on the belt. Excessive belt stretching under load can negatively affect the positioning precision as well as the dynamic behaviour of the belt. If the tension member produces an excessive post-stretch during operation of the timing belt transmission as a result of poor lay stranding quality or inadequate material properties, the pre-tension force will suffer, which in turn will reduce overall functional reliability.

Test runs with tension members for timing belts

By drawing conclusions from the above, Breco Antriebstechnik Breher GmbH & Co. KG has compiled a number of quality features for the tension members used in its products. These features are confirmed for all tension members used from the supplier, and additionally are in-company tested at regular intervals. Testing tension members from various suppliers revealed and conveys that there can be substantial differences in equal products available in the market. The following are the parameters tested:

• ­tension member bonding in the polyurethane matrix of a new timing belt and of the belt after one test run,
• ­the stretching of the timing belts encountered during or as a result of the test run, and
• ­the drop in span pre-tension force from the beginning to the end of the test run.

Figure 2: Test run configuration for the tension members (example) By way of example, Figure 2 shows a test run with a BRECOFLEX® StandardPlus timing belt of the AT 20 type with the respective pulleys.

Figure 3: Pre-tension force behaviour during a test run Figure 3 shows a typical behaviour of the pre-tension force in the timing belt transmission during a test run. The flatter the curve the better the performance of the tension member used for the timing belt. It is important to note that for the assessment of the tension member quality the test transmission must not be re-tensioned, as is otherwise applicational practice.

Comparative test results

Tension members of Ø 0.6 and Ø 0.9 diameter of a total of four suppliers were tested. These tests have shown in part considerable differences. The following are some example results.

Figure 4: Bonding of the tension members in the polyurethane matrix prior to and after the test runs with the Ø 0.6 tension member Figure 5: Loss of pre-tension power in the Ø 0.6 tension member after the test runs The diagram in Figure 4 contains the values referring to the bonding of Ø 0.6 tension members in the polyurethane matrix prior to and after the test run. Differences are relatively small. However, there is a noticeable difference between the values prior to and after the test run for supplier 3. Figure 5 shows the loss in pre-tension force of the tested Ø 0.6 tension member. The differences were substantial. Reasons for such differences could be found, amongst others, in the material composition of the tension member, in the lay stranding quality, and in the surface quality of the tension members.

Figure 6: Change in length of the Ø 0.9 tension members after completion of the test runs Figure 6 shows the change in length of the Ø 0.9 tension member as a result of the test runs. With one exception, all tested tension members are almost at the same level. Even a change in length like this, as revealed for the tension member of supplier 2 could be explained by its material composition and lay stranding quality.

Summary and conclusions

Tension members for BRECO® and BRECOFLEX®- timing belts must distinguish themselves by high bonding at minimum loss during operation, by a small overall post-stretch, and a small pre-tension loss while in operation. The results shown above by way of example illustrate that not all products available in the market meet these requirements to the same extent. Information about tear resistance, elongation at break and overall rigidity alone is not enough to sufficiently describe the tension member properties. Hence, careful product selection is crucial to ensure functional reliability and longevity of the timing belts. To meet the high quality requirements of the products, the tension members for BRECO® and BRECOFLEX® timing belts are selected, amongst others, with the described parameters in mind, and subjected to regular quality tests.

Author of this professional article is Dr. Jürgen Vollbarth, Head of Sales and Product Development / Product Management of Mulco-Partner Breco Antriebstechnik Breher GmbH & Co. KG, Porta Westfalica (Germany)

Mulco-Europe EWIV
Garbsen, 13th of September, 2010

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