DuPont Engineering Design Electronic Magazine
Improved polymer conveyor chain performance
By Karsten Faust
DuPont Engineering Polymers, Germany
Lightweight polymer conveyor chains are increasingly replacing those made of metal because they require lower drive power and, with the correct material pairing, they can operate quietly, lubricant-free and with little maintenance over long service periods. DuPont, iwis antriebssysteme GmbH & Co.KG (Munich, Germany) and the Technical University of Chemnitz (Germany) are collaborating on design and material enhancements to further extend their performance and range of application.
Chains made of highly chemical-resistant polymer provide a range of significant advantages over their metal counterparts. They are up to 40 percent lighter than comparable metal chains and therefore permit resource-saving operation. Injection molding of the individual parts enables many functions to be integrated in a single processing step, making assembly costs correspondingly low. A reduction in noise emissions of up to 80% compared to metal chains is a further benefit. Conveying systems using polymer conveyor chains deliver these benefits to a range of industrial sectors. They are most commonly used in the food, beverage and packaging industries, where lubricant-free operation and minimal abrasion are persuasive arguments.
Complex requirements beyond standard solutions
Current designs for polymer chains are, however, reaching their mechanical and tribological limits in light of the increasingly complex demands made on them, including higher tensile forces, speeds and operating temperatures as well as larger distances between the axles. Limiting factors include insufficient stiffness and strength and the fact that thermoplastics can creep (irreversibly stretch in the direction of load) when exposed to high static and constant loads.
In order to be able to use polymer chains in more complex applications, joint development work by iwis antriebsysteme, DuPont and the conveying technology department at TU Chemnitz is focusing on the development of three-dimensionally flexible polymer chains with significantly improved mechanical properties and comparable or better sliding properties.
An individual design approach
In the course of this work, a new chain geometry has been jointly developed and registered for patent protection, in which the pull elements of the chain are alternately arranged as inner and outer links, and the carrier plates are designed as separate elements. The lugs of the pull elements are oriented in the direction of load and, in addition, the wall thickness has been adjusted to correspond with the locally effective mechanical force, thereby reducing material usage. A cardan joint, formed by the bolt and pin, is retained and strengthened. Propulsion occurs on both sides through the bolts, ensuring an even transmission of power and enabling reverse operation.
Figure 1. The new pull element design (right) reduces the amount of mechanical stress compared to a standard design (left)
Load simulations using finite element analysis show that in the newly-designed pull elements – with the same tensile force – considerably lower effective stress occurs than in the standard chain geometry (red and orange sections in Figure 1). As a result, the new chains require less material for the same nominal load. Alternatively, higher tensile loads can be transmitted for the same material usage. Tests on injection molded prototypes have confirmed functionality as well as the desired increase in strength and stiffness.
Promising results with long-fiber compounds
On the material side, there are two, parallel objectives: to increase load-bearing capacity and optimize the tribological system properties. The former is achieved in principle by using fiber-reinforced polymers, which provide increased strength and stiffness. Sliding behavior is adjusted mainly through the use of modifiers. These include sliding additives such as PTFE (e.g. DuPont™ Teflon®) or silicone, which are used to reduce the coefficient of friction between the chain and guide rail.The real challenge is to improve all properties at the same time. As an initial piece of research, standard chain links made of polyoxymethylene (POM, DuPont™ Delrin® 511P), polybutylene terephthalate (PBT, DuPont™ Crastin® 600LF(low friction)) and a long glass fiber-reinforced semi-aromatic polyamide (DuPont™ Zytel® HTN LG50 HSL) were injection molded using the same trial tool. The results of tensile testing showed clear improvements in load-bearing capacity (Figure 2).
A significant factor in the dimensioning and reliable operation of conveying systems is the behavior of material under variable (pulsating) long-term stress. In order to measure this so-called creep strength, the pulsating tensile force acting on the chain during its continuous circulation is simulated on test equipment. The results confirm that the benefits of the long fiber-reinforced material are essentially retained, even under dynamic loads. By using such polymers, the permissible conveyor performance may either be significantly raised or the lifetime of the chain extended considerably when compared to today’s standard polymers.
Good tribological behavior expected
Comprehensive testing is currently being conducted at the TU Chemnitz in order to optimize the tribological system formed by the chain and guide rail. Critical areas in this case are the points of contact between the pull elements themselves, the pull element and pin, and the pull element and bolt (Figure 3). Working in close cooperation with the material developers from DuPont, the TU Chemnitz is investigating a number of other polymers (besides the above-mentioned long fiber-reinforced polymer), in which numerous reinforcing agents and additives are being tested. The aim of this work is to find the best combination of polymer, reinforcement and additive to achieve the greatest improvement in properties currently possible, with particular reference to the sliding behavior and load-bearing capacity of the conveyor chains.
There are two reasons why long fiber-reinforced polymers are expected to continue delivering good results. Firstly, the number of potentially abrasive fiber ends is many times lower than for the same content by weight of short fibers. Secondly, long glass fibers are more likely to align themselves parallel to the surface during processing than short fibers. This surface alignment of the long fibers significantly helps reduce the number of fiber ends protruding from the surface.
Practical implementation only a matter of time
The results of trials to date confirm that design and material enhancements can significantly improve the strength and stiffness of polymer conveyor chains. Once the current process of optimizing the injection molding tool for the newly developed chain geometry is concluded, the already clearly visible benefits of long glass fiber-reinforced polymers are expected to be even more pronounced.