Since the introduction of the 'Famulus' with its six electromechanically driven axes by Kuka in 1973, 6-axis robots have become an integral part of production halls - especially in the automotive industry. Current models combine the degrees of freedom of 6-axis operation with increasing payloads and a reduction in the required working space. Complex parts handling and multi-stage assembly can be accomplished by increasingly space-saving systems with smaller turning radii and larger swivel ranges. However, the smaller space requirement means additional challenges for the integration of the media and energy supply - initially in the configuration, and later also in the operation and maintenance of the robots.

Most 6-axis robot applications today use protective hoses made from specially modified polymers. They are similar in appearance to simple profiled conduits that are generally used for cable protection, but are usually specially optimized for use in robotics to ensure a long service life, among other things. Their function is to guide the cables and protect them at the same time. The most commonly used materials for the hoses are polyurethane (PUR) and polyamide (PA).

Compared to simple protective hoses, energy chains are certainly the more complex and therefore more cost-intensive variant. The chains consist of around 30 to 100 individual parts or individual links per meter, which are usually made of polyamide (PA) - with or without glass fiber reinforcement - or the thermoplastic polyoxymethylene (POM).

A detailed comparison of the systems

The comparable properties of protective conduits and 3-axis cable drag chains can be classified and evaluated according to the following criteria, among others:

Cost-effectiveness of purchase and installation

In terms of acquisition costs, protective conduits are cheaper than 3-axis cable drag chains. Differences in quality between protective conduits relate almost exclusively to the use of different materials. In contrast, the 3-axis guide chains consisting of many individual parts are more expensive; on the one hand due to the number of components used, and on the other hand because glass fiber-reinforced PA and thermoplastic POM plastics made of high-quality materials are sometimes used. In addition, the individual parts have to be assembled, which means additional production costs.

Cost-effectiveness during assembly and operation

Modular 3-axis energy chain systems are much easier to assemble, as the filling can usually be inserted into the system from the outside. This means that pre-assembled cables and hoses can be installed easily and in a few simple steps or can be replaced individually in the event of a defect. Manufacturers such as Igus have sometimes developed special opening mechanisms that ensure even faster filling and can reduce assembly time by up to 80 %. In contrast, protective hoses can often only be assembled after they have been pulled into the hose, which significantly increases the time and labor costs involved and ultimately makes the production process considerably slower and more expensive.

In the event of a fault in individual cables, the entire conduit system must always be replaced in the case of protective conduits, as connectors can only be assembled to the corresponding cable after the cable has been routed through the protective conduit. This makes electrical testing of the cables more difficult and increases downtimes and costs enormously. With modular 3-axis cable carriers, on the other hand, defective chain links can be removed and replaced individually. This reduces both repair costs and robot downtimes.

Protection of cables/hoses against external influences

Due to their material and design, protective hoses are very flexible and also dust-tight. Although the closed modular 3-axis cable drag chains are less flexible due to their specified minimum bending radius, they are much more robust and stable against external influences such as pressure or impact loads.

Compliance with the minimum bending radius

Protective conduits do not guarantee a defined minimum bending radius, which can easily lead to damage and failure of the cables during the process. With modular 3-axis cable drag chains, on the other hand, a stable stop system prevents the cables from overbending, thereby significantly increasing operational safety.

Torsion absorption

Robot cables are designed to absorb torsional loads - usually ±180° over a length of one meter. Energy chains distribute the torsional load over the entire length and prevent the cables from being overloaded by means of defined stops. Protective hoses, on the other hand, do not allow torsion due to their design. The torsional load occurring in the application is absorbed solely by a swivel joint on the sixth robot axis. A disadvantage is the short length of only about 10 to 20 cm (measured from the exit of the rotating protective hose to the strain relief), in which the entire torsion takes place. In many cases, this causes the cables to be overloaded and is a weak point of the protective conduits.

Flexibility

The high degrees of freedom are a key functional feature of 6-axis robots, which explains their widespread use in automation. As the protective conduit has no radius stops, it can be bent very tightly. On the one hand, this has the advantage that a large number of positions and angles can be approached. On the other hand, however, a major disadvantage is that bending too tightly - whether intentionally or unintentionally - can damage the installed cables. With modular 3-axis energy chain systems, the specified minimum bending radius ensures that the installed cables are not damaged.

An example of a 3-axis energy chain that has been specially developed for robotics with the aims of modularity, durability, safety and ease of assembly and has a special snap lock for faster opening.

© Igus

Emergency running properties

The emergency running properties of a protective hose are significantly higher than those of a modular 3-axis energy chain system. If the hose is kinked, it can still be operated. With a modular 3-axis energy chain system, however, the chain link is destroyed if the minimum bending radius is not reached.

Retraction systems

Conduit systems generally use a retraction system with a compression spring. The disadvantage is a sharp increase in retraction force, even with pre-tensioned systems. There are different variants of modular 3-axis energy chain systems:

• Pneumatic retraction systems:
  The advantages of retracting the energy chain using a pneumatic cylinder (the pneumatic cylinder is not actuated, but used like a gas pressure spring) and a deflection pulley are an almost constant retraction force in every position and the adjustability of the retraction force; however, this is a relatively complex system with a corresponding price level.
• Retraction forces with spring rods:
  In this solution, the retraction force is applied by two fiberglass rods; the force curve is increasing and not linear. Although this solution is inexpensive, there is virtually no possibility of adjusting the retraction force.
• Retraction system with elastomer tapes:
  Technical elastomer straps are usually used in conjunction with one or two smooth-running deflections in the retraction system. This ensures an optimum force-elongation curve for the belts and maximum service life.

Accessories

Depending on the manufacturer, there is an extensive range of accessories available for both systems, such as the aforementioned retraction systems, sliding bushings, connection elements or clamps for the 6th axis on the robot. With the modular 3-axis energy chain systems, however, the variety of accessories is greater. Igus, for example, offers connection elements with strain relief teeth for convenient installation, so that cables and hoses can be relieved with cable ties - or connection elements as an intermediate connection, with which the energy chain can be fastened or intercepted at any desired point.

Installation

The length of modular 3-axis energy chain systems can be easily adjusted during installation - links can be added to or removed from the chain depending on the application. Although protective hoses can be shortened, it is difficult to extend them. In this case, the only option is to install a new protective tube.

Ease of installation in the event of defects

Defective areas are more difficult to detect and replace on protective hoses than on 3-axle energy chains. Here, it is sufficient to remove and replace the individual defective chain link, whereas with protective hoses, the only option is to replace and refill the entire cable carrier.

Tensile load on the systems

In robot applications - especially when using retraction systems - high tensile forces are generated on the cables and cable carrier systems. The disadvantage of protective conduits is that they elongate under tensile load and therefore exert considerable tensile forces on the installed cables and conduits. Modular 3-axis cable drag chains from Igus, for example, have a trailer principle with a ball and socket connection similar to a trailer coupling. This ensures maximum stability in the tensile direction and prevents the system from elongating even under the strongest tensile load. Appropriately dimensioned systems can absorb forces of up to 1000 N in the tensile direction in continuous operation.

The conclusion of the comparison

Comparison matrix on the advantages and disadvantages of protective hoses and 3-axis energy chains for 6-axis robots (xxx = satisfactory, xx = good and x = very good; overall score from the quotient of total number of points / number of evaluation criteria)

© Image: Computer&AUTOMATION, Source: Igus

The main aspect that speaks in favor of profiled protective hoses is certainly the lower acquisition costs. However, users of 6-axis robots should consider whether this initial financial advantage will not be offset in the medium term by 3-axis energy chains and even reversed in the long term. The reliability of energy chains in operation guarantees smooth, failure-free production, which more than justifies the higher acquisition costs.

Applications with docking systems are a special challenge for robotics in the automotive sector. Here, different applications are realized with just one robot at a time. For the 6-axis robot, this means that the tool has to be changed on the sixth axis in the program sequence or the entire application has to be changed. For example, applications in which several welding guns are used are not uncommon. If different technologies are also used, the overall system becomes increasingly complex, requires more space and is potentially more susceptible to faults. This is where the 3-axis energy chain system undoubtedly has an advantage, as its modular design and versatility allow it to be perfectly tailored to the respective application.

Author:
Jörg Ottersbach is Head of Sector Management Automotive & Robotics at Igus.