Watching a Scara robot in industry can be dizzying. The horizontal articulated arm robots work rapidly across four axes. The inner and outer arms swivel horizontally. The component for gripping objects, the so-called ball screw, moves rotationally and linearly. This allows the robot arm to reach almost any point within its working radius - and at high speed. A pick-and-place cycle, for example, in which the industrial robot picks up, transports and sets down a component, only takes around a third of a second at best.

These fast movements have disadvantages, as an example from a car manufacturer in Swabia shows. The manufacturer has 40 Scara robots working in a production line - so fast that the mechanical workers wear out quickly and a lot of maintenance work is required. The robots move in 24/7 continuous operation at working speeds of up to 2000 mm/s in the horizontal plane. For this reason, the energy cables and hoses of the Scara robots are checked annually and usually replaced due to wear. An economical alternative was required.

Tripling the service life of the cable carrier

The car manufacturer's goal was to at least triple the service life of the connections. The first step on the way there: Igus, the specialist for plastic parts and cable carriers, reinforced the corrugated hose in which the power cables and hoses move with a product called 'e-rib' - a kind of skeleton made of high-performance plastic that sits in the grooves of the hose so that it can only move in one spatial direction. Instead of flopping back and forth, the hose is now stabilized. In addition, the front and rear connection points, which are attached to the hoses with cable ties, have been reinforced with two plastic supports. In combination with the e-rib, the hose is therefore stable enough to prevent kinking even during fast movements.

The car manufacturer has come up with a third improvement itself. In order to be able to follow the motor skills of a Scara robot, the hose connections must be rotatably mounted at both ends. The user has produced a swivel connection made of metal. With success: the entire system has been in use since 2017 with around 6.8 million cycles per year without being replaced. Igus is equally convinced by this combination and has set itself the goal of further developing all components into a cable carrier suitable for series production for Scara robots.

3D printers accelerate prototype construction

At the start of development, the 3800 m² test laboratory in Cologne had to be equipped with suitable equipment, including a Scara robot. The manufacturer Epson Germany agreed to provide a model from the G6 series for testing purposes. This enabled the developers to tackle the first challenge: the production of a ball-bearing rotary joint. This is where a special feature of Igus comes into play: whenever possible, the company replaces metal with lightweight, high-performance plastics that are designed to minimize friction and wear, thus helping to save drive energy; experts refer to this as tribological optimization.

The company has also invested in 3D printers to print prototypes and spare parts for users - a cost-effective alternative to injection molding, which involves the costly production of injection molds. For this reason, it made sense to print the rotary joint from high-performance plastic and then use metal balls. The developers began with tests and printed rotary joints with different filaments. At the same time, they focused on developing a holder for the rotary joint, as it is important that the cable guide does not twist during operation of the robot. For this reason, the connection point was moved to the center of the axis of rotation via a bracket.

Tests with 900 cycles per hour

During the test phase, the Scara robot moved at top speed in a protective cage. At first everything looked fine, but after a few minutes the 3D-manufactured rotary joint and the integrated ball bearing could no longer withstand the loads. In order to absorb the axial forces, two standard ball bearings from the Igus xiros series were therefore pressed into the outer shell, one on top of the other. At the same time, the design of the connection was made more compact in order to reduce leverage forces. The plug and screw principle of this design makes it possible to dispense with cable ties.

Matthias Meyer is Head of the ECS triflex & Robotics division at igus.

© igus

The engineers finally started the next tests. The robot completed 900 cycles per hour, 20,000 cycles per day, loaded with a Chainflex control line and two hoses with diameters of 4 to 6 mm. The tests showed that the design is suitable for everyday use. The new Scara Cable Solution has been available since fall 2020, either as an empty conduit or directly assembled with cables. In the next step, Igus would like to adapt the solution to the connection points of other manufacturers.