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Polyurethane timing belt drives in dynamic precision positioning axes

The T-shaped construction, consisting of a linear motor axis and a gantry axis made of carbon fibre reinforced plastic (CFRP), is a particular feature of the 2-axis linear systems from JAT. They are specifically designed for use in depaneling machines. They position electronic PCBs with a repeat precision of 2 mm by means of grippers. A CONTI® SYNCHROFLEX GEN III timing belt is used for the very fast and precise drive of the gantry axis. High rigidity, minimal total weight and the compact design were some of the defining criteria for the JAT specialists in deciding in favour of these high performance polyurethane timing belts.

Depaneling machines

N/A N/A In PCB production, large panels are populated with electronic components, soldered, checked and then separated into many small PCBs, a process which is called depaneling. In this way, about 60 to 80 PCBs for smartphones are made from a finished panel with a size of 25 by 25 inches. The machines for separating these panels are called depaneling machines. Depending on the geometry, required precision and susceptibility of the PCBs to damage, depaneling is carried out in the industry by cutting, milling, and recently also by lasers.

In 1990, after German reunification, the former development engineers from Carl Zeiss Jena founded Jenaer Antriebstechnik GmbH (JAT). Their objective was to supply dynamic and precise drive systems tailored to the application for special-purpose machines. Within a few years, the small engineering office became an internationally sought-after partner for customized drive solutions. Handling systems and dynamic single- and multi-axis systems are some of the company’s specialities. The drive systems are used in the packaging, printing, and textile industry, as well as in medical technology and the semi-conductor industry. For this market, with a very high degree of automation, JAT offers gantry systems for depaneling machines.

The X/Y axis system shown here is designed for the transfer of the finished milled PCBs out of the machine. For this, the customer installs a gripper weighing 20 kg with Z and additional rotating axis to the slide of the gantry axis. This gripper transfers the PCBs, for example, to a transport conveyor, stacking system or a workpiece carrier. The CFRP gantry system has to perform two important functions, which considerably determine the cycle time of the machine: The positioning of the gripper over the PCB and the subsequent transfer of the PCB to the next process including the return of the gripper to its original position.

JAT design engineer Bastian Sadewasser explains: “The transfer is particularly time-critical, as milling cannot be carried out at the same time. The X/Y system has to offer very high positioning accuracy and high speeds for the transfer. The axis settling times have to be very short to make it possible to achieve the positioning times. We use a very light-weight but rigid supporting structure made of carbon fibre reinforced plastic (CFRP). This reduces the required drive power of the linear motor and increases the speed of the X-axis. Therefore we also wanted a particularly light drive system for the Y-axis. It also had to be backlash free, maintenance free and sufficiently rigid.

Light-weight and rigid drive for the Y-axis

In the comparison of different drive concepts for the Y-axis such as direct drive, spindle drive and timing belt drive, the timing belt drive proved to be ideal given the technical requirements. It offers the lowest weight for this application. The servo motor can be positioned very close to the carrier axis, which has a positive effect on the natural settling behaviour of the gantry axis. By carefully selecting the motor size, and small pulley diameters it was possible to do without a conventional gear unit, eliminating backlash. The timing belt drive mass to be moved is very low.

“The stiffness of the timing belt, required to achieve the desired positioning times, proved to be a challenge. We were only able to meet the required settling times with the very stiff CONTI® SYNCHROFLEX GEN III polyurethane timing belt and with a belt tension precisely tailored to the load mass and acceleration,” Bastian Sadewasser explains.

JAT was given technical assistance in the design and configuration by the technical dealer and timing belt specialist Wilhelm Herm. Müller GmbH & Co. KG, Garbsen. Engineer and customer consultant André Schmidt from the Müller branch office in Leipzig has been advising JAT for many years and explains the principles: “The timing belt span functions in principle like a stiff spring. With the mass of the gripper attached, a typical spring-mass system is created. The settling times depend on the stiffness of the timing belt, on the load mass – in this case of the gripper – and on the motor rating. As the load mass was specified by the customer and the motor size was determined by the gearless concept, the stiffness of the timing belt was the decisive optimization parameter for meeting the requirements of speed.”

The CONTI® SYNCHROFLEX GEN III is one of the highest performing polyurethane belts of the manufacturer ContiTech Antriebssysteme GmbH. The high-performance polyurethane specially developed for this belt is harder than the standard SYNCHROFLEX timing belt. The tension is distributed to up to 30% more load-bearing teeth. Furthermore, the GEN III timing belt can reliably transmit up to 45% more tension and also offers 45% higher rigidity than the standard SYNCHROFLEX timing belt. This is achieved, amongst others, by a higher packing density of the steel cord tension members. “This was essential for achieving the desired speed and low settling times of the Y-axis,” Bastian Sadewasser adds.

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