“Pneumatic components which are robust and compact are becoming increasingly important in the life sciences sector,” says Daniel Gebert, who is responsible for quality management at Aventics GmbH in Laatzen. “The health sector increasingly needs solutions for low-pressure applications with a high degree of electrical and functional integration,” he adds. As an example of his company’s extensive product portfolio, he describes a dialysis unit valve system: “Components like this combine precision with extreme reliability. We are continually improving our products.”
In the course of the continuous further development of its products, Aventics decided to use a new type of plastic for the base plate of the system. However, in the pre-series stage, the screws often stripped their threads. Daniel Gebert launched a problem solving campaign; the engineering team took a very close look at the entire application and soon found the cause of the problem. The pneumatic straight nutrunners previously used were simply not suitable for the new base plate material. As a result of their design, tools of this type can only tighten screws with a fixed torque value and at a fixed speed. “Because of the inertia of the tool, the threads were simply stripped when the self-tapping screws were tightened,” Gebert says. Although the new plastic was more suitable from the chemical point of view, it was also more sensitive.The plastic component previously used had been thicker, tougher and stronger, and was not damaged if the extremely low specified torque value of 18 cNm (= 18 centi-Newton-metres or hundredsof a Newton-metre = 0.18 Nm) was repeatedly exceeded. The change to a new material gave Aventics cause to rethink the tightening systems previously used with a view to ensuring a secure assembly process.
Compared with hand-held pneumatic straight nutrunners, the MicroTorque system is much quieter and more precise
Although the self-tapping screws with a diameter of 0.93 mm only have four and a half thread turns, the MicroTorque spindle, type QMC41-50-HM4, completes a three-stage tightening process over this very short distance. Daniel Gebert describes the process in detail: “At the beginning of the tightening cycle, the screwdriver insert turns at a reduced speed to insert the tool into the Torx-T2 screw head. Following the completion of this insertion stage, the system switches to the rotation angle stage and the screw is tightened at 500 rpm until the head makes contact with the material surface. As soon as this point has been reached, the spindle speed is reduced drastically to 210 rpm and the screw is tightened to the final torque value of 18 centi-Newton-metres in a torque-controlled stage.” As the rotation speeds can be freely programmed in all stages, the MicroTorque system can adapt to a wide variety of tightening applications. Optical and acoustic signals on the screwdriver and the controller give the operator an overview of assembly results at all times. Up to the late autumn, the system had completed 80,000 assembly cycles without any problems.
Following the changeover to controlled, monitored tightening, the reject rate fell practicably to zero
The newly developed system from Atlas Copco can do even more, Gebert emphasizes: “The MicroTorque controller is extremely compact and the graphics functions are just fantastic. We only need to connect an ordinary PC and the controller can print detailed curves and tightening plots." Visualization is clear and unambiguous. Through the analysis of tightening plots, Aventics can now interpret the tightening operation in much greater detail and understand what happen precisely in each stage of the process. “That was just not possible with pneumatic tools.”
The graphs give a precise indication of fluctuations in insertion resistance. This can occur if the diameters of pre-drilled threaded holes in the injection-molded components are outside the specified tolerances. During screw insertion, the MicroTorque screwdriver detects changes in friction values in the micro-range.
Thanks to the three-stage tightening process, we have reduced our rejection rate drastically and are now producing more precisely than ever before
Written by Heiko Wenke