Engineers from Tölke, a packaging machinery specialist, were looking to design closure processes with more flexibility using linear-rotary motors instead of conventional cam disc technology. This need led the Tölke design team to outfit its new filling and closing line with linear-rotary motors from LinMot.
The company had already taken several measures to add flexibility to its filling and closing machines that used conventional cam-stroke technology. By using transport cups with identical outer shapes and individualized inner contours, often only the cups had to be swapped out for a product changeover. If it was necessary to swap out all of the part-specific components of a machine, then Tölke made sure that this could be done quickly by hand, with no tools required.
However, if the closing process itself needed to be modified for a product changeover, it was necessary to change out the mechanical cams involved in the screwing process. This is a time-consuming and costly process. As a result, Tölke built a carousel machine with 16 closing stations for an application with frequent product changeovers. The entire screwing process now uses a model PR01-84 linear-rotary motor at each station.
This electric motor, part of the PR01 series, was specially developed for the closing and screwing process. It combines both a linear motor and a rotary direct drive in a compact housing, each of which is controlled separately; any combination of linear and rotary motions can be implemented.
“For the rotary part of the screwing process, we have been using a rotary servomotor instead of a pneumatic motor wherever the screwing application requires a defined turn angle and a defined torque, as well as when we require a product changeover at the push of a button,” said Franz-Josef Patzelt, one of the managing directors of Tölke.
New, however, is the use of an electronic linear axis in the closer. “The cap needs to be picked up, placed on the bottle, and then guided so as to provide optimal support for the rotary motion,” said Markus Kröger, the Tölke project manager responsible for this job. “If this linear motion were controlled by a cam disc, then the heights at which the cap is picked up and placed down would be fixed, and the entire motion sequence would be defined.”
If modifications to the motion sequence were required for a product changeover, then the mechanical solution would require different closure heads or even different cam discs to be installed; the machine builder would have to integrate adjustable cam discs. In some cases, a spring would also need to be installed to compensate for the weight of the head.
“With a direct drive and an electronic stroke curve, none of this is needed anymore,” said Kröger. With the right parameter sets for the programmed motion sequence, or by invoking a predefined recipe, the motion of the linear motor can be designed as needed.”
As such, different types of closures, including screw-on and press-on caps, can be processed on the same machine. Even different press forces or thread pitches, such as those found on containers with and without safety caps, can be handled by a linear drive without any mechanical reconfiguration.
In addition, a linear-rotary motor like the PR01, with its two independently controlled axes, can start the rotary motion during the linear stroke, decoupled from the position of the turntable.
Simple changeover of the screwing station
To promote the modularization of the machine, Tölke engineers used the mechanical decoupling of the closing process from the carousel or turntable. If a screwing station was damaged, it could now be replaced in a short time, so the machine could get back to work much faster after a crash. In addition, because of the decoupling, the closing process can now be completed for all containers in the line prior to a planned machine stop.
To further reduce downtime and monitor the screwing process, the information produced by the linear-rotary drive for each individual screwing process (torque, speed, angle, vertical position, linear speed and force) can be analyzed.
“We can use the data provided by the drive to determine the number of revolutions actually performed, so that a separate check of the height of the closed container to monitor the screwing process can also be eliminated,” added Patzelt, citing a practical example. The drive data for snap-on caps can also be usefully applied for monitoring purposes. An error message can be generated if a prescribed value for the snap-on force is exceeded due to an interfering injection point.
Due to these many advantages, Tölke had already developed a solution prior to the use of the PR01 linear-rotary motor, wherein the linear motion was generated by a servomotor in conjunction with a ballscrew spindle. The ballscrew, however, had to be protected against dust, which requires additional design effort that is not necessary for the fully assembled linear-rotary motor. “The linear-rotary motor is much simpler to use, as an integrated unit, and takes up less space,” said Kröger.