Linz, Tirschenreuth (Oberpfalz), April 2019 – It is a cumbersome term that conceals a patented world first in road construction: the so-called “split oscillation drum” significantly increases the compaction quality of road rollers. The two rollers working on one axis – the so-called “drums” – are oscillating separately. The innovation has been presented for the first time at the Bauma trade fair (8 to 14 April in Munich). The new generation of rollers is being built by Hamm AG. Linz Center of Mechatronics GmbH (LCM), as development partner, has taken care on the synchronization of the two motors driving the oscillation of the split drums. This was achieved once again with a digital-hydraulic approach which generates the necessary precise distribution of the volume flow to the hydraulic motors. Thanks to the synchronously oscillating drums a perfect roadway can be realized.
“The two halves of the drum need to swing completely synchronous in order to transfer full power to the ground and to avoid cracks in the asphalt around the drum gap,” explains Project Manager Peter Janner, Hamm AG. With an operating weight of around ten tons, tandem rollers – the heavy athletes of road construction – already apply a significant surface load to the subgrade. In order to further increase the compaction performance, the drum is made to vibrate or oscillate. While a centrally mounted unbalance shaft forces the drum into a circular movement during vibration, two eccentrically mounted shafts rotating in the same direction generate the typical oscillation movement of the drum during oscillation. The compaction forces are applied tangentially. Even at lower asphalt or ambient temperatures, this ensures a homogeneous, flat road surface and tight seams and also protects the driver, machine and surrounding buildings by reducing vibrations.
Patented global novelty
As simple as the working principle of an oscillating roller is, as demanding is the synchronization of the split drums. Even if the unbalance shafts in the two drums are hydraulically driven by completely identical motors, these never work exactly synchronous. “Thus, the speed differences at the drums must be compensated by adjusting the speed of the unbalance shafts during steering movements and cornering in order to maintain synchronous vibration behaviour,” says Peter Janner. “But this is exactly what is needed to achieve optimum compaction quality,” says LCM Managing Director Gerald Schatz, outlining the challenge his development team was confronted with in spring 2016.
Stability is the goal
The data from two acceleration sensors on the left and right drum suspensions were used as the basis for the compensation of the pressure levels and the resulting phase shift. In order to be able to obtain reasonable information, these measurement signals first had to be processed in a complex filter process. After a thorough analysis at Hamm’s production site and preliminary tests in their own laboratories, the hydraulic experts at LCM quickly decided on a solution: Since both motors are driven in the same hydraulic circuit, a way had to be found to throttle one motor so that the speed could be exactly adapted to the other. “Digital hydraulics is fits ideally to this task,” explains Bernd Winkler, Business Area Manager Drives at LCM. Only digital valves make it possible to adjust even such low volume flows 50 times per second – i.e. every 20 milliseconds – robustly, energy-efficiently and economically.
Boosting as a turbo for the valves
The needed short reaction time is only possible under perfect technical conditions. “You have to boost the valves, i.e. drive them with a higher voltage during the switching. In this specific case, 24 volts instead of 12,” specifies Andreas Plöckinger, team leader for hydraulic drives. Since this was possible with the LCM boost electronics, but not with the Hamm on-board electronics, the Linz hydraulic engineers produced a special solenoid coil for the hydraulic valve in their in-house 3D printer. A special stroke of luck proved to be that very similar coils are already available on the market as series products. “Since our tests were successful, the valves can be operated with the existing on-board electronics, the jointly developed software module and the cost-effective coils,” stresses Plöckinger. “For Hamm, this is the technologically and economically optimal solution”.