Dual-drive gears eliminate play
Tom Shelley reports on an ingenious arrangement for driving the linear motions of very large, high precision machine tools
A rack drive for large machine tools eliminates backlash by using two servo planetary gearheads.
Tension between the two can be created either mechanically or electrically. Redex Andantex in France has developed a version of each type. The electrical system is named TwinDrive, while the mechanical system is DualDrive. The latter has been implemented on a new Heynumill machine from German company Heylingenstaedt to process and manufacture large format tools and moulds for a German car maker.
In the case of TwinDrive, there is an electrical pre-load so that each gear interfacing with the rack is driven by a separate servo motor. One pinion is considered to be the driving pinion and the other, the braking pinion. A special motor controller is required to handle the pre-load. The company calls this concept TwinDrive.
However, it is also possible to achieve a similar effect using only one motor, and torsionally winding one pinion against the other. One pinion is considered to be fixed, and the other spring loaded. The system does not require any special motor controller. The preload is obtained using a Redex patented preloading system based on a torsionally stiff coupling.
For additional precision, the units are made to be very stiff, by optimising the diameter of the drive pinion in such a way that the transferred torque has the best rigidity ratio. The bearings are two pre-tensioned and well spaced taper roller bearings, positioned in such a way that the pinions are carried as close as possible to the active load, and only separated by the width of the tensioning unit from the drive bearing.
For moving masses of 15 tonnes, performance is said to be 25% better than comparable conventional systems.
The gearboxes come in six sizes, with reduction ratios of from 21 to 91, and able to produce accelerations of up to 25 m/s2. Nominal tangential load at a speed of 60m/min is up to 108kN. Maximum axis weight is up to 62 tonnes and maximum speed, up to 78 m/min.
The machine tools are milling machines with worktables up to 4m wide, which can take the form of either palette tables, double tables or integrated round tables. Two DualDrive systems are used for the drives of the X and Y axes.
Another Redex product is used for the main spindle – a two-stage reduction gearbox delivering a maximum gearbox speed of 6000 rpm from a drive capacity of up to 100kW.
Ulrich Krause, sales manager of Heyligenstaedt, says of the technology: “We had to boost performance. Now we are able to achieve tolerances in magnitudes of just hundredths with perfect dynamism.”
The next Heynumill line, consisting of six machines, is in the works for an automobile manufacturer. The translation motions occur in the X-Y-Z direction, the milling capacity will be 67 kW and the speed 6000 RPM.
“We would like to use the DualDrive system also for other machine lines,” concludes Krause.
* System ensures backlash-free driving of racks for machine tools by using a mechanical, sprung torsion coupling to wind one driving pinion against the other
* The system is an alternative to using two separate, electrically driven pinion drives –which require the use of two servo motors and a special controller
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