Getting the most out of machine tools means providing the ability to process a range of different materials and to be able to perform both high and low speed cutting. Ian Carr describes how two-speed planetary gearboxes allow the cutting edge power of modern machine tools to be fully realised
Whether cutting, boring, grinding or shearing, modern machine tools have to meet what at first sight appear to be conflicting operational needs. On the one hand they need to provide fast cutting for final finish work and/or the rapid processing of soft materials; but they must also have low speed and high torque capabilities for rough cutting and hard materials.
This dual speed requirement is beyond the capabilities of a spindle motor alone, but can be created by the addition of a two-speed gearbox. With a direct drive or 1:1 gear ratio, the spindle motor runs in high speed mode, while the lower ratio will reduce speed and increase torque. Together the two speed capabilities increase the usability of the machine tool enormously.
The result is that the full cutting power is available across the widest possible speed range. Depending on the motor and gearbox specification it is possible to provide speed ranges with constant power of up to 1:20.
While there are numerous different gearbox designs, planetary gears offer a number of advantages in machine tool applications. For instance they are more compact than most alternatives because the power transmission is shared between several gears, and space saving is always useful in machine design. They are also quiet, reliable and easy to use.
Drive Lines offers the ZF Duoplan gearbox, which is ideal for use in machine tools and many other applications. One of its design features, the floating sun gear, means the Duoplan is tolerant of misalignment and concentricity issues that may arise in machine tool applications. This reduces the generation of heat and vibration during operation, improving reliability and extending the working life of the unit. Further, combining the simultaneously meshing planetary gears with a helical gearing profile assures low noise, even at high operating speeds.
Characterised by their optimised torque transmission capabilities and superior low speed performance, the ZF Duoplan can be used in horizontal or vertical configuration. They can connect straight to the spindle motor without the need for additional components, thus further promoting operational reliability.
ZF Duoplan gearboxes provide a choice of two output variants. The standard long bearing base output flange version is used for belt drives, allowing high cantilever forces. As an option, an extended output version delivers higher belt forces. The second variant is the INLINE with short output housing and angular contact bearings for direct drive applications.
As an option, the INLINE version can have through-spindle coolant delivery, which can be high-pressure (140 bar), high flow (35 litres per minute) for cooling the cutting tool in very demanding applications.
Drive Lines offers the Duoplan in seven different sizes: covering nominal input torques from 120Nm to 2,100Nm and nominal output torques from 120Nm to over 11,000Nm, with powers from 19kW to 120kW. The range accommodates motor frame sizes from 100 to 280, with the motor-gearbox combination usually fixed to the machine frame or bed via the gearbox feet.
Open and closed versions of the gearbox let machine designers optimise its interface with the motor. The open version has no adapter plate and is sealed by the motor’s shaft seal. The closed version can be supplied with a standard shaft seal or with ball bearings which prevent any axial movement.
Three backlash classes are available Class 3 provides nominal backlash of less than 30 arcmin (or 40 arcmin in the very largest ZF Duoplan models), and meets the needs of workpiece processing with constant cutting force.
Class 2 provides less than 20 arcmin of backlash, meeting the needs of milling machines and machine centres used in precision work. This class is also useful in applications that require interrupted cutting, ribbed workpieces and very tough materials.
Class 1 achieves less than 15 arcmin of backlash and is for ultra-precise work because it helps minimise resonant vibrations.
In conclusion we can say that two-speed gearboxes enable machine tools to offer a broad range of operational capabilities, often to the point of being truly universal. They can offer high speed and low speed machining, rough cutting and final finishing, standard or high torque cutting, and be used with almost all engineering materials. The ZF Duoplan meets all of the requirements of the modern machine tool designer.
Ian Carr, Managing Director of Drive Lines.