Twin rack-and-pinion drives ensure zero backlash

The latest product development in the rack-and-pinion drive range from Redex Andantex, the KRPX is an example of concurrent design between several machine tools OEMs and one of their most strategic suppliers.

To implement a TwinDRIVE system (zero backlash with two servo-reducers and electrical preload) on a moving column or gantry machine was often quite difficult because either an extension on both sides of the main frame was requested to fix each reducer or a special bracket support had to be designed, manufactured and fitted to the frame. In addition, these two kinds of extensions need at least to keep the overall stiffness the same as that of the main frame to avoid any deterioration of the dynamic performance.

With KRPX system, however, there is no need to design, integrate or machine a high-precision fixation extension to the main machine frame, meaning that implementation is far simpler and more cost-effective (up to 15 per cent less expensive) than the conventional alternatives.

Construction

Each KRPX housing is made from high-quality spheroidal graphite cast iron and it is fitted to the machine main frame using oversized screws to ensure an absolute connection.

Its structure has been optimised with FEA to avoid deflection during acceleration. A very precise keyway machined on the mounting surface allows very easy setting while ensuring the correct mounting geometry. This smart system has been designed to eliminate any risk of incorrect mounting or system deflection.

In addition, the KRPX inherits the best advantage of the TwinDRIVE, with its exceptional compound stiffness for highly demanding rack-and-pinion drive applications.

Available in the three standard universal sizes 2, 3 and 4, the KRPX is very compact, which is a definite advantage, with respective pinion heights not greater than 104, 127 and 148mm, respectively. With this new rack-and-pinion drive concept, machine tool designers can exceed market standards with maximum feed capacity up to 105,000N and stiffness on the rack up to 333,000N/mm.

The rigidity of the KRPX results from its advanced design with an integrated output pinion and reinforced output bearing. The output pinions are machined to the same diameter as the shafts, giving the optimum compromise between transmissible torque and rigidity at the rack. The output bearing consists of two preloaded and substantially oversized angular-contact roller bearings. This construction enables the bearing to support the pinion as close as possible to the load, as only the thickness of the fastening nut separates the pinion from the output bearing. This design reduces the radial bending, which is a factor responsible for up to 60 per cent of the overall distortion but is nevertheless still poorly handled by many existing systems.

The result is increased dynamic precision thanks to the design's capacity to move heavier loads at higher speeds. Its performance in terms of compound stiffness combined with its low inertia enables higher accelerations (up to 25m/s2).

As a consequence, the natural frequencies of the global drive systems in which the rack-and-pinion drive is integrated can be raised significantly.