Rack-and-pinion drives gain momentum delivering zero-backlash motion.

Atlanta racks install in modular sections, and total standard pitch error over a 157.48-in. section, for instance, is less than the accumulated error of a ballscrew the same length.

Within Atlanta's Split Pinion mechanically preloaded system, one half of a helical pinion is fixed on the shaft, and the other half slides axially along the shaft splines, while axial springs induce the preload.

The mechanically preloaded Redex DualDrive system has two reducers with two output pinions in a closed loop and provides easy preload torque setting while the pinions mesh with the rack.

As an electrically preloaded drive system, the Redex TwinDrive uses two reducers, two pinions, and two servomotors — one is a master and the other a slave.

CNC machine tool performance depends on the precision of linear-axis drive systems. Traditionally, OEMs use ballscrews or linear-motor technology to achieve this, but because of increased machine speeds and long axis lengths, ballscrews are somewhat limited by their inherent critical speed. While linear motors meet these demands, they are expensive and subject to thermal stress. Fortunately, machine tool OEMs have a third alternative — zero-backlash rack-and-pinion drives.

Although such drives are already widely incorporated, they've lost favor over the past few years due to the unavailability of standardized high-quality (AGMA 12 or greater) rack systems. As a result, competing machine tool manufacturers design their own racks, pinions, and preloaded gearboxes. Now, however, Andantex USA, in cooperation with its global affiliates, offers a modular system of racks, pinions, reducers, and lubrication systems that let OEMs build any length axis from standardized components as well as handle weights from 100 to 100,000 lb.

Atlanta, an Andantex affiliate, manufactures the off-the-shelf racks in a range of modules. These include M from 2 to 10 (diametral pitch = 25.4/M) in lengths of 19.68, 39.37, and 59.05 in. with straight (spur) or helical teeth in standard quality levels up to DIN 6h25 (~AGMA 12).

The racks link together using companion racks that ensure proper tooth spacing. Sections are standard components manufactured in lots, which makes them less expensive and quickly delivered. Standard pitch error is

0.00087 in./ft, but since the racks install in modular sections, total error over a 157.48-in.section, for instance, is less than the accumulated error of a ballscrew the same length.

Achieving zero backlash
To eliminate backlash between the rack and pinion, Atlanta offers two classes of preloaded systems. The first, which incorporates mechanical preloading, includes the company's Split Pinion and Redex DualDrive products, while the second uses electrical preloading and features the Redex TwinDrive reducer.

Mechanical systems achieve preload from spring loading or torsionally winding one pinion against another while they contact the rack. One pinion is considered fixed and the other spring loaded. One motor drives such systems, and its size does not have to increase for accommodating the preload torque. In fact, motor size is just enough to move the load and overcome normal system efficiencies.

Atlanta's split-pinion design consists of a helical pinion in two halves. One half is fixed on the shaft, and the other half slides axially along the shaft splines. Axial springs induce the preload.

The split pinion mates to the helical rack at the correct center distance, and the moving pinion half displaces axially via preload spring compression. Because of this displacement, the pinion half rotates on the helix angle until it eliminates backlash. Once that's done, the preload nut, which is turned a specific number of graduations, induces the required preload for easy and accurate settings.

Since this design does not eliminate reducer backlash, manufacturers combine the split pinion with a precise servo-worm reducer sporting less than 2 arc-min of backlash. Using one servomotor makes these systems cost effective, and they provide high positioning accuracy for small machines with axis weights up to 25,000 lb and rack forces to 3,700 lb.

The Redex DualDrive system uses two reducers with two output pinions in a closed loop. A torsional preload makes for easy preload-torque setting while the pinions mesh with the rack, and output pinions integrate with output shafts to optimize gearbox stiffness. Specially selected diameters ensure that resonant frequency is higher than 20 Hz and torque experienced by the gearbox is as light as possible.

A universal housing permits easy machine integration because the gearbox mounts to any surface. The DualDrive, combined with one servomotor, delivers precision positioning and contouring accuracy on machines with axis weights up to 100,000 lb and rack forces as high as 15,000 lb.

Unlike mechanical ones, electrically preloaded systems incorporate two servomotors and reach preload by braking one pinion against another. These systems change the magnitude and direction of the preload so that the two motors work together while accelerating large masses.

The Redex TwinDrive has two reducers, two pinions, and two servomotors. One servo motor is a master, and the other a slave. The drive's reducers are similar to the DualDrive except there is no mechanical preload.

Choosing between mechanical and electrical preload systems is a function of a machine manufacturer's preferred control scheme. Mechanical systems have a single-motor controller, while electrical ones require a more-complicated two-motor controller.