Machine Design

Small, light rack-and-pinion drive saves energy

In the past, rack-and-pinion drives were generally developed for planetary-gear units, with pinions fitted to the output shafts and matching gear racks. But the rack-and-pinion drive would put an overhung load on the gear unit that limited the size of the drive. Thus, the drives could only use 40 to 60% of the gear units’ torque.

Engineers at Stober, Maysville, Ky. wanted to solve this problem, so they developed the ZTRS drive, a compact, highly efficient rack-and-pinion drive. They developed the drive together with Atlanta, a gear-rack manufacturer based in Farmingdale, N. J.

The ZTRS drives are based on the company’s PH and PHA planetary-gear units, which are torsionally stiff and generate high torque. Ratios range from 4:1 to 121:1, and drives are available in right-angle versions and with built-in motors. But the ZTRS can be attached directly to any servomotor.

The output-bearing housing supports the rack-and-pinion drive, which relieves the load on the flange-mounted planetary gear’s bearings. This reduces the tilting moment and frees engineers to match the drive to the torque needed for their application. In general, this lets engineers use gear units a size smaller than those used in the past. The device also uses comparatively small-diameter pinions, so it takes more speed to get the required feed rate. This means the gear ratio is lower and, in many cases, a single-stage planetary gear unit can be used instead of a two-stage device.

The ZTRS reduces linear backlash by 50% to 3 arc-min, and increases linear stiffness by up to 100% compared to standard PH and PHA units. The new mounting configuration lets it use pinions with small pitch diameters.

Apart from improving linear stiffness and linear backlash, the pinion concentricity was adjusted to less than 0.01 mm. The drive is lubricated using a felt pinion built into the output-bearing housing. It can be manually relubed or hooked to an automated lubrication subsystem.

The flange-mounted planetary-gear unit offers a choice between helical PH and PHA versions. And the ZTRS-PH(A) rack-and-pinion drive comes in sizes 7, 8, and 9, and modules 2 to 6, with gearing quality 5.

© 2011 Penton Media, Inc.

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