(August 23, 2011)
In the past, rack and pinion drives were generally developed from a planetary gear unit with a pinion fitted to the output shaft and the matching gear rack. But the overhung load on the gear unit from the rack and pinion combination was then so high that it actually determined the size of the drive. Utilization of the gear units' actual torque would normally be only in the 40 to 60 percent range.
Stober Drives, Inc. wanted to overcome this disproportionality and developed a new concept for a super-compact and highly-efficient rack and pinion drive jointly with Atlanta, a premier gear rack manufacturer. The ZTRS drives are based on Stober's PH and PHA precision planetary gear units, which are designed for very high torque requirements as well as on torsional stiffness and tilting movement. The ratios range from 4-121:1 and are available in right-angle and integrated motor configurations.
"The most immediately striking feature of the new HighForce ZTRS-PH(A) rack and pinion drive is the integral output bearing housing, a highly stiff supporting assembly for the rack pinion, which relieves the load on the flange-mounted planetary gear unit bearings," said Adam Mellenkamp, product manager at Stober. "The reduced tilting moment of this construction makes it possible to base the drive design on the torque actually required."
As a rule, gear units can now be a size smaller than those used in the past, he added. But due to the comparatively small pinions, a higher output speed is necessary to obtain the required feed rate. This means that the gear ratio is lower, and in many cases a single-stage planetary gear unit can be used instead of a two-stage. This provides the outstanding high-power density that should encourage many designers to use re-designed rack and pinion drives. The flange-mounted planetary gear unit offers a choice between helical PH and PHA versions.
The ZTRS is a ready-to-install drive solution and can be attached directly to any servomotor. "Other advantages are that linear backlash is reduced by 50 percent, and linear stiffness is increased by up to 100 percent," Mellenkamp said. "The new mounting system makes it possible to use pinions with a very small pitch diameter."
Although size reduction was of central importance, other factors were also considered in designing the ZTRS. Apart from the improvement in linear stiffness and linear backlash, the pinion concentricity was adjusted to ≤ 0.01 mm. The wide variety of gear ratios and pinion teeth numbers allows optimized adaptation of the inertia, said Mellenkamp. Additionally, there is lubrication using a felt pinion, which is integrated into the cast output bearing housing for manual operation or connection to a central lubrication system. The HighForce ZTRS-PH(A) rack and pinion drive is now available in sizes 7, 8 and 9, modules 2 to 6 and gearing quality 5.
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