HD Systems has introduced a new planetary gearhead, which relies on technology borrowed from its line of harmonic drive gearing. This new gearhead, the Harmonic Planetary, utilizes a non-rigid ring gear to achieve both low backlash, and more importantly, consistently low backlash for the life of the gear.

Often, conventional planetary gearheads exhibit quite different backlash levels after a few hundred hours of use as compared to their "out of the box" levels. The conventional philosophy of planetary gearhead design dictates that all gearing elements must be as hard and rigid as possible. Low backlash is achieved by making a tight fit between gears. This tight fit is achieved in a variety of ways including adjustment of machining tolerances to create an interference fit between gears, and controlling the gear mesh of bevel-like gears with shims and washers. These methods do achieve low backlash over the initial period of use. However, the inevitable gear tooth wear that results, as when any mechanical elements are in contact, is not compensated for. This wear results in backlash levels increasing to many times their initial values. Another consequence of making "tight" gears is low efficiency due to high friction between gear teeth.

The Harmonic Planetary utilizes HD Systems' 30 years of design and manufacturing expertise in non-rigid gearing elements. The ring gear is designed to allow a small amount of elastic radial deformation. This radial deformation is just enough to provide a pre-load between the ring gear and planet gears which reduces the spacing between gear elements permitting backlash levels of 1 arc-min or less. As the gear teeth wear, this elastic radial deformation acts like a stiff spring to compensate for space between the teeth that would otherwise cause an increase in backlash. This creates very low and sustainable backlash levels with high torsional stiffness and efficiency.

The implications of the Harmonic Planetary design are important for suppliers of high precision automation equipment. Automation equipment is tested extensively before it is shipped to an end customer. The manufacturer may conduct accuracy and repeatability tests. The results of these tests allow the manufacturer to calibrate the system prior to delivery to the customer. Clearly, motion control components with wear that is not continuously compensated for will result in accuracy and repeatability characteristics that degrade over time. Therefore, the performance of the automation equipment will have quite different long-term operating characteristics as compared to the "as delivered" condition. The continuously self-compensating design of the Harmonic Planetary ensures consistent high performance over the life of the machine.