Modern ac servomotors built with high-energy density, rare-earth magnets, and powered by sophisticated controls, produce significantly higher torque at higher speeds than conventional models.
Such motors have made possible direct drives (no gear reducer) in some applications. However, direct drives work best for relatively lighter loads only. Manufacturers of PCBs, for example, use direct-drive chip shooters to rapidly locate surface-mount components. Heavier loads and the subsequent inertia mismatch with the motor can bring control and stability problems.
A simple and cost effective way to match inertia is with a gear reducer. Unfortunately, some reducers aren't up to the challenges posed by these more potent ac servomotors. Consider input speed ratings, for example. Gear reducers for use with such motors must accept input speeds to 6,000 rpm or higher, about double that of some conventional units. Other desirable attributes include high torque capacity and torsional stiffness, compact size, and high efficiency. Modern planetary gear reducers can meet all these criteria compared to fixed-axis types that sometimes fall short. To understand the reasons for this requires a basic under-standing of geartrain dynamics and gear tooth design, as well as the features that separate apparently similar units.
Because each gear mesh in fixed-axis gear-heads carries the entire torsional load, loads are limited by the weakest gear in the train. Planetary geartrains, in contrast, share the load over multiple gear meshes. This is why planetary models have higher torque capacity and torsional stiffness for a given frame size. These geometric differences also affect gear lubrication. Gears mounted on fixed axes sling lubricant away, but planetary units continuously redistribute it. Lack of lubrication limits rotational speeds of fixed-axis units.
Besides geometry, gear type is another important metric. Although parallel-shaft, fixed-axis gearboxes are generally called spur gear reducers, many actually contain helical gears. Helical gears have a larger contact ratio and can transfer higher forces with less audible noise than spur gears. However, for a given gear diameter and tooth count, spur gears can be stronger than helical types. This is because the helix angle requires that teeth be cut with a smaller root area. Less material at the root reduces load capacity and tolerance to shock loads. And peak torque capacity is a critical factor in applications where loads abruptly stop and start.
Another downside to helical gears is that they develop axial forces. These forces react into a thrust washer causing additional frictional losses, heat, and wear. Spur gears, in contrast, produce only radial forces when meshing. Installing double-helical herringbone gears is one way to eliminate axial forces in helical geartrains. But, herringbone gears are costly to make and generally used in larger gear reducers built for extremely high torque.
A practicable method for boosting torque capacity in smaller, servogear reducers is to add planet gears. Here, the load is distributed over more gear teeth. But, this is possible only for ratios of about 5:1 or lower. Higher reduction ratios require larger planet gears which limits gear count for a given package size. Increasing gear face width or optimizing tooth profiles are yet other ways to increase torque capacity of these smaller gear reducers.
Much of the noise produced by gear reducers comes from gear teeth impacting one another. Helical gears, in general, are quieter than spur gears because contact between teeth is more gradual and continuous. Both types, however, are subject to manufacturing inaccuracies. These deviations from theoretical involute curves cause transmission errors that show up as vibration and periodic changes in speed. Helical gears, because they are more difficult to produce, tend to be more susceptible to manufacturing inconsistencies than spur gears. Through careful machining, however, transmission errors can be practically eliminated and noise reduced significantly. For high-speed planetary servogear reducers, gear teeth should be precision cut, heat treated, and ground or honed to final finish. Such practices can make spur gears less noisy than helical gears.
Information for this article was provided by Dr. Gerhard Antony and Thomas Herr, Neugart USA, Bethel Park, Pa.