Spur versus planetary gearheads for dc servomotors

Dc servomotors with built-in gearheads can provide a more cost-effective solution in a smaller package than an assembly of separate components. However, determining which type of gearhead is appropriate for the job involves evaluating factors such as backlash, efficiency, torque capacity, speed, reduction ratio, and cost.

Two common gearhead or reducer designs are spur and planetary. In general, spur gearheads are simpler and less expensive than planetary units and work best for low-torque applications. Torque capacity of spur types is limited because each gear in the train bears the entire torsional load. Planetary gearheads, in contrast, share the load over multiple planet gears. Here, the input shaft drives a central sun gear which, in turn, drives the planet gears. Each of the planet gears simultaneously deliver torque to a rotating carrier plate coupled to a geared output shaft.

Also a factor determining load carrying ability is the material from which the gears are built. Sintered nickel-steel is typically used because the sintering process economically produces gears in net shape that are able to run at closer tolerances. Moreover, because the material is porous, gears hold lubricant better than steel units. However, cut-steel gears tend to be more durable and are therefore a better choice for higher-torque applications. But adequate lubrication is important regardless of gear material, especially at high speeds and loads. Here, planetary gearheads have the advantage because oil flying outward from the sun gear is captured by the planet gears and carrier plate. Spur types, on the other hand, tend to fling lubricant off and away from the gears. This is one reason planetary gear-heads have higher speed ratings.

Next, consider backlash and reduction ratio. Backlash is a measure of positional accuracy usually specified in arc-minutes. For example, a typical spur gearhead has about 10 arc-min of backlash, whereas its planetary counterpart may have about half of that. Reduction ratios for both spur and planetary gearheads range from near unity up to several hundred to one. Spur gearheads, with a single geared input shaft coupled to a geared output shaft (single stage), provide about 6:1 reduction. Planetary units, for comparison, can reach roughly 10:1 in a single stage. For higher ratios and proportionally greater output torque, multiple stages or gear sets are stacked together axially. Increasing the number of stages boosts the reduction ratio and output torque but increases overall length and lowers mechanical efficiency. Output torque is expressed as:

out = N motor

where motor is the motor torque in oz-in., N is the reduction ratio, and is the efficiency. A typical single-stage spur gearhead, for example, is about 90% efficient whereas a two-stage model is about 85% efficient. Most planetary gearheads are roughly 97% and 94% efficient for one and two-stage units, respectively.

Information for this article was provided by John Marks, Applications Engineer, Pittman, Harleysville, Pa.