Using Gearheads in DC Servomotors
Gearheads are used primarily to make controlling a large load inertia with a smaller motor inertia possible. The current and motor torque would have to be equal times greater as the reduction ratio used without a gearhead. A selection of windings in each frame size is available, which offers a variety of solutions for output requirements when combined with reduction ratios. There are unique advantages offered by each combination of gearhead and motor.
A more cost-effective solution in a smaller package compared to the assembly of separate components is provided by DC servomotors that come with built-in gearheads. Factors including torque capacity, reduction ratio, backlash and efficiency must be evaluated to determine which type of gearhead is right for the job.
Planetary and Spur Gearheads
Planetary and spur are two common reducer or gearhead designs. In general, spur gearheads work better for low torque applications, and less expensive and simpler than planetary units. As each gear bears the full torsional load, spur types have limited torque capacity. In contrast, planetary gearheads divide the load across multiple planet gears. Here, the planet gears are driven by a central sun gear driven by the input shaft. Torque is simultaneously delivered by each of the planet gears to a rotating carrier plate fixed to the output shaft.
The material from which the gears are built is another factor that determines the load carrying capacity. Sintered nickel-steel is the favored material to use, as the sintering process produces gears that are able to run at closer tolerances more economically. In addition, the porous material the gears are made from holds lubricant more efficiently than steel gears.
Higher Torque Applications
For higher torque applications, steel gears are more durable, and make a more appropriate choice. Regardless of gear material, adequate lubrication is a must, particularly at high loads and speeds. As oil projected outward from the sun gear is caught by the carrier plate and planet gears, planetary gearheads are more advantageous here. Spur types, in contrast, often propel lubricant away from and off of the gears. This is a primary reason that planetary gearheads generally get higher speed ratings.
Reduction ratio and backlash should be considered next. Backlash, which is measured in arc-minutes, measures positional accuracy. The average spur gearhead, for example, has approximately 10 arc-minutes of backlash, with its planetary counterpart only having 5 arc-minutest. Reduction ratios for both types of gearhead range from near balanced up to several hundred to one.
With a geared output shaft fixed to a single geared input shaft, spur gearheads provide about 6:1 reduction. In comparison, planetary units can achieve approximately 10:1 in a single stage. Multiple stages or gear sets are stacked together axially for greater output torque and higher ratios. The output torque and reduction ratio are boosted by increasing the number of stages, but overall length is increased, and mechanical efficiency is lowered.
For example, 90% efficiency can be achieved with a typical single-stage spur gearhead, with only 85% achieved with a two stage model. Planetary gearheads, by comparison, reach an efficiency of 97% in one-stage units and 94% in two-stage.