Motion Control - Gearheads
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Gearheads perform two key tasks in motion systems: torque multiplication and inertia matching. In torque multiplication, the gearhead provides a mechanical advantage to scale the output torque of the motor by the ratio of the gearhead. This can enable the system to use a smaller, cheaper motor for an overall cost reduction, more than offsetting the price of the gearbox.
Gearboxes can also be used for inertia matching. In this case, the inertia goes as the square of the reduction ratio. A 10:1 gearbox provides a factor of 100 mechanical advantage for inertia matching. Here, again, the effect enables a smaller, lower cost motor to be used in the system.
Although gearboxes are essential and powerful tools for good motion systems, they need to be chosen with care. It’s important to take into account factors like load, lifetime, accuracy, and efficiency when making a choice. Reduction ratio is also important. The entries below discuss these trade-offs in more detail.
A cycloidal gearbox consists of a pair of slightly eccentric gears placed side-by-side and enclosed by a ring gear. The two plates are clocked slightly relative to one another so that they alternate carrying the load, precessing around the ring gear in a cycloidal path. At any one time, a large number of teeth are in contact. This enables cycloidal gearboxes to achieve extremely high accuracy, with backlash on the order of 30 to 40 arcsec. They are also able to handle very high loads.
Cycloidal gearboxes typically require more effort to specify and install than other types of gearboxes. Before they can be installed in an axis, they typically require custom flanges and motor mounts. Motion Solutions applications engineers will work with you to help specify the right cycloidal gearbox for your application. We leverage long-standing relationships with the factory engineers and deep domain expertise with these products to help you integrate them into your design and commission the equipment.
Summarizing their properties, from worst to best:
Accuracy: spur, planetary, worm, cycloidal
Backlash: spur, planetary, worm, cycloidal
Efficiency: worm, spur, planetary, cycloidal
Lifetime: spur, worm, planetary, cycloidal
Load: spur, worm, planetary, cycloidal
For applications with higher loads or greater reliability demands, planetary gearing provides a better solution. Planetary gears consists of a central “sun” gear surrounded by multiple “planet” gears and held together by a retaining ring. The increased contact area boosts reliability. Planetary gears can handle much higher loads than spur gears. Backlash drops to 1 to 15 arcmin. Operating speed ranges from 2000 RPM to 4000 RPM, with 3000 RPM being the sweet spot.
The trade-off for that performance is that planetary designs can be expensive. The additional gears also generate more audible noise than the alternatives. Planetary gearboxes are available in 90-degree configurations by adding a bevel-gear stage.
Spur gears are functional, economical, and widely available. They are generally configured as 90-degree gearboxes. They do have some performance limitations, however, making them best suited to low-and applications.
The contact surface between gears is a single tooth. That limits the load they can handle and makes them vulnerable to failure. The single-tooth contact also limits accuracy; typical spur gearboxes have backlash of around 60 arcmin. For the right application, though, they can be very effective.
Intrinsically 90-degree gearboxes, worm gearboxes offer reduction ratios ranging from 5:1 to 100:1. These high ratios enable them to deliver very high-accuracy performance. For reduction ratios above 50:1, the effect is so extreme that they can be self locking. This feature makes them useful to prevent back driving.
On the downside, the self-locking characteristic means that their efficiencies are only about 30%. Backlash varies depending upon quality. Entry-level worm gearboxes might have backlash around 60 arcmin but for high-end versions, backlash can be as low as 1 arcmin.