Determining Speed Reducers for Robotic Applications

Tutorial contributed by Thomson Micron, LLC

Designers and specifiers of robotic systems commonly use speed reducers (or gearheads) to optimize speed, increase torque, and improve the performance and precision of the system's mechanical components. Recent innovations present engineers with more choices than ever. Choosing the gearhead for your robotic system can be daunting as multiple gearheads can be required in one system. This article will help you identify the gearhead technology and type best suited to your application.

Harmonic Gear

Gear configurations is what sets the major types of gearheads apart. There are four basic types of gearing systems used by designers: harmonic drive, cycloidal drive, parallel shaft spur drive and planetary. We will take a look at each of the four gearing systems and discuss the advantages and disadvantages of each.

Harmonic Drive

Construction:

Input -- elliptical wave generator in the center; output -- a flexible spline held in elliptical shape by wave generator has two fewer teeth than a fixed, rigid circular spline. The flexspline turns in the opposite direction of the wave generator.

Principle of Operation: As the wave generator rotates, the teeth of the flexspline and circular spline engage across the major axis of the ellipse. With each turn, the wave generator moves the flexspline two teeth back (counter clockwise) on the circular spline.

Advantages: High gear reduction (60:1 and higher) in a small envelope. Select for applications where high torque is needed in a small envelope.

Disadvantages: Not torsionally rigid. Wind-up in the flexible spline creates excessive lost motion. Difficult to obtain low ratios (<60:1). Position and velocity ripple occur.

Cycloidal Drive

Construction: Input -- high speed shaft with eccentric bearing assembly. Cycloid discs with one less lobe than the ring gear pins. Slow-speed shaft assembly. Fixed ring gear roller and pins.

Cycloidal Gear

Principle of Operation: "Ripple Effect." The high speed shaft turns the cycloid discs around the internal circumference of the fixed ring gear. With each full turn of the high speed shaft, the cycloid discs advance a distance of one lobe in the opposite direction of the ring gear. The cycloid discs transmit their movement to the slow speed shaft via the slow speed shaft pins. These pins are projected through the cycloid disc bores.

Advantages: High gear reduction in a small envelope (30:1 to 120:1 in a single stage). Good shock overload capacity.

Disadvantages: Poor efficiency due to high sliding contact. Excessive torque and speed ripple. Difficult to mount to motors.

Parallel Shaft Spur Gearing

Construction: A train of spur gears, mounted on bearing-supported shafts.

Principle of Operation: Two spur gears mesh to transmit torque.

Advantages: Low cost solution to simple speed reduction.

Disadvantages: Low torque capabilities. High backlash for the frame size. High noise, especially under load. Primary noise contributor is tooth deflection. Gearing requires periodic lubrication.

Planetary Gearing

Construction: Input -- sun gear. Fixed internal gear, output shaft carries planetary gears.

Principle of Operation: As the input sun gear turns, the three planet gears rotate and the load is shared equally across the three planet gears. The planet gear movement is transmitted to the output shaft.

Advantages: High torque output in a small envelope. Low backlash. High torsional stiffness. Load shared equally across three planet gears means longer gear life. Self-lubricating gears are maintenance-free. All parts rotate concentrically around center axis. High radial load capacity.

Disadvantages: Higher ratios (>10:1) require double stage gearheads.

Because today's designs require increasingly higher torque and precision while keeping the package size constant or smaller, most development is being focused upon the planetary gear configuration. With a more in-depth understanding of the planetary gearhead's advantages, you will understand why this technology is the focus of today's research:

* High Torque Output in a Small Package

Typically compared to parallel shaft spur gearheads where two gears mesh simultaneously, planetary gearheads transmit torque through three gear meshes. This change of gear configuration triples the torque capacity with expanding the envelope. As a result, planetary gearheads are the ideal partner for small, high speed servo and stepper motors.

* Low Backlash

With respect to gears, backlash is the distance by which a tooth space exceeds the thickness of an engaging tooth. This excess space translates to lost motion. For gearheads, backlash is comprised not only of gear error, but also manufacturing tolerances, bearing clearances and thermal expansion in the gears, housing and bearings.

* Optimal Torsional Stiffness

Motion can be lost not only to backlash, but also to torsional wind-up. This motion loss is measured after backlash has been removed. A gearhead's torsional deflection is caused by the rotational elastic deformation of the gearhead's components: output shaft, gear teeth and housings. In practice, the main source of torsional deformation is the gearhead output shaft. Torsional rigidity tests have provided the following rule-of-thumb. As the shaft diameter doubles, the torsional stiffness of the shaft increases by a factor
of 16.

* Load Shared Equally Across the Planet Gears

Gearing can increase motor bearing life. Planetary gearheads extend motor bearing life up to 900 times longer than conventional parallel shaft spur gearheads. The longer the motor bearing lasts, the longer the production line runs without downtime. Planetary gearheads extend motor bearing life by virtue of their gear configuration. Decreases in motor bearing life are typically the result of gear-induced loads on the motor shaft. With the planetary gear configuration, the torque transmitted to the sun gear is equally distributed across the three planet gears. Each planet gear mesh theoretically carries one-third of the resultant system forces, yielding a zero load on the motor pinion.

* High-Speed Input

Operating servo and stepper motors at higher speeds extracts the best performance and the highest efficiency from the motor. The planetary gearhead converts the high speed/low torque characteristic of the motor to a low speed/high torque characteristic, which is more suitable to the overall system. Using the planetary gearhead increases motor efficiency without sacrificing the performance of the mechanical components in the overall system.

* Self-Lubricating

In a gearhead, lubrication is primarily used to reduce friction and wear between the contacting surfaces of the gear teeth. With proper lubrication, gearhead life and performance are dramatically increased. Insufficient lubrication increases friction at the contacting surfaces. The friction increase manifests in localized heating to extremely high temperatures. At these elevated temperatures, the gear teeth surfaces wear and score, which can result in premature failure.

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