Gearmotors are an all-in-one combination of an electric motor and one or more pairs of gears (which is also known as a gearbox). A gearmotor simplifies combining a motor with a gear reducer system.

Planetary Gears

Planetary gears consist of one or more outer gears, or planet gears, revolving about a central, or sun gear. Typically, the planet gears are mounted on a movable arm or carrier that may rotate relative to the sun gear. These systems often use an outer ring gear or annulus, which meshes with the planet gears.

The gear ratio in this type of system is not obvious, particularly because there are several ways in which an input rotation can be converted into an output rotation.

Typically, one of these three gear wheels is held stationary; one is an input providing power to the system, while another is an output, receiving power from the system. The ratio of input rotation to output rotation is dependent upon the number of teeth in each gear, and upon which component is held stationary.

Planetary gears offer several advantages over parallel axis gears. These include high power density, the ability to achieve a large reduction in a small volume, multiple kinematic combinations, pure torsional reactions, and coaxial shafting. The disadvantages include high bearing loads, inaccessibility, and design complexity.

In a planetary gearbox arrangement, one advantage is power transmission efficiency, which is typically 3% per stage. Thus, a high proportion of the energy transmitted through the gearbox is used rather than wasted on mechanical losses inside the gearbox.

Planetary gearbox arrangements distribute load efficiently too. The transmitted load is shared between multiple planets, which greatly increases torque density. The more planets in the system, the greater load ability and the higher the torque density. This arrangement is also very stable due to the even distribution of mass and increased rotational stiffness.

Strain Wave Gearing

Strain wave gearing is an approach to speed reduction using metal elasticity (deflection) of the gear to reduce speed. (Strain wave gearing is also known as Harmonic drives–a registered trademark term of Harmonic Drive Systems Inc.) The benefits of using this approach include zero backlash, high torque, compact size, and positional accuracy.

A strain wave gear is comprised of three components: Wave Generator, Flexspline, and Circular Spline.

The Wave Generator is an assembly of a bearing and a steel disk called a Wave Generator plug. The outer surface of the Wave Generator plug has an elliptical shape machined to a precise specification. A specially designed ball bearing is pressed around this bearing plug causing the bearing to conform to the same elliptical shape of the Wave Generator plug. The Wave Generator is typically used as the input member, usually attached to a servomotor.

The Flexspline is a thin-walled steel cup. Its geometry allows the walls of the cup to be radically compliant, yet remain torsionally stiff since the cup has a large diameter. Gear teeth are machined into the outer surface near the open end of the cup (near the “brim”). The Flexspline is usually the output member of the mechanism.

The cup has a rigid boss at one end to provide a rugged mounting surface. The Wave Generator is inserted inside the Flexspline so that the bearing is at the same axial location as the Flexspline teeth. The Flexspline wall near the brim of the cup conforms to the same elliptical shape of the bearing. This causes the teeth on the outer surface of the Flexspline to conform to this elliptical shape. Effectively, the Flexspline now has an elliptical gear pitch diameter on its outer surface.

The Circular Spline is a rigid circular steel ring with teeth on the inside diameter. It is usually attached to the housing and does not rotate. Its teeth mesh with those of the Flexspline. The tooth pattern of the Flexspline engages the tooth profile of the Circular Spline (circular) along the major axis of the ellipse. This engagement is like an ellipse inscribed concentrically within a circle. Mathematically, an inscribed ellipse will contact a circle at two points. However, the gear teeth have a finite height. So there are actually two regions (instead of two points) of tooth engagement. Roughly 30% of the teeth are engaged at all times.

The pressure angle of the gear teeth transforms the output torque’s tangential force into a radial force acting on the Wave Generator bearing. The teeth of the Flexspline and Circular Spline are engaged near the major axis of the ellipse, and disengaged at the minor axis of the ellipse.

The Flexspline has two less teeth than the Circular Spline. Thus, every time the Wave Generator rotates one revolution, the Flexspline and Circular Spline shift by two teeth. The gear ratio is calculated by:

Number of Flexspline Teeth / (Number of Flexspline Teeth – Number of Circular Spline Teeth)

The tooth engagement motion (kinematics) of the strain wave gear is different than that of planetary or spur gearing. The teeth engage in a manner that allows up to 30% of the teeth (60 teeth for a 100:1 gear ratio) to be engaged at all times. This contrasts with maybe 6 teeth for a planetary gear, and 1 or 2 teeth for a spur gear.

In addition, the kinematics enable the gear teeth to engage on both sides of the tooth flank. Since backlash is defined as the difference between the tooth space and tooth width, this difference is zero in strain wave gearing.

As part of the design, the gearteeth of the Flexspline are preloaded against those of the Circular Spline at the major axis of the ellipse. They are preloaded such that the stresses are well below the material’s endurance limit.

In the gear as the gear teeth wear, this elastic radial deformation acts like a very stiff spring to compensate for space between the teeth that would otherwise cause an increase in backlash. This allows the performance to remain constant over the life of the gear.

Strain wave gearing offers high torque/weight and torque/volume ratios. The lightweight construction and single stage gear ratios of up to 160:1 allows the gears to be used in applications requiring minimum weight or volume. Small motors can exploit the large mechanical advantage of a 160:1 gear ratio to create a compact, lightweight, and low cost package.

A new tooth profile for strain wave gearing has become available in the last few years. This “S” tooth design allows more gear teeth to engage. The effect is to double torsional stiffness, double peak torque ratings, and lengthen operational life.

The “S” tooth form does not use the involute curve of a tooth. Instead, it uses a series of pure convex and concave circular arcs that match the loci of engagement points dictated by theoretical and CAD analysis.

The increased root filet radius makes the “S” tooth much stronger than an involute curve gear tooth. It will resist higher bending (tension) loads while maintaining a safe stress margin.


Content provided by Design World.