Sometimes it pays to be eccentric

There's more than one way to get high positional accuracy, minimal backlash, and high reduction rations in small packages. An unassembled harmonic-drive gear from HD Systems Inc., Hauppauge, N.Y., shows the three main components. The reducer includes a wave generator, far left, which engages the flexspline, center, with the circular spline in two regions on opposite sides of the ellipse's major axis. Harmonic-drive pancake gears from Harmonic Drive Technologies, a Peabody, Mass., harmonic-gear manufacturer, are nearly flat for applications that require high reduction ratios in small packages. HDF models, for example, are as thin as 11 mm and have torque ratings from 56 to 3,180 lb-in. with higher peak ratings. Dojen cycloidal speed reducers, from Mectrol Corp., Salem, N.H., maintain constant contact between all cam lobes and followers. This creates high shock-load capacity and consistent torsional stiffness. Designing a system that demands minimal backlash, tight space constraints, or high reduction ratios, often calls for looking beyond traditional spur or helical gearboxes. In fact, less common power-transmission devices can often solve all of these design constraints. Several technologies produce high reduction ratios in small lightweight packages. They do it using clever, reliable kinematic principles instead of conventional methods. All three methods highlighted in this article use some type of noncircular rotation to generate reductions. Harmonic gears use an elliptical disc to flex a geared ring that meshes with a stiff outer ring. Cycloidal speed reducers use cams and needle rollers to generate reductions. Bearing reducers have several sets of bearings that serve different functions, including letting the reducers carry radial and axial loads. For each device in this article, either of the three main components can act as either the input, output, or fixed member. This increases the reducers' versatility. Because the components are reversible, they can be used as speed reducers, increasers, or differentials. If an application requires the component to hold a load, however, a brake must be specified. Although all three types of reducers offer high reductions, small packages, and high accuracy compared to traditional gear reducers, each has something special to offer. Harmonic-drive gears, for example, provide high reduction ratios in a single stage and good torque-to-weight ratios. Cycloidal speed reducers offer extremely high stiffness and high shock-load capacity. And bearing reducers operate with