Harmonic drives adapt to today's applications

Graham Mackrell, UK Sales Director for Harmonic Drive, looks at the technology underpinning harmonic drives and the benefits it offers to industry.

One of the many technological spinoffs of the "space race" of the 1950s and 1960s between the USA and the USSR, the harmonic drive has become synonymous with precision motion control.

The recognised benefits of this gearing technology, invented in 1955 by the American Walter Musser, include very low or even zero backlash, high reduction ratios and an assembly that is both lightweight and robustly reliable.

Following Musser's patenting of the harmonic drive principle, which is fundamentally different to conventional epicyclic gears, the early development and production of the new precision gear was done exclusively for the US National Aeronautics and Space Administration (NASA).

Among the notable results of this collaboration over the years have been the drives for the wheels of the Lunar Rover vehicle, and motion control devices aboard satellites such as the Hubble space telescope and the International Space Station.

By the early 1970s, however, several companies, including the German and UK-based Harmonic Drive (under its original name of Harmonic Drive Systems), were putting the technology into use in many other industries for the precision control of machinery such as robots, machine tools, printing presses, MRI scanners and aviation controls.

Aerospace was still an important market, though, with Harmonic Drive gears being applied in the European aerospace industry for the first time in 1976.

One of the obvious attractions of the harmonic drive to industries such as aerospace is the compact, lightweight nature of its construction.

The design itself is ingeniously simple.

Every harmonic drive comprises just three concentrically mounted components.

These are known as the wave generator, the flexible spline (or flexspline), and the circular spline.

If we start from the motor, or driven, end of the drive, the wave generator is the component mounted onto the motor's drive shaft (for the majority of applications this would be a servomotor).

Although at first sight the generator appears to be a circular "plug" with a thin-raced ball bearing fitted around its circumference, it has in fact a precision-machined elliptical shape - a shape taken up by the bearing with the whole wave generator assembly acting, in effect, like an oval cam that bears against the inside of the flexspline.

The flexspline is really a flexible steel cylinder flanged at one end (the opposite end to that which accepts the wave generator) and with external teeth precision-machined around its circumference.

The output drive from the harmonic gear unit is mounted onto the flange, while the flexspline itself rotates within the circular spline.

The latter is a rigid steel ring, anchored to the casing of the drive unit, but this time with internal teeth precision-cut into its inner diameter.

The drive is assembled in such a way that the flexspline's external gear teeth engauge with the internal teeth of the circular spline.

The flexspline, however, being slightly smaller in diameter than the circular spline, is always machined with two fewer teeth than the circular spline - which is one of the defining characteristics of the harmonic drive.

What happens in operation is that when the wave generator is rotated by the motor, because of its shape it imparts a continuously moving elliptical wave-like motion to the inside of the flexspline.

The flexspline now takes on that elliptical shape within the circular spline so that the flexspline's teeth only engage with those of the circular Spline at either ends of the major axis of the eclipse.

On the minor axis the flexspline teeth are fully disengaged from the circular spline.

As the wave generator rotates, the "zone of engagement" progresses around the fixed teeth of the circular spline.

But because of the two-teeth difference between flexspline and circular spline, each rotation of the wave generator has the effect of rotating the flexspline two teeth in the opposite direction.

The high-speed motor input to the wave generator is thus reduced to a significantly lower output speed (in the opposite direction of rotation to that of the motor).

The actual reduction ratio is calculated by dividing the number of teeth on the flexspline by two (ie the teeth displacement per revolution).

In the HFUS range of units from Harmonic Drive, for example, single-stage reduction ratios from 50:1 to 160:1 are available.

But the harmonic drive is much more than a speed reduction device.

Because of the way in which its gear teeth engauge - at opposite ends of the induced ellipse, rather than at single points of contact as in planetary or spur gears - the drive offers exceptional positioning accuracy and virtually zero backlash.

The HFUS units, for example, have an accuracy of better than one minute of arc, with repeatability of just a few seconds of arc.

Despite their low weight and small size, these drives have a high torque capacity and torsional stiffness, making them ideal for a wide range of positioning applications.

This is because up to 30% of the total number of teeth are always engauged and taking some load at any one time.

A typical application, demonstrating many of the harmonic drive's benefits, can be found onboard the latest Airbus A340-600, the world's largest passenger aircraft.

In conjunction with Airbus Germany, Extel Systems Wedel designed and developed the special lifts used in flight to move loaded serving trolleys, weighing up to 240kg, up from their storage area beneath the passenger deck.

These lifts move the trolleys via a toothed belt arrangement driven by a pulley mechanism mounted on the lower deck.

The precision drive capability for the lifts is provided by Harmonic Drive HFUC-40 model gears, operating outside of the passenger deck area where ambient temperatures and pressures can be extremely low.

However, the gears are constructed from all stainless steel parts, eliminating any problem of differential expansion and contraction that can be found in other drives manufactured from dissimilar materials.

With each drive set measuring just 135 x 53mm and weighing only 1.7kg, the units met the design specifications for the lowest possible weight and a small footprint, coupled with minimal operating noise - a particular feature of the harmonic drive technology, which is virtually vibration-free.

With an expected minimum operating life of 20 years and with only minimal maintenance required, the harmonic drive component sets used on the A340-600 have now also been specified for the new Airbus A380.

One attribute of many harmonic drives which was not really called for by Airbus has, nevertheless, proved to be of enormous value to Reis Robotics, one of the world's leading developers and integrators of industrial robotics systems.

Its RV6L vertical, articulated arm robot, for example, incorporates a CO2 laser, the beam of which is directed from the source to the cutting head by a sequence of enclosed lenses and mirrors, or beam guides, mounted within the robot's arm.

To enable the upper robot arm to carry payloads of up to 400kg, the relatively heavy laser source of the RV6L is located in the lower part of the articulated arm, near the base mounting plate.

The laser beam, therefore, has to be focused through the articulated joint between the lower and upper arms.

This is where Harmonic Drive's HFUS technology has proved its worth.

Because of the concentric design of the drive's components, the HFUS gear component set can be supplied with a large diameter, hollow central shaft, through which the robot's laser beam can be directed.

Harmonic Drive has made this possible because the HFUS supplied to Reis Robotics features a special flexspline diaphragm, which opens outwards, rather than inwards.

The drive is supplied as a complete unit with housing and output flanges, together with cross roller output bearings between the circular spline and the housing flange to support the upper robot arm.

Innovations such as the hollow shaft design, and on-going developments in reducing their size and weight even further, should ensure that harmonic drives continue their deserved popularity for precision motion control.

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