The strain wave gear was first designed in 1957 and has been a staple of precision engineering ever since. Sixty years later, the time-tested design has received an update to make it more effective for compact and space-restricted applications. Graham Mackrell explains a new development in strain wave gearing
Sometimes, you only need one small innovation to make something revolutionary. Sliced bread, for example, simply took a stroke of inspiration for Otto Rohwedder to combine automatic bread wrapping equipment with cutting functionality.
The strain wave gear has been the pinnacle of precision gearing since the technology was first developed in 1957 by inventor and founder of Harmonic Drive, Walton Musser.
Consisting of just three crucial components — the wave generator, flexspline and the circular spline — and ensuring zero backlash as well as a high gear reduction ratio, strain wave gears have long been the standard for critical applications.
However, the changing face of industry in recent years has driven increasing demand for highly precise large hollow shaft gears. Here, the bore of the gear can be used for running power cables or data lines through.
This kind of innovation is absolutely critical for many complex applications that require digital functionality. For example, surgical robots require a constant power supply to operate. Due to the location of the precision gears in the axis of the robot, it is essential that power lines can pass through easily and safely.
Making designs and processes more convenient is a powerful catalyst for innovation. After all, sliced bread was conceived purely to address the minute irritation of manual slicing. Similarly, the lack of hollow shaft in strain wave gears prompted the development of our new WavePlus technology.
Traditionally, the wave generator of a strain wave gear is made up of a specially designed ball bearing. This bearing is necessary for the wave generator to rotate precisely, but due to its shape, it is impossible to include a large hollow shaft in the design.
WavePlus addresses this issue by redesigning the strain wave gear to use a specially designed needle bearing. This needle bearing uses cylindrical rollers with very small diameters, capable of providing the same precise motion but with a significantly reduced footprint that allows for a wider central shaft-diameter.
Something as straightforward as this adjustment revolutionises the way that high precision gearing is delivered to the aerospace, medical and military sectors. It is a design that has worked well for Harmonic Drive’s recently launched IridiumLine component sets, which have shaft diameters of up to 60 per cent of the gear size. The technology also enhances our ability to offer bespoke designs for customers.
WavePlus technology is an advancement that reinvents the strain wave gear principle for increasingly technical demands of today's modern applications. While it is only a small change, for modern engineers this may be the greatest development since sliced bread.
Graham Mackrell is managing director of precision gearing expert Harmonic Drive UK.