Micro gear grows for mass production

Edited by the Engineeringtalk editorial team Jul 3, 2002

For a world that is constantly calling for devices to become smaller with no reduction in performance or quality, Harmonic Drive has introduced an extra-extra small precision gear unit.

For a world that is constantly calling for devices to become smaller with no reduction in performance or quality, Harmonic Drive has introduced an extra-extra small precision gear unit.

Measuring just 8mm in diameter by 1mm in depth, this backlash-free gear has a reduction ratio of between 500:1 and 1000:1.

It provides a highly efficient torque increase and achieves a repeatability accuracy of a few second of arc.

Its maximum output torque is 10mNm at an output speed of 100rev/min.

To manufacture this minute gear, which is based on Harmonic Drive's unique wave generator, flexspline and circular spline construction, a special production technique has been developed.

Although conventional methods of gear production can be used down to a module of 60-100mm, depending on the tooth geometry, there are significant restrictions on optimal tooth geometry if conventional techniques are applied to manufacture a gear with a module within this range.

As a result, micro-technological methods, such as the LIGA-process, have been employed for the manufacture of the micro gear's components.

Typically, these methods are derived from processes for the production of semiconductor components and utilise lithographic techniques.

Horizontal dimensions are represented in a blocking absorber coating on a mask and are projected precisely into a photo resistant material during exposure.

In order to achieve structures with heights up to several millimetres yet with tolerances better than 1mm on horizontal dimensions it is necessary to use a synchrotron X-ray source.

This provides high-energy, short-wavelength, highly parallel X-rays that minimise refection and scattering in the photo-resist and so make high-aspect ratio, highly accurate structures possible.

However, these initial 3D structures are expensive to produce so a different, more cost-effective forming process is required to replicate the highly accurate structures on a commercial basis.

Consequently, a new production technique had to be developed for the manufacture of micro metal components.

The new technique (patent applied for) is based upon the LIGA process, with a special forming process, a "second galvanoforming".

X-ray lithography is used to produce highly accurate master moulds that can be reproduced using cost-effective mass production processes, such as plastic injection moulding.

These plastic structures serve as last moulds for the second galvanoforming step.

In this phase, metal is deposited in the last moulds by galvanoforming, so enabling the high volume production of miniaturised metal gear components.

"These precision microgears are a key enabling technology for a new generation of miniaturised devices in a wide range of application areas", says David Wheeler, Managing Director, Heason Technologies Group.

He went on, "They are ideally suited to precise positioning applications in optics, medical equipment, semiconductor manufacture, robotics, laser technology and biotechnology".