Wobbling gears achieve high ratios

Wobbling gears achieve high ratios

Tom Shelley reports on a novel gear transmission that can achieve exceptionally high reduction ratios in a small space



A gearbox that works on the same principle as a rotating coin, just prior to its falling flat, is able to achieve reduction ratios of 1,000 to 1 or more within a compact space.

Developed for space probes, including one proposed to be sent to the planet Mercury, it is also proving to be of interest to automotive engineers.

First commercial applications are expected to be in seat recliners and electric bicycles.

The basic working principle is based on nutation - the periodic variation in the precession of the axis of a spinning body. In 1728, the British astronomer James Bradley discovered that it causes a slight wobble in the rotation axis of the earth, with a periodicity of 18.6 years, although it took another 20 years to explain it. The easiest example to understand, however, is what happens to a spinning coin when it falls on one side but continues to rotate.

Because the coin is tilted relative to the surface it is rotating on, the circumference traced by the edge in contact with the surface is slightly less than the circumference of the coin. This means that for each rotation of the position of contact, a point on the edge of the coin moves further round in the direction of rotation by a small amount.

In a device developed by a small Italian company, the coin is replaced by an inclined bevel gear, and the ground by a fixed axial bevel gear. The first bevel gear is free to rotate about its axis, which is inclined to the axis of the input shaft and that of a second, fixed bevel gear. Inside the first bevel gear is a third bevel gear, coaxial with and fixed to the first. The third gear engages with a fourth bevel gear attached to the output shaft and coaxial with it.

An alternative arrangement has the first and third bevel gears mounted back to back instead of one inside the other. The input shaft then runs through the centre of the second bevel gear, which is still fixed. The third bevel gear engages with the fourth bevel gear on the output shaft as before.

In the commercial market, it has to be considered against a number of established high ratio reduction systems. There are a large number of these. The most common are probably multi stage spur gear gearboxes, planetary gearboxes and worm drives, but we should not forget harmonic belt drives, cyclo reducers, and other more exotic devices that we report on from time to time.

The main advantage of the nutating gearbox is the small space required, in which four bevel gears are able to achieve reduction rations from 10:1 to 1000:1. For large scale applications, such as power generation, in gearboxes more than 1m cube, it is thought possible to achieve reduction ratios of as much as 3000:1. A further advantage lies in the fact that at least four teeth are in contact with each other at any one time, allowing gears to be made from lower strength material and more torque to be transmitted with less noise and vibration. It is also said to be very efficient.

Marketing is being handled by Genoa based technology promoters, D'appolonia, who call it the 'Spacegear'.

The company comments, "There is an increasing number of electrically driven actuators in a car, such as: windows, rear-view mirrors, seats, windscreen wipers, antennas, spoilers, roof, hoods, oil pumps and water pumps. All these functions are or can be driven by electric motors. The size of the motors is directly linked to the torques needed to produce the required motion. If a high reduction ratio can be achieved in a limited space, smaller and faster electric motors can be used to provide the same level of mechanical power."

One application investigated is in seat recliners. Such devices, which are located in the seat body, typically comprise a planetary gear unit to reduce speed plus a worm drive plus a motor unit. A prototype of seat recliner with a spacegear is said to have been produced in collaboration with an unnamed maker of car automotive devices. For the more environmentally aware, a coaxial geared motor is also said to be currently under development for electric bicycles. While electric bicycles have yet to catch on in Europe, 400 Chinese manufacturers produced 1.6 million electric bicycles last year, and the figure for this year may reach 2.5 million, according to Wang Fenghe, chairman of the China Bicycle Association. The market in Japan is said to be 160,000 to 200,000, expected to rise to 500,000 per year.

In aerospace, a feasibility study is currently under way to investigate the possibility of using the gearbox in the European Space Administration Bepi Colombo probe, scheduled to go to the planet Mercury in 2011. The gearbox has to meet very stringent requirements in terms of weight and volume without exceeding a maximum certain diameter.

Other possible applications which D'appolonia intends to seriously study in the near future, include: the powering of water and oil pumps on car engines, driving air conditioning compressors; adjustment of mirrors, windows, and external wings; automatic safety adjustments including seat positioning devices; devices for elderly and disabled people; household appliances; robotics and marine winches.

D'Appolinia
MST Aerospace-European Space Agency Technology Forum
Andrea Barbagelata at D'Appolonia

Eureka says:

This looks to be the most compact and rugged very high ratio speed reduction gearbox we have come across so far

Pointers

* Gearbox is based on four bevel gears in an exceptionally compact arrangement

* It is possible to achieve reduction ratios of from 10:1 to 3000:1

* Because at least four teeth are in contact with each other at any one time, it is possible to use gears made from materials that are less expensive while transmitting more torque with less vibration and noise

Author
Tom Shelley

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